Ferroelectric Photovoltaic Materials Research Articles

Theoretical study of stability and optical absorption properties of ferroelectric materials ZnXO3 (X=Ge, Sn and Pb)

The chemical potential equilibrium phase diagram and optical absorption properties of ZnXO3 (X = Ge, Sn, and Pb) of LiNbO3 type (LN- ZnXO3) have been calculated by density functional theory. We find that LN-ZnXO3 (X = Ge, Sn, and Pb) do not form a stable chemical potential area with respect to the binary compounds, but could be stabilized under external pressures. The non-equilibrium chemical potential areas in LN-ZnXO3 (X = Ge, Sn, and Pb) are in a sequence of Ge > Sn > Pb. The calculated tendency of transition pressures and binary impurities compound during the synthesis of LN-ZnXO3 (X = Ge, Sn and Pb) are consistent with the experimental results, which are explained by the structure and electronic structure analysis. The calculated optical absorption property indicates that LN-ZnPbO3 could be a good ferroelectric photovoltaic material, while LN-ZnXO3 (X = Ge, Sn) are a good visible light transparent material. The diversity of the feasible transition between VBM and CBM charge with small band gap play important roles in good performance of optical absorption property in LN-ZnPbO3. These inclusions could expand the applications of LN-ZnXO3 (X = Ge, Sn, Pb).

First-principles investigation of bandgap tailoring in tetragonal Bi2FeCrO6 by magnetic ordering and B-site-cation ordering

The development of ferroelectric photovoltaic (FE-PV) materials has been limited for a long time due to their large bandgap. Many strategies for lowering the bandgap have been suggested to promote FE-PV properties. The effects of magnetic ordering and B-site-cation ordering to lower the bandgap of FE-PV are investigated in this paper using first-principles calculations. Results show that the most stable structure of tetragonal Bi2FeCrO6 ([Formula: see text]-Bi2[Formula: see text] is the AS1 structure (Fe/Cr alternate stacking ordering) with C-type antiferromagnetic ordering (defined as AC-[Formula: see text]-Bi2FeCrO6), which has a small bandgap, suggesting that AC-[Formula: see text]-Bi2FeCrO6 is among the FE-PV materials with the highest application potential.

Polarization tunable and enhanced photovoltaic properties in tetragonal-like BiFeO3 epitaxial films with graphene top electrode

Ferroelectric photovoltaic materials have attracted intensive interest due to their intriguing above-bandgap photovoltage, while the low photocurrent limits their further device applications. In this work, both enhanced and tunable photovoltaic effect (PVE) are demonstrated in sandwiched structure of Graphene/tetragonal-like BiFeO3/Ca0.96Ce0.04MnO3 (Graphene/T(-like) BFO/CCMO). The epitaxial BFO film is grown on the CCMO buffered LaAlO3 substrate by pulsed laser deposition and the mechanically exfoliated graphene is transferred directly onto the BFO film as top electrode. The optimized open circuit voltage (Voc) and short circuit current density (Jsc) are measured to be −0.88 V and 2.56 mA/cm2, respectively. Moreover, the photovoltaic response can be modulated by controllable ferroelectric polarization, whereby both the Voc and Jsc show piezoresponse-like hysteresis behaviors against voltage. The notably enhanced PVE is likely a result of the combination effects of large polarization in the T(-like) BFO film, partially unscreened depolarization field, high transmittance and conductivity of the graphene top electrode. This work clearly indicates the potential of Graphene/ferroelectric photovoltaic devices for memory and energy conversion applications.

