The optical properties of Λ-graded indium gallium nitride (InGaN) solar cells are studied. Graded InGaN well structures with the indium composition increasing to xmax and then decreasing in a Λ-shaped pattern have been designed. Through polarization doping, this naturally creates alternating p- and n-type regions. Separate structures are designed by varying the indium alloy profile from GaN to maximum indium concentrations ranging from 20% to 90%, while maintaining a constant overall structure thickness of 100 nm. The solar cell parameters under fully strained and relaxed conditions are considered. The results show that a maximum efficiency of ≅5.5 % under fully strained condition occurs for xmax = 60 % . Solar cell efficiency under relaxed conditions increases to a maximum of 8.3% for xmax = 90 % . Vegard’s law predicts the bandgap under relaxed conditions, whereas a Vegard-like law is empirically determined from the output of nextnano™ for varying indium compositions to calculate the solar cell parameters under strain.
The optical properties of periodic graded GaN/InGaN are studied. We have designed graded InGaN quantum well (QW) structures with the indium composition increasing then decreasing in a zigzag pattern. Through polarization doping, this naturally creates alternating p-type and n-type regions. Separate structures are designed by varying the number of repeating periods (1 to 3), while maintaining constant overall structure thicknesses. Calculation of the transition probabilities and the electron and hole wave-functions between the conduction band and the valence band reveals a complex energy structure which predicts the photoluminescence peaks for band to band transitions.
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