High-Efficiency, Non-doped, Pure-Blue Fluorescent Organic Light-Emitting Diodes via Molecular Tuning Regulation of Hot Exciton Excited States.

Tremendous efforts have been made on researching triplet-triplet annihilation (TTA) and thermally activated delayed fluorescence (TADF) materials for realizing high-efficiency blue organic light-emitting diodes (OLEDs) through utilizing triplet exciton conversion to the lowest singlet excited state (S1) from the lowest triplet excited state (T1). However, hot exciton conversion from the upper triplet energy level state (Tn, n > 1) to the lowest singlet excited state (S1) is an increasingly promising method for realizing pure-blue non-doped OLEDs with performances comparable to those of TTA and TADF materials. Herein, two pure-blue fluorescent emitters of donor (D)-π-acceptor (A) type, PIAnCz and PIAnPO, were designed and synthesized. The excited-state characteristics of PIAnCz and PIAnPO, confirmed by theoretical calculations and photophysical experiments, demonstrated these materials' hot exciton properties. Based on PIAnCz and PIAnPO as emission layer materials, the fabricated non-doped devices exhibited pure-blue emission with Commission Internationale de l'Eclairage (CIE) coordinates of (0.16, 0.12) and (0.16, 0.15), maximum luminescences of 10,484 and 15,485 cd m-2, and maximum external quantum efficiencies (EQEs) of 10.9 and 8.3%. Besides, at a luminescence of 1000 cd m-2, the EQEs of PIAnPO-based devices can still be high at 7.7%, and the negligible efficiency roll-off was 6.0%. The device performance of both materials demonstrates their outstanding potential for commercial application.

Highly efficient non-doped blue fluorescent OLEDs with low efficiency roll-off based on hybridized local and charge transfer excited state emitters.

Designing a donor-acceptor (D-A) molecule with a hybridized local and charge transfer (HLCT) excited state is a very effective strategy for producing an organic light-emitting diode (OLED) with a high exciton utilization efficiency and external quantum efficiency. Herein, a novel twisting D-π-A fluorescent molecule (triphenylamine-anthracene-phenanthroimidazole; TPAAnPI) is designed and synthesized. The excited state properties of the TPAAnPI investigated through photophysical experiments and density functional theory (DFT) analysis reveal that its fluorescence is due to the HLCT excited state. The optimized non-doped blue OLED using TPAAnPI as a light-emitting layer exhibits a novel blue emission with an electroluminescence (EL) peak at 470 nm, corresponding to the Commission International de L'Eclairage (CIE) coordinates of (0.15, 0.22). A fabricated device termed Device II exhibits a maximum current efficiency of 18.09 cd A-1, power efficiency of 12.35 lm W-1, luminescence of ≈29 900 cd cm-2, and external quantum efficiency (EQE) of 11.47%, corresponding to a high exciton utilization efficiency of 91%. Its EQE remains as high as 9.70% at a luminescence of 1000 cd m-2 with a low efficiency roll-off of 15%. These results are among the best for HLCT blue-emitting materials involved in non-doped blue fluorescent OLEDs. The performance of Device II highlights a great industrial application potential for the TPAAnPI molecule.