The effect of solvent additive on the charge generation and photovoltaic performance of a solution-processed small molecule: Perylene diimide bulk heterojunction solar cell

The photovoltaic performance and charge generation dynamics in thin film bulk heterojunction organic photovoltaic (BHJ OPV) devices comprising the small molecule donor 7,7′-(4,4-bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b′] dithiophene-2,6-diyl)bis(6-fluoro-4-(5′-hexyl-[2,2′-bithiophen] -5-yl)benzo[c][1,2,5]thiadiazole) (p-DTS(FBTTh₂)₂) and a perylene diimide (PDI) electron acceptor are investigated with and without the processing additive 1,8-diiodooctane (DIO). UV-vis absorption spectroscopy indicates that the use of DIO during processing increases the structural order of both p-DTS(FBTTh₂)₂ and PDI compared to films cast from chlorobenzene alone. Excitation intensity dependent broadband vis-NIR transient absorption pump-probe experiments over a dynamic range from 100 fs to 100 μs reveal that, in blends processed without DIO, essentially none of the interfacial charge transfer states generated after exciton dissociation at the donor-acceptor interface split into spatially separated charge carriers. In contrast, in blends processed with 0.4 vol% DIO, geminate recombination is significantly reduced, and spatially separated charge carriers are generated. It appears that the drastic increase in the power conversion efficiency in p-DTS(FBTTh₂)₂:PDI BHJ OPV devices upon the use of DIO, from 0.13% to 3.1%, is a consequence of the increased solid state order of both p-DTS(FBTTh₂)₂ and PDI, which leads to a significant improvement of the exciton dissociation efficiency and makes this system among the most efficient non-fullerene BHJ organic solar cells to date.