Online ISSN : 2423-8872
Print ISSN : 0029-0181
ISSN-L : 0029-0181
Volume 75 , Issue 3
Showing 1-23 articles out of 23 articles from the selected issue
  • Motoharu Imai, Naoto Umezawa
    2020 Volume 75 Issue 3 Pages 148-153
    Published: March 05, 2020
    Released: September 14, 2020

    Barium disilicide (BaSi2) has shown potential as a photo-absorber material for thin-film solar cells. The electronic properties, optical absorption coefficient, and defect properties of BaSi2 were investigated using calculations based on the density functional theory. The calculations show charge transfer from Ba to Si atoms and covalent bond formation in Si4 tetrahedra, which confirm that BaSi2 is a member of the Zintl phase. The density of states and partial charge distributions for BaSi2 revealed that the electronic states of BaSi2 can be explained based on the electronic states of Si4 and Ba atoms. The high optical absorption coefficient can be attributed to three direct transitions with energy gaps, which are slightly larger than the indirect band gap. Si vacancy, Ba substituted for Si antisite, and Si interstitial defects are dominant, but do not create a significant number of carriers. The calculated Fermi level is pinned in the middle of the band gap, indicating that bipolar doping is feasible; thus, it is easier to form p–n junctions.

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  • Naoki Ogawa, Masato Sotome, Masao Nakamura, Takahiro Morimoto
    2020 Volume 75 Issue 3 Pages 154-159
    Published: March 05, 2020
    Released: September 14, 2020

    When a photon induces an interband transition in a solid, the excited electron (hole) experiences a spatial shift if the crystal lacks an inversion symmetry, leading to a generation of directional photocurrent. This phenomenon has been known as bulk or anomalous photovoltaic effect for years, and is recently reformulated with the viewpoint of quantum mechanics as the change in the geometrical phase upon photoexcitation. It is also predicted that this so-called shift current of electron (hole) can be less-dissipative and ultrafast. We demonstrate the emergence of shift current in several organic and inorganic ferroelectrics by using extensive optical spectroscopies, whose excitation spectra nicely compared to the ones from first-principles band calculations.

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