Performance Of Organic Light-emitting Diodes Research Articles

(Invited) In silico Material Design and Realization of Highly-Efficient Organic Light-Emitting Diodes

Conversion of triplet excitons into light is very important to realize highly efficient organic light-emitting diodes (OLEDs). One solution is the use of transition-metal-based phosphorescent materials.1,2 Thermally activated delayed fluorescence (TADF)3 is another solution. Recently, we have developed various type of excellent TADF emitters using a simple high-throughput screening method. In this talk, I will present the method and the results we have obtained.4-8 The OLED performances are excellent, but further improvements are necessary to overcome efficiency roll-off problem etc. I will also show our recently proposed material design concept,9 named tilted Face-to-Face alignment with Optimal distance (tFFO), to further accelerate reverse intersystem crossing (RISC). Using this design concept, we developed a new material, TpAT-tFFO (Figure), which provided very fast RISC with a rate constant (k RISC) over 107 s-1.The results of multiscale charge transport simulations10-13 and dynamic nuclear polarization enhanced solid-state NMR14 will be also presented, if time permits.We express sincere thanks to my group members, Prof. Adachi, and his group members. This work was supported by JSPS KAKENHI Grant Numbers JP20H05840 (Grant-in-Aid for Transformative Research Areas, “Dynamic Exciton”), JP17H01231, and JP17J09631, and FIRST Program. Computation time was provided by the Super Computer System, Institute for Chemical Research, Kyoto University. NMR measurements were supported by the international Joint Usage/Research Centre (iJURC) at the Institute for Chemical Research, Kyoto University, Japan. M. A. Baldo et al. Appl. Phys. Lett. 75, 4 (1999).H. Sasabe and J. Kido, Eur. J. Org. Chem. 2013, 7653 (2013).H. Uoyama et al. Nature, 492, 234 (2012).H. Kaji et al. Nat. Commun., 6, 8476 (2015).K. Suzuki et al. Angew. Chem. Int. Ed., 54, 15231 (2015).T. Miwa et al. Sci. Rep., 7, 284 (2017).Y. Wada et al. Appl. Phys. Lett., 107, 183303 (2015).Y. Wada et al. Adv. Mater. 30, 1705641 (2018).Y. Wada, H. Nakagawa, S. Matsumoto, Y. Wakisaka, and H. Kaji, ChemRxiv, https://doi.org/10.26434/chemrxiv.9745289.v1 (2019); Nat. Photon. 14, 643 (2020).F. Suzuki et al. J. Mater. Chem. C 3, 5549 (2015).H. Uratani et al. Sci. Rep. 6, 39128 (2016).F. Suzuki et al. Sci. Rep. 8, 5203 (2018).S. Kubo et al. Sci. Rep. 8, 13462 (2018).K. Suzuki et al. Angew. Chem. Int. Ed. 56, 14842 (2017). Figure 1

Highly-efficient OLED with cesium fluoride electron injection layer

The effective electron-injecting layer (EIL) is one of the important factors in organic light-emitting diodes (OLEDs) to achieve high performance devices. Herein, we have proposed the modification in cesium fluoride thickness and demonstrated that the performance of OLED is enhanced while varying the thickness of EIL. The cesium fluoride thickness may greatly influence the electron injection ability from the cathode to electron transport layer (ETL). The optimized green phosphorescent OLED consisting CBP:Ir(ppy)3 emissive layer reveals forward viewing power efficiency (PE) of 46.3 lmW−1, current efficiency (CE) of 51.7 cdA-1 and external quantum efficiency (EQE) of 14.2%, at 100 cdm−2, which has enhancement of PE, CE, and EQE of 38.6%, 44.8%, and 46.4%, respectively at 100 cdm−2. The thickness variation of cesium fluoride EIL resulted in very balanced electron injection which is also valid for other OLED devices. The employment of optimized EIL is a favourable approach to enhance the performance in OLEDs applications.

