Synthetic Metals 123 (2001) 53±60 Photovoltaic devices based on polythiophenes and substituted polythiophenes C.O Tooa, G.G Wallacea,*, A.K Burrellb, G.E Collisb, D.L Of®cerb, E.W Bogec, S.G Brodiec, E.J Evansc a b Intelligent Polymer Research Institute, University of Wollongong, NSW 2522, Australia IFS-Chemistry, Massey University, Private Bag 11222, Palmerston North, New Zealand c BHP Steel Research Laboratories, The Broken Hill Proprietary Company Limited, P.O Box 202, Port Kembla, NSW 2505, Australia Received February 2000; accepted 15 September 2000 Abstract In recent years there has been considerable interest in the fabrication of photovoltaic devices using polymeric and organic materials This paper presents work carried out using a range of polythiophenes, including some substituted with porphyrin moieties as light harvesters Homopolymers and copolymers were investigated for their performance in photovoltaic devices, and the use of both solid polymer electrolyte and liquid electrolyte was examined Both photoelectrochemical cells and Schottky devices were investigated The best photoelectrochemical cell was fabricated using polyterthiophene which had V oc 139 mV, I sc 123:4 mA cmÀ2, ®ll factor 0:38, and energy conversion efficiency 0:02% at a halogen lamp intensity of 317 W mÀ2 The Schottky device gave a V oc 0:5 V and Isc of 0.98 mA cmÀ2 at a halogen lamp intensity of 500 W mÀ2 # 2001 Elsevier Science B.V All rights reserved Keywords: Photovoltaic devices; Polythiophenes; Conducting polymers; Porphyrins Introduction Commercial photoelectric conversion devices are made mostly from inorganic semiconductors In the last two decades, much work has focused on polymeric and organic materials [1,2] since the structure and properties of these photoactive materials can be readily controlled and they are considerably cheaper than the inorganic equivalents These studies include the incorporation of conducting polymers into photovoltaic devices Early work involved devices based on polyaniline [3], poly(p-phenylene vinylene) [4], poly(p-phenylene vinylene)/perylene heterojunction [5], poly(2-methoxy-5-(2H -ethyl-hexyloxy)-1,4-phenylene vinylene composite [6,7], poly(3-methylthiophene) [8] and polythiophene [9] In the approach described previously for poly(3methylthiophene) (P3MTh) [8], the P3MTh functions as a p-type semiconductor where holes or hole-polarons are the * Corresponding author Tel.: 61-2-4221-3127; fax: 61-2-4221-3114 E-mail address: gordon_wallace@uow.edu.au (G.G Wallace) dominant carriers that cause the measured photocurrent The holes generated by irradiation of light cause, via the external circuit, the counter electrode consisting of a Pt coating on indium tin oxide (ITO) coated glass to be positively charged, whilst the electrons move to the P3MTh/solid polymer electrolyte (front) junction The holes oxidise the electron donor iodide to generate triiodide at the counter electrode The electrons injected to the front contact reduce the triiodide back to iodide Thus, the cell converts light to electricity in a renewable process where there is no net chemical reaction Polythiophenes can be electrosynthesised by oxidative polymerisation according to Eq (1) (1) The properties of the polymer can be controlled by the judicious selection of the substituents R1 and/or R2 In addition, the type of counter-anion (AÀ) can also influence 0379-6779/01/$ ± see front matter # 2001 Elsevier Science B.V All rights reserved PII: S - 7 ( 0 ) 0 - 54 C.O Too et al / Synthetic Metals 123 (2001) 53±60 the properties of the