Substantial bulk photovoltaic effect enhancement via nanolayering

The shift current photovoltaic effect response in wurtzite and zinc blende semiconductors via first-principles calculations

Recently, the search for materials with high photoelectric conversion efficiency has emerged as a significant research hotspot. Unlike p-n junctions, the bulk photovoltaic effect (BPVE) can also materialize within pure crystals. Here, we propose wurtzite and zinc blende semiconductors without inversion symmetry (AgI, GaAs, CdSe, CdTe, SiGe, ZnSe, and ZnTe) as candidates for achieving the BPVE and investigate the factors that affect the shift current. GaAs with a wurtzite structure exhibits the highest shift current value of 31.8 μA V-2 when spin-orbit coupling is considered. Meanwhile, the peak position of the maximum linear optical conductivity and shift current in the wurtzite structure is lower than that in the zinc blende structure. In addition, we also found that strong covalency within the same main axis group element significantly influences the shift current, exemplified by wurtzite SiGe, which exhibits 15.8 μA V-2. Our research highlights the importance of a smaller band gap, reduced carrier effective mass, and increased covalency in achieving a substantial shift current response. Ultimately, this study provides valuable insights into the interplay of the structural and electronic properties, offering directions for the discovery and design of materials with an enhanced BPVE.

Enhancing shift current response via virtual multiband transitions

Materials exhibiting a significant shift current response could potentially outperform conventional solar cell materials. The myriad of factors governing shift-current response, however, poses significant challenges in finding such strong shift-current materials. Here we propose a general design principle that exploits inter-orbital mixing to excite virtual multiband transitions in materials with multiple flat bands to achieve an enhanced shift current response. We further relate this design principle to maximizing Wannier function spread as expressed through the formalism of quantum geometry. We demonstrate the viability of our design using a 1D stacked Rice-Mele model. Furthermore, we consider a concrete material realization - alternating angle twisted multilayer graphene (TMG) - a natural platform to experimentally realize such an effect. We identify a set of twist angles at which the shift current response is maximized via virtual transitions for each multilayer graphene and highlight the importance of TMG as a promising material to achieve an enhanced shift current response at terahertz frequencies. Our proposed mechanism also applies to other 2D systems and can serve as a guiding principle for designing multiband systems that exhibit an enhanced shift current response.

Light-induced shift current vortex crystals in moiré heterobilayers

Transition metal dichalcogenide (TMD) moiré superlattices provide an emerging platform to explore various light-induced phenomena. Recently, the discoveries of novel moiré excitons have attracted great interest. The nonlinear optical responses of these systems are however still underexplored. Here, we report investigation of light-induced shift currents (a second-order response generating DC current from optical illumination) in the WSe2/WS2 moiré superlattice. We identify a striking phenomenon of the formation of shift current vortex crystals-i.e., two-dimensional periodic arrays of moiré-scale current vortices and associated magnetic fields with remarkable intensity under laboratory laser setup. Furthermore, we demonstrate high optical tunability of these current vortices-their location, shape, chirality, and magnitude can be tuned by the frequency, polarization, and intensity of the incident light. Electron-hole interactions (excitonic effects) are found to play a crucial role in the generation and nature of the shift current intensity and distribution. Our findings provide a promising all-optical control route to manipulate nanoscale shift current density distributions and magnetic field patterns, as well as shed light on nonlinear optical responses in moiré quantum matter and their possible applications.

Flexible BaTiO3 Thin Film-Based Coupled Nanogenerator for Simultaneously Scavenging Light and Vibration Energies

Ferroelectric materials have a variety of properties, such as piezoelectricity, pyroelectricity, and the ferroelectric photovoltaic effect, which enable them to obtain electrical energy from various external stimuli. Here, we report a coupled nanogenerator based on flexible BTO ferroelectric films with a cantilevered beam structure. It combines the photovoltaic and flexoelectric effects in a ferroelectric materials-based coupled nanogenerator for simultaneously scavenging vibration energy and light energy, thus improving energy scavenging performance. As compared with the photovoltaic effect individually, simultaneous vibration and light illumination under a light intensity of 57 mW/cm² at 405 nm can produce a photo-flexoelectric coupling current of 85 nA, where the current peak has been enhanced by 121%. Due to the photo-flexoelectric coupling effect, the device has outstanding charging performance, where a 4.7 μF capacitor can be charged to 60 mV in 150 s. These devices have potential applications in multi-energy scavenging and self-powered sensors.

