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Nano Research
Nano Res
DOI 10.1007/s12274-015-0711-4
Enhance the Performance of Perovskite-Sensitized
Mesoscopic Solar Cells by Employing Nb-doped TiO2
Compact Layer
Xiong Yin 1, Yanjun Guo 1, Zhaosheng Xue 2, Peng Xu 1, Meng He 1 (), and Bin Liu 2 ()
Nano Res., Just Accepted Manuscript • DOI 10.1007/s12274-015-0711-4
http://www.thenanoresearch.com on January 6, 2015
© Tsinghua University Press 2015
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1
Enhance the Performance of Perovskite-Sensitized
Mesoscopic Solar Cells by Employing Nb-doped TiO2
Compact Layer
Xiong Yin 1, Yanjun Guo 1, Zhaosheng Xue 2, Peng Xu 1,
Meng He 1,*, Bin Liu 2,*
1
CAS Key Laboratory of Nanosystem and Hierarchical
Fabrication,
National
Center for Nanoscience
and
Technology, Beijing 100190, P. R. China
2 Department
of Chemical and Biomolecular Engineering,
National University of Singapore, Singapore 117576,
Singapore
Photovoltaic performance of perovskite-sensitized mesoscopic
solar cells could be effectively enhanced by employing
Nb-doped TiO2 compact layer, in comparison to device using
pristine TiO2 compact layer.
Nano Research
DOI (automatically inserted by the publisher)
Research Article
Enhance the Performance of Perovskite-Sensitized
Mesoscopic Solar Cells by Employing Nb-doped TiO2
Compact Layer
Xiong Yin 1, Yanjun Guo 1, Zhaosheng Xue 2, Peng Xu 1, Meng He 1 (), and Bin Liu 2 ()
Received: day month year
ABSTRACT
Revised: day month year
Perovskite solar cells are one of the most promising alternatives to conventional
photovoltaic devices, and extensive studies are focused on device optimization
to further improve their performance. A compact layer of TiO 2 is generally used
in perovskite solar cells to block holes from reaching the fluorine doped tin
oxide electrode. In this contribution, we engineered TiO2 compact layer by
Nb-doping, which led to solar cells with a power conversion efficiency (PCE) of
10.26%, which was obviously higher than that of the devices with the same
configuration but using a pristine TiO2 compact layer (PCE = 9.22%). The
enhancement of the device performance was attributed to the decreased
selective contact resistance and increased charge recombination resistance
resulting from Nb-doping, which was revealed by the impedance spectroscopy
measurements. The developed strategy highlights the importance of interface
optimization for perovskite solar cells.
Accepted: day month year
(automatically inserted by
the publisher)
© Tsinghua University Press
and Springer-Verlag Berlin
Heidelberg 2014
KEYWORDS
Perovskite
solar
cell,
dense
film,
spraying
pyrolysis
deposition,
Nb-doped TiO2 compact
layer,
impedance
spectroscopy
As one of the most promising alternatives to
conventional photovoltaic devices, perovskite solar
cell has been intensively investigated in the past few
years [1-6]. Great progress has been made in the
device fabrication and significant improvement of
device performance has been achieved, as evidenced
by the recently reported efficiency of 19.3%, in
comparison with 3.81%, the efficiency of the first
perovskite solar cell reported in 2009. [7, 8] Moreover,
perovskite solar cells with various configurations
have also been demonstrated. A hole-blocking layer,
in most cases, a compact layer of TiO2, is generally
used between the FTO conducting substrate and the
mesoscopic scaffold and/or perovskite layer to
prevent the holes in perovskite or hole-transporting
Address correspondence to Meng He, email: [email protected] ; Bin Liu, email: [email protected]
2
Nano Res.
layer from reaching the FTO substrate [8-15]. The
importance of the hole-blocking layer in both
perovskite solar cells and its predecessor, the solid
state dye-sensitized solar cells has been well
acknowledged [16]. Nevertheless, engineering the
hole-blocking layer to improve the device
performance has been rarely reported. In this work,
we report the Nb-doped TiO2 compact layer and its
application in perovskite solar cells with mesoscopic
TiO2 layer as the scaffold of perovskite. The devices
with the Nb-doped TiO2 compact layer could reach a
power conversion efficiency (PCE) of 10.26%, which
was obviously improved as compared to those
devices using a pristine TiO2 compact layer with an
efficiency of 9.22%.
