arXiv subjects

Yanfa Yan

Publications and source records attributed to Yanfa Yan.

17 recordsLinked to original sources

arXiv2026

\textit{Ab initio} \textit{GW}-BSE theory of optical activity in $\alpha$-quartz

We present an ab initio many-body theory of optical activity in solids within the GW-BSE framework. Dielectric spatial dispersion is formulated in two complementary forms: exciton envelope modulation and sum-over-exciton-states expansion. Our application to $\alpha$-quartz reveals that the envelope-modulated formulation captures the low-frequency region, whereas the sum-over-exciton-states formulation is essential to reproduce the correct full frequency dependence. Comparisons with the independent-particle approximation and simple local-field corrections further highlight the decisive role of excitonic many-body effects in shaping the spectral dispersion of optical activity in solids.

cond-mat.mtrl-sci↗

arXiv2026

Photoinduced metastable cation disorder in metal halide double perovskites

Lead-free perovskites have emerged as environmentally benign alternatives to lead-halide counterparts for optoelectronics. Among them, the double perovskite Cs2AgInCl6 family exhibits remarkable white-light emission with proper composition engineering, enabled by strong electron-phonon coupling and the formation of self-trapped excitons (STEs). Despite these advantages, the fundamental photo- and structural dynamics governing their excited-state behavior remain poorly understood. Here, we report a long-lived metastable phase in the Cs2AgInCl6 double perovskite family and unravel this process and the concomitant electronic and structural evolution using a suite of tools including transient optical spectroscopy, time-resolved X-ray diffraction (TR-XRD) and X-ray absorption (TR-XAS). We show that the photoinduced, transient metastable phase is associated with B-site (Ag-In) disorder, which induces a dramatically reduced optical bandgap. Supported by TR-XRD and first-principles calculations, the Ag-In disorder drives the formation of Ag-rich and In-rich domains with millisecond lifetimes, with lifetimes increasing at lower temperatures. TR-XAS further reveals that photogenerated STEs oxidize Ag+ to Ag2+, facilitating this highly temporally asymmetric order-disorder transition. Our findings demonstrate a new mechanism, mediated by hole-localized STE formation, that enables prolongation of transient light-induced states to the multi-millisecond regime in double perovskites, opening possibilities to harvesting the functional properties of metastable phases of these materials.

cond-mat.mtrl-sci↗

arXiv2025

Origin of the voltage gap and recombination losses in all-perovskite tandem solar cells

All-perovskite tandem solar cells with narrow and wide bandgap perovskite absorbers are promising candidates for low-cost and high efficiency photovoltaic applications. However, the open circuit voltage of typical tandem structures is generally smaller than the sum of the individual voltages in the single-junction form; a quantity we call the voltage gap. Subcell optimization can only begin once nonradiative losses associated with each absorber layer can be properly identified. To address this, we used absolute electroluminescence hyperspectral imaging to construct external radiative efficiency maps of each subcell within the tandem stack and compare these measurements with single junction devices. These measurements were then combined with additional electro-optical characterization and modeling to construct subcell current vs voltage curves. We find that the narrow band gap subcell contributes the most towards the voltage gap and therefore fabrication and processing efforts should focus on reducing nonradiative recombination losses within the narrow band gap absorber.

physics.optics↗

arXiv2025

Comparison of Electroluminescence and Photoluminescence Imaging of Mixed-Cation Mixed-Halide Perovskite Solar Cells at Low Temperatures

Halide perovskites have emerged as promising candidates for high-performance solar cells. This study investigates the temperature-dependent optoelectronic properties of mixed-cation mixed-halide perovskite solar cells using electroluminescence (EL) and photoluminescence (PL) hyperspectral imaging, along with current-voltage analysis. Luminescence images, which were converted to EL and PL external radiative efficiency (ERE) maps, revealed significant changes in the optoelectronic behavior of these devices at low temperatures. Specifically, we found that a significant source of heterogeneity in the low-temperature EL ERE maps below 240 K is related to local charge injection and extraction bottlenecks, whereas PL ERE maps show suppressed non-radiative recombination and significant improvements in efficiency throughout the investigated temperature range. The spatial distribution of ERE and its variation with applied current were analyzed, offering insights into charge-carrier dynamics and defect behavior. Our results reveal that while the perovskite layer exhibits enhanced ERE at low temperatures, charge injection barriers at the interfaces of the perovskite solar cells significantly suppress EL and degrade the fill factor below 240 K. These findings reveal that a deeper understanding of the performance of perovskite solar cells under low-temperature conditions is an essential step toward their potential application in space power systems and advanced semiconductor devices.

