TCO/metal/TCO structures for energy and flexible electronics

Thin Solid Films 520 (2011) 1–17

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Thin Solid Films

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Critical review

TCO/metal/TCO structures for energy and flexible electronics

C. Guillén ⁎, J. HerreroDepartamento de Energía (CIEMAT), Avda. Complutense 22, Madrid 28040, Spain

⁎ Corresponding author. Tel.: +34 91 346 6669; fax:E-mail address: [email protected] (C. Guillén).

0040-6090/$ – see front matter © 2011 Elsevier B.V. Aldoi:10.1016/j.tsf.2011.06.091

a b s t r a c t
a r t i c l e i n f o
Available online 30 June 2011

Keywords:Transparent conductive oxideMetal nanofilmMultilayer electrodeFlexible displaySolar cell

There is increasing attention paid to improving transparent conductive electrodes for applications in largearea photovoltaic devices and displays that are being developed for energy and electronics. To date,transparent and conductive oxides (TCO) based on In2O3, ZnO, or SnO2 are commonly used, but advanceddevices require new electrodes with lower resistivities than previously achieved and with optical propertiessuperior to those of the present generation. TCO/metal/TCO multilayer structures have emerged as aninteresting alternative because they provide optical and electrical characteristics globally superior to thoseattainable with a single-layer TCO or metal electrode and can be deposited at low temperatures ontoinexpensive plastic substrates. Indeed, the fabrication of thin film devices on flexible substrates hassubstantial interest for application to lightweight products and implementation of roll-to-roll depositionprocesses that can significantly reduce production costs. In this sense, organic electronics that require lowdeposition temperatures have the best chance to be the first transferred from conventional glass toinexpensive plastic substrates. The present critical review summarizes current TCO/metal/TCO researchresults, first analyzed for materials and thickness selection as a function of the optical transmittance andelectrical resistance parameters, and then analyzed according to other important properties such asmechanical reliability and thermal and humidity stability. The review concludes with a brief discussion of theresults obtained for TCO/metal/TCO structures applied as electrodes in several organic electronic devices.

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Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12. Materials and thickness selection to optimize the TCO/metal/TCO optical and electrical performance . . . . . . . . . . . . . . . . . . . . . 3

2.1. Metal layer selection for optimum electrical performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32.2. TCO layers selection for optimum optical performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52.3. Optimized TCO/metal/TCO optical and electrical performances . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

3. Additional characteristics of TCO/metal/TCO electrodes on flexible plastics and heat-resistant glass substrates . . . . . . . . . . . . . . . . . 83.1. Crystalline structure and mechanical characteristics of TCO/metal/TCO electrodes . . . . . . . . . . . . . . . . . . . . . . . . . . . 83.2. Thermal and humidity stability of TCO/metal/TCO electrodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

4. Application of TCO/metal/TCO electrodes in electronic devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114.1. Application of TCO/metal/TCO electrodes in OLED devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124.2. Application of TCO/metal/TCO electrodes in OPV devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

5. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

1. Introduction

Transparent and conductive oxide (TCO) layers are essentialcomponents for a large variety of photosensitive electronic devices,

acting as transparent electrical contacts or electrodes in flat paneldisplays, touch screens, thin film solar cells, and electrochromicdevices [1–6]. There is a wide range of requirements for such metaloxide layers depending on the specific application. First, a certainsheet resistance is needed in order to meet the electrical functionality.The range is from sheet resistances of the order of 400–700 Ω/sq(ohms per square) required for electrodes in touch screens to sheetresistances below 10 Ω/sq for large area flat panel displays and thin

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film solar cells [7]. Another very important property is thetransmittance of the layers in the spectral range of interest. Thisrange can be determined by the sensitivity of the human eye, by thequantum efficiency of the absorber material for solar cells, or by theemission spectra of the active materials used for new lightningtechnologies. In spite of the specific requisites for the differentapplications, two basic conditions are generally accepted to definemetal oxide TCOs: (1) the oxide must have a band gap energy above3.1 eV, which renders it transparent to wavelengths greater than400 nm, to transmit more than 80% of the visible light in thin filmform; and (2) the metal oxide must be susceptible to degeneratedoping so that carrier densities of 1020–1021 cm−3 can be achieved[2].

In general, TCOs are degenerate n-type semiconductors withintrinsic doping by native donors such as oxygen vacancies and/orinterstitial metal atoms and additional extrinsic doping by donorimpurities. There is an inherent limitation in the metal oxideconductivity that can be obtained by increasing the carrier concen-tration, because the Coulomb interaction between the free electronsand the ionized donor centers from which they are generatedprovides a source of scattering that is inherent to the doped material[8,9]. In addition, for metal oxides with a high number of free carriers,some absorption of the incident radiation by interaction with theelectron gas takes place around the characteristic electron plasmafrequency which increases with increasing carrier concentration[10,11]. Thus, when carrier densities become greater than2×1021 cm−3, the TCO exhibits plasma frequencies that shift fromabsorbing infrared wavelengths to visible light, reducing thetransparency in the visible region [10]. The requirement of transpar-ency and the fundamental scattering mechanism establish anabsolute limit to the TCO resistivity of about 4×10−5Ω cm [8,9]. Inpractice, it is difficult to approach this resistivity limit owing toadditional scattering mechanisms due to neutral impurities, grainboundaries, or other forms of structural disorder which depend onthe nature of the specific material and the details of the preparationprocedure [12].

Themost popular TCOs are tin-doped indium oxide (ITO), fluorine-doped tin oxide (F:SnO2 or FTO), and aluminum-doped zinc oxide (Al:ZnO or AZO) [1–6]. ITO dominates the market in high-end electronicsand it constitutes themost common usage of indium. This is due to theextraordinary combination of ITO optical and electrical properties:intrinsic band gap of 3.7 eV and electrical resistivity near2×10−4 Ω cm when deposited at temperatures above 250 °C [3].Indium, however, is a scarce metal and the increasing demandcoupledwith its natural scarcity is reflected in its market value [13]. Inthis sense, a significant drop in indium consumption by reduction ofthe ITO layer thickness can be obtained by combination with othervery thin metal films that have typical resistivities of the order of10−6Ω cm [14,15]. Indeed, ITO/Ag/ITO structures have allowed theachievement of sheet resistances below 5 Ω/sq and a visibletransmittance above 85% with overall thicknesses below 100 nm[15–18]; whereas more than 400 nm thickness would be required forsingle-layer ITO electrodes with 2×10−4Ω cm to obtain the samesheet resistance. In the ITO/Ag/ITO structures, the key element for thesheet resistance is the Ag layer which has excellent electricalproperties and can be made thin enough to provide sufficienttransmittance, but the overall transmittance of the stack is stillincreased by embedding the metal between thin ITO films thatsuppress the reflection from the Ag layer in the visible region andproduce a selective transparent effect. Analogous performance hasbeen achieved with other TCO/metal/TCO structures such as ZnO/Ag/ZnO [19], Al:ZnO/Ag/Al:ZnO [20], InZnSnOx/Ag/InZnSnOx [21], andInZnOx/Ag/InZnOx [22], all with sheet resistance below 5 Ω/sq andmaximum transmittance about 85% with less than 100 nm totalthickness. Thus, the substitution of ITO by alternative TCOs becomeseasier with TCO/metal/TCO structures because the metal interlayer

allows one to decrease the overall resistivity even though the TCOelectrical quality is not optimum.

The majority of the present production of flat panel displays andthin film solar cells is based on glass substrates which provide a rigidsupport and can withstand the temperatures required for successivemanufacturing steps [23,24]. In order to meet the expectations forgrowing renewable energy and electronic markets with lower prices,it is necessary to substitute flexible plastics for rigid glass as the basesubstrate [25–27]. This is because the handling of polymer foils ofsome tens or hundreds of micrometers in thickness is easier thanpanes of glass with similar thicknesses. Polymer foils are lighter thanglass and bendable, thus allowing low-cost mass production by roll-to-roll (R2R) based continuous fabrication processes [28,29].

Thermoplastic polymers have a price advantage, but they do notwithstand the temperatures required for some widely used produc-tion steps in semiconductor processing. Currently, it is difficult tomake high quality transparent electrodes from the commonly usedTCOs because the necessary temperatures for achieving a lowelectrical resistivity cannot be applied to the substrate. Nevertheless,the substitution of TCO/metal/TCO structures for single-layer TCOelectrodes avoids the temperature problem, because optimumelectrical conductivity can be obtained without substrate heating.This is a good solution that justifies the efforts for the more complextechnology. In fact, all the trilayer structures cited above as examplesof high performance electrodes have been prepared by sputtering atroom temperature [15–22], generally onto glass but also onto cheapplastics such as polyethylene terephthalate (PET) [20,21] or poly-ethersulphone (PES) [16].

Sputter technology is extensively used for TCO deposition on largearea glass [3,30], and is also applied for wide web R2R deposition ofTCO films on plastics [31,32]. Pilot sputter roll coater machines havebeen used successfully for the preparation of ITO/Ag/ITO structures onplastics [7,16,33–35], providing scalability for industrial applications.Additionally, the ITO/Ag/ITO electrodes grown by continuous R2Rsputtering processes have much more robust mechanical propertiesthan analogous ITO single-layers as demonstrated by bending tests[16]. The superior flexibility of ITO/Ag/ITO electrodes is attributed tothe existence of the ductile metal interlayer that provides effectiveelectrical conductivity even after the ITO is beyond its failure strain[36,37]. Thus, significant advantages in material and process temper-ature savings can be achieved with the substitution of TCO/metal/TCOstructures for single-layer TCO electrodes on both glass and plasticsupports, but the additional requirement of mechanical stabilitymakes the multilayer electrodes essential for applications on flexibleelectronic devices.

