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

Abstract

Most semitransparent organic solar cells (ST-OSCs) show a low open-circuit voltage (VOC) 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 VOC, 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 VOC 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 VOC. Especially, the narrow band-gap PCE10-2Cl having overlapping absorption with the non-fullerene acceptor IT-4F not only obtains a remarkably improved VOC, 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.