Current-voltage and capacitance study of light-induced metastabilities in CuZnSnSSe thin film solar cells

White, red and blue light-induced metastabilities in Cu2ZnSnS₄(Se₄) solar cells were investigated by temperature dependent current-voltage measurements, drive level capacitance profiling, impedance and thermal admittance spectroscopy. A set of devices were studied where white and blue light soaking at room temperature led to degradation of the device performance, while after red light soaking the solar cell efficiency did not change.

We observed a significant effect of light soaking on capacitance data measured in both low and high-temperature ranges for these devices. In particular, the net doping concentration extracted from drive-level capacitance profiling substantially increased after light soaking treatments.

Low and high-temperature capacitance steps observed in the reference capacitance-frequency spectra were assigned to Fermi level pinning and bulk defects, correspondingly. Light soaking with different-wavelength light led to a shift of both steps toward the high-frequency range, and hence a decrease in the thermal admittance activation energies.

A low-frequency 'inductive' loop was detected in the impedance spectra after light soaking, regardless of wavelength. It was proposed that the appearance of the 'inductive' loop is due to the formation of a negative electric field at the highly defected CdS/Cu2ZnSnS₄(Se₄) hetero-interface. This result also leads us to conclude that such electric field is responsible for the metastable behaviour of these devices at room temperature, while the low temperature metastable changes might have a different origin.

We also discuss the methodology for electrical characterization of the metastable solar cells in detail. (paper)