Investigation on the Interfaces in Organic Devices by Photoemission Spectroscopy

Organic semiconductors have garnered significant interest owing to their low cost, flexibility, and suitability for large-area electronics, making them vital for burgeoning fields such as flexible electronics, wearable devices, and green energy technologies. The performance of organic electronic devices is crucially determined by their interfacial electronic structure. Specifically, interfacial phenomena such as band bending significantly influence carrier injection, transport, and recombination, making their control paramount for enhancing device performance. This review investigates the interplay among molecular orientation, interfacial charge transfer, and interfacial chemical reactions as the primary drivers of interface band bending. Furthermore, it critically examines effective strategies for optimizing interfacial properties via interface engineering, focusing on interlayer insertion and template layer methods. The review concludes with a summary and outlook, emphasizing the integration of interface design with material development and device architecture to realize next-generation, high-performance organic electronic devices exhibiting improved efficiency and stability.

Liquid crystals as solid-state templates

Liquid crystals (LCs) combine the anisotropy of crystals with the fast molecular dynamics of liquids. Controlling the molecular orientation of LCs is the key enabling feature of liquid crystal displays (LCDs), a technology that has played a pivotal role in ushering in the digital age of today. Here we review controlling molecular organization in LCs over large distances for a different application: the assembly of macroscopically organized solids. The traditional approach of controlling orientational order in organic solids is growing single crystals, a process limited by slow kinetics. In this article, we review an alternate approach: the generation of organized solids through the (i) polymerization, (ii) physical gelation, and (iii) vitrification of small-molecule LCs. The generation of solids through these routes is enabled by innovations in (i) molecular design, (ii) formulation chemistry, and (iii) macroscopic alignment of LCs. Controlling molecular orientation, defects, and deformations in the precursor LC phase enables the assembly of solids with unique properties such as programmable responses to stimuli. We discuss the "organize and solidify" approach for the preparation of materials with LC order for soft robotics, chemical sensing, and lithographic patterning. Finally, we outline future challenges and opportunities in the field of liquid crystalline solids.

Domain-Wall Ferroelectric Polarons in a Two-Dimensional Rotor Lattice Model

We demonstrate the formation of ferroelectric domain-wall polarons in a minimal two-dimensional lattice model of electrons interacting with rotating dipoles. Along the domain wall, the rotors polarize in opposite directions, causing the electron to localize along a particular lattice direction. The rotor-electron coupling is identified as the origin of a structural instability in the crystal that leads to the domain-wall formation via a symmetry-breaking process. Our results provide the first theoretical description of ferroelectric polarons, as discussed in the context of soft semiconductors.

Tuning Optical Properties of Organic Thin Films through Intermolecular Interactions - Fundamentals, Advances and Strategies

In applications ranging from photon-energy conversion into electrical or chemical forms (such as photovoltaics or photocatalysis) to numerous sensor technologies based on organic solids, the role of supramolecular structures and chromophore interactions is crucial. This review comprehensively examines the critical intermolecular interactions between organic dyes and their impact on optical properties. We explore the range of changes in absorption or emission properties observed in molecular aggregates compared to single molecules. Each effect is dissected to reveal its physicochemical foundations, relevance to different application domains, and documented examples from the literature that illustrate the potential modulation of absorption or emission properties by molecular and supramolecular structural adjustments. This work aims to serve as a concise guide for exploiting supramolecular phenomena in the innovation of novel optical and optoelectronic organic materials, with emphasis on strategic application and exploitation.

Insights from Planar Heterojunctions: Understanding Charge Carrier Generation in Organic Photovoltaics

Organic photovoltaics (OPVs) have recently achieved high short-circuit current densities (JSC) approaching 30 mA/cm2 with internal quantum efficiencies surpassing 90%. In comparison to their inorganic or perovskite counterparts, a distinguishing feature of OPVs is the involvement of singlet or charge-transfer excitons in photoelectron conversion. A deeper understanding of the charge generation process with these excitons is crucial to further enhance JSC while maintaining the open-circuit voltage and fill factor. In this perspective, we provide new insights into the charge generation mechanisms and their electric field dependence derived from investigations using planar heterojunction structures and their comparison to bulk heterojunction systems. We aim to foster open discussion and collaboration within the research community to address the aforementioned challenges.

Efficient harvesting of triplet excitons via multiple fast TTA up-conversion and high-lying reverse intersystem crossing channels for efficient blue fluorescent organic light-emitting diodes

The efficient harvesting of triplet excitons is key to realizing high efficiency blue fluorescent organic light-emitting diodes (OLEDs). Triplet-triplet annihilation (TTA) up-conversion is one of the effective triplet-harvesting strategies. However, during the TTA up-conversion process, a high current density is necessary due to the competitive non-radiative triplet losses. In this study, we designed blue organic light-emitting diodes with fast TTA up-conversion and high-lying reverse intersystem crossing channels. It can be seen that the utilization of triplet excitons was greatly improved and the non-radiative triplet losses were significantly suppressed due to the availability of multiple efficient channels for triplet exciton utilization. As a result, a high efficiency blue fluorescent OLED was successfully fabricated. The maximum external quantum efficiency (EQE) reached 11.4% with CIE coordinates of (0.13, 0.11). This research provides a new route for the development of high efficiency blue fluorescent OLEDs.

Greater Influence of Density on the Electrical Properties of an Organic Semiconductor Glass Compared to Molecular Orientation

Physical vapor deposition is widely used in the fabrication of organic light-emitting diodes and has the potential to adjust the density and orientation through substrate temperature control, which may lead to enhanced electrical performance. However, it is unclear whether this enhanced property is because of the horizontal molecular orientation or the increased density. The effects of the density and orientation on the electrical properties of a potential electron transport material, (3-dibenzo[c,h]acridin-7-yl)phenyl)diphenylphosphine oxide (TPPO-dibenzacridine), were investigated. According to the gyration tensor analysis, TPPO-dibenzacridine resembled an oblate ellipsoid. Furthermore, these films exhibited the highest density when prepared at a substrate temperature of 87.5% of the glass transition temperature with an increase in density of approximately 1.5%. Variable angle spectroscopic ellipsometry measurements confirmed that the transition dipole moment direction of the dibenzacridine moiety, which is involved in the electrical properties, remained isotropic at this temperature. Although horizontal orientations are known to optimize their π-π overlap and improve the electrical properties, the lowest driving voltage was observed under these conditions, which led to the conclusion that the enhanced electrical properties of TPPO-dibenzacridine are greatly influenced by the increased density rather than by the molecular orientation.

