Designing Host Materials for the Emissive Layer of Single‐Layer Phosphorescent Organic Light‐Emitting Diodes: Toward Simplified Organic Devices

Abstract Thanks to the tremendous effort over the last 20 years, phosphorescent organic light‐emitting diodes (PhOLEDs) represent a prevalent technology. In this technology, all the high‐efficiency PhOLEDs are multi‐layer devices constituting, in addition to the emissive layer (EML), of a stack of functional organic layers. These layers play a crucial role in the device performance as they improve the injection, transport, and recombination of charges within the EML. Single‐layer PhOLEDs (SL‐PhOLEDs) represent ideal OLEDs, consisting only of the electrodes and the EML. However, reaching high‐performance SL‐PhOLED is far from easy, as removing the functional layers of an OLED stack dramatically decreases the performance. To achieve high SL‐PhOLED efficiency, the efficient injection, transport, and recombination of charges should be insured by the EML, and particularly, by the host material. In the present exhaustive review, the different molecular design strategies are analyzed, which have been used to construct high‐efficiency hosts for SL‐PhOLED. The impact of the electronic properties (triplet energy, HOMO/LUMO energy, mobility etc.) on the device characteristics (threshold voltage, electroluminescent spectrum, external quantum efficiency, etc.) are discussed. This allows to draw a structure/properties/device performance relationship map of interest for the future design of functional materials for SL‐PhOLEDs.