Novel polythiophene derivatives functionalized with conjugated side-chain pendants comprising triphenylamine/carbazole moieties for photovoltaic cell applications

We synthesized a series of polythiophenes (PTs) featuring 2-ethylhexyl-substituted terthiophene (T) or quaterthiophene (BT) as the conjugated unit in the polymer backbone with pendant conjugated tert-butyl-substituted triphenylamine (tTPA)- or carbazole (tCz)-containing moieties as side chains, namely PTtTPA, PBTtTPA, PTtCz and PBTtCz. Incorporating T and BT moieties into the polymer backbone and attaching tTPA or tCz units promoted efficient conjugation within the extended conjugated frameworks of the polymers, resulting in lower band-gap energies and red-shifting of the maximal UV-Vis absorption wavelength. The higher electron-donating ability of tTPA resulted in broader absorption bands 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 PT and the fullerene derivative by reducing the side-chain density of PT, thus providing sufficient free volume for efficient incorporation of [6,6]phenyl-C-61-butyric acid methyl ester (PC61BM) into the polymer chains. Polymer solar cells (PSCs) were fabricated by spin-coating a blend of each PT with the fullerene derivative (PC61BM) as a composite film-type photoactive layer; PBTtTPA/PC61BM-based PSCs showed superior photovoltaic (PV) performance to PTtTPA/PC61BM-based PSCs in terms of conjugation and absorption band broadness. However, PBTtCz/PC61BM-based PSCs showed inferior PV performance to PTtCz/PC61BM-based PSCs. The lower HOMO level led to a higher open-circuit voltage (V-oc; 0.74 V) and larger photo-energy conversion efficiency ((eta); 2.77%) of PTtCz/PC61BM-based PSCs.