Triplet-triplet Fusion Research Articles

Achieving Narrowband and Stable Pure Blue Organic Light-Emitting Diodes by Employing Molecular Vibration Limited Strategies in the Extended π-Conjugated Indolocarbazole Skeleton.

B- and N-heterocyclic fluorophores have reveal promising efficiency in blue organic light-emitting diodes (OLEDs) with small full-width-at-half-maximum (FWHM). However, their structural determinants for spectral broadening and operating stability are still needed to be investigated in further. Herein, a novel multi-N-heterocycles Diindolo[3,2,1jk:3',2',1'jk]dicarbazole[1,2-b:4,5-b] (DIDCz) is proposed to manipulate the emission color toward pure blue region by extending π-conjugation of the N-π-N bridge. By utilizing computed spectral technique, interrelationships between indolocarbazole (ICz)-cyclization sites and spectral broadening are defined. Molecular backbone modifications involving optimized 2,3,6,7-ICz cyclization and steric hindrance substituents are conducive to restricting swing of peripheral bonds and stretching resonances of the polycyclic aromatic hydrocarbon (PAH) frameworks, thereby contributing to the reduction of shoulder emission peaks. Consequently, the DIDCz-based chromophore exhibited narrowband blue emission with a FWHM of only 15nm, achieving highly efficient (external quantum efficiencies of 8.87% in triplet-triplet fusion fluorescence and 22.3% in sensitized fluorescence) and long-term (95% of the initial luminance of 1000cdm-2, T95 = 1545h) electroluminescence performances, showing one of the most narrowband and stable blue OLEDs among the reported PAH chromophores. The current achievements offer a new perspective to manage spectral broadening precisely based on the molecular vibration limiting technique.

Visualizing the “Hidden” Triplet–Triplet Fusion Process to Fluorescence in Typical Organic/Polymer Light‐Emitting Diodes

AbstractFluorescence emission of a typical poly(9,9‐dialkylfluorene) derivative (PF Green B)‐based polymer light‐emitting diodes (PLEDs) clearly demonstrates that it partially involves the contribution of triplet excitons by the triplet–triplet fusion (TTF) process through measuring the magneto‐electroluminescence (MEL) responses of devices at different bias conditions and temperatures. The TTF process to fluorescence intrinsically correlates with the concentration, lifetime, and polaron quenching of triplet excitons, as modulated by different bias regimes and conditions. Varying the cathode configurations of PLEDs by an additional exciton blocking layer and cathode buffer layers in PLEDs, such as a thin layer of bathocuproine, tetractylammonium bromide, or poly(ethylene glycol) dimethyl ether with metal cathodes, changes the carrier dynamics and regulates the magnitude of TTF process to fluorescence. The performance of the devices has increased due to the enhanced TTF process in fluorescence, as characterized by the measurement of MEL responses. The results in this work unveil the “hidden” component of fluorescence emission, which originates from the fusion of triplet excitons to singlet excitons in typical PLEDs. The results elucidate that TTF of triplet excitons to fluorescence emission can be a practicable mechanism that contributes to enhancing the performance of devices.

Enhancement of lifetime in blue triplet-triplet-fusion organic light-emitting diodes with fluorenyl amine- and triphenyl silane-based electron blocking layers

We report fluorenyl amine-based electron blocking layers (EBLs) N,9,9-triphenyl-N-(4′-(triphenylsilyl)-[1,1′-biphenyl]−4-yl)−9 H-fluoren-2-amine (P-pSi-DPFA), N,9,9-triphenyl-N-(3-(triphenylsilyl)phenyl)−9 H-fluoren-2-amine (P-mSi-DPFA), and N-([1,1′-biphenyl]−4-yl)−9,9-diphenyl-N-(3-(triphenylsilyl)phenyl)−9 H-fluoren-2-amine (BP-mSi-DPFA) that prevent intermolecular interactions with emissive layer (EML). Triplet-triplet fusion (TTF)-based organic light-emitting diodes (OLEDs) use anthracene derivatives as hosts, and due to the electron-rich nature of anthracene core, the recombination zone is predominantly formed at the EBL/EML interface. Therefore, it is important to control the intermolecular interaction between EBL and EML while also having an EBL with suitable hole transport characteristics and an appropriate highest occupied molecular orbital (HOMO) level. We developed TTF-based OLED achieving up to 1489 hours (LT50 at 500 cd/m2) using P-pSi-DPFA as the EBL, securing hole transport characteristics through a fluorenyl amine core, and preventing unwanted intermolecular interactions with the EML by substituting triphenyl silane into the amine core.

