Lifetime Of Organic Light-emitting Devices Research Articles

Robust Spirobifluorene Core Based Hole Transporters with High Mobility for Long-Life Green Phosphorescent Organic Light-Emitting Devices.

Using a tailored high triplet energy hole transport layer (HTL) is a suitable way to improve the efficiency and extend the lifetime of organic light-emitting devices (OLEDs), which can use all molecular excitons of singlets and triplets. In this study, dibenzofuran (DBF)-end-capped and spirobifluorene (SBF) core-based HTLs referred as TDBFSBF1 and TDBFSBF2 were effectively developed. TDBFSBF1 exhibited a high glass transition temperature of 178 °C and triplet energy of 2.5 eV. Moreover, a high external quantum efficiency of 22.0 %, long operational lifetime at 50 % of the initial luminance of 89,000 h, and low driving voltage at 1000 cd m-2 of 2.95 V were achieved in green phosphorescent OLEDs using TDBFSBF1. Further, a high-hole mobility μh value of 1.9×10-3 cm2 V-1 s-1 was recorded in TDBFSBF2. A multiscale simulation successfully reproduced the experimental μh values and indicated that the reorganization energy was the primary factor in determining the mobility differences among these SBF core based HTLs.

Organic molecules with inverted singlet-triplet gaps.

According to Hund’s multiplicity rule, the energy of the lowest excited triplet state (T1) is always lower than that of the lowest excited singlet state (S1) in organic molecules, resulting in a positive singlet-triplet energy gap (ΔE ST). Therefore, the up-converted reverse intersystem crossing (RISC) from T1 to S1 is an endothermic process, which may lead to the quenching of long-lived triplet excitons in electroluminescence, and subsequently the reduction of device efficiency. Interestingly, organic molecules with inverted singlet-triplet (INVEST) gaps in violation of Hund’s multiplicity rule have recently come into the limelight. The unique feature has attracted extensive attention in the fields of organic optoelectronics and photocatalysis over the past few years. For an INVEST molecule possessing a higher T1 with respect to S1, namely a negative ΔE ST, the down-converted RISC from T1 to S1 does not require thermal activation, which is possibly conducive to solving the problems of fast efficiency roll-off and short lifetime of organic light-emitting devices. By virtue of this property, INVEST molecules are recently regarded as a new generation of organic light-emitting materials. In this review, we briefly summarized the significant progress of INVEST molecules in both theoretical calculations and experimental studies, and put forward suggestions and expectations for future research.

Modeling Electron-Transfer Degradation of Organic Light-Emitting Devices.

The operational lifetime of organic light-emitting devices (OLEDs) is governed primarily by the intrinsic degradation of the materials. Therefore, a chemical model capable of predicting the operational stability is highly important. Here, a degradation model for OLEDs that exhibit thermally activated delayed fluorescence (TADF) is constructed and validated. The degradation model involves Langevin recombination of charge carriers on hosts, followed by the generation of a polaron pair through reductive electron transfer from a dopant to a host exciton as the initiation steps. The polarons undergo spontaneous decomposition, which competes with ultrafast recovery of the intact materials through charge recombination. Electrical and spectroscopic investigations provide information about the kinetics of each step in the operation and degradation of the devices, thereby enabling the building of mass balances for the key species in the emitting layers. Numerical solutions enable predictions of temporal decreases of the dopant concentration in various TADF emitting layers. The simulation results are in good agreement with experimental operational stabilities. This research disentangles the chemical processes in intrinsic electron-transfer degradation, and provides a useful foundation for improving the longevity of OLEDs.

Electroluminescence Stability of Organic Light-Emitting Devices Utilizing a Nondoped Pt-Based Emission Layer.