Both the pristine and Nb-doped TiO2 compact layers
were prepared via spray pyrolysis, and the details
are shown in the electronic supplementary material
(ESM). The compact layers were first characterized
with a field emission scanning electron microscope
(FESEM), and typical images are presented in Figure
1. Both pristine and Nb-doped compact layers are
smooth and homogeneous, which are significantly
different from the morphology of FTO (Fig. S1a, b).
The compact layers are formed by the stacking of
nanoparticles with the size of a few tens of
nanometers (Fig. S1c, d). No discernible difference in
morphology between the pristine and Nb-doped
compact layers was observed. Only a peak
corresponding to 101 reflection of anatase besides
reflections of FTO was observed in XRD patterns of
both pristine and Nb-doped TiO2 coated FTO
substrates (Fig. S2). X-ray photoelectron spectra (XPS)
were recorded to investigate the chemical
composition of the samples and chemical states of the
constituent elements. As revealed by the spectra
presented in Figure 1, the binding energies of Ti 2p
detected in both pristine and Nb-doped samples are
characteristic for Ti4+, and no signals of Ti 3+ and Ti2+
are identified in both samples [17, 18]. In addition,
the core level spectrum of Nb 3d was obtained from
the Nb-doped film (Fig. 1e). Two peaks with binding
energies of 207.38  0.2 eV and 210.11  0.2 eV are
detected, which are characteristics for 3d5/2 and 3d3/2
components of Nb5+, respectively [17, 18]. The atomic
concentration of Nb element in the doped compact
layer was estimated to be ~3.6% using XPS. These
results confirmed that Nb was indeed doped into
TiO2 compact layer prepared in this study.
Figure 1 SEM images of TiO2 (a) and Nb-doped TiO2 (b) compact layers; XPS core level spectra of Ti 2p (c) for TiO2 compact layer,
and XPS core level spectra of Ti 2p (d) and Nb 3d (e) for Nb-doped TiO2 compact layer.
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An ideal hole-blocking layer should block the
backward transport of electrons from FTO to
light-harvesting materials and hole-transporting
materials efficiently, on the other hand, hinder the
forward transport of electrons from light-harvesting
materials to FTO least. Therefore, the rectifying
property of a compact layer can give important
information on its performance as a hole-blocking
layer [19]. To investigate the rectifying behavior of
the pristine and Nb-doped compact layers, we
fabricated simple devices with the structure of
FTO/compact layer/spiro-MeOTAD/Au, where
metal Au was used as contact electrode [19-21].
Figure 2 (a) Current-voltage curves of devices with different
compact layers (CL) fabricated using different spraying cycles,
open shapes: TiO2 CL, solid shapes: Nb-doped TiO2 CL. The
positive potential represents that FTO substrate is positively
biased, and the current flowing from FTO to spiro-MeOTAD is
labeled as positive; (b) Chemical capacitance of blank FTO
electrode, TiO2 CL and Nb-doped TiO2 CL electrode, measured
in liquid electrolyte containing I-/I3- redox couple.
The potential between the two electrodes was
continuously increased from -1 V to +1 V [19-21]. The
thicknesses of the compact layer were controlled by
changing the number of deposition cycles. The
typical current-voltage curves of the different devices
measured in dark are shown in Fig. 2a. In the case of
devices with two cycles of deposition, a linear J-V
relationship complying with Ohm’s law was
observed, which is similar to that observed for the
devices without compact layer (not shown),
indicating that no Schottky contact was formed. We
speculate that continuous compact layer was not
formed after only two cycles of deposition.
Furthermore, the resistance of the device with TiO2
compact layer was slightly larger than that of the
device with Nb-doped TiO2 compact layer, indicating
that Nb-doping improved the conductivity of the
compact layer. After six spray pyrolysis cycles, the
deposited thin film showed significant rectification
characteristics. Further increasing the spraying cycles
to eight resulted in thin films showing a better
blocking effect on the backward electron transport
from FTO to spiro-MeOTAD, as evidenced by the
lower current density in the third quadrant of Fig. 2a.
