III-V DILUTE NITRIDE MULTI-QUANTUM WELL SOLAR CELLS FOR HIGH EFFICIENCY PHOTOVOLTAICS
Vijaya, Gopi Krishna 1986-
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Addition of a few percent of nitrogen to conventional III-V semiconductor alloys creates a surprising reduction of the host band-gap. Due to their unusual band-gap characteristics, dilute nitride III-V semiconductor alloys such as Ga(In)AsN are suitable candidates to serve as a 1eV 3rd junction sub-cell in current high efficiency multi-junction solar cells, and are projected to achieve solar cell efficiencies of ~40%. In order to overcome material constraints such as poor minority carrier lifetime and doping issues in dilute nitrides, prior work has involved insertion of quantum well (QW) nanostructures of dilute nitrides within the intrinsic region of a solar cell, nevertheless, issues of carrier extraction have persisted. In order to improve upon this design, in this work, the band structure and energy levels of dilute nitride multi-quantum-well (MQW) system are calculated, from which the absorption coefficient is evaluated as a function of strain, nitrogen concentration, and electric field strength. Fitting the simulated value of absorption to experimental results showed incomplete extraction of carriers in the multi-quantum well region. Experimental characterization of previously grown dilute nitrides revealed the presence of a parasitic nitridation of the GaAsN/GaAs interface. Improvement of QW quality was done by using a modified run-vent system reduced nitridation. Improvement of carrier extraction was proposed based on the calculations of thermally assisted resonant tunneling QW design, and preliminary devices were grown to test this design. Results included a record open circuit voltage of 636 mV, and improved carrier extraction from resonant tunneling MQW solar cells of ~1eV, enabling future fabrication of multi-junction devices with efficiencies of ~39% (1 sun) and ~49% (500 sun conc).