cond-mat.mtrl-sci↗

arXiv2025

Neural Network-Guided Symbolic Regression for Interpretable Descriptor Discovery in Perovskite Catalysts

Understanding and predicting the activity of oxide perovskite catalysts for the oxygen evolution reaction (OER) requires descriptors that are both accurate and physically interpretable. While symbolic regression (SR) offers a path to discover such formulas, its performance degrades with high-dimensional inputs and small datasets. We present a two-phase framework that combines neural networks (NN), feature importance analysis, and symbolic regression (SR) to discover interpretable descriptors for OER activity in oxide perovskites. In Phase I, using a small dataset and seven structural features, we reproduce and improve the known {\mu}/t descriptor by engineering composite features and applying symbolic regression, achieving training and validation MAEs of 22.8 and 20.8 meV, respectively. In Phase II, we expand to 164 features, reduce dimensionality, and identify LUMO energy as a key electronic descriptor. A final formula using {\mu}/t, {\mu}/RA, and LUMO energy achieves improved accuracy (training and validation MAEs of 22.1 and 20.6 meV) with strong physical interpretability. Our results demonstrate that NN-guided symbolic regression enables accurate, interpretable, and physically meaningful descriptor discovery in data-scarce regimes, indicating interpretability need not sacrifice accuracy for materials informatics.

physics.data-an↗

arXiv2023

Chiral electrons and spin selectivity at chiral-achiral interfaces

Chiral molecules can selectively transport electrons of a particular spin orientation, yet the underlying mechanism remain poorly understood. Here, we present theoretical evidence that electrons propagating through chiral materials with screw symmetry exhibit chirality themselves and carry symmetry-induced pseudo-angular momentum (PAM) which must be characterized by both spin and orbital components. The chiral electron transport is PAM polarized, with counterpropagating states exhibiting opposite PAM. At chiral-achiral interfaces, the PAM polarization is converted to spin polarization due to wave function matching, thereby establishing a novel principle for the chiral-induced spin selectivity.

cond-mat.mtrl-sci↗

arXiv2022

Subsurface Characteristics of Metal-Halide Perovskites Polished by Argon Ion Beam

Focused ion beam (FIB) techniques have been frequently used to section metal-halide perovskites for microstructural investigations. However, the ion beams directly irradiated to the sample surface may alter the properties far different from pristine, potentially leading to modified deterioration mechanisms under aging stressors. Here, we combine complementary approaches to measure the subsurface characteristics of polished perovskite and identify the chemical species responsible for the measured properties. Analysis of the experimental results in conjunction with Monte Carlo simulations indicates that atomic displacements and local heating occur in the subsurface of methylammonium lead iodide (MAPbI3) by glazing Ar+ beam irradiation (15 nm by 4 kV at 3 degree). The lead-rich, iodine-deficient surface promotes rapid phase segregation under thermal aging conditions. On the other hand, despite the subsurface modification, our experiments confirm the rest of the MAPbI3 bulk retains the material integrity. Our observation supports that polished perovskites could serve in studying the properties of bulk or buried junctions far away from the altered subsurface with care.

cond-mat.mtrl-sci↗

arXiv2022

Optical Activity of Solids from First Principles

Within the framework of independent particle approximation, the optical activity tensor of solids is formulated as from different contributions: the magnetic dipole, electric quadrupole, and band dispersion terms. The first two terms have similar counterparts in the theory of finite systems, while the last term is unique for crystals. The magnetic dipole and electric quadrupole transition moments are calculated with a sum-over-states formulation. We apply the formulation to calculate and analyze the optical rotation of elemental tellurium and the circular dichroism of $(6,4)$ carbon nanotube. Decomposed optical activity into different contributions are discussed. The calculated spectra agree well with experiments. As a showcase of achiral crystals, we calculate the optical activity of wurtzite GaN.

cond-mat.mtrl-sci↗

arXiv2022

Molecular engineering of metal-free perovskite MDABCO-NH4I3 towards enhanced ferroelectric polarization

Molecular ferroelectrics have attracted increasing interests over the past decade due to mechanical flexibility, chemical diversity, environmental friendliness, easy-processing, and lightness. The performance of molecular ferroelectrics is approaching more and more competitive to their inorganic counterparts. Despite the chemical diversity and tunability of the molecular cations or anions, molecular ferroelectrics are not abundant. In particular, physical directed design principles for new discovery or performance optimization are still lacking. Here, through first-principles calculations, we firstly reveal the importance of the molecular dipole moment of the polar cations in a molecular perovskite ferroelectric, and then propose a molecular dipole guided design rule for high-performance molecular ferroelectrics. Finally, the rule is validated by first-principles calculations. We anticipate that the rule is very useful in the field urging for new and high-performance molecular ferroelectrics.

