Quantitative Analysis of the Efficiency of OLEDs

Hybridized local and charge transfer dendrimers with near-unity exciton utilization for enabling high-efficiency solution-processed hyperfluorescent OLEDs

Achieving both high emission efficiency and exciton utilization efficiency (ηS) in hot exciton materials is still a formidable task. Herein, a proof-of-concept design for improving ηS in hot exciton materials is proposed via elaborate regulation of singlet-triplet energy difference, leading to an additional thermally activated delayed fluorescence (TADF) process. Two novel dendrimers, named D-TTT-H and D-TTT-tBu, were prepared and characterized, in which diphenylamine derivatives were used as a donor moiety and tri(triazolo)triazine (TTT) as an acceptor fragment. Compounds D-TTT-H and D-TTT-tBu showed an intense green color with an emission efficiency of approximately 80% in solution. Impressively, both dendrimers simultaneously exhibited a hot exciton process and TADF characteristic in the solid state, as was demonstrated via theoretical calculation, transient photoluminescence, magneto-electroluminescence and transient electroluminescence measurements, thus achieving almost unity ηS. A solution processable organic light-emitting diode (OLED) employing the dendrimer as a dopant represents the best performance with the highest luminance of 15090 cd m-2 and a maximum external quantum efficiency (EQEmax) of 11.96%. Moreover, using D-TTT-H as a sensitizer, an EQEmax of 30.88%, 24.08% and 14.33% were achieved for green, orange and red solution-processed OLEDs, respectively. This research paves a new avenue to construct a fluorescent molecule with high ηS for efficient and stable OLEDs.

The Role of Balancing Carrier Transport in Realizing an Efficient Orange-Red Thermally Activated Delayed-Fluorescence Organic Light-Emitting Diode

Simultaneously realizing improved carrier mobility and good photoluminescence (PL) efficiency in red thermally activated delayed-fluorescence (TADF) emitters remains challenging but important. Herein, two isomeric orange-red TADF emitters, oPDM and pPDM, with the same basic donor-acceptor backbone but a pyrimidine (Pm) attachment at different positions are designed and synthesized. The two emitters show similarly good PL properties, including narrow singlet-triplet energy offsets (0.11 and 0.15 eV) and high photoluminescence quantum yields (ca. 100 and 88%) in doped films. An orange-red organic light-emitting diode (OLED) employing oPDM as an emitter achieves an almost twice as high maximum external quantum efficiency (28.2%) compared with that of a pPDM-based OLED. More balanced carrier-transporting properties are responsible for their contrasting device performances, and the position effect of the Pm substituent leads to significantly distinct molecular packing behaviors in the aggregate states and different carrier mobilities.

A Thermally Activated Delayed Fluorescence Green OLED with 4500 h Lifetime and 20% External Quantum Efficiency by Optimizing the Emission Zone using a Single-Emission Spectrum Technique

Device optimization of light-emitting diodes (LEDs) targets the most efficient conversion of electrically injected charges into emitted light. The emission zone in an LED is where charges recombine and light is emitted from. It is believed that the emission zone is strongly linked to device efficiency and lifetime. However, the emission zone size is below the optical diffraction limit, so it is difficult to measure. An accessible method based on a single emission spectrum that enables emission zone measurements with sub-second time resolution is shown. A procedure is introduced to study and control the emission zone of an LED system and correlate it with device performance. A thermally activated delayed fluorescence organic LED emission zone is experimentally measured over all luminescing current densities, while varying the device structure and while ageing. The emission zone is shown to be finely controlled by emitter doping because electron transport via the emitter is the charge-transport bottleneck of the system. Suspected quenching/degradation mechanisms are linked with the emission zone changes, device structure variation, and ageing. Using these findings, a device with an ultralong 4500 h T₉₅ lifetime at 1000 cd m^-2 with 20% external quantum efficiency is shown.

