Organic Electronics 14 (2013) 2039–2045

Contents lists available at SciVerse ScienceDirect

Organic Electronics

journal homepage: www.elsevier .com/locate /orgel

Organic/metal hybrid cathode for transparent organiclight-emitting diodes

1566-1199/$ - see front matter � 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.orgel.2013.04.035

⇑ Corresponding author. Tel.: +82 42 860 1166; fax: +82 42 860 1029.E-mail address: jiklee@etri.re.kr (J.-I. Lee).

1 First co-author (equal contribution).

Jin Woo Huh 1, Jaehyun Moon 1, Joo Won Lee, Jonghee Lee, Doo-Hee Cho, Jin-Wook Shin,Jun-Han Han, Joohyun Hwang, Chul Woong Joo, Jeong-Ik Lee ⇑, Hye Yong ChuOLED Research Center, Electronics and Telecommunications Research Institute, Daejeon 305-700, Republic of Korea

a r t i c l e i n f o a b s t r a c t

Article history:Received 23 October 2012Received in revised form 31 March 2013Accepted 16 April 2013Available online 9 May 2013

Keywords:Transparent organic light-emitting diodesOrganic/metal hybrid cathodeCs-doped electron transport layer (Cs-ETL)/AgMicrostructure

We report a highly transparent organic/metal hybrid cathode of a Cs-doped electron trans-port layer (Cs-ETL)/Ag for transparent organic light-emitting diode (TOLED) applications.Particular attention is paid to the surface morphology on the Ag film and its influence onthe optical transparency and electrical conductivity. With the use of Cs-ETL, a smoothand continuous surface morphology of Ag film was achieved, leading to a high transmit-tance of �75% in TOLED with a low sheet resistance of 4.5 X/Sq in cathode film. We suc-cessfully applied our Cs-ETL/Ag transparent cathode to fabricate highly transparentOLEDs. Our approach suggests a new electrode structure for transparent OLED applications.

� 2013 Elsevier B.V. All rights reserved.

1. Introduction

A transparent organic light-emitting diode (TOLED) re-fers to an OLED type which emits light from the top andbottom directions of the device [1,2]. In the absence of ap-plied voltage, the TOLED is a transparent window, butwhen voltage is applied to the TOLED, it becomes is a bi-directional light-emitting device. Thus, a TOLED can beused as a light source for innovative window-like lightingluminaire systems: during the daytime, a TOLED can beused as an ordinary window, while after sunset the TOLEDcan serve as a lighting facility. Because the generated lightin a TOLED has to escape the top surface as well as the bot-tom surface, special attention has to be paid to the topelectrode, which is the cathode [3–5].

In conventional bottom-emission OLEDs, the cathodematerial is usually metallic with a thickness of approxi-mately 200 nm. Hence, the cathode surface, which is incontact with the organic layer, acts as a mirror and reflects

the generated light toward direction of the bottom, con-tributing to the intensity of the bottom emission. On theother hand, in TOLEDs, in addition to the electrical conduc-tivity, the cathode must have optical transparency. Regard-ing the electrical conductivity in an electrode, metallicmaterials offer sufficiently low resistance. However, dueto their absorption dependency on the thickness, a limita-tion exists when seeking to obtain both high transmittanceand low resistance.

To realize practical TOLEDs, the transmittance of the de-vice initially has to be improved. Therefore, a metal cath-ode is the key part to improve the transmittance of OLEDdevices [6,7]. Conventionally, LiF/Al/Ag has been used asa transparent cathode [7–9]. But, in this structure, theemission intensity may be lowered due to the absorptionloss in the cathode. To make matters worse, with cavityglass seal [10] of the cathode, a TOLED with a LiF/Al/Agcathode gives transmittance of only 60% at 550 nm becauselight loss of approximately 10% occurs at the glass capboundary due to difference in refractive indices. For highertransparency, it is crucial to minimize the absorption andto make the metal film as thin as possible. Hence, the Allayer in a conventional transparent cathode of LiF/Al/Ag

