Degradation Mechanisms in Blue Phosphorescent Organic Light-Emitting Devices by Exciton-Polaron Interactions: Loss in Quantum Yield versus Loss in Charge Balance

Efficient Microwave Irradiation-Assisted Synthesis of Benzodioxinoquinoxaline and Its Donor-Variegated Derivatives Enabling Long-Lived Emission and Efficient Bipolar Charge Carrier Transport

To enhance the usually low-charge carrier mobilities of highly twisted donor-acceptor-type compounds that exhibit thermally activated delayed fluorescence, we designed a rodlike acceptor benzodioxinoquinoxaline. This acceptor and two donor-acceptor-donor derivatives were synthesized via microwave Buchwald-Hartwig cross-coupling reactions with yields of up to 91%. The compounds exhibit three different types of photoluminescence, which is well-explained by quantum chemical calculations. Benzodioxinoquinoxaline shows blue fluorescence, with a very short lifetime of 0.64 ns. Its derivatives exhibit either green solid-state-enhanced thermally activated delayed fluorescence (SSE-TADF) or room-temperature phosphorescence (RTP) with lifetimes approaching 7 ms. When molecularly dispersed in a polymeric host, the compounds show a photoluminescence quantum yield close to 60%. The derivatives containing acridine or phenoxazine moieties exhibit bipolar charge transport. At an electric field of 5.8 × 105 V/cm, hole and electron mobilities of the phenoxazine-containing compound reach 3.2 × 10-4 and 1.5 × 10-4 cm2 V-1 s-1, respectively. Among the studied SSE-TADF-based organic light-emitting diodes, the device containing this compound shows the highest external quantum efficiency of 12.3% due to the good charge-transporting and SSE-TADF parameters of the emitter.

Critical role of electrons in the short lifetime of blue OLEDs

Designing robust blue organic light-emitting diodes is a long-standing challenge in the display industry. The highly energetic states of blue emitters cause various degradation paths, leading to collective luminance drops in a competitive manner. However, a key mechanism of the operational degradation of organic light-emitting diodes has yet to be elucidated. Here, we show that electron-induced degradation reactions play a critical role in the short lifetime of blue organic light-emitting diodes. Our control experiments demonstrate that the operational lifetime of a whole device can only be explained when excitons and electrons exist together. We examine the atomistic mechanisms of the electron-induced degradation reactions by analyzing their energetic profiles using computational methods. Mass spectrometric analysis of aged devices further confirm the key mechanisms. These results provide new insight into rational design of robust blue organic light-emitting diodes.

Control of Host-Matrix Morphology Enables Efficient Deep-Blue Organic Light-Emitting Devices

Mixing a sterically bulky, electron-transporting host material into a conventional single host-guest emissive layer is demonstrated to suppress phase separation of the host matrix while increasing the efficiency and operational lifetime of deep-blue phosphorescent organic light-emitting diodes (PHOLEDs) with chromaticity coordinates of (0.14, 0.15). The bulky host enables homogenous mixing of the molecules comprising the emissive layer while suppressing single host aggregation; a significant loss channel of nonradiative recombination. By controlling the amorphous phase of the host-matrix morphology, the mixed-host device achieves a significant reduction in nonradiative exciton decay, resulting in 120 ± 6% increase in external quantum efficiency relative to an analogous, single-host device. In contrast to single host PHOLEDs where electrons are transported by the host and holes by the dopants, both charge carriers are conducted by the mixed host, reducing the probability of exciton annihilation, thereby doubling of the deep-blue PHOLED operational lifetime. These findings demonstrate that the host matrix morphology affects almost every aspect of PHOLED performance.

Tetradentate Pt(II) Complexes with Peripheral Hindrances for Highly Efficient Solution-Processed Blue Phosphorescent OLEDs

Two tetradentate Pt(II) complexes with peripheral bulky-group hindrances [Pt(pzpyOczpy-B1) and Pt(pzpyOczpy-B2)] were synthesized and fully investigated for their structural and blue phosphorescent properties. Both X-ray crystallography and computational simulation revealed that bulky substituents incorporated into the C-pyrazolyl and C-pyridinyl positions lie out of the cyclometallated plane, thus alleviating the intramolecular distortions as well as reducing the intermolecular interaction in the solid state. In dichloromethane, their emission peaks at 460 nm with a narrow full width at half-maximum (FWHM) of less than 50 nm, and the photoluminescent quantum yields are over 95% with short decay lifetimes (<5 μs). Solution-processed blue devices are fabricated based on the two complexes. Device A based on Pt(pzpyOczpy-B1) shows excellent electroluminescent performances with the maximum current efficiency, power efficiency, and external quantum efficiency of 47.0 cd/A, 24.6 lm/W, and 22.9%, respectively. The understanding on inert peripheral hindrances provides an effective approach to designing Pt(II) complexes for high-quality blue phosphorescent emitters.

