Synthetic Metals 143 (2004) 283–287
Construction of a photovoltaic device by deposition of thin films
of the conducting polymer polythiocyanogen
V.P.S. Perera a , P.V.V. Jayaweera a , P.K.D.D.P. Pitigala a , P.K.M. Bandaranayake a ,
G. Hastings b , A.G.U. Perera b , K. Tennakone a,∗
b
a Institute of Fundamental Studies, Hantana Road, Kandy, Sri Lanka
Department of Physics and Astronomy, Georgia State University, Atlanta, GA 30303, USA
Received 9 June 2003; received in revised form 18 December 2003; accepted 18 December 2003
Abstract
A method is developed for electro-deposition of thin films of the conducting polymer polythiocyanogen on conducting tin oxide glass
or other conducting substrate by anodic discharge of SCN− ions form a solution KSCN in propylene carbonate. Films are found to be
highly stable and resistant to heat and chemical action. SEM pictures indicate that the films are uniform and free of pin hole. Band gap and
band positions are determined from optical absorption spectra and Mott–Schottky plots, respectively. A photovoltaic cell is constructed by
depositing polythiocyanogen on nanocrystalline films of n-TiO2 followed by p-CuI to form a heterojunction. Photocurrent action spectra
shows that light absorption by polythiocyanogen generates the photovoltaic response. Results suggest that polythiocyanogen could find
applications in optoelectronic devices.
© 2004 Elsevier B.V. All rights reserved.
Keywords: Polythiocyanogen; Conducting polymer; Photovoltaic cell
1. Introduction
Conducting polymers are extensively studied as potential
materials for application in optoelectronic devices [1–11].
Their low cost and easy control of properties leave more
flexibility in fabrication procedures compared to the conventional single crystal or polycrystalline inorganic semiconductors. An area where the conducting polymers could make
significant practical impact is photovoltaics [1–8]. Here the
low temperature deposition techniques without the involvement of vacuum technology become a great advantage.
Many attempts have been made to construct photovoltaic
cells with conducting polymers as the light harvesting material, which generate the carriers. Basically these systems
have a heterojunction configuration with a thin film of the
polymer interposed between two electrodes of which at least
one needs to be optically transparent. Polythiophene, polyacetylene, polyphenylene vinylene derivatives, polyaniline
and many other conducting polymers with complex organic
molecules as the monomer have been tested for photovoltaic
effects in sandwich configuration or blended with other
materials to form composite films. A simple molecule that
∗
Corresponding author. Tel.: +94-8-232002; fax: +94-8-232131.
E-mail address: [email protected] (K. Tennakone).
0379-6779/$ – see front matter © 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.synthmet.2003.12.018
readily undergoes polymerization is thiocyanogen (SCN)2 .
Cataldo [12–14] have conducted extensive investigations
to elucidate the structure of polythiocyanogen. Polythiocyanogen of general composition [Sy (CN)2 ]x was shown to
be constituted of long polyazomethine chains analogous to
that of polycyanogen or paracyanogen [15] but crosslinked
with sulfur bridges of different length depending on the
sulfur chain length in the original monomer. Although the
electronic conductivity [16] and photosensitivity [17,18] of
polythiocyanogen ([SCN]n ) was noted earlier, there are no
records describing the use of this material in an optoelectronic device. We have developed methods for deposition of
polythiocyanogen on conducting glass or other conducting
substrate and also fabricated a photovoltaic cell by coating polythiocyanogen on a nanocrystalline film of TiO2 .
This paper describes preparation of thin films of polythiocyanogen, their characterization and construction of a
photovoltaic cell.
2. Experimental
Conducting tin oxide (CTO) glass plates (0.25 cm ×2 cm,
sheet resistance 15 /sq) are cleaned by warming in a solution of KOH in propan-2-ol, rinsed with water followed by
284
V.P.S. Perera et al. / Synthetic Metals 143 (2004) 283–287
propan-2-ol and dried avoiding contamination with grease.
