Concept: Light-emitting diodes
The grand vision of manufacturing large-area emissive devices with low-cost roll-to-roll coating methods, akin to how newspapers are produced, appeared with the emergence of the organic light-emitting diode about 20 years ago. Today, small organic light-emitting diode displays are commercially available in smartphones, but the promise of a continuous ambient fabrication has unfortunately not materialized yet, as organic light-emitting diodes invariably depend on the use of one or more time- and energy-consuming process steps under vacuum. Here we report an all-solution-based fabrication of an alternative emissive device, a light-emitting electrochemical cell, using a slot-die roll-coating apparatus. The fabricated flexible sheets exhibit bidirectional and uniform light emission, and feature a fault-tolerant >1-μm-thick active material that is doped in situ during operation. It is notable that the initial preparation of inks, the subsequent coating of the constituent layers and the final device operation all could be executed under ambient air.
Stacks of potential: A cross-linkable host-guest system for state-of-the-art multi-layer device stacks is developed. The spirobifluorene-based host materials were co-cross-linked with green emitting phosphorescent (fac)-Ir(III) complexes. As a result a five-layered organic light-emitting diode (OLED) could be fabricated in which each organic layer has been deposited from solution.
High quantum efficiencies of above 30% in blue phosphorescent organic light emitting diodes are achieved by using novel pyridoindole-based bipolar host materials. A high quantum efficiency of 30.0% is obtained at 100 cd/m(2) by using the new host materials.
A series of 1,8-naphthalimide derivatives has been designed to explore their optical, electronic, and charge transport properties as charge transport and/or luminescent materials for organic light-emitting diodes (OLEDs). The frontier molecular orbitals (FMOs) analysis have shown that the vertical electronic transitions of absorption and emission are characterized as intramolecular charge transfer (ICT) for electron-donating and aromatic groups substituted derivatives. However, the ICT character of the electron-withdrawing substituted derivatives is not significant. The calculated results show that their optical and electronic properties are affected by the substituent groups in 4-position of 1,8-naphthalimide. Our results suggest that 1,8-naphthalimide derivatives with electron-donating -OCH(3) and -N(CH(3))(2) (1 and 2), electron-withdrawing -CN and-COCH(3) (3 and 4), 2-(thiophen-2-yl)thiophene (5), 2,3-dihydrothieno[3,4-b][1, 4]dioxine (6), 2-phenyl-1,3,4-oxadiazole (7), and benzo[c][1,2,5]thiadiazole (8) fragments are expected to be promising candidates for luminescent materials for OLEDs, particularly for 5 and 7. In addition, 3 and 7 can be used as promising hole transport materials for OLEDs. This study should be helpful in further theoretical investigations on such kind of systems and also to the experimental study for charge transport and/or luminescent materials for OLEDs.
Ancillary ligands in heteroleptic iridium complexes significantly influence the orientation of the transition dipole moments. Ir(ppy)3, a homoleptic iridium complex, exhibits isotropic dipole orientation, whereas the heteroleptic Ir complexes of Ir(ppy)2 tmd show a highly preferred dipole orientation (78%) in the horizontal direction. In addition, we demonstrate an unprecedented highly efficient green OLED exhibiting an EQE of 32.3% and a power efficiency of 142.5 lm/W by using an emitter with high quantum yield and horizontally oriented dipoles.
Two aggregation-induced emission active luminogens (TPE-pTPA and TPE-mTPA) were successfully synthesized. For comparison, another six similar compounds were prepared. Because of the introduced hole-dominated triphenylamine (TPA), fluorene groups with high luminous efficiency, and unconjugated linkages, the π conjugation length of the obtained luminogens is effectively restricted to ensure their blue emission. The undoped organic light-emitting diodes based on TPE-pTPA and TPE-mTPA exhibited blue or deep-blue emissions, low turn-on voltages (3 V), and high electroluminescence efficiencies with Lmax , ηC,max , and ηP,max values of up to 26 697 cd m(-2) , 3.37 cd A(-1) , and 2.40 Lm W(-1) .