Fabrication of highly efficient blue top-emission organic light-emitting diodes on different reflective electrodes

The performances of top-emission organic light-emitting diodes (TEOLEDs) with various P-dopant (PD) contents in the injection layer were studied by thinning or removing an indium tin oxide (ITO) film sputtered on the anode. On adjusting the thickness of the active TBPDA (N4,N4,N4′,N4′-tetra ([1,1′-biphenyl]-4-yl)-[1,1′-biphenyl]-4,4′-diamine) film used as hole transport layer, the International Commission on Illumination (CIE) coordinates of blue TEOLEDs did not change and the same CIE coordinates (0.14, 0.04) were maintained. The blue index of device I (PD of 3%) without an indium tin oxide (ITO) layer was 139.9 cd/A/CIEy at a current density of 10 mA/cm2. This value was 28% higher than that of the device B (PD of 2%), which had a 15-nm thick ITO film, and 19% higher than that of device E (PD of 2%), which had a 7-nm thick ITO film. Devices B, E, and I achieved similar voltages of approximately 3.9 V. Thus, in the optimized TEOLEDs with suitable PD contents, efficiency was improved by silver without the use of ITO as an anode.

A New Conjugated Polymer that Enables the Integration of Photovoltaic and Light-Emitting Functions in One Device.

Exploring the intriguing bifunctional nature of organic semiconductors and investigating the feasibility of fabricating bifunctional devices are of great significance in realizing various applications with one device. Here, the design of a new wide-bandgap polymer named PBQx-TCl (optical bandgap of 2.05eV) is reported, and its applications in photovoltaic and light-emitting devices are studied. By fabricating devices with nonfullerene acceptors BTA3 and BTP-eC9, it is shown that the devices exhibit a high power conversion efficiency (PCE) of 18.0% under air mass 1.5G illumination conditions and an outstanding PCE of 28.5% for a 1 cm2 device and 26.0% for a 10 cm2 device under illumination from a 1000 lux light-emitting diode. In addition, the PBQx-TCl:BTA3-based device also demonstrates a moderate organic light-emitting diode performance with an electroluminescence external quantum efficiency approaching 0.2% and a broad emission range of 630-1000nm. These results suggest that the polymer PBQx-TCl-based devices exhibit outstanding photovoltaic performance and potential light-emitting functions.

Effect of Carrier-Transporting Layer on Blue Phosphorescent Organic Light-Emitting Diodes

This study presented the effects of carrier-transporting layer (CTL) on electroluminescence (EL) performance of a blue phosphorescent organic light-emitting diodes (PHOLEDs) with electron transporting host based on three kinds of electron-transporting layers (ETLs) including 3-(4-biphenyl-yl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole (TAZ), diphenyl-bis[4-(pyridin-3-yl)phenyl]silane (DPPS) and 1,3,5-tri(m-pyrid-3-yl-phenyl)benzene (TmPyPB) and two kinds of hole-transporting layers (HTLs) such as 4,4′-bis[N-1-naphthyl-N-phenyl-amino]biphenyl (NPB), 1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC). The carrier recombination and exciton formation zones in blue PHOLEDs strongly depend on the carrier mobility of CTLs and the layer thickness, especially the carrier mobility. Between ETLs and HTLs, the high electron mobility of ETL results in a lower driving voltage in blue PHOLEDs than the high hole mobility of HTL did. In addition, layer thickness modulation is an effective approach to precisely control carriers and restrict carriers within the EML and avoid a leakage emission of CTL. For CTL pairs in OLEDs using the electron transporting host system, ETLs with low mobility and also HTLs with high hole mobility are key points to confine the charge in EML for efficient photon emission. These findings show that appropriate CTL pairs and good layer thickness are essential for efficient OLEDs.

Effect of the relationship between the energy levels of host and guest on EL performance of phosphorescence organic light-emitting diodes

In this work, we used five different fluorescent hosts and six phosphorescent guests to study the effect of the relationship between the energy levels of host and guest on the EL performance of phosphorescence organic light-emitting diodes (PHOLEDs). The experimental results show clearly that the difference between triplet energy levels of the used host and guest plays an important role, and the electron and hole injection barriers, respectively, from electron-transporting layer (ETL) and hole-transporting layer (HTL) into emitting layer (EML) and the mobility of ETL also have a certain influence. It can be seen that the triplet energy level of hosts should be higher than that of guests and the distance between them is as close as possible, ensuring the effective energy transfer from hosts to guests. Furthermore, it is also necessary to have low injection barriers into EML for electrons and holes, and high electron mobility of ETL, which guarantee electrons and holes injected effectively and evenly. This further explains the fundamental reason why the fabrication of high efficiency PHOLEDs must optimize the host, guest and match transporting layers. Our studies will help to establish better design rules for host-guest systems to achieve high performance PHOLEDs. Study the effect of the relationship between the energy levels of hosts and guests and the mobility of ETLs on the EL performance of PHOLEDs. • Study the effect of the relationship between the energy levels of host and guest on the EL performance of PHOLEDs. • The higher T 1 of host than that of guest and the LUMO and HOMO levels of guest within those of host are important. • Low injection barriers and high electron mobility of ETL are effective for carriers to inject and recombine in EML.

