Structure–Property Relationship of Pyridine‐Containing Triphenyl Benzene Electron‐Transport Materials for Highly Efficient Blue Phosphorescent OLEDs

Abstract

Three triphenyl benzene derivatives of 1,3,5‐tri(m‐pyrid‐2‐yl‐phenyl)benzene (Tm2PyPB), 1,3,5‐tri(m‐pyrid‐3‐yl‐phenyl)benzene (Tm3PyPB) and 1,3,5‐tri(m‐pyrid‐4‐yl‐phenyl)benzene (Tm4PyPB), containing pyridine rings at the periphery, are developed as electron‐transport and hole/exciton‐blocking materials for iridium(III) bis(4,6‐(di‐fluorophenyl)pyridinato‐N,C2′)picolinate (FIrpic)‐based blue phosphorescent organic light‐emitting devices. Their highest occupied molecular orbital and lowest unoccupied molecular orbital (LUMO) energy levels decrease as the nitrogen atom of the pyridine ring moves from position 2 to 3 and 4; this is supported by both experimental results and density functional theory calculations, and gives improved electron‐injection and hole‐blocking properties. They exhibit a high electron mobility of 10−4–10−3 cm2 V−1 s−1 and a high triplet energy level of 2.75 eV. Confinement of FIrpic triplet excitons is strongly dependent on the nitrogen atom position of the pyridine ring. The second exponential decay component in the transient photoluminescence decays of Firpic‐doped films also decreases when the position of the nitrogen atom in the pyridine ring changes. Reduced driving voltages are obtained when the nitrogen atom position changes because of improved electron injection as a result of the reduced LUMO level, but a better carrier balance is achieved for the Tm3PyPB‐based device. An external quantum efficiency (EQE) over 93% of maximum EQE was achieved for the Tm4PyPB‐based device at an illumination‐relevant luminance of 1000 cd m−2, indicating reduced efficiency roll‐off due to better confinement of FIrpic triplet excitons by Tm4PyPB in contrast to Tm2PyPB and Tm3PyPB.

Solution-Processed Blue/Deep Blue and White Phosphorescent Organic Light-Emitting Diodes (PhOLEDs) Hosted by a Polysiloxane Derivative with Pendant mCP (1,3-bis(9-carbazolyl)benzene)

“…48 that an additional electron-transport layer enhances efficiency in FIrpic devices. 59 The E T values of PmCPSi and Tm3PyPB (2.75 eV) 15,60 are sufficiently high to confine triplet excitons in the emitting layer and to prevent luminescence quenching. The electroluminescence spectra of the blue PhOLEDs are shown in Figure 4d.…”

“…10,11 Recently, high external quantum efficiency over 25% has been achieved for blue and white phosphorescent OLEDs using small-molecule hosts. [12][13][14][15] However, complicated co-evaporation techniques and precise processing controls are required for small-molecule based devices to ensure high reproducibility for commercialization of the product. Techniques that use Solution-processing, such as spin-coating or inkjet-printing, are more promising with respect to the reduction of fabrication costs and the realization of large-area displays for simple device configurations.…”

An n-type, new emerging luminescent polybenzodioxane polymer for application in solution-processed green emitting OLEDs

“…The efficiency of OLEDs is improved by incorporation of an electron-transport host material between the emissive layer and the cathode. [7][8][9][10] In general, organic materials are p-type and have signicantly higher mobility for holes in comparison to electrons, which leads to charge carrier imbalance inside the emissive layers and more oen, the exciton formation occurs at the emissive layer-electron transport layer interface. Emissive layers in OLED require a charge carrier balance for optimum efficiency and emissive materials with good electron mobilities are being developed for this purpose.…”

Thieno[3,4-c]pyrrole-4,6-dione as novel building block for host materials for red PhOLEDs

“…15,41 In contrast, the nature of the electron accepting moieties exhibits more variations. Among the most frequently used acceptors are oxadiazoles, 27,33,38,[42][43][44] phenantrolines, 31,45 pyridines, 32,46 benzimidazoles, 26,28,47 1,3,5-triazines, 39,48,49 1,2,4-triazoles, 29,38,50 phosphine oxides, [51][52][53] sulfones, 15,54 ketones, 40,41 carbolines 55,56 and benzonitriles. 11,57 The thieno [3,4-c]pyrrole-4,6-dione (TPD) (Scheme 1) has been successfully employed as electron accepting unit in donor-acceptor polymers for organic photovoltaics in combination with various donor units.…”