Light Out-coupling Efficiency Research Articles

Optically optimization of thin film light-emitting diodes based on transfer matrix

The evolution of white LEDs, both organic and inorganic, has marked a transformative era in lighting technology known as solid-state lighting. This revolution is driven by electroluminescence, where electron-hole pairs recombine to emit light. While OLEDs have achieved high internal quantum efficiency, limited out-coupling efficiency remains a challenge. Improving light out-coupling efficiency is crucial for enhancing LED performance. This paper employs the Transfer Matrix Method to analyze the optical properties of OLEDs and aims to enhance light coupling, efficiency, and overall impact of LED and OLED technologies.

Boosting External Quantum Efficiency to 12.0 % of an Ultraviolet OLED by Engineering the Horizontal Dipole Orientation of a Hot Exciton Emitter.

Currently, much research effort has been devoted to improving the exciton utilization efficiency and narrowing the emission spectra of ultraviolet (UV) fluorophores for organic light-emitting diode (OLED) applications, while almost no attention has been paid to optimizing their light out-coupling efficiency. Here, we developed a linear donor-acceptor-donor (D-A-D) triad, namely CDFDB, which possesses high-lying reverse intersystem crossing (hRISC) property. Thanks to its integrated narrowband UV photoluminescence (PL) (λPL: 397 nm; FWHM: 48 nm), moderate PL quantum yield (ϕPL: 72 %, Tol), good triplet hot exciton (HE) conversion capability, and large horizontal dipole ratio (Θ//: 92 %), the OLEDs based on CDFDB not only can emit UV electroluminescence with relatively good color purity (λEL: 398 nm; CIEx,y: 0.161, 0.040), but also show a record maximum external quantum efficiency (EQEmax) of 12.0 %. This study highlights the important role of horizontal dipole orientation engineering in the molecular design of HE UV-OLED fluorophores.

Development of Quasi-Two-Dimensional Perovskites and Their Application in Light-Emitting Diodes.

Quasi-two-dimensional (quasi-2D) perovskites have attracted much attention due to their outstanding properties, such as inherent quantum-well structure, strong dielectric and quantum confinement, large exciton binding energy, and high photoluminescence quantum yield. By virtue of these superior merits, quasi-2D perovskites have shown great potential for next-generation light-emitting diodes (LEDs). Herein, this review presents an overview of the basic properties of quasi-2D perovskites and their photoluminescence modulations by large organic cation engineering, monovalent cation engineering, halogen engineering, defect passivation engineering, and dimensionality engineering. Furthermore, the strategies of charge-transport layer optimization, interfacial engineering, light-outcoupling efficiency improvement, and operating stability improvement are summarized for fabricating high-performance quasi-2D perovskite LEDs (PeLEDs). Finally, the challenges and outlook for the future development of quasi-2D PeLEDs are unambiguously proposed.

Polymer Dispersed Liquid Crystal Imprinted by Microlens Array for Enhanced Outcoupling Efficiency of Organic Light Emitting Diode.

In this paper, we demonstrate the use of polymer dispersed liquid crystal (PDLC) imprinted with a microlens array (MLA) via solution process to improve the outcoupling efficiency of organic light emitting diodes (OLEDs). The PDLC, well known for its scattering effect, is an excellent technology for improving the outcoupling efficiency of OLEDs. Additionally, we introduce a simple spin-coating process to fabricate PDLC which is adaptable for future solution-processed OLEDs. The MLA-imprinted PDLC applied OLED shows an enhancement factor of 1.22 in outcoupling efficiency which is a 37.5% increase compared to the existing PDLC techniques without changing the electrical properties of the OLED. Through this approach, we can expect the roll-to-roll based extremely flexible OLED, and with further research on pattering PDLC by various templates, higher outcoupling efficiency is achievable through a simple UV irradiation process.

