We study the Fermi surface contribution to the nonlinear DC photocurrent at quadratic order in a spatially uniform optical field in the ultra-clean limit. In addition to shift and injection current, we find that polarized light incident on a metallic system generates an intrinsic contribution to the bulk photovoltaic effect deriving from photoinduced electronic transitions on the Fermi surface. In {velocity} gauge, this contribution originates in both the coherent band off-diagonal and diagonal parts of the density matrix, describing respectively, the coherent wave function evolution and the carrier dynamics of an excited population. We derive a formula for the intrinsic Fermi surface contribution for a time-reversal invariant chiral Weyl semimetal illuminated with circularly-polarized light. At low frequency, this response is proportional to the frequency of the driving field, with its sign determined by the topological charge of the Weyl nodes and with its magnitude being comparable to the recently discovered quantized circular photogalvanic effect. Our work presents a complete derivation for all contributions to nonlinear DC photocurrent and classify them according to the polarization of light in the presence and absence of TRS.
The material class of hybrid organic-inorganic perovskites has risen rapidly from a virtually unknown material in photovoltaic applications a short 7 years ago into a ~20% efficient thin-film solar cell material. As promising as this class of materials is, however, there are limitations associated with its poor long-term stability, non-optimal band gap, presence of environmentally-toxic Pb element, etc. We herein apply a functionality-directed theoretical materials selection approach as a filter for initial screening of the compounds that satisfy the desired intrinsic photovoltaic functionalit…
Note: This paper has been published in Physical Chemistry Chemical Physics, which can be viewed at the following URL: http://doi.org/10.1039/C5CP03102H Cs2SnI6, a rarely studied perovskite variant material, is recently gaining a lot of interest in the field of photovoltaics owing to its nontoxity, air-stability and promissing photovoltaic properties. In this work, we report intrinsic defects in Cs2SnI6 using first-principles density functional theory calculations. It is revealed that iodine vacancy and tin interstitial are the dominant defects that are responsible for the intrinsic n-type cond…
Historically, the design of hybrid solar photovoltaic thermal (PVT) systems has focused on cooling crystalline silicon (c-Si)-based photovoltaic (PV) devices to avoid temperature-related losses. This approach neglects the associated performance losses in the thermal system and leads to a decrease in the overall exergy of the system. Consequently, this paper explores the use of hydrogenated amorphous silicon (a-Si:H) as an absorber material for PVT in an effort to maintain higher and more favourable operating temperatures for the thermal system. Amorphous silicon not only has a smaller temperat…
Structurally chiral materials hosting multifold fermions with large topological number have attracted considerable attention because of their naturally long surface Fermi arcs and bulk quantized circular photogalvanic effect (CPGE). Multifold fermions only appear in metallic states, and therefore, most studies so far have only focused on the semimetals in compounds with chiral crystal structures. In this work, we show that the structurally chiral topological trivial insulators are also exotic states, which is interesting from the application point of view, owing to their natural advantage to h…
The bulk photovoltaic effect that is intimately associated with crystalline symmetry has been extensively studied in various nonmagnetic materials, especially ferroelectrics with a switchable electric polarization. In order to further engineer the symmetry, one could resort to spin-polarized systems possessing an extra magnetic degree of freedom. Here, we investigate the bulk photovoltaic effect in two-dimensional magnetic sliding ferroelectric (MSFE) systems, illustrated in VSe2, FeCl2, and CrI3 bilayers. The transition metal elements in these systems exhibit intrinsic spin polarization, and …
