A Phenomenological Figure of Merit for Photovoltaic Materials

I propose a figure of merit Γ_PV to estimate the maximum efficiency attainable by a generic non-ideal PV absorber in a planar single-junction solar cell under the non-concentrated AM1.5G spectrum. This efficiency limit complements the more idealized limits derived from fundamental physics, such as the Shockley-Queisser limit and its subsequent generalizations. Specifically, the present figure-of-merit approach yields stricter efficiency limits applicable to realistic PV absorbers with various imperfections, including finite carrier mobilities and doping densities. Γ_PV is a function of eight properties of the absorber that are both measurable by experiment and computable by electronic structure methods. They are: band gap, non-radiative carrier lifetime, carrier mobility, doping density, static dielectric constant, effective mass, and two parameters describing the spectral average and dispersion of the light absorption coefficient. Γ_PV has high predictive power (absolute efficiency error less than ±1.1%) and wide applicability range. The Shockley-Queisser limit and its generalizations are reproduced by Γ_PV. Simpler figures of merit proposed by others are also included as special cases of Γ_PV.<br/><br/>For a generic PV absorber at any stage of development, determination of its Γ_PV figure of merit helps understand whether imperfect PV performance is intrinsic to the material (inadequate bulk properties at the current stage of development), or if it “only” requires a different device structure, contact layers, or improved interface properties. I will show the outcome of this analysis for 25 PV absorbers with record PV efficiencies between 0% and 85% of their Shockley-Queisser limit. Using a local version of the Γ_PV figure of merit, a material-specific optimization strategy is laid out for any experimentally synthesized PV absorber, by specifying the bulk properties that should most urgently be improved to increase their PV efficiency.