Although in the area of the inorganic electronics, considerableefforts are being focused on the development of low-temperatureprocesses compatible with plastic substrates [23,24], the organicelectronics that require lower deposition temperatures have the bestchance to be the first transferred from glass to inexpensive plasticsubstrates with significant cost reduction by R2R production [26,29].Nowadays, the TCO/metal/TCO electrodes have been mainly appliedto organic luminescent displays (OLED) [21,36,38–43] and organicphotovoltaic devices (OPV) [16,17,22,44–47], with applications also ininorganic liquid crystal displays (LCD) [48], flexible random accessmemories [37], capacitors [49], gas sensors [50], and dye-sensitizedsolar cells [51].

There are additional electrode requirements from the viewpoint ofOLED and OPV applications. TCO adhesion, surface roughness, andwork function are very important to enhance the stability andefficiency of such devices [52–54]. Single-layer ITO electrodes have asurface roughness that tends to increase as the film thicknessincreases [55], and such surface morphology is transferred to thefunctional organic layers deposited subsequently, giving an uneveninterface which is not desirable for device efficiency and stability [52].Very thin trilayer structures with 100 nm of overall thickness have

Fig. 1. Schematic representation of the electrical behavior of TCO/metal/TCO multilayerstructures.

3C. Guillén, J. Herrero / Thin Solid Films 520 (2011) 1–17

shown lower surface roughness than analogous single-layers, which isattributed to differences in the grain growth of the films [56].Additionally, ITO/Ag/ITO structures deposited on plastic substrateshave improved the device efficiency compared to analogous single-layer ITO electrodes, owing to better electrical conductivity andmechanical stability [16,36,38,41,47]. However, the low work func-tion of ITO can result in imperfect alignment with the hole injection orextraction layers in OLEDs and OPVs [53,54]. Some attempts at ITOmodification have been successful to increase the work function, butthe corresponding decrease in the carrier density yields worseperformance despite the higher work function [57]. This suggeststhat it is important to have both a relatively high work function and ahigh carrier density. In this respect, utilization of InZnSnOx (IZTO) inIZTO/Ag/IZTO structures deposited on PET substrates provides flexibleOLEDswith better efficiency than those fabricated on ITO/PET samplesdue to the low sheet resistance of the multilayer and the high workfunction of IZTO [21]. In addition, for multicomponent oxide films, thework function and band gap energy can be controlled by changing thecomposition [2,13,58]. Concerning stability for outdoor deviceoperation, AZO/Ag/AZO electrodes have higher corrosion resistancethan ITO/Ag/ITO in saltwater immersion tests [40] and superiormoisture durability compared to ZnO/Ag/ZnO in damp-heat tests [59],although it should be noted that this is only for specific AZOdeposition conditions. The above points are indicative of theversatility of the TCO/metal/TCO structures to achieve not onlyimproved electrical performance, but also other characteristics(morphology, work function, bending endurance, and corrosionresistance) that can be adapted for each specific application in aneasier way than can be achieved for single-layer TCO electrodes.

In the following sections, TCO/metal/TCO structures will beanalyzed first for materials and thickness selection as a function ofoptical transmittance and electrical resistance. Other importantquestions are the mechanical reliability and thermal and humiditystability, which will be discussed in relation to the structural stress inthe films. Finally, the application of multilayer electrodes in severalOLED and OPV devices will be analyzed.

2. Materials and thickness selection to optimize the TCO/metal/TCO optical and electrical performance

For any transparent conductive electrode, materials and thicknessselection is focused on the need for optimizing the optical transmis-sion and the electrical conduction; both should be as large as possible.Thus, transparent conductive coatings are characterized by twoessential parameters: the sheet resistance (Rs) and the opticaltransmission (T), both of which depend on the coating thickness.The sheet resistance is defined by Rs=1 /σt, where σ is the electricalconductivity in Ω−1 cm−1 and t is the coating thickness in cm. Theunits of Rs are customarily quoted inΩ/sq to indicate that it measuresthe resistance of a square surface area independent of dimensions. Theoptical transmission is given by the ratio of the radiation intensity I0entering the coating on one side to the radiation intensity I leaving thesample on the opposite side; T= I / I0 and is directly related to exp(−αt), where α is the optical absorption coefficient measured incm−1.

Fraser and Cook [60] defined a figure of merit F=T/Rs as a tool forassessing the performance of different transparent conductor mate-rials. However, it has been noted that this definition weighs in favor ofthe sheet resistance, resulting in maximum F values at comparativelylarge film thickness and low transmittance values. Thus, Haacke [61]proposed another figure of merit, T10/Rs, to emphasize the importanceof the optical transparency. The latter definition is commonly used tocompare TCO materials, but single-layer metal electrodes cannotcompete with TCOs on this basis. A significant improvement was thesuggestion by other authors [62,63] to use the ratio σ/α, taking theinverse of the sheet resistance and the visible absorption coefficient

calculated from the total visible transmission and corrected forreflectance. All these definitions are useful for a researcher in order tocompare different coatings as a function of the thickness or otherdeposition conditions, but it is more difficult to compare the valuesgiven by different authors because of the discrepancies in themeasured transmittance. Commonly, T is taken as the transmittancevalue at the 550 nmwavelength where the human eye has maximumsensitivity, but the average value in the visible (400–800 nm) rangehas also been used. Moreover, sometimes T is referred only to thecoating (when the optical measurements aremadewith respect to thebare substrate), but other times T includes the substrate absorbance(for measurements performed with respect to air). This can decreasethe overall value by more than 10%.

Recently, it has been proposed [64] to extend the calculation of thefigure of merit from the visible to the near infrared region, because theoptimal conditions for certain applications can be reached in otherwavelengths range than the550 nmvalue typically employed. In addition,it should be considered that the TCO/metal/TCO multilayer structureswork in a differentmanner than electrodes based on single TCOs ormetalthin films. The overall sheet resistance is measured in a coplanarconfiguration on the upper layer of the stack deposited on an insulating(glass or plastic) substrate and is mainly related to the metal film, as hasbeendepicted in Fig. 1 by an electric circuitwith resistors representing theTCO and metal layers, where 1/Rs=1/Rmetal+2/RTCO. Considering atypical case in which RTCO=100Rmetal, Rs=Rmetal/1.02≈Rmetal. Theoverall optical transmission is controlled by destructive interference inthe beams reflected from the interfaces, and constructive interference inthe corresponding transmitted beams through the multilayer structureand the transparent substrate. In this manner, the optical and electricalperformances achieved with the TCO/metal/TCO electrodes can besuperior to that obtained with single TCO or metal thin films.

2.1. Metal layer selection for optimum electrical performance

Taking into account that the metal layer is the key elementdetermining the sheet resistance in the TCO/metal/TCO structures,and according to the electrical resistivity values listed in Table 1 forbulk materials, Ag is the first choice because it has the lowestresistivity of all metals, followed by Cu that has an only slightly highervalue, below 2 μΩ cm for both cases [65]. However, for metal thinfilms, the transmittance and the sheet resistance change rapidly withlayer thickness [66–69]. A thin and continuous metal film may betransparent in the visible spectral range and show good electricalconduction, but below a critical film thickness both the electricalresistivity and the optical absorption rapidly increase. This is due to atransition from a continuous film to one composed of distinct islandsof metal atoms (aggregated state) with properties that differconsiderably from the bulk metal [14]. In general, the criticalthickness for this transition depends on the substrate and depositionconditions in addition to the specific metal [70–74].

The evolution of the maximum transmittance and sheet resistancevalues as a function of metal layer thickness is illustrated in Fig. 2 forAg and Cu layers deposited by magnetron sputtering on soda lime

Table 1Electrical resistivity values of several bulk metals [65].

Metal Resistivity (μΩ cm) at 20 °C

Ag 1.6Cu 1.7Au 2.4Al 2.8Mg 4.6W 5.6Mo 5.7Zn 5.8Ni 7.8In 8.0Pt 10.0Pd 11.0Sn 11.5Cr 12.6Ta 15.5Ti 39.0


glass (SLG) substrates under the same conditions reported previously[15,73]. Both optical and electrical parameters show an abruptdecrease, steeper for the resistance, when the metal film thicknessincreases from 5 to 15 nm. The same tendency continues with furtherincreasing thickness of the metal film, but with a lower gradient thatis now softer for the resistivity. Such dependence and the existence oftwo different regions as a function of the thickness are in agreementwith the evolution of thin film metal growth proposed by otherauthors [14], which indicate that the change in slope coincides withthe transition from metal clusters to a continuous layer. According tothe data represented in Fig. 2, a sheet resistance value of 6 Ω/sq isobtained with a 10-nm-thick Ag film or with a 15-nm-thick Cu layer.The corresponding electrical resistivity values are 6 μΩ cm and9 μΩ cm for the Ag and Cu films, respectively. The resistivity decreasesto 3 μΩ cm at thicknesses of approximately 60 nm for Ag layers and130 nm for Cu. These resistivity values are in agreement with thosereported for other Ag and Cu layers within similar thickness ranges[75–77], where it has been established that specific resistivity isstrongly dependent on the deposition conditions. Although thin filmresistivity tends to decrease for increasing layer thicknesses, it usuallyremains above that corresponding to the bulk material, owing to theparticular characteristics of thin metallic films [78,79].

