High efficiency, low roll-off and superior color stability fluorescent white organic light-emitting diodes through triplet exciton management with phosphor and TADF sensitization

Abstract

Achieving high efficiency, low roll-off, high color rendering index (CRI) and stable electroluminescence (EL) spectra in fluorescent white organic light-emitting diodes (WOLEDs) remains a significant challenge. In this study, we present a novel strategy that employs both phosphor and thermally activated delayed fluorescence (TADF) materials as sensitizers for fluorescent emitters, facilitating the effective utilization of both singlet and triplet excitons for radiative emission. Triplet excitons are converted into singlet excitons through the reverse intersystem crossing (RISC) process in the TADF sensitizer and are transferred to fluorescent emitters through a carrier trapping process in the phosphor sensitizer. These approaches enable the resulting fluorescent WOLEDs to realize 100% exciton utilization and accelerate the exciton decay process, leading to enhanced device efficiency and reduced efficiency roll-off. The optimized fluorescent WOLEDs demonstrated a maximum current efficiency (CE), power efficiency (PE) and external quantum efficiency (EQE) of 50.4 cd A⁻¹, 52.8 lm W⁻¹ and 16.7%. These values remained at 36.0 cd A⁻¹, 24.5 lm W⁻¹ and 13.3% at a luminance of 1000 cd m⁻². The highest color rendering index (CRI) was 74 and the Commission Internationale de I’Eclairage (CIE) coordinates were (0.35, 0.34) at a luminance of 1000 cd m⁻². Additionally, the electroluminescence (EL) spectra exhibited good stability across a wide range of luminance from 1000 to 10 000 cd m⁻². Furthermore, we conducted a comprehensive study on exciton quenching, recombination coefficients, energy transfer and charge carrier trapping processes in the resulting fluorescent WOLEDs. The investigation aimed to evaluate the effectiveness of realizing high efficiency, low efficiency roll-off and high-quality EL spectra through TADF and phosphor sensitization.