Bandgap tailoring of tetragonal BiFeO3 with different magnetic orders calculated by first–principles calculations

Narrow bandgap and high polarization are crucial for improving the photovoltaic properties of ferroelectric photovoltaic materials. In this work, bandgaps of tetragonal BiFeO3 (t-BiFeO3) with different magnetic orders are investigated using first-principles calculations. The results show that when the G-type antiferromagnetic order converts to ferromagnetic order, the bandgap of t-BiFeO3 decreases from 1.530 eV to 1.037 eV and simultaneously the crystal still possesses large polarization (1.600 C m−2). Compared to C-type/G-type t-BiFeO3, the bandgap narrowing of A-type/FM are originated from the downward shift of Fe 3d eg antibonding states and the upward shift of Ob 2p states due to the ionicity increasing of Fe and Ob. This work provides an insight into how to improve the photovoltaic properties by tailoring the bandgap of the t-BiFeO3 ferroelectric materials.

Double perovskite Bi2FeMoxNi1-xO6 thin films: Novel ferroelectric photovoltaic materials with narrow bandgap and enhanced photovoltaic performance

Double perovskite ferroelectric Bi2FeMoxNi1-xO6 (BFMNO) thin films were fabricated using the sol-gel method. The thin films have a broad-band absorption in visible light range of 400–650 nm. With the increase of Mo content, the optical bandgap decreases from 2.19 eV to 2.07 eV. More importantly, the as-prepared BFMNO thin films have self-polarization behavior that causes the forming of the depolarization field and build-in field in the thin films. The narrow band gap and strong ferroelectric self-polarization play a crucial part in improving the photovoltaic effect. The thin film of x = 0.7 has the optimum open circuit voltage and short circuit current which are 5.41 V and 71.07 μA/cm2 under light illumination of 110 mW/cm2. In addition, the photovoltaic effect of the thin film also exhibits a good adaptability in high temperature environment. This work highlights the applications of the BFMNO thin films as a novel high-performance ferroelectric photovoltaic material.

Enhancing ferroelectric photovoltaic effect by polar order engineering.

Ferroelectric materials for photovoltaics have sparked great interest because of their switchable photoelectric responses and above-bandgap photovoltages that violate conventional photovoltaic theory. However, their relatively low photocurrent and power conversion efficiency limit their potential application in solar cells. To improve performance, conventional strategies focus mainly on narrowing the bandgap to better match the solar spectrum, leaving the fundamental connection between polar order and photovoltaic effect largely overlooked. We report large photovoltaic enhancement by A-site substitutions in a model ferroelectric photovoltaic material, BiFeO3. As revealed by optical measurements and supported by theoretical calculations, the enhancement is accompanied by the chemically driven rotational instability of the polarization, which, in turn, affects the charge transfer at the band edges and drives a direct-to-indirect bandgap transition, highlighting the strong coupling between polarization, lattice, and orbital order parameters in ferroelectrics. Polar order engineering thus provides an additional degree of freedom to further boost photovoltaic efficiency in ferroelectrics and related materials.

Inducing spontaneous electric polarizations in double perovskite iodide superlattices for ferroelectric photovoltaic materials

In this work, we use density functional theory calculations to demonstrate how spontaneous electric polarizations can be induced \textit{via} a hybrid improper ferroelectric mechanism in iodide perovskites, a family well-known to display solar-optimal band gaps, to create new materials for photoferroic applications. We first assemble three chemically distinct ($A$$A^{\prime}$)($B$$B^{\prime}$)I$_6$ double perovskites using centrosymmetric $AB$I$_3$ perovskite iodides (where $A$ = Cs, Rb, K and $B$ = Sn, Ge) as building units. In each superlattice, we investigate the effects of three types of $A$- and $B$-site cation ordering schemes and three different $B$I$_6$ octahedral rotation patterns. Out of these 27 combinations, we find that 15 produce polar space groups and display spontaneous electric polarizations ranging from 0.26 to 23.33 $\mu$C/cm$^2$. Furthermore, we find that a layered $A$-site/rock salt $B$-site ordering, in the presence of an $a^0a^0c^+$ rotation pattern, produces a chiral "vortex-like" $A$-site displacement pattern. We then investigate the effect of epitaxial strain on one of these systems, (CsRb)(SnGe)I$_6$, in layered and rock salt ordered configurations. In both phases, we find strong competition between the cation ordering schemes as well as an enhancement of the spontaneous polarization magnitude under tensile strain. Finally, using advanced functionals, we demonstrate that these compounds display low band gaps ranging from 0.2 to 1.3 eV. These results demonstrate that cation ordering and epitaxial strain are powerful ways to induce and control new functionalities in technologically-useful families of materials.

