A novel approach for the improvement of open circuit voltage and fill factor of InGaAsSb/GaSb thermophotovoltaic cells

Abstract

Heterojunction n-Al0.25Ga0.75As0.02Sb0.98/p-In0.16Ga0.84As0.04Sb0.96 thermophotovoltaic (TPV) cells were grown by molecular-beam epitaxy on n-GaSb-substrates. In the spectral range from 1 μm to 2.1 μm these cells, as well as homojunction n-p-In0.16Ga0.84As0.04Sb0.96 cells, have demonstrated internal quantum efficiencies exceeding 80%, despite about a 200 meV barrier in the conduction band at the heterointerface. Estimation shows that the thermal emission of the electrons photogenerated in p-region over this barrier can provide high efficiency for hetero-cells if the electron recombination time in p-In0.16Ga0.84As0.04Sb0.96 is longer than 10 ns. Keeping the same internal efficiency as homojunction cells, hetero-cells provide a unique opportunity to decrease the dark forward current and thereby increase open circuit voltage (Voc) and fill factor at a given illumination level. It is shown that the decrease of the forward current in hetero-cells is due to the lower recombination rate in n-type wider-bandgap space-charge region and to the suppression of the hole component of the forward current. The improvement in Voc reaches 100% at illumination level equivalent to 1 mA/cm2 and it decreases to 5% at the highest illumination levels (2–3 A/cm2), where the electron current component dominates in both the homo- and heterojunction cells. Values of Voc as high as 310 meV have been obtained for a hetero-cell at illumination levels of 3 A/cm2. Under this condition, the expected fill factor value is about 72% for a hetero-cell with improved series resistance. The heterojunction concept provides excellent prospects for further reduction of the dark forward current in TPV cells.