polymer Polythiophenes can undergo electrochemical redox reactions such as (2) where AÀ is a mobile counter-anion that can be exchanged with other anions in solution If the counter-anion is large and immobile, then the redox reaction is more likely to be represented by Eq (3) (3) where M is a cation inserted from the electrolyte solution to preserve charge balance in the polymer matrix In this work, tetrabutylammonium perchlorate was used as the electrolyte and so the counter-anion, perchlorate, would be expelled from the polymer on reduction, thus leaving the polymer backbone uncharged (Eq (2)) The neutral polymer, therefore, behaves like a p-type semiconductor We report here investigations into polymers made from commercially available 3-methylthiophene (3MTh), bithiophene (BiTh) and terthiophene (TTh); such monomers and their derivatives have been successfully electropolymerised by others [8±15] In addition, we have also investigated the use of porphyrin substituents on polythiophenes in order to enhance the light harvesting capabilities of the conducting polymer materials The use of porphyrins in photoelectric conversion is well-documented [16±20] Thus, trans-1(2H -(5H ,10H ,15H ,20H -tetraphenylporphyrinyl))-2-(3HH thienyl)ethene (I) and trans-1-(2H -(5H ,10H ,15H ,20H -tetraphenylporphyrinyl))-2-(3HHH -terthienyl)ethene (II) were prepared and electropolymerised with and without other thiophene monomers Experimental 2.1 Reagents and materials The trans-1-(2H -(5H ,10H ,15H ,20H -tetraphenylporphyrinyl))2-(3HH -thienyl)ethene (TPP-Th) (I) and trans-1-(2H -(5H ,10H , 15H ,20H -tetraphenylporphyrinyl))-2-(3HHH -terthienyl)ethene (TPP-TTh) (II) were synthesised by Burrell et al [21] at Massey University In addition 3-methylthiophene (3MTh, Aldrich), bithiophene (BiTh, Aldrich), terthiophene (TTh, Aldrich), tetrabutylammonium perchlorate (TBAP, Fluka), polyethylene oxide (PEO, Mv c.600 000, Aldrich), potassium iodide (Univar, Ajax), iodine (Univar, Ajax or Aldrich 99.8%), methanol (Univar, Ajax), acetonitrile (ACN, Univar, Ajax), dichloromethane (DCM, Univar, Ajax), isopropanol (Univar, Ajax), tetrapropylammonium iodide (Aldrich, !98%), ethylene carbonate (Aldrich 99%), and propylene carbonate (Aldrich 99%) were used as received ITO coated glass ( 40 or 10 O sqÀ1) was purchased from Delta Technologies Limited (USA), cut into required sizes, washed with Teepol, rinsed thoroughly with Milli-RO water followed by isopropanol, and allowed to dry The solid polymer electrolyte (SPE) was formulated by dissolving polyethylene oxide (1 g) in 10 cm3 of I2/KI in methanol, which was made up by dissolving I2 (0.15 g) and KI (1.5 g) in 100 cm3 of methanol In addition, a liquid electrolyte was formulated by dissolving I2 (60 mM) and tetrapropylammonium iodide (500 mM) in ethylene carbonate/propylene carbonate (1:1 by weight) A thin layer of platinum was sputter coated onto the ITO coated glass using a Dynavac Magnetron Sputter Coater Model SC100MS The sputtering was performed at a current of 50 mA and Ar pressure of  10À3 mbar Under these Ê sÀ1 A conditions the Pt thickness obtainable would be A Ê was sputter coated In addition, Al was Pt thickness of 10 A also sputter coated at 150 mA for 1±2 when fabricating Schottky devices 2.2 Equipment In summary, the main objectives of this work were to electrosynthesise photoactive coatings from thiophenebased monomers and fabricate them into photoelectrochemical and Schottky devices The electropolymerisation conditions for each polymer or copolymer were investigated and optimised