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.