2040 J.W. Huh et al. / Organic Electronics 14 (2013) 2039–2045

should be removed because it has high absorption in thevisible light region. Moreover, for an effective electroninjection from the Ag layer, modification of the electroninjection layer (EIL) corresponding to LiF should follow.According to previous several reports, in order to enhanceelectron injection in OLEDs, Cs2CO3 has been used as an n-type dopant [11–13]. Compared to pristine ETL thin films,Cs2CO3 doped organic ETL showed enhanced electroninjection and transport properties [11,14,15]. In addition,compared to reactive metals as n-type dopants, the incor-poration of Cs2CO3 can effectively eliminate diffusion ofmetal atoms into the underlying organic layer and conse-quential nonradiative recombination [11].

In this work, we present an approach which is useful forreducing the detrimental absorption effect without sacri-ficing the electrical conductance. To be specific, we havereplaced the LiF/Al under the metal (Ag) cathode by a Cs-doped organic ETL. First, we removed the Al layer becausethe Al layer is the main cause of absorption loss. Thus, LiF/Al layer was then replaced by an organic electron transportlayer (ETL). However, removing Al layer did not improvethe optical transmittance in actual measurement and itsoptical simulations alone could not explain the observedresults of our work. Thus, we investigated the surfacemicrostructure of the cathode film. Thermally depositedorganic ETL films can offer surface smoothness and opticaltransparency. In addition, with Cs2CO3 addition, enhancedelectrical properties are expected. Therefore, with anobjectivity of obtaining a smooth Ag film surface morphol-ogy which is favored in transmittance and conductivity, wehave replaced the LiF/Al film by an organic ETL. In our dis-cussion, we emphasize the importance of the microstruc-ture in analyzing and improving both transmittance andelectrical conductivity.

In this study, we investigated the surface morphology-property relationship of the cathode and offered a veryimportant yardstick in designing transparent electrodes.Finally, we demonstrated full functionality of TOLEDs hav-ing a cathode structure of Cs-ETL/Ag.

2. Experimental

The TOLED has the following stacking sequence: indiumtin oxide (ITO) (70 nm)/TAPC (55 nm)/TCTA: Firpic (7%)(5 nm)/DCzPPy:Firpic (10%) (5 nm)/BmPyPB (15 nm)/cath-ode/TAPC (60 nm). The organic material of each layer islisted in Table 1.

In the first part of the experiment, we investigated thethickness effect of the Al layer, which is a part of the cath-ode, on the transmittance of TOLEDs. The cathode structure

Table 1The full name of organics and their functions.

Abbreviation Chemical name

TAPC 1,1-bis[(di-4-tolylamino)phenyl]cyclohe

TCTA 4,40 ,400-tris(N-carbazolyl)-triphenylaminFirpic Iridium(III)bis(4,6-difluorophenyl)-pyridDCzPPy 2,6-bis(3-(carbazol-9-yl)phenyl)pyridinBmPyPB 1,3-bis(3,5-di-pyrid-3-yl-phenyl)benzen

is LiF (1 nm)/Al (X nm)/Ag (15 nm), which is formed on theETL of BmPyPB (40 nm). The Al thickness was varied, ateither 0 or 1.5 nm. Previously, in an effort to enhance thetransmittance of TOLEDs, we added a capping layer (CL)of TAPC onto the cathode surface [16]. Using optical simu-lations, we investigated the transmittance of TOLEDs as afunction of the CL thickness. The highest transmittancewas obtained at a CL thickness of 60 nm. Based on this re-sult, we fixed the CL thickness at 60 nm in this work.

