The efficiency limit of solar cells with molecular absorbers: A master equation approach

Abstract

A master equation approach to the limiting efficiencies of solar cells with molecular absorbers is presented. A number of model absorbers are analyzed that possess identical absorption spectra, but with differing numbers of electronic excited states. The Shockley–Queisser limit is reproduced for a molecule resembling a semiconductor, with an infinity of electronic levels in the excited state manifold. However, when only a few electronic states contribute to the absorption spectrum, the limiting efficiency is reduced. In the extreme case, where only a single electronic excited state participates in the absorption spectrum, the efficiency limit is 28.9%. At high energy thresholds, built-in thermal up-conversion results in solar cells with efficiencies exceeding the Shockley–Queisser curve. The analysis is applicable to any single threshold photovoltaic device, including those based on semiconductor, polymer, and small molecule absorbers.