Simple-structure organic light emitting diodes: Exploring the use of thermally activated delayed fluorescence host and guest materials

We investigate the dependence of the performance of non-doped blue light emitting devices with thermally activated delayed fluorescence (TADF) material bis[4-(9,9-dimethyl-9,10-dihydroacridine)phenyl]sulfone (DMAC-DPS) emission layer on hole and electron transport layers as well as emission layer thickness and study the underlying device physics. On this basis, efficient green and orange devices using DMAC-DPS as host material and TADF material (4s,6s)-2,4,5,6-tetra(9H-carbazol-9-yl)isophthalonitrile (4CzIPN) or 2,3,5,6-tetrakis(3,6-diphenylcarbazol-9-yl)-1,4-dicyanobenzene (4CzTPN-Ph) as emitting dopant are reported. In addition, white devices using single DMAC-DPS: 4CzTPN-Ph emission layer show the maximum external quantum efficiency of 13.4%, maximum power efficiency of 38.3 lm W⁻¹ and current-insensitive Commission Internationale de I'Eclairage (CIE) coordinates of (0.29, 0.39). Compared to the approach of combining TADF host and fluorescent dopant, the present devices enable the utilization of all excitons for light emission and the adoption of broad dopant concentration without significantly affecting device efficiency, which is important for the realization of the desired colour purity for display applications, while maintaining the advantages of simple-structure and low-cost.     

The effect of the electron-donor ability on the OLED efficiency of twisted donor-acceptor type emitters

Two twisted donor-acceptor (D-A) chemical structures, CCDMB and PCDMB, were developed as a new class of thermally activated delayed fluorescence (TADF) emitters for organic light-emitting diodes (OLEDs). Two emitters consist of 3-substituted carbazole as a first donor and trivalent boron as an electron acceptor in common, and carbazole and phenoxazine as second donors with different electron donor ability. While PCDMB with a strong phenoxazine donor decreased the lowest singlet excited state (S₁) level and thus showed a small singlet-triplet energy difference (ΔE_ST) value of 0.13 eV, resulting in effective reverse intersystem crossing (RISC), however, CCDMB with a weak donor showed a large ΔE_ST value of 0.21 eV. Efficient triplet harvesting of PCDMB was confirmed by a delayed component in transient PL decay curves of 25 wt% PCDMB-doped bis[2-(diphenylphosphino)phenyl] ether oxide (DPEPO) films. OLED devices with a CCDMB emitter showed deep-blue emission with Commission Internationale de l’Éclairage (CIE) of (0.16, 0.12) but a low maximum EQE of 5.5%, indicative of insufficient triplet harvesting. PCDMB-based devices showed green emission with CIE of (0.21, 0.45) and a high maximum EQE of 22.3%. Our study revealed the effect of the electron donor ability of structurally similar emitters on ΔE_ST values, triplet harvesting, and device efficiency.     

Theoretical insights on the electroluminescent mechanism of thermally activated delayed fluorescence emitters

Thermally activated delayed fluorescence (TADF) emitters, with internal quantum efficiency (IQE) in organic light-emitting diodes (OLED) approaching 100%, have attracted great attention recently. However, theoretical investigation on the electroluminescent mechanism of TADF emitters is quite rare. In this paper, the time-dependent density functional theory is used to study the property of excited states of the TADF emitters, and it is found that both the geometry and the electronic structure are quite dependent on the functionals. By comparing with the experimental results, a ‘hybrid’ method is adopted to study the photophysical properties of the TADF emitters. Based on the energy structure of the states, the lowest three states are found to have close relation to the electroluminescent process. The dynamics of two lowest excited states are investigated and the rate equation is used to analyze the evolution of the three states involved. A visual picture of the exciton evolution process is obtained, and one can get a better understanding of the up-conversion mechanism of TADF emitters. The analysis of the electron distribution of the transition orbitals indicates that the first singlet excited state of the molecule possesses both the charge transfer and local excitation components, which is a necessary character for a TADF emitter. The comparison of the property of two isomers indicates that the appropriate arrangement of donor groups and acceptor groups is important for a high-efficient TADF emitter.     

