Enhanced Performance of Bulk Heterojunction Solar Cells Using Novel Alternating Phenylenevinylene Copolymers of Low Band Gap with Cyanovinylene 4-Nitrophenyls

Abstract

Two novel low band gap soluble copolymers, P1 and P2, were synthesized and characterized. P1 consisted of alternating dihexyloxyphenylene and α-[[4-(diphenylamino)phenyl]methylene]-4-nitro-benzeneacetonitrile. P2 consisted of alternating dihexyloxyphenylene and α,α′-[(1,4-phenylene)dimethylidyne]bis(αZ,α′Z)-4-nitrobenzeneacetonitrile. These copolymers showed broad absorption curves with long-wavelength absorption maximum around 620 nm and optical band of 1.68 and 1.64 eV for P1 and P2, respectively. Both P1 and P2 were blended with PCBM to study the photovoltaic response of bulk heterojunction (BHJ) solar cells. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels of both P1 and P2 are well aligned with those of PCBM acceptor. This allows efficient photoinduced charge transfer and high open circuit voltage, leading to an overall power conversion efficiency (PCE) of 3.15% and 2.60% for the as cast P1:PCBM- and P2:PCBM-based devices, respectively. The PCE of the devices has been further improved up to 4.06% and 3.35% for the devices based on thermally annealed P1:PCBM and P2:PCBM blends, respectively.