Development and Mechanism Investigations of High External Quantum Efficiency, Low Turn-On Voltage TADF Green OLEDs Using 4CzIPN as the Guest Material

The present work focuses on thermally activated delayed fluorescence (TADF) materials, utilizing interface exciplex to efficiently transfer the excited-state energy to the green TADF guest material. The proposed structure is indium tin oxide (ITO)/1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC)/1,3-bis(N-carbazolyl)benzene (mCP):(4s,6s)-2,4,5,6-tetra(9H-carbazol-9-yl)isophthalonitrile (4CzIPN)/2,4,6-tris[3-(diphenylphosphinyl)phenyl]-1,3,5-triazine (PO-T2T)/lithium fluoride (LiF)(1 nm)/aluminum (Al)(100 nm). Besides, additional mCP is introduced as a blocking layer to increase the efficiency of the devices. The optimization factors include the thickness of the TAPC and PO-T2T transport layers, the doping concentration of the 4CzIPN guest material, and the doping position. As a result, a high-efficiency green organic light-emitting diode (OLED) was developed, achieving a maximum current efficiency (CEmax) of 144 cd/A, a maximum power efficiency (PEmax) of 126 lm/W, and a maximum external quantum efficiency (EQEmax) of 43.9%, along with a low turn-on voltage. We analyze the effects of doping concentration on device performance and mechanism investigations through photoluminescence (PL), TRPL, absorption spectroscopy, and impedance spectroscopy analyses. This achievement not only avoids heavy-metal pollution associated with phosphorescent OLEDs but also represents a significant breakthrough in the efficiency of green TADF OLEDs. To the authors’ knowledge, the proposed devices show the highest EQEmax data compared with the published results.