Organic light‐emitting devices (OLEDs) are expected to be adopted as the next generation of general lighting because they are more efficient than fluorescent tubes and are mercury‐free. The theoretical limit of operating voltage is generally believed to be equal to the energy gap, which corresponds to the energy difference between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) for the emitter molecule divided by the electron charge (e). Here, green OLEDs operating below a theoretical limit of the energy gap (Eg) voltage with high external quantum efficiency over 20% are demonstrated using fac‐tris(2‐phenylpyridine)iridium(III) with a peak emission wavelength of 523 nm, which is equivalent to a photon energy of 2.38 eV. An optimized OLED operates clearly below the theoretical limit of the Eg voltage at 2.38 V showing 100 cd m−2 at 2.25 V and 5000 cd m−2 at 2.95 V without any light outcoupling enhancement techniques.
A novel pure blue phosphorescent emitter FK306 with fluorinated bipyridyl ligands was developed. The I p level was determined to be 6.3 eV by photoelectron yield spectroscopy (PYS) in the solid state.The photophysical properties of a FK306/mCP film were evaluated.An 11 wt% doped film showed a peak photoluminescence at 454 nm and a high photoluminescent quantum yield (h PL ) of 78 AE 1%. A transient PL decay curve exhibited almost single-exponential decay (98%) with the phosphorescence lifetime (s p ) of 1.51 ms at room temperature. A blue OLED with a structure of [ITO (130 nm)/TAPC (40 nm)/FK306 11-20 wt% doped mCP (10 nm)/B3PyPB (50 nm)/LiF (0.5 nm)/Al (100 nm)] was fabricated and evaluated. A high power efficiency of over 30 lm W À1 and an external quantum efficiency of over 17% were observed. The CIE chromaticity diagram (x, y) was evaluated to be (0.16, 0.25), clearly indicating blue emission. These are the first decent performances using a blue emitter with bipyridyl ligands so far.
Organic light-emitting devices (OLEDs) are widely used in next-generation eco-friendly solid-state display and lighting technologies. Most key optoelectronic functions of organic films used in organic semiconductor devices are dependent on two important factors: the electronic properties of single molecules, and the molecular orientations. The molecular orientation has recently attracted considerable attention, and been recognized as a crucial parameter for determining key optoelectronic functions of organic devices, such as device lifetime, efficiency, ionization potential, and carrier mobility of semiconductor amorphous films. In this review, we discuss horizontal molecular orientation from a molecular engineering perspective considering the three essential layers of OLEDs: the hole transport layer, electron transport layer, and emissive layer. In addition, we address the future challenges of next-generation OLED materials.
Use of the intrinsic optoelectronic functions of organic semiconductor films has not yet reached its full potential, mainly because of the primitive methodology used to control the molecular aggregation state in amorphous films during vapor deposition. Here, a universal molecular engineering methodology is presented to control molecular orientation; this methodology strategically uses noncovalent, intermolecular weak hydrogen bonds in a series of oligopyridine derivatives. A key is to use two bipyridin‐3‐ylphenyl moieties, which form self‐complementary intermolecular weak hydrogen bonds, and which do not induce unfavorable crystallization. Another key is to incorporate a planar anisotropic molecular shape by reducing the steric hindrance of the core structure for inducing π–π interactions. These synergetic effects enhance horizontal orientation in amorphous organic semiconductor films and significantly increasing electron mobility. Through this evaluation process, an oligopyridine derivative is selected as an electron‐transporter, and successfully develops highly efficient and stable deep‐red organic light‐emitting devices as a proof‐of‐concept.
Molecular orientation is one of the most crucial factors to boost the efficiency of organic light-emitting devices. However, active control of molecular orientation of the emitter molecule by the host molecule is rarely realized so far, and the underlying mechanism is under discussion. Here, we systematically investigated the molecular orientations of thermally activated delayed fluorescence (TADF) emitters in a series of carbazole-based host materials. Enhanced horizontal orientation of the TADF emitters was achieved. The degree of enhancement observed was dependent on the host material used. Consequently, our results indicate that π-π stacking, CH/n (n = O, N) weak hydrogen bonds, and multiple CH/π contacts greatly induce horizontal orientation of the TADF emitters in addition to the molecular shape anisotropy. Finally, we fabricated TADF-based organic light-emitting devices with an external quantum efficiency (η ext) of 26% using an emission layer with horizontal orientation ratio () of 79%, which is higher than that of an almost randomly oriented emission layer with of 62% (η ext = 22%).
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