The photoluminescence (PL) of semiconductors can be used to determine their absorption coefficient (α) using Planck's generalized law. The standard method, suitable only for self-supported thick samples, like wafers, is extended to multilayer thin films by means of the transfer-matrix method to include the effect of the substrate and optional front layers. α values measured on various thin-film solar-cell absorbers by both PL and photothermal deflection spectroscopy (PDS) show good agreement. PL measurements are extremely sensitive to the semiconductor absorption and allow us to advantageously circumvent parasitic absorption from the substrate; thus, α can be accurately determined down to very low values, allowing us to investigate deep band tails with a higher dynamic range than in any other method, including spectrophotometry and PDS.

Low temperature post-deposition treatment of Cu2ZnSnSe4 with NaF and KF significantly improved the solar cell efficiency (from 6.4% to 7.8% and 7.7% on average, respectively) due to enhanced fill factor (from 0.58 to 0.61 and 0.62), open-circuit voltage (Voc) (from 314 mV to 337 mV and 325 mV) and short-circuit current density (from 35.3 mA⋅cm −2 to 38.3 mA⋅cm −2 and 38.6 mA⋅cm −2). Voc improvement was higher for solar cells with NaF treatment due to an increase in radiative efficiency at room temperature and shallower defect activation energy as determined by photoluminescence (PL) and temperature dependent admittance spectroscopy, respectively. In the case of KF treatment, red-shift of the PL, higher band tail density of state and donor activation energy deeper in the band gap were limiting further improvement of the Voc compared to NaF treatment.