Blue Dopant Research Articles

P‐253: Late‐News Poster: The Evaluation on the Stability of Electroluminescent Materials Using Polaron Pair Formation Kinetics

We developed the method for evaluation of the stability of electroluminescent materials by using QEMS (quantum efficiency measurement system). Relative PL (photoluminescence) intensity decay of BDs (blue dopants) with BHs (blue hosts) in solution under UV light irradiation were measured and the rate constants for polaron pair formation by electron transfer were determined through first‐order kinetics analyses. The rate constants of polaron pair formation (6.487E‐04 –1.805E‐03 min‐1 ) were inversely proportional to the lifetime of OLED (organic light‐emitting diode) devices. We expect that this evaluation method can be applied to the development of new OLED materials and longer lifetime OLED devices.

Non-conventional exciplex system by leveraging zero-radius of intramolecular energy transfer mechanism: Seamless integration of p-host and fluorescent dopant for highly efficient OLEDs

A multi-component emissive layer (EML) composed of a mixture of p- and n-type hosts and a dopant is a widely used structure in organic light-emitting diodes (OLEDs) due to its high efficiency and operational stability. However, multi-component EML devices impose limitations such as difficulty in accurately controlling the mixing ratio and profiles in the manufacturing process, high costs, and complexity in the photophysical properties of the components.In this paper, a p-type host fused with a blue fluorescent dopant, 5,9-di([1,1′-biphenyl]-2-yl)-2′-(9H-carbazole-9-yl)3,7-dimethyl-5,5a,5a1,9,9a,16a-hexahydrospiro[5,9-diaza-16b-boraindeno[2,1-b]naphtho[1,2,3-fg]anthracene-11,9′-fluorene] (DABNA-Spiro-Cz), was designed and successfully synthesized to construct a novel EML structure. Completely fused within DABNA-Spiro-Cz, the p-type host and blue fluorescent dopant can independently perform their respective roles. An exciplex is formed between n-type host and p-type host unit in the DABNA-Spiro-Cz molecule and its energy is effectively transferred to the blue fluorescent dopant unit in DABNA-Spiro-Cz. In other words, a degree of freedom is earned from a conventional 3-component exciplex-dopant system. A high external quantum efficiency of 14.9 % was achieved thanks to “zero-radius of intramolecular energy transfer in exciplex (ZETPLEX)” system, which is completely non-conventional and novel. We believe the ZETPLEX mechanism opens a new era of designing emissive materials and paves a new way of constructing a novel EML and device architecture for highly efficient OLEDs.

Modified t-butyl in tetradentate platinum (II) complexes enables exceptional lifetime for blue-phosphorescent organic light-emitting diodes

In blue phosphorescent dopants, the tetradentate platinum(II) complex is a promising material showing high efficiency and stability in devices. However, metal-metal-to-ligand charge transfer (MMLCT) formation leads to low photo-luminescence quantum yields (PLQYs), wide spectra, and intermolecular interaction. To suppress MMLCT, PtON-tb-TTB and PtON-tb-DTB are designed using theoretical simulation by modifying t-butyl in PtON-TBBI. Both materials effectively suppress MMLCT and exhibit high PLQYs of 99% and 78% in 5 wt% doped film, respectively. The PtON-tb-TTB and PtON-tb-DTB devices have maximum external quantum efficiencies of 26.3% and 20.9%, respectively. Additionally, the PtON-tb-DTB device has an extended lifetime of 169.3 h with an initial luminescence of 1200 nit, which is 8.5 times greater than the PtON-TBBI device. Extended lifetime because of suppressed MMLCT and smaller displacement between the lowest triplet and triplet metal-centered states compared to other dopants. The study provides an effective approach to designing platinum(II) complexes for long device lifetimes.

Improving high efficiency and device lifetime of blue emitters with 3H-benzo[cd]pyrene core and optimized bulky side groups

We designed and synthesized two blue fluorescent dopants, DMP-BP and DBF-BP, by introducing alkylated N-phenylaniline and alkylated dibenzo[b,d]furan-amine as side groups, based on the pyrene chromophore, 3H-benzo[cd]pyrene (BP) core.

