Charging and discharging of defect states in CIGS/ZnO junctions

The formation of photovoltaic junctions in thin film CIGS has been explored through the deposition of ZnO directly onto the CIGS by the reaction of Zn and atomic oxygen. Most of these devices exhibit strong light soaking effects in which V-oc (at 1 sun) can increase from 250 mV to over 550 mV. A 12.3% efficient ZnO/CIGS solar cell was achieved. The soaking effect, which is the main topic of this paper, was found to be bias or field driven. Detailed investigations were conducted using a 'drive and interrogate' procedure, in which the device is driven between two steady states corresponding to two different voltage biases, with brief interrogation of V-oc at various times. Data spanning the temperature range 31-125 degrees C were acquired. It was found that the V-oc versus log(t) relaxation curves can be described by a stretched exponential. and scale with t.exp(-E/kT) as a parameter, yielding E = 0.51 eV. Junction capacitance data are also reported. The underlying process, whether electronic or ionic, possesses a wide range of time constants. (C) 2000 Elsevier Science S.A. All rights reserved.