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DOE OSTI2024

Impact of symmetry breaking and spin-orbit coupling on the band gap of halide perovskites

Halide perovskite (HP) materials have recently emerged as a class of semiconductors with immense promise for various optoelectronic applications, ranging from solar cells to light-emitting diodes. One of the unique attributes of HPs is their tunable band gaps with different factors governing their value. Furthermore, the first factor is related to relativistic corrections [“mass-Darwin,” connected to the ns 2 lone pairs, and spin-orbit coupling (SOC)] that induce an orbital shift or degeneracy splitting, resulting in a band-gap reduction. The second factor involves the structural configuration: in HPs the local symmetry of each Wyckoff position tends to be broken, inducing an opening of the band gap. Based on high-throughput density functional theory calculations, this paper systematically studies a possible self-cancelation on the band-gap correction for HPs when the polymorphous configuration—structural effects—and the SOC—electronic effects—are included. Our results indicate that the nature of interplay between SOC and symmetry breaking (SB) is that they are independent decoupling effects to describe the band-gap magnitude in halide perovskites. As a result of that, we observe a transitivity of the band-gap description; i.e., if we know the band gap of halide perovskites without SB and SOC, we can independently add the effects of band-gap reduction due to SOC and band-gap opening due to SB, regardless of the order in which these effects are considered.

14 SOLAR ENERGY↗

DOE OSTI2025

Carrier Localization and Spontaneous Formation of Two-Dimensional Polarization Domain in Halide Perovskites

Halide perovskites are known for their rich phase diagram and superior performance in diverse optoelectronics applications. The latter property is often attributed to the long electron-hole recombination time, whose underlying physical mechanism has been a long-standing controversy. In this Letter, we investigate the transport and localization properties of electron and hole carriers in a prototypical halide perovskite (CsPbBr 3 ), through ab initio tight-binding nonadiabatic dynamics approach for large-scale (tens of nm size) supercell calculations, to simulate electron and ion dynamics on the same footing. Here, we found distinct structural, lattice polarization, and electron-phonon coupling properties at low (below 100 K) and high temperatures, consistent with experimental observations. In particular, at low temperature we find spontaneous formation of polar grain boundaries in the nonpolar bulk systems, which result in two-dimensional polarization patterns that serve to localize and separate electrons and holes. We reveal phonon-assisted variable-range hopping mostly responsible for low-temperature transport, and their characteristic frequency correlates with temperature-dependent phonon power spectrum and energy oscillation frequency in nonadiabatic dynamics. We answer the critical questions of long electron-hole recombination lifetime at low temperature and offer the correlation among polarization domains, electron-phonon couplings, and photocarrier dynamics.

Carrier generation & recombination↗

DOE OSTI2024

Topological polarons in halide perovskites

Halide perovskites emerged as a revolutionary family of high-quality semiconductors for solar energy harvesting and energy-efficient lighting. There is mounting evidence that the exceptional optoelectronic properties of these materials could stem from unconventional electron–phonon couplings, and it has been suggested that the formation of polarons and self-trapped excitons could be key to understanding such properties. By performing first-principles simulations across the length scales, here we show that halide perovskites harbor a uniquely rich variety of polaronic species, including small polarons, large polarons, and charge density waves, and we explain a variety of experimental observations. We find that these emergent quasiparticles support topologically nontrivial phonon fields with quantized topological charge, making them nonmagnetic analog of the helical Bloch points found in magnetic skyrmion lattices.

Science & Technology - Other Topics↗

DOE OSTI2025

An AI-accelerated pathway for reproducible and stable halide perovskites

Halide perovskites (HPs) have remarkable optoelectronic properties, and in the last decade their photovoltaic power conversion efficiency and light-emitting diode efficiency have skyrocketed. Despite the surge in research on these burgeoning materials, two key challenges in the field remain: material irreproducibility and instability. Their behavior is especially dynamic in response to environmental stressors, due to complex interactions with the perovskite crystal lattice. Here, in this review, we survey the latest achievements in HP materials research accomplished with the assistance of artificial intelligence (AI), through the implementation of automated experimentation and machine learning (ML) data analysis. Automated synthesis and characterization tackle problems with material irreproducibility by systematically controlling parameters with very high precision, creating massive datasets, and allowing methodical comparisons from which unbiased conclusions can be drawn. AI can reveal otherwise unnoticed trends, inform future experiments with the highest potential information gain, and forecast future performance. The review concludes with a forward viewpoint of how human-assisted closed-loop laboratories and shared databases allow halide perovskite materials’ processing, properties, and performance to be potentially optimized with AI, accelerating the development of highly reproducible and stable optoelectronic devices.