With respect to the optical characteristics of the metal thin films,Fig. 2 indicates transmittance values above 60% for both Ag and Culayers with thickness below 20 nm. It should be noted that these aremaximum values independent of the specific wavelength at whichthey are achieved. These results are useful to evaluate the effect ofincreasing thickness for eachmetal but, for practical applications, it is

Fig. 2. Representation of the maximum transmittance and sheet resistance valuesobtained for sputtered Ag and Cu coatings as a function of film thickness. Transmittancemeasurements were performed using the bare SLG substrate as reference.

necessary to know the dependence of the optical transmission on theradiation wavelength. In addition, not only the transmittance valuesT(%), but also the reflectance R(%), and consequently the absorptanceA(%)=100−T−R, should be known to evaluate the optical lossesdue to the metal films [80].

Transmittance and absorptance spectra corresponding to a 10-nm-thick Ag film and a 15-nm-thick Cu layer, both with the same 6 Ω/sqsheet resistance, are represented in Fig. 3 together with the spectracorresponding to the soda lime glass substrate for comparison. Similarbehavior is observed for Ag and Cu films in the near infrared, withoutsignificant absorption from 1500 to 600 nm. Therefore, the progres-sive increase in the transmittance in this range is due to a decrease inthe reflectance. Maximum transmittance is achieved around 600 nmfor the Cu film, and extends toward lower wavelengths for the Aglayers, until the absorption edge of the SLG substrate is reached in theultraviolet region around 320 nm. Fig. 3 shows that absorptanceremains low (around 10%) in the visible range for the Ag film, but anabsorption increase and proportional transmittance decrease areobserved for the Cu layer below 550 nm. In general, metals have highelectron densities, of the order of 1022–1023 cm−3, which correspondto plasma absorption in the 100–400 nm wavelength region [81,82].However, it should be considered that specific plasma frequencies forthin metallic films can differ from those of the bulk material [83].Thus, optical absorption detected at wavelengths above 400 nm for Cuthin films is associated with lower plasma frequency compared tobulk material or to Ag layers [81].

From the literature, Ag is themost commonmetal interlayer for TCO/metal/TCO electrodes (see Table 2). The table shows a compilation ofvarious trilayer structureswith reported sheet resistance below15Ω/sqand optical transmittance above 70%. The results are ordered byincreasing thickness of themetal film. All the TCO/metal/TCO structureswere prepared by sputtering the TCO layers and using sputtering orevaporation for the metal interlayer, with deposition processesperformed at room temperature (that is, without intentional substrateheating). Such preparation procedures are suitable for low-costmanufacturing on inexpensive thermoplastic substrates such as PET orPES that have beenutilized aswell as PPC (polyphtalate carbonate), PEN(polyethylene naphthalate), and PC (polycarbonate), in addition toconventional glass substrates. When various metal or TCO layerthicknesses have been tested in the same report, only the one selectedas optimum by the authors has been included in the table. For thedifferent TCOs used, it is observed that sheet resistance valuesdecreasing from 15 to 3Ω/sq are obtained with Ag film thicknessesincreasing from 8 to 20 nm, although the specific sheet resistance valuedepends on the deposition conditions [44,84–94].

Fig. 3. Optical transmittance and absorptance spectra for a 10-nm-thick Ag film and a15-nm-thick Cu layer deposited by magnetron sputtering on SLG substrates. Thesemeasurements were performed taking air as reference, and the bare SLG substrate wasalso measured for comparison.

image of Fig.�2

image of Fig.�3

Table 2Optical transmittance (T) and sheet resistance (Rs) values reported for several TCO/metal/TCO structures, some of which have been applied in OLED or OPV devices.

TCO/metal/TCO description Substrate T (%) Rs (Ω/sq) Ref. Device application

ITO 50 nm/Ag 8 nm/ITO 50 nm Glass 89 15 [44] OPVGIO 40 nm/Ag 8 nm/GIO 40 nm Glass 92.9 11.3 [84]ITO 37 nm/Ag(PdCu) 8 nm/ITO 37 nm Glass 89 10 [40] OLEDAZO 40 nm/Ag(PdCu) 8 nm/AZO 40 nm Glass 88 10 [40] OLEDITO 30 nm/Ag 8 nm/ITO 30 nm PEN 85 6.8 [85]ZnO 57 nm/Ag 9 nm/ZnO 40 nm Glass 95 7 [86]ITO 35 nm/Ag 10 nm/ITO 35 nm PET 77 10 [36] OLEDITO 35 nm/Ag 10 nm/ITO 35 nm Glass 80 9 [36] OLEDSnO2 45 nm/Ag 10 nm/ITO 45 nm Arton 85 7 [34]GZO 30 nm/Ag 10 nm/GZO 40 nm Glass 90.7 7 [87]ITO 30 nm/Ag 10 nm/ITO 30 nm Glass 90 6 [15]ITO 40 nm/Ag 10 nm/ITO 40 nm Glass 87 6 [45] OPVITO 45 nm/Ag 10 nm/ITO 45 nm Arton 86 6 [33]AZO 40 nm/Ag 10 nm/AZO 40 nm PET 85 6 [20]ITO 50 nm/AgCu-alloy 10 nm/ITO 50 nm Glass 83 5.7 [88]ITO 50 nm/Ag 10 nm/ITO 50 nm Glass 88 5 [89]ZnO 35 nm/Ag 12 nm/ZnO 35 nm PET 75 10 [90]GZO 30 nm/Ag 12 nm/GZO 30 nm PES 87.2 7 [91]IZO 30 nm/Ag 12 nm/IZO 30 nm PET 84.8 6.9 [43] OLEDAZO 40 nm/Ag 12 nm/AZO 40 nm Glass 82 7 [46] OPVGZO 40 nm/Ag 12 nm/GZO 40 nm Glass 87 6 [46] OPVITO 40 nm/Ag 12 nm/ITO 40 nm PES 89.3 4.3 [16] OPVITO 50 nm/Ag 14 nm/ITO 50 nm PET 81 11 [41] OLEDITO 70 nm/Ag 14 nm/ITO 70 nm PPC 68 6.5 [47] OPVIZTO 30 nm/Ag 14 nm/IZTO 30 nm PET 86 5 [21] OLEDIZO 40 nm/Ag 14 nm/IZO 40 nm Glass 87.7 4.2 [22] OPVITO 40 nm/Ag 15 nm/ITO 40 nm Glass 85 4.2 [48]ITO 42 nm/Ag 15 nm/ITO 42 nm Glass 85 3.3 [18]ITO 43 nm/Ag 16 nm/ITO 43 nm Glass 79.4 8.9 [92]ITO 40 nm/Ag 16 nm/ITO 40 nm Glass 86.5 4.4 [17] OPVITO 50 nm/Ag 17 nm/ITO 50 nm PET 83.2 6.7 [93] OLED [38]ITO 54 nm/Ag 20 nm/ITO 54 nm Glass 75 3.5 [94]ZnO 50 nm/Cu 5 nm/ZnO 50 nm Glass 83 10 [96]ZnO 30 nm/Cu 6 nm/ZnO 30 nm PEN 88 10 [97]AZO 40 nm/Cu 8 nm/AZO 40 nm Glass 84 9 [98]ITO 40 nm/Cu 14 nm/ITO 40 nm Glass 69 6 [45] OPVITO 30 nm/Cu 16 nm/ITO 30 nm Glass 88 6 [73]AZO 50 nm/Au 9 nm/AZO 50 nm Glass 83 12 [99]ITO 50 nm/Au 10 nm/ITO 40 nm PC 72 5.6 [100]IZO 40 nm/Au 12 nm/IZO 40 nm Glass 81.0 5.5 [22] OPV


Pure silver is generally used, but some Ag-based alloys includingCu, Au, and Pd in small proportions have also been utilized[40,88,94,95]. These reports indicate that such alloys can improvethe thermal andmoisture stability of the electrode, but it is interestingto note that multilayer corrosion resistance is also greatly influencedby the outer TCO film characteristics [40], as will be further analyzedin Section 3.2. When the optical and electrical properties of pure silverwere measured in relation to the added metallic elements in severalAg alloys [88,94], it was noticed that some Cu or Au incorporation(about 5–10 wt.%) has little influence on the transmittance and sheetresistance, but the addition of Pd should be limited to less than 2 wt.%in order to avoid a significant sheet resistance increase. However, itwas concluded that Pd–Cu is better than Cu alone to improve chemicalstability [94]. TCO/metal/TCO structures with sheet resistance below10 Ω/sq have also been obtained by using pure Cu or Au interlayers inthe 8–16 nm thickness range [96–100]. By using these metals,maximum transmittances are achieved at around 550–600 nm, buta significant transmission decrease is observed around 500 nm,analogous to that illustrated in Fig. 3 for the Cu layer in comparisonwith the Ag film. Such transmission loss has been related to plasmaabsorption, and the comparative analysis of Cu and Au interlayers hasestablished that the absorption increases and moves towards higherwavelengths in the visible region when the carrier density decreasesby the substitution of Cu for Au interlayers with the same thickness[101]. In addition, when the Cu or Au layer thickness is increased, thedecrease in the transmittance with thickness is faster than thatobserved for Ag films. This makes it necessary to lower the thicknessupper limit to maintain a high transmittance value. This fact is much

more important for other metals, and thus the maximum transmit-tance goes below 60% for Ni thickness above 10 nm [102] or Ptthickness above 6 nm [103], which are too thin for obtaining a suitabledecrease in sheet resistance values. The utilization of Al thin films hasbeen reported in multilayer ZnO/Al/ZnO coatings [104,105], but theiroptical and electrical properties were only improved after heatingabove 300 °C, for which the transmittance and resistivity reachedtypical values of Al-doped ZnO single-layer electrodes. The introduc-tion of a thin Fe layer into AZO/Fe/AZO structures has been proposedas a way to achieve ferromagnetic properties for specific applications[106], with overall transmittance and resistivity values also of theorder of those achieved with single-layer AZO electrodes.

