Ordered p-conjugated polymer backbone in amorphous blend for high efficiency nonfullerene organic photovoltaics

In & pi;-conjugated polymers, the amorphous region absent from & pi;-& pi; stacking order typically limits polymer functions compared to the crystalline region with high & pi;-& pi; stacking order. Here, we show that a benzodithiophene-thiazolothiazole copolymer containing tripropylsilyl groups (PSTz2) has a greater coplanar backbone structure when the & pi;-& pi; stacking order is absent, such as in solution and in a film blended with a nonfullerene acceptor, than when it is present in a neat film. The excitons and charge carriers generated in PSTz2 are more highly delocalized in the blend film than in the neat film, presumably through the backbone. The unconventional structural feature of PSTz2 shows higher photovoltaic performance in nonfullerene-based cells compared to its alkyl-functionalized counterpart. Our results show that it is possible to develop & pi;-conjugated polymers that perform well in amorphous blends due to the ordered backbone structure. The amorphous regions of & pi;-conjugated polymers typically show lower functionality than the crystalline regions with high & pi;-& pi; stacking order. Here, a benzodithiophene-thiazolothiazole copolymer shows a greater coplanar backbone structure when the & pi;-& pi; stacking order is absent compared to the crystalline state.

Automated summary

In this study, a benzodithiophene-thiazolothiazole copolymer with tripropylsilyl groups (PSTz2) was found to have a larger coplanar backbone structure in solutions and blended films with nonfullerene acceptors, compared to its crystalline state. The excitons and charge carriers in PSTz2 were more easily delocalized in the blend films, potentially through the backbone. This unconventional structural feature resulted in higher photovoltaic performance in nonfullerene-based solar cells compared to its alkyl-functionalized counterpart.