Electrostatically tuning radical addition and atom abstraction reactions with distonic radical ions

Although electrostatic catalysis can enhance the kinetics and selectivity of reactions to produce greener synthetic processes, the highly directional nature of electrostatic interactions has limited widespread application. In this study, the influence of oriented electric fields (OEF) on radical addition and atom abstraction reactions are systematically explored with ion-trap mass spectrometry using structurally diverse distonic radical ions that maintain spatially separated charge and radical moieties. When installed on rigid molecular scaffolds, charged functional groups lock the magnitude and orientation of the internal electric field with respect to the radical site, creating an OEF which tunes the reactivity across the set of gas-phase carbon-centred radical reactions. In the first case, OEFs predictably accelerate and decelerate the rate of molecular oxygen addition to substituted phenyl, adamantyl, and cubyl radicals, depending on the polarity of the charged functional group and dipole orientation. In the second case, OEFs modulate competition between chlorine and hydrogen atom abstraction from chloroform based on interactions between charge polarity, dipole orientation, and radical polarizability. Importantly, this means the same charge polarity can induce different changes to reaction selectivity. Quantum chemical calculations of these reactions with DSD-PBEP86-D3(BJ)/aug-cc-pVTZ show correlations between the barrier heights and the experimentally determined reaction kinetics. Field effects are consistent between phenyl and cubyl scaffolds, pointing to through-space rather than through-bond field effects, congruent with computations showing that the same effects can be mimicked by point charges. These results experimentally demonstrate how internal OEFs generated by carefully placed charged functional groups can systematically control radical reactions.

Is the Future of Materials Amorphous? Challenges and Opportunities in Simulations of Amorphous Materials

Amorphous solids form an enormous and underutilized class of materials. In order to drive the discovery of new useful amorphous materials further we need to achieve a closer convergence between computational and experimental methods. In this review, we highlight some of the important gaps between computational simulations and experiments, discuss popular state-of-the-art computational techniques such as the Activation Relaxation Technique nouveau (ARTn) and Reverse Monte Carlo (RMC), and introduce more recent advances: machine learning interatomic potentials (MLIPs) and generative machine learning for simulations of amorphous matter (e.g., MAP). Examples are drawn from amorphous silicon and silica literature as well as from molecular glasses. Our outlook stresses the need for new computational methods to extend the time- and length-scales accessible through numerical simulations.

Enhancement of Spontaneous Orientation Polarization in Glassy Organic Semiconductor Mixtures

The alignment of permanent dipole moments and the resulting spontaneous orientation polarization (SOP) are commonly observed in evaporated neat films of polar organic molecules and lead to a so-called giant surface potential. In the case of mixed films, often enhanced molecular orientation is observed, i.e., a higher degree of alignment, in comparison to neat layers, if it is diluted into a suitable (nonpolar) host. So far, different possible influences on molecular orientation have been discussed, with the most prominent probably being the so-called surface equilibration model. In this contribution, we discuss how surface equilibration can influence orientation in mixed layers and which other intermolecular interactions have to be considered to explain the observed enhancement of SOP in mixed layers.

Strategies for Controlling Emission Anisotropy in Lead Halide Perovskite Emitters for LED Outcoupling Enhancement

In the last decade, momentous progress in lead halide perovskite (LHP) light-emitting diodes (LEDs) is witnessed as their external quantum efficiency (ηext) has increased from 0.1 to more than 30%. Indeed, perovskite LEDs (PeLEDs), which can in principle reach 100% internal quantum efficiency as they are not limited by the spin-statistics, are reaching their full potential and approaching the theoretical limit in terms of device efficiency. However, ≈70% to 85% of total generated photons are trapped within the devices through the dissipation pathways of the substrate, waveguide, and evanescent modes. To this end, numerous extrinsic and intrinsic light-outcoupling strategies are studied to enhance light-outcoupling efficiency (ηout). At the outset, various external and internal light outcoupling techniques are reviewed with specific emphasis on emission anisotropy and its role on ηout. In particular, the device ηext can be enhanced by up to 50%, taking advantage of the increased probability for photons outcoupled to air by effectively inducing horizontally oriented emission transition dipole moments (TDM) in the perovskite emitters. The role of the TDM orientation in PeLED performance and the factors allowing its rational manipulation are reviewed extensively. Furthermore, this account presents an in-depth discussion about the effects of the self-assembly of LHP colloidal nanocrystals (NCs) into superlattices on the NC emission anisotropy and optical properties.

From Chains to Arrays: Substrate-Mediated Self-Assembly of Diboron Molecules

In this study, we explore the substrate-mediated control of self-assembly behavior in diboron molecules (C12H8B2O4, B2Cat2) using scanning tunneling microscopy (STM). The structural transformation of B2Cat2 molecules from one-dimensional (1D) molecular chains to two-dimensional (2D) molecular arrays was achieved by changing the substrate from Au(111) to bilayer graphene (BLG), highlighting the key role of substrate interactions in determining the assembly structure. Notably, the B-B bond in the molecular arrays on BLG is distinctly pronounced, reflecting a more refined molecular resolution with distinct electronic states than that on Au(111). Density functional theory (DFT) calculations confirm the weak interaction between B2Cat2 molecules and the BLG substrate, which facilitates the formation of 2D molecular arrays on BLG. This work demonstrates how controlling substrate properties enables the formation of 1D chains and 2D arrays, providing valuable insights for the design of next-generation molecular electronics and catalysis systems.

Boosting spontaneous orientation polarization of polar molecules based on fluoroalkyl and phthalimide units

Polar organic molecules form spontaneous polarization in vacuum-deposited films by permanent dipole orientations in the films, originating from the molecule's potential ability to align itself on the film surface during deposition. This study focuses on developing polar molecules that exhibit spontaneous orientation polarization (SOP) and possess a high surface potential. In the proposed molecular design, a hexafluoropropane (6F) unit facilitates spontaneous molecular orientation to align the permanent dipoles, and a phthalimide unit induces strong molecular polarization. Furthermore, the introduction of phthalimides into the molecular backbone raises the glass transition temperature of the molecules, leading to the suppression of molecular mobility on the film surface during film deposition and an improvement in the dipole orientation. The resulting surface potential slope is approximately 280 mV nm-1 without substrate temperature control. Furthermore, this work proposes a method using position isomers as a design strategy to tune the SOP polarity. The substitution position of the strong polar units influences the direction of the total molecular dipoles and affects the SOP polarity of the 6F-based molecules. The proposed molecular designs in this study provide wide tunability of the SOP intensity and polarity, which contributes to highly efficient organic optoelectronic and energy-harvesting devices.