2.2. TCO layers selection for optimum optical performance

For the TCO component of TCO/metal/TCO structures, ITO preparedby magnetron sputtering is the most used, mainly because it has beendeveloped extensively as single-layer electrodes in flat panel displays,electrochromic devices, and photovoltaic solar cells [1–6]. High-quality ITO single-layers are commonly obtained by annealing at hightemperature (N250 °C) during or after the deposition process, becauseheating promotes material crystallization and reduces the density ofcrystalline defects, allowing one to achieve films that are moretransparent and conductive [107]. Under these conditions, ITOelectrical resistivities around 2×10−4 Ω cm and visible opticaltransmittances above 90% have been reported for typical thicknessesin the 100–300 nm range [107–109]. ITO thin films prepared bysputtering without any substrate heating have also achieved visible

Fig. 4. Optical transmittance and absorptance spectra for ITO and AZO layers withvarious thicknesses deposited by magnetron sputtering on SLG substrates.


transmittance about 90%, and electrical resistivity of 6×10−4 Ω cmon glass and different plastic substrates [108,110,111].

Concerning alternative indium-free TCOs, that are increasinglyresearched to avoid the high cost and low supplies of indium, AZO hasbeen proven to have suitable characteristics as a transparentconductive material, with several advantages for large area applica-tions such as non-toxicity, low cost, andmaterial abundance [112]. ForAZO thin films prepared by sputtering without heating, visibletransmittance in the 85–90% range and resistivities of 1–2×10−3 Ω cm have been obtained on glass and plastic substrates[113,114], although annealing above 250 °C is required to lower theresistivity below 9×10−4 Ω cm [114].

In spite of the higher resistivity achieved by TCO films preparedwithout heating, they are useful to be applied in TCO/metal/TCOstructures because the metal conductivity dominates the overallelectrical performance. The TCO conductivity should be only sufficientto assure a good electrical connection between the metal film and anypoint on the structure surface. However, their optical characteristicsare very important for the transparency of the stack. The principalmission of the TCO layers in stacked TCO/metal/TCO electrodes is toincrease the overall transmittance in the visible spectral range byreducing reflection from the metal surface. The limitation in thetransmittance of themetal film is mainly due to reflectance losses, andconsequently it is possible to boost the transmittance by adding layersthat act as antireflective coatings. The objective is to adjust theinterference phenomena that take place between multiple reflectionsat the different interfaces to obtain theminimal value of reflectance orthe maximum value of transmittance in a certain spectral band. Forthis purpose, TCO film thicknesses in the 30–60 nm range arecommonly used and are finely adjusted to obtain maximumtransmission at some specific wavelength, typically 550 nm [40], ormaximum integral transmission in the visible spectral region (400–800 nm) [88].

Several models have been proposed for the description of theoptical properties of TCO/metal/TCO stacks [7], and simulations basedon the refractive indices, extinction coefficients, and thicknesses ofthe components have been used for the transmittance maximization[40,88]. In this sense, it should be noted that the simple application oftabulated data for simulation can be far from reality because theoptical characteristics of TCO layers depend greatly on the filmthickness and the oxygen content [88,109]. Significant increases in therefractive index of diverse metal oxide films have been achieved bydecreasing the sputtering power density or increasing the oxygen toargon partial pressure ratio in the sputtering atmosphere [115–117],and it is known that a higher refractive index gives a betterantireflective effect on the metal layer surface [118].

The optical spectra of ITO and AZO thin films deposited bysputtering at room temperature on SLG substrates, with the otherexperimental conditions reported in previous work [15,20,73], havebeen measured for film thicknesses ranging from 30 to 50 nm and arepresented in Fig. 4. All layers show high transmittance and lowreflectance in the visible and near infrared. It can be seen that theminimum absorptance value in the visible region moves towardlonger wavelengths (from about 450 to 550 nm) when the ITO filmthickness increases. The same tendency is observed for the AZO layers,and their minimum absorptance values are only slightly higher thanfor ITO films. The optical spectra show the characteristic semicon-ductor band absorption edge, with the absorptance increasing above50% in the ultraviolet region at a wavelength about 335–345 nm,corresponding to a band gap energy of 3.7–3.6 eV, similar for ITO andAZO layers of the same thickness, but the gap energy decreasesslightly when the film thickness increases.

Usually, the band gap energy of degenerately-doped metal oxidefilms is found to be higher than that of the undoped bulk material,which is reported as Eg=3.7 eV for In2O3 and Eg=3.4 eV for ZnO[119]. This is because the energy gap between the top of the valence

band and the lowest empty state in the conduction band can increasewhen the carrier concentration (N) increases due to filling of low lyingenergy levels in the conduction band with a broadening proportionaltoN2/3 [120–122]. In addition, for TCO layers prepared under the sameexperimental conditions, it has been proven that the crystallite sizedepends on the film thickness and affects the optical properties.Higher band gap energy is obtained for thinner layers with smallercrystallite sizes [113,123,124]. Such a trend is also observed for thesamples depicted in Fig. 4. In general, changes in the gap energy ofTCO thin films have been related to variations in the mean crystallitesize, the internal stress, and/or the free carrier concentration[113,122,124,125]. For these ITO and AZO layers, the electron densityis in the range of 1020–1021 cm−3 [20,110,113] and the plasmaabsorption wavelength lies in the infrared spectrum above 1200 nm[10]. Thus, no significant absorption is detected in TCO layers over awide spectral region.

Looking at the bibliographical data summarized in Table 2, it isobserved that ITO and AZO are extensively used for TCO/metal/TCOstructures, together with Ga-doped indium oxide (GIO) [84] or Ga-doped zinc oxide (GZO) [46,87,91], mixed indium zinc oxide (IZO)[22,43], indium zinc tin oxide (IZTO) [21], and even ZnO [86,90,96,97]and SnO2 [34] without extrinsic doping. By substituting GIO layers forITO, a significant increase in the optical transmission below 400 nmhas been reported [84] since GIO has a transmittance above 80% at330 nm and above 40% at 280 nm. This is superior to the correspond-ing transmittance values obtained with analogous ITO films. Thus,GIO/Ag/GIO electrodes show an important advantage for devicesemitting ultraviolet radiation [84]. This behavior of GIO has beenattributed to the exceptionally large band gap of Ga2O3, approxi-mately 4.9 eV (250 nm in wavelength) [126]. Some enhancement inoptical properties has also been attained with the substitution of GZOlayers for AZO [46], which increases the overall transmittance from82% to 87% at 550 nm. IZO and IZTO films have been utilized as a goodalternative to the ITO anode contact with higher work function valuesthat can favor hole injection into adjacent organic materials used forOLEDs [127,128].

Mixed oxide thin films and the corresponding binaries In2O3, ZnO,and SnO2 without extrinsic doping usually present higher resistivitydue to a lower carrier concentration [58], but they can be readily usedfor multilayer structures for which the TCO conductivity requirementis lower than for single-layer electrodes. Considering that the plasmaabsorption wavelength increases as the carrier concentration de-creases, higher infrared transmittance can be achieved with undopedmetal oxides. For example, by using a SnO2 film as substitute for amore conductive ITO layer, an increase of about 15% in thetransmittance at 800 nm has been obtained [34], which can beimportant for some specific applications. However, it should be notedthat the shape of the transmittance spectrum depends on the

image of Fig.�4


thickness and the oxygen content of the metal oxide layer, and sometransmittance enhancement in a specific wavelength range can alsobe achieved by adjusting the film thickness and other preparationparameters [15,40,88].

TCO deposition conditions influence the optical characteristics, butcan also modify the electrical performance of the multilayer system.Thus, there exists an optimum oxygen partial pressure for sputterdepositing ITO layers to achieve the lowest sheet resistance with ITO/Ag/ITO structures. A minimum oxygen content is required to improvethe ITO stoichiometry and suppress the interdiffusion of atoms,whereas a higher oxygen supply further oxidizes the Ag film [18,89].

2.3. Optimized TCO/metal/TCO optical and electrical performances

Multilayer structures consisting of TCO/metal/TCO offer the bestchance for achieving low resistance and high transmittance valueswith reduced film thicknesses; this means a superior performancecompared to single-layer electrodes. The optical behavior of thetrilayer structures is illustrated in Fig. 5, where the transmittance andabsorptance spectra of ITO/Ag/ITO and ITO/Cu/ITO samples depositedon SLG substrates are compared. By using 30-nm-thick ITO films, suchas those analyzed in Fig. 4, for embedding 10-nm-thick Ag or 15-nm-thick Cu layers as in Fig. 3, the same sheet resistance of 6 Ω/sq hasbeen obtained [15,73], with similar transmittance values in themeasured spectral range, except for wavelengths below 600 nm,where some absorption related to the Cu film is observed. The ITOlayers deposited on both sides of the metal film effectively decreasethe reflectance from themetal surface and promote high transmissionof the overall structure in the visible region. It should be noted that theexact shape of the resulting curve depends on the thickness of the ITOlayers embedding the metal film, with displacement of the maximumtransmittance toward large wavelengths with increasing thickness[15]. Thus, properties can be tuned for the specific application of theelectrode.