In the second part of the experiment, we replaced theLiF/Al part in the LiF/Al/Ag structure with a Cs-doped ETL(Cs-ETL). Thus, the TOLED cathode has the structure ofCs-ETL (40 nm)/Ag (15 nm). Cs2CO3 was co-evaporatedwith ETL and the doping level was varied, at 0, 10, 20 or40%. Using a TOLED device with the Cs-ETL structure, weobtained the Cs doping concentration which yields thehighest transmittance.

In the final part, we demonstrated a fully functionalTOLED with a Cs-ETL/Ag cathode. Its transmittance, ELspectrum, and current density (J)–voltage (V)–luminance(L) characteristics were compared to those of a TOLED witha LiF/Al/Ag cathode.

The fabrication processes were described in our previ-ous work [17]. All organic and metallic layers were depos-ited by thermal evaporation in a high vacuum chamberbelow 6.7 � 10�5 Pa. The fabricated OLEDs were encapsu-lated with a glass cap. In this course, a glass cap with cavitywas glued using a UV curable resin. Also a moisture getterwas placed inside the cavity glass. The electrolumines-cence spectrum was measured using a Minolta CS-1000.The current–voltage (I–V) and luminescence-voltage (L–V) characteristics were measured with a current/voltagesource/measure unit (Keithley 238) and a Minolta CS-100. Transmittance of the glass-cap encapsulated TOLEDwas measured using an UV–visible spectrophotometer(U-3501, Hitachi). The surface morphologies of the cathodefilms were investigated by means of scanning electronmicroscopy (SEM, Model: Sirion 400, Philips) and atomicforce microscopy (AFM, Model: Park System, XE-100).Sheet resistance was measured using four-point probe sys-tem ((CMT-series, CHANG MIN CO., Ltd.) The simulationswere performed using an OLED optical simulator, Sim-OLED� [18,19] to optimize the characteristics of transpar-ent OLEDs as a function of the thickness of the TAPC.Briefly, the program has inputs of the refractive indexand thickness of every layer. The program employs thinfilm optics and the dipole emission model to calculatethe optical and spectral characteristic of OLEDs. In orderto obtain realistic simulation results, we used all of themeasured optical constants (n, k) of organic materials, as

Function

xane Hole transport layerCapping layer

e Emitter hostinato-N,C20)picolinate Emitter dopant (blue)

e Emitter hoste Electron transport layer

J.W. Huh et al. / Organic Electronics 14 (2013) 2039–2045 2041

obtained using an ellipsometer (M-2000d, J.A. WoollamCo.).

3. Results and discussion

3.1. Preliminary studies on the Al insert

In TOLEDs, a transparent cathode of LiF/Al/Ag structureis frequently used [7–9]. The sheet resistance is expectedto decrease further as the thickness of the Al insert in-creases. In terms of the electrical performance of the de-vice, increasing the thickness of the Al insert can lowerthe operation voltage condition of the OLED devices. How-ever, in terms of the transmittance, increasing the Al thick-ness is not desirable. The Beer–Lambert law dictates anexponential decrease in the transmittance upon an in-crease in the film thickness. Thus, a thicker Al layer haslower transmittance. In this scheme, the transmittance ofLiF/Al/Ag is expected to decrease as the thickness of theAl insert increases.

Fig. 1 compares the simulated and measured transmit-tances of the LiF/Al/Ag structure. Here, the thicknesses ofthe LiF and Ag are fixed at 1 nm and 15 nm. The thicknessof Al (tAl) was varied, at 0 nm or 1.5 nm. In the simulations,as expected, the insertion of Al lowers the transmittance inthe entire visible wavelength range. However, in the exper-iments, the insertion of Al does not necessarily lower thetransmittance in the entire visible wavelength range.

To understand this result, we investigated the surfacemorphology of the Ag film. In thin-film optical simulations,the optical parameters of the refractive index (n) andextinction coefficient (k) as well as the thin film thicknessof the media are considered as the main variables, whereasthe actual microstructures of the thin films are not.