to produce the best photovoltaic response Electrosynthesis and testing of polymers were achieved by using an electrochemical hardware system comprising of an EG&G PAR 363 Potentiostat/Galvanostat, a Bioanalytical Systems CV27 Voltammograph, a MacLab 400 with Chart v 3.5.7/EChem v 1.3.2 software (AD Instruments), and a Macintosh computer A three-electrode electrochemical cell was used which comprised of a working electrode (Pt disc or ITO coated glass or these substrates with polymer coatings on them), a Pt mesh auxiliary electrode and a Ag/ Ag reference electrode with salt bridge Testing of devices was done at BHP Steel Research Laboratories (Port Kembla) using an EG&G PAR 263A Potentiostat/Galvanostat with associated software and a halogen lamp (317 W mÀ2) to obtain current±voltage (I±V) curves C.O Too et al / Synthetic Metals 123 (2001) 53±60 55 Scheme Photovoltaic device Subsequently, testing was done at the University of Wollongong using an halogen lamp (SoLux MR-16 from Wiko Ltd.) and a set-up comprising of a Macintosh computer/MacLab 400 with EChem v1.3.2 software (AD Instruments)/CV27 Voltammograph (Bioanalytical Systems) In general, a light intensity of 500 W mÀ2 was used unless indicated otherwise UV±VIS spectra were obtained using a Shimadzu UV1601 spectrophotometer and scanning over the range 300±1100 nm 2.3 Photovoltaic device fabrication 2.3.1 Photoelectrochemical cell The polymers and copolymers were electrodeposited on ITO coated glass and rinsed with dichloromethane or acetonitrile and then allowed to dry In general, the polymer and copolymer coatings were electroreduced at À0.4 or À0.8 V in 0.1 M TBAP/DCM or 0.1 M TBAP/ACN before being assembled as devices The device (Scheme 1) was assembled by sandwiching up to ml of SPE solution per cm2 of area between the polymer coated ITO coated glass electrode and the Pt sputtered ITO coated glass electrode, and allowed to dry for 24 h In the case of the liquid electrolyte, the device was assembled by sandwiching the liquid electrolyte between the two respective electrodes This was done with and without a border of plastic ®lm as spacer between the electrodes 2.3.2 Schottky junction devices Schottky devices were made from electropolymerised conducting polymer, e.g polyterthiophene (PTTh) in the reduced form, on ITO coated glass The polymer was then coated with a thin layer of Al by sputter coating The device is illustrated in Scheme 2.4 Photovoltaic testing The photovoltaic devices were tested by linear sweep voltammetry (LSV) The characteristics of an I±V curve are shown in Scheme Thus, the open circuit voltage (Voc) is where the current is zero, and the short circuit current (Isc) is where the voltage is zero Other characteristics of a photovoltaic device are given below The ®ll factor (FF) is given by FF voltage at peak power Vpp Âcurrent at peak power Ipp open circuit voltage Voc Âshort circuit current Isc The energy conversion ef®ciency (ECE) is given by ECE Vpp  Ipp total power of light radiating on the cell area (5) Voc  Isc  FF total power of light radiating on the cell area (6) or ECE Results and discussion 3.1 Poly(3-methylthiophene) The poly(3-methylthiophene) photovoltaic system was chosen as the starting point for our investigations because of published information already available [8] Poly(3methylthiophene) was electrodeposited on ITO coated glass from a solution consisting of 3-methylthiophene (0.5 M) in TBAP (0.1 M)/ACN The constant potential (CP) method was used at a potential of 1.4 V versus Ag/Ag and growth time was 30 The dried