MODELLING AND SIMULATION OF VERTICAL NANOWIRE SOLAR CELLS

Abstract

Better collection efficiency, large surface to volume ratio, low processing cost and better defect tolerance are the main qualities of nanowire (NW) structure which makes it favourable choice over conventional planar solar cell. In addition to above qualities one of the main advantages of NW is that it can be used in multi junction (MJ) solar cells without the restriction of lattice-matching due to their small cross-sectional area which permits lattice mismatch strain to be low at the NW surface. But still the best efficiency of experimentally reported NW solar cell is just comparable to planar one, which point to the fact, that still lot of research work has to be done in this area. In this work optimization of efficiency has been done for GaAs and silicon vertical NW solar cells. The performance of NW solar cell is benchmarked with similar dimension planar structure under same illumination condition. Effects of doping density, height, core radius, shell thickness in NW structure, n-layer thickness in planar structure and incidence light angle variation are investigated. We fmd total internal reflection in the cell when light falls on the surface at an angle larger than 00. All the efficiencies are calculated with 0.1μm Si3N4 layer (as anti-reflecting coating) over the illuminated area. Maximum solar cell height taken into consideration for all simulations (except for height study) is 3μm which is enough for complete light absorption by GaAs. Study of traps is also done for both the materials and it is found that for the given amount of impurities, NW structure has better performance compared to planar structure. Traps study shows that for trap density of I x 1018 cm 3 GaAs NW has only 4% decrement in efficiency in comparison to 33% in its planar structure while in case of silicon, these changes are 20% and 75% respectively. These results indicate the importance of NW solar cells. One of our important observations shows that for better efficiency of NW structure, the diameter of cell should be larger than the depletion width of the cell at a particular doping. Results also show that the doping should be as high as possible for better results. For the optimized GaAs NW, GaAs planar, silicon NW and silicon planar solar cell the efficiency is found to be 29.37, 23.19, 9.11 and 6.02 respectively at normal light incidence. All the simulations are done in Sentaurus, TCAD tool by Synopsys. After optimisation of GaAs and silicon cells individually, a GaAs-silicon hetro-junction tunnel diode is optimised for higher tunnel current in order to use it in the simulation of MJ solar cell comprising of GaAs NW and planar silicon solar cell as upper and lower cells respectively. The resulting efficiency is too low when compared with the individual cells of GaAs and silicon which suggest need for further work in MJ solar cell.