| OLEDs have been proved to have great application potentials in the fields of flat-panel displays and solid-state lighting,due to the advantages of high response,large view angle,low power consumptive,flexure panels and so on.In order to meet different application requirements,various kinds of device structures are developed,such as single layer,multilayer,tandem and inverted structure.In the field of flat-panel displays,the inverted structure has great compatibility with amorphous Si(a-Si)TFT backplane,making it a hot topic in current research.However,high work function material ITO is used as cathode in an inverted OLED,which implies a very large electron injection barrier at the ITO/organic interfaces,resulting in poor performance of the OLED device.Thus,it is very important to improve the electron injection,and the major strategies for enhancing carrier injection aim at:(1)injection barrier reduction by modifying the electrode work function or introducing an interfacial dipole layer/charge separation layer;(2)injection barrier narrowing by the interfacial doping to enable tunneling.Recently,an alternative and effective strategy has been proposed.It involves geometric modification of the electrode surface to reinforce the interfacial electric field for carrier injection.In particular,an inhomogeneous electrode surface may induce some local regions injecting carriers much more efficiently compared to other areas,which can be regarded as the injection hotspots.The injection hotspot effect is ascribed to the local electric field enhancement at nanoscale spots with a large local curvature,and it can largely increase the injection current based on the Fowler-Nordheim tunneling theory.For instance,according to Gauss's law,small-sized metal nanoparticles on a planar electrode rather than large-sized ones will act as the carrier injection hotspots.In this work,distinct from our previous work of using Al nanoparticles,fullerene(C60)and its derivative,phenyl-C61-butyric acid methyl ester(PCBM),are herein employed as the injection hotspots.And two main parts are presented:(1)C60 is prepared simply by thermal evaporation on ITO to enhance electron injection,which can greatly improve the device performance.Meanwhile,the influence of different film thickness on the device performance is comparatively analyzed.With the increase of C60 thickness,the device performance increases at first and then decreases.The best performance was obtain from the device with 2 nm C60.The characterization by?-? characteristics,luminescence spectra,transmittance,AFM and UPS indicates,and combined with our injection hotspot effect,we have proposed a physical model to explain the experimental phenomenon.(2)PCBM is prepared by spin-coating on ITO to enhance the electron injection.We also compare the performance of the devices with different PCBM concentrations,and the overall trend changes are similar to those based on C60.With the rotation speed of 2000,the best performance was obtained when the concentration is 1 mg/ml.This indicates that the modification layer prepared by spin-coating can also enhance electron injection.In summary,this work proposes a new method to increase electron injection in inverted bottom-emitting OLEDs by modifying ITO with fullerenes and its derivatives.We use two different modification methods to verify the feasibility of electron injection enhancement by nanoparticles acting as "hotspots".Based on the experimental results,we propose a reasonable model for this work and believe that this new method may also apply to other optoelectronic devices. |
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