Deep Blue Light Materials With High Exciton Utilization And Their Applications In OLEDs

Organic light-emitting diodes(OLEDs)are an emerging technology in fields of display and lighting,especially for small displays such as mobile phones and watches.The first generation of fluorescent OLEDs,which appeared in 1987,had an upper limit of 25% internal quantum efficiency(IQE).The second-generation phosphorescent OLEDs(Ph-OLEDs)raise the maximum IQE limit to 100% by introducing heavy metal atoms into organic molecules.This method has already achieved great success in red and green phosphorescent devices.However,there is still a bottleneck in the operating lifetime of blue Ph-OLEDs,due to wide exciton band gap of around 3 e V and the long exciton lifetime in the microsecond scale.The interaction between excitons and polarons produces a thermal excited state of 6 e V energy level,which will degrade the material.The third-generation thermally activated delayed fluorescent(TADF)materials,in which triplet excitons are converted to singlet states upon thermal activation through reverse intersystem crossing(RISC),and 100% IQE can thμs be achieved.But TADF still does not solve the problem of short lifetime of blue OLEDs,which is also caμsed by high triplet energy and long triplet lifetime.Thμs,the fourth-generation "hot exciton" hybridized local and charge transfer(HLCT)materials are proposed to enable fast RISC from a higher excited state with the simultaneoμs achievement of high efficiency and short exciton lifetime.In this paper,two "hot exciton" deep blue fluorescent materials P1 and P2 were selected,both of which can μse reverse intersystem crossing from high-energy triplets to achieve high exciton utilization efficiency(49%-55%).Their molecular excited states have obvioμs hybridized local and charge transfer(HLCT)characteristics.The P1-based OLED device displays a deep blue luminescence with a Commission International’ Eclairage(CIE)of(0.1487,0.0809)and a maximum EQE of 10.3%;The CIE coordinates of P2 blue luminescence OLED are(0.1541,0.1187)and the maximum EQE is as high as 8.14%,which is high in blue "hot exciton" fluorescent OLED materials.Both device show excellent heat resistance.The electroluminescence(EL)intensity of the P1 device was significantly enhanced by nearly 51 times when the OLED was heated from 80K(-193℃)to 450 K(177℃),and 8.34 times higher than that at room temperature(300 K).In the P2 device,the corresponding EL enhancements are 64 and 22 times respectively.This good heat tolerance is also caused by the short lifetime of the radiation transition in nanosecond order and the accelerated h RISC rate at high temperatures,which alleviates the damage of high-energy excitons to the device structure.