Root Causes of the Limited Electroluminescence Stability of Organic Light-Emitting Devices Made by Solution-Coating

Although organic electroluminescent materials have long promised the prospect of making organic light emitting devices (OLEDs) via low-cost solution-coating techniques, the electroluminescence stability of devices made by such techniques continues to be rather limited making them unsuitable for commercialization. The root causes of the lower stability of OLEDs made by solution-coating versus the more conventional vacuum-deposition remain unknown. In this work, we investigate and compare between solution-coated and vacuum-deposited materials under prolonged excitation, using the archetypical host material 4,4'-bis( N-carbazolyl)-1,1'-biphenyl as a model OLED material. Results show that solution-coated films are more susceptible to degradation by excitons in comparison to their vacuum-deposited counterparts, resulting in a faster decrease in their luminescent quantum yield. The degradation rate also depends on the choice of solvent that was used in the solution-coating process. Results also show that the decrease in quantum yield is caused by exciton-induced chemical decomposition in the material as well as some possible molecular reorganization or aggregation, both of which are induced by excitons and proceed more quickly in case of solution-coated films. The faster degradation in the solution-coated films appears to originate primarily from their different morphological makeup and not due to chemical impurities. The findings uncover what appears to be one of the fundamental root causes of the lower stability of solution-coated OLEDs in general.

Isolating Degradation Mechanisms in Mixed Emissive Layer Organic Light-Emitting Devices

Degradation in organic light-emitting devices (OLEDs) is generally driven by reactions involving excitons and polarons. Accordingly, a common design strategy to improve OLED lifetime is to reduce the density of these species by engineering an emissive layer architecture to achieve a broad exciton recombination zone. Here, the effect of exciton density on device degradation is analyzed in a mixed host emissive layer (M-EML) architecture which exhibits a broad recombination zone. To gain further insight into the dominant degradation mechanism, losses in the exciton formation efficiency and photoluminescence (PL) efficiency are decoupled by tracking the emissive layer PL during device degradation. By varying the starting luminance and M-EML thickness, the rate of PL degradation is found to depend strongly on recombination zone width and hence exciton density. In contrast, losses in the exciton formation depend only weakly on the recombination zone, and thus may originate outside of the emissive layer. These results suggest that the lifetime enhancement observed in the M-EML architectures reflects a reduction in the rate of PL degradation. Moreover, the varying roles of excitons and polarons in degrading the PL and exciton formation efficiencies suggest that kinetically distinct pathways drive OLED degradation and that a single degradation mechanism cannot be assumed when attempting to model the device lifetime. This work highlights the potential to extract fundamental insight into OLED degradation by tracking the emissive layer PL during lifetime testing, while also enabling diagnostic tests on the root causes of device instability.

Flexible inorganic light emitting diodes based on semiconductor nanowires

The fabrication technologies and the performance of flexible nanowire light emitting diodes (LEDs) are reviewed. We first introduce the existing approaches for flexible LED fabrication, which are dominated by organic technologies, and we briefly discuss the increasing research effort on flexible inorganic LEDs achieved by micro-structuring and transfer of conventional thin films. Then, flexible nanowire-based LEDs are presented and two main fabrication technologies are discussed: direct growth on a flexible substrate and nanowire membrane formation and transfer. The performance of blue, green, white and bi-color flexible LEDs fabricated following the transfer approach is discussed in more detail.

Probing the Emission Zone Length in Organic Light Emitting Diodes via Photoluminescence and Electroluminescence Degradation Analysis

The understanding and control of the emission zone in organic light emitting diodes (OLEDs) is crucial to the device operational stability. Using the photoluminescence and electroluminescence degradation data, we have developed a modeling methodology to quantitatively determine the length of the emission zone and correlate that with the degradation mechanism. We first validate the modeling results by studying the emitter concentration effect on operational stability of devices using the well-studied thermal activated delayed fluorescent (TADF) emitter (4s,6s)-2,4,5,6-tetra(9H-carbazol-9-yl)isophthalonitrile (4CzIPN), and our results are consistent with previous published data. We further applied this methodology to study the emitter concentration effect using another TADF emitter, 4-carbazolyl-2-methylisoindole-1,3-dione (dopant 1). The results show that the emission zone of the dopant 1 devices is narrower than the 4CzIPN device, leading to faster degradation. While a higher emitter concentration does not result in widening of the emission zone, we were able to widen the emission zone and hence extend the device lifetime using a mixed host.