Improving reverse intersystem crossing in exciplex-forming hosts by introducing heavy atom effect

A fast reverse intersystem crossing rate (kRISC) is an ongoing pursuit for exciplex-forming host with thermally activated delayed fluorescence (TADF) for improved performances of organic light-emitting diodes. However, design rules regarding the development of exciplex with a high kRISC are still elusive. Here, the influence of heavy atom effect on kRISC of exciplex is investigated with bromine (Br)-substituted acceptors. It is proved that heavy atom effect induced by Br atom can enhance the spin-orbital coupling of exciplex systems and thus promote the kRISC process of exciplex. Compared with the reference one (kRISC of 5.4 × 105 per second), Br-containing exciplex exhibits an increased kRISC by almost an order of magnitude, being 4.56 × 106 per second. Based on those exciplex-forming hosts, devices based on a conventional fluorescent emitter and a TADF emitter are fabricated, and it is observed that the faster the RISC process of exciplex-forming host is, the higher the device efficiency can be obtained. Those results here should provide an effective strategy to accelerate the kRISC of exciplex-forming host for advanced device performances.

Fabrication of an oxide/metal/oxide structured electrode integrated with antireflective film to enhance performance in flexible organic light-emitting diodes

Flexible organic light-emitting diodes (OLEDs) are used widely in optoelectronic devices, with possible new applications, including foldable/rollable displays. Unique features of flexible OLEDs included high brightness, low power consumption, and flexibility. The performance of a flexible OLED is frequently hindered by refractive index difference between the air and the flexible substrate medium, leading to inefficient light outcoupling. To address this issue, we developed a mechanically robust oxide/metal/oxide (OMO) structured transparent conducting electrode (TCE), integrated with silica nanoparticles–based antireflective (AR) film, to improve the light extraction efficiency of flexible OLEDs. The AR-OMO structures were prepared on polyethylene terephthalate substrates using a combination of plasma-enhanced chemical vapor deposition and magnetron sputtering. Our results show that an OLED device based on AR-OMO TCE exhibits higher luminance efficiency (LE) and total external quantum efficiency (EQEtot) than devices based on pristine OMO or an indium tin oxide structure due to the presence of AR film that suppresses waveguided-mode light loss at the air-substrate interface. The champion AR-OMO-based flexible OLED devices achieved an LE of 12.3 cd/A and an EQEtot of 5.0%. The AR-OMO device also demonstrated outstanding mechanical flexibility, retaining 100% of its initial luminance up to a bending radius of 6 mm.

Donor engineered Deep-Blue emitters for tuning luminescence mechanism in Solution-Processed OLEDs

Abstract Three novel solution-processable A-π-2D-type deep-blue emitters, namely BCz, BBFCz, and BICz, were developed to investigate their luminescence mechanisms and performances in organic light-emitting diodes (OLEDs). The emitters were uniquely designed by connecting two carbazole analogs as donors and a boron-fused unit as an electron acceptor to the benzene core; they exhibited aggregation-induced emission properties in the film states. Theoretical calculations and time-resolved photoluminescence (TRPL) experimental results indicate that the luminescence mechanism of the three emitters changed from fluorescence to thermally activated delayed fluorescence (TADF) as the donor unit was changed from carbazole to indenocarbazole. BCz was found to act like a fluorescent emitter, but BBFCz and BICz displayed TADF characteristics. Efficient reverse intersystem crossing (RISC) in BICz was confirmed by small ΔEST, Ea, and kISC/kRISC ratio. Consequently, non-doped solution-processed TADF-OLEDs based on BICz as an emitter exhibited the highest external quantum efficiency (EQE) of 10.11%, with deep-blue commission International de ĺEclairage (CIE) color coordinates (0.16, 0.08). In contrast, BCz- and BBFCz-based devices showed relatively lower EQEs of 3.44% and 6.78%, respectively. The results showed that BICz as an emitter displayed exceptional performance in a non-doped solution-processed deep-blue TADF-OLED.