Photovoltaic effect and photopolarization in Pb[(Mg1/3Nb2/3)0.68Ti0.32]O3 crystal

Ferroelectric photovoltaic materials are an alternative to semiconductor-based photovoltaics and offer the advantage of above bandgap photovoltage generation. However, there are few known compounds, and photovoltaic efficiencies remain low. Here, we report the discovery of a photovoltaic effect in undoped lead magnesium niobate-lead titanate crystal and a significant improvement in the photovoltaic response under suitable electric fields and temperatures. The photovoltaic effect is maximum near the electric-field-driven ferroelectric dipole reorientation, and increases threefold near the Curie temperature. Moreover, at ferroelectric saturation, the photovoltaic response exhibits clear remanent and transient effects. The transient-remanent combinations together with electric and thermal tuning possibilities indicate photoferroelectric crystals as emerging elements for photovoltaics and optoelectronics, relevant to all-optical information storage and beyond.

Enhanced electrical and photocurrent characteristics of sol-gel derived Ni-doped PbTiO3 thin films

Partial substitution of group 10 metal for titanium is predicted theoretically to be one of the most effective ways to decrease the band gap of PbTiO3-based ferroelectric photovoltaic materials. It is therefore of interest to experimentally investigate their ferroelectric and photovoltaic properties. In this work, we focus on the electrical and photocurrent properties of Ni-doped PbTiO3 thin films prepared via a sol-gel route. The nickel incorporation does not modify the crystalline structure of PbTiO3 thin film, but it can increase the dielectric constant, ferroelectric polarization and photocurrent, and simultaneously decrease the band gap. The maximum remnant polarization (Pr) of 58.1 μC/cm2 is observed in PbTi0.8Ni0.2O3 thin film, and its photocurrent density is improved to be approximately one order larger than that of PbTiO3 thin film and simultaneously exhibits the polarization-dependent switching characteristic, which may be a promising choice for ferroelectric photovoltaic applications.

Modified optical and magnetic properties at room-temperature across lead-free morphotropic phase boundary in (1-x)BiTi3/8Fe2/8Mg3/8O3–xCaTiO3

Perovskite compounds of the formula (1-x)BiTi3/8Fe2/8Mg3/8O3–xCaTiO3 ((1-x)BMFT-xCTO), a class of highly potential ferroelectric photovoltaic materials with a morphotropic phase boundary (MPB), have been synthesized by solid-state reaction and exhibit simultaneously visible-light response and ferromagnetic order at room-temperature (RT). The effects of structural phases and lattice distortions on electron transitions and orbital orderings are systematically investigated. The crystal lattice displays constriction with increasing x and Raman peaks show non-liner shift due to Bi3+ and Fe3+/Mg2+ replaced by Ca2+ and Ti4+ randomly, respectively. Microstructural characterizations illustrate the rhombohedral-orthorhombic MPB occurrence during the crystal growth. Optical spectroscopy analysis shows band-gaps of our samples are about 2.2eV with anomalies at the point of phase transition. Furthermore, the (1-x)BMFT-xCTO displays RT ferromagnetism when x<0.15, which can be explained by an F center theory. These results reveal rich physical phenomena and open an avenue to design promising perovskites for solar-energy conversion devices and multiferroic applications.

Interplay of Cation Ordering and Ferroelectricity in Perovskite Tin Iodides: Designing a Polar Halide Perovskite for Photovoltaic Applications.

