Novel polythiophene derivatives functionalized withconjugated side-chain pendants comprisingtriphenylamine/carbazole moieties for photovoltaic cellapplications†

We synthesized a series of polythiophenes (PTs) featuring 2-ethylhexyl-substituted terthiophene (T) orquaterthiophene (BT) as the conjugated unit in the polymer backbone with pendant conjugated tertbutyl-substituted triphenylamine (tTPA)- or carbazole (tCz)-containing moieties as side chains, namelyPTtTPA, PBTtTPA, PTtCz and PBTtCz. Incorporating T and BT moieties into the polymer backbone andattaching tTPA or tCz units promoted efficient conjugation within the extended conjugated frameworksof the polymers, resulting in lower band-gap energies and red-shifting of the maximal UV-Visabsorption wavelength. The higher electron-donating ability of tTPA resulted in broader absorptionbands and lower band-gap energies of PTtTPA and PBTtTPA as compared with PTtCz and PBTtCz.Incorporation of the T and BT moieties into the polymer backbone enhanced the compatibility of PTand the fullerene derivative by reducing the side-chain density of PT, thus providing sufficient freevolume for efficient incorporation of [6,6]phenyl-C61-butyric acid methyl ester (PC61BM) into thepolymer chains. Polymer solar cells (PSCs) were fabricated by spin-coating a blend of each PT with thefullerene derivative (PC61BM) as a composite film-type photoactive layer; PBTtTPA/PC61BM-based PSCsshowed superior photovoltaic (PV) performance to PTtTPA/PC61BM-based PSCs in terms of conjugationand absorption band broadness. However, PBTtCz/PC61BM-based PSCs showed inferior PV performanceto PTtCz/PC61BM-based PSCs. The lower HOMO level led to a higher open-circuit voltage (Voc; 0.74 V)and larger photo-energy conversion efficiency (h; 2.77%) of PTtCz/PC61BM-based PSCs