On the other hand, more spraying cycles also led to
thicker films, which are able to hinder the forward
transport of electrons more effectively. Eight spraying
cycles were thus taken as the optimum preparation
parameters in subsequent fabrication of perovskite
solar cells. It is noteworthy that Nb-doped compact
layer of 8 spraying cycles carries a slightly higher
forward and lower backward current in comparison
with its pristine counterpart. This implies that
Nb-doping results in a slightly improved rectifying
property, which will benefit the device performance.
The improved rectifying property of Nb-doped
compact layer could be explained as follows:
Nb-doping is able to increase the carrier (electron)
density, and subsequently, the Fermi level of TiO2 [16].
Therefore, a narrower but higher Schottky barrier is
resulted as compared to that in the case of pristine
TiO2. Such a higher Schottky barrier is able to block
the backward electron transport more efficiently and
the narrower barrier should benefit the forward
electron flow. The effect of Nb-doping on the optical
transmittance of the compact layers was investigated
using UV-Vis spectroscopy. Two types of compact
layers presented similar transmittance within the
wavelength range from 300 to 850 nm (see Fig. S3).
4
Nano Res.
The electrochemical properties of compact layers
were further investigated using electrochemical
impedance spectra measured in a three-electrode
system with a liquid electrolyte [22, 23]. The chemical
capacitances, fitted using the corresponding
equivalent circuit [23], of each electrode are
displayed in Figure 2b. The chemical capacitance
measures the capability of a system to accept or
release additional electrons due to the variation of its
Fermi level. It was found that the capacitance of
blank FTO electrode was independent of the applied
bias.
However,
the
capacitance
increased
exponentially at high applied bias for both pristine
and Nb-doped TiO2 compact layer, indicating that
electrons could be accumulated in both films.
Furthermore, a significantly higher chemical
capacitance was observed within the whole range of
applied bias for the Nb-doped TiO2 layer in
comparison to pristine TiO2 layer. Since the
thicknesses of both layers are very close to each other,
the higher chemical capacitance observed for
Nb-doped film implies the higher density of states
than that in the pristine TiO2 layer, which should
facilitate the electron injection from the perovskite to
compact layers.
The devices without compact layer, with pristine and
Nb-doped TiO2 compact layers, respectively, were
fabricated using the similar process and parameters.
The details of device fabrication were given in the
supporting information. The current-voltage (J-V)
characteristics were measured in dark and under
simulated solar illumination (AM 1.5G, 100 mW cm-2).
The J-V curves of the champion cells of each group
are shown in Fig. 3a, and device performance
parameters are summarized in Table 1. The devices
without compact layer presented the lowest PCE of
3.58%, with an open-circuit voltage (Voc) of 0.845 V, a
short-circuit photocurrent density (Jsc) of 9.38 mA
cm-2 and a fill factor (FF) of 0.452. After incorporating
pristine TiO2 compact layers into the devices, PCE,
Voc, Jsc and FF increased dramatically to 0.869 V, 17.45
mA cm-2, 0.608 and 9.22%, respectively. The
corresponding parameters for the cells with an
Nb-doped compact layer are 0.880 V (Voc), 18.08 mA
cm-2 (Jsc), 0.645 (FF) and 10.26% (PCE), respectively. In
comparison with the device applying a pristine TiO2
compact layer, the device with an Nb-doped compact
layer exhibited an enhancement of ~11% in
photovoltaic efficiency.
Figure 3 Photocurrent-voltage characteristic curves (a) and
incident photo-to-electron conversion efficiency (IPCE) spectrum
(b) of perovskite-sensitized solar cells with TiO2 (solid triangles)
and Nb-doped TiO2 (solid circles) compact layer; (c)
Recombination resistance (Rrec) and selective contact resistance
(Rsc) of perovskite-sensitized solar cells with TiO2 (solid triangles)
and Nb-TiO2 (solid circles) compact layer, extracted from
impedance spectra measured under illumination with applied
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Nano Res.
