1. No category

Digital Comprehensive Summaries of Uppsala Dissertations
from the Faculty of Science and Technology 1418
Exploring Electron Transfer
Dynamics of Novel Dye Sensitized
Photocathodes
Towards Solar Cells and Solar Fuels
LEI ZHANG
ACTA
UNIVERSITATIS
UPSALIENSIS
UPPSALA
2016
ISSN 1651-6214
ISBN 978-91-554-9678-4
urn:nbn:se:uu:diva-302263
Dissertation presented at Uppsala University to be publicly examined in Häggsalen,
Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, Friday, 21 October 2016 at 13:15 for
the degree of Doctor of Philosophy. The examination will be conducted in English. Faculty
examiner: Professor Yiying Wu (The Ohio State University, Department of Chemistry and
Biochemistry).
Abstract
Zhang, L. 2016. Exploring Electron Transfer Dynamics of Novel Dye Sensitized
Photocathodes. Towards Solar Cells and Solar Fuels. Digital Comprehensive Summaries of
Uppsala Dissertations from the Faculty of Science and Technology 1418. 80 pp. Uppsala:
Acta Universitatis Upsaliensis. ISBN 978-91-554-9678-4.
The design of dyes for NiO-based dye-sensitized solar cells (DSSCs) has drawn attention owing
to their potential applications in photocatalysis and because they are indispensable for the
development of tandem dye-sensitized solar cells. The understanding of the electron transfer
mechanisms and dynamics is beneficial to guide further dye design and further improve the
performance of photocathode in solar cells and solar fuel devices.
Time-resolved spectroscopy techniques, especially femtosecond and nanosecond transient
absorption spectroscopy, supply sufficient resolution to get insights into the charge transfer
processes in p-type dye sensitized solar cell and solar fuel devices. In paper I-V, several
kinds of novel organic “push-pull” and inorganic charge transfer dyes for sensitization of ptype NiO, were systematically investigated by time-resolved spectroscopy, and photo-induced
charge transfer dynamics of the organic/inorganic dyes were summarized. The excited state and
reduced state intermediates were investigated in solution phase as references to confirm the
charge injection and recombination on the NiO surface. The charge recombination kinetics is
remarkably heterogeneous in some cases occurring on time scales spanning at least six orders
of magnitude even for the same dye.
In this thesis, we also proposed a novel concept of solid state p-type dye sensitized solar cells
(p-ssDSSCs) for the first time (paper VI), using an organic dye P1 as sensitizer on mesoporous
NiO and phenyl-C61-butyric acid methyl ester (PCBM) as electron conductor. Femtosecond
and nanosecond transient absorption spectroscopy gave evidence for sub-ps hole injection from
excited P1 to NiO, followed by electron transfer from P1●- to PCBM. The p-ssDSSCs device
showed an impressive 620 mV open circuit photovoltage.
Chapter 6 (paper VII) covers the study of electron transfer mechanisms in a covalently
linked dye-catalyst (PB-2) sensitized NiO photocathode, towards hydrogen producing solar
fuel devices. Hole injection from excited dye (PB-2*) into NiO VB takes place on dual time
scales, and the reduced PB-2 (PB-2●-) formed then donates an electron to the catalyst unit.
The subsequent regeneration efficiency of PB-2 by the catalyst unit (the efficiency of catalyst
reduction) is determined to ca. 70%.
Keywords: Electron transfer, Laser spectroscopy, Femtosecond spectroscopy, Transient
absorption, NiO, DSSCs, DSSFDs, Charge separation, Solar energy conversion, Nanosecond
photolysis, Photophysics
Lei Zhang, Department of Chemistry - Ångström, Physical Chemistry, Box 523, Uppsala
University, SE-75120 Uppsala, Sweden.
© Lei Zhang 2016
ISSN 1651-6214
ISBN 978-91-554-9678-4
urn:nbn:se:uu:diva-302263 (http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-302263)
To my beloved FAMILY
and
All makers of your dreams
List of Papers
This thesis is based on the following papers, which are referred to in the text
by their Roman numerals.
I.
Ultra-fast and Slow Charge Recombination Dynamics of
Diketopyrrolopyrrole based NiO-based Dye Sensitized Solar
Cells
L. Zhang, L. Favereau, Y. Farré, E. Mijangos, Y. Pellegrin, E.
Blart, F. Odobel, L. Hammarström
Phys. Chem. Chem. Phys., 2016, 18, 18515-18527
II.
Second Generation of Diketopyrrolopyrrole Dyes for NiO
based Dye-Sensitized Solar Cells
Y. Farré*, L. Zhang*, Y. Pellegrin, E. Blart, M. Boujtita, L.
Hammarström, D. Jacquemin, F. Odobel
J. Phys. Chem. C, 2016, 120, 7923-7940
III.
A Study of Oligothiophene-Acceptor Dyes in P-Type DyeSensitized Solar Cells
E. Sheibani*, L. Zhang*, P. Liu, B. Xu, E. Mijangos, G.
Boschloo, A. Hagfeldt, L. Hammarström, L. Kloo, H. Tian
RSC Adv., 2016, 6, 18165-18177
IV.
Long-lived Charge Separated State in NiO based p-Type
Dye Sensitized Solar Cells with Simple Cyclometalated Iridium Complexes
M. Gennari, F. Légalité, L. Zhang, Y. Pellegrin, E. Blart, J. Fortage, A.M. Brown, A. Deronier, M.N. Collomb, M. Boujtita, D.
Jacquemin, Hammarström, F. Odobel
J. Phys. Chem. Lett., 2014, 5, 2254-2258
V.
Molecular-Structure Control of Electron Transfer Dynamics of Push-Pull Porphyrins as Sensitizer for NiO Based Dye
Sensitized Solar Cells
L. Zhang, L. Favereau, Y. Farre, A. Maufrov, y. Pellegrin, E.
Blart, M. Hissler, D. Jacquemin, F. Odobel, L. Hammarström
RSC Adv., 2016, 6, 77184-77194
VI.
Solid State p-Type Dye-Sensitized Solar Cells: Concept, Experiment and Mechanism
L. Zhang, G. Boschloo, L. Hammarström, H. Tian
Phys. Chem. Chem. Phys., 2016, 18, 5080-5085
VII.
Insights into the Mechanism of a Covalently-Linked Organic Dye-Cobaloxime Catalyst System for Dye Sensitized Solar
Fuel Device
P. Pati, L. Zhang, B. Philippe, R. Fernández-Terán, S. Ahmadi,
L. Tian, H. Rensmo , L. Hammarström, H. Tian
Submitted Manuscript
*The authors contributed equally
Reprints were made with permission from the respective publishers.
Contributions to Papers
I did all femtosecond and nanosecond time-resolved experiments, data analysis and steady state measurements in all papers. I also performed part of
fabrication and characterization of the solar cells in paper III and VI. I was
major responsible in writing the manuscript of paper I and V, and the draft of
photophysical analysis for other papers. I was actively participating in the
international collaborations, research plan discussion, revising and approving
the articles. (Note: I have not done the design, synthesis and characterization
of the dyes, nor the DFT calculations; I have not done the TCSPC and streak
camera test in paper VII.)
Papers not included in this thesis:
VIII.
A Small Electron Donor in Cobalt Complex Electrolyte Significantly Improves Efficiency in Dye Sensitized Solar Cells
H. Yan*, W. Yang*, L. Zhang, R. Jiang, Y. Saygili, L. Hammarström, A. Hagfeldt, G. Boschloo
Submitted Manuscript
IX.
Organic Polymer Dots as Photocatalyst for Visible Lightinduced Hydrogen Generation
L. Wang, R. Fernández-Terán, L. Zhang, D.L.A. Fernandes, L.
Tian, H. Chen, H. Tian
Angew. Chem. Int. Ed., 2016, DOI: 10.1002/anie.201607018
Accepted
X.
Acetylacetone Anchoring Group for NiO-based DyeSensitized Solar Cell, Dyes and Pigments
J. Warnan, Y. Pellegrin, E. Blart, L. Zhang, A.M. Brown, L.
Hammarström, D. Jacquemin, F. Odobel
Dyes and Pigments, 2014, 105, 174-179
XI.
Direct Observation of Ultrafast Electron Injection on
Tetranuclear Ruthenium(II) Polypyridine Sensitized TiO2
L. Zhang, J. Föhlinger, T. Liu, S. Campagna, L. Hammarström
Manuscript
XII.
Photo-induced Electron Transfer in Organic Dye Sensitized
Indium Tin Oxide: n-type vs p-type DSSCs
L. Zhang, J. Föhlinger, W. Yang, L. Hammarström, H. Tian
Manuscript
Contents
1. Introduction ............................................................................................... 13
1.1 Motivation .......................................................................................... 13
1.2 Understanding of the Electron Transfer for Solar Energy
Conversion in Photocathode..................................................................... 13
1.3 Aim of Research ................................................................................. 14
1.4 Outline of the Thesis .......................................................................... 15
2. Fundamental Concepts .............................................................................. 16
2.1 Photophysical Processes ..................................................................... 16
2.1.1 Interaction between Light and Molecules................................... 17
2.1.2 Electronic Absorption and Emission .......................................... 17
2.2 Photo-Induced Electron Transfer ....................................................... 18
2.2.1 Classical Marcus Electron Transfer ............................................ 18
2.2.2 Photo-induced Electron Transfer ................................................ 19
2.3 Photocathode Dye Sensitized Solar Cells and Solar Fuels ................. 21
2.3.1 Introduction ................................................................................ 21
2.3.2 P-type Semiconductor................................................................. 21
2.3.2 Working Principle of p-DSSCs and p-DSSFDs ......................... 22
2.3.3 Design of p-type Dyes ................................................................ 23
3. Materials and Methods.............................................................................. 27
3.1 Dyes.................................................................................................... 27
3.2 Femtosecond Laser Spectroscopy ...................................................... 28
3.2.1 Femtosecond Transient Absorption Spectroscopy ..................... 29
3.2.2 Nanosecond Laser Flash Photolysis ........................................... 31
3.3 Time-Correlated Single Photon Counting (TCSPC) .......................... 33
3.4 Data Analysis ..................................................................................... 34
3.4.1 Important Definitions before Analysis ....................................... 34
3.4.2 Global Analysis of Transient Absorption Data........................... 35
4. Photo-induced Charge Separation Processes in NiO based D-linker-A
Dyes Sensitized Solar Cells (Paper I-V) ....................................................... 37
4.1 Introduction ........................................................................................ 37
4.2 Photophysical Study in Solution ........................................................ 38
4.2.1 Determination of Anion Spectra of p-type Dyes ........................ 38
4.2.2 Transient Absorption in Solution................................................ 40
4.3 Ultrafast Electron Transfer Dynamics on NiO Surface ...................... 42
4.3.1 Hole Injection into NiO .............................................................. 42
4.3.2 Ultrafast Charge Recombination and Slow Charge
Recombination ..................................................................................... 44
4.4 Molecular-Structure Control of Electron Transfer Dynamics in
Cyclometallated p-DSSCs (Paper IV-V) .................................................. 45
4.4.1 Long-lived Charge Separated state with Cyclmetalated
Complexes ........................................................................................... 46
4.4.2 Molecular-Structure Control of Electron Transfer Dynamics of
Push-Pull Porphyrin Sensitizers .......................................................... 47
4.5 Charge Regeneration with Co(dtbpy)33+/2+ Electrolyte ....................... 49
4.6 Conclusions ........................................................................................ 51
5. Electron Transfer in p-type Solid State Dye Sensitized Solar Cells
(Paper VI) ..................................................................................................... 52
5.1 Introduction ........................................................................................ 52
5.2 Steady State Absorption and Photovoltaic Performance .................... 53
5.3 Electron Transfer Mechanism ............................................................ 55
5.4 Conclusion.......................................................................................... 59
6. Electron Transfer Mechanisms in a Covalently-Linked Organic DyeCatalyst System for Dye Sensitized Solar Fuel Devices (Paper VII) ........... 60
6.1 Introduction ........................................................................................ 60
6.2 Tracking the Excited State ................................................................. 62
6.3 Tracking the Oxidized State and Intramolecular Charge Transfer ..... 62
6.4 Tracking the Reduced State................................................................ 63
6.5 Conclusion.......................................................................................... 66
Summary and Outlook .................................................................................. 67
Svensk sammanfattning ................................................................................ 69
Acknowledgements ....................................................................................... 71
References ..................................................................................................... 73
Abbreviations and symbols
A
A
'A
ACN
BET
CB
CR
CS
CT
CV
D
DAS
DCM
DFT
DPP
DSSC
DSSFD
dtb
ET
ESA
ff
fs
FTO
FWHM
-'Gº
GSB
HOMO
IC
ICT
IPCE
IR
IRF
ISC
JSC
k
kET
electron acceptor in dye molecules
absorbance
change of absorbance
acetonitrile
back electron transfer
conduction band
charge recombination
charge separation
charge transfer
cyclic voltammetry
electron donor in dye molecules
decay associated spectra
dichloromethane
density functional theory
diketopyrrolopyrrole
dye-sensitized solar cells
dye-sensitized solar fuel device
4,4’-di-tert-butyl-2,2’-bipyridine
electron transfer
excited state absorption
fill factor
femtosecond (10-15 s)
fluorine doped tin oxide
full width at half maximum
free energy driving force
ground state bleach
highest occupied molecular orbital
internal conversion
intramolecular charge transfer
incident photon to current conversion efficiency
infrared light
instrument response function
inter-system crossing
short circuit current density
rate constant
electron transfer rate
LUMO
MLCT
NDI
NiO
NIR
ns
'OD
OPO
P
PC
PCBM
PCE
PEC
ps
PV
R
SAS
SE
SpecEchem
p-ssDSSC
TAS
TCSPC
TD-DFT
TEA
TiO2
TPA
UV
Vis
VOC
VR
VB
ZnP
H
K
O
O
Oabs
Oem
Oex
W
lowest unoccupied molecular orbital
metal-to-ligand charge transfer
naphthalene diimine
nickel oxide
near infrared
nano-second (10-9 s)
change of optical density
optical parametric oscillator
product
propylene carbonate
phenyl-C61-butyric acid methyl ester
power conversion efficiency
photoelectrochemical cells
pico-second (10-12 s)
photovoltaic
reactant
species associate spectra
stimulated emission
spectroelectrochemistry
solid state p-type dye-sensitized solar cells
transient absorption spectrum/spectroscopy
time-correlated single photon counting
time-dependent density functional theory
triethylamine
titanium dioxide
triphenylamine
ultra violet light
visible light
open circuit voltage
vibrational relaxation
valence band
zinc(II) porphyrine
molar extinction coefficient
power conversion efficiency
reorganization energy (electron transfer theory)
wavelength
absorption wavelength
emission wavelength
excitation wavelength
lifetime
1. Introduction
1.1 Motivation
I began my research in photochemistry with a enlighten supervisor from
2009, it began with a training in using the time-resolved fluorescence spectrometer (FLS 920). Since then, my interest was increasing on the photochemistry, as it provides a high resolution method to realize the macro photochemical phenomenon in molecular scale. When I started my PhD, I exposed to the concept of femtosecond chemistry (10-15 s) for the first time,
and I did not have such fast time domain and the method in my mind at that
time to study electron transfer with spectroscopy. Human beings have been
committed to the pursuit of faster and stronger, the same in microscopic field.
Ultrafast laser pulses supply a strong technique to study the process and
mechanism of chemical reactions within small time scale. The ultrafast spectroscopy also allows people to observe the atomic and molecular changes in
a transition state of a chemical reaction process through “slow motion”, it
changes our understandings of the chemical processes fundamentally and
helps us to study and predict the important intermediates during chemical
reactions.
While I started my study and research with great interest and curiosity, it
is entitled ‘Exploring electron transfer dynamics of novel dye sensitized
photocathodes: towards solar cells and solar fuels’, mainly concerns the
hole/electron transfer processes into NiO based solar cells and solar fuel
devices.
1.2 Understanding of the Electron Transfer for Solar
Energy Conversion in Photocathodes
Solar energy, as one of attractive sustainable energies, is abundant and clean.
