While magnesium fluoride (MgF2)-organic composite hole injection layers (HILs) are commercially employed in organic light-emitting diodes (OLEDs), their fundamental working principle remains elusive, hindering rational optimization. In this study, the underlying mechanisms of this comprehensive enhancement are systematically elucidated. It is demonstrated that an optimized MgF2-organic HIL dramatically reduces the driving voltage and extends operational lifetime by over an order of magnitude. Structural analysis reveals that MgF2 forms an intimate amorphous solid solution with the organic host, suppressing its own crystallization. This unique structure facilitates a powerful, synergistic dual-enhancement: i) In situ photoelectron spectroscopy confirms the suppression of interfacial band bending for barrier-free hole transport. ii) Concurrently, a quantitative analysis demonstrates a substantial, over 50-fold increase in hole carrier density via p-doping. Definitive spectroscopic and theoretical evidence proves that these benefits originate from a ground-state charge transfer from the organic host to MgF2, unequivocally establishing MgF2 as an effective p-dopant. Crucially, it is demonstrated that this entire mechanistic framework is a general and robust principle, universally applicable across chemically distinct hole transport materials. This work therefore resolves a long-standing ambiguity and establishes a foundational design strategy for next-generation OLEDs with superior efficiency and operational stability.

Perovskite light-emitting diodes (PeLEDs) represent a compelling platform for next-generation display technologies owing to their unique optoelectronic properties. Although vacuum deposition offers the scalability and uniformity required for commercialization, vacuum-deposited PeLEDs typically exhibit lower external quantum efficiencies (EQEs) than their solution-processed counterparts. Addressing this challenge requires innovative strategies to achieve carrier confinement and suppress non-radiative recombination. In this study, we prepare high-efficiency vacuum-deposited PeLEDs through the precise compositional tailoring of Cs(1-x)MA(x)PbBr(3) nanocrystals (NCs; MA = methylammonium) embedded within a wide-bandgap Cs4PbBr6 matrix via controlled co-evaporation. The addition of a small amount of PbBr2 to MABr significantly enhances the evaporation stability of the latter as well as the uniformity of the resulting films, thereby enabling controlled growth of Cs(1-x)MA(x)PbBr(3) NCs. The MA content (x) in the resulting Cs(1-x)MA(x)PbBr(3)/Cs4PbBr6 nanostructures is precisely tuned and monitored in situ using a residual gas analyzer. At the optimal x (0.19), we achieve a remarkable 1.9-fold EQE enhancement compared with that of the MA-free device. This improvement is attributed to the controlled incorporation of MA cations, which increases the average NC size and inter-particle spacing. Structural modifications enhance photoluminescence intensity and prolong exciton decay lifetimes, indicating suppression of non-radiative recombination pathways associated with surface defects.

2D halide perovskites provide an excellent system for investigating fundamental properties of excitons due to strong quantum and dielectric confinements, yet biexciton-related phenomena remain largely unexplored. Here, we report two-photon resonant Raman scattering (TRRS) in (PEA)2PbI4 (PEA = C6H5C2H4NH3), enabled by the biexciton level acting as an intermediate state. TRRS shows a distinct threshold behavior, causing an emergent sharp Raman peak superimposed on the biexciton photoluminescence spectrum. The TRRS threshold is 88.5 MW/cm2 at 10 K, which gradually increases with temperature up to 379.7 MW/cm2 at 90 K. Unlike coexisting amplified spontaneous emission, this Raman peak shifts by 2 Delta omega 1 when the excitation frequency (omega 1) is tuned by Delta omega 1 near 2329.9 meV, which is the resonant two-photon energy for the direct generation of biexcitons. Consistent with two-photon selection rules, TRRS is inactive under co-circularly polarized excitation. Our findings highlight strong nonlinear optical properties of the perovskite and underscore its potential for biexciton-based applications.

A lithography-compatible approach for site-selective contact engineering is developed by using electron-beam lithography to pattern ultrathin Nafion interlayers at the metal contacts of p-type MoTe2 field-effect transistors. The patterned Nafion films are prepared by electron-beam irradiation followed by development, forming conformal nanometer-thick layers localized exclusively at the source and drain regions. Due to its high work function, Nafion facilitates localized charge transfer that p-dopes the MoTe2 interface, thereby narrowing the Schottky barrier width for hole injection. Two-terminal and four-terminal electrical measurements allow a clear distinction between intrinsic channel properties and contact-related effects. Nafion-contacted devices show a 2-fold increase in on-state current, more linear output behavior, and field-effect mobility up to 10 cm2/V·s, compared to control devices with bare-Pt contacts. The convergence of two- and four-terminal mobilities observed in Nafion-modified transistors indicates the successful mitigation of contact resistance and Fermi-level pinning. An electron-beam dose of 10 μC cm-2 produces optimal results by achieving good pattern definition while maintaining effective interfacial doping. This site-selective patterning method offers a practical route for tailored contact engineering in 2D materials and represents a promising path toward improved p-type MoTe2 transistors for advanced electronic applications.