High-efficiency, flexible and large-area red/green/blue all-inorganic metal halide perovskite quantum wires-based light-emitting diodes

Metal halide perovskites have shown great promise as a potential candidate for next-generation solid state lighting and display technologies. However, a generic organic ligand-free and antisolvent-free solution method to fabricate highly efficient full-color perovskite light-emitting diodes has not been realized. Herein, by utilizing porous alumina membranes with ultra-small pore size as templates, we have successfully fabricated crystalline all-inorganic perovskite quantum wire arrays with ultrahigh density and excellent uniformity, using a generic organic ligand-free and anti-solvent-free solution method. The quantum confinement effect, in conjunction with the high light out-coupling efficiency, results in high photoluminescence quantum yield for blue, sky-blue, green and pure-red perovskite quantum wires arrays. Consequently, blue, sky-blue, green and pure-red LED devices with spectrally stable electroluminescence have been successfully fabricated, demonstrating external quantum efficiencies of 12.41%, 16.49%, 26.09% and 9.97%, respectively, after introducing a dual-functional small molecule, which serves as surface passivation and hole transporting layer, and a halide vacancy healing agent.

Engineering the Macrocyclic Donor Structures towards Deep-Blue Thermally Activated Delayed Fluorescence Emitters.

Deep-blue thermally activated delayed fluorescence (TADF) molecules present promising potential in organic light-emitting diodes (OLEDs), especially for display applications. Here, an efficient molecular engineering approach to modifying the donor or acceptor features of the D-π-A-configured TADF molecules for deep-blue emission is reported. By introducing oxygen and sulfone as a bridge unit onto the macrocyclic donor, two emitters, c-ON-MeTRZ and c-NS-MeTRZ, are synthesized and characterized, respectively. The reduced donor strength of c-ON-MeTRZ and c-NS-MeTRZ as compared to that of the model molecule c-NN-MeTRZ leads to blue-shifted emissions with high photoluminescence quantum yields (PLQYs) and retains TADF characters, while the new emitter c-NN-MePym with the most blue-shifted emission only exhibits a pure fluorescent nature because of the electron-accepting feature of pyrimidine that is insufficient for inducing the TADF property. In the presence of macrocyclic donors, these new emitters show high horizontal dipole ratios (Θ// = 85-89%), which are beneficial for improving the light out-coupling efficiency. Deep-blue TADF OLEDs incorporating c-ON-MeTRZ as an emitter doped in the mCPCN host achieves a high maximum external quantum efficiency (EQEmax) of 30.2% together with 1931 Commission Internationale de I'Eclairage (CIE) coordinates of (0.14, 0.13), while the counter device employing c-NS-MeTRZ as a dopant gives EQEmax of 15.4% and CIE coordinates of (0.14, 0.09). The EQEmax of c-ON-MeTRZ- and c-NS-MeTRZ-based devices can be significantly improved to 34.4 and 29.3%, respectively, with a polar host DPEPO, which stabilizes the charge transfer (CT) S1 state to give lower ΔEST for improving the reverse intersystem crossing process. The efficient TADF character, high PLQYs, and high anisotropic emission dipole ratios work together to render the superior electroluminescence (EL) efficiencies. Based on the detailed characterizations of physical properties, theoretical analyses, and comprehensive study on the corresponding devices, a clear structure-property-performance relationship has been successfully established to verify the effective molecular design strategy of modulating the macrocyclic donor characters for efficient deep-blue TADF emitters.

Enhancement of light extraction and out-coupling efficiency of thin: film stacked organic light-emitting diode display

Enhancement of light extraction and out-coupling efficiency of thin: film stacked organic light-emitting diode display

Self-Generated Buried Submicrocavities for High-Performance Near-Infrared Perovskite Light-Emitting Diode

HighlightsSynergistic effect triggers the Ostwald ripening for the downward recrystallization of perovskite to form buried submicrocavities as light output coupler.The simulation suggests the buried submicrocavities can improve the light out-coupling efficiency from 26.8% to 36.2% for near-infrared light.Light-emitting diodes yields peak external quantum efficiency increasing from 17.3% at current density of 114 mA cm−2 to 25.5% at current density of 109 mA cm−2 and a radiance increasing from 109 to 487 W sr−1 m−2 with low rolling-off.