Conversion solar energy into electricity or solar fuels with different substrate
is an important strategy to solve the energy crisis that we are facing. Dye
sensitized solar cells (DSSCs, Figure 1.1), have been approved as a low cost
and convenient approach to convert sunlight into electricity or solar fuels
since Brian O’Regan and Michael Grätzel reported in 1991.1 P-type dyesensitized solar cells (p-DSSCs) were first reported in 1999 by Lindquist and
co-workers.2 The p-type device demonstrated cathodic photocurrent through
13
an erythrosine-sensitized porous nickel oxide electrode. Recently, this type
of solar cells has attracted intense research interest from the research community,3-15 since it is the other essential half of tandem dye-sensitized solar
cells.16-18
Figure 1.1 Schematic of electron transfer processes in n/p-type DSSCs.
The electron transfer processes exist in p-type systems concerning solar energy conversion, however there are still many phenomena and problems are
not fully understood, which become one of the obstacles to further improve
solar energy conversion efficiency.19 The electron transfer processes in photocathode based dye sensitized solar cells and solar fuel devices will describe
detail in section 2.2.3. The photosensitizers (also called dyes) play an important role on the device performance. Study of electron transfer processes
of the dyes on photoelectrodes is beneficial to guide the further dye design
and further improve the performance of photocathode in solar cells and solar
fuels. Fortunately, time-resolved spectroscopy techniques supply us such
sufficient resolution to get insights into these charge transfer processes.
1.3 Aim of Research
The aim of this thesis is to develop and understand new molecule/semiconductor systems for solar cells and solar fuel devices. In contrast
to the vast amount of work on “Grätzel cells” (electron injection in n-type
materials), I focus on the photo-induced charge separation of not as well
explored photocathods (hole injection in p-type materials) based on molecular dyes in solar cells or dye and catalyst systems for solar fuels production.
In cross-disciplinary collaboration we design new molecular dye and dyecatalyst for efficient photo-induced charge separation, current collection and
fuels production.
14
1.4 Outline of the Thesis
This thesis is on the basis of several published papers, it includes the electron transfer dynamics in organic/inorganic dyes sensitized NiO based
DSSCs (Paper I-V); chapter five is an innovative new concept of solid state
p-DSSCs, both photovoltaic and electron transfer mechanism reported at the
first time (paper VI); chapter six presents a designed covalently linked dyecatalyst system for hydrogen evolution, most important results will be presented to interpret the electron transfer processes (Paper VII).
15
2. Fundamental Concepts
This chapter gives a brief summary of some fundamental concepts encountered in photophysics and electron transfer theory, and especially the interfacial electron transfer processes. Additionally, the basic principle of p-type
dye sensitized solar cells and p-type dye sensitized solar fuel devices will be
introduced.
2.1 Photophysical Processes
Photophysical processes can be described as processes induced by light excitation which do not produce an irreversible change in the covalent structure
of the system20-22. This is in contrast to photochemical reactions such as photo-induced polymerization or dissociation where reactants and products have
different chemical structures. Jablonski diagrams are qualitative or quantitative illustrations that summarize the photophysical processes taking place in
a given system, such as ground state absorption (UV-vis absorption), excited
state absorption (transient species), internal conversion (IC), intersystem
crossing (ISC), fluorescence and phosphorescence emission, and so on (Figure 2.1).
Ground state absorption
”UV-vis”
Vibrations ”IR”
Excited state absorption
Internal conversion
Intersystem crossing
E
Emission
Figure 2.1. A Jablonski diagram which illustrates the details of radiative and nonradiative processes.
16
2.1.1 Interaction between Light and Molecules
Light is understood to be both a particle and an electromagnetic radiation
field originating from an oscillating electric dipole. Light is described as the
smallest indivisible quantity (quantum) of energy, photons, where the energy
of each photon is E = hc/O. Here h is Plank’s constant, c is constant of the
speed of light and Ois the wavelength, it is the most fundamental relations in
photophysics.
Light that interacts with a solution of individually absorbing molecules
follows the Beer-Lambert law (Eq. 2.1), which relates that light with certain
energy pass through a solution of molecules and transfer its energyto the
molecules. Equation 2.1 expresses that the transmitted light is calculated by
the initial light with irradiance I0, pass the cell of thickness l, the sample’s
concentration is C. The absorbance (A) is equal to İCl, where İ is the molar
extinction coefficient or molar absorption coefficient, it is an intrinsic property of the molecules, so when an absorbing molecule designed and synthesized, the molar extinction coefficient will be a measurement of how strongly the molecule attenuate light at a given wavelength, and also determined its
light-harvesting ability.
ܶ = ‫ܫ‬଴ 10ିఌ஼௟ ‫ܫ ؠ‬଴ 10ି஺
(2.1)
In this thesis, we are primarily concerned with the light interaction with ptype dyes in visible to near infrared (NIR) region. The light absorption,
emission and photo-induced electron/energy transfer of designed p-type dyes
will be discussed both in solution and on NiO semiconductor surface. A
short pulse laser beam was generated as monochromatic light; it is an ideal
source to study the interaction of light with matter (high brightness, strong
direction and coherence), towards this thesis, it is convenient used to identify
the structure and dyes and their host systems, composition, status and change
in spectroscopy.
2.1.2 Electronic Absorption and Emission
The oscillating electric field of light can raise a molecule to its excited electronic state, a process that is called electronic absorption (Figure 2.1). In the
quantum mechanics view, the electrons in the molecule must be in one of the
defined states, each state with a defined energy. Therefore, states 1 and 2
have the energy E1 and E2 respectively. When a ground state is excited to
state 2, it absorbs energy equal to E2 – E1, corresponding to absorption with
frequency (Eq 2.2):
Qଵଶ = (‫ܧ‬ଶ െ ‫ܧ‬ଵ )/݄
(2.2)
17
Emission is related to absorption; the molecule can emit a certain frequency
of radiation when the excited state relaxes from a higher to the ground electronic state. As the certain quantized values of energy levels of the moleculesand electrons, transitions between these quantized states occur by the
photon processes. Both absorption and emission require that the photon energy given by the Planck relationship is equal to the energy separation of the
initial pair of quantum energy states.
An excited molecule relaxes via radiative and non-radiative processes
(Figure 2.1). Non-radiative processes include the internal conversion (IC),
intersystem crossing (ISC, electron spin flipping) mostly from singlet to
triplet state (S1-T1) and vibrational relaxation (VR). Radiative processes
include fluorescence and phosphorescence. Fluorescence describes electronic transitions from singlet excited states to the singlet ground state (e.g. S1-S0)
and proceeds with some environmental effects, such as a Stokes shift which
is a red shift in fluorescence band maxima due to solvent relaxation effects.
Phosphorescence on the other hand describes radiation from the triplet excited states to a singlet ground state (e.g. T1-S0). Because phosphorescence is a
spin-forbidden process, typically long lifetimes. In many organic molecules,
the triplets are pure and live much longer, up to seconds in a rigid matrix (no
O2). Transition metals mix singlets and triplets and lead to much shorter
phosphorescence lifetimes.
2.2 Photo-Induced Electron Transfer
The theory of electron transfer proposed by R.A. Marcus23-25 was inspired by
the continuous medium theory and two-sphere model described by Libby’s26
in 1956. The electron transfer theory described in Marcus' seminal paper
described the outer sphere electron transfer reaction rate constant as a function of Gibbis free energy energy of the formation of the transition state.
Over the last 50 years, the theory of electron transfer has developed significantly, and has been extended to describe proton transfer, hydrogen atom
transfer or pericyclic reactions.
2.2.1 Classical Marcus Electron Transfer
According to the Marcus Theory, the outer sphere electron transfer rate (ET),
the classical description of the free energy of reaction is using two free energy parabolas with the same width, ie. the same force constants (Figure 2.1).
The left parabola represents the free energy associated with the reactant system (R: reactants + solvent environment), while the right parabola is the
product system (P: products + solvent environment). In this thesis, the reactant system is donor-acceptor (D-A) model forming the charge separated
state (D+-A-) after the electron transfer processes. In the classical Marcus
18
theory, the abscissa is the reaction coordinate of the electron transfer, and the
vertical axis is the free energy of the reaction system. The intersection cross
of the R and P parabolas is the transition state, where R and P have the same
coordinates and energy.
G R
P
O
'G݁
'Go
q
'q
Figure 2.2. Classical schematic of Marcus model of electron transfer reactions.
The activation free energy of the reaction, ο‫ ‡ ܩ‬is the gap between the intersection point and the bottom of free energy of R parabola. Assuming no
change in entropy, the free energy becomes energy or potential energy. ο‫‡ ܩ‬
in classical Marcus theory could then be written as the approximation of
parabolic potential energy of the reactants and the products:
஛
ସ
ο‫( = ‡ ܩ‬1 +
οீ ౥ ଶ
)
஛
(2.3)
Here 'Gº is the change of the reaction Gibbs free energy, i.e. the Gibbs free
energy difference between P and R in their respective equilibrium state; O is
the reorganization energy of the reaction, ie. in the case of no electron transfer, the energy of the reactants (R) state at the equilibrium coordinate of the
products (P) state, and vice versa. O = 1/2݇ȟ‫ݍ‬ଶ, wherein k is the force constant of the Marcus free energy parabola.
2.2.2 Photo-induced Electron Transfer
Photo-induced electron transfer (PET) between molecules is of interest in a
variety of research fields, such as natural and artificial photosynthesis,
DSSCs, and other solar energy conversion devices. The Marcus Theory of
electron transfer originally described the ground state electron transfer reaction, however this thesis is also concerned with excited state electron transfer
processes within a relative rapid time domain. Excited state electron transfer
19
is one example of an electronic deactivation process. As we know, the total
Gibbs free energy change ('G) determines if the electron transfer reaction is
spontaneous, if ¨Gº<0, this reaction is exergonic process, and reaction usually proceeds at some rate, otherwise it does not. Figure 2.3 demonstrates
photoinduced reduction and oxidation electron transfer reactions in a A-B
molecule. The electron transfer processes occur between the excited state of
A and the ground state of B. In the oxidation ET reaction, the excited state
A* is the electron donor and in the reduction side, A* is the acceptor:
Ax+ + BxOxidation: A* + B
*
Ax- + Bx+
Reduction: A + B
Oxidation
Reduction
LUMO
LUMO
LUMO
LUMO
HOMO
HOMO
HOMO
A*
B
HOMO
A*
B
Figure 2.3. The orbital schematic of reduction and oxidation electron transfer process. For simplicity, excited states are described as single-electron HOMO-LUMO
excited states.
The main goal of this thesis is to observe photo-induced electron transfer
across the dye sensitized photocathode. The interfacial electron transfer is
described as ET between a discrete and localized molecular state and the
bands in the semiconductor. In p-type DSSCs or DSSFDs, the molecular
sensitizer absorbs light to its excited state and the excited state deactivated
by competition between intramolecular relaxation and hole injection into the
valence band of the p-type semiconductor (p-SC) (Figure 2.4 ĸ). Hole injection timescales are typically ultrafast, ranging from femtoseconds to tens
of picoseconds. In the case of TiO2, the corresponding electron injection into
the conduction band has been reported to have an exponential distribution of
localized band gap states, which also leadsto an exponential decay by the ET
on the TiO2 interface. Recently, d’Amario et al.27 reported for the NiO VB
edge a higher band gap state and a nonexponential shape of the band edge,
which led to two pathways for charge recombination kinetics between the
reduced dye and injection holes (NiO+): a fast pathway between “normal”
holes that recombine with the reduced dye and a slower pathway assigned to
holes in the trap states. In consideration of the complexity of the ET process
by p-type dye/semiconductors, the full description of the processes occurring
in the DSSC system is necessary to better understand of the
dye/semiconductor interaction, and eventually improve PCE in devices.
20
2.3 Photocathode Dye Sensitized Solar Cells and Solar
Fuels
2.3.1 Introduction
With more than two decades of development, the photoconversion efficiency
(PCE) of DSSCs, has reached up to 14.3% with a cobalt-electrolyte in
2015.28-32 This PCE is impressively high, and progression to this value has
been a rather slow climb over several decades, especially when compared to
emerging technologies such as perovskite-based solar cells or organic photovoltaics. The performance of p-DSSCs on the other hand is still unsatisfactory; the highest PCE of p-DSSCs reported is only 2.5%.33 One possible way
to enhance the PCE of the DSSC is to combine both n-DSSCs and p-DSSCs
into a tandem DSSCs, where the theoretical limit for PCE is predicted to
reach above 40%34. To date, however, the record PCE for a t-DSSCs is
2.42%, reported by Bach et al.16 The poor performance of the photocathode
is what limits PCE in tandem systems.
The dye is one of the crucial components of the photocathode, which influences the overall performance of the solar cell.35-38 Light absorption entails the excited state formation which subsequently injects a hole into the
nanoporous NiO film (or other p-SC). A redox mediator such as triiodide or
a cobalt or iron complex regenerates the dye and delivers electrons to the
counter electrode in order to complete the photovoltaic circuit. Furthermore,
in analogous DSSFDs, the dye also represents the heart of the device as it
generates the redox equivalents that are used to drive chemical reactions,
such as generation of hydrogen from protons or other fuels (CH4, CO, etc.)
with different substrates (H2O, CO2, etc.).39-43 Therefore, the development
and investigation of new dyes is very helpful towards the improvement of
DSSCs and p-DSSFDs performance.
2.3.2 P-type Semiconductor
P-type oxide semiconductor (p-SC) used for p-type dye sensitized solar cells,
is complement side to n-DSSCs towards t-DSSCs. A p-SC collects holes
injected from the photoexcited hole donating dyes into their valence band
(VB). A wide band gap is a crucial property for both p-SC and n-SC. Nickel
(II) oxide (NiO) is a p-SC which has a reported optical band-gap of about 3.4
eV (vs SCE)44. Low PCE values obtained with NiO p-SC can be traced to
disadvantages such as low transparency, thickness of the film and poor conductivity. We found in this thesis that the fast charge recombination between
injected holes and the reduced dye is the main drawback factors of the low
PCE.34,45-48 Much efforts have been carried out to pursue better p-SC materials,49-51 such as Cu2O and other copper delafossite (CuXO2) electrodes,
where X is usually aluminum, chromium, or gallium. However, the replace21
ment of NiO still show lower photoconversion efficiencies than those reported NiO-based DSSCs. In this thesis, all dyes are sensitized on NiO films for
p-DSSCs or p-DSSFDs. The emphasis of the research presented here is to
understand the electron transfer dynamics in novel organic/inorganic dyes
sensitized NiO systems.
2.3.2 Working Principle of p-DSSCs and p-DSSFDs
The working principle of p-DSSCs and p-DSSFDs are shown in Figure 2.4.
The charge transfer processes for p-DSSCs are described in detail (Figure
2.4a) while electron transfer processes in a p-DSSFDs are presented in a
general fashion (Figure 2.4b). Following the detailed charge transfer processes of p-DSSCs, the p-type dyes absorb light to generate the excited states
(ᬅ); it decays primarily by hole injection into the nanoporous NiO film (or
other p-SC), charge separated state forming by the hole injection (ᬆ); a
redox couple is used to regenerate the dye and delivers electrons to the counter electrode for an external circuit (ᬇ and ᬈ). The red arrows represent
the unwanted energy loss by deactivation (ᬉ) and charge recombination
processes in the semiconductor with reduced dye or redox couple (ᬊ and
ᬋ) which is much faster in p-DSSCs than conventional DSSCs and occurs
in most cases on less than 1 ns time scale, especially for process ᬊ. In the
case of p-DSSFDs, the electron transfer processes is similar to that of the pDSSCs, where the difference is that the electron equivalents are consumed
by a catalyst instead of the redox couple. The reduced catalyst will perform
catalytic reduction of substrates.
22
䠄a䠅
CB
䠄b䠅
H2
䐡
A/A-
䐤 NiO
䐣
䐟
䐠 VB
S*/S-
Load
S/S-
Cat
Conductive Glass
Conductive Glass
S/S䐥
䐢
CB
2
H+
NiO
VB
S*/S-
Load
Figure 2.4. Schematic of (a) internal working principle of p-DSSCs, dotted line is
the electron flow (green arrow): electron transfer processes involved in the operation
of the cell (green) and the unwanted deactivation and charge recombination (red).
and (b) brief working principle of p-DSSFDs, S: dye or sensitizer, A: Redox couple,
CB: Conduction band, VB: Valence band and Cat: Catalyst.
