Two-dimensional like conjugated copolymers for high efficiency bulk-heterojunction solar cell application: Band gap and energy level engineering

Three two-dimensional like conjugated copolymers PFSDCN, PFSDTA and PFSDCNIO, which consist of alternating fluorene and triphenylamine main chain, and different pendant acceptor groups (malononitrile, 1,3-diethtyl-2-thiobarbituric acid and 2-(1,2-dihydro-1-oxoinden-3-ylidene)malononitrile) with thiophene as pi-bridge, have been designed, synthesized and characterized. The structure-property relationships of the two-dimensional like conjugated copolymers were systematically investigated. The absorption spectra, band gaps, and energy levels of the polymers were effectively tuned by simply attaching different acceptor groups. As the electron-withdrawing ability of the acceptors increased, the band gaps of the polymers were narrowed from 2.05 to 1.61 eV; meanwhile, the LUMO energy levels of the polymers decreased from -3.27 to -3.75 eV, whereas their relatively deep HOMO energy levels of similar to-5.35 eV were preserved. BHJ solar cells were fabricated and characterized by using the three polymers as donor materials and the highest power conversion efficiency of 2.87% was achieved for the device based on PFSDTA:(6,6)-phenyl-C-71-butyric acid methyl ester blend.