Polythiocyanogen was deposited on CTO surfaces as follows: KSCN dried at 105 ◦ C for several hours is dissolved
in moisture free propylene carbonate (0.15 M solution). The
solution was heated to 90 ◦ C and electrolyzed under galvanostatic conditions (2 mA cm−2 ) with the CTO glass plate
as the anode and a platinum foil as the counter electrode.
Discharged SCN− ions undergo polymerization at the CTO
surface depositing a orange–yellow film on the CTO surface.
If the solution is not warmed polymerization becomes slow
and polymeric particles of (SCN)2 formed near the anode
tend to break away from the electrode surface and leach into
the solution. Polythiocyanogen in the powder form was prepared by rapid electrolysis of a solution of KSCN in propylene carbonate. Ultrasonic agitation of the solution prevented
adherence of polythiocyanogen to the anode. A compressed
pellet of the powder was used to measure the density as well
as the conductivity. Thickness of the film is deduced from
the charge that has passed through the electrolyte. To measure the resistivity of the film, the thiocyanogen coated plate
and platinum foil are immersed in a sodium sulfate solution
(0.1 M) and resistance is measured using an impedance meter (Hewlett Packard 4276A LCZ Meter). The resistivity of
the film is calculated by comparing with the resistance of a
cell of same geometry when the thiocyanogen coated plate
is replaced with a CTO glass plate. The same set up is used
measure the capacitance (C) and plot the Mott–Schottky diagram (i.e. the plot of 1/C2 versus applied voltage V). The
photoresponse of the films were examined in a three electrode configuration under potentiostatic conditions in a electrolytic medium (0.1 M Na2 SO4 ).
Nanocrystalline films of TiO2 were prepared as described
[19]. Briefly, the procedure involves spreading of a colloidal solution of titanium dioxide (prepared by hydrolysis
of titanium isopropoxide) on CTO glass plates heated to
150 ◦ C and sintered at 450 ◦ C for 30 min. After cooling,
the loose crust on the surface is wiped off with cotton wool
and the process is repeated until a film thickness reaches
∼10 ␮m. Surface area of the film was estimated by deposition of a dye of known surface coverage, extraction of the
dye and spectrophotometric estimation. Polythiocyanogen
was coated to the nanocrystalline TiO2 surface by the same
method as for CTO plates. Polythiocyanogen films coated
on CTO and nanocrystalline TiO2 surfaces and for comparison the bare CTO and TiO2 surfaces were examined
by SEM. FT-IR spectrum of films deposited on the above
substrates could not be obtained due to strong IR absorption by the CTO surface. However, to confirm that the
material deposited is polythiocyanogen, the FT-IR spectrum was obtained by scraping off the film from the CTO
surface. The photovoltaic cell was formed by deposition of
a layer of p-CuI over the polythiocyanogen deposited on
TiO2 . CuI was deposited by drop coating from a solution
of CuI in acetonitrile (6.3 × 10−3 M). A gold plated CTO
glass plate pressed onto the CuI surface served as the back
contact (construction of the cell is shown in Fig. 1). I–V
Fig. 1. Construction of the photovoltaic cell.
characteristics of the cell at 1000 W m−2 , 1.5 AM illumination were ascertained using a source meter (Keithley 2420).
3. Results and discussion
Fig. 2 shows the FT-IR spectrum of a sample of polymer scraped from the CTO surface. The spectrum has the
same general characteristics of polythiocyanogen prepared
by other methods [13]. Films deposited on CTO glass (or on
TiO2 ) were found to be highly stable and firmly adhered to
the substrate. They are resistant to concentrated nitric and
sulfuric acids but attacked by strong alkalis. The film softens
and peels off when immersed in a strong solution of sodium
sulfide. On heating no sign of chemical decomposition or
film breakdown was detected up to ∼300 ◦ C. An ∼100 nm
thick film deposited on TiO2 substrate has a conductivity
9 × 10−8 S cm−1 . Compressed pellets had a higher conductivity of 4 × 10−6 S cm−1 . Presumably rapid electrolysis introduces some dopant. In photoresponse measurements of
Fig. 2. FT-IR spectrum of the polythiocyanogen scraped off from a film
deposited on conducting tin oxide glass (T = transmittance).
