Singlet reservoir for multi-channel barrierless harvesting of keto triplet excitons for high-efficiency electroluminescence of an ESIPT fluorophore

Abstract

Efficient excited state intramolecular proton transfer (ESIPT) OLED emitters are critical for the development of cost-effective and color-stable WOLEDs due to their significant Stokes shifts, which minimizes spectral overlap. In this research, a yellow-emitting ESIPT fluorophore, HBT-PA, has been demonstrated to achieve high-efficiency OLEDs with a maximum external quantum efficiency (EQE) surpassing 5% and a current efficiency (CE) of 22.20 cd A⁻¹. This performance is attributed to the effective utilization of excitons by the S₂ singlet reservoir in the keto tautomer, enabling the harvesting of triplet excitons through multi-channel, highly efficient high-lying reverse intersystem crossing (hRISC), which is predominantly barrierless and characterized by substantial spin–orbit coupling (SOC) matrix elements of 6.7–10.9 cm⁻¹. Complementary-color WOLEDs, exhibiting remarkable color stability and performance, were fabricated based on the fluorophore's exceptionally large Stokes shift. These WOLEDs demonstrated maximum EQE and CE values of 13.57% and 16.84 cd A⁻¹, respectively. This study highlights the significant potential of HBT-PA for low-cost and large-scale production of energy-efficient WOLEDs.