Research Progress on Triarylmethyl Radical-Based High-Efficiency OLED

Perchlorotrityl radical (PTM), tris (2,4,6-trichlorophenyl) methyl radical (TTM), (3,5-dichloro-4-pyridyl) bis (2,4,6 trichlorophenyl) methyl radical (PyBTM), (N-carbazolyl) bis (2,4,6-trichlorophenyl) methyl radical (CzBTM), and their derivatives are stable organic radicals that exhibit light emissions at room temperature. Since these triarylmethyl radicals have an unpaired electron, their electron spins at the lowest excited state and ground state are both doublets, and the transition from the lowest excited state to the ground state does not pose the problem of a spin-forbidden reaction. When used as OLED layers, these triarylmethyl radicals exhibit unique light-emitting properties, which can increase the theoretical upper limit of the OLED's internal quantum efficiency (IQE) to 100%. In recent years, research on the luminescent properties of triarylmethyl radicals has attracted increasing attention. In this review, recent developments in these triarylmethyl radicals and their derivatives in OLED devices are introduced.

Sub-turn-on exciton quenching due to molecular orientation and polarization in organic light-emitting devices

The efficiency of organic light-emitting devices (OLEDs) is often limited by roll-off, where efficiency decreases with increasing bias. In most OLEDs, roll-off primarily occurs due to exciton quenching, which is commonly assumed to be active only above device turn-on. Below turn-on, exciton and charge carrier densities are often presumed to be too small to cause quenching. Using lock-in detection of photoluminescence, we find that this assumption is not generally valid; luminescence can be quenched by >20% at biases below turn-on. We show that this low-bias quenching is due to hole accumulation induced by intrinsic polarization of the electron transport layer (ETL). Further, we demonstrate that selection of nonpolar ETLs or heating during deposition minimizes these losses, leading to efficiency enhancements of >15%. These results reveal design rules to optimize efficiency, clarify how ultrastable glasses improve OLED performance, and demonstrate the importance of quantifying exciton quenching at low bias.

The improved performance and mechanism of solution-processed blue PhOLEDs based on double electron transport layers

Performance improved solution-processed blue phosphorescent organic light emitting diodes (PhOLEDs) are demonstrated by adopting a double electron transport layer (ETL) strategy, which consists of TPBi and an additional Alq₃ ETL. With the help of Alq₃ ETL, the performance of the optimal device with a double ETL is significantly enhanced. The maximum luminance of OLEDs is improved from 6787 cd m^-2 to 13 054 cd m^-2, and the maximum current efficiency is increased from 3.9 cd A^-1 to 11.4 cd A^-1. Furthermore, the difference of carrier injection in the two types of PhOLEDs is explored by using the transient electroluminescence measurement method. The results imply that double ETL can help to balance electron injection and carrier transport, reduce the interface charge accumulation, leading to a high efficiency. The PL decay of the emission layer with different ETL is detected to analyze the effect of the introduced second ETL layer and the interface on the exciton decay of the emission layer. The results show that the introduced interface in devices with a double ETL has an adverse effect on the exciton emission, which contributes to the serious efficiency roll-off of devices with a double ETL.

Triplet Harvesting by a Fluorescent Emitter Using a Phosphorescent Sensitizer for Blue Organic-Light-Emitting Diodes

The development of highly efficient blue organic light-emitting diodes (OLEDs) with good stability is currently the most important issue in OLED displays and lighting. This paper reports an efficient blue fluorescent OLED based on a deep-blue-emitting phosphorescent sensitizer [(dfpysipy)₂Ir(mpic)] and a conventional fluorescent emitter (TBPe). Efficient triplet harvesting by the fluorescent emitter occurs in the OLED because of sensitization even though the difference in the emission energy between the phosphorescent and fluorescent emissions was only 0.05 eV. These results clearly demonstrate the potential for realizing highly efficient blue fluorescent OLEDs using phosphorescent sensitizers without requiring ultraviolet-emitting phosphorescent dye.

