Thin-film Light-emitting Diodes Research Articles

Acceleration of radiative recombination for efficient perovskite LEDs

The increasing demands for more efficient and brighter thin-film light-emitting diodes (LEDs) in flat-panel display and solid-state lighting applications have promoted research into three-dimensional (3D) perovskites. These materials exhibit high charge mobilities and low quantum efficiency droop1–6, making them promising candidates for achieving efficient LEDs with enhanced brightness. To improve the efficiency of LEDs, it is crucial to minimize nonradiative recombination while promoting radiative recombination. Various passivation strategies have been used to reduce defect densities in 3D perovskite films, approaching levels close to those of single crystals3. However, the slow radiative (bimolecular) recombination has limited the photoluminescence quantum efficiencies (PLQEs) of 3D perovskites to less than 80% (refs. 1,3), resulting in external quantum efficiencies (EQEs) of LED devices of less than 25%. Here we present a dual-additive crystallization method that enables the formation of highly efficient 3D perovskites, achieving an exceptional PLQE of 96%. This approach promotes the formation of tetragonal FAPbI3 perovskite, known for its high exciton binding energy, which effectively accelerates the radiative recombination. As a result, we achieve perovskite LEDs with a record peak EQE of 32.0%, with the efficiency remaining greater than 30.0% even at a high current density of 100 mA cm−2. These findings provide valuable insights for advancing the development of high-efficiency and high-brightness perovskite LEDs.

Synergistic fabrication, characterization, and prospective optoelectronic applications of DES grafted activated charcoal dispersed PVA films

AbstractThis study investigates the synthesis, analysis, and utility of films comprising deep eutectic solvent (DES) grafted activated charcoal (AC) within a polyvinyl alcohol (PVA) matrix for optoelectronic device applications. The fabrication process involves the dispersion of DES functionalization AC into the PVA solution, followed by casting onto substrates with controlled drying. Comprehensive characterization encompassing X‐ray diffraction (XRD), scanning electron microscopy (SEM), UV–vis spectroscopy, Fourier‐transform infrared spectroscopy (FTIR), and impedance spectroscopy which discerns the films microstructure, morphology, conductance, band‐gap, and optical traits. The DES grafted AC infusion with variable concentration has significantly influenced optical absorbance and reduced the band gap indicating efficient charge mobility. Furthermore, the impedance analysis has revealed the electrical conduction of the film to be 1.8 × 10−6 Ω−1 m−1. In summary, the dispersion of DES modified AC in the PVA matrix have converted the insulating PVA to a semiconducting polymeric film with reduced band‐gap and increased absorption, which present a propitious avenue for wide array of optoelectronic devices, such as thin film transistors, photovoltaics, LEDs, photodetectors, and many such applications.

Modular slot-die coater for in situ grazing-incidence x-ray scattering experiments on thin films.

Multimodal in situ experiments during slot-die coating of thin films pioneer the way to kinetic studies on thin-film formation. They establish a powerful tool to understand and optimize the formation and properties of thin-film devices, e.g., solar cells, sensors, or LED films. Thin-film research benefits from time-resolved grazing-incidence wide- and small-angle x-ray scattering (GIWAXS/GISAXS) with a sub-second resolution to reveal the evolution of crystal structure, texture, and morphology during the deposition process. Simultaneously investigating optical properties by in situ photoluminescence measurements complements in-depth kinetic studies focusing on a comprehensive understanding of the triangular interdependency of processing, structure, and function for a roll-to-roll compatible, scalable thin-film deposition process. Here, we introduce a modular slot-die coater specially designed for in situ GIWAXS/GISAXS measurements and applicable to various ink systems. With a design for quick assembly, the slot-die coater permits the reproducible and comparable fabrication of thin films in the lab and at the synchrotron using the very same hardware components, as demonstrated in this work by experiments performed at Deutsches Elektronen-Synchrotron (DESY). Simultaneous to GIWAXS/GISAXS, photoluminescence measurements probe optoelectronic properties in situ during thin-film formation. An environmental chamber allows to control the atmosphere inside the coater. Modular construction and lightweight design make the coater mobile, easy to transport, quickly extendable, and adaptable to new beamline environments.

