GaTlAs Quantum Well Solar Cells for Sub-band Gap Absorption
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MRS Advances © 2019 Materials Research Society DOI: 10.1557/adv.2019.334
GaTlAs Quantum Well Solar Cells for Sub-band Gap Absorption Ahmed Zayan, Thomas E. Vandervelde* Renewable Energy and Applied Photonics Laboratory, Electrical and Computer Engineering Department Tufts University Medford, Massachusetts 02155, USA *[email protected]
ABSTRACT
Despite the improvements seen in efficiency of GaAs cells over the years, there remains room for improvement for it to approach the theoretical single junction limit posited by Shockley and Quiesser decades ago. One of the more pursued options is the growth of quantum wells within the structure of GaAs to enhance its photon absorption below its bandgap. Multiple Quantum Wells (MQW) have been an ongoing topic of research and discussion for the scientific community with structures like InGaAs/GaAs and InGaP/GaAs quantum wells producing promising results that could potentially improve overall energy conversion. Here, we used WEIN2K, a commercial density functional theory package, to study the ternary compound Ga1-xTlxAs and determine its electronic properties. Using these results combined with experimental confirmation we extend these properties to simulate its application to form a MQW GaAs/ Ga1-xTlxAs solar cell. Ga1-xTlxAs is a tunable ternary compound, with its bandgap being strongly dependent on the concentration of Tl present. Concentrations of Tl as low as 7% can reduce the bandgap of Ga1-xTlxAs to roughly 1.30 eV from GaAs’s 1.45 eV at room temperature with as little as a 1.7% increase in lattice constant. The change in bandgap, accompanied by the relatively small change in lattice constant makes Ga 1-xTlxAs a strong candidate for a MQW cell with little to no strain balancing required within the structure to minimize unwanted defects that impede charge collection within the device. Our GaAs photodiode with TlGaAs MQWs shows an expanded absorption band and improved conversion efficiency over the standard GaAs photovoltaic cell with dilute concentrations of Tl incorporated into the compound.
2015
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INTRODUCTION The material properties, and relative ease of fabrication have made III-V compounds ideal for many novel solar energy harvesting structures. Chief among these prospective solutions are Multiple Quantum Well (MQWs) cells, which have shown promising results in GaAs/AlGaAs, InGaAs/GaAs and InGaN/GaN devices [1]–[5]. With relative modelled efficiencies of approximately 10% higher than the standard bulk III-V single junction cell, it becomes worthwhile to research how far the efficiency envelope can be pushed in similar setups with various material choices. MQWs allow for the absorption and conversion of sub-bandgap photons into collectable electrical energy. This is mostly due to the improved carrier transport through the device
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