Direct identification of interfacial degradation in blue OLEDs using nanoscale chemical depth profiling.

Understanding the degradation mechanism of organic light-emitting diodes (OLED) is essential to improve device performance and stability. OLED failure, if not process-related, arises mostly from chemical instability. However, the challenges of sampling from nanoscale organic layers and interfaces with enough analytical information has hampered identification of degradation products and mechanisms. Here, we present a high-resolution diagnostic method of OLED degradation using an Orbitrap mass spectrometer equipped with a gas cluster ion beam to gently desorb nanometre levels of materials, providing unambiguous molecular information with 7-nm depth resolution. We chemically depth profile and analyse blue phosphorescent and thermally-activated delayed fluorescent (TADF) OLED devices at different degradation levels. For OLED devices with short operational lifetimes, dominant chemical degradation mainly relate to oxygen loss of molecules that occur at the interface between emission and electron transport layers (EML/ETL) where exciton distribution is maximised, confirmed by emission zone measurements. We also show approximately one order of magnitude increase in lifetime of devices with slightly modified host materials, which present minimal EML/ETL interfacial degradation and show the method can provide insight for future material and device architecture development.

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.

Probing twisted intramolecular charge transfer of pyrene derivatives as organic emitters in OLEDs.

Intramolecular charge transfer (ICT) plays a critical role in determining the photophysical properties of organic molecules, including their luminescence efficiencies. Twisted intramolecular charge transfer (TICT) is a process in which structural change accompanies ICT. Herein, we used time-resolved spectroscopy to study TICT in pyrene derivatives that are promising blue organic light emitting diode (OLED) emitter candidates; these derivatives show strong solvent-dependent charge-transfer (CT) behavior with unique fluorescence properties, increased fluorescence intensity in polar solvent. Slight structural changes that do not affect excited state dynamics were observed in nonpolar solvents, while polar solvents were found to affect excited state dynamics and CT characteristics, which affect their unusual fluorescence behavior. The TICT behavior of these pyrene derivatives can be modulated through structural modification. Our study provides valuable guidelines for the control of optical properties, including the luminescence efficiencies of OLED emitters that show TICT characteristics.

Multiple-Resonance Extension and Spin-Vibronic-Coupling-Based Narrowband Blue Organic Fluorescence Emitters with Over 30% Quantum Efficiency.

Achieving narrow-bandwidth emission and high external quantum efficiency (EQE) simultaneously is a challenge for next-generation blue-emitting organic light-emitting diodes (OLEDs). In this study, novel multiple-resonance thermally activated delayed fluorescence (MR-TADF) emitters are developed by fusing an indolocarbazole unit with two carbazole skeletons using para-oriented nitrogen atoms. The resulting rigid and planar π-system without electron-accepting atoms exhibits pure blue photoluminescence at 470 nm, reaching a 100% quantum yield with a full-width-at-half-maximum (FWHM) of 25 nm. Higher-level quantum chemistry calculations confirm an MR effect within the extended π-conjugation and an enhanced triplet-to-singlet crossover (104 s-1 ) through a reduced energy gap (ΔEST ) coupled with large spin-vibronic coupling mediated by low-lying triplet excited states. An OLED fabricated using the MR-TADF emitter with CIE color coordinates of (0.12, 0.16) exhibits a record high EQE of 30.9% and a small FWHM of 23 nm. With further optimization of the device structure, a high EQE of 33.8% is achieved without additional outcoupling enhancements owing to the near-perfect horizontal alignment of the emitting dipoles.

Dipole Moment- and Molecular Orbital-Engineered Phosphine Oxide-Free Host Materials for Efficient and Stable Blue Thermally Activated Delayed Fluorescence.

To utilize thermally activated delayed fluorescence (TADF) technology for future displays, it is necessary to develop host materials which harness the full potential of blue TADF emitters. However, no publication has reported such hosts yet. Although the most popular host for blue TADF, bis[2-(diphenylphosphino)phenyl]ether oxide (DPEPO) guarantees high-maximum external quantum efficiency (EQEmax ) TADF devices, they exhibit very short operational lifetimes. In contrast, long-lifespan blue TADF devices employing stable hosts such as 3',5-di(9H-carbazol-9-yl)-[1,1'-biphenyl]-3-carbonitrile (mCBP-CN) exhibit much lower EQEmax than the DPEPO-employed devices. Here, an elaborative approach for designing host molecules is suggested to achieve simultaneously stable and efficient blue TADF devices. The approach is based on engineering the molecular geometry, ground- and excited-state dipole moments of host molecules. The engineered hosts significantly enhance delayed fluorescence quantum yields of TADF emitters, as stabilizing the charge-transfer excited states of the TADF emitters and suppressing exciton quenching, and improve the charge balance. Moreover, they exhibit both photochemical and electrochemical stabilities. The best device employing one of the engineered hosts exhibits 79% increase in EQEmax compared to the mCBP-CN-employed device, together with 140% and 92-fold increases in operational lifetime compared to the respective mCBP-CN- and the DPEPO-based devices.

