21 research outputs found
Magnetic Bulk Photovoltaic Effect: Strong and Weak Field
Shift current and ballistic current have been proposed to explain the bulk
photovoltaic effect (BPVE), and there have been experiments designed to
separate the two mechanisms. These experiments are based on the assumption that
under magnetic field, ballistic current can have a Hall effect while the shift
current cannot, which is from some energy-scale arguments and has never been
proven. A recent work [Phys. Rev. B 103, 195203 (2021)] using quantum transport
formalism achieves a conclusion that shift current indeed has a Hall current,
seemingly contradicting the previous assumption and making the situation more
confusing. Moreover, the behavior of BPVE under strong magnetic field is still
unexplored. In this Letter, using a minimal 2D tight-binding model, we carry
out a systematic numerical study of the BPVE under weak and strong magnetic
field by treating the field in a non-perturbative way. Our model clearly shows
the appearance of the magnetically-induced ballistic current along the
transverse direction, which agrees with the previous predictions, and
interestingly a sizable longitudinal response of the shift current is also
observed, a phenomenon that is not captured by any existing theories where the
magnetic field is treated perturbatively. More surprisingly, drastically
different shift current is found in the strong-field regime, and the evolution
from weak to strong field resembles a phase transition. We hope that our work
could resolve the debate over the behavior of BPVE under magnetic field, and
the strong-field behavior of shift current is expected to inspire more studies
on the relation between nonlinear optics and quantum geometry
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
Discovery of enhanced lattice dynamics in a single-layered hybrid perovskite
Layered hybrid perovskites have attracted much attention in recent years due
to their emergent physical properties and exceptional functional performances,
but the coexistence of lattice order and structural disorder severely hinders
our understanding of these materials. One unsolved problem regards how the
lattice dynamics are affected by the dimensional engineering of the inorganic
frameworks and the interaction with the molecular moieties. Here, we address
this question by using a combination of high-resolution spontaneous Raman
scattering, high-field terahertz spectroscopy, and molecular dynamics
simulations. This approach enables us to reveal the structural vibrations and
disorder in and out of equilibrium and provides surprising observables that
differentiate single- and double-layered perovskites. While no distinct
vibrational coherence is observed in double-layer perovskites, we discover that
an off-resonant terahertz pulse can selectively drive a long-lived coherent
phonon mode through a two-photon process in the single-layered system. This
difference highlights the dramatic change in the lattice environment as the
dimension is reduced. The present findings pave the way for the ultrafast
structural engineering of hybrid lattices as well as for developing high-speed
optical modulators based on layered perovskites
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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
First-Principles Investigations On Raman Spectroscopy And Bulk Photovoltaic Effect
Light-matter interaction, as the name suggested, describes the interaction between the illuminating light and different states of matters. In solid-state physics, people usually focus on how light influences the energetics and dynamics of materials where light is taken as a classical electromagnetic field. In the first part of this dissertation, we will focus on the situation where the frequency of light is lower than the band gaps of materials, so that the light will not be absorbed but scattered and possibly acquire a frequency shift, which is called Raman scattering. We will show how the conventional theory of Raman scattering developed for harmonic systems can be modified to treat highly anharmonic systems. The adapted theory will be used to reproduce the experimental Raman spectra of methylammonium lead iodide-a promising material for solar energy conversion-and demonstrate the atomistic origin of its strong lattice anharmonicity. Then, in the second part of this dissertation, we will shift our attention to the scenario where the photon energy surpasses the band gap and will be absorbed by the material. We will be mainly concerned with one specific phenomenon, the bulk photovoltaic effect (BPVE), which means that a steady DC current can be generated in spatially homogeneous but inversion-broken or time-reversal-broken materials. We will develop a theory for ballistic current, an important mechanism for BPVE in addition to shift current, and then use first-principles methods to evaluate it for real materials such as BaTiO3 and MoS2. Our theory and computation show clearly that the phonon ballistic current can be as important as shift current, while the exciton ballistic current is less appreciable. Moreover, we will investigate how BPVE will behave under a uniform magnetic field, demonstrating a sizable response of shift current at weak field and a nontrivial evolution of the spectral shape going into the strong field