Anthracene derivatives with strong spin-orbit coupling and efficient high-lying reverse intersystem crossing beyond the El-Sayed rule.

The attention to materials with hot exciton channel and triplet-triplet fusion (TTF) mediated high-lying reverse intersystem crossing (hRISC) has been raised for their ability to convert non-emissive 'dark' triplets into radiative singlet excitons. This spin conversion process results in high exciton utilization efficiency (EUE) that exceeds the theoretical limits. Notably, it is known that such spin conversion processes from the high-lying excited triplet to the singlet state are facilitated by the orthogonal orbital transition effect governed by the El-Sayed's rule. In this study, an anthracene derivative with indenoquinoline substituent 7,7-dimethyl-9-(10-(4-(naphthalen-1-yl)phenyl)anthracen-9-yl)-7H-indeno[1,2-f]quinoline (2MIQ-NPA) was synthesized and analyzed to investigate whether the hRISC process occurs in these molecules, even when the El-Sayed's rule is not followed. The hRISC channels of the emitter were fully unraveled through DFT calculations and experiments, which were quantitatively subdivided using transient electroluminescence measurements. The results showed that 2MIQ-NPA, which does not follow the El-Sayed's rule and has a relatively strong spin-orbit coupling matrix element of 0.116 cm-1 between the high-lying triplet state of T4 and the lowest singlet state of S1, effectively converted triplet excitons into singlet excitons with an EUE of 64.3%, contributed by a direct hot exciton channel of 19.2% and a TTF-mediated hot exciton channel of 15.1%. Despite the low outcoupling efficiency, the non-doped device with 2MIQ-NPA achieved an excellent device performance with an external quantum efficiency of 7.0%.

Enhanced triplet-triplet fusion for high efficiency and long lifetime of multiresonant pure blue organic light emitting diodes

Multiresonant (MR) narrowband emitters became extensive attention as a blue fluorescent dopant for organic light-emitting diodes (OLEDs) due to their high color purity and large photoluminescence quantum yields (PLQY), but they show poor operational lifetime. Herein, we present a double emissive layer (DEML) architecture to enhance the efficiency and lifetime of pure blue fluorescent OLEDs for MR type emitters. Mainly the DEML are divided into recombination zone and triplet–triplet fusion (TTF) zone based on different triplet energy hosts. Notably, we achieved a tremendous TTF ratio of 32.4% for MR-type dopant, which is close to the theoretical maximum of 37.5%. Furthermore, fabricated fluorescent devices based on DEML structure manifested tremendous maximum external quantum efficiency (EQEmax) of nearly 11% and revealed a longer lifetime (LT95) of 79 h at 1000 cd/m2 due to efficient triplet harvesting, which is almost double compared to the single host device. We believe that our findings can pave the way for enhancement in EQE and lifetime for blue OLEDs.

Numerical Analysis and Optimization of a Hybrid Layer Structure for Triplet–Triplet Fusion Mechanism in Organic Light‐Emitting Diodes

AbstractIn this study, a steady state and time‐dependent exciton diffusion model including singlet and triplet excitons coupled with a modified Poisson and drift‐diffusion solver to explain the mechanism of hyper triplet–triplet fusion (TTF) organic light‐emitting diodes (OLEDs) is developed. Using this modified simulator, various characteristics of OLEDs, including the current‐voltage curve, internal quantum efficiency, transient spectrum, and electric profile are demonstrated. This solver can also be used to explain the mechanism of hyper‐TTF‐OLEDs and analyze the loss from different exciton mechanisms. Furthermore, we perform additional optimization of hyper‐TTF‐OLEDs that increases the internal quantum efficiency by ≈33% (from 29% to 40%).