Highly efficient thermally activated delayed fluorescence devices using a phosphorescent dye as host

In this work, we further researched the influence of heavy atom on the electroluminescent (EL) performances of organic light-emitting diodes (OLEDs) containing thermally activated delayed fluorescence (TADF) materials as emitters by utilizing iridium(III)bis(4’,6’-difluorophenylpyridinato)tet-rakis(1-pyrazolyl)borate (Ir(mppy)3) as the host of the assistant light-emitting layer (EML) and 2-[4-(diphenylamino)phenyl]-10,10-dioxide-9H-thioxanthen-9-one (TXO-TPA) as emitter. Though optimizing the doping concentration of TXO-TPA and the thickness of assistant EML, we have successfully improved the EL performances of TXO-TPA based device by analyzing energy transfer, exciton quenching progresses and carrier distribution within EMLs. Finally, the optimal device obtained high current efficiency and external quantum efficiency of 38.92 cd/A and 12.41% at 2000 cd/m2 (3.2 V). Meanwhile, the optimal device has a slower efficiency roll-off with maximum current efficiency of 42.43 cd/A (EQE=12.95%) with a long LT50 (the time for brightness decay to 50% of the initial brightness) lifetime of 6700 h at 2000 cd/m2.

Improving Performance of Organic Light-emitting Diodes by Tuning Molecular Orientation in Hole Transport Layer

Improving Performance of Organic Light-emitting Diodes by Tuning Molecular Orientation in Hole Transport Layer

Optical and electrical characterization of blended active materials for white OLEDs (WOLEDs)

The purpose of this work is the investigation of white light emission by blending RGB-emitting polymers and the definition of the optimal blending weight ratio, concerning the device operation characteristics and color emission. A comprehensive study of the optical properties and optoelectronic performance of white organic light-emitting diodes (WOLEDs) based on blends of poly(9,9-di-noctylfluorenyl-2,7-diyl) (PFO), poly(9,9-dioctylfluorene-altbaenzothiadiazole) (F8BT) and poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV) is reported. It was observed that using relatively low ratios, up to 5 wt% of F8BT and MDMO-PPV into the host polymer PFO, slightly affects the dielectric optical response of the blended films and the characteristic electronic absorptions of PFO. Photoluminescence (PL) studies of the blends reveal that the main mechanism is the energy transfer from PFO to F8BT constituent. However, the electroluminescence (EL) study of blends, demonstrates that, in addition to energy transfer, the individual emission of PFO, F8BT and MDMO-PPV leads to a broad emission of the produced OLED devices that covers the whole visible spectral range. As a result, an almost white emission is achieved proving the potentiality to implement such solution-processable blends for lighting applications.

The exploration of acceptor ratio in thermally activated delayed fluorescent donor for the effect on exciplex OLED

The effect of PO-T2T acceptor ratio in thermally activated delayed fluorescent donor of DMAC-DPS on exciplex organic light-emitting diodes (OLED) performance is explored herein. As the ratio decrease of PO-T2T acceptor, we found an obvious blue shift of electroluminescence (EL) spectra from 536 nm to 496 nm, while the EL efficiency maintain a high level with maximum current efficiency of 25.7–38.1 cd/A, power efficiency of 24.2–39.5 l m/W and external quantum efficiency (EQE) of 10.4–12.1%, respectively. That means a wide spectral move range of 40 nm without damaging device EQE in exciplex OLED could be realized. The strong interaction between exciplex induced from charge separation is responsible for the blue shift as acceptor ratio reduce.

