Multiple phases of Cu2ZnSnSe4 detected by room temperature photoluminescence

Cu2ZnSnSe4 based solar cells are promising but suffer from low open circuit voltage relative to their band gap. Additionally, the bandgap as extrapolated from quantum efficiency (QE) measurements varies without clear correlation to the growth conditions. Using room temperature photoluminescence, we show that different materials with different bandgaps coexist within micrometer sized areas of the absorbers. Simulations of the effect of multiple bandgaps on both the absorption and the Shockley-Queisser radiative recombination limit, explain the variations of the bandgap extrapolated from QE and the deficiencies of the solar cell parameters.