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Electronic Band Structure, Phonons and Exciton Binding Energies of Halide Perovskites CsSnX3 with X=Cl, Br, I

HUANG, LING-YI
http://rave.ohiolink.edu/etdc/view?acc_num=case1436477745

Abstract Details

2015, Doctor of Philosophy, Case Western Reserve University, Physics.
Halide perovskites (ABX3) have attracted a large amount of attention since 2009 due to their important applications in solar cells. Though high power conversion efficiencies have been achieved, the underlying reasons for the success are not fully understood. Moreover, some important issues have not yet been resolved, mainly stability issues resulting from phase transitions in halide perovskites. In this thesis, we used the Full-Potential Linear Muffin-Tin Orbital method with Quasiparticle Self-consistent GW (QSGW) to study the electronic band structures and related properties in one family of halide perovskites: CsSnX3 with X=I, Br, and Cl. We also used density functional perturbation theory to perform phonon calculations for these materials. Our first-principles calculations are compared with experimental data. The main purpose of this study is to understand the fundamental physical properties in these materials and the underlying reasons for the success of these materials in solar cells, and eventually to elucidate information about phase transitions in these materials with the goal of resolving the stability issues. Our QSGW band gaps are in good agreement with experimental gaps. An extremely interesting finding is that the halide perovskites have an “inverted” band structure; i.e. in contrast to most conventional semiconductors, their valence band has a strong Sn-s like character (antibondingly mixed with X-p) while their conduction band has Sn-p character. The unusual characteristic explains (i) the strong luminescence, (ii) the relatively weak band gap dependence on anions, (iii) why the valence band maximum occurs at the R-point, and (iv) the anomalous dependence of the band gap on lattice constants. We also find that the small effective hole mass results in high hole mobility. The free-exciton binding energy was estimated to be of the order of 0.1 meV for CsSnI3. Strong LO-TO splittings and large static dielectric constants are found. LO phonons contribute to infrared absorption spectra of α-CsSnBr3 and α-CsSnCl3 and Raman spectra of γ-CsSnI3 and M-CsSnCl3. Strong LO-plasmon couplings are expected in these materials because the plasmon frequencies are comparable to the LO phonon frequencies. We find soft phonons, which are phonons with imaginary phonon frequencies, in cubic and tetragonal phases, but not in orthorhombic and monoclinic phases. The phase transitions from the α phase to the β phase and then to the γ phase can be summarized, in terms of space groups and soft phonons, as follows Oh1 ― (M2+) ➝ D4h5 ― (Z5-) ➝ D2h16, where the M2+ and Z5- are the soft phonons in the cubic and tetragonal phases, respectively. The displacement pattern of the M2+ mode is shown in Fig. 5.4 while the displacement pattern of the Z5- mode is shown in Fig. 5.5. The notation M2+ means that the phonon at k-point M of the cubic Brillouin zone belonging to the irreducible representation 2+ of the point group of that k-point, D4h, leads to the transformation of the crystal to a new one with the D4h5 subgroup as spacegroup and similar for the second step.
Walter Lambrecht (Advisor)
143 p.
Physics

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Citations

  • HUANG, L.-Y. (2015). Electronic Band Structure, Phonons and Exciton Binding Energies of Halide Perovskites CsSnX3 with X=Cl, Br, I [Doctoral dissertation, Case Western Reserve University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=case1436477745

    APA Style (7th edition)

  • HUANG, LING-YI. Electronic Band Structure, Phonons and Exciton Binding Energies of Halide Perovskites CsSnX3 with X=Cl, Br, I. 2015. Case Western Reserve University, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=case1436477745.

    MLA Style (8th edition)

  • HUANG, LING-YI. "Electronic Band Structure, Phonons and Exciton Binding Energies of Halide Perovskites CsSnX3 with X=Cl, Br, I." Doctoral dissertation, Case Western Reserve University, 2015. http://rave.ohiolink.edu/etdc/view?acc_num=case1436477745

    Chicago Manual of Style (17th edition)

case1436477745
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This open access ETD is published by Case Western Reserve University School of Graduate Studies and OhioLINK.