Releasing the hidden shift current in the TTF-CA organic molecular solid via symmetry lowering

Abstract

Bulk photovoltaic effect, characterized by an excitation-driven unbiased spontaneous photocurrent, has attracted substantial attention mainly due to its potential for harvesting solar energy. Here, we investigate the photovoltaic characteristics of organic molecular solids and focus on the association between the photocurrent and the crystal symmetry in the exemplary case of tetrathiafulvalene-p-chloranil. We perform comprehensive first-principles calculations, including direct evaluations of the excitedstate current via real-time propagations of the time-dependent density functional theory. We find that the charge shifting in the low-temperature phase is mainly driven by the intrachain ferroelectricity, which gives rise to a photocurrent not only in the visiblelight range but also near the band-edge infrared region. The shift current that is locked in the symmetry of the high-temperature phase can be released by introducing a potential asymmetry. We suggest that organic molecular solids can be exploited via appropriate engineering to lower the symmetry, aiming at room-temperature photovoltaics.

Giant bulk photovoltaic effect driven by the wall-to-wall charge shift in WS2 nanotubes

“…The shift-current spectra were evaluated from the tight-binding Hamiltonian based on the maximally localized Wannier functions 66 using the second-order optical response formalism 39 – 41 . For the shift-current estimation, a 3 × 3 × 100 k -point grid was used in our calculations, which provide a sufficiently dense mesh, leading to the well converged value of the shift-current spectra.…”

“…A spontaneous photocurrent (shift current) is induced by the spatial charge shift between occupied and unoccupied states via optical excitation of polar materials 38 . As atomic structures, electronic band dispersions, and electric polarizations are closely intertwined in this phenomenon 39 , 40 , the fundamental question is whether any particular geometrical advantages are inherent to the 1D structure 35 . In these inspections, the justification of the perturbation-based theories and the consideration of the nonlinear effect of strong fields also need to be addressed to achieve complete understanding of the behavior of TMD nanotubes under practical application conditions.…”

First-principles identification of the charge-shifting mechanism and ferroelectricity in hybrid halide perovskites

“…The shift current spectra are expressed by: σ abc (ω) = iπe 3 ∂k a − i(A a mm − A a nn )r c mn with the Berry connections A a mm 22,46 . The shift-current spectra were calculated from the maximally localized Wannier function using the WANNIER90 package 47 with 30 × 30 × 20 k-point grids for β-MAPbI 3 and 50 × 50 × 50 k-point grids for α-FAPbI 3 .…”

“…The shift current J a is a second order response and thus can be expressed in terms of two electric field components and material-dependent response function . The shift current spectra are expressed by: where indices a , b , and c represent Cartesian directions, denotes the velocity matrix elements, and denotes the generalized derivatives, which are defined as with the Berry connections 22 , 46 . The shift-current spectra were calculated from the maximally localized Wannier function using the WANNIER90 package 47 with 30 × 30 × 20 k-point grids for β -MAPbI 3 and 50 × 50 × 50 k-point grids for α -FAPbI 3 .…”

“…The shift current model, also known as the bulk photovoltaic effect (BPVE), is derived by the intrinsic quantum-mechanical nature of the electronic band structures [22][23][24] . It is the second-order optical response that has been established through second-order perturbation theory 25 .…”