Enhanced Emitting Dipole Orientation Based on Asymmetric Iridium(III) Complexes for Efficient Saturated-Blue Phosphorescent OLEDs

Three novel asymmetric Ir(III) complexes have been rationally designed to optimize their emitting dipole orientations (EDO) and enhance light outcoupling in blue phosphorescent organic light-emitting diodes (OLEDs), thereby boosting their external quantum efficiency (EQE). Bulky electron-donating groups (EDGs), namely: carbazole (Cz), di-tert-butyl carbazole (tBuCz), and phenoxazine (Pxz) are incorporated into the tridentate dicarbene pincer chelate to induce high degree of packing anisotropy, simultaneously enhancing their photophysical properties. Angle-dependent photoluminescence (ADPL) measurements indicate increased horizontal transition dipole ratios of 0.89 and 0.90 for the Ir(III) complexes Cz-dfppy-CN and tBuCz-dfppy-CN, respectively. Analysis of the single crystal structure and density functional theory (DFT) calculation results revealed an inherent correlation between molecular aspect ratio and EDO. Utilizing the newly obtained emitters, the blue OLED devices demonstrated exceptional performance, achieving a maximum EQE of 30.7% at a Commission International de l'Eclairage (CIE) coordinate of (0.140, 0.148). Optical transfer matrix-based simulations confirmed a maximum outcoupling efficiency of 35% due to improved EDO. Finally, the tandem OLED and hyper-OLED devices exhibited a maximum EQE of 44.2% and 31.6%, respectively, together with good device stability. This rational molecular design provides straightforward guidelines to reach highly efficient and stable saturated blue emission.

Exploring the characteristic "plug-in" configuration of an adsorbed starburst molecule

In this study, the adsorption of starburst molecules (C57H48N4, SBM) deposited on Au(111) was investigated by scanning tunneling microscopy (STM). SBM molecules selectively adsorb on the face-centered-cubic regions of Au(111) in quasi one-dimensional chains. Compared with the SBM structure on hBN/Rh(111), the assembly structure of SBM on Au(111) can be clearly confirmed to be molecular chains in the "plug-in" configuration. Scanning tunneling spectroscopy (STS) revealed that the interaction between SBM molecules and Au(111) induces the modification of the molecular electronic states. The STS mapping further revealed a continuous 1D electronic state concentrated at the center of the SBM molecular chains.

High Density Two-Component Glasses of Organic Semiconductors Prepared by Physical Vapor Deposition

Physical vapor deposition (PVD) is widely utilized for the production of organic semiconductor devices due to its ability to form thin layers with exceptional properties. Although the layers in the device usually consist of two or more components, there is limited understanding about the fundamental characteristics of such multicomponent vapor-deposited glasses. Here, spectroscopic ellipsometry was employed to characterize the densities, thermal stabilities, and optical properties of covapor deposited NPD and TPD glasses across the entire range of composition. We find that codeposited NPD and TPD form high density glasses with enhanced thermal stability. The dependences of density and stability upon substrate temperature are correlated, and the birefringence of the codeposited glasses is determined by the reduced substrate temperature of mixtures. Additionally, we observe that the transformation of a highly stable and dense two-component glass into its supercooled liquid initiates from the free surface and propagates into the bulk at a constant velocity, like single component PVD glasses. All of these features are consistent with the surface equilibration mechanism.

Hexacarbazolylbenzene: An Excellent Host Molecule Causing Strong Guest Molecular Orientation and the High-Performance OLEDs

Hexacarbazolylbenzene (6CzPh), which is benzene substituted by six carbazole rings, is a simple and attractive compound. Despite the success of a wide variety of carbazole derivatives in organic light-emitting diodes (OLEDs), 6CzPh has not received attention so far. Here, excellent performances of 6CzPh are revealed as a host material in OLEDs regarding conventional host materials. Various strategies are implemented to improve the performance of OLEDs, e.g., triplet utilization by thermally activated delayed fluorescence (TADF) and phosphorescence emitters for maximizing internal quantum efficiency, and molecular orientation control for increasing outcoupling efficiency. The present host material is suited for both criteria. Robustness of the structure and sufficiently high triplet energy enables a high external quantum efficiency with a long device lifetime. Besides, the host material boosts the horizontal molecular orientations of several guest emitters. It is noteworthy that disk-shaped 4CzIPN marks the complete horizontal molecular orientations (Θh = 100%, S = -0.50). These results provide an effective way of improving efficiencies without sacrificing device durability for future OLEDs.

The effects of alkyl substitution on the aggregation of π-conjugated dyes: spectroscopic study and modelling

A family of dithienosilole-based dyes with alternating donor and acceptor conjugated groups, decorated with linear or branched alkyl chains at different positions on the backbone, have been obtained and investigated in different aggregation states. These dyes are characterized by almost panchromatic absorption and by near-IR emission, with good quantum yields in a variety of solvents with different polarity. We demonstrate that the nature and position of the alkyl substituents strongly govern the self-assembly of the dyes, whose packing is also sensitive to external stimuli, such as grinding and water addition. Thanks to computational results and theoretical modelling, we are able to interpret the results based on two possible preferential packings, characterized by distinct spectroscopic behaviour, whose abundance can be tuned according to the nature and position of the alkyl chains, as well as via external stimuli.

