Research ArticleAPPLIED PHYSICS

Enhancing ferroelectric photovoltaic effect by polar order engineering

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Science Advances  06 Jul 2018:
Vol. 4, no. 7, eaat3438
DOI: 10.1126/sciadv.aat3438
  • Fig. 1 Polar instability of BFO induced by chemical substitution.

    (A) Polarization and (B) relative permittivity hysteresis loops of La-substituted BFO films. (C) Evolution of remnant polarization and relative permittivity versus La concentration showing FE-PE phase transition. Piezoresponse images of (D) pure and (F) 20% La–substituted BFO films in as-grown states. (E and G) Piezoresponse images after writing box-in-box patterns, as depicted by the dashed squares in (D) and (F), respectively. The blue circles indicate the same locations before and after switching. The scale bar is 1 μm for all PFM images. (H) Simplified schematic representing the polarization states of the La-substituted films after electrical poling. (I) Brief temperature-composition phase diagram indicating the competing polar instability caused by the substitution of Bi by La. R, rhombohedral-like phase; O1, AFE orthorhombic phase; O2, PE orthorhombic phase. The region in between FE and AFE shows polar instability.

  • Fig. 2 Enhancement of photovoltaic effect of BFO by La substitution.

    Current density–voltage characteristics of La-substituted BFO films with (A) downward and (B) upward polarization states, respectively. The insets illustrate the device structure under test. (C) Time-dependent Jsc of different La-substituted BFO films. (D) Dependence of Voc and Jsc on the La concentration, showing the progressive enhancement when approaching the phase transition boundary.

  • Fig. 3 Optical properties of La-substituted BFO.

    (A) Absorption coefficients of La-substituted BFO films determined by spectroscopic ellipsometry. The light spectrum used for photovoltaic measurements coincides with the visible light spectrum. (B) Calculated direct and indirect transition energies of the second electronic transition indicated in (A). (C) Transient reflectivity curves of 0 and 20% La–substituted BFO films fitted by biexponential functions plus a slow amplitude component for radiative recombination. (D to G) (αhν)2 and (αhν)1/2 Tauc plots of La-substituted BFO films used for determining the direct and indirect transition energies. The green and red shaded regions indicate regions satisfying indirect and direct transition models, respectively.

  • Fig. 4 First-principles calculations of La doping in BFO.

    (A) DFT-calculated band structure of pure BFO (2 × 2 × 2 supercell). The inset zooms in the valence band edge near X and Γ points, indicating nearly direct bandgap. (B) DFT-calculated band structure of 18.75% La–substituted BFO. The inset shows the indirect bandgap feature of the valence band edge at X and Γ points. (C) Carrier lifetime enhancement factor, calculated from DFT band structures, under different substitution concentrations and for different La defect configurations. When introducing more La into BFO, the band structures show more indirect bandgap features, leading to reduced recombination rate. In most configurations, La substitution results in an indirect gap. The blue square at 18.75% La illustrates an uncommon case with a direct gap. (D) Band charge density of [001] plane for VBM at X point in pure BFO, showing the nodal sites at middle Bi atoms. (E) Band charge density of [001] plane for VBM at X point in La-doped BFO with the same nodal site positions. In this case, La atoms occupy the wave function nodal sites, leaving the energy at this k point unchanged.

  • Fig. 5 MOs and lattice distortion of FeO6.

    (A) Hybridization between 5 Fe 3d and 18 O 2p orbitals forms low-energy bonding (blue box), high-energy antibonding (red box), and intermediate nonbonding (black box) orbitals. The dashed arrow denotes weak dipole-forbidden transition, and the solid arrow denotes strong dipole-allowed transition. (B) Crystal structure of BFO with R3c symmetry showing FeO6 distortion and tilting pattern coupled to the ferroelectric polarization (blue solid arrow). The blue dotted arrow indicates the polarization rotation induced by A-site substitution. The black dotted arrows denote the displacive tendency of the Fe and O6 relative to Bi sublattice in accordance with the polarization rotation path, that is, the in-plane polarization tends to diminish.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/4/7/eaat3438/DC1

    Section S1. Carrier lifetime enhancement due to indirect bandgap.

    Fig. S1. Ferroelectric polarization characterizations of the La-doped BFO films.

    Fig. S2. Dielectric characterization of the La-doped BFO films.

    Fig. S3. Topographic and PFM images of the as-grown La-doped BFO films.

    Fig. S4. Local polarization switching of the La-doped BFO films.

    Fig. S5. Temperature-dependent P-E loops.

    Fig. S6. Photocurrent and dark current of the La-doped BFO films.

    Fig. S7. Spectrum of the halogen lamp used in the photovoltaic test.

    Fig. S8. Optical dielectric constants.

    Fig. S9. Direct-forbidden fitting and integrated spectral weight.

    Fig. S10. Complete fittings of the absorption spectra using Tauc’s rule.

    Fig. S11. Universal polar instability–enhanced FPV effect.

    Fig. S12. Evidence for polar instability in Dy-doped BFO.

    Fig. S13. Evidence for polar instability in BiFe0.5Cr0.5O3.

    References (5456)

  • Supplementary Materials

  • This PDF file includes:
    • Section S1. Carrier lifetime enhancement due to indirect bandgap.
    • Fig. S1. Ferroelectric polarization characterizations of the La-doped BFO films.
    • Fig. S2. Dielectric characterization of the La-doped BFO films.
    • Fig. S3. Topographic and PFM images of the as-grown La-doped BFO films.
    • Fig. S4. Local polarization switching of the La-doped BFO films.
    • Fig. S5. Temperature-dependent P-E loops.
    • Fig. S6. Photocurrent and dark current of the La-doped BFO films.
    • Fig. S7. Spectrum of the halogen lamp used in the photovoltaic test.
    • Fig. S8. Optical dielectric constants.
    • Fig. S9. Direct-forbidden fitting and integrated spectral weight.
    • Fig. S10. Complete fittings of the absorption spectra using Tauc’s rule.
    • Fig. S11. Universal polar instability–enhanced FPV effect.
    • Fig. S12. Evidence for polar instability in Dy-doped BFO.
    • Fig. S13. Evidence for polar instability in BiFe0.5Cr0.5O3.
    • References (5456)

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