We study the effects of using an emitting material (Pt(II) bis(3-(trifluoromethyl)-5-(2-pyridyl)pyrazolate—Pt(fppz)2) characterized by a preferred horizontal dipole alignment and a nearly unitary quantum yield regardless of concentration on the lifetime of organic light-emitting devices (OLEDs). Using such a material as a dopant in increasingly higher concentrations is found to lead to an increase in device stability, a trend that is different from that commonly observed with conventional OLED guests. The results are consistent with the newly discovered exciton–polaron-induced aggregation degradation mechanism of OLED materials. When this emitter is used as a neat emission layer, the material is already in a highly aggregated state, and the device is no longer affected by exciton–polaron interactions. The results demonstrate the potential stability benefits of using such materials in OLEDs.

Very high efficiency phosphorescent organic light-emitting devices by using rough indium tin oxide

The efficiency of organic light-emitting devices (OLEDs) is shown to significantly depend on the roughness of the indium tin oxide (ITO) anode. By using rougher ITO, light trapped in the ITO/organic wave-guided mode can be efficiently extracted, and a light outcoupling enhancement as high as 40% is achieved. Moreover, contrary to expectations, the lifetime of OLEDs is not affected by ITO roughness. Finally, an OLED employing rough ITO anode that exhibits a current efficiency of 56 cd/A at the remarkably high brightness of 105 cd/m2 is obtained. This represents the highest current efficiency at such high brightness to date for an OLED utilizing an ITO anode, without any external light outcoupling techniques. The results demonstrate the significant efficiency benefits of using ITO with higher roughness in OLEDs.

SiNx와 금속막을 이용한 플렉시블 OLED 봉지 방법

The encapsulation method of flexible organic light emitting devices (OLEDs) was investigated for the structure of ITO / 2-TNATA / NPB / Alq3 : Rubrene (1 vol.%) / Alq3 / LiF / Al / Alq3 / LiF / Al (OLED #1), on which SiNx thin film was deposited and metal film was attached to protect the damage of OLED from oxygen and moisture. The SiNx thin film was deposited by plasma enhanced chemical vapor deposition (PECVD) method using SiH4 of 20 sccm and N2 of 15~35 sccm as reactor gases. The optimum SiNx deposition condition was found to be 20 sccm SiH4 and 20 sccm N2 from the Ca test of the fabricated SiNx thin film. The life time of OLED #1, OLED #1 / SiNx 200 nm, OLED #1 / SiNx 400 nm and OLED #1 / SiNx 400 nm / metal film was 7, 12, 25, and 45 hours, respectively. In conclusion, it has been shown that the lifetime of OLEDs can be improved more than 6 times by SiNx film and a metal film encapsulation.

Simplified Organic Light-Emitting Devices Utilizing Ultrathin Electron Transport Layers and New Insights on Their Roles

The lifetime of organic light-emitting devices (OLEDs) can be limited by exciton-polaron interactions at the organic/organic interfaces. In this work, we show that simplified phosphorescent OLEDs (PHOLEDs) are subjected to this phenomenon. By reducing the exciton concentration at the emission layer (EML)/electron transport layer (ETL) interface by means of increasing the EML thickness, hence broadening the recombination zone, the device lifetime can indeed be improved. Moreover, we report a device that displays the same extended lifetime, but with only 1 nm thin ETL. Studying the roles of this ultrathin ETL in increasing device efficiency reveals that electron injection, hole blocking, and triplet exciton blocking are all important factors. Hole blocking of the ETL can be achieved by highest occupied molecular orbitals level mismatch, where a layer thickness as low as 1 nm is sufficient, or by low hole mobility of the ETL, where a much thicker layer is required (> 5 nm). This ultrathin ETL also enables devices with only 50 nm total organic stacks, which is more than 50% thinner than the typical. This structure opens up opportunities for much shorter processing time and lower fabrication costs in the OLED industry.

Study on scalable Coulombic degradation for estimating the lifetime of organic light-emitting devices

The luminance decays of organic light-emitting diodes (OLEDs) are investigated with initial luminance of 1000 to 20 000 cd m−2 through a scalable Coulombic degradation and a stretched exponential decay. We found that the estimated lifetime by scalable Coulombic degradation deviates from the experimental results when the OLEDs work with high initial luminance. By measuring the temperature of the device during degradation, we found that the higher device temperatures will lead to instabilities of organic materials in devices, which is expected to result in the difference between the experimental results and estimation using the scalable Coulombic degradation.

