Advanced Epitaxial Lift-off Processes for III-V Solar Cells

Abstract

This thesis presents a comprehensive investigation into the development of advanced epitaxial lift-off (ELO) processes and novel contacting solutions for high-efficiency III-V thin-film solar cells. The primary objective of this research is to overcome the challenges associated with the fabrication of thin-film solar cells, including improving etch rates, optimizing bending curvatures and thin-film transfer techniques. The study begins by exploring the ELO process for GaAs thin-film solar cells using hydrofluoric acid (HF), with a focus on optimizing etch rates and bending curvature without compromising the quality of the thin-film during the release. A significant achievement is the realization of a 5-fold increase in lateral etch rate, reaching up to 20 mm/h, which is the fastest reported so far. This improvement is attributed to the use of internal sacrificial stressor layers (ISSL) and the addition of acetone (ACE) to the HF solution, enabling the release of trapped air bubbles and enhancing etch dynamics. The research also investigates the development of ELO processes for complex multijunction solar cell (MJSC) architectures, such as GaInP/GaAs/GaInNAsSb triple-junction solar cells, using hydrochloric acid (HCl) etching and sidewall protection. A high-density sidewall protection layer is developed to prevent undesired etching of epi-structures from the active region during the lift-off process, ensuring selective etching of sacrificial layers. In addition to optimizing ELO processes, this study explores novel approaches for fabrication of external contacts using inkjet printing and vacuum deposition techniques. These approaches provide non-destructive electrical contacts for fragile ELO solar cells, preserving the high- quality thin-films without physical damage during measurements. The developed external contacts demonstrate reduced electrical losses compared to traditional epoxy-based contacts, making them a promising solution for high-efficiency solar cells. The thesis also presents a detailed analysis of the role of ISSL and external stressor layers in achieving optimal bending curvature and precise control over the ELO process. The study investigates the influence of ISSL and varied film thicknesses on bending curvature, achieving optimal bending with a radius of curvature (ROC) in the range of 2.9 mm to 3.5 mm. The research outcomes have significant implications for the development of high-efficiency III-V thin-film solar cells. The optimized ELO processes and novel contacting solutions can be scaled up to wafer-scale production, enabling large-scale manufacturing of high-performance solar cells. Overall, this thesis provides a comprehensive understanding of the advanced ELO processes and novel contact technologies for high-efficiency III-V thin-film solar cells, paving the way for broader adoption of this technology in various applications.