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

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.