What Makes a Good Solar Cell?

Recent years have seen a substantial efficiency improvement for a variety of solar cell technologies as well as the rise of a new class of photovoltaic absorber materials, the metal-halide perovskites. Conversion efficiencies that are coming closer and closer to the thermodynamic limits require a physical description of the corresponding solar cells that is compatible with those limits. This progress report summarizes recent work on the interdependence of basic material properties of semiconductor materials with their efficiency potential as photovoltaic absorbers. The connection of the classical Shockley-Queisser approach, with the band gap energy as the only parameter, to a more general radiative limit and to situations where nonradiative recombination dominates is discussed. The authors delineate a consistent loss analysis that enables a quantitative comparison between different solar cell technologies. In a next step, bulk material properties that influence the photovoltaic performance of a semiconductor like absorption coefficient, densities of states of the free carriers, or phonon energies are considered. It is shown that variations of these properties have a big influence on the optimized design of a solar cell but not necessarily on the achievable efficiency.