Hybridization of short-range and long-range charge transfer excited states in multiple resonance emitter

Multiple resonance (MR) thermally activated delayed fluorescence emitters have been actively studied as pure blue dopants for organic light-emitting diodes (OLEDs) because of excellent color purity and high efficiency. However, the reported MR emitter, 2,5,13,16-tetra-tert-butylindolo[3,2,1-jk]indolo[1′,2′,3′:1,7]indolo[2,3-b]carbazole (tDIDCz) based on bis-fused indolocarbazole framework could not demonstrate efficient triplet-to-singlet spin crossover. In this work, we report two isomeric MR emitters designed to promote triplet exciton harvesting by reconstructing the electronic structure of tDIDCz. To manage excited states, strong electron donors were introduced at the 2,5-/1,6-position of tDIDCz. As a result, 2,5-positions managed tDIDCz shows long-range charge transfer characteristics while preserving the MR nature. Quantum chemical calculation demonstrates direct spin-orbit coupling by long-range charge transfer and spin-vibronic coupling assisted reverse intersystem crossing by short-range charge transfer simultaneously contribute to triplet-to-singlet spin crossover. Consequently, high performance blue OLED recorded a high external quantum efficiency of 30.8% at a color coordinate of (0.13, 0.13).

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.

In Search of Hosts for Blue OLEDs: Computational Design and Experimental Validation.

Considering the difficulties associated with the conventional 'trial and error' method for a complete analysis of a giant molecular space, we took the aid of computational pathway (DFT) in screening a large space search of 780 (12×13×5) molecules to search for a host for the blue emitter. The selection process was completed in three Tiers with the conditions of highest theoretical triplet energy (>2.81 eV), aligned HOMO/LUMO levels w.r.t blue dopant (FIrpic), and position of substituents to meet the optimal requirements as host materials. Tier 1 screened twelve different imidazole heterocycle derivatives as base space groups which resulted in the selection of 4,5-diphenyl-1H-imidazole. Tier 2 process converged the search to mCN-CZ having the highest triplet energy and appropriate HOMO/LUMO level relative to FIrpic and ETL. Further, the carbazole of mCN-CZ was replaced with different aromatic hydrocarbons to find the other best compound in terms of triplet energy and HOMO/LUMO. Tier 3 resulted in another promising candidate (mCN-FL) as possible host materials. The band alignment with guest predicted mCN-FL and mCN-CZ to have optimal device performances compared to CZ-CZ and the experimentally observed device performance was in accordance with virtual screening results when TAPC was utilized as the hole transporter. The device results of mCN-CZ and mCN-FL were better than the reference host TCTA. The obtained results thus proved that a virtual screening process will be a useful tool for synthetic chemists in designing task-specific materials.

Realization of Highly Efficient InP Quantum Dot Light‐Emitting Diodes through In‐Depth Investigation of Exciton‐Harvesting Layers

AbstractQuantum dot light‐emitting diodes (QLEDs) are considered promising candidates for several optoelectronic applications; however, they are plagued by the over‐injection of electrons compared to holes, which limits device efficiency. Studies have attempted to reuse the leaked electrons and transfer recombination energies via inserting an exciton‐harvesting layer (EHL) between the emissive layer (EML) and hole transport layer (HTL). This study conducts a detailed analysis of the energy transfer mechanisms to obtain better insights into improving the device performance. First, by analyzing the electroluminescence (EL) spectra and exciton dynamics, the effect of EHLs comprising phosphorescence (PH) or thermally activated delayed fluorescence (TADF) blue dopant is compared. Through parallel incorporation of those EHLs on QLEDs and organic LEDs, the minimal contribution of the PH‐EHL to energy transfer in QLEDs is confirmed, whereas the TADF‐EHL has a significant contribution. Second, highly efficient top‐emission green QLEDs with the TADF‐EHL are achieved. They exhibit a maximum luminance (L) and current efficiency (CE) of 40700 cd m−2 and 68.0 cd A−1, respectively, which are the highest among the reported values for green‐emitting InP QLEDs. The proposed approaches are expected to provide aid in the realization of highly efficient QLEDs from the analysis to the device optimization stage.