Mixing effect of hole-injecting and hole-transporting materials on the performance and lifetime of organic light-emitting devices

A hole-injecting-transporting layer (HITL), which was prepared by in situ mixing hole-injecting and hole-transporting (HTM) materials upon evaporating in vacuum, has been attempted to reduce the number of organic layers in organic light-emitting devices (OLEDs). OLEDs with HITLs showed lower current density and luminance than those with hole-injecting layer only or hole-transporting layer only, which is attributed to the charge transport change controlled by the HITL nanomorphology made upon mixing. The highest efficiency and the longest lifetime were achieved for the OLED having the HITL with 25% HTM content, while the OLED with HTM only exhibited very poor device lifetime despite the highest efficiency.

A New AC-Driving Method for Active-Matrix OLED Displays

The lifetime of organic light-emitting devices can be influenced by the OLED material and structure, electrode material, driving mode and so on. Among all these factors, OLED driving method has been regarded as one of the most important factors, and it is well known that ac-driving mode improves the lifetime of OLED, especially pulse-driving mode with a reversed-biased voltage on OLED. In this article, a new ac-driving mode for Active-Matrix OLED is proposed on basic fabrication of two-TFT current source pixel circuit. Simulation has been done on the pixel and 1×4 matrix circuits, and simulation results have demonstrated that OLED is in reversed-biased state during recovery time. At last, a practical peripheral circuit that can provide achieve an ac-driving mode has been designed.

Improved stability of organic light-emitting diode with aluminum cathodes prepared by ion beam assisted deposition

We have fabricated highly stable organic electroluminescent devices based on spin-coated poly-p-phenylene-vynylene (PPV) thin films. The electrical properties of aluminum cathode, prepared by ion beam assisted deposition, on PPV have been investigated and compared to those by thermal evaporation. Although energetic particles of Al assisted by ArC ion may damage the organic material, I–V–L characteristics are improved by applying thin Al buffer layer. In addition, a dense Al cathode inhibits the permeation of H2O and O2 into PPV film through pinhole defects, and thus retards dark spot growth. It may be deduced from highly packed structure of Al cathode with an increase in the contact area between Al and PPV that reduce the contact resistance. In conclusion, the lifetime of organic light-emitting device (OLED) has been extended effectively by dense Al film through ion beam assisted deposition process.

Charge injection and transport model in organic light-emitting diodes with aluminum cathodes prepared by ion beam assisted deposition

We have fabricated highly stable organic electroluminescent devices based on spin-coated poly- p-phenylene-vynylene (PPV) thin films. The electrical properties of aluminum cathode, prepared by ion beam assisted deposition, on PPV have been investigated and compared to those by thermal evaporation. Although energetic particles of Al assisted by Ar + ion may damage the organic material, I– V– L characteristics are improved by applying thin Al buffer layer. In addition, a dense Al cathode inhibits the permeation of H 2O and O 2 into PPV film through pinhole defects, and thus retards dark spot growth. It may be deduced from highly packed structure of Al cathode with smaller junction resistance between Al and PPV. In conclusion, the lifetime of organic light-emitting device (OLED) has been extended effectively by dense Al film through ion beam assisted deposition process.

Prospects and applications for organic light-emitting devices

Efficient electroluminescence from an organic thin film at low drive voltage was first reported in 1987. Since then, the possibility of realizing a new generation of flat panel displays has driven an increasing volume of research in both industrial and academic laboratories. In early 1997, the improvement in the efficiency and lifetime of organic light-emitting devices leaves them poised to compete commercially as monochrome, emissive flat panels. Advances based on the unique properties of organic thin films create the potential for realizing a new generation of full color and flexible displays.