In most cases, the maximum transmittance is required to occur ataround 600 nm, in the middle of the visible range. This has beenobtained with ITO layers of 30 nm thickness for the structuresdepicted in Fig. 5. For these samples, the maximum transmittance isabove 80% including the SLG substrate (or above 90% by discountingit) at wavelengths near 600 nm, which is of the same order as thetransmittance achieved by single-layer ITO (Fig. 4) and 20% higherthan the transmittance of single-layer Ag or Cu (Fig. 3) at 600 nm. Itcan be seen that high transmittance is also obtained with both metalinterlayers in the near infrared region, being above 60% including theSLG substrate at 900 nm. However, the absorptance of Cu below

Fig. 5. Optical transmittance and absorptance spectra for trilayer electrodes composedof two ITO layers of 30 nm thickness with either a 10-nm-thick Ag film or a 15-nm-thickCu interlayer, all deposited by magnetron sputtering on SLG substrates. The sheetresistance of the multilayer electrodes is 6 Ω/sq.

550 nm is found to be about 30% for SLG/ITO/Cu/ITO (Fig. 5) and SLG/Cu (Fig. 3) samples, whereas it is below 10% for Ag-based samples.Such absorption results in a proportional decrease in the opticaltransmission. Thus, the transmittance of the SLG/ITO/Ag/ITO system issuperior in the short wavelengths region, remaining above 60% at450 nm, where the transmittance decreases to 45% for the samesystem with a Cu interlayer. The absorption related to the ITOcomponent is located around 335 nm (3.7 eV) and at this wavelength,the absorptance increases to 50% for ITO/Ag/ITO and up to 57% for theITO/Cu/ITO system.

Other authors have noted that the transmittance decay for IZO/Ag/IZO and IZO/Au/IZO electrodes in the near ultraviolet region (300–400 nm) shifts towards longer wavelengths when the metal layerthickness is increased and such displacement is faster for Au-basedthan Ag-based electrodes [22]. This is attributed to higher absorptionfor Au than for Ag interlayers in the near ultraviolet region. Similarchanges in the transmittance curves have not been directly related tothe absorption. Thus, for ITO/AgCu-alloy/ITO electrodes, the trans-mission decreases and shifts toward longer wavelengths, but thereflection increases proportionally, when the metal film thickness isincreased [88]. In this case, it was pointed out that the metal layerthickness did not affect the extinction coefficient, but considerablyinfluenced the index of refraction in the direction that thicker metalfilms resulted in a lower index [88].

The above data and those shown in Table 2 have proven thatsimilar transmittance and sheet resistance values can be obtainedwith TCO/metal/TCO structures using several TCO and metal combi-nations. High temperature deposition or annealing is not required toachieve good electrical conductivity, so TCO/metal/TCO structureshave been grownwith good characteristics on glass and thermoplasticsubstrates at room temperature.

In Fig. 6, the optical performance of ITO/Ag/ITO and ITO/Ag/AZOelectrodes prepared on SLG and PET substrates is compared. By using30-nm-thick ITO and AZO films equal to those analyzed in Fig. 4 and10-nm-thick Ag layers as in Fig. 3, a sheet resistance of 6 Ω/sq hasbeen obtained [15,20], and similar transmittance values taking intoaccount the respective substrate. At 600 nm wavelength, thetransmittance is 90% for bare SLG and decreases to 83% after trilayerdeposition, whereas it is 86% for bare PET and decreases to 78% afterthe trilayer structures were grown. Thus, 91–92% transmittance isachieved at the 600 nm wavelength for the various structuresdiscounting the substrates. By comparing the optical curves for theITO/Ag/ITO and ITO/Ag/AZO electrodes in Fig. 6, some increase intransmittance at wavelengths below 600 nm has been detected forthe ITO/Ag/AZO samples on both SLG and PET, whereas at wave-lengths above 600 nm, the transmittance is slightly higher for the ITO/

Fig. 6. Optical transmittance and absorptance spectra for trilayer electrodes composedof a 30-nm-thick ITO underlayer, a 10-nm-thick Ag interlayer, and a 30-nm-thick ITO orAZO upper layer deposited on SLG and PET substrates. The sheet resistance of themultilayer electrodes is 6 Ω/sq.

image of Fig.�5

image of Fig.�6


Ag/ITO samples. These small variations are attributed to the specificspectral dependence of the absorption and the refraction indices forthe upper ITO or AZO layer. In fact, similar enhancement in the shorterwavelength region has been reported for glass/AZO/AgPdCu-alloy/AZO structures that have about 80% transmittance at the 450 nmwavelength, whereas analogous glass/ITO/AgPdCu-alloy/ITO struc-tures have about 75% at the same wavelength [40]. For the samplesrepresented in Fig. 6, the increase in the transmittance attained withthe substitution of AZO for ITO in the upper layer is about 5% at the450 nm wavelength on SLG and PET.

Fig. 7 shows optical transmittance and sheet resistance data,summarized in Table 2 for TCO/Ag/TCO electrodes, plotted as afunction of the Ag interlayer thickness for samples deposited on glassand plastic substrates. The linear fits depicted in the figure have beenperformed with the overall data obtained on the different substrates,and it is noted that the data dispersion for analogous samples shouldbe related not only to the substrate nature, but also to the specificdeposition parameters for each film constituting the stacked struc-ture, especially for the Ag interlayer that gives widely different sheetresistance values for the same thickness, but different depositionconditions.

3. Additional characteristics of TCO/metal/TCO electrodes onflexible plastics and heat-resistant glass substrates

Although high visible transmittance and low electrical resistanceare essential parameters for efficient TCO/metal/TCO electrodes, othercharacteristics such as mechanical reliability and thermal stability canalso be decisive for applications on flexible substrates or on devicesthat require subsequent high temperature fabrication steps. From theviewpoint of manufacturing flexible electronic devices, continuouselectrode deposition processes onto unheated plastic substrates byR2R techniques are unrivaled in terms of processing cost, processingspeed, and thermal budget. However, they require mechanicalstability of the electrode properties against bending and unbendingcycles. TCO/metal/TCO electrodes have shown better mechanicalreliability than analogous single-layer TCOs for flexible electronicdevices [16,37], and analyses of their mechanical behavior as afunction of thickness, the crystalline structure, and the substratenature will be useful to achieve optimal performance.

TCO/metal/TCO electrodes have also shown better capability thansingle-layer TCOs to improve conventional electronic devices on rigidglass substrates such as supertwisted liquid crystal displays [48], andthey can be extended to other rigid devices that experience hightemperatures during fabrication [129–131]. Thus, an analysis of the

Fig. 7. Transmittance and sheet resistance values for the TCO/Ag/TCO structuresincluded in Table 2, which are listed as a function of the Ag interlayer thickness forsamples deposited on glass and plastic substrates.

trilayer electrode characteristics as a function of temperature will alsobe useful.

The durability of the TCO/metal/TCO electrodes against humidityand chemical agents present in the environment has widespreadimportance for commercial applications. Although the demands forchemical durability are greatly reduced after device sealing within aninsulated unit, trapped moisture and reactive contaminants from themanufacturing environment or window materials could potentiallyaffect the long term durability of the electrodes. Several tests havebeen designed to evaluate the effects of moisture on different TCO/metal/TCO samples [40,59,101], and the analysis of the resultsobtained as a function of the specific metal and TCO characteristicswill allow interpretation of the causes of the degradation and ways toavoid it.

3.1. Crystalline structure and mechanical characteristics of TCO/metal/TCO electrodes

With increasing interest in the development of displays on flexiblesubstrates, there is a great need for more mechanically robust flexibletransparent conductors because the TCO electrodes fail under lowermechanical strain than other device layers [132]. The maximumachievable bending and reliability of the display is dictated by thefracture properties of these thin and brittle films, which are controlledby a complex interplay between process induced defects and residualfilm stresses, cohesive properties, and adhesion to the flexiblesubstrate. As the coefficient of thermal expansion and compliance ofthe plastic substrates are larger than those of TCOs, some compressiveinternal stress is usually induced during deposition [133]. Compres-sive internal stress can be beneficial to the crack resistance of a TCOfilm, but is detrimental to the buckling delamination resistance whenit is superimposed on tensile external stresses. The internal stress andintrinsic cohesive properties depend greatly on film depositionconditions in addition to the specific substrate nature. It has beenfound that sputtering at a low working pressure on PET and PENsubstrates produces ITO films with large compressive stress, whichbecomes tensile at a higher pressure with a gradual transition througha zero-stress condition occurring at an intermediate value [133]. Asimilar decrease in the compressive stress by increasing the sputter-ing working pressure has been obtained for ZnO and Al:ZnO thin films[134]. The internal stress of ITO and AZO samples deposited on PETalso decreases when the sputtering power is decreased or the target-to-substrate distance increased [135,136].

