Research On High Efficiency Exciplex Based Organic Light-Emitting Diodes

Organic light-emitting diodes(OLED),as a new generation of display and lighting technology,are the research hotspots in recent years because it has the advantages of light and thin,low driving voltage,high luminous efficiency,fast response speed,active light emission without backlight,wide viewing angle,high contrast,flexible and flexible,etc.Among many research directions,thermally activated delayed fluorescence(TADF)is currently the main research direction in the field of light-emitting displays due to its high efficiency,no heavy metals,environmental friendly,and low cost.There are two ways to achieve thermally activated delayed fluorescence:one is to use intramolecular charge transfer between donor and acceptor fragments in delayed fluorescent materials;the other is to use intermolecular charge transfer between donor and acceptor molecules in exciplexes.These two methods have their own advantages and disadvantages.Among them,the unique advantages of exciplex are the extremely small singlet-triplet energy gap(?EST),adjustable carrier transport characteristics,and extremely small carrier injection barrier,which is extremely expected to realize high-efficiency OLED.However,due to the charge transfer between molecules,the formation mechanism of exciplexes is complicated,and there are many influencing factors(gain and loss of electron ability,molecular configuration,intermolecular contact mode,intermolecular force,molecular HOMO and LUMO distribution,molecular triplet state energy level,etc.),it is difficult to obtain exciplex based OLED with high efficiency.Therefore,the current development of OLED based on exciplexes lags behind delayed fluorescent materials.Therefore,it is necessary to further study the exciplex so that the unique advantages of the exciplex can be used to obtain high-efficiency OLED based on the exciplex.In OLED,the exciplex can be used as a host material to transfer energy to the guest to emit light,and it can also be used as a luminescent material to emit delayed fluorescence.Exciplexes face some problems in the development of these two directions.The main problems faced by exciplex as a host device are as follows:first,there are fewer new exciplexes and fewer exciplexes with high energy levels.In order to meet the needs of various devices and obtain high-efficiency blue and white light devices,it is necessary to further explore a variety of new exciplexes,especially those with higher energy levels;secondly,devices with exciplexes as the host have been able to achieve very high efficiencies.The high efficiencies are all obtained at low brightness,and further research is still needed to improve the efficiency of the device at high brightness.The main problems faced by exciplex as luminescent material devices are:first,the efficiency of exciplex as luminescent material devices is not high enough,the maximum is only 17.8%,and further research on new high-efficiency exciplex systems is needed;secondly,the current high high-efficiency exciplex as luminescent material devices are all prepared based on the vacuum vapor deposition method,with low material utilization and high cost.Further research is still needed to prepare high-efficiency exciplex as luminescent material devices using lower cost methods.In response to the above-mentioned problems,this dissertation rationally screened the exciplex donors and acceptors by studying the working mechanism and efficiency decay mode of the exciplex as the host,and proposed a method to reduce the efficiency roll-off under high brightness.High-efficiency red,green,blue,and white OLEDs based on exciplex hosts were developed with improved efficiency at high brightness.By studying the working mechanism of exciplex as luminescent material and the influence of the triplet energy level and solubility on the efficiency.Reasonably designed exciplex donor and screened the acceptor,proposed a method to improve the efficiency of the device with exciplexes,and developed a high-efficiency OLED with ternary exciplex as luminescent material based on the solution spin-coating method.The main research contents and innovations of this dissertation are as follows:1.Reasonably screen the donor and acceptor,adjust the carrier transporting characteristics,obtain an exciplex host with extremely high energy and balanced carrier transporting characteristics,and obtain a high-efficiency white OLED with stable spectrum and small efficiency roll-off under high brightness.Through reasonable screening,a new exciplex m-CBP:B4PyPPM with high triplet energy level was obtained,and it has a small ?EST,which can realize delayed fluorescence emission.By adjusting the doping ratio,a good carrier transport balance is achieved.The blue,cyan,green and red phosphorescent OLEDs based on m-CBP:B4PyPPM have maximum external quantum efficiencies as high as 26.2%,26.1%,24.7%and 26.5%.Then,by simply doping the blue or cyan phosphorescent guest together with the red phosphorescent guest into mCBP:B4PyPPM,the single emitting layer white light devices W1 and W2 were successfully prepared,and they showed high external quantum efficiencies up to 23.9%and 26.9%,even at a brightness of 5000 cd/m2,the external quantum efficiencies of the devices W1 and W2 are still as high as 19.1%and 21.0%.Later,we also studied threecolor multi-light-emitting layer devices W3 and W4 with higher color rendering index(CRI),which have a very small range of CIE color coordinate variation(±0.001,±0.000).Also at a brightness of 5000 cd/m2,the external quantum efficiencies of devices W3 and W4 are still as high as 18.9%and 21.0%.2.Research the exciton decay equation and find a way to improve the