Intrinsic Point Defects and Frenkel Pair Formation in Photovoltaic Absorber Zn$_3$P$_2$: Regulating $p$-type Conductivity through Growth and Annealing Conditions
Research Center Future Energy Materials and Systems of the Research Alliance Ruhr and Interdisciplinary Center for Advanced Materials Si（研究联盟鲁尔未来能源材料与系统研究中心及先进材料跨学科中心Si）
Organic semiconductors possess an intrinsic energetic disorder characteristic, which holds an exceptionally important role for understanding organic photovoltaic (OPV) operation and future optimization. We performed illumination intensity dependence of capacitance-voltage (C-V) measurements in PIDTDTQx:PC70BM based organic bulk heterojunction (BHJ) photovoltaics in working conditions. Energetic disorder profiles for the photo-active layer, PIDTDTQx:PC70BM, changed significantly when different interfaces were involved. The effects of energetic disorder that could be reflected from C-V profiles …
Relating Band Edge DOS Occupancy Statistics Associated Excited State Electrons Entropy Generation to Free Energy Loss and Intrinsic Voc Deficit of Solar Cells
该研究将带边 DOS 占据及激发态电子熵与太阳能电池自由能损失和本征 Voc 亏损关联，指出带边 DOS 工程是突破辐射复合损失、优化性能的新维度。
BaCd2P2: a promising impurity-tolerant counterpart of GaAs for photovoltaics
Thayer School of Engineering, Dartmouth College（达特茅斯学院塔耶尔工程学院）；Department of Chemistry, Iowa State University（爱荷华州立大学化学系）；Aiiso Yufeng Li Family Department of Chemical & Nanoengineering, UC San Diego（加州大学圣地亚哥分校艾索·于峰李家族化学与纳米工程系）；Department of Materials Science and NanoEngineering, Rice University（莱斯大学材料科学与工程及纳米工程系）；Rice Advanced Materials Institute, Rice University（莱斯大学先进材料研究所）；Ames National Laboratory, U.S. Department of Energy（美国能源 Ames 国家实验室）
The Shockley-Queisser (SQ) limit provides a convenient metric for predicting light-to-electricity conversion efficiency of a solar cell based on the band gap of the light-absorbing layer. In reality, few materials approach this radiative limit. We develop a formalism and a computational method to predict the maximum photovoltaic efficiency of imperfect crystals from first principles. Our scheme includes equilibrium populations of native defects, their carrier-capture coefficients, and the associated recombination rates. When applied to kesterite solar cells, we reveal an intrinsic limit of 20%…
The bulk photovoltaic effect (BPVE) occurs in solids with broken inversion symmetry and refers to DC current generation due to uniform illumination, without the need of heterostructures or interfaces, a feature that is distinct from the traditional photovoltaic effect. Its existence has been demonstrated almost 50 years ago, but predictive theories only appeared in the last ten years, allowing for the identification of different mechanisms and the determination of their relative importance in real materials. It is now generally accepted that there is an intrinsic mechanism that is insensitive …
Composition Anisotropy Drives Large Bulk Photovoltaic Fields Along the Non-polar Vertical Direction in 2D Hybrid Perovskite Ferroelectrics
Heidelberg University（海德堡大学）；Universität Heidelberg（海德堡大学）；NTNU Norwegian University of Science and Technology（挪威科技大学）；Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences（中国科学院福建物质结构研究所）；Technical University of Darmstadt（达姆施塔特工业大学）；Graz University of Technology（格拉茨工业大学）
Bulk photovoltaic (BPV) effect, a second order nonlinear process that generates static current under light irradiation, requires centrosymmetric broken systems as its application platform. In order to realize measurable BPV photocurrent in spatially centrosymmetric materials, various schemes such as chemical doping, structural deformation, or electric bias have been developed. In the current work, we suggest that magnetic proximity effect via van der Waals interfacial interaction, a contact-free strategy, also breaks the centrosymmetry and generate large BPV photocurrents. Using the Bi2Te3 qui…
