Single and multi-junction solar cells utilizing a 1.0 eV SiGeSn junction

Abstract

Multi-junction photovoltaic technologies lead the way to achieving ultra-high power conversion efficiencies for both space based and terrestrial concentrator applications. However, realizing a lattice matched quad-junction solar cell remains challenging due to a lack of suitable material systems able to achieve the elusive middle sub-cell band-gap energy of 1.0 eV. In this work, we present a potential candidate material to achieve this 1.0 eV sub-cell, the group–IV ternary alloy SiGeSn. Initial simulations of triple and quadruple junction solar cell designs show that this novel material system has the potential to reach efficiencies in excess of 45 and 48% respectively under concentrated illumination. We report on the electrical characterization of both single and triple junction devices based on a 1.0 eV SiGeSn junction. It is shown that the external quantum efficiency performance of both SiGeSn devices is promising given the initial state of development of the alloy, with a fitted minority electron diffusion length in the junction base shown to be limited to 5 µm. However, further material study and growth optimization is required to improve open-circuit voltage performance and the reverse bias breakdown characteristic in the SiGeSn junction. Given these short-comings we believe that group-IV ternary alloys provide a viable alternative approach to achieving highly efficient multi-junction solar cells in future.