Thin-Film LED based on AlInGaN Layers Grown on Hybrid SiC/Si Substrates

Thin-Film LED based on AlInGaN Layers Grown on Hybrid SiC/Si Substrates

Dielectric light-trapping nanostructure for enhanced light absorption in organic solar cells

Dielectric scatterers where Mie resonances can be excited in both electric and magnetic modes have emerged as a promising candidate for efficient light trapping (LT) in thin-film solar cells. We present that light absorption in organic solar cells (OSCs) can be significantly enhanced by a front-sided incorporation of a core–shell nanostructure consisting of a high-refractive-index dielectric nanosphere array conformally coated with a low-refractive-index dielectric layer. Strong forward light scattering of the all-dielectric LT structure enables the absorption in an organic semiconductor to be remarkably boosted over a broad range of wavelengths, which is attributed to interference of a simultaneous excitation of the electric and magnetic dipole resonant modes. The OSC with the LT structure shows the short-circuit current density (Jsc) of 28.23 mA/cm2, which is 10% higher than that of a flat OSC. We also explore how the LT structure affects scattering cross-sections, spectral multipole resonances, and far-field radiation patterns. The approach described in this work could offer the possibility for the improvement of characteristic performances of various applications, such as other thin-film solar cells, photodiodes, light-emitting diodes, and absorbers.

Wafer-Scale Transferrable GaN Enabled by Hexagonal Boron Nitride for Flexible Light-Emitting Diode.

Epitaxy growth and mechanical transfer of high-quality III-nitrides using 2D materials, weakly bonded by van der Waals force, becomes an important technology for semiconductor industry. In this work, wafer-scale transferrable GaN epilayer with low dislocation density is successfully achieved through AlN/h-BN composite buffer layer and its application in flexible InGaN-based light-emitting diodes (LEDs) is demonstrated. Guided by first-principles calculations, the nucleation and bonding mechanism of GaN and AlN on h-BN is presented, and it is confirmed that the adsorption energy of Al atoms on O2 -plasma-treated h-BN is over 1eV larger than that of Ga atoms. It is found that the introduced high-temperature AlN buffer layer induces sufficient tensile strain during rapid coalescence to compensate the compressive strain generated by the heteromismatch, and a strain-relaxation model for III-nitrides on h-BN is proposed. Eventually, the mechanical exfoliation of single-crystalline GaN film and LED through weak interaction between multilayer h-BN is realized. The flexible free-standing thin-film LED exhibits ≈66% luminescence enhancement with good reliability compared to that before transfer. This work proposes a new approach for the development of flexible semiconductor devices.

Construction of a Flexible Optogenetic Device for Multisite and Multiregional Optical Stimulation Through Flexible µ-LED Displays on the Cerebral Cortex.

Precisely delivering light to multiple locations in biological tissue is crucial for advancing multiregional optogenetics in neuroscience research. However, conventional implantable devices typically have rigid geometries and limited light sources, allowing only single or dual probe placement with fixed spacing. Here, a fully flexible optogenetic device with multiple thin-film microscale light-emitting diode (µ-LED) displays scattering from a central controller is presented. Each display is heterogeneously integrated with thin-film 5×10µ-LEDs and five optical fibers 125µm in diameter to achieve cellular-scale spatial resolution. Meanwhile, the device boasts a compact, flexible circuit capable of multichannel configuration and wireless transmission, with an overall weight of 1.31g, enabling wireless, real-time neuromodulation of freely moving rats. Characterization results and finite element analysis have demonstrated excellent optical properties and mechanical stability, while cytotoxicity tests further ensure the biocompatibility of the device for implantable applications. Behavior studies under optogenetic modulation indicate great promise for wirelessly modulating neural functions in freely moving animals. The device with multisite and multiregional optogenetic modulation capability offers a comprehensive platform to advance both fundamental neuroscience studies and potential applications in brain-computer interfaces.

Structural, Electronic, Elastic, and Optical Characteristics of AgZF3 (Z = Sb and Bi) Fluoro-Perovskites: Using a Computational Approach for Energy Generation.

This research is being conducted to learn more about various compounds and their potential uses in various fields such as renewable energy, electrical conductivity, the study of optoelectronic properties, the use of light-absorbing materials in photovoltaic device thin-film LEDs, and field effect transistors (FETs). AgZF3 (Z = Sb, Bi) compounds, which are simple, cubic, ternary fluoro-perovskites, are studied using the FP-LAPW and low orbital algorithm, both of which are based on DFT. Structure, elasticity and electrical and optical properties are only some of the many features that can be predicted. The TB-mBJ method is used to analyze several property types. An important finding of this study is an increase in the bulk modulus value after switching Sb to Bi as the metallic cation designated as "Z" demonstrates the stiffness characteristic of a material. The anisotropy and mechanical balance of the underexplored compounds are also revealed. Our compounds are ductile, as evidenced by the calculated Poisson ratio, Cauchy pressure, and Pugh ratio values. Both compounds exhibit indirect band gaps (X-M), with the lowest points of the conduction bands located at the evenness point X and the highest points of the valence bands located at the symmetry point M. The principal peaks in the optical spectrum can be understood in light of the observed electronic structure.