The bulk photovoltaic effect (BPVE) leads to directed photo-currents and photo-voltages in bulk materials. Unlike photo-voltages in p-n junction solar cells that are limited by carrier recombination to values below the bandgap energy of the absorbing material, the BPVE photo-voltages have been shown to greatly exceed the bandgap energy. Therefore the BPVE is not subject to the Shockley-Queisser limit for sunlight to electricity conversion in single junction solar cells and experimental claims of efficiencies beyond this limit have been made. Here, we show that BPVE energy conversion efficienci…
The bulk photovoltaic effect generates intrinsic photocurrents in materials without inversion symmetry. Shift current is one of the bulk photovoltaic phenomena related to the Berry phase of the constituting electronic bands: photo-excited carriers coherently shift in real space due to the difference in the Berry connection between the valence and conduction bands. Ferroelectric semiconductors and Weyl semimetals are known to exhibit such nonlinear optical phenomena. Here we consider chalcopyrite semiconductor ZnSnP$_2$ which lacks inversion symmetry and calculate the shift current conductivity…
Direct spatiotemporal imaging of carriers reveals a long-lived bulk photovoltaic mechanism
Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and BIST（加泰罗尼亚纳米科学与纳米技术研究所）；TU Eindhoven（埃因霍温理工大学）；University of Pennsylvania（宾夕法尼亚大学）；SPEC, CEA, CNRS, Université Paris-Saclay（巴黎萨克雷大学）；ICREA - Institució Catalana de Recerca i Estudis Avançats（加泰罗尼亚研究与高级研究学院）
High-quality Sb2Te3 films are obtained by molecular beam epitaxy on graphene substrate and investigated by in situ scanning tunneling microscopy/spectroscopy. Intrinsic defects responsible for the natural p-type conductivity of Sb2Te3 are identified to be the Sb vacancies and SbTe antisites in agreement with first-principles calculations. By minimizing defect densities, coupled with a transfer doping by the graphene substrate, the Fermi level of Sb2Te3 thin films can be tuned over the entire range of the bulk band gap. This establishes the necessary condition to explore topological insulator b…
We show that the quantum geometry of the Fermi surface can be numerically described by a 3-dimensional discrete quantum manifold. This approach not only avoids singularities in the Fermi sea, but it also enables the precise computation of the intrinsic Hall conductivity resolved in spin, as well as any other local properties of the Fermi surface. The method assures numerical accuracy when the Fermi level is arbitrarily close to singularities, and it remains robust when Kramers degeneracy is protected by symmetry. The approach is demonstrated by calculating the anomalous Hall and spin Hall cond…
Two-dimensional materials with layered structures, appropriate bandgap, and high carrier mobilities have gathered tremendous interests due to their potential applications in optoelectronic and photovoltaic devices. Here, we report the growth of BiI3 thin film with controllable atomic thickness on graphene-terminated 6H-SiC(0001) substrate by molecular beam epitaxy (MBE) method. The growth kinetic processes and crystalline properties of the BiI3 film are studied by scanning tunneling microscopy (STM). The scanning tunneling spectroscopy (STS) reveals a bandgap of 2.8 eV for monolayer BiI3 with …
Lead-free halide double perovskites with the formula of quaternary A$_2^+$B'B'$^{3+}$X$_6^-$ have recently attracted intense interest as alternatives to lead-halide-perovskite-based optoelectronic materials for their non-toxicity and enhanced chemical and thermodynamic stability. However, the understanding of intrinsic defect properties and their effects on carrier transport and Fermi level tuning is still limited. In this paper, we show that, by exploring the phase diagram of a halide double perovskite, one can control the effects of intrinsic defects on carrier trapping and Fermi level pinni…
MnBi2Te4 and MnBi4Te7 are intrinsic antiferromagnetic topological insulators, offering a promising materials platform for realizing exotic topological quantum states. However, high densities of intrinsic defects in these materials not only cause bulk metallic conductivity, preventing the measurement of quantum transport in surface states, but may also affect magnetism and topological properties. In this paper, we show by density functional theory calculations that the strain induced by the internal heterostructure promotes the formation of large-size-mismatched antisite defect BiMn in MnBi2Te4…