Fig. 2 shows the two SEM surface images of 15-nmthick Ag films which reside on LiF (Fig. 2a) and on an Al

Fig. 1. Simulated and measured transmittances of LiF/Al/Ag. tAl refers to th

layer (Fig. 2b), respectively. In both cases, the Ag filmsare granular, but the grain sizes of Ag on LiF are muchsmaller with more rough surface; The average grain sizeof the Ag film on LiF is �25 nm which is less about atenth of that of Ag film on Al, and root-mean square(rms) roughness is �15 nm while Ag film on Al has�2 nm. As described earlier, granular and rough Ag filmswith less than 20 nm in thickness experience light lossdue to surface plasmon resonance (SPR) [20–22] as wellas grain boundary scattering [23]. It is noticeable thatSPR-induced loss increases considerably with rms rough-ness of the film [20] while the scattering no longer occursto any significant extent when the size of the grainboundary of the films is below the size of the wavelengthof the light being scattered. From this, we expect thatmore rough granular surface of the Ag film on LiF experi-ences larger transmittance loss than that of Ag film on Alby SPR-induced absorption rather than scattering. Thissupports the result showing that the transmittance ofLiF/Ag is lower than LiF/Al/Ag, as shown in Fig. 1.

The surface morphology in Fig. 2 also explains thechanges in the electrical conductance of the Ag films. Theinitial stage of metallic film growth is governed by the Vol-mer-Weber mechanism [24]. At the beginning of filmgrowth, separate islands initially form after the depositionof metal. Next, when the number of islands increases, theindividual islands start to connect physically. The film be-comes continuous only when a certain thickness isreached, which corresponds to the percolation thickness.The percolation thickness is dependent on the materialsystem. In Fig. 2, one can deduce that the percolation thick-ness of Ag on LiF is much thicker than that of Ag on Al. Inthe case of Ag on LiF, the Ag granules are not only small(�25 nm) but are also rarely connected. Meanwhile, Agon Al shows much larger granules, which are mainly phys-ically connected.

e Al thickness. (Transmittance was measured under glass reference.)

Fig. 2. Surface SEM images of (a) LiF/Ag (15 nm) and (b) LiF/Al (1.5 nm)/Ag (15 nm).

2042 J.W. Huh et al. / Organic Electronics 14 (2013) 2039–2045

The surface energy difference can be used for a roughcomparison of the percolation thickness of an identicalmetallic film on a different type of support. If the surfaceenergy of the support is lower than the deposited film,there exists a driving force that exposes the surface ofthe support, which results in restraining the formation ofthe percolation network. In this context, a system whichhas a larger value of dgi=j ! Dci=j is expected to have athicker percolation thickness. Here, dgi=j ! Dci=j is the sur-face energy difference between i (deposit) and j (support).The reported surface energies of LiF, Al and Ag are 0.34 J/m2, 1.1 J/m2, and 1.3 J/m2, respectively [25]. ThedgAg=LiF ! DcAg=LiF and dgAg=Al ! DcAg=Al values are 0.96 J/m2 and 0.2 J/m2, respectiv/ely. Thus, the percolation thick-ness of Ag on LiF is thicker than that of Ag on Al. In ourmaterial systems, it is apparent that the Al insert has theeffect of thinning the percolation thickness of the depos-ited Ag. The difference in the percolation thickness canbe examined by measuring the sheet resistance. The mea-sured sheet resistance of LiF/Al/Ag was 7 W=sq! X=sq.The sheet resistance of LiF/Ag was approximately�107 W=sq! X=sq, which is too high for OLED applica-tions. This reflects the fact that percolation is establishedin Ag on Al, implying a thinner percolation thickness ofAg on an Al support.

To sum up, in a TOLED with a cathode of LiF/Al/Ag, theAl contributes by making the Ag film continuous and facil-itates the formation of percolation paths. As a result, com-pared to the Ag in the LiF case, higher transmittance andlower sheet resistance were achieved. The former is dueto the reduced scattering and the latter is due to the highdensity of the percolation paths.

