Investigating impact of operating pressure of sputter deposited ZnO thin films for window layer in solar cells

The operating pressure during RF magnetron sputtering plays a pivotal role in determining the structural, optical, and electrical properties of zinc oxide (ZnO) thin films, which are crucial for their performance as window layers in solar cell applications. The undoped ZnO thin films were deposited on glass substrates using a high-purity (99.99%) ZnO target at varying working pressures. Structural analysis through x-ray diffraction (XRD) confirmed the amorphous nature of the films. Optical transmittance measurements using UV-vis spectroscopy revealed high transparency, exceeding 90% in the visible range. Surface morphology analysis via Atomic Force Microscopy (AFM) demonstrated a reduction in surface roughness from 83.5 nm to 23.8 nm with decreasing working pressure. Photoluminescence (PL) spectroscopy exhibited near-band-edge (NBE) emissions at 400 nm along with broad red luminescence centred around 750 nm, indicating the presence of intrinsic defect states. Furthermore, spectroscopic ellipsometry was employed to determine the refractive index, carrier mobility, and charge carrier concentration, all of which were found to significantly increase as the working pressure decreased from 1.4 to 1.8 for the refractive index, from 0.46 cm² Vs⁻¹ to 1.85 × 10⁴ cm² Vs⁻¹ for carrier mobility, and from 1.79 × 10¹⁰ cm⁻³ to 2.28 × 10²⁰ cm⁻³ for charge carrier concentration. These results underscore the critical influence of sputtering pressure on the quality and functionality of ZnO thin films for optoelectronic and photovoltaic applications.