Narrow Bandgap (0.7–0.9 eV) Dilute Nitride Materials for Advanced Multijunction Solar Cells

A prominent role in the worldwide transition towards sustainable energy production is played by photovoltaics that is used to convert sunlight directly into green electricity. One of the key photovoltaic technologies is multijunction solar cell architecture based on III–V compound semiconductors, which provides the highest conversion efficiencies to date in terrestrial and space applications of solar cells. Currently, up to 47.6% conversion efficiency has been achieved under concentrated illumination with this approach. Still, despite major efforts, the milestone efficiency of 50% has not been realized. Reaching this efficiency level practically requires implementation of five or more junctions into multijunction solar cell devices, which allows more efficient utilization of the solar spectrum. In turn, this requires the development of new sub-cells and related materials. This is especially true for lattice-matched multijunction architecture, where the library of materials is more strictly limited. To this end, the thesis focuses on the development of narrow bandgap dilute nitrides and related multijunction solar cells lattice-matched to GaAs, ultimately targeting at 50% conversion efficiencies.

As the initial steps towards realization of this, four-junction solar cells employing two dilute nitride subcells were demonstrated. To this end, the bandgap of the bottom junction was shifted towards 0.9 eV. The experimental four-junction devices yielded efficiencies of up to 39% under concentration, yet with fine-tuning and higher concentration factors over 46% could be attainable.

A major part of the experimental work in this thesis involved fabrication of narrow bandgap GaInNAsSb subcells with 6–8% nitrogen concentrations for bridging the gap to Ge with lattice-matched materials. The thesis covers the progress from the first proof-of-concept narrow-gap GaInNAsSb junctions towards high performance subcells enabled by structural and epitaxial developments. Significant improvements for the performance of 0.8 eV GaInNAsSb solar cells were obtained by employing a back reflector behind the dilute nitride junction, and by optimizing the molecular beam epitaxy growth of the narrow-gap materials. The best narrow bandgap subcells presented in this work would already enable current-matching in next-generation multijunction devices with projected efficiencies exceeding 50%.