The Design and Progress of Organic Light-Emitting Diode

Organic light emitting diode (OLED) is one of the main lighting devices and can be used as mobile phone screens, so its performance enhancement is worthy of an in-depth study. This paper discusses three types of devices with different light emitting principles in the history of OLED development in chronological order and their performances, as well as ways to enhance their external quantum efficiency (EQE). For fluorescent, phosphorescent and thermally activated delayed fluorescent (TADF) OLEDs, adding additional layers and doping are effective means of improving the performance. For fluorescent OLED, the EQE can be increased up to 11.5% by adding an efficiency enhancement layer and doping the emitting layer with a new blue dopant with a higher orientation coefficient. For phosphorescent OLED, a hole transport layer is utilized to block excitons within the FIrpic-doped emissive layer leading to an EQE of 16.7%. For TADF OLED, the soluble doped TADF OLEDs is helpful at improving the quantum efficiency up to 18.3%. This paper looks forward to the maturation of these strategies and their practical application, and the identification of more technologies that can enhance the performance of OLED devices to help make it more usable.

High-efficiency blue-emission crystalline organic light-emitting diodes sensitized by “hot exciton” fluorescent nanoaggregates

Sensitizing fluorescent materials is an effective way to maximally use excitons and obtain high-efficiency blue organic light-emitting diodes (OLEDs). However, it is a persistent challenge for present amorphous thin-film OLEDs to improve photon emission under low driving voltage, severely impeding the development of OLED technology. Here, we propose a novel OLED architecture consisting of a crystalline host matrix (CHM) and embedded "hot exciton" nanoaggregates (HENAs), which effectively sensitize blue dopant (D) emission. Owing to the advantages of the crystalline thin-film route, the device exhibits largely enhanced blue photon output [Commission International de L'Eclairage coordinates of (0.15, 0.17)], with a low turn-on/operation voltage of 2.5 V (at 1 cd/m2)/3.3 V (at 1000 cd/m2), an extremely low Joule heat loss ratio (7.8% at 1000 cd/m2), and a maximum external quantum efficiency (EQE) up to 9.14%. These areal photon output features have outperformed the present amorphous thin-film blue OLEDs with high EQE, demonstrating that the CHM-HENA-D OLED is promising for future OLEDs.

Study on charge dynamics and recombination process via transient electroluminescence analysis in blue fluorescence organic light-emitting diodes

Fluorescent organic light-emitting diodes (FOLEDs) exhibit substantially different efficiency-lifetime characteristics depending on dopant materials even in the same host material. Thus, obviously charge and exciton dynamics and recombination process may vary depending on the dopants. However, an accurate understanding of charge and exciton dynamics with dopants in FOLEDS is lacking so far. Herin, triplet–triplet-fusion-based FOLEDs with two blue dopants, 2,5,8,11-tetra-tert-butylperylene (TBPe) and 2,12-di-tert-butyl-5,9-bis(4-(tert-butyl)phenyl)-5,9-dihydro-5,9-diaza-13b-boranaphtho[3,2,1-de] anthracene (t-DABNA) were investigated via transient electroluminescence (TrEL). Interestingly, the TrEL of the blue FOLEDs elucidated different charge profiles and recombination processes within the emission layers. The TrEL spikes after pulse-off and the characteristic times of EL intensity after pulse-on revealed the accumulated charge profile concerning the charge balance. In temperature-dependent TrEL, the delayed fluorescence, exciton decay time, and EL spikes after pulse-on reflected the emission mechanisms as well. The results showed that the electrical properties and the device performances of blue FOLEDs can be significantly impacted by the alignment of the energy levels of hosts and dopants, even with a low doping concentration. Furthermore, depending on the charge trap conditions, it was found that efficiency roll-off can be attributed to Dexter energy transfer from host to dopant and singlet–triplet annihilation by direct recombination on dopant molecules.

