Narrow band-gap materials with overlapping absorption simultaneously increase the open circuit voltage and average visible transmittance of semitransparent organic solar cells

Most semitransparent organic solar cells (ST-OSCs) show a low open-circuit voltage (V-OC) because of the inherent narrow band-gap of the active layer materials, which is proven to be a key limitation for the improvement of the device performance. To obtain a high V-OC, wide-band gap polymer donors, such as PM6 are often selected for ST-OSCs, but the absorption in the visible light region of such wide-band gap polymer donors is not conducive to the transparency of the devices. Therefore, rational design of active materials to improve the V-OC and device efficiency without sacrificing transparency is critical to realize ST-OSCs for practical applications. Herein, we propose a new design concept of combination of donors and acceptors with low energy levels and overlapping near-infrared absorption for high performance ST-OSCs. We incorporate chlorine, sulfur and fluorine functional atoms into the polymer PCE10 to down-shift the energy levels of the resulting polymer donors, namely PCE10-2Cl, PCE10-SF and PCE10-2F, respectively. All these functionalized polymer donors show lower energy levels than PCE10, ensuring a significantly higher V-OC. Especially, the narrow band-gap PCE10-2Cl having overlapping absorption with the non-fullerene acceptor IT-4F not only obtains a remarkably improved V-OC, but also achieves a comparable photocurrent to the blends with complementary absorption. Subsequently, the PCE10-2Cl:IT-4F-based device delivers a champion power conversion efficiency (PCE) of 10.72% among the functionalized PCE10-based devices. For the ST-OSC device with PCE10-2Cl:IT-4F, a PCE of 8.25% with an average visible transmittance (AVT) of 33% is achieved without any extra treatment, which is an outstanding performance of ST-OSCs with an AVT of over 30%. These results indicate that a rational combination of narrow band-gap donors and acceptors with overlapping absorption is a promising strategy to fabricate high performance ST-OSCs