Photoelectrochemical cells based on polymers and copolymers from terthiophene and nitrostyrylterthiophene C.A. Cutler a , A.K. Burrell b , G.E. Collis b , P.C. Dastoor c , D.L. Of®cer b , C.O. Too a , G.G. Wallace a,* a Intelligent Polymer Research Institute, University of Wollongong, North®elds Avenue, Wollongong, NSW 2522, Australia b IFS-Chemistry, Massey University, Private Bag 11222, Palmerston North, New Zealand c Department of Physics, University of Newcastle, University Drive, Callaghan, NSW 2308, Australia Received 26 June 2000; received in revised form 6 November 2000; accepted 4 December 2000 Abstract This research work was aimed at improving the photovoltaic ef®ciency of photoelectrochemical cells PECs) made from terthiophene based conducting polymers. Two approaches were considered. One was to use polyE)-3 00 -p-nitrostyryl)terthiophene) in expectation that the electron withdrawing nitro substituent would enhance charge separation. The other approach was to condition the polymers at different potentials before assembly into PECs. In this way, these polymers would have different conductivities that would be expected to have different charge transport properties. Thus, polymers and copolymer from terthiophene and nitrostyrylterthiophene have been electrochemically polymerised onto indium tin oxide ITO) coated glass. These ®lms were characterised by cyclic voltammetry. In addition, ®lms were electrochemically conditioned at a series of different potentials before they were characterised by UV±VIS spectrophotometry and then fabricated into PECs using liquid electrolyte. The photovoltaic tests show that the electron withdrawing nitro group results in charge separation being the dominant factor in the photovoltaic behaviour of these polymers, as evidenced by their behaviour being independent of the conditioning potential. In contrast, the photovoltaic behaviour of polyterthiophene PECs shows that the best performance was obtained from the fully reduced polymers, which behave as p-type semiconductors for exciton formation and charge separation. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Conducting polymer; Nitrostyrylterthiophene; Terthiophene; Photovoltaics; Photoelectrochemical cells 1. Introduction Polythiophenes are attractive candidates for use in photo- electric conversion devices as a wide variety of substituents can be readily attached to the polymer backbone providing polymers with various electronic states, conductivities, sta- bilities and other useful physical properties [1]. To date, most studies have involved poly3-methylthiophene) [2±6] or related poly3-alkylthiophenes) [7]. Recently, however, there has been an upsurge of interest in exploring the photoelectrochemicalpropertiesofpolythiophenesobtained fromalternativelysubstitutedthiophenes[8,9],bithiophenes [6,8] or terthiophenes [10]. Polythiophene grown from terthiophene has the potential to adopt a more ordered structure [11,12] than the generally amorphouspolymersgrownfromthiophene[13].Thisisdue to the fact that polymerisation through the beta position is minimised particularly in thin ®lms [1]. Polythiophenes and polythiophene oligomers with a well-ordered closely- packed structure have been shown to possess greater charge carrier mobility [13,14]. Effective charge separation and mobility are intrinsic requirements in any photovoltaic device [15±17]. Therefore, it is likely some polyterthio- phenes,inwhichthereisagreaterhomogeneousdistribution in conjugation length along the chains [18], could enhance charge mobility and would be a useful component in photo- electrochemical cells. Terthiophene also has a lower oxida- tion potential than methylthiophene, bithiophene and many of their derivatives [15], which aids in the facile electro- chemical growth of conducting polymer ®lms. The photocurrents that can be attained using polythio- phene-based electrodes have been enhanced by attaching electron acceptors such as the nitro group to the polythio- phene chain [19,20]. Direct conjugation of the electron- withdrawing nitro group with the polyterthiophene backbone due to the presence of an alkene linker [21,22] Synthetic Metals 123 2001) 225±237 * Corresponding author. Tel.: 61-2-4221-3127; fax: 61-2-4221-3114. E-mail address: gordon_wallace@uow.edu.au G.G. Wallace). 0379-6779/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII:S0379-677901)00294-6