Improving excitons utilization of blue thermally activated delayed fluorescence emitter to achieve highly efficient solution-processed hybrid white organic light-emitting diodes

5-(5,9-dioxa-13b-boranaphtho[3,2,1-de]anthracen-7-yl)-10,15-diphenyl-10,15-dihydro-5H-diindolo[3,2-a:3′,2′-c]carbazole (DBA-DI) with high triplet energy was utilized as efficient blue emission dopant to construct solution-processed thermally activated delayed fluorescence-phosphorescence (TADF-phosphorescence) hybrid white organic light-emitting diodes (WOLEDs). Meanwhile, a series of TADF-phosphorescence hybrid WOLEDs composed of bis(2-phenylquinoline) (2,2,6,6-tetramethylheptane-3,5-dionate)iridium(III) (PQ2Ir(dpm)), tris(2-(4-tolyl)-phenylpyridine)iridium (Ir(mppy)3) and DBA-DI are successfully developed by introducing the co-host system consist of poly(N-vinylcarbazole) (PVK) and 2,6-bis(3-(9H-carbazol-9-yl)phenyl)pyridine (26DCzPPy) with high color rendering index (CRI) and improving the utilization of triplet excitons. Furthermore, energy transfer processes among different emitters were investigated in detail based on absorbance spectra, photoluminescence and transient photoluminescence spectra. Ultimately, the Commission Internationale de I'Eclairage coordinates of the optimum solution-processed WOLEDs in warm-white emission region are (0.41, 0.41) and high CRI is 82. Furthermore, the same device realized outstanding efficiencies of 32.14 cd A−1, 21.00 lm W−1 and 15.1% at high brightness of 1000 cd m−2.

Highly Efficient Solution-Processed Blue Phosphorescent Organic Light-Emitting Diodes Based on Co-Dopant and Co-Host System.

The low-lying HOMO level of the blue emitter and the interfacial miscibility of organic materials result in inferior hole injection, and long exciton lifetime leads to triplet-triplet annihilation (TTA) and triplet-polaron annihilation (TPA), so the efficiencies of blue phosphorescent organic light-emitting diodes (PhOLEDs) are still unsatisfactory. Herein, we design co-host and co-dopant structures to improve the efficiency of blue PhOLEDs by means of solution processing. TcTa acts as hole transport ladder due to its high-lying HOMO level, and bipolar mCPPO1 helps to balance carriers’ distribution and weaken TPA. Besides the efficient FIr6, which acts as the dominant blue dopant, FCNIrPic was introduced as the second dopant, whose higher HOMO level accelerates hole injection and high triplet energy facilitates energy transfer. An interesting phenomenon caused by microcavity effect between anode and cathode was observed. With increasing thickness of ETL, peak position of electroluminescence (EL) spectrum red shifts gradually. Once the thickness of ETL exceeded 140 nm, emission peak blue-shifts went back to its original position. Finally, the maximum current efficiency (CE), power efficiency (PE), and external quantum efficiency (EQE) of blue phosphorescent organic light-emitting diode (PhOLED) went up to 20.47 cd/A, 11.96 lm/W, and 11.62%, respectively.

