Benzo[1,2-<i>b</i>:4,5-<i>b</i>′]dithiophene–Pyrido[3,4-<i>b</i>]pyrazine Small-Molecule Donors for Bulk Heterojunction Solar Cells

Abstract

We report on the synthesis, material properties, and bulk heterojunction (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.