Physical Property Tunning Of Transition Metal Oxide And Its Application In CdTe Solar Cell

Transition metal oxides have complex physical properties.The strong coupling among charge,spin,orbitals,and lattice has attracted a lot of attention from researchers.In transition metal oxides,the exchange interaction and strong correlation interaction between electrons have become important research objects in condensed matter physics.This promoted the application and development of materials and spintronic devices,such as giant magnetoresistance,transparent conductivity materials,ultraviolet laser,and solar cells.In CdTe thin-film solar cells,severe carrier recombination and microscopic leakage channels at the interface can significantly affect cell performance.Because of their tunable physical properties and large bandgap,transition metal oxide is an ideal candidate for buffer layer material in CdTe solar cells.In this thesis,by regulating the physical properties of several transition metal oxides,their fundamental scientific issues were experimentally investigated.By tuning the material property,their effects as buffer layers on the performance of high conversion efficiency CdTe solar cells were discussed.In Chapter ?,the magnetic order and semiconductor properties of transition metal oxides and their applications in spintronic and semiconductor devices are briefly described based on the exchange interaction and correlation interaction.The basic principles of solar cells are introduced,and the applications of transition metal oxides in CdTe solar cells are discussed.In Chapter ?,the physical properties of MgxZn1-xO thin films are tuned.By using vacuum heat treatment,the microscopic morphology,crystalline quality,defect types,and electrical conductivity of MgxZn1-xO(MZO)films were tuned.It was found that vacuum heat treatment causes the loss of MZO surface components,resulting in a rough Mg-rich layer on the film surface and causing changes in the electron affinity potential on the film surface.The physical process of vacuum heat treatment effect on MZO resistivity was investigated.It was found that after vacuum heat treatment,the defect concentration of intrinsic donor defect oxygen vacancies was increased,and the defect concentration of intrinsic accepter defect interstitial oxygen was decreased in the film.This leads to the changes in resistivity and carrier concentration of MZO films.In Chapter ?,the application of MgxZn1-xO thin films in CdTe solar cells is investigated.CdTe solar cells were fabricated by using vacuum heat-treated MZO films.Combing the experimental results and the theoretical simulation results,it was found that the carrier concentration and Mg-rich layer of the MZO films would change the band alignment,which affects the fill factor of solar cells.And the rough MZO surface would increase the trap concentration in the CdTe layer,which decreases the open-circle voltage of solar cells.Besides,new structure CdTe solar cells were fabricated.It was found that the interdiffusion between CdSe and CdS was unfavorable to the carrier collection.And the oxygen in the CdTe deposition and heat treatment atmosphere would lead to the "S-kink" phenomenon in the current density-voltage curve of solar cells.By improving the fabrication process,this phenomenon is eliminated and higher conversion efficiency is obtained.In Chapter ?,the physical properties of TiO2 thin films are tuned and their application in CdTe solar cells is investigated.By vacuum heat treatment,the concentration of oxygen vacancies in TiO2 thin films was tuned,and an electron concentration of 1018 cm-3 was obtained.CdTe solar cells were prepared with TiO2 films of different thicknesses.It was found that noncontinuous TiO2 could modify the front electrode surface and reduce the density of leakage channels,which would increase the open-circuit voltage of solar cells.While the oxygen in the subsequent fabrication process would decrease the oxygen vacancy concentration in the TiO2 layer and increase its resistance.Thus continuous TiO2 films would hinder carrier transport,and lead to the "S-kink" phenomenon in the current density-voltage curve of solar cells.In Chapter ?,the physical properties of NiO antiferromagnet are tuned and the spin-phonon coupling effect is studied.By tuning the stoichiometric ratio of NiO nanoparticles,the relative changes of Raman peak intensities of two strong first-order vibrational modes are detected.With the help of theoretical calculation,these two modes are assigned to the vibrational modes at the Brillouin zone boundary.Further temperature-dependent Raman spectra revealed anomalous changes in the Raman peak position and full width at half maximum.This phenomenon was attributed to the spin-phonon coupling effect,and the spin-phonon coupling coefficients of different size NiO nanoparticles were obtained.In Chapter ?,the main contents of this thesis are summarized,and the research perspectives of the future work are proposed.