Internal Quantum Efficiency Of Wurtzite InGaN/GaN Core-shell Nanowire Solar Cells

A high photovoltaic efficiency can be achieved in the wurtzite InGaN/GaN coreshell nanowire solar cells by taking advantage of both the large surface area to volume ratio of the nanowire structure and the band gap of InGaN,which could be continuously tuned from 0.64 to 3.43 eV with component,covering almost the entire solar spectrum.The inner quantum efficiency of a solar cell is usually estimated based on the band edge of bulk materials,not accounted for the fact that the quantum confinement of InGaN/GaN quantum wells leads to discrete levels of carriers varying with the indium component and well width.This thesis discusses the internal quantum efficiency(hereafter referred to as efficiency)and its variation with component and size in cylindrical wurtzite InGaN/GaN core-shell nanowire solar cells by taking into account the quantum confinement effect and the carrier collection probability.The approach is based on the black-body approximation and detailed balance theory.Firstly,the eigen states of electrons and holes are solved from Hamiltonian equations under the approximation of effective mass.The gap between the ground states of electron and hole determined the minimum light absorption energy and the input power in the approximation of blackbody.Then,detailed balance theory and a maximum efficiency model are adopted to solve the quasi-Fermi energy levels of electrons and holes and thus effective photo generated carriers is obtained to calculated the open circuit voltage.The product of the open circuit voltage and short circuit current is the output power.Finally,internal quantum efficiency is obtained by the ratio of output power to input power.The results are summarized as follows:When all the photo generated carrier escapes,(1)Increasing the indium component x with fixed quantum well widths and barrier thicknesses leads to increased quantum confinement effects which move the electron and hole ground states away from the band edges,while the quantum well band gap narrows,leading to a non-monotonic change in the optical absorption band gap,and the input power,output power and efficiency also show the trend of nonmonotone change.(2)Increasing indium component x from 0 to 1 for different quantum well widths and fixing the barrier thickness,it was found that the maximum output power and extreme efficiency occurred at two close x.For wide quantum well,the maximum output power and extreme efficiency were higher and the corresponding x was smaller.(3)Increasing x from 0 to 1 for different barrier thicknesses and fixing the quantum well width,the maximum output power and extreme efficiency were found to occur at two similar x.The maximum output power and extreme efficiency were lower for thicker barrier and the corresponding x was smaller.When the barrier thickness is less than the critical value,the extreme efficiency is the highest efficiency;when the barrier thickness is greater than the critical value,the highest efficiency is achieved when x is 0.In the case that part of photo generated carrier escapes,a shorter tunneling lifetime than the thermal excitation lifetime tunneling indicates that tunneling is a dominant escape mechanism.If the escape probability is taken into account,the output power and the extreme efficiency decrease while the corresponding indium component x remains unchanged.This work is expected to provide a theoretical reference for optimizing the efficiency of wurtzite InGaN/GaN core-shell nanowire solar cells.