Research on the High Light Out-Coupling Efficiency Deep-Blue Top-Emitting Organic Light-Emitting Diode through FDTD Optical Simulation.

We have studied high light out-coupling efficiency top-emitting organic light-emitting diodes (TOLEDs) under the guidance of the finite-difference time-domain (FDTD) simulation. TOLED achieves an extraordinarily high light extraction efficiency at 468 nm, in deep-blue regions, of 49.70%, which is approximately 3.5 times that of the bottom light-emitting diode (BOLED) by changing the thickness of the organic layer and the position of the light-emitting layer in the FDTD simulation. Based on the simulation results, the TOLED with ultrahigh efficiency and narrow full width at half maximum is successfully fabricated, and the maximum external quantum efficiency of TOLED is almost 3.3 times that of the BOLED, which is perfectly consistent with the FDTD simulation results. Meanwhile, the shift of the electroluminescence spectrum of the TOLED is restricted within 10° in the angular-dependence test (0° to 80°). The optimized performance of the OLED indicates a new method to develop a high-performance device under the guidance of simulation.

Anisotropic emission of orientation-controlled mixed-dimensional perovskites for light-emitting devices

Perovskite light-emitting diodes (PeLEDs) are attracting increasing attention owing to their impressive efficiencies and high luminance across the full visible light range. Further improvement of the external quantum efficiency (EQE) of planar PeLEDs is limited by the light out-coupling efficiency. Introducing perovskite emitters with directional emission in PeLEDs is an effective way to improve light extraction. Here, we report that it is possible to achieve directional emission in mixed-dimensional perovskites by controlling the orientation of the emissive center in the film. Multiple characterization methods suggest that our mixed-dimensional perovskite film shows highly orientated transition dipole moments (TDMs) with the horizontal ratio of over 88%, substantially higher than that of the isotropic emitters. The horizontally dominated TDMs lead to PeLEDs with exceptional high light out-coupling efficiency of over 32%, enabling a high EQE of 18.2%.

Enhanced performance of organic light-emitting diodes by integrating quasi-periodic micro-nano structures

To integrate a quasi-periodic micro-nano structure (PMS) to the organic light-emitting devices (OLEDs) is an efficient way to enhance the performance of OLEDs. In this paper, the PMS prepared by the phase separation of Polystyrene and Poly (methyl methacrylate) was integrated to the OLEDs with the structures of Glass/PMS/Ag (30 nm)/MoO3 (5 nm)/(NPB) (40 nm)/(Alq3) (60 nm)/LiF (0.5 nm)/Al (150 nm). The maximum luminance intensity and external quantum efficiency increased to 10700 cd/m2 and 1.11 %, which is 48 % and 44 % higher than that of 7209 cd/m2 and 0.77 % of the planar reference device. The enhanced performance of OLEDs was ascribed to the attenuation of surface plasmon polariton loss caused by the PMS, which was testified by the Finite-Difference Time-Domain (FDTD) simulation. The PMS was also transferred to the hole transfer layer (PEDOT: PSS) of OLEDs by nano-imprinting lithography with the structure of Glass/(ITO) (100 nm)/PEDOT: PSS (100 nm) (with PMS)/NPB (10 nm)/Alq3 (50 nm)/LiF (0.5 nm)/Al (100 nm). The performance was also improved by the optimized PMS and the light out-coupling efficiency increased to about 49.5 %, which is much higher than that of 28.8 % in the OLEDs with PMS Ag anode and 20 % in the planar reference devices. This suggests that the PMS can improve the OLED device performance regardless of the functional layer in which the PMS is integrated.

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 135.5cd A-1 , 135.7 lm W-1 and 41.3%, with a small roll-off of 9.7% at 5000cdm-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.

