Interpretation of admittance, capacitance voltage, and current voltage signatures in Cu In,Ga Se2 thin film solar cells

A series of Cu In,Ga Se2 CIGS thin film solar cells with differently prepared heterojunctions have been investigated by admittance spectroscopy, capacitance voltage CV profiling and temperature dependent current voltage IVT measurements. The devices with different CdS buffer layer thicknesses, with a In2S3 buffer or with a Schottky barrier junction all show the characteristic admittance step at shallow energies between 40 160 meV which has often been referred to as the N1 defect. No correlation between the buffer layer thickness and the capacitance step is found, as it would be expected if the N1 response could be related to an interface donor defect. Temperature dependent current voltage measurements show that the CdS layers are highly resistive at low temperatures where the N1 step in admittance is observed, with a dielectric relaxation frequency smaller than the resonance frequency of the N1 response. These results strongly contradict to the common assignment of the N1 response to a donor defect at or close to the heterointerface. A new explanation for the N1 response is outlined which assigns the admittance step to a non ohmic back contact acting as a second junction in the device. The effect of a Schottky contact at the Mo CIGS interface on admittance and CV measurements are substantiated with numerical device simulations. The model allows a unified explanation of characteristic admittance, CV and IVT features commonly observed in CIGS solar cells