Hering, Abigail R. [Univ. of California, Davis, CA↗

DOE OSTI2026

Supramolecular Assembly of Multielement Ribbon-like Structures Derived from Halide Perovskites

Halide perovskites are crucial materials with broad applications owing to their exceptional optoelectronic properties. Vacancy-ordered double perovskites, featuring highly tunable transition metal sites, enable controllable optoelectronic properties through multielement compositional design. Here, in this study, we introduced 18-crown-6 into the vacancy-ordered double perovskites system and developed two-dimensional ribbon-like single crystals (18C6@K) 2 {PtSnTeIrRe} 1 Cl 6 via an antisolvent supramolecular assembly method, demonstrating morphology modulation through multielement composition design. The crystals crystallize in the centrosymmetric space group $P\bar{1}$. The dumbbell-shaped structural units (crown ether@A)2MX6 pack along a and b axes to form a 2-dimensional (2D) monolayer, and these monolayers further stack along the c axis to generate the ribbon-like single crystals. Energy-dispersive X-ray spectroscopy (EDX) qualitatively confirmed the uniform distribution of the five transition metals throughout the crystal, while inductively coupled plasma atomic emission spectroscopy (ICP–AES) quantitatively verified their atomic ratios. We further investigated the origin of the morphology, distinct from the previously reported cube-like single crystals with the $R\bar{3}$ space group. When acetonitrile was used as the solvent, three-dimensional crystals with $R\bar{3}$ symmetry were obtained, whereas dimethylformamide (DMF) was essential for forming two-dimensional ribbon-like single crystals. The essential role of DMF could be ascribed to its capability to maintain a higher concentration of the building blocks. Moreover, Ir 4+ and Pt 4+ cations also played critical roles in inducing the two-dimensional ribbon-like morphology. The three-element (18C6@K) 2 {PtSnTe}1Cl 6 single crystals exhibited bright yellow emission under 375 nm laser excitation, demonstrating the tunability of optoelectronic properties of this class of material.

Zhu, Heqing [University of California, Berkeley, C↗

DOE OSTI2024

A-Site Cation Chemistry in Halide Perovskites

Metal halide perovskites are an important class of semiconductors now being implemented as photovoltaic absorbers and explored for light emission, among other device applications. The semiconducting properties of halide perovskites are deeply intertwined with their composition and structure. Specifically the symmetry, tilting, and distortions of the metal halide octahedra impact the band structure and other optoelectronic properties. In this review, we examine the various compositions of monovalent A-site cations in threedimensional (3D) halide perovskites AMX 3 (M = divalent metal; X = halide). We focus on how the A-site cation templates the inorganic metal-halide perovskite framework, resulting in changes in the crystal structure symmetry, as well as M–X bonding parameters, summarized in a comprehensive table of AMX 3 structures. The A-site cation motion, effects of alloying, and 2D Ruddlesden–Popper perovskite structures with unique A-site cations are further overviewed. Correlations are shown between these A-site cation dominated structural parameters and the resulting optoelectronic properties such as band gap. This review should serve as a reference for the A-site cation structural chemistry of metal halide perovskites and inspire continued research into less explored metal halide perovskite compositions and structures.

