Triple and Quadruple Junctions Thermophotovoltaic Devices Lattice Matched to InP
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Triple and Quadruple Junctions Thermophotovoltaic Devices Lattice Matched to InP L. Bhusal, and A. Freundlich Center for Advanced Materials and Physics Department, University of Houston, 724 SR-1, 4800 Calhoun, Houston, TX, 77204 ABSTRACT Power conversion in thermophotovoltaic (TPV) or any other photovoltaic device can be increased by implementing monolithically series connected multi-bandgap structure in the device. The main concern for multi-bandgap material is the availability of different band gaps for the optimal operation of the device. Based on the recent work, GaAsN/InAsN superlattice lattice matched to InP has shown the potential of achieving band gaps in the range of 0.65-0.35eV at 300 K, which is technologically important range for the TPV structure due to the availability of the photon energies in this range from the heat source. In this work, we will present the calculation details and results to find the maximum power generated by the multi-bandgap monolithically series connected devices. Optimized band gaps for p-i-n junction subcells were estimated by finding the optimal current to provide the maximum power through the seriesconnected double, triple and quadruple junction cells for 1350 K blackbody radiation as an incident flux. INTRODUCTION. Thermophotovoltaic (TPV) conversion of infrared (IR) radiation emanating from a radioisotope heat source is under consideration for deep space exploration. Ideally, for radiator temperatures of interest, the TPV cell must convert photons efficiently in the ~0.3–0.7 eV spectral range. Best experimental data for single junction cells are obtained for latticemismatched 0.55 eV InGaAs based devices. It was suggested, that a tandem InGaAs based TPV cell made by monolithically combining two or more lattice mismatched InGaAs subcells on InP would result in a sizeable efficiency improvement. However, from a practical standpoint the implementation of more than two subcells with lattice mismatch systems will require extremely thick graded layers (defect filtering systems) to accommodate the lattice mismatch between the sub-cells and could detrimentally affect the recycling of the unused IR energy to the emitter. The unusual large band gap lowering observed in GaAs1-xNx with low nitrogen fraction [1] has sparked the interest in the development of dilute nitrogen containing III-V semiconductors for long-wavelength optoelectronic devices (e.g. IR lasers, detector, solar cells) [2-7]. Lattice matched Ga1-yInyNxAs1-x on InP has recently been investigated for the potential use in the midinfrared device applications [8], and it could be a strong candidate for the applications in TPV devices. This novel quaternary alloy allows the tuning of the band gap from 1.42 eV to below 1 eV on GaAs and band gap as low as 0.6eV when strained to InP, but it has its own limitations. To achieve such a low band gap using the quaternary Ga1-yInyNxAs1-x, either it needs to be strained on InP, which creates further complications due to the creation of defects and short life of the dev
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