polymers were examined by UV±VIS spectrophotometry in their oxidised and reduced states Thus, the spectrum of poly(3-methylthiophene) in its Scheme Schottky device 56 C.O Too et al / Synthetic Metals 123 (2001) 53±60 Scheme Current±voltage characteristics of a photovoltaic device The maximum output of the cell is given by the product IppVpp, where Ipp is the current at peak power and Vpp is the voltage at peak power oxidised state showed the free-carrier tail at longer wavelength (1100 nm) which is characteristic of a conducting polymer After reduction, this free-carrier tail was lost; signifying a loss of conductivity The reduced poly(3methylthiophene) was fabricated into solid state devices using the SPE Voc of 52 mV, Isc of 1.23 mA cmÀ2, ®ll factor of 0.24 and ECE of  10À5 % were obtained at a halogen light intensity of 317 W mÀ2 This compares favourably with the results reported by Yohannes et al [8] of V oc 140 mV, I sc 0:35 mA cmÀ2 at a light intensity of 1000 W mÀ2 3.2 Polybithiophene PolyBiTh was electrodeposited by cyclic voltammetry (CV) and by constant potential (Table 1) From the growth CV, it was observed that BiTh began to oxidise at 0.90 V and that the current increased with increasing number of cycles, thus signifying that a conductive polymer was being formed The UV±VIS spectra of the oxidised and reduced polymer were obtained (Fig 1a and b) In the spectrum of polybithiophene, reduced at À0.4 V, the loss of the bands above Fig UV±VIS spectra of polybithiophene on ITO coated glass (a) Polymer in oxidised form; (b) polymer after electrochemical reduction at À0.4 V (vs Ag/Ag) 600 nm was observed, thus demonstrating a loss of conductivity of the polymer Table summarises the results obtained from these reduced polymers when fabricated into photoelectrochemical cells These devices were tested with a 317 W mÀ2 halogen light source In general, thinner polymer ®lms gave better photovoltaic responses The better devices were obtained from polymers produced by CV growth rather than potentiostatic growth The best device incorporated a polymer grown for cycles by CV at 100 mV sÀ1, with potential limits of À0.4 to 1.2 V (V oc 247 mV, I sc 13:4 mA cmÀ2, ®ll factor 0:33, ECE 0:0034%) The best device with potentiostatic-growth polymer was obtained at 1.1 V and grown for 15 mC cmÀ2 (V oc 234 mV, I sc 5:71 mA cmÀ2, ®ll factor 0:36, ECE 0:0015%) Table Characteristics of photoelectrochemical cells from polybithiophenea Polymer growth conditions CV at 100 mV sÀ1, À0.4 to CV at 100 mV sÀ1, À0.4 to CV at 100 mV sÀ1, À0.4 to CV at 100 mV sÀ1, À0.4 to CV at 100 mV sÀ1, À0.4 to CV at 100 mV sÀ1, À0.4 to CP at 1.1 V, 15 mC cmÀ2 CP at 1.1 V, 30 mC cmÀ2 CP at 1.1 V, 60 mC cmÀ2 CP at 1.1 V, 120 mC cmÀ2 1.1 V, 1.1 V, 1.2 V, 1.2 V, 1.4 V, 1.4 V, cycles cycles cycles cycles cycles cycles Voc (mV) Isc (mA cmÀ2) Fill factor ECE (%) 240 232 247 208 206 196 234 216 199 152 7.04 4.47 13.4 3.64 3.86 5.28 5.71 2.98 1.85 1.39 0.32 0.33 0.33 0.31 0.32 0.33 0.36 0.30 0.25 0.23 0.0017 0.0011 0.0034 0.0008 0.0008 0.0011 0.0015 0.0007 0.0003 0.0002 a Monomer solution was BiTh (10 mM)/TBAP (0.1 M)/CH3CN Polymers grown by CP were pre-reduced at À0.4 V (vs Ag/Ag) before assembly into photoelectrochemical cells Light source: halogen lamp (317 W mÀ2) CV cyclic voltammetry, and CP constant potential C.O Too et al / Synthetic Metals 123 (2001) 53±60 57 reduction of the polymer at À0.4 V, the bands above 600 nm are lost, signifying a loss of conductivity The reduced polymers were fabricated into photoelectrochemical cells with SPE and tested