V.P.S. Perera et al. / Synthetic Metals 143 (2004) 283–287
285
Fig. 4. Absorption spectrum of a film of polythiocyanogen and photocurrent action spectrum of the cell n-TiO2 /[SCN]n /p-CuI.
Fig. 3. Mott–Schottky plot for a film of polythiocyanogen deposited on
conducting glass. Measurement frequency: (a) 1.5 kHz; (b) 1 kHz.
the films an anodic signal was observed, suggesting n-type
behavior. Iodine doping increases the conductivity and the
sign of the photocurrent indicating p-type behavior. Experiments with compressed pellets of polythiocyanogen have
also demonstrated an increase in conductivity of polythiocyanogen on doping with iodine and bromine [13]. Fig. 3
shows Mott–Schottky plots at 1.5 and 1 kHz for a film of
polythiocyanogen deposited onto conducting glass. From
Fig. 3 the conduction band edge is positioned at −0.46 V versus standard calomel electrode (SCE). The positive slope of
the plots confirms n-type conductivity. Fig. 4 shows the optical absorption spectrum of the polythiocyanogen film. From
Fig. 4 the band edge is found at 550 nm, corresponding to a
band gap of 2.25 eV. Fig. 5 compares the SEM pictures of
polythiocyanogen deposited onto a CTO surface and a bare
CTO surface. Structures other than the granulites in the CTO
surface are absent in the former indicating that the polymer
deposits as an uniform interconnected matrix free of pin
holes and large irregularities in thickness. Fig. 6 compares
SEM images of TiO2 coated onto CTO glass with SEM images of polythiocyanogen deposited onto TiO2 coated CTO
glass. It is obvious from Fig. 6 that the polymer film fully
covers the rough surface of the nanocrystalline TiO2 surface.
Comparison of the photocurrent action spectrum (plot
of IPCE = incident photon to photocurrent conversion
efficiency versus wavelength) and the optical absorption
of the film shows that the photocurrent originates from
the light absorbed by the film. Fig. 7 shows the I–V characteristics of the photovoltaic cell n-TiO2 /[SCN]n /p-CuI
at 1000 W m−2 , 1.5 AM illumination. The short-circuit
photocurrent, open-circuit voltage and efficiency being
2 mA cm−2 , 325 mV and 0.3%, respectively. Fig. 8 shows
a schematic diagram illustrating the positions of the conduction and valence bands of TiO2 , [SCN]n and CuI. The
mechanism of the photovoltaic effect can be explained as
follows: photons absorbed by [SCN]n generate excitons
Fig. 5. SEM picture of (a) polythiocyanogen deposited on conducting tin oxide glass surface and (b) bare conducting tin oxide glass surface.
286
V.P.S. Perera et al. / Synthetic Metals 143 (2004) 283–287
Fig. 6. SEM picture of (a) polythiocyanogen deposited on a nanocrystalline film of TiO2 and (b) bare nanocrystalline TiO2 film.
Fig. 7. I–V characteristics of the cell n-TiO2 /[SCN]n /p-CuI measured at
1000 W m−2 , 1.5 AM illumination.
Fig. 8. Schematic energy level diagram showing band positions of TiO2 ,
[SCN]n and CuI.
which decomposes to electrons and holes at the interfaces
[SCN]n /TiO2 and [SCN]n /CuI as the diffusion length of excitons in a polymer is expected to be of the order of 10 nm,
it is unlikely that diffusion of excitons generated in the bulk
of the 100 nm film of [SCN]n contribute significantly to
the photocurrent. However, when the excitons are decomposed at the TiO2 /[SCN]n , the position of the conduction
band of TiO2 allows electron injection to n-TiO2 . The hole
remaining in [SCN]n could diffuse to the [SCN]n /TiO2
interface and pass onto CuI. Similarly, when excitons are
decomposed at the [SCN]n /CuI interface a hole is injected
to p-CuI and the electron remaining in [SCN]n diffuses
to the [SCN]n /TiO2 interfaces and passes onto TiO2 . The
rates of the above processes depend on the mobilities of
electrons and holes in [SCN]n . We have not succeeded in
measuring the mobilities of electrons and holes in [SCN]n .
