The effect of annealing and CdCl2 activation on the electrical properties of thin film ZnO buffer layers for CdSeTe/CdTe solar cells

Highly resistive ZnO thin films are effective n-type buffer layers for thin film CdSeTe/CdTe photovoltaic devices. However, high-temperature processing during device fabrication may alter their electrical properties. This study investigates the structural and electrical changes in ZnO films following different process treatments: as-deposited, annealed, and cadmium chloride (CdCl2) treated. Carrier concentration, mobility, and electrical conductivity of the films were extracted using ultra-sensitive parallel dipole-line (PDL) Hall effect measurements. Films deposited at higher substrate temperatures showed improved electrical conductivity, enabling reliable extraction of carrier concentration and mobility. Notably, 750 nm thick ZnO films deposited at 500°C exhibited carrier concentrations around 1013 cm-3, which increased by four orders of magnitude after annealing and CdCl2 treatment. The ZnO films also demonstrated strong environmental stability, showing minimal change in electrical properties after 1000 hours of damp heat and UV exposure tests. Furthermore, analysis of CdSeTe/CdTe device cross sections using transmission electron microscopy and energy-dispersive X-ray spectroscopy has provided evidence of some diffusion of oxygen from the ZnO layer into the front CdSeTe absorber after the treatment. This work has simulated the influence of annealing and CdCl2 treatment on ZnO thin films to explain how their electrical properties may change during CdSeTe/CdTe device fabrication.