9,9′-Bifluorenylidene-diketopyrrolopyrrole donors for non-polymeric solution processed solar cells

We have synthesised new materials comprised of 9,9′-bifluorenylidene and dithienodiketopyrrolopyrrole units. While 9,9′-bifluorenylidene has been primarily used in non-fullerene acceptors, when used in combination with the diketopyrrolopyrrole moiety it can form donor materials that can be used in conjunction with fullerene acceptors. The compounds differ in the substituents on the 9,9′-bifluorenylidene (protonated = 1A and dimethoxy = 1B) moiety. The structure of both the neat and blend films with PC BM were found to be strongly dependent on the processing solvent used. In particular, Grazing Incidence Wide Angle X-ray Scattering measurements of films of 1A or 1B blended with PC BM prepared from chloroform or chloroform with o-dichlorobenzene as an additive showed that the donor material had no particular ordering. However, when 1,8-diiodooctane was added to the processing solvent the 1A blends showed liquid crystalline ordering while 1B formed well-defined crystallites with three-dimensional ordering. The difference in film structure had a profound effect on the device properties. For 1A the optimised blend ratio with PC BM was 1:4, but when 1,8-diiodooctane was used as the additive the best ratio of 1A to fullerene was 1:1. The films containing the well-defined crystallites of 1B all performed poorly, which was ascribed to a lack of a percolation pathway for hole extraction. The best performing device was comprised of a 1:1 blend of 1A and PC BM, which had a power conversion efficiency of 2.6%.