Design of a highly crystalline low-band gap fused-ring electron acceptor for high-efficiency solar cells with low energy loss

A fused-ring thiophene-thieno[3,2-b]thiophene-thiophene (4T)-based low-band gap electron acceptor, 4TIC, has been designed and synthesized for non-fullerene solar cells. The utilization of the 4T center core enhances the charge mobility of 4TIC and extends its absorption band edge to 900 nm, which facilitates its function as a very efficient low-band gap electron acceptor. The rigid, conjugated framework of 4T also offers a lower reorganization energy to facilitate lower V_OC energy loss. Femtosecond transient spectroscopy showed a level of polaron generation in 4TIC results in the more efficient transfer of energetic carriers higher than that seen with the benchmarked molecule, ITIC. Film morphology analysis has also shown that 4TIC has structural order that is more prominent than that of ITIC with a multiscale phase separation in the blend with donor polymer PTB7-Th. As a result, solar cells based on PTB7-Th and 4TIC exhibit a high power conversion efficiency of 10.43% and a relatively low non-ideal photon energy loss of 0.33 V. The low band gap and small energy loss make 4TIC suitable for tandem solar cells as a back-cell to reduce the transmission loss. As a demonstration, we fabricated series connection tandem solar cells incorporating 4TIC, which exhibts a high device performance of 12.62%.