ABSTRACT 2D halide perovskites provide an excellent system for investigating fundamental properties of excitons due to strong quantum and dielectric confinements, yet biexciton‐related phenomena remain largely unexplored. Here, we report two‐photon resonant Raman scattering (TRRS) in (PEA) 2 PbI 4 (PEA = C 6 H 5 C 2 H 4 NH 3 ), enabled by the biexciton level acting as an intermediate state. TRRS shows a distinct threshold behavior, causing an emergent sharp Raman peak superimposed on the biexciton photoluminescence spectrum. The TRRS threshold is 88.5 MW/cm 2 at 10 K, which gradually increases with temperature up to 379.7 MW/cm 2 at 90 K. Unlike coexisting amplified spontaneous emission, this Raman peak shifts by 2Δω 1 when the excitation frequency (ω 1 ) is tuned by Δω 1 near 2329.9 meV, which is the resonant two‐photon energy for the direct generation of biexcitons. Consistent with two‐photon selection rules, TRRS is inactive under co‐circularly polarized excitation. Our findings highlight strong nonlinear optical properties of the perovskite and underscore its potential for biexciton‐based applications.

Desired or intended location doping in two dimensional (2D) semiconductors has been a persistent issue for 2D semiconductor based electronics along with contact resistance (RC) lowering. Such doping in 2D seems almost impossible unlike in 3D semiconductors, which use ion implantation. Furthermore, maintaining a stable doping state in 2D seems very difficult. Here, we report a strategy for intended location doping of 2D materials: hole carrier transfer from electron-beam-patterned sulfonated tetrafluoroethylene-based fluoropolymer-copolymer (Nafion) underlayer. Bottom-patterned ultrathin Nafion with a large work function excessively dopes p-type WSe2, so that its sheet resistance may become compatible for integrated circuit. Top-gated WSe2 field-effect transistor channel with Nafion support for ungated region demonstrates 7 times higher mobility than without Nafion. As bottom-patterned for contact area, Nafion directly lowers RC to ∼6 kΩ·µm, which is maintained for 2 months in air ambient and survives N2 anneal of 250 °C. Our Nafion approach for 2D doping and stable RC seems advanced and practically useful.

Organic light-emitting diodes (OLEDs) offer advantages such as high contrast, vivid colors, and flexible form factors, but their use in demanding IT and automotive displays is limited by short operational lifetimes and low luminous efficiency. Tandem OLEDs, which stack multiple electroluminescent units connected via charge generation layers (CGLs), provide a promising solution. However, the development of efficient and stable n-type CGLs (nCGLs) remains challenging, particularly due to issues with metal-doped organic semiconductors. In this study, an effective nCGL system using Ytterbium (Yb)-doped 4,5-diaza-9,9-spirofluorene (Dasf), a thermally stable host is presented. Optimization identifies 10 wt.% Yb (1:1 molar ratio) as the ideal doping level for peak performance. Photoelectron spectroscopy and density functional theory confirm strong charge-transfer complex formation between Yb and Dasf's nitrogen atoms. This interaction creates electronic states near the Fermi level through hybridization of Yb 6s orbitals with Dasf's LUMO, enhancing electron injection and transport. OLEDs incorporating this optimized nCGL show a 1.6-fold improvement in external quantum efficiency and a 1.7-fold increase in operational lifetime over single-unit devices. This study highlights Yb-doped Dasf as a promising nCGL material for high-performance tandem OLEDs.