Molecular design of blue thermally activated delayed fluorescent emitters for high efficiency solution processable OLED via an intramolecular locking strategy

Realizing high-efficiency blue emission in solution processable organic light-emitting diode (OLED) with thermally activated delayed fluorescent (TADF) emitters is still a big challenge. To suppress the non-radiative process in TADF emitters, compound DPS-BF-Ac featuring a donor-π-acceptor skeleton is prepared via intramolecular cyclization, in which diphenylsulfone (DPS), acridine (Ac) and furan derivatives are regarded as the donor, acceptor and π linker, respectively. Then, compounds DPS-Ph-Ac, DPS-OMe-Ac and DPS-OH-Ac with different π linkers are investigated to further explore the molecular structure–property relationship. All compounds show promising blue emission with a clear TADF character. Single crystal analysis demonstrate that compound DPS-BF-Ac possesses almost a perpendicular geometry between donor and acceptor groups and expanded molecular rigidity, leading to a smaller singlet–triplet energy difference and greatly improved emission efficiency. A remarkable external quantum efficiency (EQEmax) of ∼ 25 % is achieved for the DPS-BF-Ac based solution OLED, concomitant with the emission peak at ∼ 480 nm. Using DPS-BF-Ac and PO-01 as the blue and red dopant, respectively, the white OLED exhibits an EQEmax of ∼ 29 %. This research shows that intramolecular cyclization is an effective strategy for designing high efficiency solution-processable blue TADF emitter.

An Overlooked Charge‐Transfer Interaction in the Interfacial Triplet–Triplet Upconversion Process in Blue Organic Light‐Emitting Diodes

AbstractTwo low‐energy triplets can generate one singlet via triplet–triplet upconversion (TTU), and result in an exciton production yield exceeding 25% in conventional fluorescence‐based organic light‐emitting diodes (OLEDs). In most cases, since such low‐energy triplets induce no serious OLED degradation, TTU‐OLEDs are the only commercialized blue OLEDs so far. Herein, it is clarified that the charge‐transfer (CT) interaction at a hole‐transport/emitter‐layer interface is an overlooked pathway to enhance TTU yield significantly. First, a small energy offset at the interface enables the formation of a high‐energy CT exciton. Second, the lower energy triplet state originated from an anthracene moiety in the emitter layer collects the interfacial triplet CT. Third, due to the high‐density interfacial triplets formation, TTU at the interface contributes to the electroluminescence from the emitter layer or blue dopants even at low current density. This finding underlines the important role of the CT interface to exploit the full potential of TTU in pure‐blue OLEDs.

The positional effect of arylamines on pyrene core in a blue fluorescent dopant significantly affecting the performance of organic light emitting diodes

New blue fluorescent dopant materials based on pyrene moieties were synthesized by variation of the positions of N-phenylnaphthalene-1-amine functional groups to minutely control the molecular geometry and photophysical properties. A blue dopant with the functional groups at the 1, 6 position exhibited the highest photoluminescence quantum yield. 3,8-Diisopropyl-N1,N6-di(naphthalen-1-yl)-N1,N6-diphenylpyrene-1,6-diamine (DI-1,6-DAP) exhibited the best performances with a current efficiency of up to 6.89 cd/A and external quantum efficiency of 5.45% among the new synthetic dopants prepared in this study. In terms of color purity, the case of substituting arylamine groups at the 1,7 position of the pyrene core unit [N1,N7-di(naphthalen-1-yl)-N1,N7-diphenylpyrene-1,7-diamine (1,7-DAP)] was superior to the case of attaching them at the 1,6 position of the same core unit [N1,N6-di(naphthalen-1-yl)-N1,N6-diphenylpyrene-1,6-diamine (1,6-DAP)] (y < 0.098).

Influence the Addition (SiO2) Nanoparticles on Optical Properties for Methylene Blue Dye

Thin films from methylene blue dye dopant with silicon oxide nanoparticles (semiconducting) with percentages (0, 0.01, 0.03 and 0.05%) with a thickness of 0.001 nm, had been prepared in this research. The effect of silicon oxide impurities in methylene blue dye on the optical properties (absorption, absorbance coefficient, transmittance, energy band-gap for the forbidden and permissible electronic transitions, refractive index, extinction coefficient, dielectric constant with its real and imaginary particles, and photoconductivity) was investigated by Spectrophotometer route in the visible range 400-800 nm. Show that studying of pure and dopants samples with previous ratios increasing of absorption coefficient with increasing the doped ratios, in addition the optical conductivity, as for the values of the band gap for forbidden and permissible transitions, they decreased with the increase by the dopants percentage.