5
forward bias
been extensively utilized for the characterization of
liquid-state, solid-state dye-sensitized solar cells and
perovskite-sensitized solar cells to gain essential
information on the internal electrical processes and
analyze the associated parameters [22, 26-31]. EIS
was consequently employed to investigate the effect
of Nb-doping of TiO2 compact layer on the
photovoltaic performance of devices. The spectra of
various devices were measured under light
illumination at different applied biases [22, 26, 27].
Typical Nyquist plots are shown in Fig. S5a and 5b,
while the corresponding equivalent circuit is
presented in Fig. S5c [22, 26]. The fitting results of
selective contact resistance (Rsc) and recombination
resistance (Rrec) are depicted in Fig. 3c. The series
resistance Rs which is mainly attributed to FTO
substrate and wires is nearly constant for various
devices with a typical value of ~24 Ω. Rsc of both
types of devices varied slightly with the applied bias
(inset of Fig. 3c). In the whole range of applied bias, a
slight decrease of Rsc resulting from Nb-doping was
observed. On the contrary, Rrec of the device was
improved due to Nb-doping. The improved Rrec
implies less hole-electron recombination in the device
while the decreased Rsc indicates more efficient
forward carrier transport. The results obtained from
EIS data measured on complete devices are in good
agreement with rectifying properties observed for
both kinds of compact layers. The Rs and Rsc will
contribute to the total series resistances (Rseries) of the
device [25]. With the doping treatment on the
compact layer, the Rseries of device decreases, which
results in improved fill factors as shown in Table 1.
These results are in a good agreement with the
previously reported results [21, 25]. In addition, the
increase in Rrec could lead to higher Voc for Nb-doped
compact layer based devices. The doping also
yielded reduced Rsc and Rseries together with increased
Rrec, which led to improved photovoltaic parameters,
yielding an enhancement in efficiency for Nb-doped
compact layer based devices.
To illustrate the consistency of the fabrication process,
15 devices were fabricated for each kind of compact
layer using the same process and parameters.
Photovoltaic parameters of these devices are shown
in Fig. S4 and the averages over 15 devices of each
group together with the corresponding standard
deviations are summarized in Table 1. Devices of
each group exhibited consistent photovoltaic
performance, as evidenced by the rather small
standard deviations of performance parameters. The
averaged photovoltaic efficiency of the devices
applying Nb-doped compact layers is 9.64% whereas
the devices with pristine compact layers show an
averaged efficiency of 8.72%. Since the difference in
averaged efficiency is bigger than three times the
standard deviation, the enhancement of photovoltaic
efficiency caused by Nb-doping is statistically
significant [24].
The dark current-voltage characteristic curves of
various devices were also measured to investigate the
blocking effect of pristine and Nb-doped compact
layers [25]. As shown in Fig. 3a, the dark current was
significantly suppressed by applying a TiO2 compact
layer in the perovskite solar cell, and the onset of the
dark current was shifted to higher applied potential
by more than one hundred millivolts. Nb-doping
further improved the blocking performance, as
evidenced by the lower dark current and the onset at
a higher bias observed for the Nb-doped compact
layer in comparison with those of the pristine one.
The lower dark current implies less backward
transport of electrons from FTO to perovskite and/or
hole-transporting layer, and less hole-electron
recombination. Therefore, an improvement of Jsc and
Voc is expected. Indeed, as shown in Figure 3a and
Table 1, the solar cells applying Nb-doped compact
layers exhibited slightly increased Jsc and Voc.
Furthermore, the enhancement in Jsc is verified by the
incident photo-to-electron conversion efficiency
(IPCE) spectra. The IPCE spectra of devices with
various compact layers are shown in Fig. 3b. An
improvement within the wavelength range from 400
to 600 nm is observed for the Nb-doped device.
Furthermore, short-circuit currents calculated from
the IPCE spectra are well consistent with the values
obtained from J-V curves, which are listed in Table 1.
