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. A new bipolar host molecule (CzFCN) configuring hole‐transporting carbazole and electron‐transporting cyano‐substituted fluorene via a saturated carbon center is utilized to realize highly efficient electrophosphorescent blue, green, yellowish‐green, yellow, red, and white devices using a common device structure.