Creating efficient delayed fluorescence luminogens with acridine-based spiro donors to improve horizontal dipole orientation for high-performance OLEDs

Developing high-performance organic light-emitting diodes (OLEDs) with high external quantum efficiency (ηext) and low efficiency roll-off is an unremitting goal, which greatly relies on the exploration of robust luminescent materials. Herein, we report a series of new delayed fluorescence luminogens built with acridine-based spiro donors and benzophenone acceptor. These luminogens have distinct merits of excellent thermal stability, aggregation-enhanced delayed fluorescence, high photoluminescence quantum yields, preferable horizontal dipole orientation and balanced bipolar charge transport. The presence of spiro donors endows the luminogens with large horizontal orientation ratios and thus high light out-coupling efficiencies. The OLEDs based on these luminogens radiate intense green light (504–512 nm) and furnish outstanding ηext values of up to 32.2% and small efficiency roll-offs, which are among the state-of-the-art delayed fluorescence OLEDs with ηexts over 30%, demonstrating their great potentials in the fabrication of highly efficient OLEDs.

Highly efficient hybrid light-emitting transistors incorporating MoOx/Ag/MoOx semi-transparent electrodes

Light-emitting transistors (LETs) couple the switching function of a transistor with the light-emitting function of LEDs. We demonstrate improved light out-coupling efficiency in LETs using oxide – metal – oxide type semitransparent electrodes.

Fabrication of cylindrical microlens array using rotatable multi-arrayed needle coating module for light out-coupling of OLEDs

We have performed coatings of poly(methyl methacrylate) (PMMA) using a rotatable multi-arrayed needle coating module to achieve high-density and high-aspect-ratio cylindrical microlens arrays (C-MLAs) for potential applications in organic light-emitting diodes (OLEDs). It is shown that high-density C-MLAs can be fabricated by rotating the coating module in such a way that the spacing between microlenses can be reduced (tuned). To enhance the aspect ratio of C-MLAs, we have analyzed the coating behaviors by varying the material parameters such as the molecular weight of PMMA, solute concentration, and wettability of the PMMA solution. It is demonstrated that high-aspect-ratio C-MLAs can be achieved using a coating solution with a small amount of a hydrophobic solvent, in which a high concentration of low-molecular-weight PMMA is dissolved. To further enhance the aspect ratio, we have come up with a new process method; repetitive coating and pre-drying treatment in ambient air. Such a repetitive coating method is shown to boost the microlens thickness without a much compromise on the microlens width. With this scheme, we have fabricated the C-MLA with the average thickness of 22.6 μm and width of 125 μm (aspect ratio as high as 0.18) at a coating velocity of 3 mm/s. The light out-coupling efficiency of OLED with the C-MLA increases by 20%, which is in good agreement with the result by ray tracing simulations.

Quantum-Rod On-Chip LEDs for Display Backlights with Efficacy of 149lmW-1 : A Step toward 200lmW-1.

Efficient white light-emitting diodes (LEDs) with an efficacy of 200lmW-1 are much desirable for lighting and displays. The phosphor-based LEDs in use today for display applications offer poor color saturation. Intensive efforts have been made to replace the phosphor with quantum-dot-based downconverters, but the efficiency and stability of these devices are still in their infancy. Quantum rods (QRs), nanoparticles with an elongated shape, show superior properties such as relatively larger Stokes shifts, polarized emission, and high light out-coupling efficiency in the solid-state. However, these QRs usually suffer from poor optical quality for PL wavelengths <550nm. Herein, a gradient alloyed CdSe/Znx Cd1- x S/ZnS and CdSe/CdS/ZnS core/shell/shell QR downconverters showing high efficacy LEDs covering a wide color gamut are reported. These QRs show high stability and a precisely tunable photoluminescence peak. The engineered shell thickness suppresses energy transfer and thus maintains the high quantum yield in the solid-state (81%). These QR-based LEDs attain an efficacy of 149lmW-1 (@10mA) andwide color gamut (118% NTSC), which is exceedingly higher than state-of-the-art quantum dots and phosphor-based on-chip LEDs.

Efficient light-emitting diodes based on oriented perovskite nanoplatelets.

