Construction of a photovoltaic device by deposition of thin films of the conducting polymer polythiocyanogen

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1 Synthetic Metals 143 (2004) 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, a Institute of Fundamental Studies, Hantana Road, Kandy, Sri Lanka b 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-tio 2 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 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.: ; fax: address: tenna@ifs.ac.lk (K. Tennakone). readily undergoes polymerization is thiocyanogen (SCN) 2. Cataldo [12 14] have conducted extensive investigations to elucidate the structure of polythiocyanogen. Polythiocyanogen of general composition [S y (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 TiO 2. 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 /$ see front matter 2004 Elsevier B.V. All rights reserved. doi: /j.synthmet

2 284 V.P.S. Perera et al. / Synthetic Metals 143 (2004) 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/C 2 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 Na 2 SO 4 ). Nanocrystalline films of TiO 2 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 TiO 2 surface by the same method as for CTO plates. Polythiocyanogen films coated on CTO and nanocrystalline TiO 2 surfaces and for comparison the bare CTO and TiO 2 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 TiO 2. CuI was deposited by drop coating from a solution of CuI in acetonitrile ( 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 TiO 2 ) 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 TiO 2 substrate has a conductivity Scm 1. Compressed pellets had a higher conductivity of Scm 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).

3 V.P.S. Perera et al. / Synthetic Metals 143 (2004) 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 Fig. 4. Absorption spectrum of a film of polythiocyanogen and photocurrent action spectrum of the cell n-tio 2 /[SCN] n /p-cui. deposits as an uniform interconnected matrix free of pin holes and large irregularities in thickness. Fig. 6 compares SEM images of TiO 2 coated onto CTO glass with SEM images of polythiocyanogen deposited onto TiO 2 coated CTO glass. It is obvious from Fig. 6 that the polymer film fully covers the rough surface of the nanocrystalline TiO 2 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-tio 2 /[SCN] n /p-cui at 1000 W m 2, 1.5 AM illumination. The short-circuit photocurrent, open-circuit voltage and efficiency being 2mAcm 2, 325 mv and 0.3%, respectively. Fig. 8 shows a schematic diagram illustrating the positions of the conduction and valence bands of TiO 2, [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.

4 286 V.P.S. Perera et al. / Synthetic Metals 143 (2004) Fig. 6. SEM picture of (a) polythiocyanogen deposited on a nanocrystalline film of TiO 2 and (b) bare nanocrystalline TiO 2 film. Fig. 7. I V characteristics of the cell n-tio 2 /[SCN] n /p-cui measured at 1000 W m 2, 1.5 AM illumination. which decomposes to electrons and holes at the interfaces [SCN] n /TiO 2 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 TiO 2 /[SCN] n, the position of the conduction band of TiO 2 allows electron injection to n-tio 2. The hole remaining in [SCN] n could diffuse to the [SCN] n /TiO 2 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 /TiO 2 interfaces and passes onto TiO 2. 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. 8. Schematic energy level diagram showing band positions of TiO 2, [SCN] n and CuI. Fig. 9. Dark rectification curve for the cell n-tio 2 /[SCN] n /p-cui.

5 V.P.S. Perera et al. / Synthetic Metals 143 (2004) 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 TiO 2 films and the heterojunction n-tio 2 /[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. [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) [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) [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) [11] S. Chao, M.S. Wrighton, J. Am. Chem. Soc. 109 (1987) [12] F. Cataldo, J. Inorg. Organomet. Polym. 7 (1997) 35. [13] F. Cataldo, Polyhedron 19 (2000) 681. [14] F. Cataldo, Polyhedron 21 (2002) [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) [20] G.R.A. Kumara, A. Konno, K. Shiratsuchi, J. Tsukahara, K. Tennakone, Chem. Mater. 14 (2002) 954.