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



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.