3.2. Organic film as an Al replacement

In this section, we probe the possibility of replacing theAl insert with an organic material. Compared to metallicthin films, organic thin films bear lower optical absorption.For the purpose of eliminating the Al layer, we also re-moved the LiF as well because LiF is known to work almostexclusively with Al [14,26]. As mentioned before, wechoose a Cs-doped ETL (Cs-ETL) as a replacement for Al.

Thus, the new cathode structure is Cs-ETL/Ag (15 nm). Inthe current work, we focus on achieving not only high opti-cal transmittance but also low cathode sheet resistance.Regarding this task, we investigated both as a function ofthe Cs concentration, as controlled by adjusting the rela-tive the deposition amounts of the ETL and Cs.

The Fig. 3b(1) shows surface images of Ag films on un-doped and Cs-doped ETL. With undoped ETL, Ag film onundoped ETL shows granular features with more high den-sity of grain boundaries. As Cs is doped, significant graingrowth can be observed. Larger grains and less boundariesare observed in Ag films. Such distinct difference in thesurface morphology strongly indicates that the Cs additionis modifying the ETL surface to induce high wettability ofAg on the Cs-ETL. It is thought that Cs addition modifiesthe surface energy of the ETL to make Ag wetting morefavorable. Based on the results of Figs. 1 and 2, this modi-fied surface morphology due to high wettability in Cs-ETL/Ag gives the result of improvement in transmittance andelectrical conductance.

Optical and electrical properties of the Ag film wereshown in Fig. 3a. The sheet resistance decreases with a rel-atively steep slope just after Cs-doping. The reduced sheetresistance by Cs-doping is associated with enhanced con-duction owing to the increased carrier concentration aswell as modified microstructure. But then it saturates to�4.5 X/sq at Cs concentration higher than 10%. Also, thetransmittance increases up to 20% Cs and then decreasesslightly as the Cs concentration increases. The transmit-tance is still around �60% in the Cs concentration rangeof 10–40%. Referring to the SEM images above results,these observations indicate that the Ag surface smoothen-ing effect of Cs addition has a certain upper bound.Fig. 3b(2) shows the nk products from measured n and kof Ag films which reside on undoped and Cs 20% dopedETL. The Cs-doped sample exhibits lower nk product. Be-cause the amount of absorption is proportional to nk, thisresult indicates a decrease in the transmittance loss dueto relatively low absorption in Ag on the Cs-ETL system.However, the measured nk values are only apparent values.The apparent change in nk values should be attributed as amicrostructure induced effect. In other words, optics alone

Fig. 3. (a) Transmittance and sheet resistance of the Cs-ETL/Ag(15 nm) film as a function of Cs concentration (Transmittance was measured under airreference and evaluated by averaging in the wavelength range of 400–700 nm.). (b-1) Surface SEM images of Cs-ETL/Ag films depending on Csconcentration. (b-2) nk product and (b-3) transmittance of two Cs-ETL/Ag films, both undoped or Cs 20% doped, in the visible wavelength range.

J.W. Huh et al. / Organic Electronics 14 (2013) 2039–2045 2043

can only explain the transmittance change in terms of nk,which is misleading without the description of the micro-structure. From the result of Fig. 3a, we chose a Cs concen-tration of 20%, which results in a highest transmittance andlow sheet resistance of 63% and 4.5 X/sq, respectively.Regarding the quality of the transparent conductive films,one may use a figure of merit, which is defined as r/a[27]. Here r and a are the electrical conductivity and thevisible absorption coefficient, respectively. The r/a is ob-tained using the relations of �{Rs ln(T + R)}�1, in which Rs

is the sheet resistance in ohms per square, T and R is thetotal visible transmittance and reflectance, respectively.As it shows, a larger value of r/a indicates better perfor-mance of the transparent electrodes. The r/a values ofour Cs-undoped and – doped(20%) ETL/Ag films are 0.7and 1.2, respectively, which show Cs-doped (20%) ETL/Agfilms have better opto-electrical performance than that ofundoped ETL/Ag films and is comparable to that of filmof doped-ZnO.