The results (Table 2) suggest that polymer growth by CV gives better devices than polymer growth by CI or CP I±V tests, however, also demonstrate that potentiostatic and galvanostatic growth give similar results Thus, the best device had V oc 179 mV, I sc 15:1 mA cmÀ2, ®ll factor 0:35, and ECE 0:0030% 3.4 Copolymers with TPP-thiophene (TPP-Th) or TPP-terthiophene (TPP-TTh) Fig UV±VIS spectra of polyterthiophene on ITO coated glass (a) Polymer in oxidised form; (b) polymer after electrochemical reduction at À0.4 V (vs Ag/Ag) 3.3 Polyterthiophene Commercial terthiophene was easily electropolymerised onto ITO coated glass and fabricated into PV devices As previously, the currents increased with increasing number of cycles during growth of polyterthiophene by CV Electropolymerisation was also achieved by potentiostatic and constant current (CI) methods Potentiostatic growth at 1.0 V afforded a slow increase in the current with time Galvanostatic growth at mA cmÀ2 occurred at 1.11 V Examples of UV±VIS spectra of oxidised and reduced polyterthiophene (PTTh) are given in Fig 2a and b, respectively Once again, it can be seen from these spectra that, on Homopolymers from TPP-thiophene could not be successfully grown, and although homopolymers from TPPterthiophene could be grown, poor photovoltaic responses were obtained Therefore, copolymers of 3MTh, BiTh or TTh with TPP-Th or TPP-TTh, light harvesting molecules, were electrochemically synthesised on ITO coated glass and fabricated into photoelectrochemical cells in the expectation of obtaining better photovoltaic characteristics 3.4.1 Copolymer of 3-methylthiophene with TPP-thiophene (TPP-Th) 3-Methylthiophene was copolymerised with TPP-Th on ITO coated glass at 1.4 V from a comonomer solution consisting of 3-methylthiophene (0.01, 0.06, 0.5, 2.0 M)/ TPP-Th (0.01 M)/TBAP (0.1 or 0.2 M)/DCM The UV±VIS spectra of the deposits (e.g Fig 3a) exhibit peaks (423 and 523 nm) not present in the spectrum of poly(3-methylthiophene), indicative of the incorporation of the TPP-Th monomer (see Fig 3b for UV±VIS spectrum of TPP-Th monomer) This was regardless of whether the copolymer was in its oxidised or reduced state The TPP-Th/3MTh copolymerisation was further investigated in order to optimise copolymer growth Maximum TPPTh in the copolymer was sought The following comonomer Table Characteristics of photoelectrochemical cells from polyterthiophenea Polymer growth conditions Voc (mV) Isc (mA cmÀ2) Fill factor ECE (%) CV at 100 mV sÀ1, À0.4 to 1.1 V, cycles CV at 100 mV sÀ1, À0.4 to 1.1 V, 10 cycles CV at 100 mV sÀ1, À0.4 to 1.2 V, cycles CV at 100 mV sÀ1, À0.4 to 2.0 V, cycles CP at 1.0 V, 15 mC cmÀ2 CP at 1.1 V, 15 mC cmÀ2 CP at 1.0 V, 15 mC cmÀ2, reduced at V for 60 s CP at 1.1 V, 15 mC cmÀ2, reduced at V for 60 s CP at 1.0 V, 10 mC cmÀ2, reduced at V for 60 s CP at 1.1 V, 10 mC cmÀ2, reduced at V for 60 s CI at mA cmÀ2, 20 s CI at mA cmÀ2, 30 s 172 179 172 187 164 82 104 118 164 150 104 132 8.86 15.1 4.80 13.1 6.38 3.09 3.02 1.86 6.81 3.83 3.18 6.42 0.31 0.35 0.30 0.35 0.27 0.19 0.19 0.21 0.29 0.25 0.27 0.27 0.0016 0.0030 0.0008 0.0027 0.0010 0.0002 0.0002 0.0002 0.0011 0.0005 0.0003 0.0008 a Monomer solution was terthiophene (10 mM)/TBAP (0.1 M)/CH2Cl2 Polymers grown by CP or CI were pre-reduced at À0.4 V (vs Ag/Ag) unless stated otherwise Light source: halogen lamp (317 W mÀ2) CV cyclic voltammetry, CP constant potential, and CI constant current 58 C.O Too et al / Synthetic Metals 123 (2001) 53±60 Fig UV±VIS spectrum of TPP-Th/BiTh copolymer grown potentiodynamically