Graphene is a promising candidate for optoelectronic applications such as photodetectors, terahertz imagers, and plasmonic devices. The origin of photoresponse in graphene junctions has been studied extensively and is attributed to either thermoelectric or photovoltaic effects. In addition, hot carrier transport and carrier multiplication are thought to play an important role. Here we report the intrinsic photoresponse in biased but otherwise homogeneous graphene. In this classic photoconductivity experiment, the thermoelectric effects are insignificant. Instead, the photovoltaic and a photo-i…
Despite the widespread use of solution-processable hybrid organic-inorganic perovskites in photovoltaic and light-emitting applications, determination of their intrinsic charge transport parameters has been elusive due to the variability of film preparation and history-dependent device performance. Here we show that screening effects associated to ionic transport can be effectively eliminated by lowering the operating temperature of methylammonium lead iodide perovskite (CH3NH3PbI3) field-effect transistors (FETs). Field-effect carrier mobility is found to increase by almost two orders of magn…
We consider spin Hall effect in a system of massless Dirac fermions in a graphene lattice. Two types of spin-orbit interaction, pertinent to the graphene lattice, are taken into account - the intrinsic and Rashba terms. Assuming perfect crystal lattice, we calculate the topological contribution to spin Hall conductivity. When both interactions are present, their interplay is shown to lead to some peculiarities in the dependence of spin Hall conductivity on the Fermi level.
We propose a scheme for generating Rashba spin-orbit coupling and perpendicular Zeeman field simultaneously for cold fermionic atoms in a harmonic trap through the coupling between atoms and laser fields. The realization of Rashba spin-orbit coupling and perpendicular Zeeman field provides opportunities for exploring many topological phenomena using cold fermionic atoms. We focus on the intrinsic anomalous Hall effect and show that it may be observed through the response of atomic density to a rotation of the harmonic trap.
We theoretically investigated the fundamental distinction between intrinsic and extrinsic nonlinear thermal Hall effect in the presence of disorder at the second-order response to the temperature gradient in terms of the semi-classical Boltzmann equation. We found that, at low temperatures, the intrinsic contribution of the nonlinear thermal Hall conductivity is proportional to the square of temperature, whereas the extrinsic contributions (side-jump and skew-scattering) are independent of temperature. This distinct dependency on temperature provide a new approach to readily distinguish the in…
The origin of anomalous Hall effect (AHE) in magnetic materials is one of the most intriguing aspect in condensed matter physics and has been controversial for a long time. Recent studies indicate that the intrinsic AHE is closely related to the Berry curvature of occupied electronic states. In a magnetic Weyl semimetal with broken time-reversal symmetry, there are significant contributions on Berry curvature around Weyl nodes, which would lead to a large intrinsic AHE. Here, we report the large intrinsic AHE in the half-metallic ferromagnet Co3Sn2S2 single crystal. By systematically mapping o…
The electronic structure and half-metallic gap of Co$_{2}$MnSi in the presence of crystallographic defects, partial Fe substitution for Mn, and thermal spin fluctuations are studied using the coherent potential approximation and the disordered local moment method. In the presence of 5\% Co or Mn vacancies the Fermi level shifts down to the minority-spin valence-band maximum. In contrast to NiMnSb, both types of Mn antisite defects in Co$_{2}$MnSi are strongly exchange-coupled to the host magnetization, and thermal spin fluctuations do not strongly affect the half-metallic gap. Partial substitu…
Enhanced Photocurrent Response in Epitaxial 0.5PZT-0.5PFN Multiferroic Thin Films
