Spacer Engineering for 2D Ruddlesden–Popper Perovskites with an Ultralong Carrier Lifetime of Over 18 μs Enable Efficient Solar Cells

2D Ruddlesden–Popper (RP) perovskites have become emerging photovoltaic materials due to their outstanding optoelectronic properties and intrinsic structure stability. Here, two structurally similar organic spacers with conjugated and unconjugated unites, namely FuMA and THFMA, were developed to study their effects on the photophysical properties of 2D RP perovskites. A very important finding is that the 2D perovskite film (n = 4) based on FuMA with a conjugated furan unit exhibits an ultralong average carrier lifetime of 18.03 μs, which could be attributed to the enlarged dielectric constant, reduced exciton binding energy, and decreased electron–phonon coupling coefficients of the FuMA-based 2D RP perovskites. The optimized device based on the FuMA spacer achieves a high PCE of 18.00% with negligible hysteresis, much higher than that of the THFMA-based device (PCE = 13.79%). This work opens a new avenue for developing 2D RP perovskite films with ultralong carrier lifetimes for photovoltaic and other optoelectronic applications.