Boosting Fill Factor of Semitransparent Donor‐Poor Organic Solar Cells for the Best Light Utilization Efficiency

Abstract Reducing the content of light‐absorbing material in the active layer of semitransparent organic solar cells (ST‐OSCs) enhances the average visible transmittance (AVT) but sacrifices the power conversion efficiency (PCE). This dilemma is a key challenge to ST‐OSCs. Here trityl tetrakis(pentafluorophenyl) borate (TrTPFB)‐doping of the polymer donor at a suboptimal donor:acceptor (D:A) ratio is reported as an approach to enhance the light utilization efficiency (LUE) via boosting the fill factor (FF). The suboptimal D:A ratio results in not only a less efficient hole transporting network but also a narrower charge photogeneration profile. The latter widens the flat‐band region. These deteriorations make charge diffusion a crucial factor for compensating charge collection. TrTPFB‐doping mainly helps elongate the charge diffusion length to alleviate severe bimolecular recombination. As a result, the FF boosts from 65.1% to 72.2% in the film AVT increased device, which is comparable to that of the bulk heterojunction device with the optimal D:A ratio. The best LUE of 3.51% is achieved in the TrTPFB‐doped ST‐OSCs based on a ternary material combination; moreover, an LUE of 5.18% is predicted after proper optical optimization. The ST‐OSC showcases a greenhouse rooftop, which outperforms the glass one for the wheat plant.