Novel Narrow Band-Gap Copolymers Based on Pyridinthiadiazole for Semitransparent Organic Solar Cells

The performance of semitransparent organic solar cells (ST-OSCs) largely lags behind that of traditional opaque OSCs because of the limited absorption region and high energy loss. It is difficult to balance the power conversion efficiency (PCE) and average visible transmittance (AVT) in ST-OSCs. In order to maximize PCE and high AVT simultaneously, we rationally design the novel narrow-band-gap donors to endow high-performance ST-OSCs with avoiding overlap of light absorption regions and sufficient driving force. The two donor polymers combined with NFAs not only ensured energy alignment but also made good utilization of near-infrared light. In this study, two novel copolymers were designed and synthesized to pair with NFAs to achieve high-voltage and light absorption complementation for high-performance ST-OSCs. The two donor polymers, namely, PBDTF-tPTZ and PBDTF-ttPTZ, were constructed by incorporating pyridine [1,2,5]thiadiazole (PTZ) as an electron-deficient moiety and benzodithiophene (BDT) as a donor unit. Besides, 3-undecylthiophene and 3-undecylthieno[3,2-b]thiophene were introduced into the molecular skeleton as different pi-bridge units to optimize the copolymers. The two PTZ copolymers showed good light absorption complementation and energy alignment with Y6. In addition, both blend films can effectively penetrate visible-light region to guarantee high AVT and open-circuit voltage (V-OC). As a result, a high PCE of 11.2% and 8.11% was achieved in PBDTF-ttPTZ:Y6-based opaque and semitransparent devices, respectively. More importantly, the AVT of the optimal device is close to 40% and the best light utilization efficiency (LUE) is high to 3.23%, indicative of the excellent color rendering and aesthetic value of the ST-OSCs.