Hsiao-Fan Chen

Bipolar Host Materials: A Chemical Approach for Highly Efficient Electrophosphorescent Devices

The future of organic light-emitting devices (OLEDs) is drifting from electrofluorescence toward electrophosphorescence due to the feasibility of realizing 100% internal quantum efficiency. There is limited availability of transition metals (TMs) such as Ir, Os, and Pt, which are used for color-tunable phosphorescent emitters, and the use of the host-guest strategy is necessary for suppressing the detrimental triplet-triplet annihilation inherently imparted by the TM-centered emitters. The inevitable demands of organic host materials provide organic chemists with tremendous opportunities to contribute their expertise to this technology. With suitable molecular design and judicious selection of chemical structures featured with different electronic nature, the incorporation of hole-transporting (HT) and electron-transporting (ET) moieties combines the advantages of both functional units into bipolar host materials, which perform balanced injection/transportation/recombination of charge carriers and consequentially lead the OLEDs to have higher performances and low roll-off efficiencies. This review highlights recently developed bipolar host materials with the focus on molecular design strategies and the structure-property-performance relationships of various classes of bipolar host materials, which are classified into several categories according to the structural features of their constituents (HT/ET blocks and spacers).

1,3,5-Triazine derivatives as new electron transport–type host materials for highly efficient green phosphorescent OLEDs

We have synthesized three star-shaped 1,3,5-triazine derivatives—2,4,6-tris(biphenyl-3-yl)-1,3,5-triazine (T2T), 2,4,6-tris(triphenyl-3-yl)-1,3,5-triazine (T3T), and 2,4,6-tris(9,9′-spirobifluorene-2-yl)-1,3,5-triazine (TST)—as new electron transport (ET)-type host materials for green phosphorescent organic light-emitting devices. The morphological, thermal, and photophysical properties and the electron mobilities of these ET-type host materials are influenced by the nature of the aryl substituents attached to the triazene core. The meta–meta linkage between the 1,3,5-triazine core and the peripheral aryl moieties in T2T and T3T limited the effective extension of their π conjugation, leading to high triplet energies of 2.80 and 2.69 eV, respectively. Time-of-flight mobility measurements revealed the good electron mobilities for these compounds (each > 10−4 cm2V−1 s−1), following the order T3T > TST > T2T. The device incorporating T2T as the host, doped with (PPy)2Ir(acac) and 1,3,5-tris(N-phenylbenzimidizol-2-yl)benzene (TBPI) as the ET layer, achieved a high external quantum efficiency (ηext) of 17.5% and a power efficiency (ηp) of 59.0 lm W−1. For the same device configuration, the T3T-based device provided values of ηext and ηp of 14.4% and 50.6 lm W−1, respectively; the TST-based device provided values of 5.1% and 12.3 lm W−1, respectively. We ascribe the superior performance of the T2T-based devices to balanced charge recombination; we ascribe the poor efficiencies of the TST-based devices to its relatively low triplet energy (2.54 eV), which did not allow efficient confinement of the triplet excitons on the green phosphorescent emitter (PPy)2Ir(acac).

Carbazole–benzimidazole hybrid bipolar host materials for highly efficient green and blue phosphorescent OLEDs

In this study, we synthesized a series of bipolar hosts (CbzCBI, mCPCBI, CbzNBI, and mCPNBI) containing hole-transporting carbazole and electron-transporting benzimidazole moieties and then examined the morphological, thermal, and photophysical properties and carrier mobilities of these bipolar host materials. Altering the linking topology (C- or N-connectivity of the benzimidazole) changed the effective conjugation length and led to different excited-state solvent relaxation behavior. The N-connected compounds (CbzNBI, mCPNBI) possessed higher triplet energies (ET) than those of their C-connected analogues (CbzCBI, mCPCBI) by 0.23 eV. The higher values of ET of CbzNBI and mCPNBI endowed them with the ability to confine triplet excitons on the blue-emitting guest. A blue PhOLED device incorporating mCPNBI achieved a maximum external quantum efficiency, current efficiency, and power efficiency of 16.3%, 35.7 cd A−1, and 23.3 lm W−1, respectively; confirming the suitability of using N-connected bipolar hosts for the blue phosphor. The donor/acceptor interactions of the C-connected analogue resulted in a lower triplet energy, making it a suitable bipolar host for green phosphors. A green-phosphorescent device incorporating CbzCBI as the host doped with (PBi)2Ir(acac) achieved a maximum external quantum efficiency, current efficiency, and power efficiency of 20.1%, 70.4 cd A−1, and 63.2 lm W−1, respectively.