cond-mat.mtrl-sci↗

arXiv2021

Superior Photo-carrier Diffusion Dynamics in Organic-inorganic Hybrid Perovskites Revealed by Spatiotemporal Conductivity Imaging

The outstanding performance of organic-inorganic metal trihalide solar cells benefits from the exceptional photo-physical properties of both electrons and holes in the material. Here, we directly probe the free-carrier dynamics in Cs-doped FAPbI3 thin films by spatiotemporal photoconductivity imaging. Using charge transport layers to selectively quench one type of carriers, we show that the two relaxation times on the order of 1 microsecond and 10 microseconds correspond to the lifetimes of electrons and holes in FACsPbI3, respectively. Strikingly, the diffusion mapping indicates that the difference in electron/hole lifetimes is largely compensated by their disparate mobility. Consequently, the long diffusion lengths (3 ~ 5 micrometers) of both carriers are comparable to each other, a feature closely related to the unique charge trapping and de-trapping processes in hybrid trihalide perovskites. Our results unveil the origin of superior diffusion dynamics in this material, crucially important for solar-cell applications.

cond-mat.mes-hall↗

arXiv2019

Controlled Preparation, Characterization, and Bandgap Modulation of RF Sputtered Antimony Vanadium Oxide (SbVO4) Thin Films

In this paper, RF sputtered antimony vanadium oxide (SbVO4) thin films and its characterization are reported. High purity sputtering targets were fabricated by sintering a mixture of Sb2O3 and V2O5 powders. Thin films were deposited by reactive sputtering at various temperature and argon/oxygen partial pressures. Several growth parameters and surface chemistry were studied by applying numerous optical and electrochemical characterization technique. It is found that SbVO4 exhibits an indirect band gap in the range of 1.89 eV to 2.36 eV and has desirable valence band position to drive water oxidation reaction under illumination. The bandgap depends heavily on stoichiometry of the film and can be modulated by incorporating controlled amount of oxygen gas in plasma environment. Optimized SbVO4 photoanodes was designed and tested for photoelectrochemical water oxidation catalysis. Preliminary studies show that these electrodes possess n-type catalytic behavior in alkaline media. The prepared SbVO4 thin films, with typical film thickness was around 400 nm, contain nanoparticles having the sizes of 10-15 nm.

physics.app-ph↗

arXiv2019

Comparative Study of RF Sputtered Ba2(BiNb)O6 and Ba2Bi1.4Nb0.6 O6 thin films and Effect of Fluorine Doping in Their Photoelectrochemical (PEC) Performances

Bismuth based ternary oxide photocatalysts are of considerable interest in photoelectrochemical water splitting. Yet these oxides are highly stable in different environment, their relative inertness limits the available synthesis routes to obtain desired stoichiometry on the final product. This report describes a method to prepare barium bismuth niobate (specifically, Ba2(BiNb)O6 and Ba2Bi1.4Nb0.6 O6) target by sintering a mixture of individual oxides and successfully fabricate desired barium bismuth niobate thin film on TCO substrate. The surface morphology, physical, and chemical properties of thin film were systematically investigated. Compositional uniformity was obtained by sputtering in oxygen argon plasma environment and higher photocatalytic activities were observed after subsequent surface treatments. Sodium fluoride surface treatment further enhanced the photocurrent density and improved electrode stability against corrosion. This work further suggests a viable approach to improve the PEC performance of sputtered barium bismuth niobate by modulating their fundamental energy states.

physics.app-ph↗

arXiv2019

Study of RF Sputtered Antimony Alloyed Bismuth Vanadium Oxide (Sb:BiVO4) Thin Films for Enhanced Photoelectrochemical (PEC) performance from Bandgap Modulation to Thickness Optimization

