Transparent Organic Light-Emitting Diodes With Quasi-periodic Nanostructures

Organic light-emitting diodes(OLEDs)have shown the amazing applications in fall-color flat panel displays and solid-state ligiting due to their prominent advantages,including low power consumption,light weight,wide color gamut,fast response time and high contrast.As a specific type of OLED structures,transparent OLED(TOLED)has an additional merit of easy integration with oxide or organic thin-film-transistor driving circuit for active matrix transparent displays with high aperture ratio.Therefore,considerable interests have been attracted in the development of TOLEDs because their potential use in the electronic products,such as smartphones,laptops,automobile windshields,and transparent video screens.However,several challenges should be solved to fabricate TOLEDs.First of all,to realize high-performance TOLEDs,the major research topic is to develop the alternative transparent electrodes with superior optical and electrical properties.In this thesis,we choose ITO as the bottom transparent electrode of TOLEDs due to its low sheet resistance and high optical transmittance.However,transparent conductive oxides(TCOs)represented by ITO will cause serious plasma damage to the underlying organic layers during their sputtering process,so they are not suitable for the top transparent materials.In addition,for the commonly used thin metal films,a trade-off always exsits when pursuing both high optical transmittance and low sheet resistance.In this thesis,we use a metal-dielectric composite electrode(MDCE)as the top transparent electrode of TOLEDs,which can metigate the dilemma existing in thin metal films.Secondly,the total efficiencies of TOLEDs,at best,currently reach 90%of the corresponding bottom emission OLEDs.In order to improve the efficiencies of TOLED,we further optimize the MDCE multilayer of MoO3/Al-Ag-Ca:Ag/MoO3 by adding a capping layer of NPB on the outside MoO3 layer for an optical coupling layer(OCL),not only the total efficiencies of TOLEDs can be improved to the values comparable to those of conventional bottom emission OLEDs,but also the bi-directional luminance ratio of bottom emission to top emission can be optimized.In addition,the combination of the organic layer of NPB and the top dielectric layer of MoO3 will protect TOLEDs effectively,which is beneficial to stable the whole devices.The severe waveguide mode and SP losses at ITO and MDCE electrodes in TOLEDs,along with the differences of optical and electrical properties between the two electrodes will lead to asymmetry bi-directional light emission of the flat devices,the intensity of bottom emission is far more than that of top emission.In addition,the strong optical microcavity effect is present with the use of a flat MDCE structure,giving rise to the spectral distortion of angle-dependent emission characteristics and thus the poor color stability.Such problems may be mitigated by introducing quasi-periodic or quasi-random nanostructures that exhibit broadband spectral dependency of the outcoupled light.In conclusion,we present green and white TOLEDs with inverted structures by using ITO as bottom cathode and MDCE as top anode,respectively.The resulting green TOLED yields a maximum total power efficiency(PE)of 108.2 lm W-1,along with a bi-directional luminance ratio of 1.28;The resulting white TOLED yields a maximum total power efficiency(PE)of 83.5 lm W-1,along with a bi-directional luminance ratio of 1.26,showing highly enhanced and balanced light emission of both sides.By introducing quasi-periodic nanostructures,TOLEDs show superior angular color stability,and the electroluminescent spectra are independent of viewing angles.