Benzimidazole-substituted bisanthracene: a highly efficient deep-blue triplet–triplet fusion OLED emitter at low dopant concentration

Two anthracene derivatives, MAnBz and DiAnBz, have been developed as emitters for deep-blue triplet–triplet fusion (TTF) fluorescence organic light emitting diodes (OLEDs). DiAnBz bearing two anthracene units at a closed distance, separated by one phenylene moiety, shows high photoluminescent quantum yields and deep-blue emission. The remarkable maximum external quantum efficiency (EQEmax) of 6.7% and 8.3% are obtained from DiAnBz-OLEDs with pristine prompt fluorescence and TTF fluorescence models, respectively. In particular, effectively diffusive TTF characteristics still can be detected at DiAnBz-OLED with an extreme low emitter concentration of 1 wt%, whose device performance exhibited a high EQEmax of 7.0% with deep blue emission at CIE (0.148, 0.078). The diffusive TTF characteristics with 20 times shorter delayed emission lifetime (τ2 of 3–4 μs), in comparison with that of MAnBz (τ2 of 60–70 μs) in the time-resolved electroluminescence experiments. This implies that pairing and TTF of the DiAnBz triplet excitons are more effective than that of MAnBz in the light emitting layer.

Stimulated triplet–triplet fusion by carrier trap-detrap mechanism in organic light-emitting diodes

Triplet–triplet fusion (TTF) has been an efficiency-enhancing mechanism in fluorescent organic light-emitting diodes (OLEDs) caused by the collision of two triplet excitons. However, achieving a high TTF ratio in fluorescent OLEDs has been difficult despite device strategies to maximize the triplet exciton density within a narrow recombination zone near the electron blocking layer (EBL) due to charge imbalance and hole accumulation between the TTF type emitter and EBL. Based on a trap-detrap mechanism, we were able to realize an improved TTF ratio and reduce hole accumulation by adding a TTF-assisting material (TTF-AM) in the TTF emitter. The TTF-AM served as the hole transport channel, triggering hole trap and detrap while improving the hole transport character of the emitting layer. Through the process of hole detrapping, the improved hole transport properties balanced carriers and generated more triplet excitons in order to activate the TTF mechanism from low to high current density ranges. By adjusting the TTF-AM, the TTF ratio of the anthracene-based emitter was increased from 5.5/20.1% to 13.4/25.5% (low/high current density), thereby resulting in more than doubled external quantum efficiency at low current density.

Nearly 100% Exciton Utilization via Hybridized Inter‐ and Intramolecular Triplet Exciton Harvesting Channels in Blue Fluorescent Organic Light‐Emitting Diodes

AbstractA highly efficient anthracene‐based 5‐(4‐(10‐phenylanthracen‐9‐yl)phenyl)‐5H‐benzofuro[3,2‐c]carbazole (ATDBF) with nearly 100% exciton utilization efficiency is developed for highly efficient nondoped blue organic light‐emitting diodes (OLEDs) through inter‐ and intramolecular triplet exciton up‐converting mechanisms. The ATDBF emitter opens hybridized hot excitons, triplet–triplet fusion (TTF), and mixed TTF/hot exciton channels for triplet exciton to singlet exciton conversion. The molecular design adopting a benzofuro[3,2‐c]carbazole (BFCz) donor manages the singlet and triplet excited states for hybridized triplet exciton up‐conversion mechanisms, which enable a high external quantum efficiency (EQE) of 7.93% in the ATDBF blue device with color coordinates of (0.15, 0.06). This is one of the best efficiencies of the nondoped blue OLEDs with a y color coordinate below 0.06.

Realization of ultra‐high‐efficient fluorescent blue OLED

AbstractThe bilayer structure for an emitting layer (EML) was developed to improve performances of a fluorescence blue organic light emitting diode. By functionally separating the EML into the charge recombination and the triplet–triplet fusion (TTF) zone, we successfully suppressed the quenching of triplet excitons by excess carriers to make more TTF efficient and the local degradation within the EML to make the lifetime longer. In the bottom emission device with the bilayer EML, 12% of external quantum efficiency (EQE) and 450 h of LT95 were achieved. Furthermore, we achieved over 14% of EQE by optimizing the material combinations.

Computational Discovery of TTF Molecules with Deep Generative Models.

