Admittance spectroscopy of efficient CuInS2 thin film solar cells

Thermal admittance spectroscopy is applied to characterize the trap properties of high-efficiency CuInS2/CdS/ZnO thin film solar cells. To that aim, the capacitance spectrum is examined in detail. Nontrap capacitance contributions like freeze-out of free carriers and series resistance are discussed. The dependence of the resistance on the sample area is used to identify its physical origin. Simulation results of a transmission line model are in good agreement with the spreading resistance of the ZnO window layer. Defect spectra, i.e., the distribution of the deep trapping states in the band gap, are extracted from the admittance spectra, using a method established by T. Walter, R. Herberholz, C. Muller, and H. W. Schock [J. Appl. Phys, 80, 4411 (1996)]. Arrhenius data of the traps are drawn best from the spectrum of conductance versus temperature. The defect spectra do not depend on the buffer layer. Generally, they show a high constant background trap density with some broad peaks. Two new trap levels at 0.3 and 0.5 eV are found, confirmed by deep level transient spectroscopy and identified as majority carrier traps in the bulk of the p-type CuInS2 absorber. An increasing density of the trap at 0.5 eV correlates with a decreasing open-circuit voltage. (C) 2000 American Institute of Physics. [S0021-8979(00)03721-X].