Eur. Phys. J. Appl. Phys. (2006) DOI: 10.1051/epjap:2006142 T HE EUROPEAN P HYSICAL JOURNAL APPLIED PHYSICS Side chain effects on photoinduced absorption and photovoltaic performance of low bandgap thienylene vinylene and phenylene vinylene copolymers L.H. Nguyen 1, a , S. G¨ unes 1 , H. Neugebauer 1 , N.S. Sariciftci 1 , K. Colladet 2 , S. Fourier 2 , T.J. Cleij 2 , L. Lutsen 3 , J. Gelan 2, 3 , and D. Vanderzande 2, 3 1 Linz Institute for Organic Solar Cells (LIOS), Johannes Kepler University, Altenbergerstr. 69, 4040 Linz, Austria 2 Hasselt University, Institute for Materials Research (IMO), Agoralaan, Building D, 3590 Diepenbeek, Belgium 3 IMEC, Division IMOMEC, Wetenschapspark 1, 3590 Diepenbeek, Belgium Received: 21 July 2006 / Accepted: 6 October 2006 Published online: 6 December 2006 – c  EDP Sciences Abstract. In this work low bandgap thienylenevinylene and phenylene vinylene copolymers, which possess either 3,4-ethylenedioxylthiophene (EDOT) groups (Polymer 1) or long alkyl side chains (Polymer 2) were investigated and compared in photoinduced electron transfer properties and photovoltaic performance. The results show that the interaction of the photoexcited polymers with an electron acceptor ([6,6]-phenyl C61 – butyric acid methyl ester (PCBM)) leads to charge generation and transfer for both polymers. We found that the long alkyl side chain in Polymer 2 instead of the EDOT group in Polymer 1 enhances the open circuit voltage (VOC ) but lowers the short circuit current (ISC ). On the other hand the long alkyl side chain in Polymer 2 significantly improves the solubility and enhances processability for solar cells fabrication. Optimization of the chemical structure of these low bandgap polymers could lead to a spectral improvement of photocurrent generation in organic solar cells. PACS. 85.60.-q Optoelectronic devices – 85.60.Bt Optoelectronic device characterization, design, and modeling – 85.60.Dw Photodiodes; phototransistors; photoresistors 1 Introduction Organic photovoltaic devices are promising candidates as renewable source of electrical energy because of its ease in fabrication and low production cost as well as light weight and flexibility. The best reported devices are today able to achieve 2.5–5% power efficiency [1,2] employing the bulk heterojunction concept which ensures a large interfacial area for efficient charge generation. However, the demand for overcoming the mismatch of the absorption spectrum and the solar emission may continue to enhance the photo- voltaic performance. Low bandgap conjugated polymers, which absorb at longer wavelengths, have appeared as a new approach and still remain the focus of strong inter- est for organic solar cells application because they may improve the efficiency compared to other traditional ma- terials as polyphenylene-vinylenes and polythiophenes. One of the most promising strategies to tailor the en- ergy levels of conjugated polymers is the donor-acceptor route concept from which the interaction between alter- nating electron rich donors and electron deficient accep- a e-mail: lehuong.nguyen@jku.at; Lhnguyen@gmx.at tors will result in a compressed band gap [3–5]. In this work we used conjugated polymers based on bis-(1-cyano- 2-thienylvinylene)phenylene whose structure consists of a central dialkoxyphenylene core p-disubstituted by two thiophene derivatives through a cyanovinylene linker. We investigated and compared these two polymers which pos- sess either EDOT (namely Polymer 1, P 1) or long alkyl side chain C 8 H 17 on the thiophene part (namely Poly- mer 2, P 2) towards photoinduced electron transfer and photovoltaic performance. A further decrease of the oxi- dation potential was obtained by the incorporation of the more electron rich EDOT (P 1) while the long alkyl side chain on the thiophene part played the role of extra sol- ubilizing chains [5]. The results show that the interaction of the photoexcited polymers with an electron acceptor ([6,6]-phenyl C61 – butyric acid methyl ester (PCBM)) leads to charge generation and transfer for both polymers. We found that the long alkyl side chain in P 2 instead of the EDOT group in P 1 enhances the open circuit voltage (V OC ) but lowers the short circuit current (I SC ). A corre- lation between the photophysical study, achievable solar cell performances and corresponding nanomorphologies is drawn in conclusion. Article published by EDP Sciences and available at http://www.edpsciences.org/epjap or http://dx.doi.org/10.1051/epjap:2006142