Enhancing Light Outcoupling Efficiency via Anisotropic Low Refractive Index Electron Transporting Materials for Efficient Perovskite Light-Emitting Diodes

Thanks to the extensive efforts toward optimizing perovskite crystallization properties, high-quality perovskite films with near-unity photoluminescence quantum yield are successfully achieved. However, the light outcoupling efficiency of perovskite light-emitting diodes (PeLEDs) is impeded by insufficient light extraction, which poses a challenge to the further advancement of PeLEDs. Here, an anisotropic multifunctional electron transporting material, 9,10-bis(4-(2-phenyl-1H-benzo[d]imidazole-1-yl)phenyl) anthracene (BPBiPA), with a low extraordinary refractive index (ne) and high electron mobility is developed for fabricating high-efficiency PeLEDs. The anisotropic molecular orientations of BPBiPA can result in a low ne of 1.59 along the z-axis direction. Optical simulations show that the low ne of BPBiPA can effectively mitigate the surface plasmon polariton loss and enhance the photon extraction efficiency in waveguide mode, thereby improving the light outcoupling efficiency of PeLEDs. In addition, the high electron mobility of BPBiPA can facilitate balanced carrier injection in PeLEDs. As a result, high-efficiency green PeLEDs with a record external quantum efficiency of 32.1% and a current efficiency of 111.7 cd A-1 are obtained, which provides new inspirations for the design of electron transporting materials for high-performance PeLEDs.

Synergetic Modulation of Steric Hindrance and Excited State for Anti-Quenching and Fast Spin-Flip Multi-Resonance Thermally Activated Delayed Fluorophore

Multi-resonance thermally activated delayed fluorescence (MR-TADF) materials hold great promise for advanced high-resolution organic light-emitting diode (OLED) displays. However, persistent challenges, such as severe aggregation-caused quenching (ACQ) and slow spin-flip, hinder their optimal performance. We propose a synergetic steric-hindrance and excited-state modulation strategy for MR-TADF emitters, which is demonstrated by two blue MR-TADF emitters, IDAD-BNCz and TIDAD-BNCz, bearing sterically demanding 8,8-diphenyl-8H-indolo[3,2,1-de]acridine (IDAD) and 3,6-di-tert-butyl-8,8-diphenyl-8H-indolo[3,2,1-de]acridine (TIDAD), respectively. These rigid and bulky IDAD/TIDAD moieties, with appropriate electron-donating capabilities, not only effectively mitigate ACQ, ensuring efficient luminescence across a broad range of dopant concentrations, but also induce high-lying charge-transfer excited states that facilitate triplet-to-singlet spin-flip without causing undesired emission redshift or spectral broadening. Consequently, implementation of a high doping level of IDAD-BNCz resulted in highly efficient narrowband electroluminescence, featuring a remarkable full-width at half-maximum of 34 nm and record-setting external quantum efficiencies of 34.3 % and 31.8 % at maximum and 100 cd m-2, respectively. The combined steric and electronic effects arising from the steric-hindered donor introduction offer a compelling molecular design strategy to overcome critical challenges in MR-TADF emitters.

Sterically Crowded Donor-Rich Imidazole Systems as Hole Transport Materials for Solution-Processed OLEDs

Imidazole, being an interesting dinitrogenic five-membered heterocyclic core, has been widely explored during the last several decades for developing various fascinating materials. Among the different domains where imidazole-based materials find wide applications, the area of optoelectronics has seen an overwhelming growth of functional imidazole derivatives developed through remarkable design and synthesis strategies. The present work reports a design approach for integrating bulky donor units at the four terminals of an imidazole core, leading to the development of sterically populated imidazole-based molecular platforms with interesting structural features. Rationally chosen starting substrates led to the incorporation of a bulky donor at the four terminals of the imidazole core. In addition, homo- and cofunctional molecular systems were synthesized through a suitable combination of initial ingredients. Our approach was extended to develop a series of four molecular systems, i.e., Cz3PhI, Cz4I, Cz3PzI, and TPA3CzI, containing carbazole, phenothiazine, and triphenylamine as known efficient donors at the periphery. Given their interesting structural features, three sterically crowded molecules (Cz4I, Cz3PzI, and TPA3CzI) were screened by using DFT and TD-DFT calculations to investigate their potential as hole transport materials (HTMs) for optoelectronic devices. The theoretical studies on several aspects including hole reorganization and exciton binding energies, ionization potential, etc., revealed their potential as possible candidates for the hole transport layer of OLEDs. Single-crystal analysis of Cz3PhI and Cz3PzI established interesting structural features including twisted geometries, which may help attain high triplet energy. Finally, the importance of theoretical predictions was established by fabricating two solution-process green phosphorescent OLED devices using TPA3CzI and Cz3PzI as HTMs. The fabricated devices exhibited good EQE/PE and CE of ∼15%/56 lm/W/58 cd/A and ∼13%/47 lm/W/50 cd/A, respectively, at 100 cd/m2.

Organoboron-based multiple-resonance emitters: synthesis, structure-property correlations, and prospects

Boron-based multiple-resonance (MR) emitters exhibit the advantages of narrowband emission, high absolute photoluminescence quantum yield, thermally activated delayed fluorescence (TADF), and sufficient stability during the operation of organic light-emitting diodes (OLEDs). Thus, such MR emitters have been widely applied as blue emitters in triplet-triplet-annihilation-driven fluorescent devices used in smartphones and televisions. Moreover, they hold great promise as TADF or terminal emitters in TADF-assisted fluorescence or phosphor-sensitised fluorescent OLEDs. Herein we comprehensively review organoboron-based MR emitters based on their synthetic strategies, clarify structure-photophysical property correlations, and provide design guidelines and future development prospects.

Recent synthetic strategies of small heterocyclic organic molecules with optoelectronic applications: a review

Over the past few years, there have been tremendous developments in the design and synthesis of organic optoelectronic materials with appealing applications in device fabrication of organic light-emitting diodes, superconductors, organic lasers, organic field-effect transistors, clean energy-producing organic solar cells, etc. There is an increasing demand for the synthesis of green, highly efficient organic optoelectronic materials to cope with the issue of efficiency roll-off in organic semiconductor-based devices. This review systematically summarized the recent progress in the design and synthesis of small organic molecules having promising optoelectronic properties for their potential applications in optoelectronic devices during the last 10-year range (2010-early 2021).