The method to calculate the PCE, is the same for both p-DSSCs and nDSSCs, where the efficiency (K) depends upon the following contributions:
short-circuit photocurrent density (JSC), open-circuit photovoltage (VOC) and
fill factor (ff) divided by the incoming incident solar power (Pin), shown in
Equation (2.4)
K=
௃ೄ಴ ή௏ೀ಴ ή௙௙
௉೔೙
Optimizing any of three factors will enhance the PCE of the p-DSSCs, the
best reported p-DSSCs showed JSC = 7.65 mA cmí2, VOC = 645 mV, and ff =
0.51, rendering a PCE of 2.51%.52 The fast charge recombination between
injected holes and the reduced dye (ᬊ) seems to be the main loss factor in
NiO DSSCs. and I also focused on the dye regeneration (ᬇ) as it’s the main
forward electron transfer process for a higher PCE. In order to enhance the
efficiencies of the p-DSSCs or p-DSSFDs, detailed charge transfer dynamics
of each process should be understood and known how to control the design
of the dye molecules for better device performance.
2.3.3 Design of p-type Dyes
Dyes or photosensitizers are the most crucial parts of dye sensitized solar
energy conversion devices, as the dyes undertake the task of light harvesting
23
and extract electrons from the p-SC substrate (or, alternatively the process
can be regarded as hole injection). Some breakthrough result of p-DSSCs
were achieved by employing a triphenylamine-based or additional secondary
electron acceptor dyes with anchor-donor-linker-acceptor (Anch-D-ʌ/linkerA) configuration reported by Sun and Hammarström in 20086,7 and 200953.
Subsequently, much effort has been devoted to the optimization of the dye
structure using different subunits. In general, the p-type dyes should obey the
following factors for p-type devices:
x The dye should have at least one appropriate anchoring group to bind
the dye strongly on the p-SC surface.
x The energetic properties of the dye should allow the excited state dye
to inject holes into NiO VB and have enough driving force for subsequent dye regeneration.
x The dye should be photochemically, thermally and electrochemically
stable and also resist self-assembly and aggregation quenching.
In addition to the above requirements54, the effect of the dye molecular structure on the electron transfer rate of each step also needs consideration. Systematic photophysical studies of the effect structural variations imparted to
dye molecules on electron transfer properties will serve as a significant guide
for more efficient p-type dyes. Furthemore, in analogous solar fuel devices,
the understanding of the electron transfer processes of the photosensitizer
also represents the heart of the device to generate the energy-rich electrons
to be converted into an energy-rich substance, such as hydrogen gas.
The general molecular structure Anch-D-S/linker-A of p-type dyes is
shown in Figure 2.5. An anchoring group will help dyes bind strongly to the
nanoporous semiconductor surface. The commonly used anchoring groups
are -COOH, -H2PO3, and -SO3H55-57. Selecting a suitable anchoring group
will decrease the degradation of the dye on film and could enhance the
charge injection ability as well, such as Wang and coworkers58 introduced
the pyridine ring as the anchoring group to instead the carboxylic group of
P1, the pyridine anchoring group positively shifted the VB energy of NiO
and slow down the charge recombination. Pavone and coworkers59 report a
first-principles study whether the possibility to control and select only the
most convenient dye binding modes could boost the performances of pDSSCs. They summarized that those forming the lowest interfacial electrostatic dipole (via dye adsorption) are those with the highest potential for pDSSCs. D-S/linker-A model is well known, in which the most electron density of the highest occupied molecular orbital (HOMO) will locate on the
donor (D) and the most electron density of the lowest unoccupied molecular
orbital (LUMO) is located on the acceptor (A) moiety, and conjugated S
spacer or non-conjugated linker will be a bridge to locate the excess electron
far away from the NiO holes, which will also slow down the charge recombination because of the weak electronic coupling.
24
Donor
Anchoring Group
S/linker
Electron
Withdrawing/Secondary
Acceptor
Figure 2.5. General molecular structure of p-type photosensitizer.
All elements of the Anch-D-S/linker-A dye could be varied and modified
independently. In order to tune the energetic or driving force of the dye molecule, different approaches have been carried out. Both organic and metalorganic chromophores used for novel p-type dyes, including ruthenium
dyes14,60,61, porphyrin dyes62, coumarin dyes63, triarylamine dyes64, diketopyrropyrrole dyes45,65, squaraine dyes66 and so on, were introduced to develop impressive p-type dyes for higher PCE.
O
C8H17 N
O
CN
O
NH
O
CN
S
OCH3
O
O
N
N
Figure 2.6. Acceptor structures used in this thesis, from left to right is naphthalene
diimide (NDI), thiophene-dicyanovinylene (tCN2), phenyl-C61-butyric acid methyl
ester (PCBM) and napthoilene-1,2-Benzimidazole (NBI).
Different design approaches can be separated via the electronic coupling
strength, the push-pull approach is the most common one, where the groups
are strongly electronically coupled. It is designed as a dye linked with an
electron withdrawing group, which affects the intrinsic properties of the dye
(absorption, electrochemistry, etc.); In other cases it is a distinct weakly
electronically coupled acceptor to the dye and in yet other cases it is a separated layer. Those cases of electronic couplings are always expressed by the
approximate redox potentials, which could be obtained from both electrochemical measurement and computational method (TD-DFT: Timedependent density functional theory), there will be a little difference as different methods of obtaining.
The push-pull method exhibits charge transfer (CT) transitions which
iswell suited to induce charge transfer reactions in DSSCs. It exhibits a large
dipole moment in the excited state, that may favor electronic coupling for
hole injection. At the same time, the resulting anion radical will have electron density shifted on the acceptor part, which can be expected to reduce the
electronic coupling for recombination. Here we need clearly realize that the
excited state in general is not equal to HOMO-LUMO excitation, but that the
25
qualitative trend of asymmetric charge distribution will hold in both excited
and reduced state. However, this kind of dyes has a short molecular distance,
leading to faster charge recombination. The weakly coupled acceptor instead
will accept the electron from the reduced dye and prolong the distance of
hole/electron pair. Finally, the separate layer cases will prevent the back
electron transfer so as to retard the charge recombination. All those approaches should be designed with enough driving force for hole injection
and charge regeneration with certain redox mediators. In this thesis, all of
those approaches have been applied in the dye design. The acceptors or electron withdrawing groups are employed to separate the electrons from the
injected holes, so as to retard the charge recombination. Some acceptors used
in this thesis are summarized in Figure 2.6. Ultrafast charge recombination
were found in some push-pull systems, and the appropriate acceptor, such as
NDI, gave long-lived charge separated state and corresponding higher photovoltaic performances, as described in the later chapters of this thesis.
26
3. Materials and Methods
In this chapter, all materials (Dyes or Catalysts) and experimental techniques
will be outlined in detail. The first part will include all kinds of dyes categorized by the chromophore, which will help readers to have an intuitive view
of the whole thesis structure. The second part is experimental techniques on
the femtosecond laser spectroscopy and nanosecond laser photolysis which
was used to understand the motion of hole/electron transfer dynamics on the
sensitized semiconductor photocathode system, the last part will also describe the data analysis procedures briefly.
3.1 Dyes
The dyes used in this thesis were designed and synthesized externally. In this
part, all dyes were divided into two categories according to the chromophore,
including metal free organic and cyclometalated dyes (Figure 3.1 and Figure
3.2). The metal free organic dyes are shown in Figure 3.1, they can be subcategorized to two families, one is mainly based on diketopyrrolopyrrole
chromophore and derived to two generations of seven Anch-D-S/linker-A
dyes (Coded as DPPBr, DPPCN2, DPP-NDI and 1-4) which were studied
in paper I and paper II. The other family is triphenylamine derivatives (Coded as P1, E1, E2 and PB-1) and a covalently linked dye-catalyst system
(Coded as PB-2). P1 was employed to propose the new concept of solid state
p-type dye sensitized solar cells with phenyl-C61-butyric acid methyl ester
(PCBM) as the electron conductor in paper VI, E1 and E2 were reported as
novel triphenylamine based dyes compared with P1 in paper III, PB-2 were
designed to develop a novel efficient photoelectrochemical (PEC) cells for
hydrogen evolution in paper VII.
27
(c) TPA Dyes
(a) DPP Generation I
C4H9
N O
C4H9
N O
HO
S
O
Br
O N
DPP Br C4H9
C2H5
HO
HO
S
O N
C4H9
C2H5
O
C2H5
C4H9
N O
S
O
HOOC
C2H5
C2H5
O N
C4H9
C2H5
DPP NDI
C6H13
C6H13 C6H13
C6H13
S
S
S
S
S
C6H13
C6H13
N
CN
S
CN
DPPCN2
HOOC
HOOC
O
O
N
N C8H17
O
O
HO2C
HO2C
HO2C
P1
S NC
CN
E2
CN
S NC
HO2C
N
Br
O N
C4H9
C2H5
2
N
HOOC
N
C2H5
C4H9
N O
N
Br
O N
C4H9
C2H5 HO2C
C2H5
C4H9
N O
N
CN
S NC
C6H13
C6H13 C6H13
CN
S
S
S
S
S
CN
C6H13
C6H13
(d) Solar Fuels
C2H5
C4H9 HO2C
N O
1
N
C6H13
(b) DPP Generation II
N
N
E1
HOOC
HO2C
O
HO2C
O N
C4H9
C2H5
C2H5
C4H9
N O
HO
3
O N
C4H9
C2H5
O
N C8H17
O
O
N
O
O N
N
O H
PB 1
CN
S NC
HO
O
N C8H17
O
O
N
O
4
N
O
N
O
O N
N
O H
NN
N
N Cl
Co N O
NCl
N OH
PB 2
Figure 3.1. Structures of organic Anch-D-S/linker-A photosensitizers.
The metal complex dyes illustrated in Figure 3.2 include three categories of
metal organic chromophores. The cyclometalated iridium complexes were
reported in Paper IV as a simple dye structure giving a long-lived charge
separation state. Zinc(II) porphyrin complexes were designed to compare the
electron transfer dynamics by molecular structure variations (Paper V).
PO3H2
PO3H2
PO3H2
N
H2O3P
Ir
N
N
N
H2O3P
N
N
N
H2O3P
N
CN
CN
OMe
N
N
H2O3P
IrDPQCN2
IrPhen
OMe
Ir
N
N
PO3H2
N
H2O3P
Ir
N
H2O3P
PO3H2
PO3H2
IrBpystyryl
OMe
OMe
C12H25O
HO2C
N
N
OC12H25
Zn
C12H25O
ZnPref
HO2C
C12H25O
N
N
N
OC12H25
HO2C
N
N
C12H25O
OC12H25
Zn
N
N
OC12H25
C12H25O
N
NO2 HO2C
N
OC12H25
Zn
C12H25O
ZnP TPA NO2
N
N
O
O
N
N C8H17
O
O
OC12H25
ZnP NDI
Figure 3.2. Sturecures of cyclometalated photosensitizers.
3.2 Femtosecond Laser Spectroscopy
In the end of the 20th century, because of research on ultrafast light-matter
interaction becomes increasingly viable, the rapid development and matures
of ultrafast lasers in science and technology.. LASER technique presents the
acronym for “Light Amplification by Stimulated Emission of Radiation”,
which is one of the most important development technologies on the interaction of light and matters, with ultra-short pulses (<10-15 s) generation, we can
28
achieve tools with sufficient time resolution to insight the chemical reaction
world, which is always extremely short time scales (femtosecond to picosecond). Ultrafast lasers supply a spectroscopic technique to study the dynamic
processes in materials or chemical reactions, this spectroscopic technique is
so named as ultrafast laser spectroscopy. The research used ultrafast spectroscopy gained rapid development after Prof. Ahmed Zewail was awarded
the Nobel Prize at 1999. He gave his nobel lecture on “Femtochemistry:
Atomic-Scale Dynamics of the Chemical Bond Using Ultrafast Lasers”,
from this moment, ‘Femtochemistry’ became one of the areas of physical
chemistry that studies chemical reactions on extremely short timescales (approximately 10-15 s). Ultrafast laser methods include many different techniques, such as ultrafast transient absorption, time-resolved photoelectron
spectroscopy and two-photon photoelectron spectroscopy, multidimensional
spectroscopy, ultrafast imaging and femtosecond fluorescence up-conversion,
as one of the ultrafast laser methods, ultrafast transient absorption spectroscopy (pump-probe transient absorption spectroscopy). They have developed
to a powerful set of techniques for probing and characterizing the electronic
and structural properties of short-lived excited states (transient states) of
photochemically/photophysically relevant molecules.
3.2.1 Femtosecond Transient Absorption Spectroscopy
Time-resolved pump-probe transient absorption spectroscopy is a sensitive
technique to study the time evolution of the dynamics of the excited states
and the lifetimes of the short living species by measuring the time dependent
transient absorption signal.
A classical pump-probe transient absorption experiment set-up schematic
is presented in Figure 3.3. Two spatially overlapped laser pulses are incident
into a sample, two pulses with varying time delay are so called pump and
probe, respectively. The pump pulses with intense and quasi-monochromatic
light is used to as excitation the sample. The weaker and quasimonochromatic or spectrally broad probe pulse is then applied to monitor the
photoinduced optical changes. Two transient absorption setups were used in
this thesis, one is the UV-vis probe 1 kHz system, which is then one mostly
used in this thesis, and the other is the mid-IR probe 3 kHz system. The basic
set-up for the two systems is similar, but the mid-IR probe system is different in the probe optics.
The goal of transient absorption experiments is to extract the optical density (OD) of the transient absorption from ultra-short excited new species. In
this way the OD can be gained from the difference of the probe beam in
front of the sample and after the sample with delay time W (Eq. 3.1). In the
real case, from the measured OD of the TA we subtract the OD of the absorption, measured for the unexcited sample
29
௉
௉బ
௉
௉బ
οܱ‫ܦܱ = ܦ‬௔௙௧௘௥ െ ܱ‫ܦ‬௕௘௙௢௥௘ = െ(݈‫݃݋‬ଵ଴ )௔௙௧௘௥ െ (െ݈‫݃݋‬ଵ଴ )௕௘௙௢௥௘ =
݈‫݃݋‬ଵ଴ (
௉್೐೑೚ೝ೐ (O)
௉ೌ೑೟೐ೝ (O,ఛ)
)
(3.1)
Transient absorption spectroscopy is sufficient to resolve the character of the
charge separated state or radical anions in different time domain, my colleagues53 reported an improved photovoltaic performance in organic system
with appended electron acceptor, naphthalene diimide (NDI), it has been
proved to be a reliable electron acceptor; its radical anion is characteristic at
475 nm and 605 nm peaks on the transient absorption spectroscopy. When
NDI is designed in a p-type sensitizer with a chromophore and bridge linker
in feasible energetic level, and if the excited state chromophore extract electron out to form the NDI anion, the hole/electron pair will be separated and
spectroscopic will probe the lifetime of the charge recombination and charge
regeneration with or without available redox mediator from 100 fs to ms
time delay.
PD
mirrors at
different levels
Detector
sam
ple
ߣൗ െ plate
2
Filter
Beam splitter
Chopper
Lens
Iris
Mirror
CaF2
Delay
line
LASER
TOPAS
Figure 3.3.Schematic picture of the fs TA set-up, a Coherent Legend Ti:Sapphire
amSOLILHUN+]Ȝ QPFWHM 100 fs). The output is split to form pump and
probe beams. Desired pump wavelengths were obtained with a TOPAS-white, and
with neutral density filters, the energy of each pulse was kept between 200 and 500
nJ. The white light continuum probe was obtained by focusing part of the 800 nm
light on a moving CaF2 plate (UV-vis probe) or Ti:Sapphire (NIR probe). Polarization of the pump was set at magic angle, 54.7° relative to the probe. Instrumental
response time depends on pump and probe wavelengths, but are usually about 150
fs.
Determination of the change of optical density
To determine the change of optical density, we can consider what contributes to the transient absorption spectroscopy (TAS). Before time zero, the
time delay is in negative part, which means the probe is before the pump, so
the 'OD signal is zero. At positive delay times when the probe is after the
pump with a certain positive time delay, there will be mainly three contribu30
tions to the TAS (Figure 3.4): Ground state bleach (GSB) is the loss of the
ground state S0, it shows negative signal on the differential spectrum, mostly
the pump wavelength will be chosen after the absorption maximum, and
pump scattering will cut about 20-50 nm in this range. Excited state absorption (ESA) is one important contribution, which is normally shown as positive 'OD signal to the blue and red of the GSB range, and is due to the absorbance of the excited state (S1-Sn) or of any intermediate that is formed
from the excited state. Stimulated emission (SE) is a radiative transition
from the excited state to a lower state and stimulated by the probe pulse. It
shows negative peak close to the GSB, which normally overlayed with the
ESA in the red part. Dynamic Stokes shift may be observed at the initial time
delay. If the red part ESA is strong and broad spectra features, it always difficult to distinguish the SE signal.
