Comparison of Minority Carrier Lifetime Measurements in Superstrate and Substrate CdTe PV Devices

We discuss typical and alternative procedures to analyze time-resolved photoluminescence (TRPL) measurements of minority carrier lifetime (MCL) with the hope of enhancing our understanding of how this technique may be used to better analyze CdTe photovoltaic (PV) device functionality. Historically, TRPL measurements of the fast recombination rate (t ₁) have provided insightful correlation with broad device functionality. However, we have more recently found that t ₁ does not correlate as well with smaller changes in device performance, nor does it correlate well with performance differences observed between superstrate and substrate CdTe PV devices. This study presents TRPL data for both superstrate and substrate CdTe devices where both t ₁ and the slower TRPL decay (t ₂) are analyzed. The study shows that changes in performance expected from small changes in device processing may correlate better with t ₂. Numerical modeling further suggests that, for devices that are expected to have similar drift field in the depletion region, effects of changes in bulk MCL and interface recombination should be more pronounced in t ₂. Although this technique may provide future guidance to improving CdS/CdTe device performance, it is often difficult to extract statistically precise values for t ₂, and therefore t ₂ data may demonstrate significant scatter when correlated with performance parameters.