14 SOLAR ENERGY↗

DOE OSTI2024

Intrinsic Limits of Charge Carrier Mobilities in Layered Halide Perovskites

Layered halide perovskites have emerged as potential alternatives to three-dimensional (3D) halide perovskites due to their improved stability and larger material phase space, allowing fine tuning of structural, electronic, and optical properties. However, their charge carrier mobilities are significantly smaller than those of 3D halide perovskites, which has a considerable impact on their application in optoelectronic devices. Here, we employ state-of-the-art approaches to unveil the electron-phonon mechanisms responsible for the diminished transport properties of layered halide perovskites. Starting from a prototypical A M X 3 halide perovskite, we model the case of n = 1 and n = 2 layered structures and compare their electronic and transport properties to the 3D reference. The electronic and phononic properties are investigated within density functional theory (DFT) and density functional perturbation theory (DFPT), while transport properties are obtained via the Boltzmann transport equation. The vibrational modes contributing to charge carrier scattering are investigated and associated with polar-phonon scattering mechanisms arising from the long-range Fröhlich coupling and deformation-potential scattering processes. Our investigation reveals that the lower mobilities in layered systems primarily originate from the increased electronic density of states at the vicinity of the band edges, while the electron-phonon coupling strength remains similar. Such an increase is caused by the dimensionality reduction and the break in octahedra connectivity along the stacking direction. Our findings provide a fundamental understanding of the electron-phonon coupling mechanisms in layered perovskites and highlight the intrinsic limitations of the charge carrier transport in these materials. Published by the American Physical Society 2024

Cucco, Bruno (ORCID:0000000331564143)↗

DOE OSTI2026

Photophysical Properties and Phase Behavior of Ultrawide Photovoltaic Bandgap Cesium–Lead-Based Triple Halide Perovskites

Metal halide perovskite films in the top cell of triple-junction tandems require bandgaps around 2.0 eV to achieve current matching, assuming that the middle absorbing layer is the commonly used FAPbI 3 composition and the bottom cell has a bandgap around 1.1 eV. Unfortunately, mixed organic/inorganic metal halide perovskites that have the necessary Br content to reach a bandgap of 2.0 eV segregate into iodine-rich and bromine-rich phases under illumination, limiting their obtainable voltage. Previous reports have shown improved photostability using either Cs-based inorganic compositions or Cl incorporation on the X-site. Here, we investigate the inorganic triple halide compositional space CsPb(I 1–x–y Br y Cl x ) 3 where bandgaps near 2.0 eV are expected based on the knowledge that CsPbI2Br has a bandgap of 1.90 eV. Incorporation of Cl occurs readily for x ≤ 0.07–0.10 within perovskites with a Br content of 0.3 ≤ y ≤ 0.42. When x >0.1, X-ray diffraction and photoluminescence (PL) measurements indicate that multiple compositional phases form. We hypothesize that the variable sizes of the three halide ions are not supported within the rigid Cs lattice, resulting in the formation of multiple compositional phases. The photoluminescence quantum yield of the single-phase compositional space–CsPb(I 1–x–y Br y Cl x ) 3 where x ≤ 0.07was typically 0.001–0.004%, most likely as a result of a high defect density, including mobile iodine species. PL light-soaking measurements of many perovskite compositions with bandgaps in the range of 1.89–2.05 eV demonstrate that phase segregation occurs when initial bandgaps are above 1.95 eV regardless of halide content: indicating further iodide oxidation and corresponding migration under illumination. The conclusion is that further compositional or additive engineering is necessary for the development of inorganic triple halide compositions that accomplish the elusive goal of fabricating high-quality and photostable 2.0 eV films for use in multijunction tandems.

Electrical conductivity↗

DOE OSTI2025

Dynamic nanodomains dictate macroscopic properties in lead halide perovskites

Lead halide perovskites have emerged as promising materials for solar energy conversion and X-ray detection owing to their remarkable optoelectronic properties. However, the microscopic origins of their superior performance remain unclear. Here we show that low-symmetry dynamic nanodomains present in the high-symmetry average cubic phases, whose characteristics are dictated by the A-site cation, govern the macroscopic behaviour. We combine X-ray diffuse scattering, inelastic neutron spectroscopy, hyperspectral photoluminescence microscopy and machine-learning-assisted molecular dynamics simulations to directly correlate local nanoscale dynamics with macroscopic optoelectronic response. Our approach reveals that methylammonium-based perovskites form densely packed, anisotropic dynamic nanodomains with out-of-phase octahedral tilting, whereas formamidinium-based systems develop sparse, isotropic, spherical nanodomains with in-phase tilting, even when crystallography reveals cubic symmetry on average. We demonstrate that these sparsely distributed isotropic nanodomains present in formamidinium-based systems reduce electronic dynamic disorder, resulting in a beneficial optoelectronic response, thereby enhancing the performance of formamidinium-based lead halide perovskite devices. By elucidating the influence of the A-site cation on local dynamic nanodomains, and consequently, on the macroscopic properties, we propose leveraging this relationship to engineer the optoelectronic response of these materials, propelling further advancements in perovskite-based photovoltaics, optoelectronics and X-ray imaging.