For ITO layers deposited on PET, acrylic (AC), and glass, the frictionforce on PET increases almost linearly as the sputtering power isincreased, whereas for glass and AC it increases only slightly withincreasing power. The friction force of ITO films grown on PET and ACis always greater than on glass [137]. The adhesion test performed forsuch ITO/PET samples as a function of the oxygen pressure usedduring deposition closely follows the dependence of the electricalresistivity, both the friction force and the resistivity increase at highoxygen pressures. In this case, the link between the mechanicalproperties and the electrical characteristics seems to be via oxygenvacancies in the ITO films as oxygen concentration changes. Thisindicates that the mechanical properties are influenced by the filmdensity [137].

The determination of TCO crystalline parameters is useful toestablish the evolution of internal stress as a function of the substratetype, temperature, or other deposition conditions. The displacementof experimental diffraction peak positions (d) with respect to thosetabulated in powder diffraction standards (d0) is used to calculate thecompressive or tensile lattice distortion as Δd=(d−d0) /d0[110,113]. Fig. 8 shows X-ray diffraction data obtained for severalelectrodes prepared at room temperature on SLG and PET substratesunder the same experimental conditions as previously reported[15,20]. By using 30-nm-thick ITO and AZO films such as those

image of Fig.�7

Fig. 8. X-ray diffraction data corresponding to several electrodes composed of 30-nm-thick ITO or AZO films and 10-nm-thick Ag layers deposited on SLG and PET substrates. Thevertical lines represent angular positions reported by the Joint Committee of the Powder Diffraction Standards (JCPDS).


analyzed in Fig. 4 and a 10-nm-thick Ag layer as shown in Fig. 3, thesame sheet resistance of 6 Ω/sq has been obtained for ITO/Ag/ITO andAZO/Ag/AZO trilayer electrodes on both substrates. The ITO filmdeposited onto the bare SLG or PET did not exhibit any diffractionpeak, while for the AZO layer, the (002) peak corresponding to thehexagonal ZnO structure [138] was identified. For the ITO/Ag/ITOelectrodes, the diffractograms revealed the (400) peak of cubic In2O3

and the (111) peak corresponding to cubic Ag [138]; whereas for theAZO/Ag/AZO samples, the (101) peak of hexagonal ZnO appears. Dueto the low process temperature used for deposition and the low layerthickness, TCO films with low crystallinity are expected, especially forITO which is reported to require typical thickness above 100 nm tocrystallize at room temperature [139,140]. However, ITO/Ag/ITOsamples show the (400) diffraction peak of In2O3, and this isattributed to a better crystallization of the upper ITO film eventhough it is only 30 nm thick. Other authors have indicated similarbehavior for ITO/Au/ITO electrodes, where the Au layer appears to beeffective in crystallizing the upper ITO film with about 30–45 nmthickness [100,101,141]. The same effect is also observed for the AZO/Ag/AZO samples in Fig. 8, which have shown a better crystallization ofZnO with (101) texture than AZO layers deposited onto bare SLG orPET substrates.

It is known that amorphous structures have inherently lowercohesive strength than crystalline materials due to a larger freevolume. Thus, enhancement of the crystallinity of TCO/metal/TCOmultilayers can be beneficial for their mechanical stability. Forcrystalline ITO films deposited on PET at 75–80 °C, the cohesionstrength is almost three times higher than for amorphous ITO films ofthe same thickness deposited at b35 °C [142].

For each experimental peak identified in Fig. 8, the standardpowder angular position has been depicted by a solid vertical line inthe figure. Note that the experimental peaks are located at lowerangles than expected from the powder files [138]. Some increase inthe experimental interplanar spacings (d400 for ITO and d002 for AZO)with respect to standard values is detected, suggesting an elongationof unit cells (along the a-axis for ITO or the c-axis for AZO) in thedirection perpendicular to the films, and the existence of in-planecompressive stress. Single-layer TCOs have shown similar latticedistortion that decreased when either the film thickness or thepreparation temperature was increased [143–145]. Higher internalstress is usually associated with lower crystalline quality [146,147].

Evolution of the TCO sheet resistance as a function of the substratecurvature radius or increasing tensile strain is a commonly usedindicator for the practical design of reliable devices on flexiblesubstrates [148,149]. Electrical measurements performed for both ITO[148] and AZO [149] films deposited with various thicknesses on PET

revealed the existence of a critical strain for which the resistancestarted to increase dramatically because the conductive layer failedmechanically. The critical strain is found to depend on film thickness.The sheet resistance of the thinnest layers increases sharply at thehighest threshold strain, while for thicker films the resistanceincreases at a lower strain with a smaller slope.

A simple model has been proposed that describes the increasingresistance in cracked TCO layers in terms of a small volume ofconducting material within each crack [148]. According to this model,the volume of bridgingmaterial within the crack is smaller for thinnerfilms, but the conducting path between fragments remains due to aductile sublayer at the interface between the TCO and the flexiblesubstrate. This is consistent with studies of thin ceramic films onductile substrates where the threshold strain for cracking was foundto be inversely proportional to the film thickness [150].

In the design of single-layer TCO electrodes for flexible applica-tions, there is a trade-off between using a thick TCO layer to reducethe sheet resistance and using thinner films that can withstandgreater strain in the substrate. However, this can be overcome withthe utilization of TCO/metal/TCO electrodes that have betterelectrical performance with a significant TCO thickness reduction.The addition of a ductile metal underlayer was also found to beuseful in PET/Al/ITO structures that show superior mechanicalstability compared to PET/ITO samples [151,152]. A recent analysisof the influence of the Ag layer thickness on the mechanicalreliability of PET/Ag/ITO structures, with 150-nm-thick ITO films,indicates that both the crack and delamination resistances aresignificantly improved only above a certain metal thickness [153].For 5-nm-thick Ag film, weak adhesion at the interfaces wasobserved, but the adhesion improved as the Ag film thickness wasincreased to 10 and 15 nm. The tensile stress at cracking was higherfor the 15-nm-thick Ag underlayer. The improved crystallinity of theITO film by the metal underlayer was thought to be the primaryfactor for enhancing crack resistance since significant grain growthwas detected in the 15-nm-thick Ag film [153].

The superior mechanical reliability of the TCO/Ag/TCO electrodesin comparison with single-layer TCOs has been established bystandard bending tests performed for samples deposited on flexiblesubstrates [16,21,36,37,43,91]. The results obtained for PET/ITO/Ag/ITO samples as a function of the Ag interlayer thickness show thatimproved performance is achieved with at least 8-nm-thick Ag films[36]. The insertion of such an Ag interlayer enhances the bendingendurance as a function of the substrate curvature radius. In addition,the performance of ITO/Ag/ITO structures after many cycles ofbending showed increased improvement over ITO. This indicatesthat the ductility of the Ag layer can provide effective electrical

image of Fig.�8


conductivity even after the ITO films are beyond their failure strain[36].

3.2. Thermal and humidity stability of TCO/metal/TCO electrodes

Although TCO/metal/TCO electrodes have as a principal advantagethe demonstration of high quality even at room temperaturedeposition, thus allowing low cost manufacturing on inexpensiveplastics, the effect of post-deposition thermal treatments has beenanalyzed for multilayer electrodes prepared on glass substrates as away to improve optical and electrical characteristics and/or thermalstability for applications that require high temperature processes.Heat treatments up to 300–400 °C in vacuum or nitrogen arecommonly used to enhance the optical, electrical, and structuralqualities of TCO layers prepared at room temperature. The annealingproduces significant crystallite growth and electronic mobilityenhancement. The carrier concentration also increases due to moreeffective extrinsic doping and/or oxygen vacancy creation[108,109,114]. In this respect, the annealing atmosphere stronglyinfluences oxygen adsorption and desorption processes. Oxygenadsorption contributes to a decrease in electrical conductivity afterannealing in oxidizing atmospheres above 400 °C [109,114]. AZO ismore sensitive than ITO or SnO2 to thermal oxidation [154,155].

Concerning the behavior of silver thin films in oxidizing atmo-sphere, the formation of Ag2O or AgO phases has been reported [156].In addition, changes in the film morphology due to island growth andagglomeration processes during annealing and/or oxidation have astrong effect on silver and copper layer degradation [156–160]. Themetal electrical resistivity is expected to increase linearly as thetemperature rises due to phonon scattering when the void densityremains constant; deviations from linearity during thermal rampinghave been related to an increase in surface roughness due to theinitiation of agglomeration in the metal thin films [161].

The critical onset temperature for agglomeration was found to beabout 250 °C in vacuum for 20-nm-thick Ag films deposited directlyonto bare glass substrates, whereas it increases to 350 °C for glass/ITO/Ag and 450 °C for glass/Ag/ITO samples. This indicates that theupper ITO layer is efficient in suppressing the surface diffusion of Ag[94]. In addition, the ITO deposition conditions have been shown toaffect the Ag layer stability. The best performance with annealingtemperature was achieved by adjusting the oxygen concentration ofthe working gas during the ITO sputtering [89]. The inclusion of smallproportions (1–2 wt.%) of metallic elements such as Pd or Cu in the Aglayer is effective to increase the onset temperature and the activationenergy for agglomeration [40,94], resulting in a significant improve-ment in the thermal stability of ITO/Ag-based alloy/ITO electrodes[94]. However, it should be noted that for unannealed samples, the

Fig. 9. Optical (transmittance and absorptance spectra) and structural (X-ray diffractograms)10-nm-thick Ag interlayer, and a 30-nm-thick ITO or AZO upper layer as-grown and after h

optical and electrical performances achieved by trilayer systems withAg-based alloys are inferior to that obtained with pure Ag interlayers[33,94].

