QUANTUM EFFICIENCY OF CdTe SOLAR CELLS IN FORWARD BIAS

When the quantum efficiency of a CdS/CdTe solar cell is measured under forward voltage, the measurement is likely affected by several factors including (1) the voltage dependence of the collection efficiency, (2) series resistance of the cell and instrumentation, (3) the electrical barrier of the back contact, (4) photoconductive effects in the CdS and CdTe, and (5) any secondary barrier in the primary junction. Each of these effects has a distinct signature, but without careful attention to these signatures, misinterpretation is possible and perhaps common. The approach here is to numerically simulate effect (1), and then progressively add effects (2), (3), (4), and (5). In each case, we show the characteristic signature of the apparent quantum efficiency when voltage is varied, we discuss the effect of bias light on the quantum efficiency measurement, we compare when possible with analytical modeling, and we show the impact of each effect on the cell’s current-voltage curves, particularly the light/dark crossover. Although the presentation will be specific to CdTe cells, the principles should at least in part apply to other solar cells as well.