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

Abstract

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: ∼−3.15 eV, E g ec = 2.28 eV ) as well as identical thin film absorption behaviour ( λ 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 λ e=568 nm (with a fluorescence quantum yield of Φ 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 OC, as high as 900 mV and power conversion efficiency, η 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.