Ultrafast Carrier Dynamics of CdTe: Surface Effects

Cadmium telluride has been an important absorber material used in solar cells for decades given its near-optimum band gap and lower cost for device fabrication. However, the overall efficiency has been low compared to the theoretical limit. One major contributor to the problem of the relatively low open-circuit voltage is the high surface recombination of photogenerated carriers. In this contribution, time-resolved pump–probe reflectivity was used to study the carrier dynamics of CdTe(111) under the influence of selected surface conditions. It was found that surfaces after the processes of chemical etching and thermal oxidation for short durations exhibit bulk-like transient reflectivity and a clearly reduced surface recombination velocity, according to a model calculation. The comparative study indicates that the origins of carrier traps include the midgap defect states that are associated with the uncapped Te surface atoms, namely the Te antisites and Te interstitials, and subsurface damages resulting from polishing and ion sputtering. The present results suggest that the formation of a thin oxide layer through annealing after chemical etching may be beneficial for surface improvement for CdTe-based solar cells.