Two-dimensional halide perovskites are attracting attention due to their structural diversity, improved stability, and enhanced quantum efficiency compared to their three-dimensional counterparts. In particular, Dion-Jacobson (DJ) phase perovskites exhibit superior structural stability compared to Ruddlesden-Popper phase perovskites. The inherent quantum well structure of layered perovskites leads to highly anisotropic charge transport and optical properties. Therefore, controlling the preferred crystal orientation (parallel or perpendicular) is crucial for optimizing device performance. This work presents a rational strategy to control parallel and perpendicular crystal growth in C6N2H16PbI4 (4AMPPbI4)-based DJ phase perovskite thin films. We demonstrate that crystal orientation depends on crystal growth rates, which can be controlled by varying the solvent composition, antisolvent, and annealing temperature. Direct and inverse photoelectron spectroscopy reveals that the electronic structure of 4AMPPbI4, including its work function, ionization energy, and electron affinity, is orientation-dependent. Different orientations significantly affect carrier transport as confirmed by single-carrier devices. This study highlights the critical role of crystal orientation in DJ phase perovskites for designing high-performance optoelectronic devices.

Organic ferroelectric crystalline polymer, P(VDF-TrFE) has attracted broad attentions due to its lead-free benefits and process convenience. However, it has a long-standing drawback, its process limit in crystalline film thickness, whose minimum is almost fixed as ∼100 nm. Hence, operation voltage of any P(VDF-TrFE)-based ferroelectric memory field-effect transistors (FeFETs) has always been over 10 V. Here, innovatively thinned ∼20 nm P(VDF-TrFE) crystalline layers are fabricated on Pt and Au gate, empowering FeFETs with two dimensional (2D) MoTe2 channel to operate under minimum 3 V pulse. Such thin crystalline layer is achieved through spin-coating after initial growth of 5 nm-thin crystalline seed layer, P(VDF-TrFE)-brush. This ultrathin P(VDF-TrFE)-brush effectively inhibits the de-wetting problem of P(VDF-TrFE)-solution during spin-coating, leading to good surface-energy matching and pinhole-free conformal coating of classical P(VDF-TrFE). As a result, 3–4 V pulse operations of p-MoTe2 nonvolatile memory FETs are nicely realized without leakage current loss. These numbers may be regarded as one of the lowest values in report.

Organic-inorganic hybrid perovskites (OIHPs) are a promising class of materials that rival conventional semiconductors in various optoelectronic applications. However, unraveling the precise nature of their low-energy electronic structures continues to pose a significant challenge, primarily due to the absence of clear band measurements. Here, we investigate the low-energy electronic structure of CH3NH3PbI3 (MAPI3) using angle-resolved photoelectron spectroscopy combined with ab initio density functional theory. We successfully visualize the electronic structure of MAPI3 near the bulk valence band maximum by using a laboratory photon source (He Iα, 21.2 eV) at low temperature and explore its fundamental properties. The observed valence band exhibits a highly isotropic and parabolic band characterized by small effective masses of 0.20-0.21 me, without notable spectral signatures associated with a large polaron or the Rashba effect, subjects that are intensely debated in the literature. Concurrently, our spin-resolved measurements directly disprove the giant Rashba scenario previously suggested in a similar perovskite compound by establishing an upper limit for the Rashba parameter (αR) of 0.28 eV Å. Our results unveil the unusually complex nature of the low-energy electronic structure of OIHPs, thereby advancing our fundamental understanding of this important class of materials.

A quantum state of matter that is forbidden to interact with photons and is therefore undetectable by spectroscopic means is called a dark state. This basic concept can be applied to condensed matter where it suggests that a whole band of quantum states could be undetectable across a full Brillouin zone. Here we report the discovery of such condensed-matter dark states in palladium diselenide as a model system that has two pairs of sublattices in the primitive cell. By using angle-resolved photoemission spectroscopy, we find valence bands that are practically unobservable over the whole Brillouin zone at any photon energy, polarization and scattering plane. Our model shows that two pairs of sublattices located at half-translation positions and related by multiple glide-mirror symmetries make their relative quantum phases polarized into only four kinds, three of which become dark due to double destructive interference. This mechanism is generic to other systems with two pairs of sublattices, and we show how the phenomena observed in cuprates, lead halide perovskites and density wave systems can be resolved by the mechanism of dark states. Our results suggest that the sublattice degree of freedom, which has been overlooked so far, should be considered in the study of correlated phenomena and optoelectronic characteristics. The identification of dark states-quantum states that do not interact with photons-in real materials may help to address many unsolved issues in condensed-matter physics. Now, they have been identified in palladium diselenide.

