Polycrystalline oxide materials exhibit semiconductor properties due to grain boundary (GB) and grain characteristics, which enrich the variety of applications. However, how to regulate the energy band structure of grains and the potential barriers at GBs through defect engineering is crucial to achieve a high performance electronic device. Herein, it is found that Fe3+ ions can change the grain energy band structure of CaCu3Ti4O12 (CCTO) materials, which enhances the linearization of the resistance–temperature curve (ln ρ–1000/ T) in the high temperature region. First principles calculation indicates that Fe3+ doping narrows the forbidden band and induces new impurity energy levels in the forbidden band, which matches the conclusion that the resistivity–temperature dependence of grains shifts toward the low-temperature region as derived from impedance spectroscopy. This shift results in no monotonic variation in grain resistivity within the application temperature region, thus enhancing the linearity of the ln ρ–1000/ T curve of CCTO materials in the high temperature region. In addition, Fe3+ ions can modulate the activation energy of CCTO materials in a wide range by changing the activation energy of GBs, which broadens the temperature range of CCTO. The significance of this work lies not only in achieving linearization of CCTO materials for high temperature thermistor application, but more importantly, the method presented here provides an avenue for the study of polycrystalline semiconductor materials.
Based on the density functional theory, the energy band and electronic structure of β-CuGaO2 are calculated by the modified Becke-Johnson plus an on-site Coulomb U (MBJ + U) approach in this paper. The calculated results show that the band gap value of β-CuGaO2 obtained by the MBJ + U approach is close to the experimental value. The calculated results of electronic structure indicate that the main properties of the material are determined by the bond between Cu-3d and O-2p energy levels near the valence band of β-CuGaO2, while a weak anti-bond combination is formed mainly by the O-2p energy level and Ga-4s energy level near the bottom of the conduction band of β-CuGaO2. The β-CuGaO2 thin film is predicted to hold excellent photovoltaic performance by analysis of the spectroscopic limited maximum efficiency (SLME) method. At the same time, the calculated maximum photoelectric conversion efficiency of the ideal CuGaO2 solar cell is 32.4%. Relevant conclusions can expand β-CuGaO2 photovoltaic applications.
Solid‐solution ceramics show a potential in the field of electronic devices. In particular, it is necessary to investigate the photoluminescence and conductive properties of CaEuNbMoO8 (CENMO) solid solution ceramics. It is demonstrated that the sintered ceramics appear as CaMoO4 phase with scheelite structure. The results of X‐ray photoelectron spectroscopy show that the abnormal reduction of Eu3+→Eu2+ occurs in the ceramics, which is due to the formation of VCa′′$V_{Ca}^{^{\prime\prime}}$ vacancy caused by the substitution of Ca2+ by Eu3+, making it act as electron donors. The change of local symmetry of Eu3+ ions in ceramics results in a strong red fluorescence, and the charge transition between Eu, Nb, and Mo ions and O ions in the host lattice results in the near‐ultraviolet (near‐UV) excitation. As a result, CaEuNbMoO8 ceramic materials have promising applications in both near‐ultraviolet excited light‐emitting diodes and thermistors.
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