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Advanced functional materials: Bipolar multifunctional delayed fluorescent materials for high performance monochromatic / warm white OLED devices

wallpapers News 2020-08-13

organic light-emitting diodes (OLEDs) have the advantages of self illumination high contrast flexibility light weight are widely used in the field of smart phone display. The development of OLED is inseparable from the improvement of luminous material efficiency. Compared with more mature phosphorescent materials thermally activated delayed fluorescence (TADF) materials have low cost can achieve 100% exciton utilization in OLED devices through rapid reverse inter system crossing (RISC) process which has become a hot spot in OLED research field in recent years. However some reported tadf-oled systems still have some defects such as carrier transport imbalance serious exciton annihilation so on which make these OLEDs generally have serious efficiency roll off at high brightness which greatly limits their practical application.

for this reason Professor Zhao zujin academician team of Tang benzhong of South China University of technology etc. put forward effective design strategies for luminescent materials. The team introduced a large steric substituent into the molecule which effectively inhibited the interaction luminescence quenching between molecules in the aggregated state; selected appropriate intensity of electron donor (d) acceptor (a) units designed a twisted D-A skeleton so that the material had bipolar transmission delayed fluorescence characteristics at the same time. This strategy not only improves the solid-state luminescence efficiency exciton utilization but also suppresses the exciton annihilation process. It is worth mentioning that the material properties meet the design requirements of high-efficiency OLED light-emitting materials host materials at the same time so that the material can be used not only as a light-emitting material in OLED devices but also as a host material in phosphorescent OLED devices.

in this work the team designed synthesized systematically studied a new green light material sbf-bp-dmac. Compared with the soluble state sbf-bp-dmac exhibited enhanced fluorescence (AEE) delayed fluorescence in the aggregation state. Subsequently the team systematically studied the electroluminescent properties of sbf-bp-dmac prepared a series of OLED devices. The maximum current efficiency power efficiency external quantum efficiency of undoped sbf-bp-dmac devices are 67.2 CDA – 1 65.9 LMW – 1 20.1% respectively while the maximum efficiency of doped sbf-bp-dmac devices is 79.1 CDA – 1 70.7 LMW – 1 24.5%. In addition the team used IR (tptpy) 2acac as the object sbf-bp-dmac as the main body to prepare high efficiency orange light devices spectral stable warm white light devices. Among them the switching voltage of orange light device is only 2.3V the maximum efficiency is 88.0 CDA – 1 108.0 LMW – 1 26.8% the maximum brightness is more than 100000 CDM – 2; the maximum emmetropic efficiency of warm white device is 69.3 CDA – 1 45.8 LMW – 1 21.0%. The efficiency roll off of the above devices is very small at high brightness which has the potential for further practical application.

in addition the team clarified the reasons for the high efficiency low efficiency roll off of sbf-bp-dmac devices by studying the transmission characteristics of sbf-bp-dmac the energy transfer charge trapping behavior in phosphorescent devices etc. The results show that the excitons are mainly composed on the sbf-bp-dmac then undergo an efficient RISC f ü rster energy transfer process to emit orange light on IR (TPY) 2acac. In this process the non radiative deactivation processes such as triplet triplet exciton annihilation are effectively suppressed which improves the efficiency stability of the device at high brightness.

the materials designed in this work combine the characteristics of AEE TADF bipolar carrier transport can meet the requirements of efficient host materials guest materials at the same time which provides a useful reference for the development of OLED material system.

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