Highly efficient double-doped solid-state white light-emitting electrochemical cells

We report highly efficient, solid-state, white light-emitting electrochemical cells (LECs) based on a double-doped strategy, which judiciously introduces an orange-emitting guest, [Ir(ppy)2(dasb)]+(PF6−), into a single-doped emissive layer comprised of an efficient blue-green emitting host, [Ir(dfppz)2(dtb-bpy)]+(PF6−), and a red-emitting guest, [Ir(ppy)2(biq)]+(PF6−), to improve the balance of carrier mobilities and, thus, to enhance the device efficiency. Photoluminescence (PL) measurements show that the single-doped (red guest) and the double-doped (red and orange guests) host–guest films exhibit similar white PL spectra and comparable photoluminescence quantum yields, while the device efficiencies of the double-doped white LECs are twofold higher than those of the single-doped white LECs. Therefore, such enhancement of the device efficiency is rationally attributed to the improved balance of carrier mobilities of the double-doped emissive layer. Peak external quantum efficiency and peak power efficiency of the double-doped white LECs reached 7.4% and 15 lm W−1, respectively. These efficiencies are amongst the highest reported for solid-state white LECs and, thus, confirm that the double-doping strategy is useful for achieving highly efficient white LECs.

Improving the balance of carrier mobilities of host–guest solid-state light-emitting electrochemical cells

We report efficient host-guest solid-state light-emitting electrochemical cells (LECs) utilizing a cationic terfluorene derivative as the host and a red-emitting cationic transition metal complex as the guest. Carrier trapping induced by the energy offset in the lowest unoccupied molecular orbital (LUMO) levels between the host and the guest impedes electron transport in the host-guest films and thus improves the balance of carrier mobilities of the host films intrinsically exhibiting electron preferred transporting characteristics. Photoluminescence measurements show efficient energy transfer in this host-guest system and thus ensure predominant guest emission at low guest concentrations, rendering significantly reduced self-quenching of guest molecules. EL measurements show that the peak EQE (power efficiency) of the host-guest LECs reaches 3.62% (7.36 lm W(-1)), which approaches the upper limit that one would expect from the photoluminescence quantum yield of the emissive layer (∼0.2) and an optical out-coupling efficiency of ∼20% and consequently indicates superior balance of carrier mobilities in such a host-guest emissive layer. These results are among the highest reported for red-emitting LECs and thus confirm that in addition to reducing self-quenching of guest molecules, the strategy of utilizing a carrier transporting host doped with a proper carrier trapping guest would improve balance of carrier mobilities in the host-guest emissive layer, offering an effective approach for optimizing device efficiencies of LECs.

Anionic iridium complexes for solid state light-emitting electrochemical cells

Two anionic iridium complexes Na[Ir(2-(p-tolyl)pyridine)2(CN)2] (A1) and Na[Ir(2-phenylquinoline)2(CN)2] (A2) were synthesized and characterized for use in light-emitting electrochemical cells (LECs). The photophysical and electrochemical studies show that the emission wavelengths and the LUMO energy levels of these complexes are governed by the conjugation length of the cyclometalated ligands. Single-layer LEC devices incorporating anionic complexes were fabricated and tested. Suffering from poor solubility of the anionic complexes, the device performances were rather limited due to the lack of film uniformity. The solubility issue was circumvented by using 18-crown-6 as an additive, efficiently eliminating the cluster structure formed by ion-dipole (sodium ion–lone pair of the nitrile ligand) interaction. The maximum brightness and peak external quantum efficiency achieved by crown-mediated LEC were up to 69 cd m−2 and 1.38%, respectively. The device efficiencies exhibit a great dependence with the LUMO level of the complex, suggesting substantial importance of the electron injection process.

