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 efficiencies [highest power conversion efficiency (PCE) of 2.6%] in a standard organic photovoltaic architecture, when combined with PC71BM after a brief thermal annealing, than the S-containing molecule 1 (highest PCE of 1.0%). Studies of the resulting morphologies of BHJs based on 1 and 2 showed that one-atom substitution could engender important differences in the solubilities, which then influenced the crystal orientations of the small molecules within this thin layer. Brief thermal annealing resulted in rotation of the crystalline grains of both molecules to more energetically favorable configurations.