Te Layer to Reduce the CdTe Back-Contact Barrier

The CdTe valence-band maximum (VBM) at 5.8 eV below the vacuum level is mismatched with potential contacting metals such as Ni (work function ~5.2 eV) for the p-type CdTe commonly used in solar cells. Consequently, downward band bending in the CdTe forms both a hole barrier and a reverse field for electrons. Simulations reported here explain that a Te layer, which has been shown experimentally to enhance CdTe cell performance, is effective because it divides the larger CdTe barrier into two smaller ones. Comparison of simulations with experimental data implies a Te VBM of 5.40-5.45 eV, which is intermediate between the CdTe VBM and the Ni work function. An important consequence is a major reduction in the need for copper (Cu) at the back contact; with the Te layer, Cu in smaller amounts can be used for doping bulk CdTe instead of the larger concentration needed to reduce the band bending. The minimum thickness for the Te layer to enhance performance is calculated to be ~20 nm, which is consistent with experimental variations in thickness and is also near the depletion width for the 10 ¹⁸ cm ^-3 Hall density of Te in those experiments. It also predicts that a Te layer used in combination with CdMgTe (CMT), whose conduction-band expansion can reflect electrons away from back interface, can further enhance cell performance by mitigating CMT's small valence-band expansion.