Materials Science↗

DOE OSTI2024

Beyond Ion Migration in Metal Halide Perovskites: Toward a Broader Photoelectrochemistry Perspective

Ion migration is a broad term used to account for the degradation of halide perovskite materials and devices. However, ion mobility is only one piece of the full picture-mobile ions/defects are first created, then transported, and eventually annihilated or immobilized. In this Perspective, we summarize emerging work that shows how tractable photochemical and Faradaic reactions provide a continuous source of ions to migrate. Furthermore, we discuss strategies to fundamentally manipulate ion migration by targeting specific electrochemical and reduction/oxidation mechanisms. This highlights the important role of defect photoelectrochemistry, as well as the soft nature of the perovskite lattice, in ion migration and self-healing. We conclude that distinguishing more detailed processes involved in “ion migration”, with an emerging focus on the reactions that form mobile ionic defects, is necessary to greatly improve the stability of devices and open up more technological applications.

defects↗

DOE OSTI2025

Ferroelastic Domain-Induced Electronic Modulation in Halide Perovskites

Lead halide perovskites have emerged as promising materials for optoelectronic applications due to their exceptional properties. In the all-inorganic CsPbBr 3 perovskites, ferroelastic domains formed during phase transitions enhance bulk transport and emissive efficiency. However, the microscopic mechanisms governing carrier dynamics remain poorly understood. In this study, we employ cathodoluminescence (CL) and micro-Raman spectroscopy to image and investigate the electronic properties of the ferroelastic domain walls in CsPbBr 3 single crystals. CL measurements reveal a reduced emissive yield and a slight redshift in emission at the domain walls. Further, micro-Raman studies provide spatially resolved mapping of vibrational modes, exhibiting second-order phonon modes localized at the domain boundaries. In conclusion, our findings suggest that electron–phonon coupling at the twin domain walls plays a critical role in facilitating efficient charge separation, thereby improving the optoelectronic performance of the CsPbBr 3 perovskites.

Carrier dynamics↗

DOE OSTI2026

From Filamentary Failure to Durable Halide Perovskite Memristors

Halide perovskites have emerged as promising materials for memristive devices. While their pronounced electrochemical reactivity and fast ionic mobility enable numerous advantages including low-voltage operation and fast switching, the same features also render perovskite-based memristors vulnerable to metallic shunts and poor endurance, limiting their practical applications. Here, we elucidate both the resistive switching and failure mechanisms in perovskite memristors with a fluorine-doped tin oxide (FTO)/methylammonium lead triiodide (MAPbI3)/Ag structure and demonstrate a strategy to substantially enhance device durability. As opposed to commonly invoked filamentary mechanisms, electrical, structural, and spectroscopic analyses reveal that resistive switching arises from interfacial barrier modulation by reversible Ag redox reactions that drive electrochemical doping/dedoping within the perovskite. Device failure, however, originates from metallic Ag0 filamentation that ultimately forms permanent conductive pathways. Introducing an ultrathin Al2O3 interlayer at the inert-electrode interface improves device endurance by more than 30-fold, exceeding 15,000 switching cycles, without compromising other performance metrics. Interfacial characterization indicates that the Al2O3 layer modifies wettability of Ag deposits, promoting planar island growth rather than through-film filamentation. These findings establish a clear link between interfacial electrochemistry, metal precipitation behavior, and memristor reliability, highlighting inert-electrode interfacial engineering as an effective pathway toward durable perovskite-based memristors.

