Effect of capping group on the properties of non-polymeric diketopyrrolopyrroles for solution-processed bulk heterojunction solar cells

Abstract

Abstract Solution-processable non-polymeric organic semiconductors are attractive for opto-electronic applications due to their relatively simple synthetic reproducibility and characterization, and enhanced capability for fine-tuning of their properties. We report the synthesis, charge transport, photophysics, and photovoltaic properties of three non-polymeric materials based upon bisarylthiophenyl diketopyrrolopyrrole [DPP(ThAr)2]. The DPP(ThAr)2 molecules are comprised of solubilizing alkyl groups, 2-ethylhexyl (for 1a) and 2-octyldodecyl (for 1b and 2), which are capped with electron accepting moieties - fluorenone (1) or benzothiadiazole (2). While the materials could be prepared under standard Suzuki-Miyaura cross-coupling conditions, a simple direct arylation afforded 2 and 1b in good yields. We found 1a with a short alkyl chain lacked sufficient solubility for solution processing but 1b and 2 are solution processable and form good quality films. All three materials exhibited ambipolar charge transport in field effect transistors, with 1b showing balanced charge mobilities of about 10−3 cm2 V−1 s−1. Bulk heterojunction solar cells of 1b or 2 with PC71BM were found to have high open-circuit voltages (up to 0.9 V) with power conversion efficiencies of up to 4.5% and 2.6%, respectively.