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

Abstract

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 sp 3 ‐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 cm 2 V −1 s −1 ). The meta–meta configuration of the fluorene‐based acceptors allows the bipolar hosts to retain relatively high triplet energies [ E T = 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 η ext of 15.1%, 17.9%, 17.4%, 18%, and 20% for blue (FIrpic), green [(PPy) 2 Ir(acac)], yellowish‐green [ m ‐(Tpm) 2 Ir(acac)], yellow [(Bt) 2 Ir(acac)], and red [Os(bpftz) 2 (PPhMe 2 ) 2 , OS1], respectively. In addition, combining yellowish‐green m ‐(Tpm) 2 Ir(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.