FZU - Institute of Physics of the Czech Academy of Sciences（捷克科学院物理研究所）；Catalan Institute of Nanoscience and Nanotechnology, ICN 2, CSIC and The Barcelona Institute of Science and Technology (BIST)（加泰罗尼亚纳米科学与技术研究所，西班牙国家研究委员会与巴塞罗那科学与技术学院）
We have performed bulk-sensitive spin-resolved photoemission spectroscopy in order to clarify the intrinsic spin-resolved electronic states of half-metallic ferromagnet CrO2. We used CrO2 epitaxial films on TiO2(100), which shows a peak at 1 eV with a clear Fermi edge, consistent with the bulk-sensitive PES spectrum for CrO2. In spin-resolved spectra at 40 K, while the Fermi edge was observed in the spin up (majority spin) state, no states at the Fermi level (EF) with an energy gap of 0.5 eV below EF was observed in the spin down (minority spin) state. At 300 K, the gap in the spin down state …
Chiral anomaly induced negative magnetoresistance (NMR) has been widely used as a critical transport evidence on the existence of Weyl fermions in topological semimetals. In this mini review, we discuss the general observation of the NMR phenomena in non-centrosymmetric NbP and NbAs. We show that NMR can be contributed by intrinsic chiral anomaly of Weyl fermions and/or extrinsic effects, such as superimposition of Hall signals, field-dependent inhomogeneous current flow in the bulk, i.e. current jetting, and weak localization (WL) of coexistent trivial carriers. Such WL controlled NMR is heav…
We demonstrate the capability of growing high quality ultrathin films of the topological insulators Bi2Se3 and Bi2Te3 using molecular beam epitaxy. Unlike previous growth techniques, which often pin the Fermi energy in the conduction band for ultrathin samples, our samples remain intrinsic bulk insulators. We characterize these films using in-situ angle resolved photoemission spectroscopy (ARPES), which is a direct probe of bandstructure, and ex-situ atomic force microscopy. We find that the conduction band lies above the Fermi energy, indicating bulk insulating behavior with only the surface …
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 …
The establishment of a rigorous theory on thermodynamics of light management in photovoltaics that accommodates various loss mechanisms as well as wave-optical effects in the absorption and reemission of light is at stake in this contribution. To this end, we propose a theoretical framework to calculate the open-circuit voltage enhancement resulting from photon recycling ($ΔV^{\mathrm{PR}}_{\mathrm{oc}}$) with rigorous wave-optical treatment. It can be applied to both planar thin-film and nanostructured single-junction solar cells. We derive an explicit expression for $ΔV^{\mathrm{PR}}_{\mathr…
Combined in-situ x-ray photoemission spectroscopy, scanning tunnelling spectroscopy and angle resolved photoemission spectroscopy of molecular beam epitaxy grown Bi2Te3 on lattice mismatched substrates reveal high quality stoichiometric thin films with topological surface states without a contribution from the bulk bands at the Fermi energy. The absence of bulk states at the Fermi energy is achieved without counter doping. We observe that the surface morphology and electronic band structure of Bi2Te3 are not affected by in-vacuo storage and exposure to oxygen, whereas major changes are observe…
We perform time- and angle-resolved photoemission spectroscopy of a prototypical topological insulator Bi$_2$Se$_3$ to study the ultrafast dynamics of surface and bulk electrons after photo-excitation. By analyzing the evolution of surface states and bulk band spectra, we obtain their electronic temperature and chemical potential relaxation dynamics separately. These dynamics reveal strong phonon-assisted surface-bulk coupling at high lattice temperature and total suppression of inelastic scattering between the surface and the bulk at low lattice temperature. In this low temperature regime, th…
Topological surface states with intrinsic magnetic ordering in the MnBi$_2$Te$_4$(Bi$_2$Te$_3$)$_n$ compounds have been predicted to host rich topological phenomena including quantized anomalous Hall effect and axion insulator state. Here we use scanning tunneling microscopy to image the surface Dirac fermions in MnBi$_2$Te$_4$ and MnBi$_4$Te$_7$. We have determined the energy dispersion and helical spin texture of the surface states through quasiparticle interference patterns far above Dirac energy, which confirms its topological nature. Approaching the Dirac point, the native defects in the …