Impact of Intermittent Deposition on Spontaneous Orientation Polarization of Organic Amorphous Films Revealed by Rotary Kelvin Probe

The control of the molecular orientation and resultant polarization is essential for improving the performance of organic optoelectronic devices. Conventionally, the substrate temperature and deposition rate are tuned to control the molecular orientation of vapor-deposited films. In this study, we proposed a novel method, referred to as "intermittent deposition", in which the polarization direction and magnitude are controlled by introducing intervals during physical vapor deposition. The rotary Kelvin probe measurement of the Alq3 and TPBi films clearly showed a time-dependent decrease in the surface potential owing to the surface relaxation of the molecular orientation immediately after deposition. Through a series of intermittent depositions, in which the deposition shutter is repeatedly opened and closed at certain intervals, a relaxed surface layer was built up, and we could control the polarization magnitude. For the Alq3 film, even the polarization direction was switched. The proposed new deposition method is applicable to general organic molecules, not limited to polar molecules, thereby potentially tuning the conduction properties of organic devices and fabricating novel devices.

Controlling Spontaneous Orientation Polarization in Organic Semiconductors─The Case of Phosphine Oxides

Upon film growth by physical vapor deposition, the preferential orientation of polar organic molecules can result in a nonzero permanent dipole moment (PDM) alignment, causing a macroscopic film polarization. This effect, known as spontaneous orientation polarization (SOP), was studied in the case of different phosphine oxides (POs). We investigate the control of SOP by molecular design and film-growth conditions. Our results show that using less polar POs with just one phosphor-oxygen bond yields an exceptionally high degree of SOP with the so-called giant surface potential (slope), reaching more than 150 mV nm-1 in a neat bis-4-(N-carbazol(yl)phenyl)phenyl phosphine oxide (BCPO) film grown at room temperature. Additionally, by altering the evaporation rate and substrate temperature, we are able to control the SOP magnitude over a broad range from 0 to almost 300 mV nm-1. Diluting BCPO in a nonpolar host enhances the PDM alignment only marginally, but combining temperature control with dipolar doping can result in highly aligned molecules with more than 80% of their PDMs standing upright on the substrate on average.

Revealing the Orientation Selectivity of Tetrapyridyl-Substituted Porphyrins Constrained in Molecular "Klotski Puzzles"

Understanding and controlling molecular orientations in self-assembled organic nanostructures are crucial to the development of advanced functional nanodevices. Scanning tunneling microscopy (STM) provides a powerful toolbox to recognize molecular orientations and to induce orientation changes on surfaces at the single-molecule level. Enormous effort has been devoted to directly controlling the molecular orientations of isolated single molecules in free space. However, revealing and further controlling molecular orientation selectivity in constrained environments remain elusive. In this study, by a combination of STM imaging/manipulations and density functional theory calculations, we report the orientation selectivity of tetrapyridyl-substituted porphyrins in response to various local molecular environments in artificially constructed molecular "Klotski puzzles" on Au(111). With the assistance of STM lateral manipulations, "sliding-block" molecules were able to enter predefined positions, and specific molecular orientations were adopted to fit the local molecular environments, in which the intermolecular interaction was revealed to be the key to achieving the eventual molecular orientation selectivity. Our results demonstrate the essential role of local molecular environments in directing single-molecule orientations, which would shed light on the design of molecular structures to control preferred orientations for further applications in molecular nanodevices.

Depletion of two-level systems in highly stable glasses with different molecular ordering

Recent findings of structural glasses with extremely high kinetic and thermodynamic stability have attracted much attention. The question has been raised as to whether the well-known, low-temperature “glassy anomalies” (attributed to the presence of two-level systems [TLS] and the “boson peak”) persist or not in these ultrastable glasses of much lower configurational entropy. To resolve previous contradictory results, we study a particular type of ultrastable glass, TPD, which can be prepared by physical vapor deposition in a highly-stable state with different degrees of layering and molecular orientation, and also as a conventional glass and in crystalline state. After a thorough characterization of the different samples prepared, we have measured their specific heat down to 0.4 K. Whereas the conventional glass exhibits the typical glassy behaviour and the crystal the expected Debye cubic dependence at very low temperatures, a strong depletion of the TLS contribution is found in both kinds of ultrastable glass, regardless of their layering and molecular ordering.

The Key Role of Molecular Packing in Luminescence Property: From Adjacent Molecules to Molecular Aggregates

The luminescence materials act as the key components in many functional devices, as well as the detection and imaging systems, which can be permeated in each aspect of modern life, and attract more and more attention for the creative technology and applications. In addition to the diverse properties of organic luminogens, the multiple molecular packing at aggregated states frequently offers new and/or exciting performance. However, there still lacks comprehensive analysis of molecular packing in these organic materials, resulting in an increased gap between molecular design and practical applications. In this review, from the basic knowledge of organic compounds as single molecules, to the discernable property of excimer, charge transfer (CT) complex or self-assembly systems by adjacent molecules, and finally to the opto-electronic performance of molecular aggregates, the relevant factors to molecular packing and practical applications are discussed.

Studying Molecular Rearrangement of P1 Dye at a Passivating Alumina Surface Using Vibrational Sum-Frequency Generation Spectroscopy: Effect of Atomic-Level Roughness

The effect of roughness and thickness of alumina layers, mimicking the passivation layer commonly used in dye-sensitized photoelectrodes, on the molecular adsorption of P1 dye, 4-(bi(4-(2,2-dicyano-vinyl)-thiophene-2-yl]-phenyl]-aminobenzoic acid) has been studied using surface-sensitive vibrational sum frequency generation(VSFG) spectroscopy. The VSFG spectra reveal the formation of poorly ordered dye layers on relatively rough surfaces where XPS measures a higher dye loading. Furthermore, these poorly ordered dye molecules are responsible for the generation of trapped electronic states as probed by successive photoluminescence (PL) measurements. Surface sensitive VSFG spectroscopy in combination with XPS and PL measurements provide complementary spectral information on ordering of the adsorbed dyes, their density on the surface and electronic states of the adsorbed monolayer which are prerequisite for improving our understanding of molecularly functionalized photoelectrodes and their further development.

