The Study Of Preparation And Charge Generation Mechanism Of Conducting Polymer/metal Oxide Charge Generation Layer

Since organic light-emitting devices(OLEDs)are current-driven devices,the luminance of the devices is determined by the current through the devices.In other words,large current is required to achieve high luminance.However,large current can reduce the operating life-time of devices.To overcome this issue,Kido et al proposed tandem organic light-emitting diodes(TOLEDs),which comprised two or more light-emitting units interconnected by charge generation layer(CGL).In TOLEDs,charges generated by CGL can be injected into the adjacent light-emitting units,so that multi-photons can be generated when an electron-hole pair is injected.Thus,high luminance can be obtained at relatively low current,as a result the luminous efficiency and lifetime of the devices can be simultaneously improved.Most TOLEDs are prepared by vacuum deposition method,but this method has the demerits of waste of materials,long production cycle and limited device size.On the other hand,the solution-processing methods can solve these problems.So far,the studies on solution-processed TOLEDs have focused on improving the luminous efficiency of the devices,nevertheless,the underlying charge generation mechanism is largely unexplored.In this thesis,we report the properties of solution-processed two-and three-unit tandem polymer light emitting devices in an inverted structure.More importantly,the charge generation and injection mechanism for the poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate)(PEDOT:PSS)/zinc oxide(Zn O)CGL is elucidated.The paper is divided into the following four parts:(1)The influences of surface morphology,conductivity and thickness of PEDOT: PSS layer on the properties of the devices employing the PEDOT:PSS/Al or CGL/Al hole injection contact were investigated.PEDOT:PSS layers on top of SY-PPV are flat and uniform,and have the RMS roughness of 1.7 nm as determined by atomic force microscope(AFM).PVP AI 4083 and PH-1000 PEDOT:PSS with different conductivities were employed to prepare the devices with PEDOT:PSS/Al or CGL/Al hole injection contact to investigate the effect of PEDOT:PSS conductivity on device properties.The luminous efficiency of the device using PVP AI 4083 PEDOT: PSS(5.7 cd/A)is higher than the counterpart device using PH-1000 PEDOT:PSS(2.3 cd/A),which may be related to the intensified luminescence quenching strength of PH-1000 PEDOT:PSS layer.Furthermore,the influence of the PVP AI 4083 PEDOT:PSS layer thickness on the characteristics of the devices with CGL/Al hole injection contact is studied.The devices with a 30 or 60 nm PEDOT:PSS layer show the luminous efficiency of 5.7 cd/A,which is much higher than that of the devices devoid of a PEDOT: PSS layer(0.001 cd/A),indicating that PEDOT:PSS layer plays an essential role in the charge generation and injection process of the PEDOT:PSS/ZnO CGL.(2)The effects of surface morphology,preparation method and thickness of ZnO layer on the properties of devices employing the ITO/CGL election injection contact were investigated.ZnO layers on top of SY-PPV/PEDOT:PSS are flat and uniform and have the RMS roughness of 2 nm.The ethanolamine additive used in the preparation process of ZnO layer strongly degrades hole injection properties of the underlying PEDOT:PSS layer,which can be attributed to the n-doping effect of ethanolamine to PEDOT:PSS,lowering the its work-function.The devices with a 10 or 30 nm ZnO layer show the luminous efficiency of 2.3-2.5 cd/A,in stark contrast the devices without a ZnO layer is not glowing,revealing the indispensable role of ZnO layer in the CGL operation.(3)The above bottom-and top-light emitting units are assembled to prepare TOLEDs.The driving voltage,electroluminescence spectra and luminous efficiency of TOLEDs are approximately equal to the superposition of the respective property of the light emitting units,indicating efficient charge generation and injection properties of the PEDOT:PSS/ZnO CGL.In addition,the TOLEDs with the PEDOT:PSS/ZnO and V2O5/PEDOT:PSS/ZnO CGL show similar characteristics,revealing that the use of the PEDOT:PSS/ZnO CGL allows the simplification of the CGL configuration without compromising device properties.On this basis,we first report TOLEDs comprising three polymer light-emitting units.The driving voltage,the luminous efficiency and the electroluminescence spectra are approximately equal to the sum of the respective property of the constituent three light emitting units,indicating that the PEDOT:PSS/ZnO CGL is robust and the increase in the number of light emitting units does not degrade the performance of the devices.(4)The charge generation process in CGL is studied by measuring the I-V and C-V characteristics of various interconnects-based devices and modeling the temperature-dependent I-V characteristics.The I-V measurements indicate that only the devices with the PEDOT:PSS/ZnO interconnect show the large current,which illustrate that the charge generation occurs merely in the PEDOT:PSS/ZnO heterojunction.In addition,the C-V characteristics of PEDOT:PSS/ZnO based devices are similar regardless of the presence of a poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-(4,4'-(N-(4-butylphenyl)(TFB)layer or not,verifying that the TFB/PEDOT:PSS interface has insignificant impact on charge generation process.Furthermore,temperature-dependent I-V characteristics are simulated by the Richardson-Schottky thermal emission model,yielding that the barrier height for the charge generation and injection is 0.73 eV,which is approximately equal to the reported energetic difference of ca.0.8 eV between HOMO level of PEDOT:PSS and conduction band minimum of ZnO.All the experimental results and modeling strongly support that the charge is only generated at the PEDOT:PSS/ZnO interface.Thus,the charge generation process of the PEDOT:PSS/ZnO CGL can be described as the thermal emission of electrons from the HOMO level of PEDOT:PSS to the conduction band minimum of ZnO.