Enhanced photovoltaic performance of silicon-based solar cell through optimization of Ga-doped ZnO layer

Abstract

In the present study, the impact of deposition pressure and substrate temperature of Ga-doped Zinc Oxide (GZO) thin film and the photovoltaic performance of this structure as a transparent conductive oxide (TCE) layer in silicon-based solar cell were investigated. Implementing a single target of GZO, the structural, optical, and electrical properties of 350 nm thick GZO thin films with various deposition pressure (5 mTorr, 10 mTorr, 15 mTorr and 20 mTorr) at room temperature (RT) and substrate temperature (RT, 150 °C, 200 °C, 250 °C) at 15 mTorr deposition pressure were fabricated using RF magnetron sputtering technique. The aim here was to find out the GZO films with the optimum pressure and substrate temperature to incorporate them into solar cell as a TCE layer. The X-ray diffraction (XRD) and atomic force microscopy (AFM) techniques were used to determine the structural properties of all samples. The optical transmission measurements were performed using spectroscopic Ellipsometer and the band gap values were calculated by Tauc plot using optical transmission data. In addition, the electrical characterization of the GZO samples were analyzed by the Van der Pauw method and Hall measurements. Finally, the most promising GZO thin film was determined based on the structural and optoelectrical characterization. The findings indicated that the XRD pattern of all the prepared films was dominated by (002) preferential orientation irrespective of the deposition pressure and substrate temperature. The AFM measurements showed that all the samples had a dense surface morphology regardless of the deposition pressures, but the surface morphology of the samples was clearly changed upon increasing substrate temperatures. The transmission values of the film did not significantly alter (∼82%) when the deposition pressures except for the substrate temperature of 200 °C (86%) were changed. The band gap values were calculated between 3.30 eV and 3.36 eV, which can be associated with enhancement of crystalline quality of the films. The lowest resistivity and the highest carrier concentration values belonged to the film fabricated at 15 mTorr@200 °C by 2.0 × 10−3 Ω.cm and 1.6 × 1020 cm−3, respectively. Both increasing the deposition pressure (up to 15 mTorr) and substrate temperature (up to 200 °C) contributes to improving the crystallite size, widening the optical band gap, lowering the resistivity, and increasing the carrier concentration. In order to evaluate and compare the effect of both deposition pressure and substrate temperature, Silicon-based solar cells were fabricated using the most promising layers (15 mTorr@RT, 15 mTorr@200 °C). The cell performance with the GZO thin film as a TCE layer showed that varying both the pressure and substrate temperature of the GZO film contributed to enhancing the solar cell parameters. Thus, the conversion efficiency increased from 9.24% to 12.6% with the sequential optimization of pressure and temperature. It can be concluded that the pressure applied during the deposition and substrate temperature had a significant impact on the properties of GZO thin films and its photovoltaic performance of solar cell used as TCE layer.