and pre-reduced at À0.4 V (vs Ag/Ag) in 0.1 M TBAP/ACN for 60 s Fig (a) UV±VIS spectrum of copolymer of 3MTh/TPP-Th on ITO coated glass Comonomer solution: 3MTh (1 M)/TPP-Th (0.01 M)/TBAP (0.2 M)/DCM; copolymerisation potential: 1.4 V (vs Ag/Ag) Copolymerisation time: 30 (b) UV±VIS spectrum of TPP-Th dip coated onto a glass slide from a mM TPP-Th/DCM solution ratios of TPP-Th/3MTh were investigated: 10/10, 10/20, 10/ 30 mM In all cases, polymer deposition on ITO coated glass was possible at 1.8 V and above but the deposits were brittle and tended to ¯ake off Devices were fabricated from the deposits after pre-reduction It was found that these devices were not as good as the device made previously from electrodeposition of copolymer from TPP-Th (10 mM)/3MTh (2 M) at 1.4 V This is probably due to the higher potential required to form the copolymer when higher ratios of TPP-Th were used Photoelectrochemical cells were assembled from these reduced copolymers with SPE and tested The best result was obtained from the copolymer grown from a comonomer solution of TPP-Th (0.01 M)/3MTh (2 M)/TBAP (0.1 M)/ DCM This device had V oc 83 mV, I sc 1:67 mA cmÀ2, ®ll factor 0:23 and ECE 0:0001% at a halogen lamp intensity of 317 W mÀ2 These results are better than those obtained from the poly(3-methylthiophene) homopolymer 3.4.2 Copolymers of TPP-Th and BiTh Copolymers of TPP-Th and bithiophene (BiTh) were electrosynthesised on ITO coated glass 10 mM TPP-Th and 10 mM BiTh were used and cyclic voltammetry was performed from À0.8 or 0.0 V to 1.2, 1.4 or 1.6 V CV during growth indicated that currents increased with each cycle, thus con®rming conducting polymer growth Potentiostatic growth also produces electroactive material With all deposition methods, the deposits tended to be brittle and ¯aked off the electrodes The best results were obtained when the limit was 1.4 V or the polymers were grown at a constant potential of 1.4 V For comparison, deposits grown at 1.2 and those at 1.4 V were fabricated into devices after pre-reduction UV±VIS spectra were run on the reduced deposits Typically a major peak at 438 nm, ascribed to the porphyrin substituent, is apparent in the spectra (e.g Fig 4); reduced polyBiTh itself has a major peak at 474 nm (Fig 1b) These copolymers were fabricated into devices with SPE, and I±V test results indicate that CV growth produces the best devices In addition, for CP growth, thinner ®lms give better results The best device had V oc 204 mV, I sc 1:79 mA cmÀ2, ®ll factor 0:30, and ECE 0:0003% These results are not as good as those obtained from polyBiTh alone 3.4.3 Copolymers of TPP-terthiophene (TPP-TTh) and BiTh Electrocopolymerisation was performed by potentiodynamic and potentiostatic methods It was apparent from the CV during growth that the current increased with subsequent cycles, thus indicating the growth of a conductive polymer In addition, the cathodic peak potential shifted cathodically with increasing number of cycles The UV±VIS spectrum of the copolymer (reduced form) is shown as Fig and is substantially different from the spectra of polyBiTh or poly(TPP-TTh) Increasing the BiTh content in the comonomer solution from 0.1 to 0.2 M gave better copolymers Once again, CV growth of the copolymers produced the best devices with SPE The best solid state device had V oc 200 mV, I sc 5:77 mA cmÀ2, ®ll factor 0:31, Fig UV±VIS spectrum of copolymer of BiTh and TPP-TTh in its reduced form C.O Too et al / Synthetic Metals 123 (2001) 53±60 59 from copolymer grown potentiodynamically were better than