Explanation of red spectral shifts at CdTe grain boundaries

We use cathodoluminescence spectrum imaging to investigate the nanoscale properties of CdTe thin-films for solar cells deposited by close-spaced sublimation. Luminescence emission is detected (bands) at ∼1.32 eV and ∼1.50 eV, which are consistent with Z- and Y-bands. For the grains in the as-deposited films, there is a significant redshift in the transition energies near the grain boundaries. The high grain boundary recombination velocity and the donor-acceptor pair (DAP) mechanism of the Z-band transition account for the contrast between grain boundaries and the grain interior. By applying DAP theory, we estimate the concentration of the shallow donor species participating in the Z-band transition to be ∼1017 cm−3.