Fig. 4a shows the surface roughness of the Ag films on aCs (20%)-ETL and on LiF/Al according to SEM results asshown in Fig. 4b. As mentioned previously, the SEM imagesshow that the Ag film on the Cs-ETL is almost featureless,whereas the morphology of the Ag film on Al is granular.The AFM scan diagrams revealed that the peak-to-peakroughness of Ag films on the Cs-ETL does not exceed3 nm, while that on LiF/Al exceed 11 nm. The surface mor-phology of Ag on the Cs-ETL is not only smoother but alsomore continuous. Such a feature indicates the high wetta-bility of Ag on the Cs-ETL, which leads to an improvementin the transmittance due to both less scattering- and lessSPR-induced light loss as well as lower sheet resistancedue to the dense percolation paths in the Ag film on theCs-ETL, as mentioned above. The rms roughness values ofAg films on a Cs-ETL and on LiF/Al were found to be

0.6 nm and 1.9 nm, while the sheet resistances were4.5 X/Sq and 7 X/Sq, respectively.

3.3. TOLED with a Cs-ETL/Ag cathode

Fig. 5 demonstrates a TOLED bearing our new cathodestructure of Cs-ETL/Ag. The figure shows the transmittance(Fig. 5a), EL spectra (Fig. 5b) and the J–V–L relationship(Fig. 5c and d) of TOLEDs with an Ag cathode on theCs(20%)-ETL and LiF/Al structures. Fig. 5a shows that thetransmittance of the TOLED with the Cs-ETL/Ag cathodeis higher than that of the TOLED with the LiF/Al/Ag cathodewithout a change in the overall transmittance curve shapesthroughout the visible wavelength range. The Cs-ETL/Agcathode led to an improvement of up to 20% in the visiblewavelength range. Transmittance of over 70% was achievedat 550 nm by means of conventional glass encapsulation.The inset of Fig. 5a shows an actual image of our TOLEDwith the Cs-ETL/Ag cathode. The institutional logo isclearly observable. Fig. 5b shows that replacing LiF/Al witha Cs-ETL does not induce any change in the EL spectra ofthe bottom and top side emissions. These results implythat replacing LiF/Al with an organic ETL introduces anenhancement of the transmittance without any significantchange in the internal optics of the TOLED.

Fig. 5c and d compare the J–V and L–V characteristics oftwo TOLEDs with different cathode structures. In the J–Vplot, the two samples exhibit current levels that are nearlyidentical to the applied voltage. The L–V plot shows thatthe TOLED with the Cs-ETL/Ag structure shows animprovement in the luminance of the top emission. Thisimprovement in the top side is thought to have its originin the higher transmittance in the top cathode of the Cs-ETL/Ag structure, as shown in Fig. 5a. Similar J–V character-istics or better L–V characteristics imply that the OLED

Fig. 4. (a) The AFM line-scan across the Ag films on Cs(20%)-ETL and LiF/Al (The deep points in the AFM scan diagram LiF/Al/Ag represent the open granularborder regions.). (b) The Ag film morphology on (a) Cs(20%)-ETL and LiF/Al.

Fig. 5. (a) The transmittances, (b) EL spectra and (c) J–V (d) L–V characteristics of TOLEDs with Cs(20%)-ETL/Ag or LiF/Al/Ag cathodes. (Transmittance wasmeasured under air reference. Inset in (a) is the photograph of the institutional logo through the TOLED device.)

