We have investigated the influence of the spectral albedo on the power output of bifacial solar cells. We adapted the Shockley-Queisser radiative flux balance framework to account for a variation of the spectrum and intensity of the incoming light. We find that the ideal band gap and the maximum efficiency depend on the spectral albedo of the surroundings and that optimal cell performance cannot be assessed when only accounting for a spectrally independent albedo. With a spectral albedo model, we predict that the power output for a bifacial silicon solar cell surrounded by green grass is 3.1% higher than for a wavelength-independent albedo, and even 5.2% higher for white sand. We experimentally verify this trend for silicon heterojunction solar cells and we derive the ideal spectral albedo.

The mathematical dependence of bandgap-voltage offset on Auger and radiative recombination is derived. To study the recombination near the intrinsic limit, we manufacture thin silicon heterojunction test structures designed to minimize surface recombination, and to measure voltages and effective lifetimes near the Auger and radiative limit. Open-circuit voltages over 760 mV were measured on 50-μm-thick structures, leading to bandgap-voltage offsets at open-circuit down to 0.35 V. The Auger and radiative recombination represents over 90% of the recombination at open-circuit. This dominance also holds at the maximum power point, giving pseudo-fill factors of 86%. We demonstrate the potential of thin silicon devices to reach high voltages, and bandgap-voltage offsets in line with the best reported for direct bandgap materials such as gallium indium phosphide and gallium arsenide.

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For commercially-viable solar-grade silicon, thinner wafers and surface saturation current densities below 1 fA cm−2, are required to significantly increase the practical efficiency limit of solar cells.

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