介质覆盖Ag阴极的顶发射有机发光二极管出光效率增强机制及耦合表面等离子体激元研究

Mechanism Study and the Role of Coupled Surface Plasmon Polaritons in Optical Out-coupling Enhanced Top-emitting OLEDs with a Dielectric Capped Ag Cathode

采用经典的光学传输矩阵法和微腔理论计算具有不同厚度的功能有机薄膜和电极形式的顶发射有机发光二极管光学的耦合出光效率。根据仿真计算结果,制备了绿色荧光Alq_(3)顶发射器件并进行了相关测试。研究发现,采用银作为半透明阴极并蒸镀NPB有机覆盖层,相对于无覆盖层的器件来说,出光效率大大增强,该增强效应采用近场光学理论的偶极子辐射与银电极相互作用理论,即耦合表面等离子体激元理论,解释为是金属Ag薄膜阴极增强的透射、变化的微腔增益以及最主要的出光银阴极上的耦合表面等离子体激元效应三个因素的综合作用。

In the field of Organic Light-Emitting Diode(OLED)display application,most manufacturing companies have made a transition to TEOLED technology,due to the nearly 100%aperture rate of Thin Film Transistor(TFT)driven Active Matrix OLED(AMOLED).TEOLEDs can be fabricated on any solid substrates like silicon wafers or glasses.A TEOLED is constructed using a semi-transparent top cathode such as ITO or Ag thin film and a reflective bottom metal anode.ITO is a perfect transparent anode material.It may be used as cathode.ITO’s work function,however,is fairly high,which does not suit electron injection,decreasing the performance of the device.Other problematic issues in using ITO include indium,a very scarce and expensive element,sputtering that damages the organic functional layer,and brittleness which cannot be used in flexible devices.Hence,it is important to explore alternative cathode.Ag is often used as a replacement of ITO for its relatively high optical transparency and good ductility.In general,a dielectric with a high refractive index is deposited on the surface of the metal cathode to promote the out-coupling efficiency.It has been reported that the light extracting efficiency will be increased if an Ag cathode is coated by index matching materials such as inorganic ZnSe,ZnS,MgF_(2),SiO_(2),ITO,ZnO,TiO_(2),organic Alq_(3),NPB,BCP and so on.The degree of enhancement varies widely with different types of material and thicknesses.Some researchers did the transmittance measurement for cathode with different thicknesses of a capping layer,and found that,despite the transmittance through the cathode being at the maximum value,the total device light extracting efficiency is far from the optimal value and sometimes just the opposite.Leo,et al.showed that this efficiency enhancement was mainly due to low absorption and high reflection,which lead to a high extraction because of the micro-cavity effect.Another paper argued that the enhancement of the efficiency is closely related to suppression of Surface Plasmon Polaritons(SPPs),but these authors did not elaborate on the working physics.It is therefore very important to identify various contributing factors in optical out-coupling.In this paper,using a combination of classic optics and SPP theory,we study the various possible contributing factors on enhancing optical out-coupling efficiency in a TEOLED.The classical optical transfer matrix method and micro-cavity theory is applied to compute optical out-coupling of Top-Emitting OLEDs(TEOLEDs).Optical paths of various functional organic films and electrode structures are included in the calculations.Fluorescent Alq_(3) green TEOLEDs were fabricated and tested based on theoretical calculations.For TEOLEDs with Ag cathodes plus NPB coatings,we experimentally observed significant enhancements in optical out-couplings compared to that of with bare Ag cathode.The highest EQE is at 45 nm capping layer.We also found that thin-film optics theory is insufficient to explain the observed enhancement.To explain the experimental results,we incorporated near-field radiation of electric dipole and its interactions with Ag cathode film,i.e.,SPPs theory,coupled SPPs in a dielectric/metal/dielectric(D/M/D)structure.The near-field excited SPPs widely exist in the OLEDs,an essential factor which leads to the low quantum efficiency of OLEDs.In normal BEOLED devices,SPPs occur on the reflecting cathode,and in TEOLED,SPPs occur on the reflecting anode and semi-transparent metal cathode.Therefore,to recover SPPs loss in TEOLED is especially significant.When the metal film is thinner than the decay depth of the SPP mode,the emitter can activate SPP onto both sides of the cathode,named as SPP2(bottom)and SPP1(top),and they will interact each other to give rise to coupled modes under various upper capping layer(DMD).When the capping layer reaches a certain thickness,the coupling level gets to the top.Based on the reciprocity principle of radiation transfer,SPP1 can be easily recovered to light under the moment match condition,vice versa.More in-depth research reveals that,strictly,it is incorrect to refer to the SPP modes as coupling one to another respectively;rather they are in an integration of coupled SPPs on both sides.Therefore,we demonstrate that optical out-coupling enhancement has three contributing factors in a TEOLED:an increased transmittance of cathode Ag,a varied internal micro-cavity gain,and the dominant improved SPP coupling on both sides of the Ag film cathode.Such devices with a dielectric capping layer(DMD)increase the optical out-coupling and the efficiencies of TEOLEDs with very simple means,which is very significant for the low-cost commericialization application in display area.