Thanks to tremendous efforts of the last twenty years, Phosphorescent Organic Light-Emitting Diodes (PhOLEDs) represent nowadays a prevalent technology. In this technology, all the high-efficiency PhOLEDs are multi-layer devices (ML-PhOLEDs) constituted, in addition to the Emissive Layer (EML), of a stack of functional organic layers. These layers have a crucial role in the device performance. Single-Layer PhOLEDs (SL-PhOLEDs) represent ideal OLEDs, only constituted of the electrodes and the EML. However, reaching high-performance SL-PhOLED is far from an easy task as removing the functional layers of an OLED stack dramatically decreases the performance. To reach a high PhOLED efficiency without the different functional layers, the efficient injection, transport and recombination of charges should be insured by the EML and more particularly by the host material. In the present exhaustive review, we analyse the different molecular design strategies, which have been used to construct high-efficiency hosts for SL-PhOLED. This allows to draw a structure/properties/device performance relationship map of interest for the future design of functional materials for SL-PhOLEDs.
Financing ANR (n°19-CE05-0024), Picture from Jean-Francois Bergamini-ISCR