Electrochemical impedance spectroscopy (EIS) has
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Research
Table 1 Photovoltaic parameters of perovskite-sensitized solar cells with different compact layers (CLs)
Device
Voc / V a
Jsc / mA cm-2 b
FF c
PCE / % d
Jsc / mA cm-2 e
Without CL
0.845
9.38
0.452
3.58
-
TiO2 CL (best)
0.869
17.45
0.608
9.22
17.20
Average ± SD f
0.847 ± 0.014
17.27 ± 0.36
0.596 ± 0.013
8.72 ± 0.28
-
Nb-TiO2 CL (best)
0.880
18.08
0.645
10.26
17.72
Average ± SD f
0.862 ± 0.014
17.49 ± 0.42
0.640 ± 0.010
9.64 ± 0.30
-
a
Voc: Open-circuit voltage, b Jsc: Short-circuit photocurrent density, c FF: Fill factor, d PCE: Power conversion efficiency, e Jsc:
calculated from IPCE, f SD: standard deviation
In conclusion, Nb-doped TiO2 compact layers were
prepared and applied as hole-blocking layers in
perovskite-sensitized solar cells. Investigations on
rectifying properties revealed that Nb-doping
improved not only the blocking effect but also the
forward electron transport. Devices with pristine and
Nb-doped TiO2 compact layers are also investigated
with electrochemical impedance spectroscopy. It was
found that Nb-doping of compact layers resulted in
improved Rrec and decreased Rsc of the device,
confirming the results obtained from investigations
on rectifying properties. Accordingly, an obvious
enhancement on photovoltaic efficiency was
observed for the devices applying Nb-doped
compact layers in comparison with the ones with
pristine compact layers. These results show that
modification of the compact layer is an understudied
avenue where improvements in perovskite solar cells
can be achieved.
UV-Vis spectra of FTO, pristine and Nb-doped TiO2
compact layers, photovoltaic performance of
perovskite-sensitized solar cells, Nyquist plots of
electrochemical
impedance
spectra
of
perovskite-sensitized solar cells) is available in the
online
version
of
this
article
at
http://dx.doi.org/10.1007/s12274-***-****-*
(automatically inserted by the publisher).
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Acknowledgements
This work was financially supported by the National
Natural Science Foundation of China (Grant Nos.
21103032 and 51272049), the Major State Basic
Research Development Program of China (973
Program) (No. 2011CB932702) and SInberise
R279-000-393-592. Dr. X. Yin thanks Mr. Jin Fang for
help on IPCE measurements and Au evaporation.
Electronic Supplementary Material: Supplementary
material (Experimental methods, FESEM images and
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Nano Res.
Electronic Supplementary Material
Enhance the Performance of Perovskite-Sensitized
Mesoscopic Solar Cells by Employing Nb-doped TiO2
Compact Layer
Xiong Yin 1, Yanjun Guo 1, Zhaosheng Xue 2, Peng Xu 1, Meng He 1 (), and Bin Liu 2 ()
EXPERIMENTAL METHODS
Device fabrication
Fluorine-doped SnO2 (FTO) substrate was firstly etched using Zn powder and HCl (2 M) to form the desired
electrode patterns (15 mm ×10 mm). The resultant FTO substrates were ultrasonically cleaned with detergent,
deionized water, acetone, ethanol and 2-propanol, respectively. TiO2 compact layer was then deposited on the
cleaned FTO substrate by spraying 0.2 M Ti(i-OPr)2(acac)2 at 450 °C. In the case of the Nb-doped TiO2 compact
layer, 0.01M Niobium (V) ethoxide was added into the Ti4+ precursor solution. Prior to spraying, the solution
was ultrasonically mixed for 30 min. The thickness of the compact layers was controlled by changing the
number of spray cycles. The mesoporous TiO2 was spin-coated on the surface of the compact layer using the
commercial TiO2 paste (Solaronix T/SP) with a diluted concentration. These electrodes were sintered at 125 °C
for 10 min, 325 °C for 15 min, 375°C for 15 min, 450 °C for 20 min and 500 °C for 15 min, respectively. The
thickness of mesoporous TiO2 was ~450 nm. The PbI2 (Sigma-Aldrich) solution in DMF was dropped on the
mesoporous film, and then spin-coated at 5500 rpm for 30 s in a dry Ar box. The film was then dried at 75 °C
for 30 min. The film was finally immersed in the CH 3NH3I isopropanol solution (10 mg mL-1) for 20 s. After that,
it was heated at 75 °C for 30 min to form perovskite-sensitized TiO2 electrode. The hole transferring layer was
prepared by spin-coating a solution containing 72.3 mg Spiro-MeOTAD, 28.8 µ l 4-tert-butylpyridine and 0.06 M
lithium bis(trifluoromethylsulfonyl)imide per ml of chlorobezene on the top of perovskite layer at 4000 rpm for
45s. Finally, Au contact electrode (~80 nm) was deposited on the HTL using thermal evaporation at a pressure
of 2 × 10-6 mbar.
