Organic bulk heterojunction solar cells based on solution processable small molecules (A-π-A) featuring 2-(4-nitrophenyl) acrylonitrile acceptors and phthalimide-based π-linkers

A novel small molecule-based organic donor SM [(2Z,2'Z)-3,3'-(((1E,1'E)-(2-cyclohexyl-1,3-dioxoisoindoline-4,7-diyl)bis(ethene-2,1-diyl))bis(4,1-phenylene))bis(2-(4-nitrophenyl)-acrylonitrile)] featuring 2-(4-nitrophenyl) acrylonitrile as the acceptor and a pi-conjugation bridge composed of phthalimide and styryl units, with an A-pi-A type structure, has been synthesized. It showed a long wavelength absorption band having an absorption maximum around 635 nm and the optical bandgap was 1.63 eV, which is lower than most reported conjugated polymers, including poly(3-hexylthiophene) (P3HT). The photovoltaic properties were investigated by constructing bulk heterojunction organic solar cell devices using SM as the electron donor and fullerene derivatives, i.e. PC60BM and PC70BM as the electron acceptors with the device architecture ITO/PEDOT:PSS/SM:PC60BM or PC70BM/Al. The effect of the SM/fullerene derivative weight ratio and the processing solvent were carefully investigated to improve the performance of the organic solar cells. The optimized organic solar cell with SM:PC60BM and SM:PC70BM cast from THF solvent, at a weight ratio of 1 : 3 showed power conversion efficiencies (PCEs) of about 1.70% and 2.56%, respectively. The enhanced value of PCE for the BHJ photovoltaic device based on PC70BM is related to the better absorption of PC70BM in the visible region compared to that of PC60BM. The SM:PC70BM blends cast from a DIO-THF mixture and subsequent thermal annealing exhibited improved PCEs of 3.68% and 4.14%, respectively.