PoTA-LEC · Engineering Phosphorescent and TADF small molecules for white Light-emitting Electrochemical Cells

Engineering Phosphorescent and TADF small molecules for white Light-emitting Electrochemical Cells

Light-emitting electrochemical cells (LECs) are getting much attention as single-layered, cost-effective, and moderate performing devices. LEC’s active layer contains an emitter and an electrolyte (ionic liquid or a mixture of ion conductive polymer and inorganic salts). To date, the best emitters are conjugated polymers (CP) or an ionic transition metal complex (iTMC) with average efficiencies of >10 cd/A, and stabilities >1000 h. However, they still encounter serious hitches: i) CPs only harvest singlet excitons (25%) upon recombination (75% triplet excitons are dark), require multistep synthesis/purification, and feature a low device self-stability (spontaneous phase separation); ii) iTMCs harvest both singlet and triplet excitons, but they are based on Ir(III). As alternative Cu(I) complexes have been proposed, reaching moderate efficiencies/stabilities due to their poor redox behavior. Hence, the LEC community exercised to find rainbow emitters like quantum dot (QD), small molecule (SM), and perovskites (P). Since QDs and P contain toxic metal ions (Cd+2 and Pb+4), SMs are the next generation of sustainable emitters due to easy synthesis, high photoluminescence quantum yields (PLQYs) and redox stability, and good carrier motilities. To date, rainbow phosphorescence and thermally activated delayed fluorescence (TADF) SM emitters have not been explored towards realizing stable and efficient white SM-LECs. In contrast, phosphorescence SMs with PLQYs ca. 10 % covering the visible range are known, while red- or white-emitting TADF-LECs have not been set yet. PoTA-LEC aims at developing a full-fledged phosphorescent and TADF SM design to achieve highly performing rainbow LECs. Based on the SM-LEC and -OLED prior arts, PoTA-LEC focuses on two action lines: i) ionic/neutral phosphorescent SMs with peripheral bulky groups and covalently linked host moieties to enhance PLQYs and device efficiency, and ii) ionic/neutral rainbow TADF emitters used as single emitters or hosts. Optimized monochromatic LECs was targeted.

Data: CORDIS, © European Union