Characterization
The morphology of the compact layers was observed with a field emission scanning electron microscope
(FESEM, Hitachi S-4800). X-ray photoelectron spectroscopy (XPS) measurements were conducted with an
ESCA Lab250xi spectrometer using Al Kα (1486.6 eV) irradiation as X-ray source. All the spectra were
calibrated to the binding energy of the adventitious C1s peak at 284.8 eV. The optical transmittance of the blank
FTO, TiO2 and Nb-doped TiO2 compact layer on FTO substrate was measured by using a UV-Vis
spectrophotometer (Model Hitachi U3010, Japan). The rectifying behaviors of compact layer were detected
using CHI 660D workstation at scanning range from -1.0 V to 1.0 V with a scanning rate of 0.075 V s-1. The
photocurrent-voltage (J-V) measurements of perovskite-sensitized solar cells were recorded by a Keithley 2420
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source meter with a solar simulator (Oriel Newport, 150 W, AM 1.5) as the light source. The J-V curves were
measured by reverse scan (forward bias (1.1V) to short circuit (0V)). The light intensity was calibrated using a
Si reference cell (Oriel Newport PN91150V). IPCE measurements of the devices were performed using a 300 W
xenon lamp (Newport 66902), a lock-in amplifier (Newport Merlin digital 70140) and a monochromator
(Cornerstone CS260). Electrochemical impedance spectra were recorded using a Zahner workstation (Zennium
IM6, Germany). Impedance spectroscopy was performed in a three-electrode cell with the blank FTO or
different compact layer as the working electrode, the Ag/AgCl and Pt wire as the reference electrode and
counter electrode, respectively. I-/I3- redox couple was dissolved in anhydrous acetonitrile and used as the
electrolyte (0.5 mol L-1 LiI, 0.05 mol L-1 I2). The impedance characterization was performed at negative potential
in the range of 0 to -1.0 V with the amplitude of the AC signal of 10 mV and the frequency ranging between 10
MHz and 0.1 Hz. The impedance measurements of perovskite-sensitized solar cells were recorded at an
applied DC potential bias with a sinusoidal AC potential perturbation of 20 mV in a frequency range of 2 MHz
to 0.05 Hz. The applied DC potential increased from 0 to 0.8 V with a step of 0.1 V. The resultant impedance
spectrum was analyzed using the ZView software (Scribner Associates Inc.) and the corresponding equivalent
circuit. All the measurements were performed under ambient conditions.
Figure S1. SEM images of blank FTO substrate (a: low magnification; b: high magnification), TiO 2 (c) and
Nb-doped TiO2 (d) compact layers (high magnification) after 8 spray cycles.
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Figure S2. XRD patterns of the blank FTO substrate, as-prepared TiO2 and Nb-doped TiO2 compact layers; *
indicates 101 reflection of anatase.
Figure S3.
UV-Vis transmittance of blank FTO, TiO2 and Nb-TiO2 compact layer on FTO substrate
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Figure S4. Photovoltaic performance of 15 perovskite-sensitized solar cells with TiO2 compact layer (CL) and
Nb-doped TiO2 CL, respectively; Voc: open-circuit voltage, Jsc: short-circuit photocurrent density, FF: fill factor,
PCE: power conversion efficiency
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Figure S5. Nyquist plots of perovskite-sensitized solar cells with TiO2 compact layer (a) and Nb-doped TiO2
compact layer (b); Symbols represent the experimental data and solid lines are fitted curves; (c) the equivalent
circuit used in the study.
Address correspondence to Meng He, email: [email protected] ; Bin Liu, email: [email protected]
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