Selective area doping and contact resistance (RC) lowering in two dimensional (2D) semiconductors have been persistent issues for 2D semiconductor based electronics. Such doping in 2D seems almost impossible unlike in 3D semiconductors, which use ion implantation. Furthermore, maintaining a low and stable RC in 2D seems very difficult. Here, we report a strategy for area selective doping of 2D materials: electron-beam patterning of sulfonated tetrafluoroethylene-based fluoropolymer-copolymer (Nafion) underlayer with a large work function. Patterned-underlayer selectively increases the hole density of p-type WSe2, whose sheet resistance becomes compatible for integrated circuit. Top-gated WSe2 field-effect transistor channel, as modulated with Nafion area, demonstrates 7 times higher mobility than without Nafion. As patterned for contact area, Nafion directly lowers RC to ~ 6 kΩ-µm, which is maintained for 2 months in air ambient and survives N2 anneal of 200 o C. Our Nafion approach for selective 2D doping and stable RC seems advanced and practically useful.

In this study, spin-to-charge conversion (SCC) of various topological materials with ferromagnet is investigated using spintronic terahertz (THz) emission spectroscopy. Compared with other topological materials, significantly large THz emission is observed for topologically nontrivial phases of Bi1-xSbx (x > 0.2) that predominantly originates from the topological surface state. When Bi is superposed above a certain stoichiometry of Bi1-xSbx, it plays a crucial role in generating a highly spin-split state and enhancing the spin-mixing conductance, resulting in colossal THz emission. This proves that improving the SCC efficiency through interface engineering is a useful strategy to design a powerful spintronic device. Collectively, this study proposes a methodology for systematically analyzing SCC efficiency or spin Hall angle using THz emission spectroscopy and offers an efficient structure for future spintronic devices.

Two-dimensional Ruddlesden–Popper (2DRP) perovskites are promising owing to their excellent environmental stability and competitive efficiency. During the fabrication process, 2DRP perovskites were often unintentionally exposed to light in the laboratory. However, the influence of light illumination on the surface structure of 2DRP during fabrication is unclear. Herein, the photodegradation of 2DRP perovskite (phenethylammonium lead iodide, PEA 2 PbI 4 ) is comprehensively investigated using x-ray photoelectron spectroscopy, scanning electron microscopy, atomic force microscopy, and photoluminescence spectroscopy. We observed that only high-energy light, including that from a blue laser, air mass 1.5G, and notably, the daily used fluorescent lamp (FL) in the laboratory, significantly degraded PEA 2 PbI 4 . However, the red laser and ultraviolet-filtered FL, which had low energies, did not cause photodegradation. From this systematic study, we can explain the discrepancies in the surface morphologies previously studied. For instance, randomly oriented nanorod or rough surface of PEA 2 PbI 4 mostly stems from photodegradation. We observed that photodegradation occurred more strongly when the films were illuminated during annealing than when they were illuminated after all fabrication processes were completed. We suggest that this difference stems from the completeness of the structure and the photodegraded PbI 2 passivation effect. Our study provides two key guidelines for the fabrication of PEA 2 PbI 4 films. The daily-use FL in the laboratory must be avoided for high-quality samples, and dark conditions are highly recommended, at least during the annealing process.

Recently, LG Display completed the "EX Technology" development and is prioritizing technological innovation. This technology includes deuterium technology and personalization algorithms, which increase screen brightness (luminance) by 30% compared to existing OLEDs while producing more accurate and natural colors. In this paper, we will describe how deuterium technology can improve the lifetime of OLED products and the degradation mechanism of deuterated materials. By replacing the C‐H bond with a C‐D bond, isotope species exhibit electrochemical stability, leading to a longer device lifetime.

Polymorphic phase transition between semi-conducting 2H and semi-metallic 1T ' in MoTe2 has garnered sig-nificant interest due to its wide applicability for memory device. Recently, it has been reported that charge doping is a prospective method for facilitating practical application of phase transition. In particular, the posi-tively doped system with lower energy barrier between two phases is more advantageous than the negatively doped system. However, although various methods for negative charge-mediated phase transition have been developed, study on positive charge-mediated phase transition is scarce due to the requirement of high charge density. Herein, we report positive charge-mediated phase modulation in MoTe2 synthesized by molecular beam epitaxy. The structural phase of MoTe2 is significantly influenced by the stoichiometric ratio of molybdenum to tellurium, and the tellurium adatoms make 1T' phase more stable than 2H phase. Photoelectron spectroscopy reveals that positive charge doping by electron transfer to tellurium adatoms is crucial in the determination of phase. Finally, based on the energy difference and energy barrier between two phases, the doping effects during and after synthesis are verified. This study on the structure modulation of MoTe2 provides physical insight for phase transition as well as a basis for the development of nanoscale electronic devices.

