Effects of Moisture-Proof Back Passivation Layers of Al₂O₃ and Al_<i>x</i>Ti_1–<i>x</i>O_<i>y</i> Films on Efficiency Improvement and Color Modulation in Transparent a-Si:H Solar Cells

Abstract

In this study, the back passivation layers (BPLs) were developed to protect hydrogenated amorphous silicon (a-Si:H) thin films of transparent solar cells from humidity and contaminants. Metal oxide compound films with Al (Al2O3) and Ti (AlxTiyOz (ATO)) were fabricated by plasma-enhanced atomic layer deposition for the BPLs on transparent solar cells. The BPLs of Al2O3 films applied to the transparent solar cells were deposited in different thicknesses to evaluate the performance, and the ATO film thickness was fixed at 30 nm. Even at the thinnest thickness of 30 nm, the water vapor transmission rates of BPLs were very low at 1.96 × 10–3 (Al2O3) and 1.23 × 10–2 g/m2·day (ATO). In addition to moisture protection, measures of cell performance, including open-circuit voltage and short-circuit current density, were improved by blocking the leakage current and through the optical interference effects of the BPLs. The solar cell with ATO BPLs exhibited an increase in efficiency of more than 12% compared with those of conventional reference cells. Furthermore, by varying the refractive index and thickness of the BPLs, the reflection and transmission spectra were modulated to implement various cell colors without serious loss in cell efficiency.