efficiency rolloff under high brightness.It is proposed to improve the efficiency of the device under high brightness with the exciplex host and the phosphorescent assistant host,and obtain the low efficiency roll-off under high brightness.By studying the exciton decay equation,it is found that increasing the radiative transition rate and reducing the TTA and TPQ rates can achieve low efficiency roll-off.TTA and TPQ will increase nonlinearly with the increase of exciton density and charge trapping density.Therefore,we choose to use the exciplex host to achieve a balanced carrier transport by adjusting the ratio of the donor to the acceptor,and to minimize the charge trapping in the light-emitting layer.Secondly,in order to reduce the accumulation of excitons and trapped charges at high current density,we add a phosphorescent assistant host to the traditional host-guest system to form an energy funnel system,where the assistant host can be used as a platform for exciton storage and transfer and reducing the possibility of excitons accumulating at the triplet energy level of the phosphorescent light-emitting guest.By using the m-CBP:B4PYPPM exciplex as host,the green and red phosphorescent OLEDs exhibited external quantum efficiencies of 25.9%and 27.8%,respectively.The maximum external quantum efficiency of the green and red phosphorescent OLEDs using the blue phosphorescent molecule FIrpic as the assistant host is almost the same as that of the undoped green and red phosphorescent OLEDs,which are 25.8%and 27.6%,respectively.The most important thing is that the external quantum efficiency of green and red PhOLEDs at the brightness of 10000 cd/m2 has increased from 12.8%and 11.1%to 20.2%and 22.4%,respectively,and the efficiency roll-off is from 22.0%and 47.1%decreased to 7.0 and 6.9%,respectively.The concept presented here will provide a good reference for future design of high-efficiency PhOLED with low efficiency roll-off.3.Designed and synthesized a new type D-A small molecule donor material with enhanced electron donating ability.Based on the solution method and the vacuum vapor deposition method,both the high-efficiency OLED based on exciplex as luminescent material and host can be obtained.According to the formation principles and laws of exciplex,we designed and synthesized a new type of D-A type electron donor TPAFPO,which has enhanced electron donating ability,suitable HOMO and LUMO energy levels,and higher triplet energy levels.We chose PO-T2T as the acceptor material according to the formation rule of exciplex.The exciplex TPAFPO:PO-T2T exhibits a smaller ?EST and obvious delayed fluorescence luminescence.The exciplex as luminescent material device of TPAFPO:PO-T2T shows a green emission peak at 540 nm,and the maximum external quantum efficiency is as high as 17.0%.At that time,the maximum efficiency of exciplex as luminescent material reported in the literature was 17.8%,and the exciplex as luminescent material efficiency of our exciplex TPAFPO:PO-T2T had reached the international advanced level at that time.In view of the high efficiency of exciplex as luminescent material devices,such an efficient system can naturally become a good host.We selected the near-infrared delayed fluorescent material APDC-DTPA as the doped guest.The device shows a near-infrared emission peak at 690 nm and also exhibits an external quantum efficiency as high as 9.2%.In addition,considering the low material utilization rate,high cost of the vacuum vapor deposition method and the good solubility of TPAFPO in chlorobenzene.We tried to study the solution-based exciplex TPAFPO:POT2T as luminescent material and the host device.Their emission peaks are 536 nm and 688 nm,and their maximum external quantum efficiencies are 15.1%and 8.1,respectively.%,the luminescence and efficiency are close to the devices prepared by the vacuum vapor deposition method.4.A new type of small molecule donor material was designed and synthesized.The fluorinated pyridine group was introduced to enhance the solubility,and a solutionprocessed high-efficiency ternary exciplex as luminescent material OLED was obtained through a ternary strategy.In order to meet the requirements of high-efficiency exciplex and solution-processed device preparation,we designed and synthesized a new type of small molecule donor TPA-3,introduced fluorinated pyridine functional group,it has suitable HOMO and LUMO energy levels,higher triplet energy level and good solubility.Using PO-T2T as the acceptor,TPA-3:PO-T2T exhibits a smaller ?EST and obvious delayed fluorescence luminescence.The maximum external quantum efficiency of the TPA-3:PO-T2T based solution processed bulk exciplex device is 14.4%,and 11.0%at a brightness of 1000 cd/m2.Subsequently,we chose two donors,3CzFDPhTz and 9PhFDPhTz,to further study the ternary exciplex.The fluorescence quantum yields of TPA-3:PO-T2T,TPA-3:3CzFDPhTz:PO-T2T and TPA-3:9PhFDPhTz:PO-T2T solidstate films were measured to be 0.57,0.84 and 0.85,respectively.Using TPA3:3CzFDPhTz and TPA-3:9PhFDPhTz as EML,the maximum external quantum efficiency of interfacial exciplex device is as high as 18.8%and 24.0%.Using TPA3:3CzFDPhTz:PO-T2T and TPA-3:9PhFDPhTz:PO-T2T as EML,the maximum external quantum efficiency of bulk exciplex device is as high as 17.7%and 21.4%,and the high external quantum efficiency of 12.2%and 14.5%is still maintained at a brightness of 1000 cd/m2.As far as we know,such high efficiency surpasses all exciplex based OLEDs prepared by solution-processed method and vacuum vapor deposition method reported in the literature at that time.The research results in this chapter can provide a good reference for future high-efficiency exciplex as luminescent material OLED design.