Modeling and Analysis of Multi-junction Solar Cells

In this paper a methodology to evolve an analytical design of a high efficiency multi-junction solar cell is presented. The multi-junction selected for the study is a four-junction tandem solar cell structure, ZnO-ZnSe-Si-Ge. These materials have wide range of band gap from 3.33 to 0.67 eV, so as to absorb maximum solar radiation. ZnO is used as top layer to work as transparent conductive oxide layer to minimize the optical reflection loss; ZnSe is used to match the band gap between ZnO and silicon to minimize transmission loss. Other materials, Silicon and germanium that have lower band gap absorb most of longer wave length photons and minimize transmission losses. An analytical model is developed and implemented using Matlab script file. Model is used to analyze various parameters to estimate short circuit current, open circuit voltage, and fill factor and efficiency of different junctions. Further efficiency enhancement of multi-junction solar cell is done at different conditions such as optimization of PN diode, adding an intrinsic layer to make PN as PIN diode and finally to add tunneling layer to make PIN as p-n-i-p⁺. Analytical efficiency of 76.69 % is obtained with optimization, intrinsic and tunneling layers at 1 SUN irradiation (incident power Pin=1 kW/m²).