Electronically active defects in the Cu2ZnSn(Se,S)4 alloys as revealed by transient photocapacitance spectroscopy

Transient photocapacitance (TPC) spectra were obtained on a series of Cu2ZnSn(Se,S)(4) absorber devices with varying Se: S ratios, providing bandgaps (E-g) between 1 eV and 1.5 eV. Efficiencies varied between 8.3% and 9.3% for devices with E-g <= 1.2 eV and were near 6.5% for devices with E-g >= 1.4 eV. The TPC spectra revealed a band-tail region with Urbach energies at or below 18 meV for the first group, but in the 25-30 meV range for the higher band-gap samples. A deeper defect band centered near 0.8 eV was also observed in most samples. We identified a correlation between the Urbach energies and the voltage deficit in these devices. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4754834]