| Crystalline organic semiconductors have the advantages of high carrier mobility and high thermal stability,and are ideal media for constructing advanced organic lightemitting diodes(OLEDs).However,due to problems such as crystalline organic material systems and crystalline OLED preparation methods,the performance of current crystalline OLED devices,especially blue-emitting crystalline OLEDs lags far behind amorphous OLEDs.Although weak epitaxy growth(WEG)has been verified to prepare low driving voltage,efficient emission of crystalline fluorescent OLED.However,limited by the material system,the external quantum efficiency(EQE)of the device is still limited to less than 5%.Therefore,the research topic of this paper is based on the wide bandgap fluorescent material 2FPPICz crystalline film prepared by WEG technology as the crystalline host matrix(CHM).From the host-guest doping technology to the successful application of the "thermal exciton" sensitization strategy,a new device structure of "thermal exciton" nanoaggregates combined with crystalline thin films" was finally developed.The device efficiency is greatly improved and surpasses all blue-emitting crystalline OLEDs reported so far.The new device realizes the effective injection and transmission of large currents,and exhibits unparalleled light output characteristics,making crystalline OLEDs a truly high-efficiency device.The main research content of the paper is as follows:1.A high-quality CHM was prepared by using WEG technology,and triplettriplet annihilation(TTA)material(PIMPy)was introduced into the CHM as a guest material to prepare a CHM-TTA device.The TTA effect in the CHM-TTA device was verified by transient EL spectroscopy.At the same time,as the concentration of PIMPy guest molecules increases,the TTA effect in CHM-TTA devices also increases.Thanks to the efficient energy transfer between 2FPPICz and PIMPy guests and the TTA effect,the CHM-TTA device(15 wt%PIMPy)achieves a low efficiency roll-off and a maximum EQE of 4.2%,CIE color coordinates(0.15,0.88)with a deep blue emission.2.To further improve the EQE of crystalline devices,the "hot exciton" material PAC is used as a sensitizer(HES)to sensitize the blue dopant material DSA-Ph(D)with high fluorescence quantum yield(PLQY),and a sensitized crystalline device(CHM-HES-D OLED)was fabricated.It is shown that partial excitons can be formed directly on the PAC sensitizer,realizing the efficient utilization of triplet excitons.In the meantime,effective energy transfer can be carried out between the PAC sensitizer and D.The CHM-HES-D device achieves a low driving voltage(3.8 V at 1000 cd/m2),a full width at half maximum(FWHM)of 50 nm and CIE(0.14,0.15).with blue emission.Compared with the 2FPPICz CHM device doped only with the fluorescent guest DSA-Ph(CHM-D),the maximum EQE of the device was increased from 1.8%to 6.2%when the "hot exciton" sensitizer was introduced,indicating that the sensitization strategy can be successfully implemented in the crystalline system.At the same time,due to the high mobility of the crystalline film,the CHM-HES-D device exhibits higher light output characteristics and a lower Joule heat loss ratio than amorphous reference OLED with DSA-Ph as an emitter.3.Although the "hot exciton" material sensitization strategy can achieve a breakthrough in the efficiency of crystalline OLED devices,some excitons are still formed in the 2FPPICz CHM,resulting in the inability to utilize some triplet excitons.To solve this problem,we prepared nano-sized "hot exciton" aggregates sensitizer,and prepared a new type of high-performance crystal state OLED device(CHMHENA-D),by making the nanoaggregates evenly embedded in the 2FPPICz CHM by alternate growth.The research shows that the diameter and height of the nanoaggregate in the "single growth cycle" is about 40-70 nm and 4-7 nm,respectively,and the vertical projected area on the 2FPPICz crystalline film accounts for about 20%;which verify the PAC nanoaggregates sensitizer are amorphous,while 2FPPICz can continue to grow along the crystalline film not covered by nanoaggregates and can maintain good crystal integrity.The 2FPPICz/PAC organic heterojunction effect can not only enhance the conductance of the device but also bend the LUMO energy level of the PAC(a few nanometers in contact with 2FPPICz)downward(by 0.12 eV downward).The energy level of the PAC nanoaggregates can be almost completely aligned with the LUMO energy level of the electron transport layer after bending,and the electrons can be directly injected into the nanoaggregates with almost no potential barrier,rather than on the 2FPPICz crystalline host.On the other hand,the holes are transported through the high-transport performance crystalline film,and after entering the nanoaggregates,they recombine with electrons to form excitons.Based on this novel device structure design,crystalline OLEDs achieve extremely low turn-on voltage(2.5 V),low driving voltage(3.3 V@1000 cd/m~2),maximum EQE=9.14%,and CIE(0.15,0.17),and FWHM of 50 nm with blue emission.It achieves low Joule heat loss(7.8%)and high photon output,surpassing all high-EQE amorphous blue(CIEy<0.3)OLED devices reported in the literature.These results suggest that the strategy of combining "hot exciton" nanoaggregates with crystalline thin films is an effective way to develop nextgeneration high-performance OLEDs. |