White Organic LED with a Luminous Efficacy Exceeding 100 lm W−1 without Light Out‐Coupling Enhancement Techniques

Luminous efficacy (LE), which is given by the ratio of luminous flux to power, is commonly used to measure the power consumption of a light source. Unfortunately, the LE of white organic light‐emitting diodes (OLEDs) still lags behind those of inorganic LED for practically used (>100 lm W^?1). In this paper, an ultraefficient white OLED is discussed based on a newly designed thermally activated delayed fluorescent exciplex host. The resulting white OLED delivers an unusually high forward‐viewing LE of 105.0 lm W^?1 and external quantum efficiency (EQE) η_ext of ≈30% (without using any optical out‐coupling techniques). As far as it is known, specifically, these efficiencies are the highest values among the published white OLEDs to date. Two‐color warm white emission is realized with Commission International de I'Eclairage coordinates of (0.40, 0.48) at a brightness of 1000 cd m^?2. Furthermore, the well‐matched energy alignment endows the device with an extremely low turn‐on voltage (≈2.5 V). Such high efficiencies and excellent device performance should benefit from the advantages of exciplex material solely used as the host. Therefore, this study anticipates that the findings have great potential to boost the LE of OLEDs, and more importantly, fulfill the power efficacy requirement for lighting applications.     

A periphery cladding strategy to improve the performance of narrowband emitters,achieving deep-blue OLEDs with CIEy < 0.08 and external quantum efficiency approaching 20%

Deep-blue thermally activated delayed fluorescence (TADF) emitters with National Television System Committee (NTSC) standard (0.14, 0.08) and narrowband emission have been one of the challenging issues for organic light-emitting diodes (OLEDs). In this work, a novel molecular design strategy, periphery cladding, was used to design and synthesize three deep-blue multi-resonance TADF emitters, N,N, 5,9-tetraphenyl-5,9-dihydro-5,9-diaza-13b-boranaphtho[3,2,1-de]anthracen-7-amine (PAB), 2,12-di-tert-butyl-5,9-bis(4-(tert-butyl)phenyl)-N,N-diphenyl-5,9-dihydro-5,9-diaza-13b-boranaphtho[3,2,1-de]anthracen-7-amine (2tPAB) and 2,12-di-tert-butyl-N,N,5,9-tetrakis(4-(tert-butyl)phenyl)-5,9-dihydro-5,9-diaza-13b-boranaphtho[3,2,1-de]anthracen-7-amine (3tPAB). By cladding large steric hindrance tert-butyl unit at periphery of multi-resonance emitter, the intermolecular interactions were suppressed, thus reducing aggregation-induced emission quenching and improving the PLQY of emitter. 3tPAB with full periphery cladding exhibited higher PLQY (74.7%). As a result, the device based on 3tPAB acquired the best performance by using 1,3-di(9H-carbazol-9-yl)benzene (mCP) with low polarity as host. The maximum external quantum efficiency (EQE_max), CIE coordinates, and full-width at half-maximum (FWHM) of 3tPAB-based device were 19.3%, (0.141, 0.076), and 26 nm, respectively. To our knowledge, this is the first report about narrowband TADF OLEDs with single host that EQE_max approaches 20% while CIE coordinates meet the NTSC blue-light standard.     

Novel Blue Bipolar Thermally Activated Delayed Fluorescence Material as Host Emitter for High‐Efficiency Hybrid Warm‐White OLEDs with Stable High Color‐Rendering Index

A novel thermally activated delayed fluorescence (TADF) molecule, PHCz2BP, is synthesized and used to construct high performance organic light‐emitting diodes (OLEDs) in this work. PHCz2BP is not only the neat emitting layer for efficient sky‐blue OLED, with very high peak external quantum efficiency/power efficiency (EQE/PE) values of 4.0%/6.9 lm W^?1, but also acts as a host to sensitize high‐luminance and high‐efficiency green, orange, and red electrophosphorescence with the universal high EQEs of >20%. More importantly, two hybrid white OLEDs based on the double‐layer emitting system of PHCz2BP:green phosphor/PHCz2BP:red phosphor are achieved. To the best of the knowledge, this is the first report for three‐color (blue–green–red) white devices that adopt a TADF blue host emitter and two phosphorescent dopants without any other additional host. Such simple emitting systems thus realized the best electroluminescent performance to date for the WOLEDs utilizing the hybrid TADF/phosphor strategy: forward‐viewing EQEs of 25.1/23.6% and PEs of 24.1/22.5 lm W^?1 at the luminance of 1000 cd m^?2 with the color rendering indexes of 85/87 and warm‐white Commission Internationale de L'Eclairage coordinates of (0.41, 0.46)/(0.42, 0.45), indicating its potential to be used as practical eye‐friendly solid‐state lighting in future.