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

Abstract

Thin-film Cu(In,Ga)Se2 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.

Spatially Resolved Identification of Shunt Defects in Thin Film Solar Cells via Current Transport Efficiency Imaging Combined with 3D Finite Element Modeling

“…The inhomogeneously distributed shunt defects appear as very bright (red‐dashed circles) or dark spots (black‐dashed circles) in both the experimental and the simulated f T images, revealing their individual local responses to terminal differential changes. Note that a similar phenomenon of the increased photocurrent transport efficiency of a shunted cell is observed in the CIGS module, which is the result of the series connection of the cells . Combined with the mathematical relation deduced above, the ohmic and nonohmic shunt defects can thereby be identified from the nonshunted regions.…”

Imaging photocurrent collection losses in solar cells

“…7 This method is universally valid and yields the differential local photocurrent collection efficiency f pc;loc . [8][9][10] Hence, an image of f pc;loc depicts the probability that a locally generated differential photocurrent arrives at the terminals of the solar cell at a given bias situation. Though being simple, general, and quantitative, the method only images a differential and not the integral situation.…”

“…The details of the measurement and calculation procedure have been published previously. 8,9 For our purpose, we have to calculate the total externally collected photocurrent I coll;loc resulting from the generated photocurrent at position r according to…”

“…This decrease results from the decrease in the differential diode resistance towards higher voltages compared to the series resistances within the device. 9 Following the imaging of f pc;loc , we determine the total local photocurrent collection efficiency F pc;loc . This quantity is calculated by integration of f pc;loc over the different illumination intensities, as described by Eq.…”