We derive upper bounds on the magnitude of shift photocurrent generation of materials in two limiting cases: the flat-band limit of almost-isolated systems such as molecular crystals, and the wide-band limit of one-dimensional or quasi-one-dimensional materials such as ferroelectric polymers or other materials with chain-like motifs. These bounds relate the magnitudes of the shift current bulk photovoltaic effect to materials parameters. In both cases, we find that ratio of electron hopping amplitudes to the band gap plays a vital role in maximizing the amount of nonlinear response. Furthermore, by using the Wannier function formalism, we quantify the effect of long-range hopping amplitudes, showing how delocalization of electronic states gives rise to larger photocurrents. These results inform the design and selection of new materials for large shift current generation.

Despite a long history of research into nonlinear response theory, there has been no systematic investigation into the maximum amount of nonlinear optical response attainable in solid-state materials. In this work, we present an upper bound on the second-order response functions of materials, which controls the shift current response. We show that this bound depends on the band gap, bandwidth, and geometrical properties of the material in question. We find that delocalized systems generally have larger responses than more localized or isolated ones. As a proof of principle, we perform first-principles calculations of the response tensors of a wide variety of materials, finding that the materials in our database do not yet saturate the upper bound. This suggests that new large shift current materials will likely be discovered by future materials research guided by the factors mentioned in this work.

Ferroelectric materials, because of their robust spontaneous electrical polarization, are widely used in various applications. Recent advances in modelling, synthesis and characterization techniques are spurring unprecedented advances in the study of these materials. In this Review, we focus on thin-film ferroelectric materials and, in particular, on the possibility of controlling their properties through the application of strain engineering in conventional and unconventional ways. We explore how the study of ferroelectric materials has expanded our understanding of fundamental effects, enabled the discovery of novel phases and physics, and allowed unprecedented control of materials properties. We discuss several exciting possibilities for the development of new devices, including those in electronic, thermal and photovoltaic applications, and transduction sensors and actuators. We conclude with a brief survey of the different directions that the field may expand to over the coming years. 2016 Macmillan Publishers Limited, part of Springer Nature. All rights reserved.

We show that the carrier recombination rate of noncentrosymmetric materials can be strongly modified by spin-orbit coupling. Our proposed mechanism involves the separation of conduction and valence bands into their respective spin components, which changes the transition dipole moments between them. The change in the carrier recombination can be either positive or negative in sign, or vary depending on the location of carriers in the Brillouin zone. We have performed a large scale DFT screening study to identify candidate materials that display this effect. We have selected three materials, Pb4SeBr6, ReTe3Br5, and CsCu(BiS2)2, which span the range of behaviors, and discuss their electronic band structure in greater detail. We find transition dipole moment enhancement factors of up to three orders of magnitude, reflecting the physical impact of spin-orbit coupling on the carrier lifetime. Therefore, further explorations of the spin-orbit coupling and lattice symmetry could prove to be useful for manipulating the photophysics of materials.

The bulk photovoltaic effect is a long-known but poorly understood phenomenon. Recently, however, the multiferroic bismuth ferrite has been observed to produce strong photovoltaic response to visible light, suggesting that the effect has been underexploited. Here we present three polar oxides in the LiNbO3 structure that we predict to have band gaps in the 1-2 eV range and very high bulk photovoltaic response: PbNiO3, Mg1/2Zn1/2PbO3, and LiBiO3. All three have band gaps determined by cations with d10s0 electronic configurations, leading to conduction bands composed of cation s orbitals and O p orbitals. This both dramatically lowers the band gap and increases the bulk photovoltaic response by as much as an order of magnitude over previous materials, demonstrating the potential for high-performing bulk photovoltaics.

Based on first-principles calculations, we predict a substantial increase in the optical dielectric function of LiAsSe2 under pressure. We find that the optical dielectric constant is enhanced threefold under compression along all three axes by 3%. This enhancement is mainly due to the dimerization strength reduction of the one-dimensional (1D) As-Se chains in LiAsSe2, which significantly alters the wave function phase mismatch between two neighboring chains and changes the transition intensity. By developing a tight-binding model of the interacting 1D chains, the essential features of the low-energy electronic structure of LiAsSe2 are captured. Our findings are important for understanding the fundamental physics of LiAsSe2 and provide a feasible way to enhance the material optical response that can be applied to light harvesting for energy applications.

Here we develop a microscopic approach aimed at the description of a suite of physical effects related to carrier transport in, and the optical properties of, halide perovskites. Our theory is based on the description of the nuclear dynamics to all orders and goes beyond the common assumption of linear electron-phonon coupling in describing the carrier dynamics and band gap characteristics. When combined with first-principles calculations and applied to the prototypical MAPbI₃ system, our theory explains seemingly disparate experimental findings associated with both the charge-carrier mobility and optical absorption properties, including their temperature dependencies. Our findings demonstrate that orbital-overlap fluctuations in the lead-halide structure plays a significant role in determining the optoelectronic features of halide perovskites.

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.

The correlation between the shift current mechanism for the bulk photovoltaic effect (BPVE) and the structural and electronic properties of ferroelectric perovskite oxides is not well understood. Here, we study and engineer the shift current photovoltaic effect using a visible-light-absorbing ferroelectric Pb(Ni$_{x}$Ti$_{1-x}$)O$_{3-x}$ solid solution from first principles. We show that the covalent orbital character dicates the direction, magnitude, and onset energy of shift current in a predictable fashion. In particular, we find that the shift current response can be enhanced via electrostatic control in layered ferroelectrics, as bound charges face a stronger impetus to screen the electric field in a thicker material, delocalizing electron densities. This heterogeneous layered structure with alternative photocurrent generating and insulating layers is ideal for BPVE applications.

Spontaneous polarization and inversion symmetry breaking in ferroelectric materials lead to their use as photovoltaic devices. However, further advancement of their applications are hindered by the paucity of ways of reducing bandgaps and enhancing photocurrent. By unravelling the correlation between ferroelectric materials' responses to solar irradiation and their local structure and electric polarization landscapes, here we show from first principles that substantial bulk photovoltaic effect enhancement can be achieved by nanolayering PbTiO3 with nickel ions and oxygen vacancies ((PbNiO2)x(PbTiO3)(1-x)). The enhancement of the total photocurrent for different spacings between the Ni-containing layers can be as high as 43 times due to a smaller bandgap and photocurrent direction alignment for all absorption energies. This is due to the electrostatic effect that arises from nanolayering. This opens up the possibility for control of the bulk photovoltaic effect in ferroelectric materials by nanoscale engineering of their structure and composition.