ABSTRACT
Several challenges to the development of high-eciency organic light-emitting diodes (OLEDs) are addressed by the use of a host material. ese include prevention of concentration quenching (a form of self-quenching), providing ecient charge injection, charge
Asanga B. Padmaperuma 10 .1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 235
10 .2 Design Criteria for Host Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 238
10 .3 A Chemist’s Best Friend: Computational Prediction . . . . . . . . . . . . . . . . . . . . 239 10 .3 .1 Electronic Density Distributions and Molecular Orbital Energies . . . 240 10 .3 .2 Excited-State Energy Levels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 242 10 .3 .3 Mobility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 244 10 .3 .4 Computational Design of Host Molecules . . . . . . . . . . . . . . . . . . . . . . . 246
10 .4 Physical and Chemical Characterization of Host Materials . . . . . . . . . . . . . . 247 10 .4 .1 Electrochemical Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 248 10 .4 .2 Use of Unipolar Devices to Characterize Charge Transport . . . . . . . 249 10 .4 .3 Relationship between Charge Transport in the Host
and the Emission Zone Location . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 250
10 .5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 252
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 252
transport, and exciton transfer, and ensuring good stability. In practice, the host material is optimized for charge injection from the hole transport layer (HTL) and electron transport layer (ETL) and charge transport to facilitate charge recombination and exciton formation. is suggests that the host highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) should be similar to the ETL LUMO and HTL HOMO to facilitate ecient electron and hole transfer, respectively. In practice, some energy level mismatch is typically present, but the energy level mismatch should be small to ensure low device operating voltage.