Highly Efficient Ultra-Thin EML Blue PHOLEDs with an External Light-Extraction Diffuser

In this study, various diffusers are applied to highly efficient ultra-thin emission layer (EML) structure-based blue phosphorescent organic light-emitting diodes (PHOLEDs) to improve the electroluminescence (EL) characteristics and viewing angle. To achieve highly efficient blue PHOLEDs, the EL characteristics of ultra-thin EML PHOLEDs with the various diffusers having different structures of pattern-shape (hemisphere/sphere), size (4~75 μm), distribution (surface/embedded), and packing (close-packed/random) were systematically analyzed. The diffusers showed different enhancements in the overall EL characteristics of efficiencies, viewing angle, and others. The EL characteristics showed apparent dependency on their structure. The external quantum efficiency (EQE) was enhanced mainly by following the orders of pattern, size, and shape. Following the pattern size, the EQE enhancement gradually increased; the largest-sized diffuser with a 75 μm closed-packed hemisphere (diffuser-1) showed a 1.47-fold EQE improvement, which was the highest. Meanwhile, the diffuser with a ~7 μm random embedded sphere with a low density (diffuser 5) showed the lowest 1.02-fold-improved EQE. The reference device with ultra-thin EML structure-based blue PHOLEDs showed a maximum EQE of 16.6%, and the device with diffuser 1 achieved a maximum EQE of 24.3% with a 5.1% wider viewing angle compared to the reference device without a diffuser. For the in-depth analysis, the viewing angle profile of the ultra-thin EML PHOLED device and fluorescent green OLEDs were compared. As a result, the efficiency enhancement characteristics of the diffusers show a difference in the viewing angle profile. Finally, the application of the diffuser successfully demonstrated that the EL efficiency and viewing angle could be selectively improved. Additionally, we found that it was possible to realize a wide viewing angle and achieve considerable EQE enhancement by further investigations using high-density and large-sized embedded structures of light-extraction film.

Dissymmetric Triaryltriazines: Small Mass Columnar Glasses

Columnar liquid crystals with very small molecular masses that form anisotropic glasses well above room temperature are obtained by mixed dissymmetric substitution of sym-triazine with ester-bearing phenyl and phenanthryl or tetrahelicenyl moieties. The combination of low molecular symmetry with configurational flexibility and short polar ester moieties stabilizes the mesophase over large temperature ranges and induces pronounced calorimetric glass transitions within the anisotropic fluid despite the smallness of the molecules. In contrast to more symmetrical homologs, no ester tails longer than ethyl are necessary to induce the liquid crystalline state, allowing for the near-absence of any insulating and weight-increasing alkyl periphery. Films drop-cast from solution show in all cases emission spectra that do not show significant change of fluorescence emission upon annealing, indicating that the columnar hexagonal mesoscopic order is obtained directly upon deposition from solution and is resistant to crystallization upon annealing.

Composition Dictates Molecular Orientation at the Heterointerfaces of Vapor-Deposited Glasses

Physical vapor deposition (PVD) can prepare organic glasses with a preferred molecular orientation. The relationships between deposition conditions and orientation have been extensively investigated in the film bulk. The role of interfaces on the structure is less well understood and remains a key knowledge gap, as the interfacial region can govern glass stability and optoelectronic properties. Robust experimental characterization has remained elusive due to complexities in interrogating molecular organization in amorphous, organic materials. Polarized soft X-rays are sensitive to both the composition and the orientation of transition dipole moments in the film, making them uniquely suited to probe molecular orientation in amorphous soft matter. Here, we utilize polarized resonant soft X-ray reflectivity (P-RSoXR) to simultaneously depth profile the composition and molecular orientation of a bilayer prepared through the physical vapor deposition of 1,4-di-[4-(N,N-diphenyl)amino]styryl-benzene (DSA-Ph) on a film of aluminum-tris(8-hydroxyquinoline) (Alq3). The bulk orientation of the DSA-Ph layer is controlled by varying deposition conditions. Utilizing P-RSoXR to depth profile the films enables determination of both the bulk orientation of DSA-Ph and the orientation near the Alq3 interface. At the Alq3 surface, DSA-Ph always lies with its long axis parallel to the interface, before transitioning into the bulk orientation. This is likely due to the lower mobility and higher glass transition of Alq3, as the first several monolayers of DSA-Ph deposited on Alq3 appear to behave as a blend. We further show how orientation at the interface correlates with the bulk behavior of a codeposited glass of similar blend composition, demonstrating a straightforward approach to predicting molecular orientation at heterointerfaces. This work provides key insights into how molecules orient during vapor deposition and offers methods to predict this property, a critical step toward controlling interfacial behavior in soft matter.

Crystalline organic thin films for crystalline OLEDs (II): weak epitaxy growth of phenanthroimidazole derivatives

The ordered molecular arrangement of crystalline organic semiconductors facilitates high carrier mobility and light emission in organic light-emitting diode (OLED) devices. It has been demonstrated that the weak epitaxy growth (WEG) process is a valuable crystallization route for fabricating crystalline thin-film OLEDs (C-OLEDs). Recently, C-OLEDs based on crystalline thin films of phenanthroimidazole derivatives have exhibited excellent luminescent properties such as high photon output at low driving voltage and high power efficiency. Achieving effective control of organic crystalline thin film growth is crucial for the development of new C-OLEDs. Herein, we report the studies on morphology structure and growth behavior of the phenanthroimidazole derivative WEG thin films. The oriented growth of WEG crystalline thin films is determined by channeling and lattice matching between the inducing layer and active layer. Large-size and continuous WEG crystalline thin films can be obtained by controlling the growth conditions.

Synthesis of 2,5,8-Tris(1-phenyl-1H-benzo[d]imidazol-2-yl)benzo[1,2-b:3,4-b':5,6-b″] Trithiophenes and Their Spontaneous Orientation Polarization in Thin Films

To investigate the relationship between molecular structures and spontaneous orientation polarization (SOP) in organic thin films, 2,5,8-tris(1-phenyl-1H-benzo[d]imidazol-2-yl)benzo[1,2-b:3,4-b':5,6-b″] trithiophene (TPBTT) and its ethyl derivative (m-ethyl-TPBTT) were synthesized. Variable angle spectroscopic ellipsometry and two-dimensional grazing-incidence wide-angle X-ray scattering showed that the vacuum-deposited films of TPBTT and m-ethyl-TPBTT had a higher degree of molecular orientation parallel to the substrate compared with that of prototypical 2,2',2″-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi) due to the larger π-conjugated benzotrithiophene core. However, TPBTT films showed a lower SOP of +54.4 mV/nm than did the TPBi film (+77.3 mV/nm), indicating that the molecular orientation alone did not determine the SOP. In contrast, m-ethyl-TPBTT showed a larger SOP of +104.0 mV/nm in the film. Quantum chemical calculations based on density functional theory suggested that the differences in the stable molecular conformation and the permanent dipole moments between TPBTT and m-ethyl-TPBTT caused the differences in SOP. These results suggest that the simultaneous control of the orientational order and conformation of the molecules is important to achieving a large SOP in films.

