Study of Charge Transport Properties in a ZnO/CdS/Cu(In,Ga)Se2 Solar Cell via Admittance Spectroscopy

The authors investigate the charge carrier transport properties in a thin-film heterojunction Cu(In,Ga)Se₂ (CIGS) solar cell device via coordinated capacitance-voltage and admittance spectroscopic measurements. The CIGS absorber width was measured using temperature dependent depletion width in the freeze-out regime and its acceptor density extracted from conventional capacitance-voltage analysis in the non-freeze-out regime. The bias-dependent modified dielectric relaxation in the Cu(In,Ga)Se₂ solar cell was exploited to extract the absorber's resistivity, hole mobility, and their temperature dependency, all by admittance spectroscopy. In the temperature range of 133-300 K, the extracted hole mobility in the CIGS device increases from 7.55 × 10^-3 to 4.08 cm²/V s with temperature. The temperature dependent resistivity and mobility show Arrhenius behavior with an activation energy ≈120 meV. The thermally activated behavior of mobility is related to the potential fluctuation encountered by the holes during their transport, which enables admittance spectroscopy to directly measure the potential fluctuation in the CIGS polycrystalline material.