Properties of Arsenic-Doped ZnTe Thin Films as a Back Contact for CdTe Solar Cells

As-doped polycrystalline ZnTe layers grown by metalorganic chemical vapor deposition (MOCVD) have been investigated as a back contact for CdTe solar cells. While undoped ZnTe films were essentially insulating, the doped layers showed significant rise in conductivity with increasing As concentration. High p-type carrier densities up 4.5 × 10¹⁸ cm^-3 was measured by the Hall-effect in heavily doped ZnTe:As films, displaying electrical properties comparable to epitaxial ZnTe single crystalline thin films in the literature. Device incorporation with as-deposited ZnTe:As yielded lower photovoltaic (PV) performance compared to reference devices, due to losses in the open-circuit potential (V_OC) and fill factor (FF) related to reducing p-type doping density (N_A) in the absorber layer. Some minor recovery observed in absorber doping following a Cl-free post-ZnTe:As deposition anneal in hydrogen at 420 °C contributed to a slight improvement in V_OC and N_A, highlighting the significance of back contact activation. A mild CdCl₂ activation process on the ZnTe:As back contact layer via a sacrificial CdS cap layer has been assessed to suppress Zn losses, which occur in the case of standard CdCl₂ anneal treatments (CHT) via formation of volatile ZnCl₂. The CdS sacrificial cap was effective in minimising the Zn loss. Compared to untreated and non-capped, mild CHT processed ZnTe:As back contacted devices, mild CHT with a CdS barrier showed the highest recovery in absorber doping and an ~10 mV gain in V_OC, with the best cell efficiency approaching the baseline devices.