Fluorene‐Based Asymmetric Bipolar Universal Hosts for White Organic Light Emitting Devices

Abstract

Two new bipolar host molecules composed of hole‐transporting carbazole and electron‐transporting cyano (CzFCN) or oxadiazole (CzFOxa)‐substituted fluorenes are synthesized and characterized. The non‐conjugated connections, via an sp3‐hybridized carbon, effectively block the electronic interactions between electron‐donating and ‐accepting moieties, giving CzFCN and CzFOxa bipolar charge transport features with balanced mobilities (10−5 to 10−6 cm2 V−1 s−1). The meta–meta configuration of the fluorene‐based acceptors allows the bipolar hosts to retain relatively high triplet energies [ET = 2.70 eV (CzFOxa) and 2. 86 eV (CzFCN)], which are sufficiently high for hosting blue phosphor. Using a common device structure – ITO/PEDOT:PSS/DTAF/TCTA/host:10% dopants (from blue to red)/DPPS/LiF/Al – highly efficient electrophosphorescent devices are successfully achieved. CzFCN‐based devices demonstrate better performance characteristics, with maximum ηext of 15.1%, 17.9%, 17.4%, 18%, and 20% for blue (FIrpic), green [(PPy)2Ir(acac)], yellowish‐green [m‐(Tpm)2Ir(acac)], yellow [(Bt)2Ir(acac)], and red [Os(bpftz)2(PPhMe2)2, OS1], respectively. In addition, combining yellowish‐green m‐(Tpm)2Ir(acac) with a blue emitter (FIrpic) and a red emitter (OS1) within a single emitting layer hosted by bipolar CzFCN, three‐color electrophosphorescent WOLEDs with high efficiencies (17.3%, 33.4 cd A−1, 30 lm W −1), high color stability, and high color‐rendering index (CRI) of 89.7 can also be realized.

Efficient fluorescence/phosphorescence white organic light-emitting diodes with ultra high color stability and mild efficiency roll-off

“…The efficiency of most of the reported highly efficient WOLEDs even drops to single digit at high brightness of 5000 or 10 000 cd/m 2 . [17][18][19] Besides, the spectra of most of the reported WOLEDs usually exhibits a big change with different brightnesses, which implies that the color of the WOLEDs under the maximum luminous efficiency is not really white emission as some reports suggest.…”

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

“…All derivatives exhibited irreversible oxidation, as commonly observed for carbazole based materials, owing to the instability of the formed cations. 66 The HOMO levels were calculated from the onset of the oxidation peaks relative to the oxidation of ferrocene and span a narrow range between À5.65 eV and À5.70 eV (Fig. S9, ESI †), indicating no significant barrier for hole injection from the adjacent layers.…”

High efficiency and low driving voltage blue/white electrophosphorescence enabled by the synergistic combination of singlet and triplet energy of bicarbazole derivatives

“…In general, the bipolar molecules containing both p-type and n-type moieties can facilely achieve high performance even in the most challengable blue phosphorescent OLEDs (PhOLEDs). By choosing reasonable linkage, the resulted materials with both electron donating/withdrawing groups can minimize intramolecular charge transfer (ICT) and thus gain high triplet energy (E T ) level as well as matched HOMO/LUMO levels for carrier injections [13][14][15][16][17][18]. For example, Lee's group reported a new bipolar host molecule CbBPCb (E T = 2.77 eV), which can achieve very high EQE of 30.1% in sky-blue PhOLEDs [19].…”