Facile preparation of small molecules for bulk heterojunction solar cells

Abstract

Two solution-processable diketopyrrolopyrrole (DPP)–pyrene–DPP small molecules PYDPP-EH and PYDPP-BO with 2-ethylhexyl (EH) and 2-butyloctyl (BO) side chains at DPPs, respectively, are synthesized for organic photovoltaic devices with only two step reactions from commercially available materials without using toxic stannyl intermediates and potentially dangerous lithiation reactions. Compared with PYDPP-BO or the pyrene–DPP–pyrene type SMs with very long N-alkyl substitutes to ensure their solubility reported by other groups, the shorter but more 2-ethylhexyl groups endow PYDPP-EH with not only enough solubility to be solution-processed but also stronger π–π interactions, more ordered self-aggregation as well as better intermolecular charge transport in films, which induce the blend films with [6,6]-phenyl-C₆₁-butyric acid methyl ester (PC₆₁BM) with higher hole mobility and the solar cells with more efficient photocurrent response and a higher power conversion efficiency of 4.64%. This study manifests that subtly changing molecular structure with shorter but more alkyl chains can enhance photovoltaic performance.