Effects of alkyl-substituent length on photovoltaic performance of bulk heterojunction solar cells utilizing non-peripherally octaalkyltetrabenzotriazaporphyrins

Abstract

We report alkyl-substituent length dependences of optical and electronic characteristics, crystal structures, and photovoltaic performances of bulk-heterojunction organic solar cells utilizing non-peripherally alkyl-substituted tetrabenzotriazaporphyrin (CnTBTAPH2). By extending the pentyl (n = 5) to octyl (n = 8) substituents, the molecular alignment was affected and the intercolumnar packing was probably modified from pseudo-hexagonal structure to 2D rectangular lattice. Furthermore, the devices utilizing the C5TBTAPH2 and C6TBTAPH2 discotic macrocycles with the short-alkyl-substituents exhibited a relatively high photovoltaic performance with a power conversion efficiency of around 4.7% in comparison with those utilizing the long-alkyl-substituted macrocycles. These differences in the device performance could be understood by taking the hole mobility and highest occupied molecular orbital energy level into consideration.