High efficiency thin-film solar cells for space applications: challenges and opportunities

Abstract

In theory high efficiency thin-film III-V solar cells obtained by the epitaxial lift-off (ELO) technique offer excellent characteristics for application in space solar panels. The thesis describes several studies that investigate the space compatibility of the thin-film solar cell design developed at Radboud University. The main focus of the thesis is the effect of the harsh space environment (electron and proton radiation, harsh UV, temperature cycling, high vacuum) on the interactions between the semiconductor (GaAs) solar cell and the gold contacts and copper handling and stabilization foil. Accelerated ageing tests with a substrate-based model system show that the device performance may be severely affected by in diffusion of copper. Further investigation shows that two different degradation mechanisms can be distinguished. At relatively low test temperatures (< 250°C) the contact metals merely intermix without a large effect on the electrical performance of the solar cell, while at higher test temperatures (= 250°C) intermixing of the metals and recrystallization of the metals with the semiconductor occurs. This recrystallization process allows for rapid diffusion of copper into the active device structure, where it diminishes the solar cell performance. Several metals were tested for their suitability as diffusion barriers. It is shown that Ni barrier layers offer the potential to reduce device degradation as a result of copper diffusion. However, the mechanism by which the Ni barriers operate makes them unsuitable for application in thin-film solar cells. Further investigation of potential barrier materials is therefore necessary. Finally experiments with thin-film cells on gold and copper carriers have shown that there is a definite potential for application of thin-film solar cells in space, but that further investigations and adaptations are required. Particularly the effects of thermally induced stress should be studied in more detail.