PHC host materials for simplified PhOLED devices

Since the discovery of organic light-emitting diodes in 1987, technology has evolved to increase the efficiency of these light-emitting devices by modifying their operating principle via the nature of the energy transfers involved in the devices. In just 35 years, four types of OLED have been developed: fluorescent OLEDs, phosphorescent OLEDs (PhOLEDs), thermally activated delayed fluorescence emission OLEDs (TADF-LEDs) and hyperfluorescent OLEDs (HF-OLEDs).

By 2023, the most efficient devices in terms of emission and stability will be PhOLEDs, in which the emissive layer is a host matrix doped with a phosphorescent emitter. By appropriately matching the energy levels between the matrix and the dopant, virtually all the excitons formed in the matrix when the device is switched on are transferred to the dopant, which is the diode's emitter. As a result, the device's internal quantum efficiency can reach 100%.

The properties required for an organic semiconductor to be used as a host matrix in a PhOLED are: (i) a good match between the HOMO and LUMO energy levels at the Fermi level of the electrodes and/or the HOMO and LUMO levels of the intermediate layers to facilitate charge injection into the matrix, (ii) balanced electron and hole mobility to optimize the recombination of electron-hole pairs, (iii) good thermal and morphological stability to enable vacuum deposition and to avoid matrix decomposition or evolution when the diode is in operation, and finally (iv) good matching of the matrix energy levels to the dopant and, in particular, the S ¹ and T ¹ levels of the matrix must be higher than the S ¹ and T ¹ levels of the dopant to enable full transfer of singlet and triplet excitons from the matrix to the dopant.

Based on these prerequisites, fantastic progress has been made since the first PhOLED was described in 1998. On the one hand, researchers have sought to simplify design, manufacturing methods and costs, as well as ease of recycling. On the other hand, the search for stable and efficient host matrices has been a highly developed field of research.

Today, most host matrices leading to efficient devices are based on small electron-donor (D: carbazole, diphenylamine...) or electron-acceptor (A : phosphine oxide, sulfone, oxadiazole...) generally of the D-A, D-π-A (π for π system) or D-spiro-A (spiro for spiro junction) type, enabling external quantum yields approaching 30% for the three main colors of red, green and blue. The downside of

of these D- and A-unit-based host matrices is that they all contain...