The Effect of Solvent Additive on the Charge Generation and Photovoltaic Performance of a Solution-Processed Small Molecule:Perylene Diimide Bulk Heterojunction Solar Cell

Abstract

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