Alternating Copolymers of Dithienyl-s-Tetrazine and Cyclopentadithiophene for Organic Photovoltaic Applications

Abstract

As a new emerging electron deficient building block, s-tetrazine (Tz) shows high electron affinity, which is even higher than the commonly used benzothiadiazole units. This property makes Tz a very promising electron withdrawing unit for low band gap conjugated polymers, especially for organic solar cell materials. We report the synthesis and property of five alternating copolymers of s-tetrazine and cyclopenta[2,1-b:3,4-b′]dithiophene (CPDT), which are bridged with a thiophene unit. Methyl, hexyl, and/or 2-ethylhexyl groups are introduced onto thiophene and CPDT units to tune the solubility, UV absorption, frontier molecular orbital energy levels, and interchain stacking property of the resulting polymers. These polymers are stable up to 220 °C and decompose to dinitrile compounds with the breaking of Tz linkage at a higher temperature. Efficient bulk heterojunction solar cells were fabricated by blending these polymers with (6,6)-phenyl-C71-butyric acid methyl ester (PC71BM), and they reached a power conversion efficiency up to 5.53% under simulated AM 1.5 G irradiation of 100 mW/cm2. The morphological structures of the active layers from different polymers or under different processing conditions were then analyzed by atomic force microscopy (AFM) and correlated with their device performance.