Eisenbarth, T.; Unold, T.; Caballero, R.; Kaufmann, C.A.; Schock, H.-W.: Interpretation of admittance, capacitance-voltage, and current-voltage signatures in Cu(In,Ga)Se2 thin film solar cells. Journal of Applied Physics 107 (2010), p. 034509/1-12
10.1063/1.3277043
Abstract:
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.