Quantitative Analysis of the Separate Influences of Material Composition and Local Defects on the V_oc of PV Devices: An Exemplary Study on CIGS

A lot of effort is required in order to close the efficiency gap between laboratory record Cu(In,Ga)Se-2 (CIGS) solar cells and commercially produced modules. An obstacle easily overlooked is the fact that reduced module performance may have multiple root causes. Here, we show how to isolate two main factors that influence the open circuit voltage (V-oc) of photovoltaic devices, the material composition and the local defects, here shunts in particular, i.e., regions of significantly lowered resistivity. In order to characterize the material composition, we used electroluminescence spectroscopy. Additionally, we investigated the effect of local defects on the V-oc, by the use of thermography. Which of both factors dominates the resulting V-oc depends on the insolation intensity under which the device is operated. We established a model in order to estimate the V-oc as a function of the luminescence peak wavelength, the quantification of the thermal hot spots caused by local defects, and the insolation intensity. Subsequently, we investigated how these factors affect the electrical conversion efficiency of the device. The combination of these two measurement techniques results in a more conclusive evaluation of the electrical parameters, thereby enabling an improved quality assessment to allow a successive process optimization. This approach is also applicable to other thin film PV technologies such as perovskite and organic solar cells.