Bandgap Fluctuations Observed by EL in Various Cu(In,Ga)(Se,S)2 PV Modules

We applied a recently published electroluminescence method for calculating the lateral bandgap distribution to copper indium gallium (di) selenide (CIGS) modules of three manufacturers. First, we used a CIGS module with a known apparent bandgap distribution to calibrate a bandgap imaging method at the electroluminescence setup at the University of Ljubljana. Results of the method newly calibrated at the system are in agreement with the known apparent bandgap distribution. Image analysis of ten CIGS modules of three manufacturers reveals spatial bandgap fluctuations of different types and intensities across the modules. Average apparent bandgap and its standard deviation vary from 0.99 to 1.14 eV and 4 to 22 meV, respectively. Observed fluctuation patterns often match between the modules of the same manufacturer, indicating that the fluctuations originate from systematic spatial variations in the production process. Variety of fluctuations indicates that they should be investigated both between different module types and within a single type. A standard deviation of 22 meV leads to an estimated loss in the efficiency of 0.26% (absolute); thus, such bandgap fluctuations constitute an optimization potential for the manufacturer.