Solvent Annealing Effects in Dithieno[3,2-b:2',3'-d]pyrrole–5,6-Difluorobenzo[c][1,2,5]thiadiazole Small Molecule Donors for BHJ Solar Cells

Low-bandgap small molecule (SM) donors that can be solution-processed with fullerene acceptors (e.g. PC61/71BM) are proving particularly promising in bulk-heterojunction (BHJ) solar cells. Compared to their π-conjugated polymer counterparts, SM donors are well defined (monodispersed) and more synthetically modular –with relatively wide ranges of bandgaps achievable in stepwise couplings of various donor and acceptor motifs. However, the optimization of SM-fullerene morphologies and BHJ device efficiencies relies more specifically on the use of processing additives, post-processing thermal or solvent vapor annealing (SVA) approaches, and achieving adequate interpenetrating networks and structural order in BHJ thin films can be challenging. In this report, we examine the correlated effects of molecular structure and post-processing SVA on the BHJ solar cell performance of a set of π-extended SM donors composed of dithieno[3,2-b:2',3'-d]pyrrole (DTP) and 5,6-difluorobenzo[c][1,2,5]thiadiazole ([2F]BT) units. In these systems (SM1-3), the introduction of additional alkyl substituents and unsubstituted thiophene rings on the peripheral unit groups critically impacts the effects of SVA steps on BHJ solar cell efficiency. We show that the more π-extended and alkyl-substituted analogue SM3 stands out –with BHJ device efficiencies of ca. 6% obtained from SVA with CS2– while SVA-treated SM3-based active layers also show the most favorable ordering and carrier mobility patterns. However, unlike numbers of SM donors reported in recent years, DTP–[2F]BT SM analogues are in general not prone to dramatic performance variations in BHJ thin films cast with processing additives. Our results indicate that the role of SVA steps is not independent of the molecular structure of the SM donors used in the BHJ solar cells.