Donor–Acceptor Small Molecules for Organic Photovoltaics: Single-Atom Substitution (Se or S)

Two isostructural low-band-gap small molecules that contain a one-atom substitution, S for Se, were designed and synthesized. The molecule 7,7′-[4,8-bis(2-ethylhexyloxy)benzo[1,2-b:4,5-b′]dithiophene]bis[6-fluoro-4-(5′-hexyl-2,2′-bithiophen-5-yl)benzo[c][1,2,5]thiadiazole] (1) and its selenium analogue 7,7′-[4,8-bis(2-ethylhexyloxy)benzo[1,2-b:4,5-b′]dithiophene]bis[6-fluoro-4-(5′-hexyl-2,2′-bithiophen-5-yl)benzo[c][1,2,5]selenodiazole] (2) are both based on the electron-rich central unit benzo[1,2-b:4,5-b′]dithiophene. The aim of this work was to investigate the effect of one-atom substitution on the optoelectronic properties and photovoltaic performance of devices. Theoretical calculations revealed that this one-atom variation has a small but measurable effect on the energy of frontier molecular orbital (HOMO and LUMO), which, in turn, can affect the absorption profile of the molecules, both neat and when mixed in a bulk heterojunction (BHJ) with PC71BM. The Se-containing variant 2 led to higher efficie...