Research On Fabrication And Properties Of Colloidal Quantum-dot Based Tandem Light Emitting Diodes

Colloidal quantum-dot(CQD)based Light-emitting diode(QLED)is an ideal candidate for next-generation flat-panel displays and solid-state lighting due to its unique properties of tunable emission-color,high color-saturation,and solution processability.With unremitting efforts of industry and academia,performance of QLED has made great progress in recent years.For example,to date,the reported external quantum efficiencies(EQEs)of red,green,and blue QLED have all exceeded 20%,which are comparable to the most efficient organic light-emitting diodes(OLEDs).However,QLED still has shortcomings in realizing display applications.First,the short lifetime of blue device limits practical application of QLED.In addition,the patterning technology of CQD based emitting-layer is still not mature,which limits large-scale industrialization of QLED.Finally,because regular QLED can only be driven by direct current,it cannot directly match civilian50 Hz alternating current electricity,which complicates the device driving circuit and causes power loss.Aiming at above three technical problems,in this dissertation,by taking the QLED-based tandem structure as research object,three new types of tandem light-emitting diodes(LEDs)are developed,and the optoelectronic properties and working mechanism of the devices are also studied in detail.Further improvement strategies of performance of single-junction QLED are also explored in this thesis.The main research contents of the thesis are as follows:First,to solve the problems of immature emitting layer patterning technology and short lifetime of blue QLED,organic and CQD hybrid multi-functional tandem LED is developed,in which bottom yellow QLED and top blue OLED are connected in series by an IZO transparent intermediate connecting electrode.By analyzing device structure and working principle,the achievable functionalities of the device driven in series and parallel are discussed,respectively.The research found that the functionality of full-color tunable or white light emission can be realized under parallel or series driving,respectively.The working principle and driving method of device for realizing full-color tunable functionality are studied in detail.The optoelectronic performance of the device in realizing full-color tunable functionality is measured and analyzed,and the color gamut of 63%NTSC(National Television Standards Committee)is realized by optimizing the device structure.In addition,the driving method and performance of the device for realizing white light emission functionality are also studied.By utilizing full-color tunable functionality and introducing long-lifetime blue OLED,the device can be used for display applications without needing to pattern emitting layer,and its lifetime will also not be limited by blue QLED.Secondly,to further improve color gamut and brightness of full-color tunable device,an emitting-layer patterning-free pixel tandem LED is developed,in which bottom yellow QLED and top blue OLED are serially connected by a semitransparent Al(2 nm)/IZO(40 or 80 nm)/Ag(20 nm)/IZO(5 nm)intermediate connecting electrode.The analysis results of optical interference theory show that this intermediate connecting electrode has color-selecting and enhancing functionalities.Subsequently,color-selecting and enhancing performance of the device are investigated by adjusting the thickness of underlying IZO in intermediate connecting electrode and functional layer of blue OLED,respectively.The research results indicate that when the thickness of IZO is 40 or 80 nm,the device can convert emitting color of yellow QLED to pure red or green,and the color purity of blue emission can also be significantly improved when reducing functional layer thickness of blue OLED.On this basis,a pixel tandem LED with color gamut of110%NTSC is successfully fabricated,and the achievable maximum brightness of red,green and blue emissions can reach 18759,64180 and 35200 cd/m~2,respectively.Third,to solve the problem that regular QLED cannot be driven by alternating current,high-performance bipolar tandem QLED is further developed.On the basis of regular QLED,Al and Ag bridging-electrode based coplanar bipolar QLED are first fabricated,and their working mechanism and optoelectronic properties are also studied in detail.By characterizing optoelectronic properties of electron transport layer,it is demonstrated that in Al-based device,electron injecting is achieved by extracting hole from bridging electrode.Subsequently,by studying the current density-voltage characteristic curve,it is proved that in Ag-based device,electron is directly injected from cathode through the conductive channel formed by Ag~+migrating.On this basis,bipolar tandem QLEDs with bottom emitting and double-side emitting are successfully fabricated,and the optoelectronic properties of the devices are studied in detail.The research results indicate that the highest EQE achievable for top and bottom light-emitting unit of bipolar tandem QLED can reach 23.8%and 22.1%,respectively,which are comparable to the most efficient regular QLEDs.Finally,to further improve performance of tandem device,the sputtering buffer-layer free transparent QLED and binary CQD emitting-layer based high-efficiency QLED are also developed in this thesis.In the study of transparent QLED,the Zn Mg O:PVP(Polyvinylpyrrolidone)hybrid electron transport layer is developed.The effects of introducing PVP on optoelectronic and physical properties of Zn Mg O electron transport layer,such as surface morphology,surface defects,electron transport efficiency and compactness,are studied in detail.The research results show that this hybrid electron transport layer can improve charge injection balance of device,suppress exciton quenching of emitting layer,and resist sputtering damage.On this basis,the sputtering buffer-layer free transparent QLED with a total EQE of 10.63%is successfully fabricated.In the study of binary CQD emitting-layer based QLED,the emitting layer with mixed red and blue CQD is developed.The quantum yield enhancement mechanism in binary CQD emitting layer is analyzed in detail.Through the characterization and analysis of quantum yield,it is proved that the quantum yield of emitting CQD in binary CQD emitting layer is higher.By introducing binary CQD emitting layer,QLED with an EQE as high as 21.65%is successfully achieved.By adopting the above-mentioned fabrication strategies of electron transport layer and CQD emitting layer,the performance of tandem device is expected to be further improved.These novel tandem devices developed in this thesis not only have the potential to provide new solutions to problems facing in industrialization of QLED,but also have potential to be applied in transparent and double-side displays.