Several authors have reported an improvement in the optical andelectrical characteristics after annealing in vacuum or nitrogen attemperatures between 200 and 400 °C for the trilayer structures ITO/Ag/ITO [18,73,89], ZnO/Ag/ZnO [160], GZO/Ag/GZO [87], ZnO/Cu/ZnO[96], and ITO/Au/ITO [141].

The enhancement of the electrical conductivity of TCO/metal/TCOstructures after annealing has been related to grain growth in both theTCO and the metal layers as determined by X-ray diffraction andelectron or atomic-forcemicroscopymeasurements [18,87,96,141,160].This results in a lower density of grain boundaries which act as barriersfor carrier conduction. Moreover, the influence of the metal layerthickness should also be considered since the mobility in TCO/metal/TCO electrodes is limited by the combined effects of grain boundaryscattering and interface scattering. When the metal interlayer is verythin, interface scattering dominates [85,90]. Regarding optical proper-ties, the visible transmittance usually increases and the absorptiondecreases after heat treatment [141]. This has also been related to TCOrecrystallization [87,141] since grain boundaries are responsible forlight absorption.

The electrical conductivity of ITO/Ag/ITO [18,94] and ZnO/Ag/ZnO[160] structures decreased when the annealing temperature wasincreased above 450 °C, slightly in vacuum or nitrogen and moreabruptly in air or oxygen environments [18,160]. For such deteriora-tion, two primary reasons have been found: one is a decrease in Agpurity due to oxygen diffusion from the annealing atmosphere[18,160] or interdiffusion at Ag/ITO interfaces [89], and the other isagglomeration of Ag atoms that breaks the continuity of themetal film[94]. Both phenomena, the decrease in purity and the agglomeration,affect interfacial properties and decrease the optical transmittance byincreasing light scattering [89,94,160].

Apart from variation in themaximumvalues, a displacement of thetransmittance curve toward shorter wavelengths has been observedafter heat treatments above 200 °C [18,94,141,160]. The degree of theshift is smaller for samples heated in air than for samples annealed invacuum or nitrogen atmospheres [18,160]. Spectroscopic ellipsome-try and optical calculations show that the changes are caused by adecrease in the TCO refractive index [18]. The decrease in refractiveindex of metal oxide films is attributed to lower packing densities orstress relaxation in the lattice caused by the out diffusion of oxygenduring heat treatment [162–164].

Typical changes in the optical and structural characteristics oftrilayer electrodes induced by annealing are illustrated in Fig. 9 forSLG/ITO/Ag/ITO and SLG/ITO/Ag/AZO samples as-deposited at roomtemperature and after heat treatment at 350 °C in flowing nitrogen.

data corresponding to trilayer electrodes composed of a 30-nm-thick ITO underlayer, aeating to 350 °C in nitrogen.

image of Fig.�9

Fig. 10. Schematic illustration of a simple device with a TCO/metal/TCO anode, an HTL(hole transport layer), an ETL (electron transport layer), and a metal cathode.


These samples are the same as those analyzed in Fig. 6, consisting of30-nm-thick ITO and AZO layers with an embedded 10-nm-thick Agfilm. The sheet resistance is 6 Ω/sq and remains unchanged afterannealing. It can be seen that the transmittance curve shifts towardslower wavelengths after heat treatment. This results in highertransmittance values in the near ultraviolet region due to a significantdecrease in the absorptance below 450 nm, together with an increasein the TCO band gap. The transmittance decrease in the infraredregion is related to a decrease in the TCO refractive index. Since mostLCD panels experience temperatures of about 300 °C during fabrica-tion processes in nitrogen [1], the transmittance shift phenomenashould be taken into account in order to prevent undesirabledeviation of color coordinates from original designs [18].

For the results depicted in Fig. 9, the optical transmittance at400 nm increases from 45% to 55% after annealing and it remainsabove 40% at 350 nm for the annealed samples. An absorptance of 50%is achieved at a wavelength near 335 nm with the as-depositedelectrodes and around 325 nm after heat treatment, indicating a TCOband gap widening of about 0.1 eV. A similar increase in band gap hasbeen reported for single-layer ITO and AZO samples and related tograin growth promoted by thermal treatment [108,114]. Fig. 9 showsthat heating at 350 °C has produced a strong recrystallization of theIn2O3 layers, resulting in higher diffraction peaks near the standardpowder angular positions [138]. This indicates a decrease in latticedistortion with respect to the as-deposited conditions. No significantchanges are observed in the intensity of the diffraction peakscorresponding to Ag or ZnO after annealing, although the displace-ment of the (002) peak toward its standard position also indicates adecrease in ZnO lattice distortion. Thus, the evolution of the opticaland structural characteristics of these trilayer electrodes with thermaltreatment is related to recrystallization and reduction of structuralstress in the TCO layers, whereas the metal characteristics remainessentially unaltered by the heating.

Various single-layer TCOs have been exposed simultaneously tomoist atmospheres (above 70% humidity) at moderate temperatures(generally below 90 °C) to evaluate the stability of their optical andelectrical parameters [165–168]. For samples that degraded, thosedeposited on soda lime glass degraded more than those on alkali-freeglass [165,166] suggesting that sodium accumulation at the interfacebetween the coating and the glass substrate contributes to darkeningand/or delamination of the TCO layer [165]. In addition, it has beenfound that stability depends on the layer deposition conditions,mainly on the oxygen to argon partial pressure ratio for the samesputtered material [167].

Multilayer TCO/metal/TCOelectrodeswere also found to be sensitiveto the atmosphere, and can be unstable under moisture attack. Inparticular, for Ag-based multilayers, aggregation and/or migration ofsilver atoms was found to be responsible for optical and electricaldegradation of the electrodes with humidity [40,59], in a similar way asindicated above for thermal degradation at high temperatures. Theworsening of the TCO/Ag/TCO characteristics seems to be causedprimarily by agglomeration (not chemical reaction) of the Ag interlayer,and the resulting disruption of the TCO overlayer [169]. The agglom-eration is governedby temperature andhumidity and their effects onAgsurface diffusivity, but it is also controlled by the surrounding layers, thesubstrate, and particulate contamination through their effects on thesurface free energy, diffusivity, and transport of silver [170]. Discolor-ation spots have been detected in ITO/Ag/ITO samples exposed to thehumidity. The samples become opaque after 17 h of saltwaterimmersion testing. AZO/Ag/AZO electrodes, in which the AZO layersare grown in oxygen-free atmospheres (pure Ar) resulting in strong(002) peak intensities [40], exhibit better performance, remainingtransparent after 200 h of immersion. Degradation detected in ZnO/Ag/ZnO samples, grown under pure Ar conditions, caused disruption ofoverlying films [171]. Moisture penetrated into the top ZnO coating toreach the Ag interlayer and enhanced the migration of silver, lowering

the interfacial adhesion forcebetween the layers. As a result, the topZnOfilm, which was in compressive stress, peeled [171]. Delamination wassuppressed by using AZO filmswith a lower compressive stress [59]. Forother TCO/Ag/TCO electrodes exposed to 90–95% relative humidity at40–60 °C, discoloration spots were eliminated or reduced by introduc-ing a small amount of Pd [172] or Ti [173] into the silver layer. Theaddition of these elements is effective to increase the activation energyfor silver agglomeration. With respect to the utilization of other metalinterlayers, ITO/Au/ITO structures exhibit good stability in air at relativehumidity of 90% and temperature of 60 °C; the normalized sheetresistance remained constant over an exposure time of 600 h [101].

4. Application of TCO/metal/TCO electrodes in electronic devices

It has been established that suitable transmittance and sheetresistance values can be achieved with several TCO/metal/TCOcombinations, but the choice of the best electrode material for aspecific application depends also on the work function value of theTCO in relation with the energy level of the next layer in the deviceconstruction, because it is desirable that the contact interfacepresents the smallest possible barrier to carrier transport. Up tonow, the TCO/metal/TCO electrodes have been mainly applied inOLEDs [21,36,38–43] and OPVs [16,17,22,44–47], as has been alreadypointed out in the Introduction. For these devices, the trilayerelectrode is commonly used as an anode in the configuration that isrepresented in Fig. 10, where two or more active organic layers arestacked between the upper TCO film of the anode and anothermetallic film used as cathode. In the simplest case, only two organiclayers are utilized, one as a hole transport layer (HTL) and the otheras an electron transport and emission layer (ETL) to constitute theOLED [40]. These are analogous to the donor and acceptor layersused for OPV devices [44]. This is only a simplified representationbecause for bulk heterojunction solar cells, one photoactive layercomposed of interconnected networks of electron donors andacceptors is utilized [16,17,22,45–47], whereas more constituentsare also added next to the anode to act as a hole injection layer (HIL)or next to the cathode to act as an electron injection layer (EIL) inother OLEDs [21,36,41,43]. The application of trilayer structures ascathodes has been reported in semitransparent OLEDs that usesingle-layer TCO anodes [39,42].

