Monodispersed vs. polydispersed systems for bulk heterojunction solar cells: the case of dithienopyrrole/anthracene based materials

The article reports on the properties of a new class of arylene–ethynylene semiconductors incorporating anthracene and the bridged bithiophene dithienopyrrole. Two monodispersed structures were synthesised: the first with a dithienopyrrole core bound to two anthracenyl–ethynyl side groups namely the 2,6-bis(anthracen-9-ylethynyl)-4-(2-ethylhexyl)-4H-dithieno[3,2-b:2′,3′-d]pyrrole (ADA); in the second structure the anthracene core was functionalised with two dithienopyrrolylethynyl groups, obtaining 9,10-bis((4-(2-ethylhexyl)-4H-dithieno[3,2-b:2′,3′-d]pyrrol-2-yl)ethynyl)anthracene (DAD). The properties of these materials were compared with those of the corresponding polymer: poly[4-(2-ethylhexyl)-4H-dithieno[3,2-b:2′,3′-d]pyrrole-2,6-diylethynylene-anthracen-9,10-diylethynylene] (polyAD). Devices employing PC61BM as an electron acceptor revealed that the monodispersed materials (ADA and DAD) were better performing than polyAD, seemingly due to the better homogeneity of the donor–acceptor blend, as revealed by AFM. The PCE value (1.3%) obtained with DAD ranks among the highest reported for non-polymeric small molecule-based BHJ solar cells constructed without the use of additives or annealing processes, thus demonstrating that ethynylene-containing electron-rich systems are promising donors for organic solar cell applications.