The Study Of Carrier Transportation Of Silicon Nanowire Solar Cells

We are living in a era of energy source crisis,the solar energy is an excellent substitute of traditional energy because of its cleanliness,renewability,wide range of application and low cost.Recently silicon nanowire solar cells have attracted much attention by researchers for its low reflectivity and special light absorption ability.In this thesis the author focuses on the problem that the carrier transportation distance of axial structure solar cells is too long so the carrier might be recombined before they reach the junction area and the open circuit voltage of device is affected critically.A new solution has been developed,that is fabricating axial structure PEDOT/SiNWs solar cells with PEDOT only covering on the top of nanowires and radial structure PEDOT/SiNWs solar cells with PEDOT sticking on the whole side wall by using two kinds of PEDOT with different wetting property and surface tension.By adjusting the carrier transportation route,the photogenerated carriers were separated and collected more effectively and the recombination was suppressed so that the performance of nanowire cells was improved.This thesis includes two major issues as follows:The light coupling of single silicon nanowire with different radii and the reflection spectra of silicon nanowires with different parameters(period,radius,length)were simulated using Finite difference time domain method(Lumerical FDTD).In the single nanowire,the light intensity is much stronger than that in the planar silicon,and 125 nm is the optimal radius among the absorption spectrum of silicon.As for the silicon nanowires array,after analyzing the data,a conclusion was found that for the hexagonal close packed structure when the period of array is 500 nm and the radius of nanowires is equal to one fourth of period,the fill rate is 0.196,the average reflectivity is the lowest among all band.It is a vital conclusion about the optical design of silicon nanowire arrays and it offers reliable theoretical basis for the experiments.The silicon nanowire array were etched with microsphere template.And the radial heterojunction silicon nanowire solar cells were fabricated by using two kinds of PEDOT with different wetting property and surface tension.The simulation result of field distribution in the nanowire is cooperated with the data of reflectivity,surface morphology,light current and Quantum Efficiency measurement.Comparing the performances of radial and axial devices,it turns out that the open circuit voltage and the PCE of radial devices were improved obviously for the optimization of carrier transportation route and the reduction of recombination.A power conversion efficiency of 12.46%(26.7% increase)and an open circuit voltage of 0.525V(7.4% increase)were achieved,which is a good result.Radial structure cells avoid the disadvantage of axial structure ones that the recombination of carriers on the side wall is critical.Thanks to the structure changing of heterojunction,the open circuit voltage and power conversion efficiency were dramatically improved and it offers important reference for wide application of silicon nanowire solar cells.