High-efficiency Blue And White Fluorescent Devices With Simple Structure Based On TADF Materials

Organic light-emitting diode(OLED)has been widely used in flat-panel displays and solid-state lighting due to its wide viewing angle,fast response,flexible bending,low power consumption and light weight.Organic light-emitting diodes produce singlet and triplet excitons with a ratio of 1: 3 under the electrical excitation.The traditional fluorescent materials have wide variety,and emission spectra covering the entire visible light region,and good stability.However,the traditional fluorescent material can only use singlet excitons for light emission,and the triplet excitons accounting for 75% of all excitons are wasted by non-radiative dissipation,which greatly limits device efficiency.In order to overcome the limit of low internal quantum efficiency inherent for fluorescent materials,phosphorescent materials have been developed.Phosphorescent materials can simultaneously utilize singlet and triplet excitons for light emission and as a result the internal quantum efficiency of organic light emitting devices based on phosphorescent materials can reach 100%.However,the stability of the blue phosphorescent device needs to be further improved.In addition,high-cost phosphorescent materials containing iridium,platinum and other precious metals limit their use in device preparation.Thermally activated delayed fluorescence(TADF)materials considered to be the third generation of organic optoelectronic materials after fluorescent materials and phosphorescent materials can use singlet and triplet excitons for light emission in the absence of precious metals,which have attracted wide attention.Among them,the blue TADF material DMAC-DPS has shown superior light-emitting properties in the doped and non-doped devices.This thesis mainly studies the characteristics of the devices based on the blue TADF material DMAC-DPS,which mainly includes the following parts:(1)The characteristics of non-doped devices based on DMAC-DPS are studied.The influences of different hole transport layers,electron transport layers,and the thickness of the light-emitting layer on the device characteristics have been investigated.Device with the structure of ITO/MoO3(5 nm)/mCP(40 nm)/DMAC-DPS(30 nm)/ SPPO13(50 nm)/CsF(1 nm)/Al(150 nm)shows the maximum external quantum efficiency of 14.3% and the power efficiency of 26.8 lm W-1.In addition,measurements on the I-V characteristics of the unipolar carrier devices indicate that the holes are the majority carriers and as a result the carrier recombination region is close to the interface of DMAC-DPS layer and electron transport layer.(2)On the basis of the results on non-doped devices,a series of DMAC-DPS: TBPe devices with the general structure of ITO/MoO3(5 nm)/mCP(40 nm)/DMAC-DPS: TBPe(30 nm)/SPPO13(50 nm)/CsF(1 nm)/Al(150 nm)using different TBPe concentration are prepared.The I-V-B characteristics,luminous efficiency-current density properties and electroluminescence spectra of DMAC-DPS: TBPe devices are studied.The maximum external quantum efficiency of the DMAC-DPS: 1.0% TBPe device is 12.7%,close to that of the DMAC-DPS non-doped device(14.3%).The Commission Internationale de I'Eclairage(CIE)coordinates of(0.15,0.25)are measured for DMAC-DPS: TBPe devices,which are better than(0.16,0.27)for DMAC-DPS non-doped devices.The generation and evolution of excited states in DMAC-DPS: TBPe devices are discussed.(3)In order to avoid the influences of multiple energy loss mechanisms in TADF sensitized fluorescence devices,we prepare all-TADF devices by incorporating the green TADF material(4s,6s)-2,4,5,6-tetra(9H-carbazol-9-yl)isophthalonitrile(4CzIPN)and the yellow TADF material 2,3,5,6-tetrakis(3,6-diphenylcarbazol-9-yl)-1,4-dicyanobenzene(4CzTPN-Ph)into DMAC-DPS.The maximum external quantum efficiency and power efficiency of the green and yellow devices with the general structure of ITO/MoO3(5 nm)/mCP(40 nm)/DMAC-DPS: TADF(30 nm)/SPPO13(50 nm)/CsF(1 nm)/Al(150 nm)are 10.9%,32.1 lm W-1 and 11.0%,36.6 lm W-1.Moreover,the combination of TADF host and dopant materials allows the use of broad dopant concentration to achieve the desired colour purity without significantly affecting device efficiency.In addition,we also find that the emission of 4CzTPN-Ph:DMAC-DPS film has a blue-shift of about 30 nm compared to that of CBP: DMAC-DPS,which can be attributed to different dipole interactions between host and guest materials.(4)White emission can be achieved in DMAC-DPS:4CzTPN-Ph devices with reduced 4CzTPN-Ph concentration.Among the devices,0.4% 4CzTPN-Ph device shows the maximum external quantum efficiency of 14.7%,while 0.8% 4CzTPN-Ph device exhibits current-insensitive CIE coordinates of(0.29,0.39),which is the close to white emission.We measure the photoluminescence spectrum and transient photoluminescence spectrum of DMAC-DPS:0.4% 4CzTPN-Ph thin films.The emission at 550 nm,shown in the EL spectrum of DMAC-DPS:10% 4CzTPN-Ph devices,can be also observed in the photoluminescence spectrum and the decay profile of the delayed component of the emission can be fitted with a double exponential model,yielding life-time values of 0.7 and 2.6 ?s,which are similar to those of 4CzTPN-Ph obtained from mCBP: 4CzTPN-Ph film,excluding that the emission comes from the exciplex formed between DMAC-DPS and 4Cz TPN-Ph.The generation and evolution of excited states in DMAC-DPS: TADF devices are discussed,which is further compared to that in TADF sensitized fluorescence devices.The almost identical external quantum efficiency of DMAC-DPS and DMAC-DPS: 4CzTPN-Ph devices indicates that all the excitons produced by the host and guest materials can be used for light emission.