A model for the open circuit voltage relaxation in Cu(In,Ga)Se₂ heterojunction solar cells

¹ Faculty of Physics, University of Oldenburg, 26111 Oldenburg, Germany
² Institut für Physikalische Elektronik, Universität Stuttgart, Pfaffenwaldring 47, 70569 Stuttgart, Germany

Corresponding authors: thorsten.meyer@uni-oldenburg.de uwe.rau@ipe.uni-stuttgart.de

Received: 29 January 1999
Revised: 22 July 1999
Accepted: 22 July 1999
Published online: 15 October 1999

Abstract

This article investigates the electronic transport properties of ZnO/ CdS/ Cu(In,Ga)Se₂ heterojunction solar cells during and after illumination or forward bias in the dark. We observe a relaxation of the open circuit voltage under constant illumination as well as a relaxation of the voltage drop over the device under constant forward bias current in the dark. Both phenomena are accompanied by an increase of the sample capacitance. We introduce a general quantitative model concept for the open circuit voltage relaxation and related effects in heterojunction devices that explains the phenomena as a consequence of the persistent capture of charge carriers within the space charge region. We apply our concept to develop a specific quantitative model for the observed metastablity in Cu(In,Ga)Se₂ heterojunction solar cells.