Pyridine-substituted triazine as an acceptor for thermally activated delayed fluorescence emitters showing high efficiency and low roll-off in organic light-emitting diodes

In thermally activated delayed fluorescence (TADF) molecular design, triazine (TRZ) is of the most widely used acceptor core with three phenyl rings protected. In this work, by substituting the peripheral phenyl group with electrophilic pyridine rings, a new acceptor 2-phenyl-4,6-di(pyridin-3-yl)-1,3,5-triazine (PyTRZ) was designed and two novel TADF emitters 10-(4-(4,6-di(pyridin-3-yl)-1,3,5-triazin-2-yl)phenyl)-10H-phenoxazine (PXZ-PyTRZ) and 10-(4′-(4,6-di(pyridin-3-yl)-1,3,5-triazin-2-yl)-[1,1′-biphenyl]-4-yl)-10H-phenoxazine (PXZ-Ph-PyTRZ) were accordingly exploited. Both compounds exhibit obvious TADF emitters, while their kinetic processes related to exciton radiation exhibit some obvious differences and deeply affect their overall performances in organic light-emitting diodes (OLEDs). The optimized device by using PXZ-Ph-PyTRZ as the emitter achieves a superior maximum external quantum efficiency (EQE) of 22.2% than that of PXZ-PyTRZ (18.5%), whereas their device efficiency roll-offs show different tendencies with increasing current densities. Particularly, owing to a shorter decay lifetime of 1.9 μs, the PXZ-PyTRZ-based device achieves an impressive EQE of 13.1% at an ultra-high luminance of 10,000 cd/m2, which is comparable with state-of-the-art high-performance OLEDs. These results demonstrate a bright prospect of the PyTRZ moiety in exploiting TADF emitters; more importantly, we provide a new method of deriving TRZ cores for TADF molecular design.

Impact of secondary donor units on the excited-state properties and thermally activated delayed fluorescence (TADF) efficiency of pentacarbazole-benzonitrile emitters.

The performance of organic light-emitting diodes based on thermally activated delayed fluorescence emitters depends on the efficiency of reverse intersystem crossing (RISC) processes, which are promoted by a small energy gap between the lowest singlet (S1) and triplet (T1) excited states and large spin-orbit couplings. Recently, it was proposed that the introduction of secondary donor units into 2,3,4,5,6-penta(9H-carbazol-9-yl)benzonitrile (5CzBN) can significantly increase the mixing between triplet states with charge-transfer (CT) and local-excitation characteristics and consequently increase the spin-orbit couplings. Here, the results of long-range corrected density functional theory calculations show that the main impact on the RISC rates of substituting 5CzBN with secondary donors is due to a decrease in adiabatic singlet-triplet energy gaps and intramolecular reorganization energies rather than to a change in spin-orbit couplings. Our calculations underline that at least two singlet and three triplet excited states contribute to the ISC/RISC processes in 5CzBN and its derivatives. In addition, we find that in all emitters, the lowest singlet excited-state potential energy surface has a double-minimum shape.

Circularly Polarized Thermally Activated Delayed Fluorescence Emitters in Through-Space Charge Transfer on Asymmetric Spiro Skeletons.

This work describes a strategy to produce circularly polarized thermally activated delayed fluorescence (CP-TADF). A set of two structurally similar organic emitters SFST and SFOT are constructed, whose spiro architectures containing asymmetric donors result in chirality. Upon grafting within the spiro frameworks, the donor and acceptor are fixed proximally in a face-to-face manner. This orientation allows intramolecular through-space charge transfer (TSCT) to occur in both emitters, leading to TADF properties. The donor units in SFST and SFOT have a sulfur and oxygen atom, respectively; such a subtle difference has great impacts on their photophysical, chiroptical, and electroluminescence (EL) properties. SFOT exhibits greatly enhanced EL performance in doped organic light-emitting diodes, with external quantum efficiency (EQE) up to 23.1%, owing to the concurrent manipulation of highly photoluminescent quantum efficiency (PLQY, ∼90%) and high exciton utilization. As a comparison, the relatively larger sulfur atom in SFST introduces heavy atom effects and leads to distortion of the molecular backbone that lengthens the donor-acceptor distance. SFST thus has lower PLQY and faster nonradiative decay rate. The collective consequence is that the EQE value of SFST, i.e., 12.5%, is much lower than that of SFOT. The chirality of these two spiro emitters results in circularly polarized luminescence. Because SFST has a more distorted molecular architecture than SFOT, the luminescence dissymmetry factor (|glum|) of circularly polarized luminescence of one enantiomer of the former, namely, either (S)-SFST or (R)-SFST, is almost twice that of (S)-SFOT/(R)-SFOT. Moreover, the CP organic light-emitting diodes (CP-OLEDs) show obvious circularly polarized electroluminescence (CPEL) signals with gEL of 1.30 × 10-3 and 1.0 × 10-3 for (S)-SFST and (S)-SFOT, respectively.

