The Fabrication of Flexible Transparent Electrodes and Surface Treatment for Organic Light-Emitting Devices

  • 王 振道

Student thesis: Doctoral Thesis


Organic Light Emitting Diodes (OLEDs) have been developed as a future display technology due to many advantages such as fast response time high contrast and application to be incorporated in lightweight display Indium tin oxide (ITO) is the most commonly used material for transparent conducting electrodes (TCEs) for OLEDs due to its excellent optical transmittance (typically 85% in the visible wavelength region) and low sheet resistance (as low as 10Ω/square) However ITO has a high cost due to the scarcity of indium and its fragility is a significant drawback for use in OLEDs In addition ITO requires a high processing temperature and the migration of indium limits the lifetime of OLEDs These drawbacks limit the application of the ITO TCEs Alternative TCEs are thus needed to replace ITO electrodes In this research we choose silver grids based TCEs as anode in OLEDs The thesis are divided into three parts In the first part of this thesis we fabricate the silver grids TCEs via the thermal deposition method The proposed grid shows low sheet resistance and a good figure of merit The sheet resistance decreased from 688 to 3 37Ω/square when the thickness was increased from 30 to 70 nm The samples are characterized in terms of the contact angle to calculate the surface energy and polarity The surface energy and polarity of the samples increased from 8 15 to 58 029 mJ/m2 and 0 024 to 0 067 respectively when the sulfur content was increased from 6 67 to 9 26% (thickness increased from 50 to 70 nm) The fabricated Ag grid transparent conducting films show good optical and electrical characteristics and have potential for application in optoelectronics Secondly we fabricate the MoO3/Ag grids/MoO3 (MAM) flexible TCEs to smooth the surface morphology of silver grids The proposed structure also improves transparency compared with that of the traditional tri-layer electrode (dielectric/metal film/dielectric) by using metallic grid patterns (dielectric/metal grids/dielectric) The MoO3 layer will decrease the transmittance although it smoothes the surface roughness of the silver grids Therefore we replaces the MoO3 layer with zinc oxide (ZnO) to fabricate the ZnO/Ag grids/ZnO (ZAZ) structure via thermal deposition We find the crystallization and electrical optical and mechanical characteristics of ZAZ TCEs are compared with those of MAM and ZnO/Ag film/ZnO TCEs It is found that the improvement in electrical characteristics is due to the crystallization of ZnO film In the third part of this thesis we discuss the surface treatment on MoO3 layer and the silver grids When the silver grids with an UV ozone treatment duration of 15 s the Ag2O thin films do not grow completely and current-voltage characteristics are poor However a 30 s UV-ozone treatment yielded good-quality Ag2O thin films The Ag2O thin films were reconverted into the AgOx phase with further increases in UV-ozone exposure time The Ag2O work function is nearly 5 0 eV which decreases the injection barrier of the silver grids (~4 7 eV) and MoO3 (~5 3 eV) Nevertheless excessive treatment time leads to the production of AgOx thin films and an increase in the work function to 5 3 eV the same as the highest occupied molecular orbital energy of MoO3 which causes a work function mismatch The work function mismatch between the Ag grids and the MoO3 layer results in a high injection barrier decreasing OLED performance The electrical properties of the electrodes and devices apparently depend on the composition of the silver oxide buffer layer as determined using X-ray photoelectron spectroscopy The surface and optical properties of the TCEs were also investigated The results show that the OLED devices with the proposed TCEs have better roll off and current efficiency compared to traditional ITO-based devices We also demonstrate the performance of OLEDs with hexylphosphonic acid (HPA) or UV-ozone treatment on their MoO3 anode buffer layers The OLEDs with a PA treated (5 mM @1 h) MoO3 layer have lower turn-on voltage and low current efficiency roll-off under high operating current The hole-only device (ITO/ MoO3 with PA or UV-ozone treatment/NPB/Al) was fabricated to calculate the active energy via temperature dependent I-V measurement When the devices were operated at high temperatures the activation energy of the UV-ozone treated and untreated hole-only devices became nonlinear However the activation energy of the PA treated devices had a more stable performance at high temperatures The interfacial resistance of the untreated hole-only devices and the PA and UV-ozone treated devices were calculated by Admittance Spectroscopy (AS)
Date of Award2019
Original languageEnglish
SupervisorSheng-Yuan Chu (Supervisor)

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