Sample
P0
P
(power in)
(power out)
Excited State
Absorption
(ESA)
෰S1 generated
ESA
GSB
Absorbance:
A = -log T = log P0/P
SE
Stimulated
Emission (SE)
Ground State
Bleach (GSB) ෰S0 lost
Figure 3.4.Simplified diagram for different contributions of the transient absorption
spectra.
3.2.2 Nanosecond Laser Flash Photolysis
Nanosecond laser flash photolysis is a pump-probe technique, in which the
sample is firstly excited by a strong pulse (pump) with nanosecond pulse
width. This strong pulse pumps to increase the excited state population of
molecules. Xe arc lamp source is used as the probe light with certain time
delay after the nanosecond pump light (see detail set-up in the Figure 3.5).
31
OPO
Xe arc
lamp
hQ
PMT & CCD
hQ
Sample
Figure 3.5. A simplified schematic for nanosecond laser photolysis set-up, a Qswitched YAG laser with an optical parametric oscillator (OPO) that delivered ca.
10 ns pulses at 10 pulses/second. Pulse energies were 2-10 mJ/pulse at the sample.
The spectrometer (Edinburgh Instruments LP920-K) used a 450 W Xe-lamp to generate probe light. For single wavelength traces the probe signal was detected using a
monochromator, a P928-type PMT and a digital oscilloscope while transient spectra
were collected using a spectrograph and a CCD.
Nanosecond photolysis used in this thesis is mainly focused on to monitor
the long-lived charge separation state recombination between the reduced
dye and injected holes, which is normally short and mainly recombine at <1
ns time scale for p-type DSSCs, with additional electron acceptor designed
to give D-Slinker-A molecular dyes could retard the charge recombination
dramatically as expected, such as the DPP derivatives. Meanwhile, the nanosecond transient absorption could also determine the charge regeneration
efficiency for DSSCs, especially for the system use in a cobalt electrolyte,
since the cobalt electrolyte is known regenerate dyes to the ground state at >
Ps, nanosecond transient absorption can be useful to give a reliable charge
regeneration efficiency. To approach a quantitative comparison of the data
for the different dyes, and with and without electrolytes, we used the
weighted average of logarithmic lifetimes from a biexponential fit to the data
(Eq. 3.2)67
݈‫ = > ߬ < ݃݋‬σ ‫ܣ‬௜ ݈‫߬(݃݋‬௜ )
(3.2)
where Ai is the fractional amplitude for each component. The more common
definition of average lifetime, < ߬ >= σ ‫ܣ‬௜ ߬௜ overemphasizes longer lifetime components and has the related, unsatisfactory problem that <W> z
(<k>)-1. These two problems are solved by the definition in eq. 3.2, which
gives equal weight to each decadic time scale just like plots of data on a
logarithmic time scales do.
Analysisusing the KWW-function (stretched exponential function, eq.
3.3)68-71 that is commonly used in DSSCs research gave unsatisfactory fits to
our data.
ఉ
‫ݐ(ݏܾܣ = )ݐ(ݏܾܣ‬଴ )݁‫ ݌ݔ‬ቀെ൫‫ݐ‬ൗ߬൯ ቁ
32
(3.3)
Also, the regeneration reaction is not expected to follow a stretched exponential function, and comparison of average KWW lifetimes with and without electrolytes can therefore be problematic. Moreover, the average KWW
lifetime is calculated using the ill-behaved * function. For cases where ȕ <
0.3, as has been reported for NiO systems,27 the average lifetimes can be
quite long, orders of magnitude longer than the time point at which the original data no longer shows a detectable signal. In such a case the average
lifetime calculated has no physical relevance. For these reasons we prefer
not to use the KWW function.
3.3 Time-Correlated Single Photon Counting (TCSPC)
Time-correlated single photon counting allows us to measure the time that a
molecule stays in its excited state following excitation (Figure 3.6), by
measuring fluorescence lifetime. The distinct advantages of TCSPC technique are sensitivity, data accuracy and precision. Generally, one of the inputs to the TCSPC electronics (either ‘start’ and ‘stop’) will be a pulse generated by a single photon. Single photons can be detected by photodetectors
with intrinsically high gain. Repeating this process many times will give the
decay data, fs to ps repetition frequency could give high resolution of at least
100 ps excited state lifetime. The commonly used organic fluorophores lasts
in a time scale from hundred picoseconds to some tens of nanoseconds in
solution or on solid films.
Monochromator
Sample
PMT
CFD
Start
CFD
LASER
Detector
Stop
MCA
TAC
ADC
Figure 3.6. A simplified schematic for TCSPC. A pulsed laser is used as excitation
light. A part of itpasses through the sample, the other part goes to the electronics as
‘sync’ signal, the light emitted from excited sample goes through the monochromator and appropriate filter for selection certain wavelength and PMT amplified the
signal to the constant fraction discriminator (CFD). Then the time-to-amplitude
converter (TAC) collect the emitter as ‘start’ and after delay time a ‘stop’ command
from the reference signal. Analog to digital converter (ADC) and multi-channel
analyzer (MCA) translate all signal to a data output and the detector will give a plot
of counts vs. delay time.
33
3.4 Data Analysis
Data analysis will focus on the data processing of the time-resolved spectroscopy towards novel photosystems. The absorption of a photon by the
complex that can be studied in real time by optical techniques, femtosecond
transient absorption is as described, a powerful techniques to monitor the
electron/hole transport in the solar energy conversion systems. By this method, one records the transient absorption ('OD (O, t)) as a function of the
probe wavelength O and of the delay time t. The data is often analyzed by a
sum of exponential functions as in Eq. (3.4)
οOD (O, ‫ = )ݐ‬σ௜ ܱ‫ܦ‬௜ (O)݁ ି௧Τఛ೔
(3.4)
Here i corresponds to the size of the basis expansion used to describe the
data, ODi (O) stands for the preexponential coefficient related to each probe
wavelength and Wi is the time constant for each exponential decay. For multidimensional time-resolved datasets dealing with in this thesis, OD is amplitude as a function of wavelength.
In the analysis process, we should firstly consider some important definitions in the time-resolved spectroscopy which significantly affect the analysis of results, such as instrument response function (IRF), chirp correction
and coherent artifacts.
3.4.1 Important Definitions before Analysis
IRF of the time-resolved spectroscopy is defined as the convolution of the
reaction-inducing laser pulse and the probe pulse in ultrafast spectroscopy or
the reflected/scattering excitation light in time-resolved fluorescence spectroscopy. It was always different significantly and the timing characteristic
of each instrument is depending on the excitation wavelength.
Another important definition we need understand is chirp, it causes the
frequency changes in time when the laser pulse goes through media, the
supercontinuum is chirped due to self-phase modulation and group velocity
dispersion. As the chirp happened in different frequency, it make the probe
enter the sample at different times for a certain time delay, i.e. the blue probe
always arrives at the sample later than the red probe. his will affect the time
resolution of the measurements. So reliable correction is essential to resolve
the chirp, for femtosecond transient absorption, chirp always shows in the
initial <10 ps, third degree of polynominal fitting give a chirp line with
standard error about 10 fs (Figure 3.7).
34
1,6
Delay Time (ps)
1,2
0,8
0,4
0,0
Femtosecond TA data
Fitting
-0,4
400
500
600
Wavelength (nm)
700
Figure 3.7. Chirp line of femtosecond TA and fitting.
Coherent artifacts give undesired signals from two pulses overlapping
around time zero that for a resonant response of the molecular system, nonresonant contributions give a rise to additional signals. Artifacts in the ultrafast transient absorption spectroscopy is always realize coming from nonlinear optical effects, which is mostly causes by three reasons72,73: firstly, it
may be caused by application of very short laser pulses (shorter than 100 fs);
The other is the application of a spectrally very broad probe pulse, the white
light continuum in the 300-1100 nm range is always together with temporal
chirp; Last one is a sensitive detection system which allows observation of
optical density change on the order of 0.0005, unwanted signal of relatively
low amplitude came to competitive with the weak TA signals. The artifacts
are intrinsic in certain measurements and detection system; however, it can
be reduced or avoided by understanding the testing system, such as controlling certain enough power of the pump laser pulse or optimizing the sample
concentration and using white light with small temporal chirp.
3.4.2 Global Analysis of Transient Absorption Data
The time resolved transient absorption measurements give three dimensional
data sets, which presents the change of absorbance as a function of wavelength and delay time. Chirp correction should be carried out to find a proper
chirp line for each measurement, to correct the original data to time zero.
There are typically different experimental artifacts in the ultrafast optical
spectroscopy, which hinder the straightforward analysis. Thus, the data analysis procedures need to include algorithms for instrument response deconvolution, wavelength dependence of the time-zero position of the recorded
signal and of the width of the instrument response function (IRF) and coherent artifact analysis. More detailed data analysis in this thesis is conducted
on a lab written MATLAB script with all correction of the chirp, IRF and
artifacts.74,75 Before the global analysis, it must be determined how many
processes happened in the measured sample and how many species may be
observed. Here it needs to be noted that the electron transfer processes may35
be of non-exponential character, the time scale for each process is usually
overlap in multi-exponential.4,11,52 Each time constant derived does not reflect an individual process or sub-population, but represents the time scales
of a heterogeneous process, and the precise values should not be overinterpreted. The number of exponents used to fit data is somewhat arbitrarily
chosen as the minimum number of exponents needed to give representative
fit. Firstly, certain enough probe wavelengths choose to fit and determine
certain exponentials for global fitting, this process can be used to have a
rough view of the sample system, while it could determine the reaction rate
for each process approximately. In next step, those rates will be locked for
each species to make the global fitting.
Decay associated spectra (DAS) and Species Associated Spectra (SAS)
The global analysis result consists of a sum of exponential decay times. The
decay associated spectra (DAS) reflect changes in absorbance associated
with each time constant (Eq 3.2). It indicates the spectra relative to each
component in the global fit and presents each component as spectra features
in certain time delay (Figure 3.8). Recalculated the DAS amplitudes, we can
obtain the species associated spectra (SAS), which present the ‘pure’ absorbance of each species and allows for a more straight and direct observation.
For the calculation of the SAS, a consecutive model was assumed.
20
' Abs. u 10
3
0
-20
-40
W1 = 2.05 ps
W2 = 11.6 ps
-60
W3 = 149 ps
W4 = Inf ps
-80
400
450
500
550
Wavelength (nm)
600
650
Figure 3.8. An example of decay associated spectra (DAS) from global analysis of
an organic dye DPPBr in dichloromethane, three exponentials were used and the
lifetime for each component is 2.05 ps, 11.6 ps and 149 ps, respectively.
36
4. Photo-induced Charge Separation Processes
in NiO based D-linker-A Dyes Sensitized
Solar Cells (Paper I-V)
In this chapter, several kinds of novel Anch-D-S/linker-A p-type dyes were
systematically investigated by time-resolved spectroscopy, and photoinduced
charge separation mechanisms of the dyes based on different chromophores
were summarized towards the design of more efficient dye and the development of higher efficiency photocathode for p-type solar cells and tandem
cells.
4.1 Introduction
In order to improve the photon-to-electricity conversion efficiency (PCE),
the design rules of p-type dyes should follow the normal guidelines, which
have been demonstrated in chapter 2. In this chapter, paper I and paper II
present several diketopyrrolopyrrole (DPP) based Anch-D-S/linker-A type
derivatives. DPP pigments have been proved to begood light harvesting and
emission chromophores and also have well applied as optical and electrical
applications in chemosensors76, two-photon absorption77 and organic photovoltaic devices78. Papar I and II present two series of DPP based p-type dyes,
there are four ‘push-pull’ DPP dyes in total (Figure 4.1), coded as DPPBr,
DPPCN2, 1 and 2. DPPBr and DPPCN2 are functionalized with a 4thiophene carboxylic acid as anchoring group and a bromo (DPPBr) or vinyldicyano (DPPCN2) as the electron withdrawing group. 1 and 2 use a
bis(benzoic acid) aminophenyl moieties as the anchoring group and the same
bromo used as the electron withdrawing group. Push-pull dyes present large
dipole moment in the excited state, such as the reported DFT calculations of
the first DPP series65. The other three dyads are coded as DPP-NDI, 3 and 4,
they all consist of a DPP unit, which is linked to a naphthalene diimide (NDI)
as the secondary electron acceptor. PI-NDI has been a successful dyad
linked with an secondary electron acceptor, which achieved a higher NiO
based DSSCs performance, VOC 0.35 V and JSC 1.7 mA cm-2.53 Detailed photophysical studies are carried out on two generations of dyes/NiO systems,
37
the mechanism comparison will be carried out on the different anchoring
groups, linkers and electron acceptors.
Paper III present two novel designed triphenylamine (TPA) based oligothiophene-acceptor p-type dyes (E1 and E2), the long six oligothiophene
used as the linker to prolong the distance between the donor and acceptor,
aiming to obtain long-lived charge separated state and render better PCE of
solar cells. This approach has been proven efficiently and achieved the world
record of p-DSSCs and tandem solar cells by Bach and co-workers.16,33,79
TAS was used to investigate both solution phase and sensitized NiO films.
The classical p-type dye, P1,10 is used as a reference. As a result, the E2 dye
showed longer lived charge separation compared with the simple DPP dyes.
Paper IV and V present two series of cyclometalated dyes. Photophysical
measurements carried out to study the molecular-structure control of the
electron transfer dynamics. Long-lived charge separated states were found
by the simple cyclometalated iridium complexes (paper IV), which makes
them compatible with other redox mediators than I3-/I-, such as a cobalt electrolyte, which reaches high open circuit voltage. In paper V, three novel
push-pull porphyrine zinc dyes are also employed to study the charge transfer mechanisms vs. photovoltaic properties.
4.2 Photophysical Study in Solution
The understanding of all excited states and radical features of dye molecules
significantly guides the realization of the electron transfer processes on semiconductors. Spectroelectrochemistry (SpecEchem), steady state UV-vis,
time-resolved fluorescence and TAS are main techniques to study the photophysical properties in the solution phase.
4.2.1 Determination of Anion Spectra of p-type Dyes
Spectroelectrochemistry (SpecEchem) is one of most useful methods to detect the anion radical of the dyes in solution. As the sensitivity property of
the anion, this measurements is better to be carried out in an inert gas atmosphere, glove box should be the best working environment. Electrochemical
reversibility of the anion radical should first be guaranteed. As the weak
stability or irreversibility of the anion radical of some dyes in solution,
SpecEchem method maybe inefficient for detection. An external donor could
be used to reduce the dyes in solution and flash-quench experimentcan be
used to detect the anion spectra in an inert gas saturated solution.
38
C2H5
C2H5
C4H9
N O
HO2C
S
HO2C
HO2C
C4H9
N O
CN
CN
DPPCN2
C2H5
C4H9
N O
N
Br
O N
C4H9
C2H5
1
S
O N
C4H9
C2H5
C2H5
N
C4H9
O
S
Br HO2C
O N
C4H9
C2H5
DPP Br
HO2C
N
HO2C
2
Br
O N
C4H9
C2H5
C2H5
C4H9
O
N
HO2C
N
O
C4H9
C2H5
HO2C
O
O
S
DPP NDI
N
N C8H17
O
O
C2H5
C4H9
N O
N
O
O N
C4H9
C2H5
HO2C
HO2C
3
O
N
N C8H17
O
O
C2H5
N
HO2C
C4H9
N O
O
O N
C4H9
C2H5
O
N
N C8H17
O
O
4
Figure 4.1. Molecular structures of DPP derivatives in paper I and II.
The determination of the DPP anion spectra is used as an example (Figure
4.2). The irreversibility of DPP reduction under electrochemical conditions
precluded SpecEchemdetermination of the radical anion absorption spectrum. Instead, we used the nanosecond laser flash-quench method in the
presence of an external electron donor, triethylamine (TEA). The difference
spectra of DPPBr and DPPCN2 recorded with TEA solution added in Ar
saturated dichloromethane (DCM), Figure 4.2a, shows the radical anion
spectra in the presence of TEA They exhibit clear difference from those of
the dye excited states without TEA (Figure 4.2b).