Monoclinic scheelite bismuth vanadate (BiVO4) is a promising photoanode for water splitting yet the PEC performance is limited due to its relatively higher (2.4 eV) band gap. Here, we successfully decreased its the band gap to 1.72 eV by controlled antimony alloying. Low bandgap antimony alloyed bismuth vanadium oxide (Sb:BiVO4) thin film was prepared by RF sputtering of high purity homemade target, fabricated by solid-state reaction using a mixture of Sb2O3, Bi2O3, and V2O5 powders with desired stoichiometric ratios. Several growth parameters, powder crystallography, post-deposition effects, and surface treatments, thickness dependence, effect of electrolytes on photocorrosion were studied along with its optical and electrochemical characterization. We discovered that Sb:BiVO4 is a direct band gap material in the visible light range (1.72 eV) and a valence band position suitable for driving water oxidation reaction under illumination. Furthermore, hole diffusion length is increased with antimony alloying and achieved optimum thickness of 400 nm for higher photocurrent. The controllably prepared Sb:BiVO4 particles are having the sizes of 10-15 nm in room temperature deposition and can be grown up to 0.5 microns under air annealing.

physics.app-ph↗

arXiv2019

Symbolic Regression Discovery of New Perovskite Catalysts with High Oxygen Evolution Reaction Activity

Symbolic regression (SR) is an emerging method for building analytical formulas to find models that best fit data sets. Here, SR was used to guide the design of new oxide perovskite catalysts with improved oxygen evolution reaction (OER) activities. An unprecedentedly simple descriptor, {\mu}/t, where {\mu} and t are the octahedral and tolerance factors, respectively, was identified, which accelerated the discovery of a series of new oxide perovskite catalysts with improved OER activity. We successfully synthesized five new oxide perovskites and characterized their OER activities. Remarkably, four of them, Cs0.4La0.6Mn0.25Co0.75O3, Cs0.3La0.7NiO3, SrNi0.75Co0.25O3, and Sr0.25Ba0.75NiO3, outperform the current state-of-the-art oxide perovskite catalyst, Ba0.5Sr0.5Co0.8Fe0.2O3 (BSCF). Our results demonstrate the potential of SR for accelerating data-driven design and discovery of new materials with improved properties.

cond-mat.mtrl-sci↗

arXiv2017

First-Principles Understanding of the Electronic Band Structure of Copper-Antimony Halide Perovskite: The Effect of Magnetic Ordering

We report the understanding of the electronic band structure of $Cs_4CuSb_2Cl_{12}$ perovskite through first-principles density-functional theory calculations. We find that the most stable state has the antiferromagnetic configuration where each $[CuCl_6]$ octahedral chain along the [010] direction is antiferromagnetic. The reasonable band structure of the compound can be obtained only if both the correct magnetic order and the improved exchange interaction of the Cu $\it{d}$ electrons are taken into account.

cond-mat.mtrl-sci↗

arXiv2017

Electronic Band Structures and Excitonic Properties of Delafossites: A $\textit{GW}$-BSE study

We report the band structures and excitonic properties of delafossites $CuMO_2$ (M = Al, Ga, In, Sc, Y, Cr) calculated using the state-of-the-art $\textit{GW}$-BSE approach. We find that all the delafossites are indirect band gap semiconductors with large exciton binding energies, varying from 0.24 to 0.44 eV. The lowest and strongest exciton, mainly contributed from either Cu 3$\textit{d}$ $\rightarrow$ Cu 3$\textit{p}$ (Al, Ga, In) or Cu 3$\textit{d}$ $\rightarrow$ M 3$\textit{d}$ (M = Sc, Y, Cr) transitions, is always located at $L$ point of the rhombohedral Brillouin zone. Taking the electron-hole effect into account, our theoretical band gaps exhibit nice agreement with experiments.

cond-mat.mtrl-sci↗

arXiv2017

Parity-Forbidden Transitions and Their Impacts on the Optical Absorption Properties of Lead-Free Metal Halide Perovskites and Double Perovskites

Using density-functional theory calculations, we analyze the optical absorption properties of lead (Pb)-free metal halide perovskites (AB$^{2+}$X$_3$) and double perovskites (AB$^+$B$^{3+}$X$_6$) (A = Cs or monovalent organic ion, B$^{2+}$ = non-Pb divalent metal, B$^+$ = monovalent metal, B$^{3+}$ = trivalent metal, X = halogen). We show that, if B$^{2+}$ is not Sn or Ge, Pb-free metal halide perovskites exhibit poor optical absorptions because of their indirect bandgap nature. Among the nine possible types of Pb-free metal halide double perovskites, six have direct bandgaps. Of these six types, four show inversion symmetry-induced parity-forbidden or weak transitions between band edges, making them not ideal for thin-film solar cell application. Only one type of Pb-free double perovskite shows optical absorption and electronic properties suitable for solar cell applications, namely those with B$^+$ = In, Tl and B$^{3+}$ = Sb, Bi. Our results provide important insights for designing new metal halide perovskites and double perovskites for optoelectronic applications.

cond-mat.mtrl-sci↗