Discotic materials for organic solar cells: Effects of chemical structure on assembly and performance

Discotic materials have attracted remarkable interest for application in organic solar cells. We analyze a series of phenyl-substituted hexa-peri-hexabenzocoronenes (HBCs) with residue modifications blended with perylenediimide (PDI) as donor material. The effect of differing alkyl chain lengths of 6, 8, 12 and 16 carbon atoms, introduction of a triple bond linker between HBC core and residual phenyl group and a swallow tailed dialkylphenyl chain on the device performance is investigated. Detailed insight to device physics and morphology is gained by analysis of photoluminescence quenching, transient photovoltage and photocurrent decay experiments and atomic force microscopy. Complementary the investigations explain why using short alkyl side chains higher currents and consequently increased device performance can be achieved. We report an external quantum efficiency of over 27% for devices based on discotic molecules. In this work the molecular assembly and its impact on performance in photovoltaic devices is studied. The study also reveals pathways to further increase the quantum yield of small molecule organic solar cells. (C) 2009 Elsevier B.V. All rights reserved.