We present a computational workflow based on quantum chemical calculations and generative models based on deep neural networks for the discovery of novel materials. We apply the developed workflow to search for molecules suitable for the fusion of triplet-triplet excitations (triplet-triplet fusion, TTF) in blue OLED devices. By applying generative machine learning models, we have been able to pinpoint the most promising regions of the chemical space for further exploration. Another neural network based on graph convolutions was trained to predict excitation energies; with this network, we estimate the alignment of energy levels and filter molecules before running time-consuming quantum chemical calculations. We present a comprehensive computational evaluation of several generative models, choosing a modification of the Junction Tree VAE (JT-VAE) as the best one in this application. The proposed approach can be useful for computer-aided design of materials with energy level alignment favorable for efficient energy transfer, triplet harvesting, and exciton fusion processes, which are crucial for the development of the next generation OLED materials.

Pulsed excitation method provides path forward for tracing triplet excitons in OLEDs

Measurements of angle-resolved delayed electroluminescence in the setting of triplet-triplet fusion use singlet excitons to study the kinetics of nonemissive triplets.

Achieving High Efficiency at High Luminance in Fluorescent Organic Light-Emitting Diodes through Triplet-Triplet Fusion Based on Phenanthroimidazole-Benzothiadiazole Derivatives.

Achieving high efficiency at high luminance is one of the most important prerequisites towards practical application of any kind of light-emitting diode (LED). Herein, we report highly emissive organic fluorescent molecules based on phenanthroimidazole-benzothiadiazole derivatives capable of maintaining high external quantum efficiency (EQE) at high luminance enabled by triplet-triplet fusion (TTF) in doped organic LEDs. The PIBzP-, PIBzPCN-, and PIBzTPA-based devices showed EQEs of 8.27, 9.15, and 8.64 %, respectively, at luminance of higher than 1000 cd m-2 , with little efficiency roll-off.

Exploiting novel electron-deficient moiety 2,5-diazarcarbazole to functionally construct DPA-containing electron transporting materials for highly efficient sky-blue fluorescent OLEDs

A span-new electron-deficient moiety 2,5-diazarcarbazole (25NCz) with high T1 = 2.77 eV was firstly designed and synthesized, which had enormous potential to construct organic electronic materials in photoelectric field. Herein, 25NCz as large rigid-type periphery functionalized group, was introduced to develop novel electron transporting materials (ETMs) by grafting large rigid-type π-conjugated core anthracene. Two novel ETMs p-S25NCzDPA and p-D25NCzDPA were designed and developed. Due to the dual characteristics of rigidity and electron-accepting of the 25NCz, as expected, these two ETMs possessed good thermal stability and high electron mobility. When triplet-triplet fusion (TTF) sky-blue fluorescent OLEDs based on these two novel ETMs were manufactured, they exhibited superior performances. In particular, the maximum external quantum efficiency (EQE) reached 7.45% and the EQE kept 7.45% at the luminance of 50,000 cd m−2 for p-S25NCzDPA based device. This work rationally demonstrated that the significance of newly developed electron-deficient moiety 25NCz in designing ETMs for high-performance OLEDs.

Design Strategy of Anthracene-Based Fluorophores toward High-Efficiency Deep Blue Organic Light-Emitting Diodes Utilizing Triplet-Triplet Fusion.

In contrast to the red and green regions, conventional fluorescent emitters continue to serve as blue emitters in commercialized organic light-emitting diodes. Many researchers have studied anthracene moieties as blue emitters, given their appropriate energy levels and good emission properties. We herein report two new deep blue-emitting anthracene derivatives that include p-xylene as moieties connecting the anthracene cores to side groups. We enhanced the efficiency by maximizing triplet-triplet fusion (TTF) without sacrificing emission color. The large steric hindrance imposed by the methyl groups of p-xylene creates a perpendicular geometry between p-xylene and the neighboring aromatic rings. Any extension of π-conjugation is thus disrupted, and the isolated core anthracene moiety emits a deep blue color with a high photoluminescence quantum yield. Moreover, the extensive steric hindrance suppresses vibration and rotation because the molecules are rigid. The high horizontal dipole ratio attributable to the large aspect ratio increases the outcoupling efficiency of the emitted light. Furthermore, the charge mobility and triplet harvesting ability are enhanced by decreasing the bulkiness of the side groups. Molecular dynamics simulation revealed that the bulkiness of the side group significantly impacted molecular density, which in turn affected the charge transport and TTF. We used two molecules, 2PPIAn (containing a phenyl side group) and 4PPIAn (containing a terphenyl side group), to form nondoped emission layers that exhibited maximum external quantum efficiencies of 8.9 and 7.1% with Commission Internationale de L'Eclairage coordinates of (0.150, 0.060) and (0.152, 0.085), respectively.

