Robust Processing of Small-Molecule:Fullerene Organic Solar Cells via Use of Nucleating Agents

The power conversion efficiency (PCE) of small-molecule bulk heterojunction solar cells is highly sensitive to the “ink” formulation used to produce the photoactive layer. Here we demonstrate that the addition of nucleating agents renders device fabrication notably less susceptible to the ink composition, promising a route toward more robust processing of efficient devices over large areas and enabling more facile materials screening. We selected as a model system blends of 7,7-[4,4-bis­(2-ethylhexyl)-4<i>H</i>-silolo­[3,2-<i>b</i>:4,5-<i>b</i>]­dithiophene-2,6-diyl]­bis­[6-fluoro-4-(5-hexyl-[2,2-bithiophen]-5-yl)­benzo­[<i>c</i>]­[1,2,5]­thiadiazole]­(<i>p</i>-DTS­(FBTTh₂)₂) as the donor and [6,6]-phenyl C₇₁-butyric acid methyl ester (PC₇₁BM) as the acceptor because this is one of the small-molecule OPV blends with a device performance that is most sensitive to ink formulation, especially when used with the processing aid diiodooctane (DIO). Addition of DIO is essential to obtain high device performances; however, a notable increase in device performance is only achieved over a very narrow DIO content regime. Use of nucleating agents drastically changes this situation and leads to well-performing devices even at extreme levels of DIO. We thus start to address here one of the great challenges in organic solar cell research: the fact that, too often, only a very limited composition range leads to high efficiency devices. This means that for every new donor or acceptor a multitude of formulations have to be tested, including in combination with processing aids, to ensure that promising materials are not overlooked. The use of nucleating agents, thus, promises to render materials discovery more straightforward as this dependency of device performance with composition can be reduced