Effects of Sub-gap Absorption on Photovoltaic Performance

The theoretical maximum photovoltaic efficiency is bounded by the Shockley-Queisser model which assumes that the photovoltaic material has step-function like absorptance, with zero absorption below its bandgap and perfect absorption above. However, typical photovoltaic materials exhibit absorption with an exponential dependence below its bandgap, known as the"Urbach tail" whose steepness is dependent on the Urbach parameter. Using a modified detailed balance model that accounts for sub-gap absorption we show that photovoltaic performance is strongly affected by the magnitude of the Urbach parameter.