Unveiling the Root Cause of the Efficiency-Lifetime Trade-Off in Blue Fluorescent Organic Light-Emitting Diodes

The origin of efficiency-lifetime trade-off in triplet–triplet fusion (TTF) type blue fluorescent organic light-emitting diodes (OLEDs) was investigated and the device structure to resolve the issue was developed. The efficiency and lifetime were simultaneously improved in the blue OLEDs by developing a multilayer hole transport stack which can adjust carrier densities and recombination zone in the emitting layer (EML). It was found that electron leakage from EML and high spatial density of excitons in the vicinity of the electron blocking layer for high TTF rates by narrow recombination zone are the detrimental factors for efficiency-lifetime trade-off. A multilayer hole transport stack employing a deep highest occupied molecular orbital hole transport layer and an electron blocking layer combined with an appropriate hole blocking layer simultaneously improved the power efficiency by 16% at 500 cd/m2 and lifetime by almost 100% (from 73 h up to 145 h). In addition, the low efficiency in the low luminance region was also completely controlled, resulting in negligible efficiency variation in the entire luminance range.

Highly Efficient Organic Blue Electroluminescent Materials and Devices with Mesoscopic Structures.

Due to the difficulty in achieving high efficiency and high color purity simultaneously, blue emission is the limiting factor for the performance and stability of OLEDs. Since 2003, we have been working on organic light-emitting diodes (OLEDs), especially on blue light. After a series of molecular designs, novel strategies have been proposed from different aspects. At first, highly efficient deep blue emission could be achieved through molecular design with highly twisted structure to suppress fluorescence quenching and redshift. Deep blue emitters with high efficiency in solid state, a twisted structure with aggregation induced emission (AIE) characteristics was incorporated to inhibit molecular aggregation, and triplet-triplet fusion (TTF) and hybridized localized charge transfer (HLCT) were adopted to increase the ratio of triplet exciton used. Secondly, a highly efficient blue OLED could be achieved through improving charge transport. New electron transport materials (ETMs) with wide band gap were developed to control charge transport balance in devices. Thirdly, a highly efficient deep blue emission could be achieved through a mesoscopic structure of out-coupling layer. A mesoscopic photonic structured organic thin film was fabricated on the top of metal electrode by self-aggregation in order to improve the light out-coupling efficiency.

Blue fluorescent OLED materials and their application for high-performance devices

The authors applied two technologies to improve the efficiency of fluorescent blue organic light-emitting diodes (OLEDs). First, an efficiency-enhancement layer (EEL) was introduced to boost triplet–triplet fusion (TTF). Second, new blue dopants with a higher orientation factor in the emitting layer were developed. Consequently, the external quantum efficiency (EQE) was increased up to 11.5% with Commission Internationale de l’Eclairage (CIE) 1931 color coordinates of (0.138, 0.092). The reported results may lead to EQEs that exceed 14% with fluorescent blue emitters.

Direct Imaging of Anisotropic Exciton Diffusion and Triplet Diffusion Length in Rubrene Single Crystals

We visualize exciton diffusion in rubrene single crystals using localized photoexcitation and spatially resolved detection of excitonic luminescence. We show that the exciton mobility in this material is strongly anisotropic with long-range diffusion by several micrometers associated only with the direction of molecular stacking in the crystal, along the b axis. We determine a triplet exciton diffusion length of 4.0 ± 0.4 μm from the spatial exponential decay of the photoluminescence that originates from singlet excitons formed by triplet-triplet fusion.

Effect of an External Magnetic Field on the Up-Conversion Photoluminescence of Organic Films: The Role of Disorder in Triplet-Triplet Annihilation

We have investigated the influence of the magnetic field on the triplet-triplet annihilation process in organic films using a model multicomponent system for blue delayed up-conversion photoluminescence. In such a way, we have derived simple analytical expressions to estimate the overall annihilation probability, outlining the peculiar role played by the disorder and demonstrating that the triplet-triplet fusion in solid films is a diffusion limited process.