Unique Curve for the Radiative Photovoltage Deficit Caused by the Urbach Tail.

The photovoltage Voc of high-performance solar cells such as GaAs and metal halide perovskites is affected by the exponential Urbach tail in the absorption spectrum, with an energy parameter EU. It has been observed that increasing Urbach energy decreases the maximum photovoltage that can be achieved. On the basis of detailed balance and reciprocity of absorption and emission, we present a calculation that shows that the voltage loss due to the exponential tail in an absorption obeys a universal relation ΔVocrad = (kBT/q) ln(1 - EU/kBT) for EU < kBT, independently of the bandgap.