Tuning the energy gap and charge balance property of bipolar host by molecular modification: Efficient blue electrophosphorescence devices based on solution-process

Three bipolar hosts composed of electron-accepting diphenylphosphine oxide and electron-donating carbazole/triphenylamine have been synthesized and characterized. With structural topology modification, the particular physical properties of the materials can be subtly optimized, such as the thermal stability, singlet-triplet energy gap and charge balance ability. Both DFT calculation and experiment results demonstrate that the introduced triphenylamine can effective minimize the HOMO-LUMO energy gap, while the carbazole units can prevent the excited energy loss and keep high triplet energy (E-T = 3.0 eV) due to the enhanced molecular rigidity. As a result, solution-processed blue PHOLEDs exhibited a high current efficiency of 25.2 cd A(-1) and a power efficiency of 11.5 lm W-1, which implies that the unique molecular modulation is very cost-effective and competitive for the device performance improving. (C) 2015 Elsevier B.V. All rights reserved.