2044 J.W. Huh et al. / Organic Electronics 14 (2013) 2039–2045

structure used in the TOLED with the LiF/Al/Ag cathode isapplicable to a TOLED with a Cs-ETL/Ag cathode, whichalso implies that the Cs-ETL/Ag cathode structure is feasi-

ble for use in highly efficient TOLEDs. The technical impor-tance of Figs. 4 and 5 is that the replacement of Al with aCs-ETL makes the Ag surface smoother and more

J.W. Huh et al. / Organic Electronics 14 (2013) 2039–2045 2045

continuous, which then improves the optical transmit-tance and electrical characteristics of the TOLED.

From these results, we found that an improvement ofthe optical and electrical properties can be achieved byapplying the Cs-ETL/Ag cathode structure. This indicatesthat our organic/metal hybrid cathode structure is a poten-tial candidate as a cathode in efficient transparent OLEDs.

4. Conclusion

Designing a highly transparent cathode is of primeimportance in transparent OLED applications. For this rea-son, we designed a new organic/metal hybrid cathode witha Cs-ETL/Ag layer to obtain highly transparent efficientTOLEDs. In a conventional LiF/Al/Ag transparent cathode,the Al induces the formation of highly granular Ag films,which can potentially deteriorate the optical transmittanceand sheet resistance. By replacing the LiF/Al with an organ-ic ETL layer, it is possible to obtain a smooth continuous Agsurface morphology, which plays a significant part inreducing the light loss due to scattering and SPR, andenhancing the transmission. By doping Cs into the ETL,compared to LiF/Al/Ag cathode TOLEDs, better L–V charac-teristics are obtained. Also, identical EL spectra are ob-tained. In terms of TOLED processing, our cathode designis highly attractive because it obviates the need to modifythe OLED stack structure. Our approach for an organic/me-tal transparent electrode may form the basis of highly effi-cient TOLED applications.

Acknowledgements

This work was supported by IT R&D program of MOTIE/KEIT, Rep. of Korea (KI002068, Development of Eco-Emo-tional OLED Flat-Panel Lighting) and ICT R&D program2013 of MSIP (Ministry of science ICT & Future Panning)(10041416, Development of Light and Space AdaptableDisplay).

References

[1] G. Gu, V. Bulovic, P.E. Burrows, S.R. Forrest, Transparent organic lightemitting devices, Appl. Phys. Lett. 68 (1996) 2606–2608.

[2] X. Zhou, M. Pfeiffer, J.S. Huang, J. Blochwitz-Nimoth, d.S. Qin, A.Werner, J. dreshsel, B. Maennig, K. Leo, Low-voltage invertedtransparent vacuum deposited organic light-emitting diodes usingelectrical doping,, Appl. Phys. Lett. 81 (2002) 922–924.

[3] H. Riel, S. Karg, T. Beierlein, B. Ruhstaller, W. Rieb, Phosphorescenttop-emitting organic light emitting devices with improved lightoutcoupling, Appl. Phys. Lett. 82 (2003) 466–468.

[4] R.B. Pode, C.J. Lee, d.G. Moon, J.I. Han, Transparent conducting metalelectrode for top emission organic light-emitting devices: Ca–Agdouble layer, Appl. Phys. Lett. 84 (2004) 4614–4616.

[5] B.J. Chen, X.W. Sun, S.C. Tan, Transparent organic light-emittingdevices with LiF/Mg:Ag caathode, Opt. Exp. 13 (2005) 937–941.

[6] Vasicek, Optics of Thin Films, North-Holland, Amsterdam, 1960.[7] L.S. Hung, C.W. Tang, M.G. Mason, P. Raychaudhuri, J. Madathil,

Application of an ultrathin LiF/Al bilayer in organic surface-emittingdiodes, Appl. Phys. Lett. 78 (2001) 544–546.

[8] C.W. Chen, P.Y. Hsieh, H.H. Chiang, C.L. Lin, H.M. Wu, C.C. Wu, Top-emitting organic light emitting devices using surface-modified Aganode, Appl. Phys. Lett. 83 (2003) 5127–5129.

