Investigations of the main loss mechanisms in Cu2ZnSn(S,Se)4 solar cells spray-coated from water-ethanol based ink: Reducing the density of defects to reach efficiencies close to 10%

In order to comprehend the main limitations occurring in our 8.6% CZTSSe devices spray-coated from a water/ethanol ink, we have employed temperature dependent admittance spectroscopy and capacitance-voltage profiling. This technique has revealed a large density of charge carriers in the dark (1016-1017 cm^-3) and two main defect levels: One fairly close to the middle of the band-gap, and another one much shallower (130 eV from the valence band maximum). A controlled tuning of the Sn content in the sprayed ink appears to offer an efficient leverage on the density of these defects: By adding only a few percent of Sn, the density of charge carriers in the dark can be reduced by more than one order of magnitude. This improvement of our process has led us to the fabrication of devices showing efficiencies close to 10% under AM1.5G.