Research Of OLEDs Based On Thermally Activated Delayed Fluorescence And Doped Hole Injection Layer

After decades of rapid development,organic light-emitting diodes(OLEDs)have gradually entered the application stage.However,there still exists many issues in OLED,such as device performance of blue emitters,manufacturing cost,the production yield of large-area display panel etc.,need to be solved.In conventional OLED,there exists a large energy level offset between the ITO and hole transport layer(HTL),which will lead to imbalanced charge transfer and carrier injection and hence result in a low luminous efficiency,high driving voltage,and fast degradation of OLEDs.To resolve this problem,a number of methods such as ITO modification and insertion of of charge-injection buffer layers have been used.For an ideal hole injection layer(HIL),it is necessary to possess a suitable work function and good performance in hole injection and transport.One of the efficient ways to improve the carrier injection and transport is by the doped HIL.Currently,"first-generation" fluorescence emitters have advantages on relatively long operation lifetime and low cost.However,merely 25%of excitons can be harvested for luminescence,resulting in a lower efficiency as compared to phosphorescent emitters.To improve the efficiency of OLED."second-generation"phosphorescence emitters made of noble metal ions were employed to harvest not only 25%exciton from singlet and but also 75%exciton from triplet states.However,they suffer from a very short lifetime of blue emitters,high cost and severe efficiency roll-off at high current densities."Third-generation" thermally activated delayed fluorescence(TADF)materials may be an alternative solution for the development of high-performance blue OLED devicesTherefore,my thesis mainly focuses on the research of new kinds of doped HILs and synthesis of new TADF blue emitters for high-performance OLED devicesFirstly,we discuss the development of an efficient and stable HIL,F4-TCNQ doped TS-CuPC via a solution process,in OLEDs.The p-type-doped TS-CuPC film demonstrates an improved hole mobility and a better surface morphology with the help of F4-TCNQ.This composite film forms a cascade energy level alignment,which would effectively facilitate hole transport.In addition,the aqueous solution processed TS-CuPC:F4-TCNQ precursor is close to neutral with good stability for avoiding the electrode erosion.As a result,the fabricated OLEDs employing TS-CuPC:F4-TCNQ as HIL show a maximum power efficiency of 46.4 lm W-1,which is significantly higher than the conventional device based on PEDOT:PSS which has a maximum power efficiency of 26.3 lm W-1.Secondly,we report a design strategy regarding the synthesis of a series of star-shaped,structurally controlled singlet-triplet splitting for blue TADF emitters,namely CZ-TRZ,MeCZ-TRZ,and 2MeCZ-TRZ,having three carbazoles as a donor and a triazine as an acceptor architecture.This system is composed of three donor units,carbazole derivatives at para-position,which are directly fused to the strong triazine acceptor backbone via phenyl ring with a methyl group to increase the steric effect as well as control the torsion angle.The electronic interaction between the donor and acceptor unit plays a crucial role in the TADF mechanism as well as for tuning the emission peak.The OLED devices exhibit excellent performance with maximum external quantum efficiencies(EQEs)of 10.9,14.4 and 15.9%at 473,484,484 nm for CZ-TRZ,MeCZ-TRZ,and 2MeCZ-TRZ,respectively.Importantly,the CIE coordinates of the device with CZ-TRZ were(0.172,0.176)which is closer to the standard deep blue emission(0.150,0.150).