Electrochemical and Photovoltaic Properties of Electropolymerized Poly(thienylsilole)s Joshua C. Byers, Paul M. DiCarmine, Mahmoud M. Abd Rabo Moustafa, Xin Wang, Brian L. Pagenkopf, and Oleg A. Semenikhin* Department of Chemistry, The UniVersity of Western Ontario, London, Ontario, N6A 5B7, Canada ReceiVed: May 12, 2009; ReVised Manuscript ReceiVed: October 3, 2009 Electrochemical and photoelectrochemical properties were studied of a series of donor-acceptor materials based on polythiophene modified with silole moieties. The materials were prepared by electrochemical anodic polymerization of 2,5-bis([2,2′-bithiophen]-5-yl)-1,1-dimethyl-3,4-diphenylsilole and 2,5-bis([2,2′-terthiophen]- 5-yl)-1,1-dimethyl-3,4-diphenylsilole, as well as copolymerization of these monomers with 2,2′-bithiophene. Photocurrent measurements showed that introduction of silole resulted in a considerable enhancement of the photovoltaic properties of silole-containing materials and especially the fill factor. However, as demonstrated by Mott-Schottky measurements, electropolymerized silole-containing materials showed a substantial degree of disorder and high density of states in the midgap, which negatively affected their photovoltaic properties. Atomic force microscopy (AFM) and phase imaging revealed the presence of phase segregation and heterogeneity of the silole-containing materials. Interestingly, introduction of siloles suppressed the cathodic (n-type) doping typical for polythiophenes. This work demonstrates that siloles show great promise as electron- acceptor groups for all-organic solar cells; however, further work is required to optimize the properties and performance of poly(thienylsilole)-based materials. 1. Introduction Organic photovoltaics have become an area of intense research over the past few years due to major environmental concerns and the desire to replace expensive silicon with readily available, inexpensive, and easily processable organic materials. The most promising organic solar cells to date are built using the so-called donor-acceptor concept, whereby the efficiency of an organic photoactive material is dramatically increased by adding acceptor units which facilitate the dissociation of photogenerated excitons and decrease the rate of recombination of photoexcited electrons and holes. 1,2 While the most efficient organic photovoltaic cells to date utilize fullerenes and fullerene derivatives such as PCBM as acceptors, 3-7 all-polymer devices have emerged as attractive alternatives (see, e.g., review 8 by Kroon et al. and references therein). However, polymer solar cells investigated so far typically suffered from poor transport of photoexcited carriers. 9,10 Specifically, the transport problems manifest themselves through a low fill factor (FF). 9 What this means is that one typically needs to apply quite high internal electric fields to efficiently separate and collect photoexcited carriers in such cells; as a result, the cells cannot produce high photocurrents at voltages close to the open-circuit voltage V OC . Recently, we developed new donor-acceptor materials based on electropolymerized thiophenes modified with silole units in the main chain. 11 It was shown that introduction of silole, which is an electron acceptor relative to thiophene, 12-14 resulted in a remarkable increase in the photoefficiency of the polymer material as compared to the nonmodified polybithiophene. Importantly, the cells using the silole-containing material also showed a quite high fill factor, in contrast with nonmodified polymer based cells. In this paper we describe in detail our studies of the electrochemical and photoelectrochemical properties of a series of thiophene-silole donor-acceptor materials. Atomic force microscopy (AFM) was used to study the morphology and mechanical properties of the films, as well as the extent of phase segregation and heterogeneity of the silole-containing materials. 2. Experimental Section Equipment and Apparatus. Three separate 5 mL cells were used for electrochemical polymerization, electrochemical, and photoelectrochemical measurements, respectively. The cells were three-electrode Pyrex glass cells without separation of the anodic and cathodic compartments. The working electrode used for polymer deposition and measurement was a 2 mm platinum disk pressed into a Teflon holder. The counter electrodes were platinum wires. Silver pseudoreference electrodes were used. They were stored in monomer-free solutions containing the corresponding solvent and 0.1 M Bu 4 NPF 6 . The potential of such electrodes in acetonitrile was found to be +0.05 V versus SCE. 15 All the potentials in this paper are given versus the silver pseudoreference electrodes. The photoelectrochemical measure- ments were performed using a 20 mW, 405 nm laser diode model LD1510 (Power Technology, Little Rock, AR). The photon flux used was measured using a calibrated Si photodiode and was found to be 1.8 × 10 17 s -1 · cm -2 after correcting for the difference in the diameters of the laser beam and the electrode (3 and 2 mm, respectively). This flux at 405 nm corresponds to a power of 90 mW cm -2 . No correction for the light absorption by the cell walls and electrolyte was made. In all electrochemical, photoelectrochemical, and impedance mea- surements, the electrode was polarized using a model PAR 263A potentiostat-galvanostat (Princeton Applied Research) con- trolled by a version 2.8 CorrWare electrochemical software (Scribner Associates Inc.). For Mott-Schottky measurements, the potentiostat was coupled to a model 1250 frequency response analyzer (Solartron) controlled by a version 2.8 ZPlot software (Scribner Associates Inc.). AFM measurements were performed using a Multimode AFM (Veeco Metrology) equipped with a Nanoscope IV controller (Veeco). Silicon PointProbe tips * Corresponding author. E-mail: E-mail: osemenik@uwo.ca. J. Phys. Chem. B 2009, 113, 15715–15723 15715 10.1021/jp904428p CCC: $40.75  2009 American Chemical Society Published on Web 11/10/2009