The dark I–V curve for the cell in the forward and reverse bias is presented in Fig. 9, rectification characteristics
needed for functioning as a photovoltaic device is evident.
On prolonged illumination, both short-circuit photocurrent and open-circuit voltage undergo a slow decay as is
found in other photovoltaic devices based on CuI [20]. On
Fig. 9. Dark rectification curve for the cell n-TiO2 /[SCN]n /p-CuI.
V.P.S. Perera et al. / Synthetic Metals 143 (2004) 283–287
replenishing the CuI overlayer (i.e. dissolution of original
layer in acetonitrile and deposition of new CuI layer), the
same photovoltaic response reappears indicating that there
is no hysteresis in the polymer film. Decay of photocurrent
and open-circuit voltage, originate almost entirely from the
deterioration of CuI.
4. Conclusion
We have devised a method for deposition of thin films
of the conducting polymer [SCN]n on conducting tin oxide glass or other conducting substrate. Films are found be
highly resistant to chemical action and temperature. Again
the films deposited by this method are uniform and largely
free of irregularities or pin holes. Films of [SCN]n were
also deposited on nanocrystalline TiO2 films and the heterojunction n-TiO2 /[SCN]n /p-CuI demonstrated good photovoltaic response. We believe that further characterization and
other studies on thin films of [SCN]n could lead to practical
applications.
References
[1] L.A.A. Pettersson, L.S. Roman, O. Inganas, J. Appl. Phys. 86 (1999)
487.
287
[2] T. Yohannes, T. Solomon, O. Inganas, Synth. Met. 82 (1996) 215.
[3] R. Valaski, A.F. Bozza, L. Micaroni, I.A. Hummlelgen, J. Solid State
Electrochem. 4 (2000) 390.
[4] C.M. Ramsdale, J.A. Barker, A.C. Arias, J.D. MacKenzie, R.H.
Friend, N.C. Greenham, J. Appl. Phys. 92 (2002) 4266.
[5] M.P.T. Christiaans, M.M. Weink, P.A. van Hal, J.M. Kroon, R.A.J.
Janssen, Synth. Met. 101 (1999) 265.
[6] N.A. Anderson, E. Hao, X. Ai, G. Hastings, T. Lian, Chem. Phys.
Lett. 347 (2001) 304.
[7] G. Yu, J. Gao, J.C. Hummelen, F. Wudl, A.J. Heeger, Science 270
(1995) 1789.
[8] C.O. Too, G.G. Wallace, A.K. Burrell, G.E. Collis, D.L. Officer,
E.W. Boge, S.G. Brodie, E.J. Evans, Synth. Met. 123 (2001)
53.
[9] G. Wang, X. Hu, T.K.S. Wong, J. Solid State Electrochem. 5 (2001)
150.
[10] R.F. Service, Science 279 (1998) 1135.
[11] S. Chao, M.S. Wrighton, J. Am. Chem. Soc. 109 (1987) 2197.
[12] F. Cataldo, J. Inorg. Organomet. Polym. 7 (1997) 35.
[13] F. Cataldo, Polyhedron 19 (2000) 681.
[14] F. Cataldo, Polyhedron 21 (2002) 1825.
[15] F. Cataldo, Eur. Polym. J. 35 (1999) 571.
[16] F. Cataldo, P. Fiordiponti, Polyhedron 12 (1993) 279.
[17] M. Bragadin, G. Capodaglio, P. Cescon, G. Scarponi, F. Pucciarelli,
J. Electroanal. Chem. 122 (1981) 393.
[18] M. Bragadin, G. Scarponi, G. Capodaglio, F. Ossola, V. Bartocci, F.
Pucciarelli, Mol. Liq. Cryst. 121 (1985) 345.
[19] K. Tennakone, G.R.R.A. Kumara, A.R. Kumarasinghe, K.G.U. Wijayantha, P.M. Sirimanne, Semicond. Sci. Technol. 10 (1995) 1689.
[20] G.R.A. Kumara, A. Konno, K. Shiratsuchi, J. Tsukahara, K. Tennakone, Chem. Mater. 14 (2002) 954.