Wide-Bandgap Metamorphic Solar Cells Grown by Molecular Beam Epitaxy

For the past decade, III-V-based triple-junction solar cells have held the record for solar cell efficiency by collecting a broad portion of the solar spectrum while minimizing thermalization loss. Recent work has led to an impressive 44.4% efficiency under concentration, but relatively little room remains for further improvements. As such, there has been much interest in increasing to 4-6 junctions in order to collect a larger portion of the solar spectrum and surpass 50% efficiency. Maximizing efficiency in such designs requires a top junction with bandgap energy (Eg ) of 2.0-2.2 eV. However, few III-V materials possess such a Eg, posing an important challenge in the development of 4-6 junction devices. Furthermore, the only materials with such wide Eg that are lattice-matched to conventional substrates contain A1, which has been shown to degrade efficiency through oxygen incorporation. Here, I investigate InyGa1- yP (y=0.42-0.30) as an alternative wide-Eg option that has a direct bandgap in the desired range but is lattice-mismatched to conventional substrates of GaAs and GaP. While mismatched growth introduces threading dislocations to the epitaxial device layers that increase non-radiative recombination and reduce solar cell efficiency, it has been shown that minimal efficiency degradation occurs if the threading dislocation density (TDD) can be kept at or below mid-106 cm-2. I thus developed GaAsxP1-x graded buffers on both GaAs and GaP to enable suitably low TDD in the desired InyGa1-yP device layers. To evaluate the effect of the starting substrate, I grew nominally identical GaAs0.65P0.35 solar cells on both GaAs and GaP substrates and found that growth on GaP substrates resulted in ∼4x higher TDD compared to growth on GaAs. GaP substrates are relatively immature compared with GaAs, possessing a 20x higher starting TDD that likely accounts for the increased TDD in the final devices. As a result, the GaAs 0.65P0.35 cells on GaP exhibited ∼40 mV lower open-circuit voltage (Voc) compared to those on GaAs; regardless, both devices performed well with Voc of 1.24 and 1.28 V, respectively. Having established high quality starting templates with appropriate TDD, I next grew wide-Eg In yGa1-yP solar cells with Eg of 1.93-2.23 eV, all maintaining TDD ≤ mid-106 cm-2. Devices on GaAs with Eg of 1.93-2.06 eV possessed high Voc values of 1.37-1.49 V. I then grew novel InyGa 1-yP (y=0.18-0.30) solar cells on GaP with Eg=2.12-2.23 eV, representing the widest-E g InyGa1- yP solar cells to date. However, the Voc values of 1.42-1.46 V indicate significant non-radiative recombination that must be reduced to obtain higher efficiency. Based on analysis of current-voltage characteristics, quantum efficiency measurements, and TDD, it appears that as the InyGa1-yP composition is tuned closer to GaP, the growth conditions to obtain high-quality material need to be adjusted. For instance, as the Ga-content increases with increasing Eg it is likely that the appropriate growth conditions more closely resemble those of GaP than those of In 0.49Ga0.51P. The results here indicate that with continued growth and device optimization, metamorphic InyGa 1-yP will be a promising candidate for the top cell in future 4-6 junction devices.