Dilute Nitride Multijunction Solar Cells Grown by Molecular Beam Epitaxy

Solar cells generate green energy directly from sunlight. The energy conversion efficiency of solar cells depends strongly on materials used as absorbers and the cell architecture. Currently, the best solar cells convert sunlight energy to electricity with an efficiency of up to 46%. This thesis focuses on the development of dilute-nitride materials and related solar cells, which are one of the most promising approaches for achieving even higher efficiencies. Applications for these cells include concentrated photovoltaic and space power systems. In particular, the thesis focuses on developing solar cell materials based on GaInNAsSb, which can provide efficient light absorption and energy conversion for a photon energy range of 0.8 eV- 1eV, typically challenging for conventional III-V semiconductors. The GaInNAsSb semiconductor materials were synthesized by molecular beam epitaxy.

The experimental work of this thesis explored the dependence of the fabrication parameters on the GaInNAsSb material and solar cell properties. It was observed that for many of the growth parameters even a slight change of the value can have a significant effect on the solar cell performance. A N incorporation model was developed to help the iteration process for growth parameter tuning. For optimized growth conditions, nearly ideal current generation for GaInNAsSb based material was achieved. Based on external quantum efficiency measurements it was possible to collect up to ~90% of the photons in the spectral range of the GaInNAsSb junction. In addition, an excellent fill factor of 0.7 and voltages in the range of 0.5 V for a 1 eV GaInNAsSb junction were measured.

Simulation based on a state-of-the-art GaInP/GaAs double junction cell, a commercial GaInP/GaAs/Ge triple junction cell and GaInNAsSb single junction cells studied in this thesis, revealed that a GaInP/GaAs/GaInNAsSb/Ge cell at the one sun concentration can have 1.7 percentage points higher efficiency than GaInP/GaAs/GaInNAsSb cell. In addition, the estimated efficiency of a four junction cell at 300 suns would be 3.6 percentage points higher than for a GaInP/GaAs/GaInNAsSb cell. The optimized single junction GaInNAsSb cell was experimentally tested into a GaInP/GaAs/GaInNAsSb cell in this work. The one sun efficiency of the cell under AM1.5G spectral conditions was 31% and the efficiency of the cell at 70 suns concentration was 37-39%. The one sun result is 91% of the projected efficiency. The results under concentrated conditions are expected to be improved by optimizing of the cell top grid design, layer structure and interfaces. As future concepts are concerned, a nanopatterned moth eye antireflection coating was fabricated on top of the GaInP/GaAs/GaInNAs cell, which was then compared to a cell that had a traditional two layer TiO2/SiO2 coating. The moth eye nanostructure had a low average reflection of 2% in the spectral range of 400-1700 nm, being less than half of the reflectance of the TiO2/SiO2 coating. For future work, the absorption loss for the nanostructure coating at wavelengths below 500 nm needs to be reduced.