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, π-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%.