Donor-acceptor alternating copolymers as donor materials for bulk-heterojunction solar cells: effects of molecular structure on film morphology and device performance.

Three photovoltaic-applicable donor-acceptor (D-A) alternating copolymers including poly{(9,9-dihexyl-9H-fluorene-2,7-ylene)-alt-2-(2,6-bis((E)-2-(5-bromo-3,4-dihexylthiophen-2-yl)vinyl)-4H-pyran-4-ylidene) malononitrile} (PFTMT), poly{(10-hexyl-10H-phenothiazine-3,7-ylene)-alt-2-(2,6-bis((E)-2-(5-bromo-3,4-dihexylthiophen-2-yl)vinyl)-4H-pyran-4-ylidene) malononitrile} (PPTMT) and poly{(2,20-bithiophene-5,50-ylene)-alt-2-(2,6-bis((E)-2-(5-bromo-3,4-dihexylthiophen-2-yl)vinyl)-4H-pyran-4-ylidene)malononitrile} (PDTTMT), were blended with [6,6]-phenyl-C(61)-butyric acid methyl ester (PCBM) to serve as active layers for photovoltaic applications. The effects of extrinsic (blend ratio and solvent) and intrinsic factors (donor materials) on the morphologies of this series of active layers were investigated by atomic force microscopy (AFM) and transmission electron microscopy (TEM). It was found that the PFTMT:PCBM active layers show distinct phase segregation with large PCBM clusters above 100 nm and are strongly affected by solvent evaporation rate in higher blend ratios. In contrast, the PPTMT:PCBM active layers are homogeneous and not affected by blend ratios and solvents, while the PDTTMT:PCBM active layers show an interpenetrating network initially formed at the blend ratio of 1:1. These results indicate that the polymer-PCBM repulsions arising from the molecular structure of the polymers play a significant role in determining the resulting morphologies of the blend films. Strong PFTMT-PCBM repulsion leads to large-scale phase segregation, while weak repulsions in PPTMT-PCBM and PDTTMT-PCBM favor small-scale phase segregation only. The best photovoltaic power conversion efficiencies are obtained from PDTTMT-based solar cells with the PDTTMT:PCBM blend ratio of 1:3 and nanoscale phase separation of the active layer, where a good balance is formed between a large donor-acceptor interface and the continuous paths of donor and acceptor phase for opposite charge carrier transport to their corresponding electrodes.