Studies On Electronic Structure And Optical Properties Of In-doped GaN

In the paper, we use the Density Functional Theory (DFT) methods and the basic equations of solar cells to calculate the electronic structure and optical properties of In-doped GaN, and study the performance of InGaN solar cells. The main works as follows:(1) We introduce the electronic properties and optical properties of GaN, and the research status of GaN device applications and the doping-related issues, and introduce the first-principles and the mechanism of solar cells.(2) The electronic structure and optical properties of the pure wurtzite GaN and In-doped GaN systems are calculated. The results indicate that pure GaN and In-doped GaN are direct band-gap semiconductors. With the doping concentration increase, the upper valence bands occurs tiny movement to low energy, the bottom of conduction bands move towards the Fermi level, the band gap becomes smaller. Meanwhile, the conduction bands drift towards the low energy occur 0.2eV, led to the band gap from the undoped 1.898eV reduced to 1.757eV,1.561eV,1.369eV, respectively. Meanwhile, the top of valence bands energy level becomes wider, localized weaken, but conduction bands almost no change. The absorption coefficient peaks are shift towards low energy, it is a red-shift.(3) We studies the InxGa1-xN alloys with the doping concentrations were 12.5%, 25%,50%,75%, respectively. We calculate the density of states and optical properties. The results show that the energy band structure decreases with the increasing concentrations of In-doped, but the calculated values are smaller than experimental values. It is a common phenomenon, because GGA calculation method is a ground state theory, but the band gap belongs to the excited states. Thus, the calculated results are lower than the experimental values. However, this phenomenon does not affect the theoretical analysis of electronic structure and optical properties of InGaN. Meanwhile, the covalent bonds become weaker, increase the degree of charge transfer, because In atomic radius greater than the radius of Ga atoms. The absorption spectra occurs red-shift phenomenon.(4) Using wxAMPS, simulate the photovoltaic properties of InxGa1-xN single-junction solar cells. We calculated open circuit voltage, short-circuit current, fill factor and conversion efficiency, and so on. The results indicate that the conversion efficiency of the cell increases with the p-layer thickness gradually from 0.05 um to 0.07 um, then the conversion efficiency decreases with the p-layer continues to 0.25 um. When In-doping concentration increases within the range from 0.5 to 0.7, the short circuit current density increases. When the indium concentration at 0.63, the maximum conversion efficiency is 19.8%, the fill factor is 0.82. Therefore, the InGaN solar cells can improve the performance.