14 SOLAR ENERGY↗

DOE OSTI2025

Control over Banded Morphologies and Circular Dichroism in Chiral Halide Perovskites

Chiral halide perovskites (c-HPs) merge the chirality of organic cations with the semiconducting properties of metal halide frameworks, creating a family of chiral semiconductors with tunable chiroptoelectronic behavior. Here, we describe the impact of periodic banded morphologies of textured c-HP ( R/S -NEA) 2 PbBr 4 films (NEA = 1- (1-naphthyl)ethylammonium) on their chiroptical behavior. Due to the interplay between the crystalline and glassy phases, the c-HP film growth is driven by rhythmic precipitation, producing a distinctive controllable radial banded pattern with the ( R/S -NEA) 2 PbBr 4 inorganic planes oriented parallel to the substrate. The banded morphology can be controlled, as evidenced by the growth temperature dictating the ridge-to-ridge spacing as well as the density of banded regions. The resulting circular dichroism (CD) spectral shape, intensity, and polarity vary in a seemingly random manner across processing conditions. However, these spectral features can be explained by considering key features of the banded morphology, such as refraction of the incident light due to surface morphology, birefringence, and stacked, rotated crystallites. These effects cannot be canceled by averaging front and back CD spectra of c-HP films, and our model incorporating these effects reproduces all observed CD spectra remarkably well. The control over the c-HP morphology and prediction capabilities of our CD modeling leads to further understanding of this class of semiconductors and the possibility of exploiting structural features for light polarization control akin to enhanced metamaterials.

14 SOLAR ENERGY↗

DOE OSTI2024

Spectroelectrochemical insights into the intrinsic nature of lead halide perovskites

Abstract Lead halide perovskites have emerged as a new class of semiconductor materials with exceptional optoelectronic properties, sparking significant research interest in photovoltaics and light-emitting diodes. However, achieving long-term operational stability remains a critical hurdle. The soft, ionic nature of the halide perovskite lattice renders them vulnerable to various instabilities. These instabilities can be triggered by factors such as photoexcitation, electrical bias, and the surrounding electrolyte/solvent or atmosphere under operating conditions. Spectroelectrochemistry offers a powerful approach to bridge the gap between electrochemistry and photochemistry (or spectroscopy), by providing a comprehensive understanding of the band structure and excited-state dynamics of halide perovskites. This review summarizes recent advances that highlight the fundamental principles, the electronic band structure of halide perovskite materials, and the photoelectrochemical phenomena observed upon photo- and electro-chemical charge injections. Further, we discuss halide instability, encompassing halide oxidation, vacancy formation, ion migration, degradation, and sequential expulsion under electrical bias. Spectroelectrochemical studies that provide a deeper understanding of interfacial processes and halide mobility can pave the way for the design of more robust perovskites, accelerating future research and development efforts. Graphical Abstract

Min, Seonhong↗

DOE OSTI2025

In Situ Electrochemistry of Buried Interfaces in Metal Halide Perovskites: Probing Energy Bands, Halide Redox Activity, and Kinetics

Control over charge injection and extraction processes across buried interfaces is fundamental for all (opto)electronic multilayer device platforms, necessitating detailed understanding of local structural and chemical differences that promote defect formation, distort energetic band-edge alignments, and alter charge transport processes. Herein, the implementation of a low-cost electroanalytical methodologies’ tool suite is described to quantitatively characterize buried interfaces and redox reactions in printable, mixed electrical–ionic, and redox-active metal halide perovskites and a prototypical hole-transporting nickel oxide (NiO x ) thin film. The objective is to demonstrate the power of electrochemical methodologies to improve the nanoscale understanding of complex interfaces within optoelectronic devices by providing case studies on how to: i) differentiate between electronic and chemical properties in NiO x contacts; ii) measure changes in reversibility of halide redox reactions via NiO x surface states; iii) assess energy alignment and charge transport across (modified) buried interfaces; and iv) quantify defects at buried interfaces that change with modifiers and differences in perovskite processing, including increasing defect concentrations when films are slot-die-coated versus spin-cast. The collective approach addresses major challenges in understanding the precise energy landscape and interface reactivity under relevant electric fields that mimic operando conditions (away from equilibrium) and across length scales in thin film device formats.

