Analysis of Cu(In,Ga)Se2 thin-film modules by electro-modulated luminescence

Thin-film Cu(In,Ga)Se-2 modules are investigated by electro-modulated luminescence with bias illumination. The large signal analysis enables the determination of the average current density/ voltage (J/V) characteristics of individual cell-stripes in a module, without contacting each individual cell. It was found that the characteristics determined from electro-modulated photo-luminescence measurements differ from the characteristics determined without bias illumination via electro-luminescence. As a reason for this, we found the non-linear relation of the local current to the local voltage which makes the determination of the cell voltage via an average of the local voltages determined from electro-luminescence images incorrect especially at high injection currents. A second analysis method using the small signal response of electro-modulated photo-luminescence was also analyzed. Recently, a method based on such a small signal analysis was demonstrated that yields the local photocurrent collection efficiency. This contribution presents the first application of this method to solar modules with series connected cells rather than the previously shown analysis of single cells. We compare two ways to evaluate the photocurrent collection efficiency from the small signal analysis and discuss how applicable these measurements are to modules. We find that the photocurrent collection efficiency of modules gives valuable information about how defective cells act within a module and which parts of the module limit the overall current through the module. (C) 2016 AIP Publishing LLC.