Influence of substrate bias and post-deposition Cl treatment on CdTe film grown by RF magnetron sputtering for solar cells

Abstract

CdTe thin films were grown by RF magnetron sputtering at deposition pressures of 1.5–4 Pa and substrate potentials of 8.8, 0, −8.2, −18.9, −28.3, −38.1, −48.0, and −98.7 V. All as-grown CdTe thin films had undergone CdCl₂ treatment in dry air. The deposition pressure had a large influence on the crystalline quality, morphology, and grain size. At 1.5–3 Pa, CdTe thin films with good crystalline quality and column morphology were successfully obtained. The substrate potential also influenced the phase composition, grain size, morphology, microstress, and roughness of the obtained CdTe thin films. The phase composition, morphology, grain size, microstress, and crystalline quality changed significantly after CdCl₂ annealing treatment. For CdTe grown at 3 Pa, the grain size reached a maximum when the Cl treatment time was 20 min; for CdTe thin films deposited at 2 Pa, grain size was the greatest when the Cl treatment time was 43 min. The maximum grain size corresponded to the best performance of the CdTe solar cell. CdTe thin films grown at different substrate potentials were subjected to Cl treatment at 400 °C for 43 min. The internal structure was observed using scanning electron microscopy of sample cross-sections sliced by a focused ion beam. After Cl treatment, voids were seen at the CdTe grain boundaries, and at the interfaces CdS/F:SnO₂ and CdS/CdTe. The CdTe film grown at a substrate potential of −18.9 V produced the best solar cell, with device parameters of η = 12.78%, V_oc = 779 mV, J_sc = 22.91 mA cm⁻², and FF = 71.62%.