High-quality halide perovskite single crystals ensure the existence of stable biexcitons, which are important for investigating many-body exciton physics and biexciton-based device applications. Recently, the biexciton binding energy (phi XX) in CH3NH3PbBr3 (MAPbBr3) single crystals was reported as-3.9 meV as deter-mined from the spectral distance between the exciton peak and the high-energy onset of the biexciton peak based on photoluminescence (PL) spectroscopy. However, the exciton PL peak is usually red-shifted compared with the true exciton level that is typically determined by the absorption measurement. Consequently, phi XX was significantly underestimated in the recent work. Here, we precisely locate the biexciton level by utilizing giant two-photon excitation specific to the biexciton resonance. We also independently estimate the true exciton level based on the steeply varying power exponent of the exciton PL by tuning the input wavelength for multiphoton excitation. As a result, the actual exciton level is determined to be-2251 meV at 10 K, thereby yielding a newly estimated phi XX = 10.2 (+/- 1) meV. This phi XX value is larger than a theoretical prediction based on variational calculations, which can be presumably attributed to polarization of the ionic components in MAPbBr3.

Halide perovskites (HPs) are fascinating materials whose optoelectronic properties are arguably excitonic. In the HP family, biexcitons are known to exist only in low dimensions where exciton-exciton binding is strongly enhanced by quantum and dielectric confinements. In this paper, however, we show that they indeed do exist in three-dimensional (3D) bulk CH3 NH3 PbBr3 (MAPbBr3 ) single crystals if the pristine crystal quality is ensured for subtle binding of two excitons. The existence of biexcitons is clearly evidenced below 30 K with a binding energy of ∼3.9 ± 0.3 meV according to i) exciton-biexciton population dynamics, ii) giant resonant two-photon excitation of biexcitons, iii) inverted Boltzmann-type spectral feature, and iv) zero degree of circular polarization in the biexciton photoluminescence. Because of the polariton effect, the two-photon resonance occurs at the excited biexciton state from which longitudinal-transverse splitting is calculated to be 3.7 meV. The discovery of the 3D biexcitons underscores the very quality of HP crystals for generating various many-body excitonic phases in MAPbBr3 and its analogues towards the improved understanding of their fundamental properties and highly efficient optoelectronic applications. This article is protected by copyright. All rights reserved.

Halide perovskites (HPs) are fascinating materials whose optoelectronic properties are arguably excitonic. In the HP family, biexcitons are known to exist only in low dimensions where exciton-exciton binding is strongly enhanced by quantum and dielectric confinements. In this paper, however, it is shown that they indeed do exist in 3D bulk CH3 NH3 PbBr3 (MAPbBr3 ) single crystals if the pristine crystal quality is ensured for subtle binding of two excitons. The existence of biexcitons is clearly evidenced below 30 K with a binding energy of ≈3.9 ± 0.3 meV according to i) exciton-biexciton population dynamics, ii) giant resonant two-photon excitation of biexcitons, iii) inverted Boltzmann-type spectral feature, and iv) zero degree of circular polarization in the biexciton photoluminescence. Because of the polariton effect, the two-photon resonance occurs at the excited biexciton state from which longitudinal-transverse splitting is calculated to be 3.7 meV. The discovery of the 3D biexcitons underscores the very quality of HP crystals for generating various many-body excitonic phases in MAPbBr3 and its analogues toward the improved understanding of their fundamental properties and highly efficient optoelectronic applications.

Gamma-ray irradiation alters the material properties of organic semiconductors, especially the electronic structure. These changes due to ionization can be useful in dosimetry. Although there are some device-based reports, the effect of gamma-ray irradiation on the electronic structure of organic semiconductors is still unclear. In this study, we investigated the electronic structures of representative organic semiconductor films, namely, n-type [6,6]-phenyl-C-61-butyric acid methyl ester (PCBM) and p-type poly(3-hexylthiophene-2,5-diyl) (P3HT), after gamma-ray irradiation generated by a Cs-137 source. The X-ray and ultraviolet photoelectron spectra of the PCBM and P3HT films were measured for various gamma-ray doses. In both PCBM and P3HT, chemical interaction with atmospheric oxygen, assisted by high-energy photons, led to significant oxidation. However, the degree of oxidation of PCBM was considerably higher than that of P3HT. The oxidation also affects the valence electronic structures. The possible chemical structures of the oxidized PCBM and P3HT are estimated using density functional theory calculations. (C) 2021 Elsevier Ltd. All rights reserved.