Solid-state light-emitting electrochemical cells employing phosphor-sensitized fluorescence

We report highly efficient phosphor-sensitized solid-state light-emitting electrochemical cells (LECs) utilizing a phosphorescent cationic iridium complex [Ir(dFppy)2(SB)]+(PF6−) as the host and a fluorescent cationic dye (R6G) as the guest. Photophysical studies show that R6G retains a high photoluminescence quantum yield (PLQY) in highly polar media, revealing its suitable use as an emitting guest in an ionic host matrix. Such solid-state LECs achieve quantum efficiency (cd A−1) efficiency, and power efficiency up to 5.5% photon/electron, 19 cd A−1 and 21.3 lm W−1, respectively. The device quantum efficiency achieved is among the highest reported for fluorescent LECs and is higher than one would expect from the PLQY of the R6G fluorescent dye in the host film, thus indicating that phosphor-sensitization is useful for achieving highly efficient fluorescent LECs. Moreover, using narrow-band fluorescent emitters, such as R6G (FWHM, ∼50 nm), is effective in improving the color saturation of solid-state LECs based on cationic complexes.

Bis(diphenylamino)-9,9 '-spirobifluorene functionalized Ir(III) complex: a conceptual design en route to a three-in-one system possessing emitting core and electron and hole transport peripherals

Conceptual design of a three-in-one (luminescence chromophore with electron and hole transports) system was demonstrated by a functionalized Ir(III) complex 3, in which 4,5-diazafluorene and bis(diphenylamino) serve as electron and hole transporting sites, respectively. The poor emission quantum yield of 3 was systematically examined via a series of photophysical studies in combination with theoretical approaches. The far lifting of the π-electron from -NPh2 renders virtually no 3MLCT contribution to the lowest transition in the triplet manifold as compared with that of the parent model 2 without amino substituents. With an empirical approach, we conclude that an energy gap law may account for the major deactivation process. A light-emitting electrochemical cell (LEC) device based on 3 shows peak EQE, peak current efficiency and peak power efficiency at 2.4 V of 0.020%, 0.013 cd A−1 and 0.017 lm/W, respectively. The low device efficiencies are in accordance with the low PL quantum yield, stemming from the ligand-centered radiationless deactivation. The conceptual design presented here should provide valuable information for future progress en route to an ideal three-in-one system suited for OLEDs.

Peripheral modification of 1,3,5-triazine based electron-transporting host materials for sky blue, green, yellow, red, and white electrophosphorescent devices

A systematic comparison of the physical properties and the dual-role (host and electron transport) applications of star-shaped 1,3,5-triazine-based ET-type hosts (T2T, 3N-T2T, 3P-T2T, and oCF3-T2T) with differential peripheral groups was reported. The introduction of N-heterocyclic polar peripheries onto a 1,3,5-triazine core gave evident benefits to the electron injection/transport properties, rendering efficient PhOLEDs with a simpler device configuration feasible. Among these hosts, 3P-T2T can serve both as a promising host and electron-transport material for various Ir-based electrophosphorescence devices. These PhOLEDs configured with the same device structure exhibited low operation voltages with a maximum ηext of 8%, 15.7%, 16.9%, 16.4% and 10.8% for sky blue (FIrpic), green [(PPy)2Ir(acac)], yellow [(Bt)2Ir(acac)], red [(Mpq)2Ir(acac)] and white [FIrpic + 0.5 wt%(Mpq)2Ir(acac)], respectively.

Using a double-doping strategy to prepare a bilayer device architecture for high-efficiency red PhOLEDs

A simple, bilayered, red phosphorescent organic light-emitting device featuring a doubly-doped emitting layer comprising of the novel hole-transporting host DTAF, the electron-transporting host 27SFBI, and the emitter Os(bpftz)2(PPhMe2)2 covering the interfacial region provides an unusually high current of ca. 1560 mA cm−2 at 8.5 V, a maximum brightness of 32 700 cd m−2, external quantum efficiencies as high as 12.3% (10.9% at 1000 cd m−2), and a power efficiency of 13.5 lm W−1. This concise device architecture is very cost-effective and competitive for practical applications.