Increased Light Extraction of Thin-Film Flip-Chip UVB LEDs by Surface Texturing.

Ultraviolet light-emitting diodes (LEDs) suffer from a low wall-plug efficiency, which is to a large extent limited by the poor light extraction efficiency (LEE). A thin-film flip-chip (TFFC) design with a roughened N-polar AlGaN surface can substantially improve this. We here demonstrate an enabling technology to realize TFFC LEDs emitting in the UVB range (280-320 nm), which includes standard LED processing in combination with electrochemical etching to remove the substrate. The integration of the electrochemical etching is achieved by epitaxial sacrificial and etch block layers in combination with encapsulation of the LED. The LEE was enhanced by around 25% when the N-polar AlGaN side of the TFFC LEDs was chemically roughened, reaching an external quantum efficiency of 2.25%. By further optimizing the surface structure, our ray-tracing simulations predict a higher LEE from the TFFC LEDs than flip-chip LEDs and a resulting higher wall-plug efficiency.

An Efficient Blue-Emission Crystalline Thin-Film OLED Sensitized by "Hot Exciton" Fluorescent Dopant.

Crystalline thin-film organic light-emitting diodes (C-OLEDs) can achieve a large light emission and a low Joule-heat loss under low driving voltage due to the high carrier mobility of the crystalline thin films. However, it is urgent for the C-OLEDs to improve their external quantum efficiency (EQE). Here, a novel strategy is proposed using a doped "hot exciton" material to sensitize a high PLQY blue emitter in C-OLEDs. Benefiting from the capability of the "hot exciton" material harnessing triplet/singlet excitons, the C-OLED exhibits an efficiency breakthrough with a maximum EQE of 6.2%, a much enhanced blue photon output with pure blue emission Commission International de L'Eclairage (CIE) (0.14, 0.15), a low turn-on/operation voltage of 2.6 V(@1cd m-2 )/3.8V (@1000cd m-2 ), and a maximum power efficiency (PE) of 9.4lm W-1 . This work unlocks the potential of C-OLEDs for achieving high photon output with high EQE.

Charge Transfer Dynamics of Doped Graphene Electrodes for Organic Light-Emitting Diodes.

Atomically thin graphene has attracted immense attention as a future transparent electrode for flat-panel displays owing to its excellent conductivity, optical transparency, and flexibility. In particular, a graphene doping process is essential for implementing graphene-based high-performance devices, and the development of a transparent cathode with a low work function is required to simplify the integration process of thin-film transistors and organic light-emitting diodes (OLEDs) into active matrix displays. In this study, a transparent n-doped graphene cathode is proposed for implementing inverted OLEDs through two types of cesium (Cs)-based doping techniques: a dipping method using wet chemicals and an evaporation method under a vacuum atmosphere. The changes in the chemical structures and work functions of the n-doped graphene electrodes, as well as their surface morphologies and transmittances, were systematically investigated. The n-type doping mechanism of graphene was investigated, and a close relationship between the electrical charge transfer characteristics of graphene transistors and the formation of C-O-Cs complexes was revealed. Finally, an effective Cs-doped graphene electrode was developed, exhibiting a dramatically decreased work function while maintaining high transmittance; therefore, the Cs-doped graphene cathode was successfully integrated with inverted OLEDs with a bottom-light emission structure that exhibited enhanced external quantum efficiency of graphene cathode-based OLEDs. Thus, our findings provide a better understanding of the doping strategies and potential of n-doped graphene as a transparent cathode for developing high-performance future displays.

Disentangling the Orientations of Spectrally Overlapping Transition Dipoles in Dense Dye Layers

The transition dipole orientations of dye assemblies in heterostructures have a crucial impact on the efficiency of novel optoelectronic devices such as organic thin-film transistors and light-emitting diodes. These devices are frequently based on heterojunctions and tandem structures featuring multiple optical transitions. Precise knowledge of preferred orientations, spatial order, and spatial variations is highly relevant. We present a fast and universal large-area screening method to determine the transition dipole orientations in dye assemblies with diffraction-limited spatial resolution. Moreover, our hyperspectral imaging approach disentangles the orientations of different chromophores. As a demonstration, we apply our technique to dye monolayers with two optical transitions sandwiched between two ultrathin silicate nanosheets. A comprehensive model for dipole orientation distributions in monolayers reveals a long-range orientational order and a strong correlation between the two transitions.

