Blue emission in sterically shielded multiresonant thermally activated delayed fluorescence emitters

Abstract

Multi-resonant TADF materials are a promising class of emitters capable of addressing the BT.2020 industry requirement for blue emission in electroluminescent displays as they simultaneously show narrowband emission and can harvest both singlet and triplet excitons to produce light. However, these emitters are typically planar and prone to aggregation and their moderately large singlet–triplet energy gap (ΔE_ST) leads to slow upconversion kinetics resulting in severe efficiency roll-off in the device. In this study we present a molecular design that simultaneously results in an emitter having a faster reverse intersystem crossing rate constant (k_RISC) and suppressed aggregation in the film state. Mes-tDABNA emits at λ_PL of 465 nm as 4 wt% doped films in SF3-RZ and has a short delayed lifetime of 45.2 μs. Vacuum-deposited OLEDs with Mes-tDABNA showed blue emission at CIE coordinates of (0.13, 0.15) and a maximum external quantum efficiency, EQE_max, of 18.4%. Unsurprisingly, these devices suffered from rather strong efficiency roll-off (EQE₁₀₀₀ of 5.6%). With the aim of addressing this efficiency roll-off, hyperfluorescent devices containing DMAC-DPS as a TADF sensitizer were fabricated, which showed an improved EQE_max of 23.1% at CIE coordinates of (0.13, 0.17) and milder efficiency roll-off (EQE₁₀₀₀ of 12.7%). These devices showed one of the highest EQE₁₀₀₀ based on DABNA-based emitters to date.