Optical anisotropy of CsPbBr3 perovskite nanoplatelets

The two-dimensional CsPbBr3 nanoplatelets have a quantum well electronic structure with a band gap tunable with sample thicknesses in discreet steps based upon the number of monolayers. The polarized optical properties of CsPbBr3 nanoplatelets are studied using fluorescence anisotropy and polarized transient absorption spectroscopies. Polarized spectroscopy shows that they have absorption and emission transitions which are strongly plane-polarized. In particular, photoluminescence excitation and transient absorption measurements reveal a band-edge polarization approaching 0.1, the limit of isotropic two-dimensional ensembles. The degree of anisotropy is found to depend on the thickness of the nanoplatelets: multiple measurements show a progressive decrease in optical anisotropy from 2 to 5 monolayer thick nanoplatelets. In turn, larger cuboidal CsPbBr3 nanocrystals, are found to have consistently positive anisotropy which may be attributed to symmetry breaking from ideal perovskite cubes. Optical measurements of anisotropy are described with respect to the theoretical framework developed to describe exciton fine structure in these materials. The observed planar absorption and emission are close to predicted values at thinner nanoplatelet sizes and follow the predicted trend in anisotropy with thickness, but with larger anisotropy than theoretical predictions. Dominant planar emission, albeit confined to the thinnest nanoplatelets, is a valuable attribute for enhanced efficiency of light-emitting devices.

Mesogenic Groups Control the Emitter Orientation in Multi-Resonance TADF Emitter Films

The use of thermally activated delayed fluorescence (TADF) emitters and emitters that show preferential horizontal orientation of their transition dipole moment (TDM) are two emerging strategies to enhance the efficiency of OLEDs. We present the first example of a liquid crystalline multi-resonance TADF (MR-TADF) emitter, DiKTa-LC. The compound possesses a nematic liquid crystalline phase between 80 °C and 110 °C. Importantly, the TDM of the spin-coated film shows preferential horizontal orientation, with an anisotropy factor, a, of 0.28, which is preserved in doped poly(vinylcarbazole) films. Green-emitting (λEL =492 nm) solution-processed OLEDs based on DiKTa-LC showed an EQEmax of 13.6 %. We thus demonstrate for the first time how self-assembly of a liquid crystalline TADF emitter can lead to the so-far elusive control of the orientation of the transition dipole in solution-processed films, which will be of relevance for high-performance solution-processed OLEDs.

Flexible plasmonic nanocavities: a universal platform for the identification of molecular orientations

The molecular orientation provides fundamental images to understand molecular behaviors in chemistry. Herein, we propose and demonstrate sandwich plasmonic nanocavities as a surface-selection ruler to illustrate the molecular orientations by surface-enhanced Raman spectroscopy (SERS). The field vector in the plasmonic nanocavity presents a transverse spinning feature under specific excitations, allowing the facile modulation of the field polarizations to selectively amplify the Raman modes of the target molecules. It does not require the knowledge of the Raman spectrum of a bare molecule as a standard and thus can be extended as a universal ruler for the identification of molecular orientations. We investigated the most widely used Raman probe, Rhodamine 6G (R6G) on the Au surface and tried to clarify the arguments about its orientations from our perspectives. The experimental results suggest concentration-dependent adsorption configurations of R6G: it adsorbs on Au primarily via an ethylamine group with the xanthene ring lying flatly on the metal surface at low concentrations, and the molecular orientation gradually changes from "flat" to "upright" with the increase of molecular concentrations.

Improving Electroluminescence Efficiency by Linear Polar Host Capable of Promoting Horizontal Dipole Orientation for Dopant

In doped organic light-emitting diodes (OLEDs), the host materials play an important role in emitting layers. Most studies about host materials mainly focus on their energy levels and carrier transport behaviors, while less attention is paid to their influence on the dipole orientation of dopants, which closely associate with the light out-coupling efficiency (ηout ) of the device. Herein, a linear polar host material (l-CzTRZ) consisting of carbazole donor, triazine acceptor, and the conjugated para-terphenyl skeleton is developed and its crystal and electronic structures, thermal and electrochemical stabilities, optical property, and carrier transport ability are investigated. l-CzTRZ prefers ordered horizontal orientation and favors electron transport in neat film. More importantly, it can promote horizontal dipole orientation for the dopants via dipole-dipole interaction, furnishing an excellent horizontal dipole ratio of 91.5% and thus a high ηout of 43% for the phosphorescent dopant (PO-01-TB). Consequently, the OLED with l-CzTRZ host and PO-01-TB dopant attains state-of-the-art electroluminescence efficiencies of 135.5 cd A-1 , 135.7 lm W-1 and 41.3%, with a small roll-off of 9.7% at 5000 cd m-2 luminance. The presented significant impact of the host on the dipole orientation of the dopant shall enlighten the design of host materials to improve OLED performance.

Process Engineered Spontaneous Orientation Polarization in Organic Light-Emitting Devices

Polar molecules with appreciable permanent dipole moments (PDMs) are widely used as the electron transport layer (ETL) in organic light-emitting devices (OLEDs). When the PDMs spontaneously align, a macroscopic polarization field can be observed, a phenomenon known as spontaneous orientation polarization (SOP). The presence of SOP in the ETL induces considerable surface potential and charge accumulation that is capable of quenching excitons and reducing device efficiency. While prior work has shown that the degree of SOP is sensitive to film processing conditions, this work considers SOP formation by quantitatively treating the vapor-deposited film as a supercooled glass, in analogy to prior work on birefringence in organic thin films. Importantly, the impact of varying thin-film deposition rate and relative temperature is unified into a single framework, providing a useful tool to predict the SOP formation efficiency for a polar material, as well as in blends of polar materials. Finally, in situ photoluminescence characterization and efficiency measurements reveal that SOP-induced exciton-polaron quenching can be reduced through an appropriate choice of processing conditions, leading to enhanced OLED efficiency.

