Semi-transparent solar cells: strategies for maximum power output in cities

Semi-transparent photovoltaics (STPVs) are a promising form of building-integrated photovoltaics for urban green energy generation. By modulating visible light absorption, STPVs can exhibit both high power conversion efficiency (PCE) and average visible transmittance (AVT). While the maximum PCE for an opaque cell is 33%, the maximum PCE for a highly transparent STPV (70% AVT) has been reported as ∼24% by Lunt in 2012. We found that the maximum PCE for STPVs with the same transparency can actually exceed this limit, reaching 28% through band selective (BS) absorption of certain visible wavelengths. This BS method also increases the maximum light utilization efficiency (LUE) from 20% to 23%. Besides performance limits, studying harvestable irradiance for STPVs in urban environments is essential for accurate power output predictions, yet such analyses are rarely found. We analysed solar irradiance in 16 cities over a decade, deriving empirical spectra for both sunny and cloudy conditions. The maximum harvestable irradiance for completely transparent PVs in cities deviates from the AM 1.5G standard (∼570 W/m²), yielding ∼460 W/m² under clear skies and ∼50 W/m² under overcast conditions, with infrared (IR) accounting for 85–90% of invisible irradiance. The corresponding maximum output power intensity ranges from 150 to 250 W/m² (sunny), depending on the absorber's transparency. Our findings reveal that organic materials with IR bandgaps (0.9–1.4 eV) and high AVT are ideal for high-performance STPVs. Examining functional layers shows that some charge extraction layers and encapsulants can impair PCE by blocking invisible light, while metal electrodes could restrict overall transparency unless nanopatterned or thinned. These results offer comprehensive guidance for material scientists and energy researchers in optimizing and analysing STPVs.