Electronic properties of the Cu2ZnSn(Se,S)4 absorber layer in solar cells as revealed by admittance spectroscopy and related methods

Admittance spectra and drive-level-capacitance profiles of several high performance Cu2ZnSn(Se,S)(4) (CZTSSe) solar cells with bandgap similar to 1.0-1.5 eV are reported. In contrast to the case for Cu(In,Ga)(S,Se)(2), the CZTSSe capacitance spectra exhibit a dielectric freeze out to the geometric capacitance plateau at moderately low frequencies and intermediate temperatures (120-200 K). These spectra reveal important information regarding the bulk properties of the CZTSSe films, such as the dielectric constant and a dominant acceptor with energy level of 0.13-0.2 eV depending on the bandgap. This deep acceptor leads to a carrier freeze out effect that quenches the CZTSSe fill factor and efficiency at low temperatures. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4729751]