Addressing Fundamental Limits to Blue OLED Stability

Phosphorescence and triplet-triplet annihilation are two of the leading approaches to realizing more efficient and stable blue organic light emitting devices. To address the stability of blue phosphorescence, we modulate the exciton lifetime of the emitters without drastically decreasing the light extraction efficiency using plasmonic light extraction structures. The results are compared to all-optical studies suggesting very strong dependence of OLED stability on excitonic lifetime.<br/> <br/>Increasing the fraction of singlet excitons generated from triplet-triplet annihilation (TTA) remains the major challenge confronting applications of solid-state excitonic upconversion in OLEDs. We will describe two recent efforts addressing this problem. First, we investigate rapid intersystem crossing (ISC) of high energy triplets (T₂) to the desired singlet state (S₁). Generating more singlet excitons is expected to improve efficiencies, but this approach comes with a tradeoff. Rapid ISC can exacerbate losses due to faster relaxation of the original triplet states (T₁) back to the ground state (S₀), as well as faster relaxation of T₂→T₁. We describe the computational design and experimental characterization of new TTA materials with predicted ISC rates more than three orders of magnitude faster than ISC in the classic material 9,10-diphenylanthracene (DPA). Experimentally, we find that the upconversion efficiency <i>doubles</i> in the limit of large enhancements in ISC, confirming that tuning the T₂→S₁ ISC rate can indeed provide a strategy for the systematic improvement of TTA materials, and demonstrating promising new materials for applications. Second, we show that the tailorability and highly porous, but ordered structure of metal-organic frameworks (MOFs) combine to minimize inter-triplet exchange coupling and engineer effective spin mixing between singlet and quintet triplet-triplet pair states. We demonstrate singlet-quintet coupling in a pyrene-based MOF, NU-1000. An anomalous magnetic field effect is observed from NU-1000 corresponding to an induced resonance between singlet and quintet states that yields an increased fusion rate at room temperature under a relatively low applied magnetic field of 0.14 T. Our results suggest that MOFs offer particular promise for engineering the spin dynamics of multi-excitonic processes and improving their upconversion performance.