Linking structural properties with functionality in solar cell materials – the effective mass and effective density of states

Understanding crucial material properties making a certain material suitable for a specific application requires a clear methodology linking microscopic parameters with macroscopic functionality. This paper provides such a methodology for the relevant example of the effective mass or effective density of states in semiconductors used as photovoltaic absorber materials. This example is inspired by the recent suggestion that low effective masses are a crucial ingredient explaining the high efficiency and in particular the high open-circuit voltages in metal-halide perovskite solar cells. We critically examine this claim and discuss the conditions that affect the relation between basic properties of a band structure with photovoltaic or more generally optoelectronic functionality. While low effective masses may indeed have a beneficial effect on the open-circuit voltage in the non-radiative limit, using low effective masses as a generic criterion for high optoelectronic functionality would be an oversimplification. Instead the effective mass should always be considered in combination with other material or device specific properties such as the type of band gap (direct or indirect), the doping level and the dominant carrier scattering mechanism.