Owing to its ideal semiconducting band gap and good carrier-transport properties, the fully inorganic perovskite CsSnI3 has been proposed as a visible-light absorber for photovoltaic (PV) applications. However, compared to the organic-inorganic lead halide perovskite CH3NH3PbI3, CsSnI3 solar cells display very low energy conversion efficiency. In this work, we propose a potential route to improve the PV properties of CsSnI3. Using first-principles calculations, we examine the crystal structures and electronic properties of CsSnI3, including its structural polymorphs. Next, we purposefully order Cs and Rb cations on the A site to create the double perovskite (CsRb)Sn2I6. We find that a stable ferroelectric polarization arises from the nontrivial coupling between polar displacements and octahedral rotations of the SnI6 network. These ferroelectric double perovskites are predicted to have energy band gaps and carrier effective masses similar to those of CsSnI3. More importantly, unlike nonpolar CsSnI3, the electric polarization present in ferroelectric (CsRb)Sn2I6 can effectively separate the photoexcited carriers, leading to novel ferroelectric PV materials with potentially enhanced energy conversion efficiency.

Lattice dynamics, electronic structure, and optical properties of LiBeSb: A hexagonal ABC-type hyperferroelectrics

The recently discovered hexagonal ABC-type hyperferroelectrics, in which the polarization persists in the presence of the depolarization filed, exhibit a variety of intriguing and potentially useful properties [Garrity et al., Phys. Rev. Lett. 112, 127601 (2014)]. For the existing prototype of LiBeSb, we present detailed first-principles calculations concerning the lattice dynamics, electronic structure, and optical properties. An unstable longitudinal optic mode in the high-symmetry structure and a large polarization of 0.5 C/m2 in the polar phase are reported, including the remarkable dependence of Born effective charges on structural distortion. Using the HSE06 hybrid functional, we predict that LiBeSb has a small band-gap of 1.5 eV and shows dominant asymmetric covalent bonding character. Importantly, we find that there are remarkable absorptions in the whole visible spectrum. These features, combined with the enhanced carrier mobility, make LiBeSb as well as the whole family of hexagonal ABC-type hyperferroelectrics as promising candidates for ferroelectric photovoltaic materials with large bulk photovoltaic effect in the visible spectrum.

Ruddlesden–Popper perovskite sulfides A3B2S7: A new family of ferroelectric photovoltaic materials for the visible spectrum

Perovskite ferroelectric materials exhibit the novel ferroelectric photovoltaic effect, where photon-excited electron–hole pairs can be separated by ferroelectric polarization. Especially, semiconducting ferroelectric materials with small band gaps (Eg) have been extensively studied for applications in solar energy conversion. Traditional route for creating semiconducting ferroelectrics requires cation doping, where Eg of the insulating perovskite ferroelectric oxides are reduced via substitution of certain cations. But cation doping tends to reduce the carrier mobility due to the scattering, and usually lead to poor photovoltaic efficiency. In the present work, based on first-principles calculations, we propose and demonstrate a new strategy for designing stoichiometric semiconducting perovskite ferroelectric materials. Specifically, we choose the parent non-polar semiconducting perovskite sulfides ABS3 with Pnma symmetry, and turn them into ferroelectric Ruddlesden–Popper A3B2S7 perovskites with spontaneous polarizations. Our predicted Ruddlesden–Popper Ca3Zr2S7 and other derived compounds exhibit the room-temperature stable ferroelectricity, small band gaps (Eg<2.2eV) suitable for the absorption of visible light, and large visible-light absorption exceeding that of Si.

Enhancement of Photocurrent in Ferroelectric Films Via the Incorporation of Narrow Bandgap Nanoparticles

A novel nanostructured ferroelectric photovoltaic material, consisting of the ferroelectric lead zirconate titanate (PZT) film and Ag(2) O semiconductor nanoparticles of comparatively narrow bandgap, has demonstrated a remarkable enhancement in the photovoltaic effects and the highest light-electricity conversion efficiency among those PZT-based photodiodes previously reported. This work sheds light on the design and enhanced performance of new optoelectronic and solar energy devices.