Complementary Hydrogen Bonding and Block Copolymer Self-Assembly in Cooperation toward Stable Solar Cells with Tunable Morphologies

We report the synthesis and characterization of a polythiophene diblock copolymer selectively functionalized with 1-n-hexylisoorotic acid moieties (P4) and a 2,6-diaminopyridine tethered fullerene derivative (PCBP). Self-assembly between P4 and PCBP through three-point complementary hydrogen bonding interactions is utilized to control and stabilize blend morphologies. These interactions have been studied both in solution and in solid state by H-1 NMR and UV-vis spectroscopies as well as optical and atomic force microscopies (AFM). Solar cells employing P4 blended with different weight ratios of PCBP and phenyl-C61-butyric acid methyl ester (PCBM) were fabricated and tested. The best power conversion efficiencies (PCEs) were observed in devices made from P4/PCBP blends (10/8 by wt) and ternary blends of P4/PCBP/PCBM (10/4/4 by wt) as active layers. Thermal stabilities of these solar cells were studied in detail by aging tests, and corresponding morphological changes were closely monitored by absorption spectroscopy, optical microscopy, AFM, and X-ray analyses. The three-point complementary hydrogen bonding interactions between P4 and PCBP, in cooperation with block polymer self-assembly, were found to not only improve the thermal stability of solar cells significantly but also lead to tunable active layer morphologies. Nanostructures with long-range order were identified in blend films employing P4, which has never been observed before in conventional polymer/fullerene bulk heterojunction (BHJ) films.