Emerging Optoelectronic Devices Based on Microscale LEDs and Their Use as Implantable Biomedical Applications.

Thin-film microscale light-emitting diodes (LEDs) are efficient light sources and their integrated applications offer robust capabilities and potential strategies in biomedical science. By leveraging innovations in the design of optoelectronic semiconductor structures, advanced fabrication techniques, biocompatible encapsulation, remote control circuits, wireless power supply strategies, etc., these emerging applications provide implantable probes that differ from conventional tethering techniques such as optical fibers. This review introduces the recent advancements of thin-film microscale LEDs for biomedical applications, covering the device lift-off and transfer printing fabrication processes and the representative biomedical applications for light stimulation, therapy, and photometric biosensing. Wireless power delivery systems have been outlined and discussed to facilitate the operation of implantable probes. With such wireless, battery-free, and minimally invasive implantable light-source probes, these biomedical applications offer excellent opportunities and instruments for both biomedical sciences research and clinical diagnosis and therapy.

Direct Optical Patterning of Nanocrystal-Based Thin-Film Transistors and Light-Emitting Diodes through Native Ligand Cleavage

Developing techniques of advanced patterning for colloidal nanocrystals (NCs) is essential to construct high-performance electronic/optoelectronic devices. However, conventional NC patterning methods require complicated photolithography processes and/or delicately designed photocrosslinkers. Here, we develop a direct, photoresist-free method to pattern functional NCs while avoiding any photosensitive molecules. In our approach, the solubility of NCs is switched through cleaving native organic ligands under UV light exposure. Developing with the mother solvent produces fine patterns with feature sizes (as low as 10 μm) comparable to that of conventional photolithography. Importantly, the obtained patterns of NCs allow postpatterning ligand exchange. As representatives, we demonstrate patterned NC-based thin-film transistors (TFTs) and quantum dot light-emitting diodes (QLEDs). The In2O3 NC-based TFTs are turned from inactive to active using the patterning process and display substantially enhanced electronic performances upon ligand exchange with inorganic ligands. Furthermore, exquisite fluorescence quantum dot (QD) patterns are enabled. The QLEDs fabricated with ZnCl2-treated green QD patterns exhibit electroluminescence performances that are comparable to those of unpatterned QLEDs. Our strategy offers a powerful yet simple patterning technique for fabricating low-temperature thin-film electronics and optoelectronics, which is expected to be a versatile and extensible approach for solution-processed NC-based thin-film device manufacturing.

Colocalized, bidirectional optogenetic modulations in freely behaving mice with a wireless dual-color optoelectronic probe

Optogenetic methods provide efficient cell-specific modulations, and the ability of simultaneous neural activation and inhibition in the same brain region of freely moving animals is highly desirable. Here we report bidirectional neuronal activity manipulation accomplished by a wireless, dual-color optogenetic probe in synergy with the co-expression of two spectrally distinct opsins (ChrimsonR and stGtACR2) in a rodent model. The flexible probe comprises vertically assembled, thin-film microscale light-emitting diodes with a lateral dimension of 125 × 180 µm2, showing colocalized red and blue emissions and enabling chronic in vivo operations with desirable biocompatibilities. Red or blue irradiations deterministically evoke or silence neurons co-expressing the two opsins. The probe interferes with dopaminergic neurons in the ventral tegmental area of mice, increasing or decreasing dopamine levels. Such bidirectional regulations further generate rewarding and aversive behaviors and interrogate social interactions among multiple mice. These technologies create numerous opportunities and implications for brain research.

