Phonon-Assisted Ballistic Current From First Principles Calculations

The bulk photovoltaic effect (BPVE) refers to current generation due to illumination by light in a homogeneous bulk material lacking inversion symmetry. In addition to the intensively studied shift current, the ballistic current, which originates from asymmetric carrier generation due to scattering processes, also constitutes an important contribution to the overall kinetic model of the BPVE. In this letter, we use a perturbative approach to derive a formula for the ballistic current resulting from the intrinsic electron-phonon scattering in a form amenable to first-principles calculation. We then implement the theory and calculate the ballistic current of the prototypical BPVE material \ch{BaTiO3} using quantum-mechanical density functional theory. The magnitude of the ballistic current is comparable to that of shift current, and the total spectrum (shift plus ballistic) agrees well with the experimentally measured photocurrents. Furthermore, we show that the ballistic current is sensitive to structural change, which could benefit future photovoltaic materials design

Ultrafast vibrational control of organohalide perovskite optoelectronic devices using vibrationally promoted electronic resonance

Vibrational control (VC) of photochemistry through the optical stimulation of structural dynamics is a nascent concept only recently demonstrated for model molecules in solution. Extending VC to state-of-the-art materials may lead to new applications and improved performance for optoelectronic devices. Metal halide perovskites are promising targets for VC due to their mechanical softness and the rich array of vibrational motions of both their inorganic and organic sublattices. Here, we demonstrate the ultrafast VC of FAPbBr3 perovskite solar cells via intramolecular vibrations of the formamidinium cation using spectroscopic techniques based on vibrationally promoted electronic resonance. The observed short (~300 fs) time window of VC highlights the fast dynamics of coupling between the cation and inorganic sublattice. First-principles modelling reveals that this coupling is mediated by hydrogen bonds that modulate both lead halide lattice and electronic states. Cation dynamics modulating this coupling may suppress non-radiative recombination in perovskites, leading to photovoltaics with reduced voltage losses

Phonon-Assisted Ballistic Current from First-Principles Calculations.

The bulk photovoltaic effect (BPVE) refers to current generation due to illumination by light in a homogeneous bulk material lacking inversion symmetry. In addition to the intensively studied shift current, the ballistic current, which originates from asymmetric carrier generation due to scattering processes, also constitutes an important contribution to the overall kinetic model of the BPVE. In this Letter, we use a perturbative approach to derive a formula for the ballistic current resulting from the intrinsic electron-phonon scattering in a form amenable to first-principles calculation. We then implement the theory and calculate the ballistic current of the prototypical BPVE material BaTiO_{3} using quantum-mechanical density functional theory. The magnitude of the ballistic current is comparable to that of the shift current, and the total spectrum (shift plus ballistic) agrees well with the experimentally measured photocurrents. Furthermore, we show that the ballistic current is sensitive to structural change, which could benefit future photovoltaic materials design

Bulk photovoltaic effect in hexagonal LuMnO3 single crystals

Hexagonal manganites, such as h-LuMnO3, are ferroelectric and have a narrow electronic band gap of ≈1.5eV. Here we report on the photoresponse of h-LuMnO3 single crystals. It is found that the short circuit photocurrent density (Jsc) and the open circuit voltage (Voc) are dependent on the direction of the polarization plane of a linearly polarized impinging light. Its angular dependence indicates the contribution of bulk photovoltaic effect to the short circuit photocurrent. It is also observed that a switchable drift photocurrent, originating from the depoling field of the ferroelectric and thus tunable (<10%) by its polarization direction, also contributes to Jsc. Although its presence precludes accurate determination of the bulk photovoltaic tensor elements and Glass coefficients, some bounds can be established. The Glass coefficients are found to be significantly larger than those obtained in BiFeO3. We argue that the smaller band gap of h-LuMnO3, its distinctive bipyramidal crystal field, and electronic configuration (3d4 vs 3d5), account for the difference and suggest a path towards ferroelectrics of higher photoconversion efficiency.Financial support from the Spanish Ministry of Science, Innovation and Universities, through the “Severo Ochoa” Programme for Centers of Excellence in R&D (FUNFUTURE CEX2019-000917-S), PID2020-118479RBI00 (AEI/FEDER, EU) (AEI/FEDER, EU), and PID2019- 107727RB-I00 (AEI/FEDER, EU) projects, and from Generalitat de Catalunya (2017 SGR 1377) is acknowledged. I.F. acknowledges RyC Contract RYC-2017-22531. Project supported by a 2020 Leonardo Grant for Researchers and Cultural Creators, BBVA Foundation. The theoretical component of this work (A.M.S. and A.M.R.) was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, under Award No. DE-FG02-07ER46431. Computational support was provided by the National Energy Research Scientific Computing Center (NERSC), a U.S. Department of Energy, Office of Science User Facility located at Lawrence Berkeley National Laboratory, operated under Contract No. DE-AC02- 05CH11231.The experimental and theoretical contributions of Y.S. are financially supported by China Scholarship Council (CSC), respectively with No. 201806410010. The work of Y.S. has been done as a part of her Ph.D. program in Materials Science at Universitat Autònoma de Barcelona.With funding from the Spanish government through the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000917-S).Peer reviewe

