What is the maximum theoretical efficiency limit for a single-junction solar cell under standard conditions, and which principle sets this limit?

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The maximum efficiency for a single-junction solar cell under standard one-sun illumination is set by the Shockley-Queisser limit, which comes from a detailed-balance analysis of how a single absorber converts light to electricity. In this framework, photons with energy below the bandgap aren’t absorbed, while photons above the bandgap create electron–hole pairs but lose the excess energy as heat. Radiative recombination acts as the fundamental loss channel, so the open-circuit voltage is limited by the bandgap energy and the balance between generation and recombination. When you optimize for the standard solar spectrum, the best achievable efficiency is about 33–34%. The Carnot limit is for heat engines, the Davies limit concerns multi-junction concepts, and the Edison limit isn’t the standard bound used for single-junction photovoltaics.