Modeling of temperature dependence of Λ-graded InGaN solar cells for both strained and relaxed features

Abstract

The temperature dependence of Λ-graded InGaN solar cells is studied through simulation using nextnano software. Λ-Graded structures have been designed by increasing and then decreasing the indium composition in epitaxial InGaN layers. Due to polarization doping, layers of p-type and n-type doping arise without the need for impurity doping. Different individual structures are designed by varying the indium alloy profile from GaN to maximum indium concentrations, xmax, ranging from 20% to 90% pseudomorphically strained to a GaN substrate and from 20% to 100% for completely strain-relaxed materials and linearly decreases back to GaN. For InxGa1-xN with x>∼0.9, if the material is strained to the GaN lattice constant, it is predicted to have a negative band gap. So this case is not considered here. The temperature dependence of the electrical and optical properties as they relate to the solar efficiency of the Λ-graded structures under relaxed and strained conditions are studied. Additionally, the dimensionless absorption coefficients are fitted and plotted as functions of the band gap under both strained and relaxed conditions at different temperatures. As a result, the generation rates as functions of the penetration depth within a cell can be calculated in order to obtain the solar cell parameters including efficiency for each Λ-graded structure at different temperatures. Under the strained condition, the xmax, where the solar cell efficiency reaches its maximum for each temperature, decreases as the temperature increases. At the same time, under the relaxed condition, at low temperatures (T = 100–400 K), xmax is 100%, that is, grading to InN results in maximum efficiency, while at higher temperatures (T = 500–800 K), xmax decreases with the increasing temperature.