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.