Photoinduced Rippling of Two-Dimensional Hexagonal Nitride Monolayers

Inducing structural changes and deformation using noninvasive methods, such as ultrafast laser technology, is an attractive route to multiple optomechanical and optoelectronic applications. Here, we show how photon excitation could accumulate in-plane stress and induce long-wavelength ripples in two-dimensional (2D) materials. Numerical results based on first-principles calculations and a continuum model predict that long-range nanoscale rippling could emerge under photon excitation in hexagonal nitride single atomic sheets. The photosoftened transverse acoustic mode dominates the out-of-plane distortion of the sheet, and the resultant rippling pattern strongly depends on the boundary condition. We reveal that the wavelength and height of the ripple scale as I^-1/3 and I^1/6, respectively, where I is the incident light energy flux. Our findings based on multiscale theory and simulations elucidate the interplay between carrier excitation, phonon dispersion, and long-range mechanical deformations, which could find potential usage in flexible electronics and electromechanical devices.

Use of metamodels for rapid discovery of narrow bandgap oxide photocatalysts

New photocatalysts are traditionally identified through trial-and-error methods. Machine learning has shown considerable promise for improving the efficiency of photocatalyst discovery from a large potential pool. Here, we describe a multi-step, target-driven consensus method using a stacking meta-learning algorithm that robustly predicts bandgaps and H2 evolution activities of photocatalysts. Trained on small datasets, these models can rapidly screen a large space (>10 million materials) to identify promising, non-toxic compounds as candidate water splitting photocatalysts. Two effective compounds and two controls possessing optimal bandgap values (∼2 eV) but not photoactivity as predicted by the models were synthesized. Their experimentally measured bandgaps and H2 evolution activities were consistent with the predictions. Conspicuously, the two compounds with strong photoactivities under UV and visible light are promising visible-light-driven water splitting photocatalysts. This study demonstrates the power of machine learning and the potential of big data to accelerate discovery of next-generation photocatalysts.

Strongly enhanced and tunable photovoltaic effect in ferroelectric-paraelectric superlattices

Ever since the first observation of a photovoltaic effect in ferroelectric BaTiO₃, studies have been devoted to analyze this effect, but only a few attempted to engineer an enhancement. In conjunction, the steep progress in thin-film fabrication has opened up a plethora of previously unexplored avenues to tune and enhance material properties via growth in the form of superlattices. In this work, we present a strategy wherein sandwiching a ferroelectric BaTiO₃ in between paraelectric SrTiO₃ and CaTiO₃ in a superlattice form results in a strong and tunable enhancement in photocurrent. Comparison with BaTiO₃ of similar thickness shows the photocurrent in the superlattice is 10³ times higher, despite a nearly two-thirds reduction in the volume of BaTiO₃ The enhancement can be tuned by the periodicity of the superlattice, and persists under 1.5 AM irradiation. Systematic investigations highlight the critical role of large dielectric permittivity and lowered bandgap.

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.

Large Bulk Piezophotovoltaic Effect of Monolayer 2H-MoS2

The bulk photovoltaic effect in noncentrosymmetric materials is an intriguing physical phenomenon that holds potential for high-efficiency energy harvesting. Here, we study the shift current bulk photovoltaic effect in the transition-metal dichalcogenide MoS₂. We present a simple automated method to guide materials design and use it to uncover a distortion to monolayer 2H-MoS₂ that dramatically enhances the integrated shift current. Using this distortion, we show that overlap in the Brillouin zone of the distributions of the shift vector (a quantity measuring the net displacement in real space of coherent wave packets during excitation) and the transition intensity is crucial for increasing the shift current. The distortion pattern is related to the material polarization and can be realized through an applied electric field via the converse piezoelectric effect. This finding suggests an additional method for engineering the shift current response of materials to augment previously reported methods using mechanical strain.

Inorganic photovoltaic cells based on BiFeO3: spontaneous polarization, lattice matching, light polarization and their relationship with photovoltaic performance

Inorganic ferroelectric perovskite oxides are more stable than hybrid perovskites. However, their solar energy harvesting efficiency is not so good. Here, by constructing a series of BiFeO3-based devices (solar cells), we investigated three factors that influence the photovoltaic performance, namely, spontaneous polarization, terminated ion species in the interface between BiFeO3 and the electrode, and polarized light irradiation. This work was carried out under the framework of the density functional theory combined with the non-equilibrium Green's function theory under a built-in electric field or finite bias. The results showed that (1) the photocurrent is larger only under a suitable electronic band gap rather than larger spontaneous polarization; (2) the photocurrent reaches the largest value in the Bi3+ ion-terminated interface than in the case of Fe3+ or O2- with the SrTiO3 electrode; (3) the photocurrent can be largely enhanced if the polarized direction of the monochromatic light is perpendicular to the spontaneous polarization direction. These results would deepen the understanding of some experimental results of BiFeO3-based solar cells.

