| Owing to their good chemical stability and excellent optical performance, wide band-gap SnO2 thin films have been widely applied in gas sensors, dye-sensitized solar cells, transparent electrodes, and catalytic materials. Generally, intrinsic SnO2 has poor conductivity and is not sufficient to achieve the requirements for the optical, electrical, and gas sensing fields. By incorporating appropriate amount of elements, the optical and electrical properties of SnO2 can be optimized and adjusted. Thus researches on this kind of films will be promising subjects. Since the rare earth elements have unique 4f electrons structure, they have good luminescence properties. Meanwhile, the rare earth atoms are polarizable and can form defects by filling the space or replacing initial atoms in SnO2 lattice. Due to different electron structure, the photoelectric properties of SnO2 thin films can be changed after doping. However, there are rare theoretical reports about this kind of doping. Calculations based on the density functional theory can provide valuable information for the study of electronic structure and properties of rare earth elements doped SnO2, and the performance of the doped system can be predicted.First, SnO2 thin films under different annealing temperature were fabricated using sol-gel method. The factors affecting their photovoltaic performance were then analyzed. Secondly, a series of rare earth elements (La, Eu, Ce) with different doping concentration are introduced to study their effects on the optical properties of SnO2 thin films. On this basis, Ce and Cu were co-doped in SnO2 thin films, and the underlying mechanism was analyzed. The details are as the following:(1) The influence of annealed temperature on the structure, optical and electrical properties of the SnO2 thin film is investigated. The results show that with the increase of annealed temperature, the atoms can obtain enough energy to nucleate in film surface and move to low-energy position, decreasing the defects in film. The crystalline quality is improved and the surface becomes smoother. As a result, the light absorption and reflection loss gets weakened, while the transmittance is increased. In addition, the increased grain size weakens the grain boundary scattering and barrier, improving the film conductivity. However, when the annealed temperature is too high, the defects in film will increase and the optical and electrical characteristics get worse. The optimal temperature is 500℃.(2) The structural, optical and electrical properties of La-doped SnO2 are studied. Besides, La doping doesn’t change the film phase. As the La doping content increases, grain size, film crack and band gap is decreased. After La doping, no new luminescence peak occurs because of the f-f transition of La. The empty and stable electronic structure of the La 4f electron orbit leads to the increased disorder of ion arrangement in SnO2, which increases the vacancy concentration and improves the ion conductivity. The first-principles calculation shows that La doping induces accepter level at the top of valence band, which connects with valence band and forms new degenerate energy level. The Fermi level shifts up into conduction band, leading to the decrease of band gap and the shift of the imaginary part of dielectric functionand absorption spectrum to lower energy.(3) As increasing the Eu doping concentration, the interplanar spacing of the film changed largely, which inhibits the growth of crystal nucleus and increases the surface diffusion barrier. Then, the separation of photoelectrons from vacancies can be effectively promoted and the light transmittance is further improved. The first-principle computation shows that the Eu density of states at Fermi energy level is relatively low and the electronic transition rate decreases, leading to the weaker light absorption and higher transmittance. The Sn4+ locates at the symmetry lattice sites in SnO2 crystal, while when the Eu3+substitutes for the Sn4+ site, Eu3+ locates at the asymmetric center field and the transition strength of 5D0→7F2 is bigger than 5D0→7F1.(4) With Ce doping, most of the SnO2 films suffer from the tensile stress, except for the 0.5%Ce doped one. Compared with the La and Eu doped films, the Ce doped ones exhibit better transmittance and conductivity. With Ce3+ doping and generating Cesn3+, defects, the adjacent O atom can divorce from the oxygen lattice and leave the oxygen vacancy, increasing the amount of carriers. The PL peak at 403 nm might correspond to the electron transition from the donor level formed by oxygen vacancy to the valence band and the transition generates violet light emission. After Ce3+ doping, strong blue emission peaks at 482 nm can be seen from the spectra, which belongs to the electron transition between 5d-excited state and 4f state of Ce3+ ion. However, when the Ce3+ ion concentration is high, there is a great amount of Ce3+ at grain boundary layer, which leads to dispersion and affects the photoluminescence.(5) With the Cu and Ce co-doping, the acceptor defects of Cusn3+ and Cesn3+ are generated and the resistivity of the films increased in some doping percentage. Meanwhile, the transmittance and the band gap decreases and the PL intense reduces compared with the Ce doped SnO2 film. The first-principle calculation indicates that the Cu impunities induce an acceptor level near the valence band maximum. After Ce co-doping, the large hybridization between the defect level and impunity level leads the conduction band shift to the lower energy side, which can further decrease the band gap and ionization energy.With doping the rare earth elements, the crystalline structure of SnO2 changes little and the grain size decreases with increasing the doping concentration. With La doping, the conductivity is improved and there is no new peak in the PL spectrum. With Eu doping, the transmittance is enhanced and the PL peaks blue shift. With Ce doping, both the conductivity and transmittance are improved, while the Ce, Cu co-doping work the opposite effect.
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