Semitransparent organic solar cells with light utilization efficiency of 4% using fused-cyclopentadithiophene based near-infrared polymer donor

Semi-transparent organic solar cells (ST-OSCs) have attracted significant attention for various applications such as building-integrated photovoltaics and smart window technologies. Although various low-band gap non-fullerene acceptors have been developed, near-infrared (NIR) absorbing polymeric donors have been rarely reported for highly efficient ST-OSCs. In this study, we synthesize cyclopentadithiophene (CPDT) core-based NIR-absorbing polymer donors, PL1 and PL2. These polymer donors show strong NIR absorption capabilities with high transmittance in visible wavelength region and deep-lying HOMO levels for cascade energy level alignment with low-bandgap non-fullerene acceptors. The opaque device with PL2 and PCBM exhibits higher power conversion efficiency of 11.62% than that of the device with PL1 (PCE: 7.54%) due to lower charge-carrier recombination loss and balanced charge-carrier mobilities. Most significantly, incorporation of PL2 into ST-OSCs results in a PCE of 9.91 %, average visible transmittance of 40.4 % and light utilization efficiency (LUE) of 4.00 %. This work achieves the one of the highest LUE in ST-OSCs based on new NIR-absorbing polymer donors.