Charge‐Transport Processes in Host–Dopant Organic Semiconductors

Abstract

AbstractHost–dopant systems are the foundation for designing the light‐emission zones of organic light‐emitting diodes (OLEDs). An efficient OLED design has to consider the detailed charge transport processes in a host–dopant system in order to regulate charges for optimal distribution of excitons. It is reported that two charge transport pathways, Frenkel–Poole type transport via either host sites or dopant sites and charge hopping between host and dopant sites, are at play. The activation energy barrier in the hopping transport is identified as the energy level offset at the host–dopant interface, and is found consistent with the energy offset in the highest occupied molecular orbitals directly measured by ultraviolet photoemission spectroscopy. Dopant concentration and the host–dopant energy offset are identified as the key parameters dictating charge conduction path in the system. Practical equations are derived to calculate carrier mobility as a function of dopant concentration and dopant–host molecular energy offset.