Bulk photovoltaic effect in hexagonal LuMnO 3 single crystals

Hexagonal manganites, such as h-LuMnO3, are ferroelectric and have a narrow electronic band gap of ≈ 1.5 eV. Here we report on the photoresponse of h-LuMnO3 single crystals. It is found that the short circuit photocurrent density (Jsc) and the open circuit voltage (Voc) are dependent on the direction of the polarization plane of a linearly polarized impinging light. Its angular dependence indicates the contribution of bulk photovoltaic effect to the short circuit photocurrent. It is also observed that a switchable drift photocurrent, originating from the depoling field of the ferroelectric and thus tunable (<10%) by its polarization direction, also contributes to Jsc. Although its presence precludes accurate determination of the bulk photovoltaic tensor elements and Glass coefficients, some bounds can be established. The Glass coefficients are found to be significantly larger than those obtained in BiFeO3. We argue that the smaller band gap of h-LuMnO3, its distinctive bipyramidal crystal field, and electronic configuration (3d4 vs 3d5), account for the difference and suggest a path towards ferroelectrics of higher photoconversion efficiency.Financial support from the Spanish Ministry of Science, Innovation and Universities, through the “Severo Ochoa” Programme for Centers of Excellence in R&D (FUNFUTURE CEX2019-000917-S), PID2020-118479RBI00 (AEI/FEDER, EU) (AEI/FEDER, EU), and PID2019-107727RB-I00 (AEI/FEDER, EU) projects, and from Generalitat de Catalunya (2017 SGR 1377) is acknowledged. I.F. acknowledges RyC Contract RYC-2017-22531. Project supported by a 2020 Leonardo Grant for Researchers and Cultural Creators, BBVA Foundation. The theoretical component of this work (A.M.S. and A.M.R.) was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, under Award No. DE-FG02-07ER46431. Computational support was provided by the National Energy Research Scientific Computing Center (NERSC), a U.S. Department of Energy, Office of Science User Facility located at Lawrence Berkeley National Laboratory, operated under Contract No. DE-AC02- 05CH11231.The experimental and theoretical contributions of Y.S. are financially supported by China Scholarship Council (CSC), respectively with No. 201806410010. The work of Y.S. has been done as a part of her Ph.D. program in Materials Science at Universitat Autònoma de Barcelona.Peer reviewe

Li iontronics in single-crystalline T-Nb2O5 thin films with vertical ionic transport channels.

Acknowledgements: This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement no. 737109. Funding has been provided by the Alexander von Humboldt Foundation in the framework of the Alexander von Humboldt Professorship to S.S.P.P. endowed by the Federal Ministry of Education and Research. The electrochemical theory of Z.J. and A.K. was supported by the US Department of Energy, Office of Science, Basic Energy Sciences, under award no. DE-SC0019281. F.N.S. acknowledges funding from the Faraday Institution CATMAT project (FIRG016). The oxide structure and phase transition theory of A.M.S. and A.M.R. was supported by the Office of Naval Research, under grant N00014-20-1-2701. The authors acknowledge computational support from the National Energy Research Scientific Computing Center (NERSC) of the DOE and the High-Performance Computing Modernization Office (HPCMO) of the US Department of Defense (DOD). Use of the APS at Argonne National Laboratory was supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences, under contract No. DE-AC02-06CH11357. We thank C. Guillemard at ALBA synchrotron and J. H. Jin at GERI for their assistance with gating of XANES and TEM samples, respectively.The niobium oxide polymorph T-Nb2O5 has been extensively investigated in its bulk form especially for applications in fast-charging batteries and electrochemical (pseudo)capacitors. Its crystal structure, which has two-dimensional (2D) layers with very low steric hindrance, allows for fast Li-ion migration. However, since its discovery in 1941, the growth of single-crystalline thin films and its electronic applications have not yet been realized, probably due to its large orthorhombic unit cell along with the existence of many polymorphs. Here we demonstrate the epitaxial growth of single-crystalline T-Nb2O5 thin films, critically with the ionic transport channels oriented perpendicular to the film's surface. These vertical 2D channels enable fast Li-ion migration, which we show gives rise to a colossal insulator-metal transition, where the resistivity drops by 11 orders of magnitude due to the population of the initially empty Nb 4d0 states by electrons. Moreover, we reveal multiple unexplored phase transitions with distinct crystal and electronic structures over a wide range of Li-ion concentrations by comprehensive in situ experiments and theoretical calculations, which allow for the reversible and repeatable manipulation of these phases and their distinct electronic properties. This work paves the way for the exploration of novel thin films with ionic channels and their potential applications

