Introduction, Characterization and Efficiency Optimization of an AlxGa1-xAs / AlxIn1-xAs Heterojunction Solar cell with GaxIn1-xAs Back Surface Field (BSF) Layer Using ADEPT/F

Abstract

Energy conversion efficiency is a major issue for photovoltaic cells today. Researchers are continuously trying to improve the efficiency level of photovoltaic devices by introducing new materials and advanced concepts. The target is to reach a high efficiency level within affordable cost, which will lead to a mass generation of electricity using photovoltaic devices. In this work, a III-V heterojunction solar cell has been introduced and characterized, which uses an AlxGa1-xAs/AlxIn1-xAs heterojunction as the working p-n junction. ADEPT/F, a 1D simulation software, was used throughout the whole work for the simulation of light J-V characteristics for different designs. Energy conversion efficiency for each design was calculated from its corresponding light J-V characteristics curve. An illumination level of 1000 W/m2 (AM1.5G standard) and a concentration level of 1 sun was considered for all the simulations in the work. The photovoltaic cell has an n-on-p structure, where the n-type Al0.7Ga0.3As (x=0.7) layer acts as an emitter, and the p-type Al0.48In0.52As (x=0.48) layer serves as the base (absorber). The base thickness was kept at 10 μm in the first simulation, and an energy conversion efficiency of 6.78% was obtained. Then, a lightly doped third layer (p-type) of Ga0.67In0.33As was introduced (in contact with the base layer). This resulted in an efficiency of 15.3%, which was further increased to 16.79% after allowing a very high doping level (1×1019 cm-3) in the Ga0.67In0.33As layer. Germanium (Ge) substrate (p-doped) was used for the structure. Afterwards, efficiency variation with change in device parameters (layer thickness and doping concentration) was investigated for the device. Variation in efficiency was plotted against a particular changing parameter, keeping every other parameter fixed at some default value. After analysing the variation curves, two optimized designs were proposed, which yield 19.57% and 20.56% efficiency, respectively. In the later part of the work, variation in energy conversion efficiency was studied by changing the alloy composition at different layers of the device. Simulations were done for different combinations of alloy composition at device layers. The study was conducted by analyzing the efficiency values resulting for different combinations. The best results were obtained for x= 0.9, 0.48 and 0.9 in the top, middle and 9 bottom layer, respectively. For optimized values of layer thickness and doping concentration at different layers, this particular combination of alloy composition yielded an efficiency of 21.39%. This particular design for the device had some drawbacks in practical fabrication. These drawbacks were addressed with appropriate solutions, and a number of changes were brought in the highest efficiency design. The modified design, which is fully feasible for fabrication, yields an efficiency of 17.03%. Considering the high fabrication cost of III-V solar cells, a thin film solar cell design was proposed, which is only 2 μm thick (excluding the substrate thickness), and yields an efficiency of 12.16%. This efficiency was raised to 13.84% by using InP substrate, instead of Ge.