copolymer generated in other ways The performance of the device also depended upon the thickness of the copolymer grown, with poor performance obtained from very thin or very thick ®lms In addition, reducing the copolymer at À0.8 V (versus Ag/Ag) gave better results than reduction at À0.4 or V The best device gave V oc 185 mV, I sc 15:9 mA cmÀ2, ®ll factor 0:28 and ECE 0:0026% Fig UV±VIS spectrum of copolymer of terthiophene and TPP-TTh in its reduced form Reduction potential: À0.8 V (vs Ag/Ag) 3.5 Photoelectrochemical cells assembled using liquid electrolyte and ECE 0:0011% Once again, the results obtained from the copolymers were not as good as those obtained from the polyBiTh homopolymer The best performing polymers and copolymers in solid state photoelectrochemical cells were utilised in fabricating new cells that incorporated a liquid electrolyte These cells were tested and a summary of their characteristics is given in Table When liquid electrolyte was used, much higher currents were obtained, and the gain in ef®ciency was 2±7.6 times The best liquid electrolyte device was fabricated using potentiodynamically-grown polyterthiophene, which had V oc 139 mV, I sc 123:4 mA cmÀ2, ®ll factor 0:38 and ECE 0:0205% 3.4.4 Copolymer of TPP-terthiophene (TPP-TTh) and terthiophene Copolymers of terthiophene with TPP-terthiophene were electrosynthesised on ITO coated glass and fabricated into photoelectrochemical cells Potentiodynamic and potentiostatic methods were used The UV±VIS spectrum of the reduced copolymer is given in Fig and is different from the spectra of the respective reduced homopolymers The I±V test results (Table 3) indicate that all the copolymers are comparable to poly-TTh as a material for solid state photoelectrochemical cells Generally, devices made 3.6 Schottky devices Schottky devices (Scheme 2) were fabricated, and tested in the same way as for the photoelectrochemical devices Table Characteristics of photoelectrochemical cells from copolymer of terthiophene and TPP-TTha Polymer growth conditions À1 CV at 100 mV s , À0.8 to 1.2 V, 10 cycles CV at 100 mV sÀ1, À0.8 to 1.4 V, 10 cycles CV at 100 mV sÀ1, À0.8 to 1.6 V, 10 cycles CV at 100 mV sÀ1, À0.8 to 1.8 V, cycles CV at 100 mV sÀ1, À0.8 to 2 V, cycles CV at 100 mV sÀ1, À0.4 to 2 V, cycles CP at 1.1 V, 30 mC cmÀ2 CP at 1.1 V, 30 mC cmÀ2, reduced at V for 60 s CP at 1.1 V, 90 mC cmÀ2 Voc (mV) Isc (mA cmÀ2) Fill factor ECE (%) 26 158 186 173 185 160 120 46 62 0.42 4.59 8.48 12.2 15.9 2.01 1.20 1.29 1.40 0.04 0.25 0.23 0.30 0.28 0.20 0.21 0.14 0.31  10À6 0.0007 0.0021 0.0020 0.0026 0.0002 0.0001  10À6 0.0001 a Monomer solution was terthiophene (10 mM)/TPP-TTh (10 mM)/TBAP (0.1 M)/CH2Cl2 Unless stated otherwise, copolymers grown by CP were prereduced at À0.8 V (vs Ag/Ag) before fabrication into devices Light source: halogen lamp (317 W mÀ2) CV cyclic voltammetry, and CP constant potential Table Comparison of characteristics of photoelectrochemical cells assembled with SPE or liquid electrolytea Type of cell Growth conditions Voc (mV) Isc (mA cmÀ2) Fill factor ECE (%) à CV, À0.4 Ditto CV, À0.4 Ditto CV, À0.8 Ditto CV, À0.8 Ditto 247 163 187 139 185 169 218 174 13.4 32.1 13.1 123.4 15.9 27.9 4.74 17.9 0.33 0.43 0.35 0.38 0.28 0.35 0.30 0.32 0.0034 0.0071 0.0027 0.0205 0.0026 0.0052 0.0010 0.0031 Poly-BiTh Poly-BiTh à Poly-TTh Ãà Poly-TTh à Copol TPP-TTh/TTh Ãà Copol TPP-TTh/TTh à Copol TPP-TTh/BiTh Ãà Copol TPP-TTh/BiTh Ãà a to 1.2 V, cycles to 2.0 V, cycles to 2.0 V, cycles to 1.8 V, cycles Light source halogen lamp (317 W mÀ2) CV cyclic voltammetry