0,8
DPPCN2 Ar 100 ns
DPPBr Ar 100 ns
60
40
0,4
'Abs. (mOD)
'Abs. (mOD)
80
DPPBr DCM 3 mM TEA Ar 2 Ps
DPPCN2 DCM 3 mM TEA Ar 2 Ps
0,0
20
0
-20
-0,4
(a)
400
450
500
550
600
650
Wavelength (nm)
700
750
-40
(b)
400
500
600
Wavelength (nm)
700
Figure 4.2. Transient absorption spectra of the DPPBr and DPPCN2 in DCM upon
excitation at 532 nm, (a) with 3 mM trimethylamine (TEA) and (b) without TEA.
39
For the DPP-NDI dye, the first reduction of DPP-NDI is localized on the
NDI unit and is electrochemically reversible.65 Therefore, the difference
absorption spectrum of the reduced DPP-NDI in DCM solution was collected by using the SpecEchemmethod. The reduced NDI exhibits a board absorption in the visible region with a distinct peak at 475 nm, a weak and
broad absorption peak at 610 nm (Figure 4.3). This spectrum feature is in
good agreement with the reported NDI- absorption.80
1,4
Absorbance (AU)
1,2
1,0
0,8
0,6
0,4
0,2
0,0
300
400
500
600
700
800
900
1000 1100
Wavelength (nm)
Figure 4.3. Spectroelectrochemical measurements of DPP-NDI upon reduction at
1.0 V vs. Ag/AgCl in dichloromethane (DCM); the arrow indicates the changes
observed. Inset: the spectral data plotted as difference spectra.
4.2.2 Transient Absorption in Solution
As demonstrated in chapter 3, the TAS always includes three main features,
the positive excited state absorption (ESA), negative ground state bleach
(GSB) and stimulated emission (SE). However, there are also many other
photophysical processes depending on the charge separation in dyes. Femtosecond and nanosecond TAS in solution phase are applied to determine the
spectra of the singlet and triplet excited states of certain dyes as the reference
to study the spectra recorded on semiconductor surface. Figure 4.4 presents
the femtosecond and nanosecond TAS of DPPBr in DCM. The SE shows a
strong dynamic Stokes shift with a time constant of ca. 1.5 ps, which is
mainly caused by the solvent relaxation, being responsible for the initial
broad and negative signal between 450 nm to 650 nm separated into distinct
bleach and stimulated emission bands, respectively. Nanosecond TAS show
different spectra from the singlet excited spectrum and also is the absence of
emission as well as the absorption bands around 560 nm and 380 nm (see
footnote I). The transients decay with W ~10.8 Ps in Ar-purged DCM, while it
I
Note that the relative magnitude of excited-state absorption and emission in a ns experiment
depends on the instrument geometry, and that the resulting magnitude of any spontaneous
emission cannot be compared to stimulated emission in a fs pump–probe experiment.
40
is strongly quenched to 320 ns in air-saturated solvent (Figure 4.5). All these
features are consistent with a triplet excited state formed by intersystem
crossing from the singlet excited state.
20
20
'Abs. (mOD)
3
40
' Abs. u 10
30
-5 ps
0.002 ps
0.5 ps
2.5 ps
13 ps
70 ps
360 ps
1900 ps
60
0
10
0
DPPBr DCM Ar 50 ns
DPPBr DCM Ar 20 Ps
DPPBr DCM O2 50 ns
-20
-10
-40
400
450
500
550
600
650
Wavelength (nm)
700
400
450
500
550
600
Wavelength (nm)
650
700
Figure 4.4. Femtosecond (left) and nanosecond (right) TAS of DPPBr in DCM
(femtosecond time unit: ps. Oex = 505 nm, nanosecond Oex = 532 nm)
DPPBr DCM Air 550 nm
DPPBr DCM Ar 550 nm
'Abs. (mOD)
30
20
W½Air=320 ns
W½Ar=10.8 Ps
10
0
0,1
1
Time (Ps)
10
Figure 4.5. The kinetic traces of DPPBr in DCM with and without oxygen probed at
550 nm.
The secondary electron acceptor NDI anion was detected in solution phase,
the initially TAS of 4 rapidly exhibits spectral signatures of an intramolecular charge separated state where DPP is oxidized by NDI: DPPƔ-NDIƔ-.
Figure 4.6 shows the transient spectra of 3 and 4 in DCM 10 ps after excitation, where clearly a charge separated state can be observed for 4, coexisting with the remaining initial excited state: DPP ground state bleach is
accompanied by one NDIƔ- absorption peak at 480 nm and a smaller one at
~610 nm, overlapping with the strong remaining stimulated emission and the
absorption peak at ~440 nm from oxidized DPP radical cation.
41
30
2
20
3
0
' Abs. u 10
' Abs. u 10
3
10
0
-10
-4
-6
Dye4 480 nm
Dye3 480 nm
-8
-20
Dye4 10 ps
Dye3 10 ps
-30
-2
400
450
500
550
600
Wavelength (nm)
650
700
-10
0
2
4
1
10
2
10
Delay time (ps)
3
10
Figure 4.6. Transient absorption spectra (left) for 3 and 4 in DCM. Both spectra, 3
(solid) and 4 (dashed) are recorded at a 10 ps delay after excitation with 525 nm
(500 nJ/pulse). Right is the kinetic traces at 480 nm probe for 3 (red) and 4 (blue).
4.3 Ultrafast Electron Transfer Dynamics on NiO
Surface
In order to study the electron transfer dynamics of dyes on a semiconductor,
the dyes were easily immobilized on the NiO by the so-called sensitization
processes. Femtosecond and nanosecond TAS were employed for various
dye/NiO systems. TAS give clear evidence of the hole injection, charge
transfer processes and charge recombination time scale. Multi-exponential
hole injection and recombination kinetics are typical for dye-sensitized NiO
and the processes often overlap. It is important to realize that each time constant derived does not reflect an individual process or sub-population, but
represents the time scales of a heterogeneous process, and the precise values
should not be over-interpreted. The number of exponents used to fit data is
somewhat arbitrarily chosen as the minimum number of exponents needed to
give representative fitting.
4.3.1 Hole Injection into NiO
Hole injection is always a continuous process spreading in a range of time
constants, it should be resolved by more careful observation of the change of
transient absorption by comparing both the singlet and triplet excited state in
solution and the new species TAS on NiO film. When the excited dye start
inject holes into NiO, the new species, reduced dye, will somehow present
different absorption spectra compared to the singlet one, the combined spectra will exhibit isosbestic point shifting in the differential spectra, kinetic
traces probed close to the isosbestic point will include more population of
the charge injection process.4,46,48 On the other hand, if there is not isosbestic
point shift on the TAS, but TAS present dramatically changed spectra compared with the singlet TAS, especially no build-up of excited state species,
42
injection is determined to be equal to or faster than the fastest detected component.15,81
Figure 4.7 shows the TAS of NiO/2, the isosbestic point shift around 570
nm, the kinetic traces and a sum of multi-exponential fitting for dye 2 are
probed at 570 nm and 625 nm, the time constants obtained from global fit of
the transient data are reported.45 Hole injection occurs on a time scale of 110 ps, but rapid charge recombination (~50 ps) depletes the charge separated
state.
YF43_NiO
1
0.3 ps
0.5 ps
8
0
1 ps
' Abs. u 10
3
100 ps
500 ps
1900 ps
4
' Abs. u 10
3
10 ps
0
-1
-2
-3
-4
-4
570 nm
625 nm
-5
-8
-6
400
450
500
550
600
Wavelength (nm)
650
700
-2
0
2
4
1
10
2
10
Delay time (ps)
3
10
Figure 4.7. Transient absorption spectra (left) of NiO/2 in the presence of 0.1 M
LiClO4. Right is the kinetic traces probed at 570 nm and 625 nm, solid line is the
multi-exponential fit. (Oex = 525 nm).
The design of the anchor group is further important for hole injection and
recombination, one would expect that a longer distance and lower degree of
conjugation to the active chromophore would give slower ET (both injection
and recombination). In two series of DPP dyes, we are comparing the results
of DPP with thiophene and triphenylamine (TPA) anchoring groups (DPPBr, 1 and 2) (Paper II), TPA group acting as a donor in the second DPP series, it increase the CT by a factor of 2. We note that this increase is moderate as the computed charge separation remains limited to less than 1.5 Å.
The DFT results indicate that the presence of two carboxylic groups that
diminish the donating power of the amine. When increasing the linker length
(from 2 to 1) the large dihedral angle of the biphenyl units prevents an efficient direct electronic communication between the TPA and the DPP, and
the excited-state is mostly localized on the DPP unit with a negligible CT
effect. This lack of CT may be detrimental for DSSCs, but the excited-state
is now well separated from the anchoring group, which is consistent with the
original idea of design to limit the charge recombination which is known to
be a key limiting parameter in p-DSSCs. In photophysical studies, both 1
and 2 exhibit 1-10 ps hole injection and ultrafast charge recombination between the injected hole with reduced DPP. Given the multiexponential kinetics we see no significant difference in the dynamics of either hole injection
or recombination between 1 and 2, in spite of the extra phenyl spacer in 1.
43
However, compared to DPP-Br, which has a different and shorter anchoring
group, the majority of recombination for 1 and 2 occurs on almost an order
of magnitude shorter time scale.
4.3.2 Ultrafast Charge Recombination and Slow Charge
Recombination
According to the global fitting results for dye 1-4, the excited state dyes 1
and 2 are quenched by hole transfer to NiO on the 1-10 ps time scale, the
subsequent recombination of the reduced DPP anion with the injected hole in
NiO is also rapid, which most likely is the reason for the poor photovoltaic
performance of dyes 1 and 2 as sensitizers in p-type DSSCs.
Figure 4.8. Schematic of electron transfer processes of DPP dyes on NiO.
Appending a secondary electron acceptor to replace the electron withdrawing bromo or vinyldicyano group, TAS of dyes DPP-NDI, 3 and 4 show
intramolecular charge transfer (ICT) from reduced DPPƔ- anion to the secondary electron acceptor forming the reduced NDIƔ- unit for all three dyes: a
strong peak around 480 nm and a weaker one at 605 nm. First, this suggests
that hole injection from excited DPP is faster than intramolecular charge
separation (W = 60-140 ps, see Figure 4.8). Second, the subsequent electron
transfer from DPPƔ- to the NDI unit must be at least as fast as hole injection.
Third, the NDIƔ- signal remains almost constant until the end of the optical
delay (2 ns), giving an evidence for the strong increase in charge separation
lifetime of DPP-NDI, 3 and 4 compared to other dyes.
The majority of charge recombination in the push-pull dyes occurs with
an ultrafast <100 ps in paper I-III, the additional secondary acceptor prolong
the charge recombination, and there is large amplitude left at >2 ns. Those
longer time scale recombination will be monitored by the nanosecond transient absorption spectroscopy.
The kinetic trace for NiO/DPP-NDI probed at 480 nm is shown in Figure
4.9. A biexponential fitting to the data trace resulted in W1 ȝVDm44
plitude) and W2 = 26 ȝV 7KH ZHLJKWHG DYHUDJH RI ORJDULWKPLF OLIetimes then gives <W> = 140 ȝV (Eq. 3.2). NiO/3 and NiO/4 signals are very
long-lived compared to other dye/NiO systems52,66,82,83 and decay on a time
scale of up to 1 ms, very similar to the results of the best performing NiO
DSSCs.79 Table 4.2 also presents the time constants of dyads with electrolyte,
which we will discussed later in section 4.5.
NiO/DPP-NDI LiClO4 480 nm
Biexp. fit
'Abs. (mOD)
6
4
2
0
1E-7
1E-6
1E-5
1E-4
1E-3
Time (ms)
0,01
0,1
1
Figure 4.9. Nano-second transient absorption kinetic for NiO/DPP-NDI, showing
the long-lived charge separarated state. Excitation at 532 nm, 2 mJ/pulse, a shortpass
filter at 500 nm was used before the monochromator to reduce scattered laser light.
Table 4.1. Time constants from ns transient absorption measurements of NiO/DPPNDI, NiO/3 and NiO/4 with and without electrolyte.
LiClO4
CoIII
Lifetime
NiO/DPPNDI
NiO/3
NiO/4
W1
49 Ps/37%
49 Ps/51%
47 Ps/58%
W2
260 Ps/63%
570 Ps/49%
476 Ps/42%
W1
4.5 Ps/92%
21 Ps/87%
24 Ps/89%
W2
75 Ps/8%
445 Ps/13%
410Ps/11%
4.4 Molecular-Structure Control of Electron Transfer
Dynamics in Cyclometallated p-DSSCs (Paper IV-V)
Two series of cyclometalated D-S/linker-A p-type dyes have been used to
investigate the charge transfer mechanism in simple Ir complexes and the
effect of the molecular structure changes. Paper IV shows three simple iridium (III) complexes. As a strong oxidant Ir complexes are favorable to efficiently inject a hole in the VB of NiO. In paper V, the classical porphyrins
45
are modified in varied approaches and photophysical studied the charge
transfer processes in detail.
4.4.1 Long-lived Charge Separated state with Cyclmetalated
Complexes
Nanosecond TAS has been used to determine the rate of charge recombination of Ir complexes (Figure 4.10), named IrPhen, IrDPQCN2 and
IrBpystyryl. Phosphonic acids are chosen instead of the carboxylic acid as
anchoring group, because it shows more stable bonding with semiconductor
surface in aqueous medium. Variation of the diamine ligands N^N provide a
change of the orbital energetics and absorption properties of the complexes.
PO3H2
H2O3P
PO3H2
PO3H2
N
Ir
N
H2O3P
N
H2O3P
N
N
Ir
N
N
N
OMe
Ir
N
N
N
CN
CN
IrDPQCN2
PO3H2
N
H2O3P
H2O3P
IrPhen
PO3H2
PO3H2
OMe
N
N
H2O3P
IrBpystyryl
OMe
OMe
Figure 4.10. Sturctures of the iridium complexes.
For three complexes, the triplet state were measured in a deaerated acetonitrile solution and all complexes gave similar features, which characterized a
broad positive band spanning from 450-650 nm and narrow band below 400
nm. For IrBpystyryl, the latter feature is replaced by a net ground state
bleach around 410 nm (Figure 4.11). The purple solid lines in Figure 4.11
are the difference spectra of reduced state of Ir complexes. It is obtained in
solution, in the presence of an electron donor N,N,N’N’-tetramethyl-1,3propanediamine (TMPDA). All three complexes shows rather weak and
indistinct features, a band around 430 nm and weaker feature in the remainder of the visible region, in agreement with a previously reported spectrum
of a reduced cyclometalated Ir complex.84
46
'Abs. (mOD)
4
2
0
6
10 ns
30 ns
50 ns
100 ns
Reduced 2 Ps
4
2
0
8
6
'Abs. (mOD)
10 ns
30 ns
50 ns
100 ns
Reduced 2 Ps
'Abs. (mOD)
6
4
2
0
10 ns
20 ns
30 ns
50 ns
Reduced 2 Ps
-2
-4
-2
(a)
400
-2
450
500
550
600
650
(b)
400
700
450
Wavelength (nm)
500 550 600 650
Wavelength (nm)
-6
700
(c)
400
450
500 550 600 650
Wavelength (nm)
700
Figure 4.11. Nanosecond TAS of IrPhen (a), IrDPQCN2 (b) and IrBpystyryl (c)
recorded in CH3CN upon excitation at 460 nm. The purple solid line in each image
is the reduced-ground state difference spectra obtained upon photoreduction with
TMPDA (the TMPDA+ contribution is subtracted).
While three complexes sensitized on the NiO film, TAS exhibit clearly different spectra from that in solution (Figure 4.12), but similar to those reduced state spectrum in solution, we therefore assign these spectra to the
reduced complexes formed by hole injection from the excited Ir complexes
into NiO. The multiexponential decay of the species is similar for the three
complexes and are fitted with a sum of three exponents with approximately
equal amplitudes: IJ1 = 50-70 ns, IJ2 §1.0 ȝs and IJ3 = 5-30 ȝs. The time scale
is much slower than normal. In fact, this is the first time that such a longlived charge separated state was monitored on NiO with very simple cyclometalated sensitizers.