Solution-processed planar perovskite light-emitting diodes (LEDs) promise high-performance and cost-effective electroluminescent devices ideal for large-area display and lighting applications. Exploiting emission layers with high ratios of horizontal transition dipole moments (TDMs) is expected to boost the photon outcoupling of planar LEDs. However, LEDs based on anisotropic perovskite nanoemitters remain to be inefficient (external quantum efficiency, EQE <5%) due to the difficulties of simultaneously controlling the orientations of TDMs, achieving high photoluminescence quantum yields (PLQYs) and realizing charge balance in the films of assembled nanostructures. Here, we demonstrate efficient electroluminescence from an in situ grown perovskite film composed of a monolayer of face-on oriented nanoplatelets. The ratio of horizontal TDMs of the perovskite nanoplatelet film is ~84%, which leads to a light-outcoupling efficiency of ~31%, substantially higher than that of isotropic emitters (~23%). In consequence, LEDs with a peak EQE of 23.6% are achieved, representing highly efficient planar perovskite LEDs.

Active Control of Spontaneous Orientation Polarization of Tris(8‐hydroxyquinolinato)aluminum (Alq3) Films and Its Effect on Performance of Organic Light‐Emitting Diodes

AbstractVacuum‐deposited amorphous organic films with an anisotropic amorphous structure are known to exhibit spontaneous orientation polarization (SOP). In this study, amorphous structures of tris(8‐hydroxyquinolinato)aluminum (Alq3) films are controlled by substrate temperature (Tsub) during vacuum deposition. The effect of density, molecular orientation, and SOP of Alq3 films on the performance of organic light‐emitting diodes (OLEDs) is comprehensively investigated. The SOP of Alq3 films is significant at low Tsub and becomes weaker as Tsub increases. The presence of SOP certainly improves electron injection from a metal cathode and, therefore, increases the current in OLEDs. External quantum efficiencies (EQEs) of OLEDs are increased by ≈40% when Tsub for the Alq3 deposition is increased from 260 to 328 K. The increased EQEs are attributed to several factors, which include not only increased photoluminescence quantum yields and light outcoupling efficiencies but also suppressed singlet‐polaron annihilation originating from the reduced SOP. Operational stability of OLEDs is also affected by Tsub and becomes the highest when Alq3 layers are vacuum‐deposited at Tsub = 299 K. These findings indicate the importance of controlling organic amorphous structures and SOP in films to maximize OLED performance and contribute to a better understanding of working mechanisms of OLEDs.

Enhancing the light out-coupling efficiency of organic light-emitting devices with random corrugated structures

The mismatch of refractive index at the interfaces of substrate, electrode/organic material will cause nearly 80% light losses in the organic light-emitting device (OLED). To solve this problem, a light extraction layer with random corrugated structures is fabricated with a quite simple method to enhance the light out-coupling efficiency of OLED. Compared with the planar OLED, about 21% enhancement of current efficiency is achieved in the corrugated OLED. This improvement comes from the scattering effect of polymer phase separation film, which can suppress surface plasmon polariton losses and reduce waveguide mode optical losses. In addition, the structures can decrease the angular dependence of the emission spectra in the planar OLED. With these advantages, the random corrugated structures provide the potential to be applicable to OLED for high-efficiency lighting and full-color display.

Controlling the Chain Orientation and Crystal Form of Poly(9,9-dioctylfluorene) Films for Low-Threshold Light-Pumped Lasers

Neat thin films of semiconducting polymers are attractive as efficient gain media toward optically pumped lasers. However, the optical loss and out-coupling of isotropic polymer thin films are far from being rationally regulated from the perspective of chain orientation and crystal form. Herein, we accomplished a simultaneous control of both chain orientation and crystal form in large-area highly ordered poly(9,9-dioctylfluorene) (PFO) thin films through epitaxial crystallization. The well-arranged PFO lamellae naturally shape a low-loss, graded-index waveguide to provide spatially distributed optical feedback. Moreover, much more horizontally oriented dipoles significantly enhance the light out-coupling efficiency. Thus, the only 65 nm-thickness oriented PFO films demonstrate excellent amplified spontaneous emission with a low excitation threshold (7.1 μJ/cm²) and a narrow full width at half-maximum (2.2 nm) measured in the perpendicular direction of PFO chains. This strategy opens an effective pathway to prepare high-performance polymer thin-film lasers and electrically pumped polymer lasers.