Erratum: “Comparison of organic light-emitting diode performance using the spectroradiometer and the integrating sphere measurements” [AIP Adv. 10, 095011 (2020)

First Page

Multiscale molecular simulations of the morphological evolution of small-molecule organic solar cells during the vacuum codeposition process

The mesoscale morphologies of organic small molecular films fabricated via vacuum deposition processes are critical to the performance of small-molecule solar cells and organic light-emitting diodes. In the present study, the morphological evolution of the active layer of DPDCPB:${\mathrm{C}}_{70}$ small-molecule solar cells during vacuum codeposition processes was revealed by a series of GPU-accelerated coarse-grained molecular-dynamics simulations. The ${\mathrm{C}}_{70}$ and DPDCPB molecules were coarsened into ellipsoids and bonded ellipsoids, respectively. The interactions between ellipsoids were described by the Gay-Berne formulation and were parametrized to reproduce potential energy surfaces from all-atom atomistic simulations using a genetic algorithm. Due to the significantly reduced overall degrees of freedom, this coarse-grained scheme allowed us to simulate the vacuum codeposition processes and monitor the morphological evolution of systems with system length scales compatible with those of the experiments (\ensuremath{\sim}30 nm). Our simulations indicate that the film morphologies are closely correlated with the DPDCPB:${\mathrm{C}}_{70}$ blending ratio. High ${\mathrm{C}}_{70}$ concentration leads to a rough film surface, which is in accordance with experimental observations and can be attributed to the strong self-aggregation behavior of ${\mathrm{C}}_{70}$ molecules. The morphological property analysis indicates that the rough film surface has an almost negligible impact on the DPDCPB/${\mathrm{C}}_{70}$ domain percolations, and the device with the optimal deposition ratio should give the most balanced hole/electron transfer in respective DPDCPB/${\mathrm{C}}_{70}$ domains. The present study demonstrates that by using the ellipsoid-based coarse-grained model, it is possible to study the morphological evolution of small-molecule organic thin film during vacuum deposition processes with molecular scale details, which can provide valuable insights for experimental teams to further optimize device fabrication protocols for the next generation of organic optoelectronic devices.

Comparison of organic light emitting diode performance using the spectroradiometer and the integrating sphere measurements

In this study, we present the comparison of device performance measurements for organic light emitting diodes using a spectroradiometer through the viewing angle and integrating sphere, widely used for device measurements. The mean calculation method using these results was applied to convert the spectroradiometer (under different viewing angles) data to match with the integrating sphere measurements. The conversion of the spectroradiometer based quantum efficiency and electroluminescence data from all different angular emission patterns was similar to that of the integrating sphere data within a reasonable range of deviation. As such, it is possible to reduce the recurring costs and required time between these two measurement techniques by bypassing the integrating sphere measurement.

Dependence of the performance of light-emitting diodes on the molecular weight of the electroluminescent polymer PFO-MEH-PPV

Controlled synthesis of the electroluminescent polymer PFO-MEH-PPV (poly[(9,9-dioctyl-2,7-divinylenefluorenylene)-alt-co-(2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene)]) provided samples of varying molecular weight (Mw) in the range 20–360 kDa, as determined by gel-permeation chromatography and light scattering. The samples were used as the active layers in organic light-emitting diodes (OLEDs), and the performance of the devices was examined as a function of Mw. Turn-on voltages fell in the range 1.92–2.78 V, luminances varied from 231 to 5826 cd/m2, and luminous efficacies ranged from 0.06 to 0.90 lm/W. The emitted colour was found to vary from green to yellow as Mw increases. Optimal performance was attained by using PFO-MEH-PPV with Mw = 100 kDa. To help reveal how Mw determines the performance of OLEDs, relative quantum yields of photoluminescence in solutions and films were measured, and films were characterized by atomic force microscopy and transmission electron microscopy.