Robust Spirobifluorene Core Based Hole Transporters with High Mobility for Long-Life Green Phosphorescent Organic Light-Emitting Devices

Using a tailored high triplet energy hole transport layer (HTL) is a suitable way to improve the efficiency and extend the lifetime of organic light-emitting devices (OLEDs), which can use all molecular excitons of singlets and triplets. In this study, dibenzofuran (DBF)-end-capped and spirobifluorene (SBF) core-based HTLs referred as TDBFSBF1 and TDBFSBF2 were effectively developed. TDBFSBF1 exhibited a high glass transition temperature of 178 °C and triplet energy of 2.5 eV. Moreover, a high external quantum efficiency of 22.0 %, long operational lifetime at 50 % of the initial luminance of 89,000 h, and low driving voltage at 1000 cd m^-2 of 2.95 V were achieved in green phosphorescent OLEDs using TDBFSBF1. Further, a high-hole mobility μ_h value of 1.9×10^-3  cm²  V^-1  s^-1 was recorded in TDBFSBF2. A multiscale simulation successfully reproduced the experimental μ_h values and indicated that the reorganization energy was the primary factor in determining the mobility differences among these SBF core based HTLs.

F4-TCNQ on Epitaxial Bi-Layer Graphene: Concentration- and Orientation-Dependent Charge Transfer at the Interface

Bi-layer epitaxial graphene (BLG) on 6H-SiC(0001) (EG/SiC) was grown and modified by thermal deposition of the molecular electron acceptor tetrafluoro-tetra cyano quinodimethane (F4-TCNQ). The surface-modified system, F4-TCNQ/EG/SiC, was studied by X-ray photoelectron spectroscopy (XPS) and angle-resolved polarized Raman spectroscopy (ARPRS). XPS results indicate that bonding of deposited F4-TCNQ molecules depends on their concentration. Although bonding through the cyano groups is present at all concentrations, charge transfer from graphene to fluorine is evident only at sub-monolayer concentrations. The corresponding change in bond character is coupled with a change in molecular orientation. Raman spectroscopy not only provides results consistent with the findings from the XPS study but also reveals a significant degree of molecular stacking above the monolayer concentration. Thus, both the variation of the acceptor concentration and the number of graphene layers provide further handles to manipulate charge and doping that may be useful in device applications.

Designing Glass and Crystalline Phases of Metal-Bis(acetamide) Networks to Promote High Optical Contrast

Owing to their high tunability and predictable structures, metal-organic materials offer a powerful platform to study glass formation and crystallization processes and to design glasses with unique properties. Here, we report a novel series of glass-forming metal-ethylenebis(acetamide) networks that undergo reversible glass and crystallization transitions below 200 °C. The glass-transition temperatures, crystallization kinetics, and glass stability of these materials are readily tunable, either by synthetic modification or by liquid-phase blending, to form binary glasses. Pair distribution function (PDF) analysis reveals extended structural correlations in both single and binary metal-bis(acetamide) glasses and highlights the important role of metal-metal correlations during structural evolution across glass-crystal transitions. Notably, the glass and crystalline phases of a Co-ethylenebis(acetamide) binary network feature a large reflectivity contrast ratio of 4.8 that results from changes in the local coordination environment around Co centers. These results provide new insights into glass-crystal transitions in metal-organic materials and have exciting implications for optical switching, rewritable data storage, and functional glass ceramics.

Substituent engineering of the diboron molecular architecture for a nondoped and ultrathin emitting layer

Owing to the high technology maturity of thermally activated delayed fluorescence (TADF) emitter design with a specific molecular shape, extremely high-performance organic light-emitting diodes (OLEDs) have recently been achieved via various doping techniques. Recently, undoped OLEDs have drawn immense attention because of their manufacturing cost reduction and procedure simplification. However, capable materials as host emitters are rare and precious because general fluorophores in high-concentration states suffer from serious aggregation-caused quenching (ACQ) and undergo exciton quenching. In this work, a series of diboron materials, CzDBA, iCzDBA, and tBuCzDBA, is introduced to realize the effect of steric hindrance and the molecular aspect ratio via experimental and theoretical studies. We computed transition electric dipole moment (TEDM) and molecular dynamics (MD) simulations as a proof-of-concept model to investigate the molecular stacking in neat films. It is worth noting that the pure tBuCzDBA film with a high horizontal ratio of 92% is employed to achieve a nondoped OLED with an excellent external quantum efficiency of 26.9%. In addition, we demonstrated the first ultrathin emitting layer (1 nm) TADF device, which exhibited outstanding power efficiency. This molecular design and high-performance devices show the potential of power-saving and economical fabrication for advanced OLEDs.

Influence of Surface Roughness on the Dynamics and Crystallization of Vapor-Deposited Thin Films

The substrate roughness is a very important parameter that can influence the properties of supported thin films. In this work, we investigate the effect of surface roughness on the properties of a vapor-deposited glass (celecoxib, CXB) both in its bulk and in confined states. Using dielectric spectroscopy, we provide experimental evidence depicting a profound influence of surface roughness on the α-relaxation dynamics and the isothermal crystallization of this vapor-deposited glass. Besides, we have verified the influence of film confinement on varying values of surface roughnesses as well. At a fixed surface roughness value, the confinement could alter both the dynamics and crystallization of vapor-deposited CXB.

Molecular Design of Diazo Compound for Carbene-Mediated Cross-Linking of Hole-Transport Polymer in QLED with Reduced Energy Barrier and Improved Charge Balance

Polymeric hole-transport materials (HTMs) have been widely used in quantum-dot light-emitting diodes (QLEDs). However, their solution processability normally causes interlayer erosion and unstable film state, leading to undesired device performance. Besides, the imbalance of hole and electron transport in QLEDs also damages the device interfaces. In this study, we designed a bis-diazo compound, X1, as carbene cross-linker for polymeric HTM. Irradiated by ultraviolet and heating, a poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt(4,4'-(N-(4-butylphenyl))] (TFB)/X1 blend can achieve fast "electronically clean" cross-linking with ∼100% solvent resistance. The cross-linking reduced the stacking behaviors of TFB and thus led to a lower hole-transport mobility, whereas it was a good match of electron mobility. The carbene-mediated TFB cross-linking also downshifted the HOMO level from -5.3 to -5.5 eV, delivering a smaller hole-transport energy barrier. Benefiting from these, the cross-linked QLED showed enhanced device performances over the pristine device, with EQE, power efficiency, and current efficiency being elevated by nearly 20, 15, and 83%, respectively. To the best of our knowledge, this is the first report about a bis-diazo compound based carbene cross-linker built into a polymeric HTM for a QLED with enhanced device performance.