(spectro)electrochemistry↗

DOE OSTI2024

Metal Oxide vs Organic Semiconductor Charge Extraction Layers for Halide Perovskite Indoor Photovoltaics

Halide perovskite indoor photovoltaics (PVs) are highly promising to autonomously power the billions of microelectronic sensors in the emerging and disruptive technology of the Internet of Things (IoT). However, how the wide range of different types of hole extraction layers (HELs) impacts the indoor light harvesting of perovskite solar cells is still elusive, which hinders the material selection and industrial–scale fabrication of indoor perovskite photovoltaics. In the present study, new insights are provided regarding the judicial selection of HELs at the buried interface of halide perovskite indoor photovoltaics. This study unravels the detrimental and severe light–soaking effect of metal oxide transport layer–based PV devices under the indoor lighting effect for the first time, which then necessitates the interface passivation/engineering for their reliant performance. This is not a stringent criterion under 1 sun illumination. By systematically investigating the charge carrier dynamics and sequence of measurements from dark, light–soaked, interlayer–passivated device, the bulk and interface defects are decoupled and reveal the gradual defect passivation from shallow to deep level traps. Thus, the present study puts forward a useful design strategy to overcome the deleterious effect of metal oxide HELs and employ them in halide perovskite indoor PVs.

14 SOLAR ENERGY↗

DOE OSTI2025

Design principles of spacer cations for suppressing phase segregation in 2D halide perovskites

Suppression of photoinduced halide segregation in mixed halide perovskites remains a significant challenge for their application as wide bandgap semiconductors in solar cells. In addition to stability issues, halide segregation leads to a loss in power conversion efficiency in solar cells and a shift in emission wavelength in light-emitting devices. However, employing low-dimensional halide perovskites, such as two-dimensional (2D) or quasi-2D structures, offers a strategy to mitigate this segregation. Here, we have systematically studied how the molecular structure and binding configuration of spacer cations, ranging from linear alkyl chains to aromatic structures, affect photoinduced halide segregation across both Ruddlesden–Popper (RP) and Dion–Jacobson (DJ) frameworks in 2D mixed halide perovskites (Br : I = 50 : 50). Aromatic spacer cations within the DJ perovskite configuration were found to suppress segregation most effectively. For example, the halide segregation rate in a 2D mixed halide perovskite film with the DJ phase using the aromatic spacer cation 1,4-phenylenedimethanammonium (PDMA) was 9.3 × 10 −4 s −1 —an order of magnitude lower than that observed with linear 2D RP perovskites employing butylammonium (BA) as the spacer cation (6.1 × 10 −3 s −1 ). Spectroscopic studies detailing the influence of spacer cation selection in mixed halide perovskites for suppressing phase segregation are discussed.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2024

3D Lead-Organoselenide-Halide Perovskites and their Mixed-Chalcogenide and Mixed-Halide Alloys

Here, we incorporate Se into the 3D halide perovskite framework using the zwitterionic ligand: SeCYS ( + NH 3 (CH 2 ) 2 Se - ), which occupies both the X - and A + sites in the prototypical ABX 3 perovskite. The new organoselenide-halide perovskites: (SeCYS)PbX 2 (X=Cl, Br) expand upon the recently discovered organosulfide-halide perovskites. Single-crystal X-ray diffraction and pair distribution function analysis reveal the average structures of the organoselenide-halide perovskites, whereas the local lead coordination environments and their distributions were probed through solid-state 77 Se and 207 Pb NMR, complemented by theoretical simulations. Density functional theory calculations illustrate that the band structures of (SeCYS)PbX 2 largely resemble those of their S analogs, with similar band dispersion patterns, yet with a considerable band gap decrease. Optical absorbance measurements indeed show band gaps of 2.07 and 1.86 eV for (SeCYS)PbX 2 with X=Cl and Br, respectively. We further demonstrate routes to alloying the halides (Cl, Br) and chalcogenides (S, Se) continuously tuning the band gap from 1.86 to 2.31 eV–straddling the ideal range for tandem solar cells or visible-light photocatalysis. The comprehensive description of the average and local structures, and how they can fine-tune the band gap and potential trap states, respectively, establishes the foundation for understanding this new perovskite family, which combines solid-state and organo-main-group chemistry.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