Pure Hydrocarbons: An Efficient Molecular Design Strategy for the Next Generation of Host Materials for Phosphorescent Organic Light-Emitting Diodes

Abstract

Fantastic progresses have been made since the discovery of phosphorescent organic light-emitting diodes (PhOLEDs) in 1998. 1 In the emissive layer of a PhOLED, a heavy-metal complex (emitter) is dispersed as a guest in an organic semiconductor (host) in order to harvest both singlet and triplet excitons. This host / guest technique is essential to reach high performance PhOLEDs. Undoubtedly, designing high-efficiency host materials for red, green and blue phosphors has been the main driving force of the field. Nowadays, the most efficient PhOLEDs report very high External Quantum Efficiency (EQE) above 27% for the three colours. The molecular design of these high-efficiency host materials is based on small functional units, which have a crucial role in the device performance. These functional units are constructed on the assembly of electron-rich and / or electron-poor molecular fragments (such as carbazole, phenylamine, phosphine oxide, oxadiazole, pyridine, sulfone etc), which allows to finely tune the molecular orbitals energy levels (HOMO and LUMO) and the mobility of the charge carriers (hole and electrons). However, these molecular fragments incorporate heteroatoms, which in turn increase the molecular design complexity and production cost of the target materials and also create potential instability in the device. Indeed, the fragile C-N, C-P and C-S bonds of such heteroatoms-based hosts are involved, in part, in the OLED instability. As the instability of OLEDs is one of the most important problem to address at the current stage of development, developing new generations of host materials, without heteroatoms, has appeared as an important challenge in the field. In recent years, this new generation of host materials, called Pure HydroCarbons (PHC), only incorporating carbon and hydrogen atoms has drawn the attention of the scientific community. PHC host materials only contain benzene rings, one of the most robust and inert building unit, which are assemble with suitable linkages to adjust their energy levels. However, reaching high performance PhOLEDs with PHC-based hosts has been a real challenge. In the present account, thanks to a structure-properties relationship study, we recount the history of PHC-based host materials for PhOLEDs from the first example published in 2005, 2 to the recent advances, which have shown that PHC hosts can overpass the performance of heteroatoms-based hosts. 3 Our group has contributed over the years to unravel the design principles of PHC hosts in order to improve the devices performance. In the light of recent results, PHC hosts now enter a new era and this work shows that the PHC design strategy is promising for the future development of OLED industry as a high-performance and lowcost option.

Simplified Green-Emitting Single-Layer Phosphorescent Organic Light-Emitting Diodes with an External Quantum Efficiency > 22%

“…2 This achievement has been possible thanks to the development of hundreds of host materials for the last 20 years. [15][16][17][18][19][20] Beside molecular engineering, the architecture of the device itself is of great importance to maximize the conversion of electric energy into luminous energy. [21][22][23] In this context, the most performant PhOLEDs to date are constructed on a multi-layer (ML) stack, which insures a good injection, transport and recombination of holes and electrons in the EML.…”

Efficient monomolecular white emission of phenothiazine boronic ester derivatives with room temperature phosphorescence

“…28 The applied substitution engineering allows a shift in IP PE and EA PE in relatively narrow ranges. After comparison of the average IP PE and EA PE values of the studied compounds with those of other organic semiconductors, 29 it can be concluded that they can exhibit good hole and electron injecting properties in the appropriate device structure.…”

Spiro-configured dibenzosuberene compounds as deep-blue emitters for organic light-emitting diodes with a CIEy of 0.04

“…5,12,[22][23][24][25] However, whatever the nature of the emitters, achieving high efficiency OLEDs in this range of wavelengths has been one of the most difficult problems to solve for the last twenty years. For example, in phosphorescent OLEDs, which are the most mature technology in the field, very high efficiency red, green and sky-blue devices have been achieved (EQE above 25%) 13,[22][23][24][25][26][27] but deep blue and violet OLEDs are still unattainable. In terms of CIE coordinates, different standards can be found in the literature.…”