Are pure hydrocarbons the future of host materials for blue phosphorescent organic light-emitting diodes?

Abstract

The fantastic development of phosphorescent organic light-emitting diodes (PhOLEDs) has been undoubtedly driven by the molecular design of high-efficiency host materials for red, green and blue phosphors. Fine tuning the electronic and physical properties of the host materials has allowed very high External Quantum Efficiencies (EQEs) to be reached, reported nowadays above 30% for the three colours. The most used molecular design strategy consists of judiciously assembling small functional units to maintain a high triplet energy level (E_T), essential to ensure efficient energy transfer from the host to the guest phosphor. To adjust the molecular orbital energy levels and the charge transport, these functional units display electron-donating (for hole transport) and electron-accepting (for electron transport) capabilities. Hundreds of host materials have been reported in the literature since the discovery of PhOLEDs in 1998. However, they all have a common characteristic: they incorporate heteroatoms. However, although these functional units are highly beneficial for the device performance, several works have highlighted their potential instability in the device. As the stability of blue-emitting PhOLEDs is a central concern in the field, finding alternatives to heteroatom-based hosts is a mandatory step. In recent years, pure hydrocarbon (PHC) materials, only built with carbon and hydrogen atoms, have shown real potential for the next generation of PhOLEDs. In the light of the literature, it appears nowadays that the PHC design strategy is very promising for the future development of the OLED industry as a high-performance and low-cost option. This is the purpose of the present Chemistry Frontiers.