The recombination rate is very similar between the three complexes, suggesting that the above, common feature is responsible for the slow recombination rather than the details of the remote ligand structure. Also, the recombination rate constant is apparently not predominantly controlled by Gibbs
free energy of the process, as this varies a lot between the complexes, but by
the electronic coupling, which is certainly weak for the back reaction.61,85
100 ns
1 Ps
10 Ps
100 ns
1 Ps
20 Ps
4
2
5
4
2
0
0
500
600
Wavelength (nm)
700
0
-5
-10
(a)
400
200 ns
1 Ps
20 Ps
10
'Abs. (mOD)
6
'Abs. (mOD)
'Abs. (mOD)
6
-15
(b)
450
500
550
600
650
Wavelength (nm)
700
(c)
400
500
600
Wavelength (nm)
700
Figure 4.12. Nanosecond TAS of IrPhen (a), IrDPQCN2 (b) and IrBpystyryl (c)
recorded on NiO. The inset is the absorbance decay probed at 440 nm or 600 nm.
(Oex = 460 nm).
4.4.2 Molecular-Structure Control of Electron Transfer
Dynamics of Push-Pull Porphyrin Sensitizers
The porphyrin dyes in p-DSSCs still show poor efficiency. Therefore, the
study of the charge transfer processes of porphyrin dyes on NiO is helpful to
47
the design of more efficient dyes. In this work (Paper V)86, the porphyrin
dyes were modified with ethynyl moieties linked porphyrin with an electron
withdrawing group (Figure 4.13). In order to red shift both Soret and Qbands of porphyrin and to inverse the relative intensities of the vibronic
bands of the lowest-lying singlet excited state. ZnPref has one carboxylic
group as the anchoring group and a phenyl linked the ethynyl. ZnP-TPANO2 has two carboxylic acid units and triphenylamine groups as the electron
donor and phenyl nitro group as the electron withdrawing group, as similar
in organic D-S/linker-A sensitizers, ZnP-NDI dyad was designed with an
additional electron acceptor, NDI.
C12H25O
N
HO2C
N
OC12H25
Zn
C12H25O
HO2C
C12H25O
N
N
N
OC12H25
HO2C
OC12H25
N
N
Zn
C12H25O
N
C12H25O
N
NO2 HO2C
N
N
O
N
C12H25O
OC12H25
ZnP TPA NO2
ZnPref
OC12H25
N
Zn
O
N
N
O
O
OC12H25
ZnP NDI
Figure 4.13. Molecular structure of designed porphyrin dyes.
Figure 4.14 shows the transient absorption spectra of the Zn-porphyrin dyes
on the NiO surface. The transient absorption spectra of NiO/ZnPref and
NiO/ZnP-NDI present similar features with a fast recovery of the ground
state bleach and the small stimulated emission signal observed at ~660 nm
during the initial few picoseconds. However, more transient signals remain
after 1.9 ns for NiO/ZnP-NDI than for NiO/ZnPref. The transient spectrum
of NiO/ZnP-TPA-NO2 is similar to that of NiO/ZnPref and decays almost
completely within 2 ns.
3
0
0.3 ps
0.5 ps
1 ps
10 ps
100 ps
500 ps
1900 ps
-5
-10
400
450
500
550
Wavelength (nm)
600
650
' Abs. u 10
' Abs. u 10
3
4
5
3
2
1
0.3 ps
0.5 ps
1 ps
10 ps
100 ps
500 ps
1900 ps
40
30
3
5
' Abs. u 10
0.3 ps
0.5 ps
1 ps
10 ps
100 ps
500 ps
1900 ps
6
10
20
10
0
-1
0
-2
400
450
500
550
Wavelength (nm)
600
650
700
400
450
500
550
600
650
700
Wavelength (nm)
Figure 4.14. Femtosecond transient absorption spectra of porphyrin dyes on the NiO
film with 0.1 M PC/LiClO4 electrolyte after excitation at 560 nm: (left) NiO/ZnPref,
(middle) NiO/ZnP-TPA-NO2, and (right) NiO/ZnP-NDI.
The corresponding band shape is hard to be observed for the other two dyes
as the remaining signal magnitude is so weak. There is no clear evidence of
NDI reduction, yet the charge separated state is clearly more long-lived with
the ZnP-NDI dyad. These reactions have been further investigated on ns - ȝV
time scale. The transient absorption after a 10 ns laser flash in the Soret band
at 450 nm are shown in Figure 4.15. We could obtain detectable transient
48
signals although most of the excited species had decayed on the < 2 ns time
scale, before the time of observation. With added PC solvent on the NiO
film, the signal for ZnP-NDI broadens a little compared to that without solvent, but there are still clear bleaches of the Soret and Q-bands from the
porphyrin, so if the electron goes out to NDI acceptor, it is only in some of
the molecules, resulting in a broadened spectra at 480 nm by the absorption
of the NDI-.
8
6
4
'Abs. (mOD)
'Abs. (mOD)
4
2
0
-2
-4
2
0
-2
-6
400
500
600
Wavelength (nm)
700
400
500
600
Wavelength (nm)
700
Figure 4.15. Nanosecond TA spectra at t = 50 ns of porphyrin dyes on NiO
films with (right) and without PC solvent (left), excitation at 450 nm: ZnPref
(black), ZnP-TPA-NO2 (blue), ZnP-NDI (red).
The driving force is large for electron transfer from ZnPƔ- to NDI, -'Gº |
0.85 eV, however it is surprising that ZnPƔ- does not reduce NDI even on the
ns time scale. Nevertheless, the NDI group apparently stabilizes the charge
separated state so that more of it survives onto the ns time scale.
4.5 Charge Regeneration with Co(dtbpy)33+/2+
Electrolyte
Slower charge recombination and faster charge regeneration efficiency by
the redox couple are expected for all designed dyes in DSSCs, in this chapter,
some D-Slinker-A dyes are compatible with Co(dtbpy)33+/2+ electrolyte, we
summarized the charge regeneration effect of
some dyes with
Co(dtbpy)33+/2+ electrolyte.
As aforementioned, NiO/DPP-NDI, NiO/3 and NiO/4 present long-lived
charge recombination on ns-ms time scale (Table 4.1), Addition of 0.1 M
Co(dtbpy)33+ rapidly regenerates the dye (Figure 4.16), as seen from the
much faster NDIƔ- decay kinetics; > 85% of the signal decayed with W = 4.5 ȝV7KH UHVXOW LPSOLHV WKDW G\HUHJHQHUDWLRQLV UDWKHUHIILFLHQW! DV
judged from the kinetics, which is most likely the main reason for improvement of the photovoltaic properties of these dyes compared to 1 and 2. The
49
slightly better performance of NiO/3 than NiO/4 could be related to a slower
(570 Ps) component in the absence of Co(dtbpy)33+ and more proportion of
this component than that of NiO/4 (470 Ps) and NiO/DPP-NDI. Control
experiments of NiO/DPP-NDI suggest that addition of Co(dtbpy)33+/2+ does
not change the recombination kinetics: addition of only Co(dtbpy)32+ has no
effect, and addition of only Co(dtbpy)33+ has the same effect as adding both
Co(dtbpy)32+ and Co(dtbpy)33+ (Figure 4.17).45
1,2
1,2
Normalized'A
1,0
Normalized 'A
NiO/3 LiClO4 480 nm
NiO/3 LiClO4 50 ns
NiO/3 CoIII 480 nm
Biexp. fit
Biexp. fit
NiO/4 LiClO4 50 ns
0,8
0,6
0,8
0,4
0,4
0,2
0,0
0,0
(a)
400
(b)
450
500
550
600
Wavelength (nm)
650
700
0,0
1,2
0,4
0,8
Time (ms)
1,2
1,6
NiO/4 LiClO4 480 nm
Normalized 'A
NiO/4 CoIII 480 nm
Biexp. fit
Biexp. fit
0,8
0,4
0,0
(c)
0,0
0,4
0,8
1,2
1,6
Time (ms)
Figure 4.16. Normalized ns transient absorption spectra of NiO/3 and NiO/4 and the
recombination, regeneration of each sample with or without Co(dtbpy)33+ electrolyte
(excitation at 532 nm). Note that Co(dtbpy)32+ absorbs more than Co(dtbpy)33+ does
at 480 nm, which explains the positive signal at the end of the observed decay.
50
Normalized 'OD
1,0
0,5
0,0
NiO/DPPNDI with CoII 480 nm
NiO/DPPNDI with CoIII 480 nm
NiODPPNDI with LiClO4 480 nm
NiO/DPPNDI with CoII/III 480 nm
0,01
0,1
1
Time (Ps)
10
100
Figure 4.17. Normalized kinetic traces of NiO/DPP-NDI with and without electrolyte at 480 nm, after excitation at 532 nm. (Open circles and open squares is data
with Co(dtbpy)33+ and Co(dtbpy)32+, green dot and blue triangles is the traces for
inert LiClO4 and Co(dtbpy)33+/2+).
4.6 Conclusions
Several series of novel Anch-D-S/linker-A p-type dyes have been systematically investigated by time-resolved spectroscopy for detailed electron transfer dynamics in dye sensitized NiO solar cells.
x Femtosecond transient absorption experiments demonstrate ultrafast hole
injection (~200 fs + ps components) of all organic and cyclometalated
photosensitizers.
x For the first time monitored a long-lived charge separated state on NiO
in a cyclometalated sensitizers (sub-Ps), and the appended NDI unit in
organic D-linker-A sensitizers presents very slow charge recombination
time scale to 50-500 Ps time scale.
x Co(dtbpy)33+/2+ electrolyte work efficiently on regeneration process for
slow recombination (>10 Ps)
51
5. Electron Transfer in p-type Solid State Dye
Sensitized Solar Cells (Paper VI)
In this chapter, solid state p-DSSCs have been proposed for the first time,
using an organic dye P1 as sensitizer on mesoporous NiO and phenyl-C61butyric acid methyl ester (PCBM) as electron conductor. Femtosecond and
nanosecond transient absorption spectroscopy has given evidence for sub-ps
hole injection from excited P1 to NiO, followed by electron transfer from
P1Ɣ- to PCBM. The p-ssDSSCs device has shown a promising open circuit
photovoltage, 620 mV.
5.1 Introduction
In the typical p-DSSCs, a liquid electrolyte containing a redox couple is
employed to undertake the internal electron transfer between photocathode
and counter electrode. Iodide/triiodide (I-/I3-) was initially used as the redox
couple in NiO-based p-DSSCs;2 However, the open circuit voltage generated
(VOC <150 mV)7,87 is unsatisfactory due to the small energy difference between the VB of NiO (0.5 V vs. normal hydrogen electrode (NHE)) and the
redox potential of I-/I3- (E0 = 0.33 V vs. NHE).11 In order to improve the VOC
of p-DSS&V LQFUHDVLQJ ǻ( E\ XVLQJ D UHGR[ FRXSOH ZLWK D PRUH QHJDWLYH
potential than I-/I3- is a desirable strategy. Gibson et al.53 used tris(ditertbutylbipyridine)cobalt(II/III) to obtain a VOC = 0.35 V. Bach and co-workers
reported two redox couples, tris(1,2-diaminoethane)cobalt(III)/(II)88 and
tris(acetylacetonato)iron(III)/(II)33 with E0 of -0.02 V and -0.2 V vs. NHE,
respectively, in liquid p-DSSCs and obtained the VOC more than 600 mV.
However, redox couples with too negative potential are quite sensitive to
oxygen and thus the solar cell fabrication has to be carefully carried out under O2-excluding conditions, making the fabrication process more complicated; moreover, like most of liquid solar cells, the electrolyte leaking problem is still a drawback in the liquid p-DSSCs. In order to completely abandon the liquid phase and still achieve a satisfactory VOC in a p-DSSC, in this
work (Paper VI), we propose and also prove the concept of solid state pDSSCs (p-ssDSSCs) (Figure 5.1). Here NiO is employed as the p-type semiconductor, an organic dye P1 as photosensitizer harvesting photons and an
air-stable solid electron conductor, PCBM, instead of liquid redox couple
52
electrolyte, is employed as solid electron transfer material. Photovoltaic
properties and time-resolved transient absorption spectroscopy are introduced to get insight into the mechanism of the proposed p-ssDSSC model.
Femtosecond and nanosecond transient absorption spectroscopy elucidated
the electron transfer mechanism in this system.
CL-NiO
SnO2:F
NC
CN
100 nm Al
N
PCBM
P1
S
NC
PCBM
CN
-
HOOC
Dye
NL-NiO
S
+
Normal Glasss
hv
Figure 5.1. Schematic drawing of p-ssDSSCs configuration and structures of the
compounds P1 and PCBM used in this study.
5.2 Steady State Absorption and Photovoltaic
Performance
UV-vis absorption spectra of NiO films sensitized/coated with different
components are shown in Figure 5.2. The sample NiO-P1/PCBM exhibits a
new peak compared to NiO-P1, around 700 nm. By comparison with the
NiO/PCBM sample, this can be assigned to PCBM absorption. The E0-0 transition energy of PCBM can be estimated to be 1.8 eV by the absorption spectrum on NiO film. Therefore, the potential of PCBM*/PCBMƔ- is determined
to be 1.4 V vs. NHE (potential of PCBM/PCBMƔ- is ~-0.4 V vs. NHE89),
which is more positive than the VB potential of NiO, 0.5 V vs. NHE, implying the hole injection from PCBM* is also thermodynamically feasible (Figure 5.3).
53
1,2
Absorbance
1,0
NiO-P1
NiO/PCBM
NiO-P1/PCBM
0,8
0,6
0,4
0,2
0,0
400
500
600
700
Wavelength / nm
800
Figure 5.2. The UV-vis absorption of different components on NiO films (200 nm
CL-NiO+700 nm NP-NiO).
V vs. NHE
P1/P1-0.8
-0.4
PCBM/ PCBM-
0
0.3
I-/I3-
0.5
VOC
VOC
NiO VB
1.4
P1-/P1*
Figure 5.3. Potential diagram of p-DSSCs based on PCBM or I-/I3-.
The photovoltaic performance of p-ssDSSCs was measured. The photocurrent density-photovoltage (J-V) curves of devices and the corresponding
incident photon-to-electron conversion efficiency (IPCE) spectra are displayed in Figure 5.4. With P1 as photosensitizer, the p-ssDSSC device, NiOP1/PCBM, rendered a Jsc RI ȝ$āFP-2 and a VOC of 620 mV, which are
significantly improved in comparison to those generated from NiO/PCBM
device. Also, one can see the IPCE around 710 nm is also enhanced in NiOP1/PCBM device, which means the photo-conversion efficiency from excited PCBM is also increased, overall, the experiments suggest that P1 dye
plays important role(s) in NiO-P1/PCBM device to improve the photovoltaic
performance: acting as photosensitizer and as blocking layer.
54
JSC / μA.cm-2
60
0,6
(a)
NiO/PCBM
NiO/P1/PCBM
50
40
30
IPCE / %
70
(b)
NiO/PCBM
NiO-P1/PCBM
0,4
0,2
20
10
0
0,0
0,2
0,4
VOC / V
0,6
0,0
400
500
600
Wavelength/nm
700
800
Figure 5.4. J-V curves (a) and IPCE spectra (b) of NiO/PCBM and NiO-P1/PCBM
devices.
5.3 Electron Transfer Mechanism
The ultrafast femtosecond spectroscopy measurements are therefore carried
out to investigate the interface charge transfer processes. For both NiO-P1
and NiO-P1/PCBM, the fs-TAS after excitation at 525 nm (Fig. 5.5a and b)
give similar TA at initial delay times. Already the spectrum at 0.3 ps can be
described by a mix of the initial singlet excited P1 (1*P1) and of the P1●formed by photo-induced hole injection. This data is in agreement with previously reported data.10
55
8
' Abs. u 10
3
4
0
-2
0.3 ps
2 ps
100 ps
1900 ps
-6
-10
400
450
500
550
600
Wavelength (nm)
650
700
8
' Abs. u 10
3
4
0
-2
0.3 ps
5 ps
100 ps
-6
500 ps
1900 ps
400
3
450
500
550
600
Wavelength (nm)
650
700
0.5 ps
5 ps
500 ps
' Abs. u 10
3
1900 ps
2
1
0
-1
450
500
550
600
Wavelength (nm)
650
700
Figure 5.5. Transient absorption spectra for NiO-P1 (upper), NiO-P1/PCBM (middle) and NiO/PCBM (bottom) dry films (Oex = 525 nm, fwhm a100 fs, 500 nJ/pulse;
the solid lines are spectra at the given time delays after excitation, the dotted lines
show spectra at intermediate times).