Solution-processed white OLEDs with power efficiency over 90 lm W-1 by triplet exciton management with a high triplet energy level interfacial exciplex host and a high reverse intersystem crossing rate blue TADF emitter.

Solution-processed white organic light-emitting diodes (WOLEDs) have shown much lower device efficiency than their vacuum-deposited counterparts, due to the lack of triplet exciton management in a single-emissive-layer device structure, which will induce triplet-triplet annihilation (TTA) and triplet-polaron annihilation (TPA). Here, two kinds of solution-processed WOLEDs, including thermally activated delayed fluorescence (TADF)/phosphorescence hybrid WOLEDs and all-TADF WOLEDs, with high power efficiency are developed by using a high triplet energy level (T1) interfacial exciplex as a host and a high reverse intersystem crossing (RISC) rate TADF emitter as a blue dopant for triplet exciton management. The interfacial exciplex host with high T1 can ensure that triplet excitons transfer from the host to the blue emitter, and the blue TADF emitter with high RISC rate (1.15 × 107 s-1) can rapidly upconvert triplet excitons to singlet ones to avoid TTA and TPA. The solution-processed TADF/phosphorescence hybrid and all-TADF WOLEDs exhibit maximum external quantum efficiencies of 31.1% and 27.3%, together with maximum power efficiencies of 93.5 and 70.4 lm W-1, respectively, which are the record efficiencies for solution-processed WOLEDs, and quite comparable to those of most vacuum-deposited counterparts.

Symmetric “Double Spiro” Wide Energy Gap Hosts for Blue Phosphorescent OLED Devices

AbstractWide energy gap materials dispiro[fluorene‐9,9′‐anthracene‐10′,9″‐fluorene] (SAS) and dispiro[xanthene‐9,9′‐anthracene‐10′,9″‐xanthene] (XAX) containing double spiro–carbons, are introduced as hosts for blue phosphorescent organic light‐emitting diodes (PHOLEDs). Both SAS and XAX are free of heteroatomic exocyclic bonds, which are implicated in limiting the stability of blue PHOLEDs. The materials are synthesized in gram‐scale quantities through short and efficient paths. They have large energy gaps (≥5.0 eV) between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) and correspondingly have high triplet energies in solid state (ET ≈ 3.0 eV). Analysis of devices using SAS and XAX as host materials with the blue phosphorescent dopant fac‐tris(N,N‐di‐p‐tolyl‐pyrizinoimidazol‐2‐yl)iridium(III) (Ir(tpz)3), shows that charges are transported and trapped by the dopant, which subsequently forms excitons directly on the phosphor. As a result, luminescence quenching pathways are suppressed which leads to blue phosphorescent devices with high (≈18%) external quantum efficiency. Thus, SAS and XAX serve as promising host materials, with high triplet energies suitable for blue PHOLEDs.

New blue phosphorescent Pt(II) complex with pyridyltriazole-based tetradentate ligand for organic light-emitting diodes

We designed and synthesized Pt-Pytz, a blue phosphorescent Pt (II) complex having 6,6-(1-methoxyethane-1,1-diyl)di (2-(3-(trifluoromethyl)-1H-1,2,4-triazol-5-yl)pyridine) as a tetradentate ligand, and investigated its photophysical, electrochemical, and electroluminescent properties. The complex showed an emission maximum at 449 nm in dichloromethane solution, with a high photoluminescence quantum efficiency of 0.81. Furthermore, the photoluminescence spectrum of Pt-Pytz in the film state contains no additional excimer emission in the long-wavelength region, indicating suppressed molecular aggregation in the solid state. An organic light-emitting diode based on Pt-Pytz as a blue dopant (10% doping) achieved the best external quantum efficiency of 7.0% with a power efficiency of 6.4%, a current efficiency of 11.2 cd/A, and the CIE coordinates of (0.146, 0.218).