The ability to controllably manipulate the laser-induced ultrafast magnetic dynamics is a prerequisite for future high speed spintronic devices. The optimization of devices requires the controllability of the ultrafast demagnetization time, , and intrinsic Gilbert damping, . In previous attempts to establish the relationship between and , the rare-earth doping of a permalloy film with two different demagnetization mechanism is not a suitable candidate. Here, we choose Co/Ni bilayers to investigate the relations between and by means of time-resolved magneto-optical Kerr effect (TRMOKE) via adju…
The experimental availability of ultra-high-mobility samples of graphene opens the possibility to realize and study experimentally the "hydrodynamic" regime of the electron liquid. In this regime the rate of electron-electron collisions is extremely high and dominates over the electron-impurity and electron-phonon scattering rates, which are therefore neglected. The system is brought to a local quasi-equilibrium described by a set of smoothly varying (in space and time) functions, {\it i.e.} the density, the velocity field and the local temperature. In this paper we calculate the charge and sp…
Spin Hall effect in metallic Pt is studied with first-principles relativistic band calculations. It is found that intrinsic spin Hall conductivity (SHC) is as large as $\sim 2000 (\hbar/e)(Ω{\rm cm})^{-1}$ at low temperature, and decreases down to $\sim 200 (\hbar/e)(Ω{\rm cm})^{-1}$ at room temperature. It is due to the resonant contribution from the spin-orbit splitting of the doubly degenerated $d$-bands at high-symmetry $L$ and $X$ points near the Fermi level. By modeling these near degeneracies by effective Hamiltonian, we show that SHC has a peak near the Fermi energy and that the vertex…
Intrinsic Spin Filter Effect in a $d$-wave altermagnet KV$_2$Se$_2$O with Open Fermi Surface
School of Science, Sun Yat-sen University（中山大学理学院）；Department of Physics and Astronomy, University of Florence（佛罗伦萨大学物理与天文学系）；Key Laboratory of Artificial Micro- and Nano-structures of Ministry of Education, School of Physics and Technology, Wuhan University（武汉大学物理科学与技术学院人工微结构和量子调控教育部重点实验室）；The State Key Laboratory for Refractories and Metallurgy, Wuhan University of Science and Technology（武汉科技大学耐火材料与冶金国家重点实验室）；Laboratory for Computational Physical Sciences (MOE), State Key Laboratory of Surface Physics, and Department of Physics, Fudan University（复旦大学物理系表面物理国家重点实验室计算物理科学教育部重点实验室）；Guangdong Provincial Key Laboratory of Magnetoelectric Physics and Devices, Sun Yat-sen University（中山大学广东省磁电物理与器件重点实验室）
Electronic properties of zigzag boron-carbon-nitride (BCN) nanoribbons, where the outermost C atoms on the edges of graphene nanoribbons are replaced by B or N atoms, are theoretically studied using the first-principles calculations. We show that BCN nanoribbons are metallic, since several bands cross the Fermi level. For BCN nanoribbons in a rich H$_2$ environment, the so-called nearly free electron state appears just above the Fermi level because of the intrinsic giant Stark effect due to the internal electric field of a transverse dipole moment. The position of the nearly free electron stat…
Weyl fermions in an external magnetic field exhibit the chiral anomaly, a non-conservation of chiral fermions. In a Weyl semimetal, a spatially inhomogeneous Weyl node separation causes similar effect by creating an intrinsic pseudo-magnetic field with an opposite sign for nodes of opposite chirality. In the present work we study the interplay of external and intrinsic fields. In particular, we focus on quantum oscillations due to bulk-boundary trajectories. When caused by an external field, such oscillations are a proven experimental technique to detect Weyl semimetals. We show that the intri…
Intrinsic Electronic Structure and Nodeless Superconducting Gap of $\mathrm{YBa_{2} Cu_{3} O_{7-δ} }$ Observed by Spatially-Resolved Laser-Based Angle Resolved Photoemission Spectroscopy