Analysis of the Bias-Dependent Split Emission Zone in Phosphorescent OLEDs

From s-polarized, angle-dependent measurements of the electroluminescence spectra in a three-layer phosphorescent organic light-emitting diode, we calculate the exciton distribution inside the 35 nm thick emission layer. The shape of the exciton profile changes with the applied bias due to differing field dependencies of the electron and hole mobilities. A split emission zone with high exciton densities at both sides of the emission layer is obtained, which is explained by the presence of energy barriers and similar electron and hole mobilities. A peak in the transient electroluminescence signal after turn-off and the application of a reverse bias is identified as a signature of a split emission zone.

Enhanced light out-coupling efficiency of quantum dot light emitting diodes by nanoimprint lithography

Extracting light from quantum dot light emitting diodes (QLEDs) by applying optical-functional nanostructures inside and outside the devices is essential for their commercial application in illumination and displays. In this paper, we demonstrate the highly effective extraction of waveguided light from the active region of QLEDs by embedding internal grating patterns fabricated using a nanoimprint lithography technique. The grating couples out waveguide mode power into the substrate without changing the device's electrical properties, resulting in an increase in both the external quantum efficiency and luminous efficiency for a green QLED from 11.13% to 13.45%, and 29 010 cd m-2 to 44 150 cd m-2, respectively. The observed improvement can be ascribed to the elimination of the waveguide mode by the grating nanostructures introduced in the device. Furthermore, the finite-difference time-domain (FDTD) simulation also demonstrated that the power loss due to the waveguide mode was reversed. The results indicate that internal nano-scattering pattern structures are attractive for enhancing the out-coupling efficiency of QLEDs.

High light-quality OLEDs with a wet-processed single emissive layer

High light-quality and low color temperature are crucial to justify a comfortable healthy illumination. Wet-process enables electronic devices cost-effective fabrication feasibility. We present herein low color temperature, blue-emission hazards free organic light emitting diodes (OLEDs) with very-high light-quality indices, that with a single emissive layer spin-coated with multiple blackbody-radiation complementary dyes, namely deep-red, yellow, green and sky-blue. Specifically, an OLED with a 1,854 K color temperature showed a color rendering index (CRI) of 90 and a spectrum resemblance index (SRI) of 88, whose melatonin suppression sensitivity is only 3% relative to a reference blue light of 480 nm. Its maximum retina permissible exposure limit is 3,454 seconds at 100 lx, 11, 10 and 6 times longer and safer than the counterparts of compact fluorescent lamp (5,920 K), light emitting diode (5,500 K) and OLED (5,000 K). By incorporating a co-host, tris(4-carbazoyl-9-ylphenyl)amine (TCTA), the resulting OLED showed a current efficiency of 24.9 cd/A and an external quantum efficiency of 24.5% at 100 cd/m². It exhibited ultra-high light quality with a CRI of 93 and an SRI of 92. These prove blue-hazard free, high quality and healthy OLED to be fabrication feasible via the easy-to-apply wet-processed single emissive layer with multiple emitters.

Manipulating Refractive Index in Organic Light-Emitting Diodes

In a conventional organic light-emitting diode (OLED), only a fraction of light can escape to the glass substrate and air. Most radiation is lost to two major channels: waveguide modes and surface plasmon polaritons. It is known that reducing the refractive indices of the constituent layers in an OLED can enhance light extraction. Among all of the layers, the refractive index of the electron transport layer (ETL) has the largest impact on light extraction because it is the layer adjacent to the metallic cathode. Oblique angle deposition (OAD) provides a way to manipulate the refractive index of a thin film by creating an ordered columnar void structure. In this work, using OAD, the refractive index of tris(8-hydroxyquinoline)aluminum (Alq3) can be tuned from 1.75 to 1.45. With this low-index ETL deposited by OAD, the resulting phosphorescent OLED shows nearly 30% increase in light extraction efficiency.