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

Abstract

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 1H 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.