Investigation on operation robustness of p-type low-temperature polycrystalline silicon thin-film transistor-based micro light-emitting diode pixel circuit using pulse width modulation under component fluctuation

In this study, we have investigated the operation robustness of a p-type low-temperature polycrystalline silicon (LTPS) thin-film transistor (TFT)-based micro light-emitting diode (µLED) pixel circuit adopting pulse width modulation (PWM) under circuit component fluctuations. The wavelength shift of µLEDs, depending on the current density, was suppressed by implementing PWM. The PWM pixel circuit controlled the emission time with constant µLED current in the simulated and measured results. In addition, the wavelength shift was suppressed below 0.48% within the 10-bit grayscale range. Furthermore, the component tolerance of the pixel circuit was investigated by simulating the error rate of µLED emission time with varying threshold voltage, mobility, subthreshold swing, and capacitance. The pixel circuit exhibited a robust operation with a maximum error rate of 4.0% under a component fluctuation of ±10%. Consequently, the µLED pixel circuit adopting PWM suppressed the wavelength shift of µLEDs and demonstrated robust circuit operation under component fluctuation.

Enhanced mechanical and gas barrier performance of plasticized cellulose nanofibril films

Abstract Cellulose nanofibrils (CNF) are mixed with plasticizers; sorbitol and glycerol, through high-pressure homogenization to prepare multifunctional biohybrid films. The resulting plasticized films obtained after solvent evaporation are strong, flexible and demonstrate superior toughness and optical transparency. The oxygen barrier properties of the biohybrid films outperform commercial packaging materials. The sorbitol-plasticized CNF films possess excellent oxygen barrier properties, 0.34 cm3·μm/m2·day·kPa at 50 % relative humidity, while significantly enhancing the toughness and fracture strength of the films. CNF films plasticized by 20 wt.% of sorbitol and glycerol could before rupture, be strained to about 9 % and 12 %, respectively. The toughness of the plasticized films increased by ca. 300 % compared to the pristine CNF film. Furthermore, the water vapor barrier properties of the biohybrid films were also preserved by the addition of sorbitol. CNF films plasticized with sorbitol was demonstrated to simultaneously enhance fracture toughness, work of fracture, softening behavior while preserving gas barrier properties. Highly favorable thermomechanical characteristics were found with CNF/sorbitol combinations and motivate further work on this material system, for instance as a thermoformable matrix in biocomposite materials. The unique combination of excellent oxygen barrier behavior, formability and optical transparency suggest the potential of these CNF-based films as an alternative in flexible packaging of oxygen sensitive devices like thin-film transistors or organic light-emitting diode displays, gas storage applications and as barrier coatings/laminations in packaging applications, including free-standing films as aluminium-replacement in liquid board and primary packaging, as replacement for polyethylene (PE) in wrapping paper, e. g. sweats and confectionary.

Crystalline organic thin films for crystalline OLEDs (I): orientation of phenanthroimidazole derivatives

Prepared by weak epitaxy growth, fluorescent phenanthroimidazole derivative 2FPPICz molecules form crystalline thin films. The orientation is resolved, providing a structure foundation for crystalline thin-film organic light-emitting diodes.

A dual-channel optogenetic stimulator selectively modulates distinct defensive behaviors

SummaryImplantable devices and systems have been emerging as powerful tools for neuroscience research and medical applications. Here we report a wireless, dual-channel optoelectronic system for functional optogenetic interrogation of superior colliculus (SC), a layered structure pertinent to defensive behaviors, in rodents. Specifically, a flexible and injectable probe comprises two thin-film microscale light-emitting diodes (micro-LEDs) at different depths, providing spatially resolved optical illuminations within the tissue. Under remote control, these micro-LEDs interrogate the intermediate layer and the deep layer of the SC (ILSC and DLSC) of the same mice, and deterministically evoke distinct freezing and flight behaviors, respectively. Furthermore, the system allows synchronized optical stimulations in both regions, and we discover that the flight response dominates animals' behaviors in our experiments. In addition, c-Fos immunostaining results further elucidate the functional hierarchy of the SC. These demonstrations provide a viable route to unraveling complex brain structures and functions.

Adduct-based p-doping of organic semiconductors.

Electronic doping of organic semiconductors is essential for their usage in highly efficient optoelectronic devices. Although molecular and metal complex-based dopants have already enabled significant progress of devices based on organic semiconductors, there remains a need for clean, efficient and low-cost dopants if a widespread transition towards larger-area organic electronic devices is to occur. Here we report dimethyl sulfoxide adducts as p-dopants that fulfil these conditions for a range of organic semiconductors. These adduct-based dopants are compatible with both solution and vapour-phase processing. We explore the doping mechanism and use the knowledge we gain to 'decouple' the dopants from the choice of counterion. We demonstrate that asymmetric p-doping is possible using solution processing routes, and demonstrate its use in metal halide perovskite solar cells, organic thin-film transistors and organic light-emitting diodes, which showcases the versatility of this doping approach.