To distinguish hole injection and recombination, we used DAS obtained
from a global analysis of the TAS data (Figure 5.6). A spectral similarity of
components indicated that they represented the same process. However,
because both injection and recombination are multi-exponential, we found
that they overlapped and both could in many cases contribute to the same
DAS component. Moreover, the TAS of the excited and reduced dyes are
56
often similar, and NiO signals contribute negligibly. Therefore, stimulated
emission from the dye excited state was very important to distinguish the
processes.
_
6
(
)
4
3
' Abs. u 10
2
0
-2
W = 0.82 ps
-4
1
W = 9.6 ps
2
W = 246 ps
-6
3
W = Inf ps
-8
4
400
450
500
550
Wavelength (nm)
600
650
700
6
4
2
3
' Abs. u 10
0
-2
-4
W = 0.63 ps
1
-6
W2 = 10 ps
W3 = 260 ps
-8
-10
W4 = Inf ps
400
450
500
550
Wavelength (nm)
600
650
700
Figure 5.6. An example of DAS of NiO-P1 and NiO-P1/PCBM.
The formation and decay of charge separation is illustrated by the kinetic
traces at 480 nm (see Figure 5.7) where large qualitative differences between
the samples are obvious. For NiO/PCBM, the initial signal of the PCBM
singlet excited state is positive, and a 300 fs component is assigned to vibrational relaxation. Then the spectrum decays with little change of shape, with
SVDQG§SVWLPHFRQVWDQWVWRUHVXOWLQa net bleach around 480 nm,
which is attributed to hole injection into NiO. For NiO-P1, the initial signal
at 480 nm is mainly a P1 ground state bleach. The bleach recovery follows
two ultrafast components with time constants of ~200 fs and 1.3 ps (Figure
5.7). These are attributed to hole injection, based on the shift in absorption
peaks and decay of stimulated emission, as discussed above.
57
Normalized ' A
1
0
-1
NiO-P1/PCBM
NiO-P1
NiO/PCBM
-2
0
10
1
2
10
Time (ps)
3
10
10
Figure 5.7. Comparison kinetics traces of all three systems probed at 480 nm, blue
diamond is NiO/PCBM, red square is NiO-P1/PCBM and black cycle is NiO-P1,
solid line is multi-exponential fit for each kinetic trace.
15.5 ps
-P1/PCBM-
PCBM
-P1/PCBM
0.2-1.32 ps
19.7 ps
+ 1% inf.
-P1
46.7 ps
393 ps
-P1Two
-P1-/PCBM
PCBM
Two-
With PCBM employed as electron conductor on the NiO-P1 in NiOP1/PCBM, the bleach recovery follows essentially the same time constants
as without PCBM, of about 300 fs and 1.9 ps. However, the magnitude of
bleach recovery is greater, and during a 47 ps component a positive absorption grows in. We conclude that the faster two components mainly form
NiO+-P1Ɣ- and that most of the charge shift to form NiO-P1/PCBMƔ- is represented by the 47 ps component (Figure 5.8). After that, a small decay
component is observed with ca. 120 ps time constant, which is similar to that
observed for NiO-PCBM. This indicates partial recombination, but most of
the signal remains at 0.1-2 ns, indicating a long-lived charge separation due
to the PCBM acceptor layer. The fraction of direct PCBM excitation is small
compared to that of P1 and cannot be directly seen in the transient absorption
data. The enhanced IPCE around 710 nm due to PCBM is, however, clear
evidence for additional charge separation generated from PCBM excitation
that leads to photocurrent generation.
NiO/NiO+
Figure 5.8. General charge transfer scheme with approximate time constants for
each system. Solid arrows denote positive process (hole injection and electron transfer), dashed arrows present recombination processes.
58
The PCBM●- signal is further detected with a 10 ns flash and probed around
1000 nm, where PCBM●- shows a characteristic absorption maximum.90-92
From Figure 5.9, a clear instrument-limited characteristic absorption rise
followed by a decay on the microsecond time scale can be observed in the
NiO-P1/PCBM sample, but not in the NiO-P1 sample. This proves that electrons are transferred to reduce the PCBM, which leads to better performance
than for NiO/PCBM.
NiO-P1/PCBM 1000 nm
Single exponential fit
ΔAbs. (mOD)
0,8
0,4
0,0
0
2000
4000
Time (ns)
6000
8000
Figure 5.9. NIR kinetics traces of NiO-P1/PCBM at 1000 nm with excitation at 532
nm (10 mJ/pulse).
5.4 Conclusion
In this chapter, we prove the new concept of a solid state p-type dye sensitized solar cells (p-ssDSSCs) for the first time. The photovoltaic data have
shown the feasibility of this concept. Ultrafast transient absorption spectroscopy was employed to successfully investigate the electron transfer processes in this system. An electron transfer from P1●- formed after hole injection
into NiO and electron transfer from P1●- to the electron conductor PCBM
can be dynamically feasible.
59
6. Electron Transfer Mechanisms in a
Covalently-Linked Organic Dye-Catalyst
System for Dye Sensitized Solar Fuel Devices
(Paper VII)
The electron transfer mechanisms were investigated for a covalent dyecatalyst sensitized NiO photocathode, towards hydrogen production solar
fuel devices. In-situ click chemistry was applied to link the organic push-pull
sensitizer (PB-1) and cobaltoxime catalyst to a dye-cobaltoxime (PB-2).
This chapter will illustrate the evaluation procedure of the electron transfer
processes in the photoelectrochemical (PEC) cells and guide the design of
more efficient solar fuel systems.
6.1 Introduction
A dye sensitized solar fuel device (DSSFD) is kind of photoelectrochemical
cell93,94 based on molecular photosensitizer/catalyst sensitized large band gap
semiconductor for solar fuel production.40,95-100 For an efficient DSSFD, the
intrinsic photocurrent match between dye/catalyst sensitized photocathode
and photoanode is desirable. Photoanodes have been intensively investigated
and rapidly developed,100-106 with a record high photocurrent density of 3
P$āFP-2 at pH 8. 102
However, the progress of photocathode is still unsatisfactory,107-109 which
causes the performance of the final DSSFD to be far from ideal.110,111 Most
of the reported photocathodes based on the dye/catalyst concept adopted the
co-sensitization method reported by Sun and co-workers,110,111 which is beneficial for intimate interaction between dye and catalyst. However, the cosensitized catalysts occupy partial sites of semiconductor surface and also
stand nearby the surface, leading to reduced dye loading and fast recombination between injected holes from dye and electrons in reduced catalyst,
which therefore limit the obtainable photocurrent. Wu and co-workers reported a coordination system between a Ru dye and cobaltoxime catalyst on
NiO with ALD Al2O3 coating, which somewhat reduced the aforementioned
issue.108 However, the axial coordination bond of a pyridine group on the
cobalt center becomes unstable upon reduction.112
60
So far, there is no covalently linked dye-catalyst system on a photocathode reported for H2 evolution. Study of such a covalent system would be
good for us to understand the different interfacial electron transfer processes
as compared to other systems and to guide us to design and develop a more
efficient photocathode in the future. With this motivation in mind, a designed covalent push-pull dye-catalyst photosensitizer was investigated to
give insight into the electron transfer processes (Figure 6.1).
CN
S
NC
CN
HO
S
N
O
NC
O
N
PB 1
HO
N
NH
O
O
N
O
N
NH
O
N3
N Cl N O
Co
H
N Cl N O
PB 2
N
N
N
Cl
N Co
N
Cl
O
N
O H
Co N3
hQ
e-
NiO
h+
H+
H+
eH
H
Figure 6.1. (upper)The molecular structure of dye (PB-1), catalyst (Co-N3) and dyecatalyst (PB-2) used in this study. (bottom) Schematic view of the charge transfer
reactions happening on NiO/dye-catalyst (PB-2) system.
In order to determine the all charge transfer processes, which are really implemented in PB-2/NiO (Figure 6.1), ultrafast transient absorption spectroscopy measurements (TAS) were performed. Different species were distinguished by sensitization with PB-1 and PB-2 on variety semiconductor films,
including ZrO2, TiO2 and NiO.
61
6.2 Tracking the Excited State
We need to understand the dynamics and spectral signatures of the excited
states of the new dyes to interpret our data. The ultrafast excited state dynamics in solution phase is often affected by many factors, such as the most
obvious solvent effects and molecular collisions. This is different from a
molecule bound to the semiconductor film, where also conformational flexibility could be reduced and dye-dye interactions may be more important.
This may bring difficulties in distinguishing the excited state by comparing
with solution data. Therefore, PB-1 and PB-2 were bound to ZrO2 films, into
which electron injection cannot occur. Figure 6.2 present the TAS of PB1/ZrO2 and its species associated spectra (SAS). Three main excited-state
features can be observed in the early-time PB-1/ZrO2 TAS: a) the GSB
DURXQGaQPZKLFKLVSDUWO\FRYHUHGE\WKHSXPSVFDWWHULQJȜex = 540
nm); b) SE giving a neat negative signal around 630 nm, the SE peak shows
a dynamic Stokes shift, red-shifting by about 10 nm during the initial ca. 2
ps; c) ESA resulting in three peaks at 420 nm, 565 nm and 700 nm, respectively, separated by the GSB and SE.
10
8
N-1
i-1
'Ai = 6 j=i cj 3m=1(1-Wm/Wj)
8
6
3
4
2
-5 ps
0.5 ps
3 ps
10 ps
30 ps
250 ps
1000 ps
1900 ps
0
-2
-4
400
450
500
550
600
Wavelength (nm)
650
700
' Abs. u 10
' Abs. u 10
3
6
4
2
0
-2
'A1 (W1 = 0.36 ps)
-4
'A3 (W3 = 42 ps)
'A2 (W2 = 4.5 ps)
'A4 (W4 = 475 ps)
-6
400
c5 (W5=Inf)
450
500
550
600
650
700
Wavelength (nm)
Figure 6.2. Transient absorption spectra of PB-1/ZrO2 (left) and its species associated spectra (SAS, right) in the presence of propylene carbonate, excitation at 540 nm
with 200 nJ/pulse pump.
6.3 Tracking the Oxidized State and Intramolecular
Charge Transfer
PB-1/TiO2 film was used to detect if there is charge injection into TiO2, and
to get a reference spectrum for the oxidized state of PB-1 for comparison
with PB-2/ZrO2 samples where intramolecular electron transfer to the catalyst unit may lead to the oxidized dye. TAS and relative global analysis present that excited PB-1 is both thermodynamically and kinetically capable of
injecting an electron to the conduction band of TiO2, hence forming the oxidized dye. It is consistent with the new species observed in PB-2/ZrO2, it
could be concluded that intramolecular charge transfer (ICT) to cobaloxime
62
occurs in the PB-2/ZrO2 system. The reaction is illustrated by the comparison shown in Figure 6.3, where the kinetic traces of PB-1/TiO2 and PB2/ZrO2 probed at 625 nm show almost the same rise process that is completed at ~30 ps, due to excited state electron transfer (to TiO2 and cobaloxime,
respectively), while the kinetics for PB-1/ZrO2 is much slower.
Normalized ' A
1
0.5
0
PB-1/NiO 625 nm
PB-2/ZrO2 625 nm
PB-1/TiO
625
nmnm
ZrO2/PB2
625
2
-0.5
PB-1/ZrO
625 nm
TiO2/PB1
2 625 nm
0
2
4
10
1
2
10
Delay time (ps)
10
3
Figure 6.3. Kinetic traces of PB-1/NiO (open circle), PB-2/ZrO2 (open diamond),
PB-1/TiO2 (upwards open triangle) and PB-2/ZrO2 (leftwards open triangle) probed
at 625 nm, solid line is the multiexponential fit.
6.4 Tracking the Reduced State
With the confirmation of the ICT effect on PB-2/ZrO2, PB-1 and PB-2 sensitized on NiO photocathodes, PB-1/NiO and PB-2/NiO, respectively, were
also investigated with TAS. Figure 6.4 gives the TAS of PB-1/NiO and PB2/NiO, and they both exhibit similar excited state TAS and the relaxation on
the time scale of a few ps as for the other samples. However, after ca. 30 ps a
visual comparison of two TAS of PB-1/NiO and PB-2/NiO shows that they
present features very different from the excited state (on ZrO2) and oxidized
state PB-1 (on TiO2).
63
5
5
4
4
3
3
3
1
0
-5 ps
0.5 ps
3 ps
10 ps
30 ps
250 ps
1000 ps
1900 ps
-1
-2
-3
-4
-5
400
450
500
550
600
Wavelength (nm)
650
700
' Abs. u 10
' Abs. u 10
3
2
2
1
0
-5 ps
0.5 ps
3 ps
10 ps
30 ps
250 ps
1000 ps
1900 ps
-1
-2
-3
400
450
500
550
600
Wavelength (nm)
650
700
Figure 6.4. Transient absorption spectra of PB-1/NiO (left) and PB-2/NiO (right) in
the presence of propylene carbonate, excitation at 540 nm with 200 nJ/pulse pump.
By inspecting the data in the region dominated by the initial stimulated
emission and formation of new species around 625 nm, it is clear that the
kinetic trace of PB-1/NiO probed at 625 nm shows a much faster rise than
the trace for PB-1/ZrO2 (see Figure 6.3), and also faster than the rise of the
oxidized state of PB-2/ZrO2 and PB-1/TiO2. This suggests that a different
species is formed in PB-1/NiO, which is assigned to the reduced state of PB1 after hole injection into NiO surface. The main life time for hole injection
from PB-1 to NiO is 1.2 ps.
The reduced PB-1 shows only weak TAS features. Nevertheless, the Species Associated Spectra (SAS) from a global, multi-exponential fit to each
data set (Figure 6.5 and Table 6.1) give evidence for intramolecular electron
transfer in PB-2/NiO (see also Figure 6.6). Ultrafast hole injection into NiO
is shown by the first two component constants; and the third component of
SAS for PB-1/NiO (W3 = 29 ps) and PB-2/NiO (W3 = 11 ps) is instead very
similar to the 475 ps component for PB-1/ZrO2. For PB-2/NiO the even
shorter lifetime than for PB-1/NiO can be explained by intramolecular
electron transfer to the cobaloxime unit in parallel to hole injection,
consistent with the results on ZrO2 above; presumably this is followed
by an even faster hole shift from the oxidized dye to NiO. Finally, the
weak SAS of the W4 and W5 components are assigned to the reduced dye, the
recombines with NiO holes in a typical, multi-exponential fashion.48 Note
that the reduction species of the cobaloxime unit itself is not expected to give
rise to detectable TAS signals.113
64
8
6
8
c 3 i-1 (1-Wm/Wj)
'Ai = 6N-1
j=i j m=1
c 3 i-1 (1-Wm/Wj)
'Ai = 6N-1
j=i j m=1
6
4
4
' Abs. u 10
' Abs. u 10
3
3
2
0
-2
-4
'A1 (W1 = 0.28 ps)
0
'A1 (W1 = 0.26 ps)
-2
'A2 (W2 = 3.9 ps)
'A2 (W2 = 3.3 ps)
-6
'A3 (W3 = 29 ps)
'A3 (W3 = 11.1 ps)
-4
'A4 (W4 = 160 ps)
'A4 (W4 = 335 ps)
-8
-10
400
2
c5 (W5=Inf)
450
500
550
600
650
c5 (W5=Inf)
-6
500
450
400
700
600
550
700
650
Wavelength (nm)
Wavelength (nm)
Figure 6.5. Species associated spectra (SAS) of PB-1/NiO (left) and PB-2/NiO
(right) from global fits of the transient absorption data.
Table 6.1. Time constants obtained from global fitting of the femtosecond TA data
of PB-1 and PB-2 on NiO (relative amplitudes at 625 nm probe wavelength are
given in brackets).
