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 effe cts 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. Detail ed investigations were conducted using a 'drive and interrogate' procedure, in which the device is driven between two steady states corresponding to t wo different voltage biases, with brief interrogation of V-oc at various ti mes. 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 b y a stretched exponential. and scale with t.exp(-E/kT) as a parameter, yiel ding E = 0.51 eV. Junction capacitance data are also reported. The underlyi ng process, whether electronic or ionic, possesses a wide range of time con stants. (C) 2000 Elsevier Science S.A. All rights reserved.