Structures and electronic properties of domain walls in BiFeO3 thin films

Domain walls (DWs) in ferroelectrics are atomically sharp and can be created, erased, and reconfigured within the same physical volume of ferroelectric matrix by external electric fields. They possess a myriad of novel properties and functionalities that are absent in the bulk of the domains, and thus could become an essential element in next-generation nanodevices based on ferroelectrics. The knowledge about the structure and properties of ferroelectric DWs not only advances the fundamental understanding of ferroelectrics, but also provides guidance for the design of ferroelectric-based devices. In this article, we provide a review of structures and properties of DWs in one of the most widely studied ferroelectric systems, BiFeO₃ thin films. We correlate their conductivity and photovoltaic properties to the atomic-scale structure and dynamic behaviors of DWs.

Ferroelectric Oxides for Solar Energy Conversion, Multi-Source Energy Harvesting/Sensing, and Opto-Ferroelectric Applications

Photoferroelectrics belong to a unique material family that exhibits both photovoltaic and ferroelectric effects simultaneously. The photovoltaic effect is the only known direct method of converting light into electricity and is the basis of solar cells. The ferroelectric effect can induce piezoelectric and pyroelectric effects, which are the working principles of widely used kinetic and thermal sensors, transducers, actuators, and energy harvesters. For a long time, photoferroelectric research was restricted to theoretical investigations only because of either the wide band gap (Eg ), which is not able to effectively absorb visible light, or to the weak ferroelectricity caused by a narrow Eg . Recent scientific breakthroughs, however, have opened doors for the development of practical applications. In this article, emerging concepts of creating balanced photovoltaic and ferroelectric properties for photoferroelectrics, as well as those of novel applications in future devices, are presented.

Colossal mid-infrared bulk photovoltaic effect in a type-I Weyl semimetal

Broadband, efficient and fast conversion of light to electricity is crucial for sensing and clean energy. The bulk photovoltaic effect (BPVE) is a second-order nonlinear optical effect that intrinsically converts light into electrical current. Here, we demonstrate a large mid-infrared BPVE in microscopic devices of the Weyl semimetal TaAs. This discovery results from combining recent developments in Weyl semimetals, focused-ion beam fabrication and theoretical works suggesting a connection between BPVE and topology. We also present a detailed symmetry analysis that allows us to separate the shift current response from photothermal effects. The magnitude and wavelength range of the assigned shift current may impact optical detectors, clean energy and topology, and demonstrate the utility of Weyl semimetals for practical applications.

Anomalous Photovoltaic Effect in Centrosymmetric Ferroelastic BiVO4

The anomolous photovoltaic (APV) effect is an intriguing phenomenon and rarely observed in bulk materials that structurally have an inversion symmetry. Here, the discovery of such an APV effect in a centrosymmetric vanadate, BiVO₄, where noticeable above-bandgap photovoltage and a steady-state photocurrent are observed in both ceramics and single crystals even when illuminated under visible light, is reported. Moreover, the photovoltaic voltage can be reversed by the stress modulation, and a sine-function relationship between the photovoltage and stress directional angle is derived. Microstructure and strain-field analysis reveal localized asymmetries that are caused by strain fluctuations in bulk centrosymmetric BiVO₄. On the basis of the experimental results, a flexoelectric coupling via a strain-induced local polarization mechanism is suggested to account for the APV effect observed. This work not only allows new applications for BiVO₄ in optoelectronic devices but also deepens insights into the mechanisms underlying the APV effect.

Phonon Influence on Bulk Photovoltaic Effect in the Ferroelectric Semiconductor GeTe

The shift current (SHC) has been accepted as the primary mechanism of the bulk photovoltaic effect (BPVE) in ferroelectrics, which is much different from the typical p-n junction-based photovoltaic mechanism in heterogeneous materials. In the present work, we use first-principles calculations to investigate the SHC response in the ferroelectric semiconductor GeTe, which is found possess a large SHC response due to its intrinsic narrow band gap and high covalency. We explore the changes of SHC response induced by phonon vibrations, and analytically fit current versus vibrational amplitude to reveal the quantitative relationships between vibrations and the SHC response. Furthermore, we demonstrate the temperature dependence of the SHC response by averaging the phonon vibration influence in the Brillouin zone. Our investigation provides an explicit experimental prediction about the temperature dependence of BPVE and can be extended to other classes of noncentrosymmetric materials.

Energy Harvesting Research: The Road from Single Source to Multisource

Energy harvesting technology may be considered an ultimate solution to replace batteries and provide a long-term power supply for wireless sensor networks. Looking back into its research history, individual energy harvesters for the conversion of single energy sources into electricity are developed first, followed by hybrid counterparts designed for use with multiple energy sources. Very recently, the concept of a truly multisource energy harvester built from only a single piece of material as the energy conversion component is proposed. This review, from the aspect of materials and device configurations, explains in detail a wide scope to give an overview of energy harvesting research. It covers single-source devices including solar, thermal, kinetic and other types of energy harvesters, hybrid energy harvesting configurations for both single and multiple energy sources and single material, and multisource energy harvesters. It also includes the energy conversion principles of photovoltaic, electromagnetic, piezoelectric, triboelectric, electrostatic, electrostrictive, thermoelectric, pyroelectric, magnetostrictive, and dielectric devices. This is one of the most comprehensive reviews conducted to date, focusing on the entire energy harvesting research scene and providing a guide to seeking deeper and more specific research references and resources from every corner of the scientific community.