W1/ps
Sample
W2/ps
W3/ps
W4/ps
Winf
PB-1/NiO
0.28/-34%
3.3/-22%
29/14%
335/22%
f/8%
PB-2/NiO
0.26/-36%
3.9/-23%
11/27%
160/10%
f/4%
To summarize the results of time-resolved spectroscopy, for PB-2/NiO
(Scheme 6.1), hole injection from the excited PB-2 to the valence band of
NiO occurs on the dual time scales of W | 1 ps and W | 30 ps. The reduced
dye is regenerated by electron transfer to the catalyst, forming a reduced
catalyst with a rate constant of ca. 3×109 s-1, which is rapid enough to compete favorably with dye-NiO recombination.
k1
NiO/Dye*-Cat
NiO/[Dye]*-Cat
k3
k4
+
+
-
k6
k7
+
NiO /Dye-Cat
k1=1.0 x 1010 s-1
NiO/Dye -Cat
NiO /Dye -Cat
hv
k2
k5
-
-
k2=5.0 x 1010 s-1
k3=1.0 x 1012 s-1
k4=3.3 x 1010 s-1
k5 > 5.0 x 1010 s-1
k6=3.3 x 109 s-1
k7=3.3 x 109 s-1 (50%)
NiO/Dye-Cat
inf (50%)
Figure 6.6. The charge separation and recombination processes and the corresponding rate constants of PB-2/NiO upon light illumination.
65
6.5 Conclusion
In this chapter, we investigate the electron transfer mechanisms in the PEC
cells using the covalently linked dye-catalyst PB-2 photocathode-catalyst by
time-resolved spectroscopy. Hole injection from PB-2* into NiO VB takes
place on dual time scales, with W | 1 ps and W | 30 ps, and the reduced PB-2
(PB-2Ɣ-) formed then donates an electron to the catalyst unit. The subsequent
regeneration efficiency of the excited dye moiety of PB-2* by the catalyst
(also called the efficiency of catalyst reduction) is determined to be ca. 75%
from a comparison of the kinetics with PB-1 on NiO. The efficient catalyst
reduction from excited dye could be explained by the long lifetime of the
excited state and the large over-potential to reduce catalyst from excited
state. This result suggests, however, that the excited dye would be a more
ideal species to reduce the catalyst due to the longer lifetime as compared to
the reduced dye-/NiO+ state in this system. Therefore, slowing down the hole
injection to make the catalyst reduction happen first could be a good strategy
to increase the efficiency of catalyst reduction, thus improving the proton
reduction.
66
Summary and Outlook
Tracing the history of human use of solar energy, people used to produce
bronze concave mirror for focusing sunlight to light a fire as early as three
thousand years ago, the pace never stopped. In modern times, the fossil energy crisis woke up numerous scientists dedicated to the development and
use of clean and renewable solar energy. Dye sensitized solar cells are one of
remarkable promising technologies to convert solar energy to electricity and
fuels. P-type dye sensitized solar cells work as the photocathode to fabricate
solar cells and particularly tandem DSSCs, and it can be also applied in the
development of photoelectrochemical devices.
In fact, the power conversion efficiencies (PCE) for p-type photocathode
based devices are much lower than those of the Grätzel type solar cells (ntype DSSCs); there is still a long way to further improve the PCE of p-type
devices. The design of dyes is crucial, in order to avoid empirically blind
design and make clear understanding of photo-induced charge transfer processes between certain dyes and semiconductor surfaces. This thesis demonstrates this approach by combining various high resolution time-resolved
spectroscopies, such as femtosecond and nanosecond transient absorption
spectroscopy, TCSPC and time-resolved fluorescence. These techniques
have been explained in Chapter 3 in detail, the integrated research platform
in our group allows us to understand the electron transfer processes on timescales from femtoseconds to seconds and on length-scales from parts of
Ångströms to that of macroscopic devices.
Chapter 4 describes the photophysical investigation of several series of
organic “push-pull” and inorganic dye sensitizers. Dyes designed with pushpull approach exhibit stronger electronic coupling with the NiO surface, but
always fast charge recombination and lower charge regeneration efficiency
which leads to poor photovoltaic performance. It was found that an additional secondary electron acceptor could separate the injected holes and reduced
dyes in a longer distance. It helps to retard the charge recombination process
to such milliseconds time scale; at this time scale, both iodide/triiodide and
cobalt electrolyte could effectively regenerate the reduced dye to the ground
state. Thus the design approaches of “push-pull” and separate acceptor are
often good ideas, they do not always work efficient, and that the best dyes
are still far from sufficient for a really good p-DSSC. The main reason is
rapid recombination between the reduced dye and injectied holes in NiO,
which may provide a possibility to optimize NiO nanoparticles or develop
67
alternative p-type semiconductors. Actually, many scientists have noticed
the importance of finding alternative materials and also putting them in practice. Together with the maturing dye design approaches, it is expected to be
the next breakthrough in this area.
In Chapter 5, we prove the new concept of a solid state p-type dye sensitized solar cells (p-ssDSSCs) for the first time. In order to search for fundamental solutions to solve serious issues caused by a liquid electrolyte, such
as leakage and solvent evaporation, and to somewhat impact the solar cell
efficiency and stability we abandoned the liquid electrolyte, and instead used
a solid state electron conductor, PCBM. Femtosecond and nanosecond transient absorption studies proved that electron transfer from the reduced dye,
formed after hole injection into NiO, to the electron conductor PCBM is
dynamically feasible. The low efficiency of the device shown in this work
could be due to the unsatisfactory lifetime of P1Ɣ-, slow dye regeneration
from PCBM or potential charge recombination by PCBM and NiO. The
future work could focus on optimizing and improving the efficiency by seeking a dye/material with long lived charge separation state or a new electron
transport material with highly efficient dye regeneration ability. The development of highly efficient p-ssDSSCs will also pave the way to construct all
solid state tandem dye sensitized solar cells.
At last, we successfully broaden our eyes to investigate the electron transfer mechanisms in an organic covalently linked dye-cobaloxime (PB-2) sensitized photocathode for hydrogen production. According to fs transient absorption spectroscopy measurement, hole injection from PB-2* into NiO VB
takes place on dual time scales, with W | 1 ps and W | 30 ps, and the reduced
dye subunit (PB-2-·) formed then donates an electron to the catalyst unit
with a rate constant of ca. 3×109 s-1. The subsequent regeneration efficiency
of PB-2 by the catalyst unit (the efficiency of catalyst reduction) is determined to be ca. 75%. Although the electron transfer mechanisms exhibit that
PB-2 is an efficient dye-catalyst for a good photocathode, the photocurrent is
still far from that of state-of-the-art photoanodes due to both fast recombination of the reduced dye with injected holes and moreover because the poor
proton reduction capability of cobaloxime catalyst.
Therefore, as for p-type DSSCs, we need to find and develop new alternative meso-porous p-type semiconductors and design better dyes to retard the
charge recombination to obtain a long-lived charge separated state; using a
more efficient proton reduction catalyst is definitely a good strategy to boost
the photocurrent of the photocathode in PEC cell; Using a carbon dioxide
reduction catalyst instead of proton reduction catalyst in PB-2 may create a
photocathode for light driven carbon dioxide reduction in the future.
68
Svensk sammanfattning
Sedan länge har människan använt solen som billig och outtömlig energiskälla. En av de senaste utvecklingarna inom solenergiforskningen är färgämnessolceller av så kallad n-typ. Dessa solceller består i allmänhet av en halvledare som är färgad med ett färgämne, en fotoanod. Efter ljusabsorbtion ger
färgämnet ifrån sig en elektron till halvledarens ledningsband, vilken sedan
kan användas för att generera ström. För att sluta strömkretsen behövs en
katod och en elektrolyt och ett möjligt sätt att förbättra solcellens effekt är att
även göra katoden fotoaktiv. Principen är den samma som i n-typ solcellen
förutom att färgämnet i princip accepterar en elektron från halvledaren, så
kallad hålinjicering. Desvärre är fotokatoderna hittills mycket mindre effektiva än fotoanoderna. Eftersom inte alla absorberade fotoner leder till en
elektronövergång som kan användas i strömkretsen begränsas effektiviteten
av solcellen. Målet för denna avhandling är att identifiera anledningen till
den mycket sämre effekten hos katoderna samt att försöka utveckla förbättrade katoder.
För att kunna förstå processerna efter ljusabsorptionen används tidsupplöst absorptions- och emissionsspektroskopi vilka är beskrivna i kapitel 3. På
detta sätt kan vi följa elektronöverföringsprocesser på en tidsskala från några
femtosekunder (10-15 s) upp till timmar vilket motsvarar distanser från bråkdelar av en Ångström (10-10 m) upp till makroskopiska apparater som t.ex.
solceller.
I kapitel 4 undersöks fotofysiken hos olika färgämnen på halvledaren
nickeloxid (NiO) vilket är en vanlig halvledare för p-typ färgämnessolceller.
En del av de använda färgämnen är oorganiska komplex samt organiska
färämnen med ”push-pull”-effekt. De sistnämnda molekylerna är konstruerade så att deras elektronmoln förlyttar sig från en molekyldel till en annan
efter att ha exciterats med ljus, vilket på så sätt underlättar hålinjiceringen i
halvledaren. Dessa molekyler har en mycket bra elektronisk koppling till
halvledaren vilket leder till snabb hålinjektion men desvärre också till snabb
rekombination av elektron-hål-paret. Ett sätt att förlängra rekombinationstiden är att använda en till elektronacceptor längre bort från halvledarytan.
Det större avstånd mellen elektronen och hålet leder till att rekombinationen
sker först efter några mikrosekunder vilket är mycket långsamare jämfört
med färgämnen utan tillagd elektronacceptor. På denna tidsskala kan elektrolyen ta emot elektronen och på så sätt slutar strömkretsen. Trots dessa förbättringar är fotokatodernas effekt fortfarande mycket mindre än i n-typ sol69
celler eftersom rekombinationen fortfarande sker mycket fortare än de eftersökta elektronöverföringsprocesser. Förutom att ändra på designen av färgämnen är det också möjligt att förändra strukturen eller sorten av halvledaren,
vilket dock inte undersöktes i detta arbete.
I dagsläget använder de flesta färgämnessolceller elektrolyter som är lösta
i ett lösningsmedel. Detta medför dock en risk för läckage eller att lösningsmedlet ångas bort med tiden. I båda fallen torrläggs solcellen vilket leder till
att den slutar fungera. I kapitel 5 undersöks för första gången möjligheten att
ersätta den flytande elektrolyten med en elektronledare i fast tillstånd. Det i
detta arbete använda systemet undersöktes med transient absorption på femtonsekunds till nanosekundstidskala. Det kunde bevisas att laddningsseprationen sker mellan NiO och färgämnet samt att färgämnet kan ge elektronen
vidare till elektronledaren, dock är effektiviteten liten. Huvudanledningen till
detta är, som i kapitel 4, att rekombinationen mellan elektronen och hålet är
snabbare än elektronöverföringen till elektronledaren. Eftersom detta är det
första undersökta system av denna sort finns fortfarande stort utrymme för
förbättringar genom att välja andra färgämnen och elektronledare.
I sista kapitlet, kapitel 6, ändras fotokatodernas användning från vanliga
solceller till bränsleceller vilka kan spjälka vatten till syrgas och vätgas. Fotokatoden har då till uppgift att reducera protoner till vätgasen. För att kunna
göra det behövs en katalysator vilken behöver elektroner för att kunna katalytsera denna reaktion. I detta arbete undersöktes ett system där katalysatorn
är kovalent länkad till ett färgämne på ytan av halvledaren. De genomförda
transienta absortpionsmätningar visar att färgämnet injicerar ett hål till halvledaren. Detta följs sedan av en elektronöverföring från färgämnet till katalysatorn i tre av fyra fallen. Dessvärre finns även i detta system en effektiv och
snabb rekombination mellan elektronen, färgämnet och hålet i halvledaren
vilket leder till att den undersökta fotokatoden är sämre än ”state–of-the-art”
fotoanoder. Även för detta system bör framtidens utvecklingsfokus vara att
minimera rekombinationen. En till möjlighet är att ersätta katalysatorn med
en som är effektivare.
Översatt av Jens Föhlinger
Uppsala 2016
70
Acknowledgements
First of all, I would like to thank my main supervisor, Prof. Leif Hammarström, thank you giving me this opportunity to go abroad, I appreciate to
have a real scholar-supervisor during my PhD, you inspired me a lot by your
powerful knowledge network, kindness, patience, direct management and
your great and generos support all the time. I learned very much in four
years, including your rigorous scientific attitude and actively exploratory
spirit.
Dr. Haining Tian, to be my assistant supervisor from the third year, thank
you for bringing and sharing so much of your knowledge on solar cells and
solar fuel devices and being like a big brother to initiate the fruitful collaborations.
Prof. Jan Davidsson, to be my official first assistant supervisor and the head
of the department, thanks your help and colorful management.
I would like to thank my collaborators in and outside Sweden, Prof Fabrice
Odobel and your group guys: We have a great cooperation and thanks for
your inspiration to me. Yan Hao, Palas B. Pati, Lei Wang, Wenxing Yang,
Roger Jiang, many thanks to your trust and happy daily talks. Deep thanks
also to other co-authors of my papers during my PhD: Prof Anders Hagfeldt,
Prof Denis Jacquemin, Prof. Gerrit Boschloo, Prof. Håkan Rensmo, Prof.
Lars Kloo, Bo Xu, Esmaeil Sheibani, Ludovic Favereau, Edgar Mijangos,
Yann Pellegrin, Errol Blart, Yoann Farré, Marcello Gennari.
Zietz Burkhard, Emad Mukhtar, Allison B. Michelle, Jonas Petersson, Mohammad Mirmohades, Luca D’Amario and Jens Föhlinger, you guys to be
talented on laser spectroscopy and data analysis, I feel very fortunate to be
good friends for helping anytime. Thank you all to help a guy from almost
zero-based on both data processing and laser technologies. Special thanks to
Jens Föhlinger, thank you share your broad knowledge, nice discussion,
kindness help and collaboration.
Here I also thanks other guys of the laser team for your generous help, Erik
Göransson, Liisa Antila, Abhinandan Makhal, Starla Glover, Ahmed ElZohry, Ricardo Fernández-Terán, Mariia Pavliuk.
Thanks to our fyskem group member, to Reiner Lomoth, Ana Morandeira,
Daniel Streich, Prateek Dongare, Jonas Sandby Lisau, Alexandr Nasedkin,
Tianfei Liu, Jacinto Sa, Daniel Fernandes, Todd Markle, Marc Bourez, Yun
71
Chen, Jutta Toscano, Mohamed Qenawy, Annabell Bonn, Lei Tian, Shihuai
Wang and Pedram Ghamgosar.
Thanks guys of administrators, ITs and technician: Susanne Söderberg, Åsa
Furberg, Jessica Stålberg, Anna Guturic, Diana Bernelind, Mikael Österberg, Patrik Lindahl and Sven Johansson, you supply me the freedom learning and studying environment.
I would also like to thank all guys from Swedish Consortium of Artificial
Photosynthesis (CAP), it brings great experts together to be a world leading
organization. I am very proud to learn here and thanks everyone for transfering your knowledge to me.
In addition, thanks my Chinese friends: First to my former supervisor: Prof.
Yu Fang, you are the pathfinder of my life and helped me choose my topic of
PhD studying; Prof Jing Liu and Prof. Junxia Peng to be my guarantors of
my PhD scholarship; Prof. Liping Ding and Prof Gang He, for happy stay
and talks; My big school brother: Mingtian Zhang, even if we never met, it
does not affect you to be my role model; My big school sister: Yu Zhang and
Hongyan Wang; My beginning roommates: Chi Zhang and Hao Jiang; My
first co-plane friends: Jinbao Zhang and Wen Huang, do you remember the
interesting scene for our first-timers on the plane?! My close friends: Wenxing Yang, Yuan Fang, Shaohui Chen, Yuan Liu, Chunmeng Yu, Haonan
Peng, Hongyue Wang, Jinxing Huo and your wife Yingying Zhu, thanks you
guys stay with me in all the ups and downs; Thanks to Ke Fan, Fusheng Li,
Xiao Huang, Xiaoliang Zhang, Jinwen Song, Gang Duan, Wenfei Dong,
Song Chen and Meiyuan Guo.
Finally, I would like to sincerely appreciate my PARENTS for raising me up
with your great love and suffered unimaginable hardship. Thanks my most
beautiful WIFE, Shan Li, you’re pregnant and our baby is growing up with
my doctoral dissertation, thanks also my wife’s PARENTS, for your always
understand and support. Thanks to my big sister and your family. I love you
all forever!
At last, I would to express my grateful to the financial supports for my PhD
study, research projects and conferences: the PhD scholarship from China
Scholarship Council (CSC); the research funding from Swedish Energy
Agency, Knut and Alice Wallenberg Foundation and Swedish Research
Council (VR); travel grant of the Anna Maria Lundin scholarship from
Smålands Nation and the Liljewalches scholarship.
72
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