Evaluation of recombination processes using the local ideality factor of carrier lifetime measurements

The mechanisms that limit the performance of a solar cell can be often identified by an assessment of the solar cell's local ideality factor m. Typically, m is extracted from the current-voltage curve of a completed solar cell and plotted as a function of voltage. In this study, m is extracted from photoluminescence measurements of the effective carrier lifetime and plotted against the excess carrier concentration Δn or the implied open-circuit voltage Voci. It is shown that a plot of m(Δn) or m(Voci) is a powerful way to analyse recombination processes within a silicon wafer, where its main advantage is that it can be determined from wafers that have neither metal contacts nor a p-n junction. With an m(Δn) plot, one can readily identify a range of Δn (or voltage) that is dominated by a single recombination mechanism, or that constitutes a transition from one dominant mechanism to another. One can also identify the dominating recombination mechanisms at a cell's maximum power point. In this paper we demonstrate the application of extracting an m(Δn) curve, and we show how it is affected by Shockley-Read-Hall and Auger recombination in the bulk, and by fixed charge in a dielectric coating.