Thiophene-containing poly(arylene-ethynylene)-alt-poly(arylenevinylene)s:: Synthesis, characterisation and optical properties

This work describes the synthesis and characterisation of two types of thiophene-containing poly(arylene-ethynylene)-alt-poly(arylene-vinylene)s (PAE-PAV) copolymers, whose repeating units (-Ph-C C-Th-CH=CH-Ph-CH=CH-)(n), 5, and (-Th-C C-Ph-C C-Th-CH=CH-Ph-CH=CH-)(n), 8a-c, consist, respectively, of a 1:2 and a 2:2 ratio of triple bond/double bond moieties. Comparison of their photophysical, electrochemical and photovoltaic properties has been carried out. Although similar electrochemical data (HOMO: - 5.43 eV, LUMO: similar to-3.15 eV, E-g(cc) = 2.28 eV) as well as identical thin film absorption behaviour (lambda(a) = 500 nm, E-g(opt) = 2.10 eV) were obtained for both types of materials, significant differences in their thin film photoluminescence behaviour and photovoltaic properties were observed. While polymer 5 shows a fluorescence maximum at lambda(e) = 568 nm (with a fluorescence quantum yield of Phi(f) = 7%), a total fluorescence quenching was observed in 8. Far better photovoltaic performance was obtained from solar cells (set up: ITO/PEDOT:PSS/active layer/LiF/Al; active layer consisting of 5 or 8b as donor and PCBM as acceptor in a 1:3 ratio by weight) designed from 5 than from 8b. Open circuit voltage, V proportional to, as high as 900 mV and power conversion efficiency, eta(AM1.5), around 1.2% were obtained. This can be attributed to the 1:2 triple bond/double bond ratio as well as the grafting of shorter octyloxy and 2-ethylhexyloxy side chains in 5 and to its comparatively higher molecular-weight. (c) 2005 Elsevier Ltd. All rights reserved.