Deep-red light-emitting electrochemical cells based on phosphor-sensitized thermally activated delayed fluorescence

Solid-state light-emitting electrochemical cells (LECs) show the advantages of a simple fabrication process, low-voltage operation, and compatibility with inert electrodes. However, even phosphorescent deep-red LECs still suffer from limited device efficiencies. In this work, we demonstrate efficient deep-red LECs based on phosphor-sensitized thermally activated delayed fluorescence (TADF). A phosphorescent ionic transition metal complex (iTMC) is used as the host and a deep-red TADF emitter is employed as the guest. With rapid intersystem crossing (ISC) to promote intramolecular singlet-to-triplet energy transfer in the iTMC, efficient host-guest Forster energy transfer ensures harvesting both singlet and triplet excitons on the host molecules. In addition, the effective reverse intersystem crossing (RISC) process can further recycle the guest triplet excitons coming from the host-guest Dexter energy transfer and direct triplet exciton formation on the guest under electrical excitation. Therefore, host-guest deep-red LECs doped with a 0.25 wt% guest achieve a peak EQE of up to 5.11%, which is among the highest reported for deep-red LECs. Analysis of the device efficiency by evaluating related device parameters implies that more than 73% of the excitons on the guest molecules can be gathered for electroluminescence (EL). It approaches 3x EL efficiency of fluorescent devices, which can only harvest 25% excitons for light emission, and thus confirms an efficient RISC process to recycle the guest triplet excitons. These results reveal that phosphor-sensitized TADF is useful for achieving highly efficient fluorescent deep-red LECs. However, triplet-triplet annihilation on the guest still hinders the improvement of the device efficiency of the phosphor-sensitized TADF LECs when the guest doping concentration or device current is higher.