Dynamics of photoexcited carrier relaxation and recombination in CdTe/CdS thin films

Efficiency-limiting defects in photovoltaic devices are readily probed by time-resolved spectroscopy. This paper presents the first direct optical measurements of the relaxation and recombination pathways of photoexcited carriers in the CdS window layer of CdTe/CdS polycrystalline thin films. We utilize the complimentary techniques of femtosecond time-resolved differential absorption (TRDA) and picosecond time-resolved photoluminescence (TRPL) to determine the relaxation and recombination mechanisms in these films. Comparison between samples with systematic variations in their CdS layers leads to several conclusions. Photoexcited holes are very rapidly captured by deep traps where they remain until recombining with electrons in shallow traps. The hole trapping states are most likely produced by tellurium substituting for sulfur in the CdS lattice. We postulate that reduction of tellurium diffusion into CdS during growth may increase the spectral response of CdTe/CdS solar cells for photon energies above 2.5 eV by reducing or eliminating these defect sites.