Organic light-emitting diodes (OLEDs) under constant current operation suffer from a decrease of luminance accompanied by an increase of driving voltage. We report a way to greatly improve the stability of OLEDs having a green emitter exhibiting thermally activated delayed fluorescence (TADF), (4s,6s)-2,4,5,6-tetra(9H-carbazol-9-yl) isophthalonitrile (4CzIPN), by introducing ultrathin (1 to 3 nm) interlayers of 8-hydroxyquinolinato lithium (Liq) between hole-blocking layer and its surrounding emissive and electron-transport layers. Under constant current operation starting at a luminescence of 1,000 cd/m(2), the time to reach 90% of initial luminance (LT90) increased eight times, resulting in LT90 = 1,380 hours after insertion of the interlayers. Combining this new concept and mixed host system, LT95 was further extended to 1315 hours that is 16 times of reference device. This is the best value reported for TADF-based OLEDs and is comparable to the operational lifetimes of well-established phosphorescence-based OLEDs. Thermally stimulated current measurements showed that the number of deep charge traps was reduced with the insertion of the ultrathin Liq interlayer, indicating that reducing the number of deep traps is important for improving the operational lifetime and that exciton-polaron annihilation may be a source of the device degradation.
Pyrene-based π-conjugated materials are considered to be an ideal organic electro-luminescence material for application in semiconductor devices, such as organic light-emitting diodes (OLEDs), organic field-effect transistors (OFETs) and organic photovoltaics (OPVs), and so forth. However, the great drawback of employing pyrene as an organic luminescence material is the formation of excimer emission, which quenches the efficiency at high concentration or in the solid-state. Thus, in order to obtain highly efficient optical devices, scientists have devoted much effort to tuning the structure of pyrene derivatives in order to realize exploitable properties by employing two strategies, 1) introducing a variety of moieties at the pyrene core, and 2) exploring effective and convenient synthetic strategies to functionalize the pyrene core. Over the past decades, our group has mainly focused on synthetic methodologies for functionalization of the pyrene core; we have found that formylation/acetylation or bromination of pyrene can selectly lead to functionalization at K-region by Lewis acid catalysis. Herein, this Minireview highlights the direct synthetic approaches (such as formylation, bromination, oxidation, and de-tert-butylation reactions, etc.) to functionalize the pyrene in order to advance research on luminescent materials for organic electronic applications. Further, this article demonstrates that the future direction of pyrene chemistry is asymmetric functionalization of pyrene for organic semiconductor applications and highlights some of the classical asymmetric pyrenes, as well as the latest breakthroughs. In addition, the photophysical properties of pyrene-based molecules are briefly reviewed. To give a current overview of the development of pyrene chemistry, the review selectively covers some of the latest reports and concepts from the period covering late 2011 to the present day.
The development of efficient organic light-emitting diodes (OLED) and organic photovoltaic cells requires control over the dynamics of spin sensitive excitations. Embedding heavy metal atoms in π-conjugated polymer chains enhances the spin-orbit coupling (SOC), and thus facilitates intersystem crossing (ISC) from the singlet to triplet manifolds. Here we use various nonlinear optical spectroscopies such as two-photon absorption and electroabsorption in conjunction with electronic structure calculations, for studying the energies, emission bands and ultrafast dynamics of spin photoexcitations in two newly synthesized π-conjugated polymers that contain intrachain platinum (Pt) atoms separated by one (Pt-1) or three (Pt-3) organic spacer units. The controllable SOC in these polymers leads to a record ISC time of <~1 ps in Pt-1 and ~6 ps in Pt-3. The tunable ultrafast ISC rate modulates the intensity ratio of the phosphorescence and fluorescence emission bands, with potential applications for white OLEDs.
Courtship signals of many animal species have highly conspicuous traits for successful mating. Fireflies utilize their bioluminescent light as visual courtship signals. In addition to bioluminescent light emission, firefly’s structural role to extract the light from its lantern also drives conspicuous optical signaling. Recently, ultrastructures of firefly lanterns have become of much interest and inspired highly efficient light management. Here we report the unique optical function of hierarchical ultrastructures found in a firefly (Pyrocoelia rufa) and further the biological inspiration for highly efficient organic light-emitting diode applications. The hierarchical structures comprise longitudinal nanostructures and asymmetric microstructures, which were successfully mimicked by using geometry-guided resist reflow, replica molding, and PDMS oxidation. The external quantum efficiency (EQE) of bio-inspired OLEDs is enhanced up to 61 %. The bio-inspired OLEDs clearly show side-enhanced super-Lambertian emission with a wide-viewing angle. Highly efficient light extraction and wide-angle illumination exhibit the hierarchical structures improve the recognition of optical courtship signals in a wide-angle range. Furthermore, this biological inspiration provides a new paradigm for designing functional optical surfaces in lighting or display applications.