Benzo[1,2-b:4,5-b’]dithiophene-Pyrido[3,4-b]pyrazine Small-Molecule Donors for Bulk Heterojunction Solar Cells

Abstract

We report on the synthesis, material properties and BHJ solar cell characteristics of a set of π-conjugated small-molecule (SM) donors composed of benzo[1,2-b:4,5-b′]dithiophene (BDT) and pyrido[3,4-b]pyrazine (PP) units – examining the perspectives of alkyl-substituted PP acceptor motifs in SM designs. In these systems (SM1-4), both the type of side chains derived from the PP motifs and the presence of ring-substituents on BDT critically impact (i) molecular packing, and (ii) thin-film morphologies and charge transport in BHJ solar cells. With the appropriate side-chain pattern, the ring-substituted analogue SM4 stands out: achieving efficiencies of ca. 6.5% with PC71BM, and fine-scale morphologies comparable to those obtained with some of the best-performing polymer donors in BHJ solar cells. 1H-1H DQ-SQ NMR analyses are used to examine the distinct self-assembly pattern of SM4, expected to factor into the development of the BHJ morphology.