Thieno[3,4-c]pyrrole-4,6(5H)-dione Polymers with Optimized Energy Level Alignments for Fused-Ring Electron Acceptor Based Polymer Solar Cells

Rapid advances have been recently demonstrated in polymer solar cells (PSCs) with fused-ring electron acceptors (FREAs), which have low bandgap and high electron mobility. Semiconducting polymer donors with medium bandgap to complement the absorption and proper energy level alignments to minimize energy loss are preferred in this system, but there are few studies on them. Here, we explore thieno[3,4-c]pyrrole-4,6(5H)-dione (TPD) based polymers for high performance PSCs with FREAs. A new TPD polymer, PMOT16, is developed with 4-methoxyl thiophene as conjugated side chains on the benzo[1,2-b:4,5-b′]dithiophene unit. PMOT16 exhibits lower energy levels and enhanced interactions compared to the thiophene counterpart, PBDTT-6ttTPD. However, in PSCs with ITIC as the acceptor, PMOT16 shows inferior performance to PBDTT-6ttTPD on short circuit current (J_SC) and fill factor. When IDIC with lower energy levels is employed as acceptor, PMOT16 PSCs show decent power conversion efficiencies (PCEs) of around 10% with low energy loss, which surpasses that of PBDTT-6ttTPD due to increase of open circuit voltage. It is found that the lower J_SC and inferior PCE in PMOT16:ITIC are ascribed to the approaching of energy levels between PMOT16 and ITIC. Our studies highlight the potential of TPD based polymers for high performance FREA-PSCs and the necessity for tuning energy level alignments.