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 VOC 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%.