Tunable metal-insulator transition, Rashba effect and Weyl Fermions in a relativistic charge-ordered ferroelectric oxide

Controllable metal-insulator transitions (MIT), Rashba-Dresselhaus (RD) spin splitting, and Weyl semimetals are promising schemes for realizing processing devices. Complex oxides are a desirable materials platform for such devices, as they host delicate and tunable charge, spin, orbital, and lattice degrees of freedoms. Here, using first-principles calculations and symmetry analysis, we identify an electric-field tunable MIT, RD effect, and Weyl semimetal in a known, charge-ordered, and polar relativistic oxide Ag2BiO3 at room temperature. Remarkably, a centrosymmetric BiO6 octahedral-breathing distortion induces a sizable spontaneous ferroelectric polarization through Bi3+/Bi5+ charge disproportionation, which stabilizes simultaneously the insulating phase. The continuous attenuation of the Bi3+/Bi5+ disproportionation obtained by applying an external electric field reduces the band gap and RD spin splitting and drives the phase transition from a ferroelectric RD insulator to a paraelectric Dirac semimetal, through a topological Weyl semimetal intermediate state. These findings suggest that Ag2BiO3 is a promising material for spin-orbitonic applications.

Shift current photovoltaic effect in a ferroelectric charge-transfer complex

Shift current is a steady-state photocurrent generated in non-centrosymmetric single crystals and has been considered to be one of the major origins of the bulk photovoltaic effect. The mechanism of this effect is the transfer of photogenerated charges by the shift of the wave functions, and its amplitude is closely related to the polarization of the electronic origin. Here, we report the photovoltaic effect in an organic molecular crystal tetrathiafulvalene-p-chloranil with a large ferroelectric polarization mostly induced by the intermolecular charge transfer. We observe a fairly large zero-bias photocurrent with visible-light irradiation and switching of the current direction by the reversal of the polarization. Furthermore, we reveal that the travel distance of photocarriers exceeds 200 μm. These results unveil distinct features of the shift current and the potential application of ferroelectric organic molecular compounds for novel optoelectric devices.The bulk photovoltaics refers to an effect whereby electrons move directionally in non-centrosymmetric crystals upon light radiation. Here, Nakamura et al. observe this effect in a ferroelectric organic charge-transfer complex, which shows large diffusion distance of photogenerated electrons over 200 µm.

Large Bulk Photovoltaic Effect and Spontaneous Polarization of Single-Layer Monochalcogenides

We use a first-principles density functional theory approach to calculate the shift current and linear absorption of uniformly illuminated single-layer Ge and Sn monochalcogenides. We predict strong absorption in the visible spectrum and a large effective three-dimensional shift current (∼100  μA/V^{2}), larger than has been previously observed in other polar systems. Moreover, we show that the integral of the shift-current tensor is correlated to the large spontaneous effective three-dimensional electric polarization (∼1.9  C/m^{2}). Our calculations indicate that the shift current will be largest in the visible spectrum, suggesting that these monochalcogenides may be promising for polar optoelectronic devices. A Rice-Mele tight-binding model is used to rationalize the shift-current response for these systems, and its dependence on polarization, in general terms with implications for other polar materials.

Optical Writing of Magnetic Properties by Remanent Photostriction

We present an optically induced remanent photostriction in BiFeO_{3}, resulting from the photovoltaic effect, which is used to modify the ferromagnetism of Ni film in a hybrid BiFeO_{3}/Ni structure. The 75% change in coercivity in the Ni film is achieved via optical and nonvolatile control. This photoferromagnetic effect can be reversed by static or ac electric depolarization of BiFeO_{3}. Hence, the strain dependent changes in magnetic properties are written optically, and erased electrically. Light-mediated straintronics is therefore a possible approach for low-power multistate control of magnetic elements relevant for memory and spintronic applications.

Enhancement of the Bulk Photovoltaic Effect in Topological Insulators

We investigate the shift current bulk photovoltaic response of materials close to a band inversion topological phase transition. We find that the bulk photocurrent reverses direction across the band inversion transition, and that its magnitude is enhanced in the vicinity of the phase transition. These results are demonstrated with first principles density functional theory calculations of BiTeI and CsPbI_{3} under hydrostatic pressure, and explained with an analytical model, suggesting that this phenomenon remains robust across disparate material systems.