Hidden phases and colossal insulator-metal transition in single-crystalline T-Nb2O5 thin films accessed by lithium intercalation

Fast migration of lithium (Li)-ions in oxide materials is fundamental to the operation of Li-ion batteries. The intercalation of Li-ions into oxides can further lead to emergent electronic property changes. Some of the fastest Li-ion conductors are 4d oxides, and of these, the niobium oxide polymorph T-Nb2O5 is especially interesting with its two-dimensional fast ion migration channels. However, the growth of single-crystalline T-Nb2O5 films is challenging due to its stability over only a limited synthesis temperature window, the existence of many other polymorphs, and its large orthorhombic unit cell. Here, we first demonstrate the epitaxial growth of single domain T-Nb2O5 thin films, critically with the ion channels oriented perpendicular to the film's surface. We show that the insertion of just a small amount of Li using ionic liquids results in conversion of the initially insulating film to a metallic state with a colossal change in resistivity of almost eleven orders of magnitude. In situ experiments, in conjunction with theoretical calculations, reveal a series of transitions between distinct crystal and electronic structures as the lithium content is systematically increased. These include hidden phases that have not previously been identified. Furthermore, replacing the Au electrode with a Li-oxide electrode allows for a significant reduction of the gate voltage at which metallization takes place. Our study opens a new path towards the exploration of hidden phases and the development of novel electrochemically controlled electronic devices

Li iontronics in single-crystalline T -Nb 2 O 5 thin films with vertical ionic transport channels

Acknowledgements: This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement no. 737109. Funding has been provided by the Alexander von Humboldt Foundation in the framework of the Alexander von Humboldt Professorship to S.S.P.P. endowed by the Federal Ministry of Education and Research. The electrochemical theory of Z.J. and A.K. was supported by the US Department of Energy, Office of Science, Basic Energy Sciences, under award no. DE-SC0019281. F.N.S. acknowledges funding from the Faraday Institution CATMAT project (FIRG016). The oxide structure and phase transition theory of A.M.S. and A.M.R. was supported by the Office of Naval Research, under grant N00014-20-1-2701. The authors acknowledge computational support from the National Energy Research Scientific Computing Center (NERSC) of the DOE and the High-Performance Computing Modernization Office (HPCMO) of the US Department of Defense (DOD). Use of the APS at Argonne National Laboratory was supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences, under contract No. DE-AC02-06CH11357. We thank C. Guillemard at ALBA synchrotron and J. H. Jin at GERI for their assistance with gating of XANES and TEM samples, respectively.The niobium oxide polymorph T-Nb2O5 has been extensively investigated in its bulk form especially for applications in fast-charging batteries and electrochemical (pseudo)capacitors. Its crystal structure, which has two-dimensional (2D) layers with very low steric hindrance, allows for fast Li-ion migration. However, since its discovery in 1941, the growth of single-crystalline thin films and its electronic applications have not yet been realized, probably due to its large orthorhombic unit cell along with the existence of many polymorphs. Here we demonstrate the epitaxial growth of single-crystalline T-Nb2O5 thin films, critically with the ionic transport channels oriented perpendicular to the film’s surface. These vertical 2D channels enable fast Li-ion migration, which we show gives rise to a colossal insulator–metal transition, where the resistivity drops by 11 orders of magnitude due to the population of the initially empty Nb 4d0 states by electrons. Moreover, we reveal multiple unexplored phase transitions with distinct crystal and electronic structures over a wide range of Li-ion concentrations by comprehensive in situ experiments and theoretical calculations, which allow for the reversible and repeatable manipulation of these phases and their distinct electronic properties. This work paves the way for the exploration of novel thin films with ionic channels and their potential applications