Controllably Tuning Excited‐State Energy in Ternary Hosts for Ultralow‐Voltage‐Driven Blue Electrophosphorescence

Abstract

Phosphorescent organic light-emitting diodes (PHOLEDs), with 100 % theoretical internal efficiency, are being rapidly developed as a most promising approach to meet the urgent and extensive demand of energy-efficient and portable digital terminals and lighting sources.[1] Thanks to the recent breakthrough of highly efficient blue PHOLEDs [2] and outcoupling technologies, PHOLEDs in full color can already realize extremely high efficiencies that approach those of fluorescent tubes (about 70 Lm W À1 ).[3] Nevertheless, as the hosts in the emitting layers (EMLs) should have higher triplet excited energy levels (T 1 ) to confine the excitons on phosphorescent guests, [4] the highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) energy gaps in PHOLEDs are often much larger than their fluorescent counterparts, which consequently result in poor energy-level alignment and thus higher driving voltages. This drawback not only complicates the design of driving circuit, but also directly reduces power efficiency (PE). [5] Thus, the low-voltage-driving high-efficiency PHOLED remains the biggest challenge. Su, Kido, et al. have reported green PHOLEDs with extremely low operating voltages of 2.18 V for onset and 2.41 V at 100 cd m À2 through good management of the interfacial contact between electron transporting layers and anodes.[5] However, there are only a few blue PHOLEDs that achieve low driving voltages; for example, applicable luminance at a driving voltage of less than 3 V.[6] The formidable challenge is the high barriers for carrier injection and transportation deriving from the prerequisite of extremely high T 1 of the hosts, for example, 2.85 eV (0.2 eV higher than that of blue phosphor iridium-(III)bis(4,6-(difluorophenyl)pyridinato-N,C2)picolinate (FIrpic; Scheme 1). This issue actually reflects the intrinsic contradiction between optical and electrical properties of the materials. As the singlet excited energy levels (S 1 ) are related to HOMO-LUMO energy gaps, the challenge has actually evolved into an original scientific problem: how to controllably adjust S 1 without changing T 1 . Unfortunately, as most of the modification approaches have the same effects on both singlet and triplet states, [7] only few hosts possess both of the energy difference between S 1 and T 1 (DE ST ) of less than 0.4 eV and T 1 of more than 2.85 eV.[6b]We recently reported several ambipolar ternary aryl phosphine oxide (APO) hosts based on indirect linkage. [6b] Their DE ST was tuned to 0.45 eV, and low driving voltages and high efficiencies were realized. However, the indirect linkage can hardly afford multiple modifications. We believe that the Scheme 1. The energy-transfer process in phosphorescent doping systems, and molecular structures of DBFxPOCzn.

Structure–property relationship of 4-substituted-spirobifluorenes as hosts for phosphorescent organic light emitting diodes: an overview

“…Using an insulating heteroatom is also an efficient strategy to obtain high E T materials with short π-conjugated pathway. Thus, silicium (silane), [44][45][46][47][48][49] and phosphoryl group (P=O), 40,44,47,[50][51][52][53] may act as an effective breaking point of the π-conjugation between the main core of the molecule and the outer groups linked to the heteroatom (phenyl for example in the case of the efficient electron transporting diphenylphosphine oxide unit). More recently, another efficient molecular design based on a π-conjugation breaking induced by an insulating spiro bridge (Called Donor-Spiro-Acceptor design) has been also successfully introduced in literature.…”

Properties modulation of organic semi-conductors based on a donor-spiro-acceptor (D-spiro-A) molecular design: new host materials for efficient sky-blue PhOLEDs

“…4 In addition, in the last fifteen years, it has been demonstrated that highly efficient blue emission should not be obtained in OLED devices using pure organic singlet exciton emissive layers (EML) but should require more sophisticated EML in which a blue phosphor is doped in an organic host material to harvest both singlet and triplet excitons. 2,[16][17][18][19][20][21][22] Thus, when designing bipolar hosts, one should carefully consider the selection of the donor/acceptor pair and their relative positions within the dye. [14][15][16][17][18] One of the weakest links of this technology remains the design of highly efficient host materials for blue phosphors.…”

“…[30][31][32][33] This 'steric hindrance' strategy is most of the time performed through the incorporation of a sterically hindered ortho linkage efficiently leading to a π-conjugation restriction. 20 For example, in the 9'triphenylsilanyl-9'H- [9,3',6',9"]ter-carbazole (Sit-Cz), the direct connexion of the silane to a carbazole unit through the nitrogen atom renders this carbazole electro-deficient (HOMO: -5.54 eV, LUMO :-2.3 eV, E T : 3.0 eV). 31 Using an insulating heteroatom is also an efficient strategy to obtain high E T materials with short π-conjugated pathway.…”

Novel Design of Iridium Phosphors with Pyridinylphosphinate Ligands for High-Efficiency Blue Organic Light-emitting Diodes

“…The introduction of phosphoryl (P = O)moiety will improve the coordination fields of the ancillary ligand to make a hypsochromic shift. More importantly, P = O is a strong electron-withdrawing group capable of polarizing the molecule, which has been widely used in bipolar host materials and electron transport materials based on its excellent electron transport property52535455565758596061. In our former publications, we also reported some efficient OLEDs with Ir(III) complexes containing the electron transport P = O moiety386263.…”