Photoelectrochemical cells with: (Ã) SPE or (ÃÃ) liquid electrolyte 60 C.O Too et al / Synthetic Metals 123 (2001) 53±60 The best Schottky device was obtained using the reduced form of polyterthiophene grown by CV at 100 mV sÀ1 between the potential limits of À0.8 to 1.1 V for 10 cycles This device had a Voc of 0.5 V and Isc of 0.98 mA cmÀ2 under a halogen lamp intensity of 500 W mÀ2 The variation of Voc with light intensity showed that the open circuit voltage increased linearly (R2 0:9943, degrees of freedom) with increasing light intensity within the range of 200± 1000 W mÀ2 This compares favourably with results reported by Kaneko and Yamada [9] for Schottky devices incorporating polythiophene tested at a xenon lamp intensity of 640 W mÀ2 Their devices had Voc of 0.13 V and Isc of 0.25 mA cmÀ2 for the reduced form of polythiophene whereas, for the oxidised form of polythiophene, Voc was 1.07 V and Isc was 1.35 mA cmÀ2 Conclusions A summary of the best results for photoelectrochemical devices is given in Table Signi®cant improvement in Voc and Isc as compared to the devices described by Yohannes et al [8] from poly-3MTh have been obtained The best devices provided Isc values which are at least 43±45 times higher than that published by Yohannes et al., given that the light source used had an intensity only one third of that used by the previous workers In general, polymers containing BiTh or TTh produced the best photoelectrochemical devices The use of liquid electrolyte greatly enhances the ef®ciency in comparison to SPE devices The best device made was from poly-TTh This device had V oc 139 mV, Isc 123:4 mA cmÀ2, ®ll factor 0:38, and efficiency 0:0205% A Schottky device was successfully made from polyterthiophene from which a Voc of 0.5 V and Isc of 0.98 mA cmÀ2 was obtained under a light intensity of 500 W mÀ2 Acknowledgements We wish to thank the Australian Research Council and BHP Limited for ®nancial support of this project We are also grateful to the New Zealand Public Good Science Fund (MAU809) and the Massey University Research Fund (GEC) References [1] M Kaneko, Photoelectric conversion by polymeric and organic materials, in: H.S Nalwa (Ed.), Handbook of Organic Conductive Molecules and Polymers, Vol 4, Wiley, New York, 1997, (Chapter 13) [2] T.A Skotheim, R.L Elsenbaumer, J.R Reynolds (Eds.), Handbook of Conducting Polymers, 2nd Edition, Marcel Dekker, New York, 1998 [3] S.-A Chen, Y Fang, Polyaniline Schottky barrier: effect of doping on rectification and photovoltaic characteristics, Synth Metals 60 (1993) 215±222 [4] R.N Marks, J.J.M Halls, D.D.C Bradley, R.H Friend, A.B Holmes, The photovoltaic response in poly(p-phenylene vinylene) thin-film devices, J Phys.: Condens Matter (1994) 1379±1394 [5] J.J.M Halls, R.H Friend, The photovoltaic effect in a poly(pphenylenevinylene)/perylene heterojunction, Synth Metals 85 (1997) 1307±1308 [6] J.J.M 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J Roncali, Conjugated poly(thiophenes): synthesis, functionalization, and applications, Chem Rev 92 (1992) 711±738 [16] T.J Schaafsma, Organic solar cells using porphyrin assemblies on semiconductor... electrosynthesise photoactive coatings from thiophenebased monomers and fabricate them into photoelectrochemical and Schottky devices The electropolymerisation conditions for each polymer or copolymer were... polymer Polythiophenes can undergo electrochemical redox reactions such as (2) where AÀ is a mobile counter-anion that can be exchanged with other anions in solution If the counter-anion is large