[9] C.-C. Wu, C.-L. Lin, P.-Y. Hsieh, H.-H. Chiang, Methodology foroptimizing viewing characteristics of top-emitting organic light-emitting devices, Appl. Phys. Lett. 84 (2004) 3966–3968.

[10] P.E. Burrows, V. Bulovic, S.R. Forrest, L.S. Sapochack, d.M. McCarty,M.E. Thompson, Appl. Phys. Lett. 65 (1994) 2922–2924.

[11] S.Y. Chen, T.Y. Chu, J.F. Chen, C.Y. Su, C.H. Chen, Stable invertedbottom-emitting organic electroluminescent devices with moleculardoping and morphology improvement, Appl. Phys. Lett. 89 (2006)053518. 3pp.

[12] A.H. Sommer, Hypothetical mechanism of operation of the Ag–O–Cs(S-1) photocathode involving the peroxide Cs2O2, J. Appl. Phys. 51(1979) 1254–1255.

[13] A. Band, A. Albu-Yaron, T. Livneh, H. Cohen, Y. Feldman, L. Shimon, R.Popovitz-Biro, V. Lyahovitskaya, R. Tenne, Characterization of Oxidesof Cesium, J. Phys. Chem. B 108 (2004) 12360–12367.

[14] T. Hasegawa, S. Miura, T. Moriyama, T. Kimura, I. Takaya, Y. Osato, H.Mizutani, Novel electron-injection layer for top-emission OLEDs, SIdInt. Symp. digest Tech. Papers 35 (2004) 154–157.

[15] Y. Li, d.-Q. Zhang, L. duan, R. Zhang, L.-D. Wang, Y. Qiu, Elucidation ofthe electron injection mechanism of evaporated cesium carbonatecathode interlayer for organic light-emitting diodes, Appl. Phys. Lett.90 (2007) 012119. 3pp.

[16] J.W. Huh, J. Moon, J.W. Lee, d.-H. Cho, J.-W. Shin, J.-H. Han, J. Hwang,C.W. Joo, H.Y. Chu, J.-I. Lee, The optical effects of capping layers onthe performance of transparent organic light emitting diodes, IEEEPhotonics J. 4 (2012) 39–47.

[17] J. Lee, J-I Lee, J-W Lee, H.Y. Chu, Interlayer engineering with differenthost material properties in blue phosphorescent organic light-emitting diodes, ETRI J. 33 (2011) 32–38.

[18] K.A. Neyts, Simulation of light emission from thin-film microcavities,J. Opt. Soc. Am. A 15 (1998) 962–971.

[19] M. Furno, R. Meerheim, M. Thomschke, S. Hofmann, B. Lüssem, KarlLeo, Proc. SPIE 7617 (2010) 761716. 12pp.

[20] S.-K. Kim, H.S. Ee, W. Choi, S.H. Kwon, J.H. Kong, Y.H. Kim, H. Kwon,H.G. Park, Appl. Phys. Lett. 98 (2011) 011109. 3pp.

[21] S.A. Maier, H.A. Atwater, J. Appl. Phys. 98 (2005) 011101. 10pp.[22] d.K. Gramotnev, S.I. Bozhevolnyi, Nat. Photonics 4 (2010) 83–91.[23] W.H. Flygare, T.d. Gierke, Light scattering in noncrystalline solids

and liquid crystals, Annu. Rev. Mater. Sci. 4 (1974) 255–285.[24] Norbert Kaiser, Review of the fundamentals of thin film growth,

Appl. Opt. 41 (2002) 3053.[25] S.O. Kasap, P. Capper, Handbook of Electronic and Photonic

Materials, Springer, 2006.[26] G. Li, C.-W. Chu, V. Shrotriya, J. Huang, Y. Yang, Efficient inverted

polymer solar cells, Appl. Phys. Lett. 88 (2006) 253503. 3pp.[27] R.G. Gordon, Criteria for choosing transparent conductors, MRS Bull.

(August) (2000) 52–57.