Purely Spin-Vibronic Coupling Assisted Triplet to Singlet Up-Conversion for Real Deep Blue Organic Light-Emitting Diodes with Over 20% Efficiency and y Color Coordinate of 0.05.

Finding narrow-band, ultrapure blue thermally activated delayed fluorescence (TADF) materials is extremely important for developing highly efficient organic light-emitting diodes (OLEDs). Here, spin-vibronic coupling (SVC)-assisted ultrapure blue emitters obtained by joining two carbazole-derived moieties at a para position of a phenyl unit and performing substitutions using several blocking groups are presented. Despite a relatively large singlet-triplet gap (∆EST ) of >0.2 eV, efficient triplet-to-singlet crossover can be realized, with assistance from resonant SVC. To enhance the spin crossover, electronic energy levels are fine-tuned, thereby causing ∆EST to be in resonance with a triplet-triplet gap (∆ETT ). A sizable population transfer between spin multiplicities (>103 s-1 ) is achieved, and this result agrees well with theoretical predictions. An OLED fabricated using a multiple-resonance-type SVC-TADF emitter with CIE color coordinates of (0.15, 0.05) exhibits ultrapure blue emissions, with a narrow full-width-at-half-maximum of 21 nm and a high external quantum efficiency of 23.1%.

Improved Efficiency and Lifetime of Deep-Blue Hyperfluorescent Organic Light-Emitting Diode using Pt(II) Complex as Phosphorescent Sensitizer.

Although the organic light-emitting diode (OLED) has been successfully commercialized, the development of deep-blue OLEDs with high efficiency and long lifetime remains a challenge. Here, a novel hyperfluorescent OLED that incorporates the Pt(II) complex (PtON7-dtb) as a phosphorescent sensitizer and a hydrocarbon-based and multiple resonance-based fluorophore as an emitter (TBPDP and ν-DABNA) in the device emissive layer (EML), is proposed. Such an EML system can promote efficient energy transfer from the triplet excited states of the sensitizer to the singlet excited states of the fluorophore, thus significantly improving the efficiency and lifetime of the device. As a result, a deep-blue hyperfluorescent OLED using a multiple resonance-based fluorophore (ν-DABNA) with Commission Internationale de L'Eclairage chromaticity coordinate y below 0.1 is demonstrated, which attains a narrow full width at half maximum of ≈17 nm, fourfold increased maximum current efficiency of 48.9 cd A-1 , and 19-fold improved half-lifetime of 253.8 h at 1000 cd m-2 compared to a conventional phosphorescent OLED. The findings can lead to better understanding of the hyperfluorescent OLEDs with high performance.

Holistic Approach to the Mechanism Study of Thermal Degradation of Organic Light-Emitting Diode Materials.

The design of stable organic light-emitting diode materials is the key to long lifetime displays under various stressful conditions. Elucidating the degradation mechanism of the materials at the molecular level provides useful information for securing high stability. Previous works based on experiments or computations disclosed only a part of the whole degradation process. Here, we propose a holistic approach to the systematic analysis of the degradation mechanism by combining experimental mass analysis and computation in a semi-automated fashion. The mass analysis identifies molecular weights of feasible products from degradation reactions. Then, the computational analysis goes through initiation, propagation, and termination phases. The initiation phase determines radical fragments and reactive sites, triggering the propagation process. In the propagation phase, we subsequently perform intermediate sampling, reaction network construction, and kinetic analysis. As a proof of concept, this approach was applied to the thermal degradation problem during the sublimation purification process. Two major pathways were successfully elucidated with full atomistic details.

Degradation of blue-phosphorescent organic light-emitting devices involves exciton-induced generation of polaron pair within emitting layers.

Degradation of organic materials is responsible for the short operation lifetimes of organic light-emitting devices, but the mechanism by which such degradation is initiated has yet to be fully established. Here we report a new mechanism for degradation of emitting layers in blue-phosphorescent devices. We investigate binary mixtures of a wide bandgap host and a series of novel Ir(III) complex dopants having N-heterocyclocarbenic ligands. Our mechanistic study reveals the charge-neutral generation of polaron pairs (radical ion pairs) by electron transfer from the dopant to host excitons. Annihilation of the radical ion pair occurs by charge recombination, with such annihilation competing with bond scission. Device lifetime correlates linearly with the rate constant for the annihilation of the radical ion pair. Our findings demonstrate the importance of controlling exciton-induced electron transfer, and provide novel strategies to design materials for long-lifetime blue electrophosphorescence devices.