Prediction And Optimization Of Electrical And Optical Properties Of Doped SnO₂

Tin dioxide(SnO₂)transparent conductive thin films with low resistivity and high transmittance have been widely used in photovoltaic system,energy-saving buildings,semiconductor gas sensing detection and other industrial fields.However,when the film resistance decreases,the transmittance of the film will also decrease,which becomes an important factor restricting the application of SnO₂thin films.Based on semiconductor theory,element doping can contribute more carriers or vacancies,which is an important technical path to improve the optical and electrical properties of SnO₂.However,there is no uniform analysis on the mechanism of SnO₂doped with different elements.How to improve the growth rate of large area FTO(SnO₂:F)thin films and the hydrogen reduction resistance of FTO thin films when they are used in a-Si thin film solar cells are the short boards in the preparation of FTO thin films at present.In this thesis,the first principles CASTEP program was used to systematically study the non-metallic doped SnO₂and the metal-nonmetal dual doped SnO₂.Based on this,the optical and electrical properties were predicted.FTO thin films and AZO/FTO bilayers were prepared by aerosol assisted chemical vapor deposition.The effect of H₂O₂on the growth rate of FTO thin films and the hydrogen reduction resistance of AZO/FTO bilayers were studied.The results show that:(1)The band structure of SnO₂is changed after doping nonmetallic elements,and the conductivity of SnO₂can be improved by doping appropriate concentration of F element.After F element doping,Fermi level enters the conduction band,which makes SnO₂have metal conductivity.F element doped SnO₂material has the highest reflectivity,followed by N,C,S and B element doped system.The conductivity of FTO is affected by both carrier concentration and impurity scattering.When F atom concentration was 12.5at.%,carrier concentration can reach 6.46×10¹⁹cm^-3,and relative conductivity of SnO₂system can reach 1.13 when F atom concentration is 8.33 at.%.(2)The synergistic effect mechanism of SnO₂doped with nonmetal(F)and metal(Mo,V,W,Ta,Nb)was discussed.In the SnO₂:(F-M)system,the carrier concentration of Sn₂₂W₂O₄₇F is the highest,which can reach 6.33×10¹⁹cm^-3.Based on the F-V double element doping system,the conductivity of Sn₂₃WO₄₇F is the strongest with the relative conductivity of 1.39.The intersubband of d-orbitals of doped metal atoms results in different amplitudes of dielectric function spectra in the range of 0 e V?2 e V.In the visible light range,the absorption coefficients of SnO₂:(F-M)system from high to low are:F-Nb,F-Mo,F-V,F-W,F-Ta.(3)The optical and electrical properties of SnO₂doped with Ta and W atoms were compared and the mechanism of the improvement of electrical properties of F-Ta(W)doped SnO₂was analyzed.The conductivity of metal doped SnO₂increases first and then decreases due to the influence of impurity scattering.The carrier concentration of Sn₂₁W₃O₄₈is the highest,reaching 6.39×10¹⁹cm^-3.The conduction band of SnO₂is shifted down by F atom,which is helpful to enhance the conductivity of SnO₂.The carrier concentration of F-Ta(W)doped SnO₂increases with the increase of Ta(W)atom concentration,while the relative carrier mobility and relative conductivity degrade.The plasma resonance frequency of Sn₂₂W₂O₄₈is close to the visible region;Sn₂₃Ta O₄₈and Sn₂₂Ta₂O₄₈have higher reflectivity in the infrared region.(4)H₂O₂contributed to the preparation of FTO thin films with high transmittance and low resistance,and could significantly improve the growth rate of the films.The growth rate of the films increased from 6.04 nm/s(0M-H₂O₂)to 8.36 nm/s(0.08M-H₂O₂)with the increase of H₂O₂concentration.Appropriate concentration of H₂O₂(0.04M)was helpful to the growth of SnO₂crystal with better lattice.The optical and electrical properties of FTO film are the best:the transmittance is 79.87%,the thin layer resistance is 3.97?/?,and the quality factor is 2.66×10^-3?^-1.(5)In the hydrogen ionomer environment,SnO₂in AZO/FTO bilayers is not reduced,and AZO/FTO films have the ability to resist hydrogen reduction.After hydrogen ion treatment,the sheet resistance of FTO increases to 55.7?/?.The surface sheet resistance of AZO/FTO bilayers increased to 9.8?/?,after deposition of 108 nm AZO thin films.