Transient phenomena in Cu(In,Ga)Se2 solar modules investigated by electroluminescence imaging

Industrially fabricated thin-film modules are investigated by electroluminescence (EL) photography. We observe metastable transients in a series of successive images recorded during application of forward bias immediately after the Cu(In,Ga)Se-2 modules were kept in the dark for several hours. Our experiments are conducted in the dark either under constant current (monitoring the module voltage) or vice versa. For both situations, the EL intensities increase with time, whereas we observe a decrease of the overall module voltage (at fixed current) or an increase of the current (at fixed voltage). We ascribe our observations to the simultaneous decrease of the bulk series resistance R-s and the reduction of recombination currents (an increase in the junction resistance R-j) during the forward bias soaking. A quantitative analysis of our data shows that the reduction of R-s we observe is much stronger than the increase of R-j. We also show that the bulk series resistance can strongly adulterate the determination of the sheet resistance of the front ZnO from EL images. (C) 2012 Elsevier B.V. All rights reserved.