The band alignment between the TCO and the adjacent organic layerplays a crucial role in determining the device performance, but differentfeatures must be considered for OLEDs and OPVs according to theinjection or extraction mechanisms that are illustrated in Fig. 11. In thecase of OLEDs, it is clear that a key parameter of the TCO/organicinterface is the injection barrier between the electrode and the organicsemiconductor. This is determinedby theenergydifference between theelectrode work function and the highest occupied molecular orbital

image of Fig.�10

Fig. 11. Band diagram representation for a TCO anode and an adjacent organic material.The TCO anode is used for hole injection in OLEDs or for photogenerated hole extractionin OPVs.


(HOMO) of the organic material. The lower the barrier, the moreefficient the injection of holes across the interface and hence the betterthe device performance. This effect is less important for carrierextraction in OPVs [54], where the carrier has to go from the HOMO tothe electrode Fermi level. In practice, the energy barrier at the TCO/organic interface should be as small as possible for both carrier injectionand extraction [54]. In general, the work function of TCO/metal/TCOelectrodes is derived from the TCO upper layer [85,90], as has beenverified by several measurements showing the same results for ITO/Ag/ITO and ITO/Ni/ITO samples as for the analogous single-layer ITOelectrodes [40,95,102]. However, Ni-doped ITO samples showed higherwork function than pure ITO films [174,175]. A good alternative is theutilization of IZO or IZTO for the TCO layers because they have higherwork function values than ITO [127,128].

Recent investigations are also focused on transparent and flexiblecapacitors and non-volatile memory devices, which are steps towardthe realization of more complex transparent and flexible electronicsystems. Fully transparent resistive random access memory (TRRAM)devices based on ITO/ZnO/ITO/Ag/ITO multilayered structures fabri-cated on flexible PES substrates exhibit high transparency up to 80%,superior flexibility performance due to the bottom trilayer electrode,and excellent resistive switching characteristics and reliable dataretention properties verified at room temperature and under thermalstress [37]. Moreover, capacitors based on Pt/Bi3NbO7/AZO/Ag/AZOmultilayered structures grown on PES substrates have suitabledielectric and leakage properties with high mechanical stability forflexible electronic applications [49].

Table 3Device parameters, including luminance (L), current efficiency (CE), and power efficiency (

Anode description J (mA/cm2) V (V)

ITO 37 nm/Ag(PdCu) 8 nm/ITO 37 nm 550 13ITO 350 13AZO 40 nm/Ag(PdCu) 8 nm/AZO 40 nm – –

ITO 40 nm/Ag 10 nm/ITO 40 nm 300 8.9ITO 300 10.3IZO 30 nm/Ag 12 nm/IZO 30 nm 200 12ITO 20 12ITO 50 nm/Ag 14 nm/ITO 50 nm 125 11ITO 100 nm 17 11IZTO 30 nm/Ag 14 nm/IZTO 30 nm 47 11ITO 33 11ITO 50 nm/Ag 17 nm/ITO 50 nm 300 4.9ITO 150 nm 300 5.8

4.1. Application of TCO/metal/TCO electrodes in OLED devices

Efficient light emitting devices using small molecules were firstdeveloped in 1987 [176]. These devices used a novel structure withseparate hole and electron transport layers such that recombinationand light emission occurred in the middle of the organic films. Thisresulted in a reduction in the operating voltage and improvements inthe luminescence efficiency, leading to the current era of OLEDresearch and device production. Molecules commonly used in OLEDsinclude organometallic chelates like Alq3 (tris-(8-hydroxyquinoline)aluminum) which is widely utilized as a green emitter and electrontransport material, with other fluorescent dyes chosen to obtain lightemission at different wavelengths [177–179]. A number of materialshave been selected for their charge transport properties; for example,CuPc (copper phthalocyanine), α-NPD (N-(1-naphthyl)-N-(phenyl)benzidine), and NPB (N,N′-bis(naphthalen-1-yl)-N,N′-bis(phenyl)benzidine) are commonly used for hole injection layers and holetransport layers [177,179]. In 1997, the first flexible OLEDs with smallmolecules were fabricated on ITO-coated PET substrates [180]. Atpresent, ITO films are also widely utilized as single-layer anodes inflexible OLEDs [181–183], but the use of TCO/metal/TCO multilayeranodes with a lower sheet resistance is emerging as a successful wayto improve the mechanical stability and the device efficiency. Table 3summarizes the performance of several OLEDs made using multilayeranodes in comparison with the same device using a single-layer ITOanode. The multilayer electrode characteristics can be found inTable 2.

Multilayer transparent electrodes consisting of ITO/Ag/ITO withvisible transmittance above 80% and sheet resistance of about 10 Ω/sq have been used as anodes in the device configuration substrate/anode/CuPc/NPB/Alq3/metal-cathode for both glass and PET sub-strates [36,41]. They have achieved a higher efficacy than employinga single-layer ITO anode. The luminance and current density values asa function of voltage show that the difference between devices withITO/Ag/ITO vs. ITO anodes increases for voltages above 6 V or currentdensities above 10 mA/cm2, where ohmic losses in themore resistivesingle-layer anode become significant. The same current density isachieved at a lower voltage with the multilayer electrode [36,41].There appears to be no influence of the anode type on the maximumcurrent efficiency (in cd/A units), but at a luminance of 104 cd/m2 thepower efficiency (in cd/W) of samples with ITO/Ag/ITO electrodesimproves by 22% compared to those with ITO anodes [36]. For otherdevices, the magnitude of improvement in performance depends onthe particular geometry and resistance of the anode transmission linein the OLED. The reduction in ohmic losses in the anode is especiallyadvantageous for high brightness and passive matrix displays wherehighermomentary brightness is necessary and ITO transmission linesare long and narrow. In addition, the application of trilayer electrodes

PE) values reported for several OLED devices with TCO/metal/TCO electrodes.

L (cd/m2) CE (cd/A) PE (cd/W) Ref.

38,000 [40]29,000 "– – – "

4.5 0.51 [36]4.5 0.44 "

10,000 [43]800 "5040 4.1 0.37 [41]450 2.6 0.24 "10,000 [21]7000 "

1.6 0.33 [38]1.3 0.22 "

image of Fig.�11

Fig. 12. Band diagram illustration for several TCOs, for which the work function values(Φ) have been obtained from reported experimental data [40,127,128].


for flexible OLEDs can reduce the drive voltage, the heat generated bythe devices, and can increase the device efficiency and operationlifetime [41].

By using a silver-based alloy including palladium and copper, ITO/AgPdCu-alloy/ITO and AZO/AgPdCu-alloy/AZO electrodes with similarvisible transmittance above 80% and sheet resistance below 10 Ω/sqwere also obtained. These electrodes have been tested and comparedwith single-layer ITO anodes in the device configuration glass/anode/α-NPD/Alq3/metal-cathode [40]. By applying an ITO/AgPdCu-alloy/ITO anode, the current density and the luminescence are higherover the voltage range of 10–15 V, and the normalized luminescentintensity spectra of the OLED device at 14 V has a peak around 530 nmthat is sharper for the trilayer than for the ITO single-layer. However,the device with an AZO/AgPdCu-alloy/AZO anode did not emit lightand burned out. In order to explain this behavior, the work functionvalue of the multilayer transparent electrode was established thesame as for the upper TCO layer,−3.7 eV vs.−4.8 eV for ITO [40]. Theenergy difference between the bottom of the conduction band of TCOand the HOMO of α-NPD (located at −5.5 eV) is larger for AZO thanfor the ITO-based anode. Therefore, the reason for the failure of theAZO-based electrode is related to the high energy barrier to carriertransport at the AZO/α-NPD interface. Thus, the energy imparted bythe applied voltage leads to dielectric breakdown of the OLED device[40].

Several TCOs, including ITO/Ag/ITO [38], IZO/Ag/IZO [43], andIZTO/Ag/IZTO [21], have been used as multilayer anodes in flexible

Table 4Device parameters, including short circuit current (Jsc), open circuit voltage (Voc), fill factor (TCO/metal/TCO electrodes.

Anode description Active layer Jsc (mA/cm2)

ITO 40 nm/Ag 10 nm/ITO 40 nm P3HT:PCBM 8.49ITO 40 nm/Cu 14 nm/ITO 40 nm 7.11ITO 80 nm 7.41AZO 40 nm/Ag 12 nm/AZO 40 nm P3HT:PCBM 9.41AZO 500 nm 9.20GZO 40 nm/Ag 12 nm/GZO 40 nm P3HT:PCBM 9.86GZO 500 nm 8.00ITO 40 nm/Ag 12 nm/ITO 40 nm P3HT:PCBM 9.30ITO 80 nm 9.61ITO 70 nm/Ag 14 nm/ITO 70 nm P3HT:PCBM 9.58ITO 150 nm 3.33IZO 40 nm/Ag 14 nm/IZO 40 nm P3HT:PCBM 8.13IZO 40 nm/Au 12 nm/IZO 40 nm 7.04ITO 40 nm/Ag 16 nm/ITO 40 nm P3HT:PCBM 9.22ITO 150 nm 7.53ITO 50 nm/Ag 8 nm/ITO 50 nm CuPc–C60 2.89ITO 100 nm 2.81

OLEDs grown on PET substrates. They exhibit higher current densityand luminance in the 5–12 V range than analogous devices made withsingle-layer ITO anodes. The ITO/Ag/ITO and IZO/Ag/IZO multilayershave a sheet resistance of 7 Ω/sq and a maximum transmittance of85% [38,43]. IZTO/Ag/IZTO electrodes are even better at 5 Ω/sq and86% transmittance [21]. In addition, for this IZTO (5 wt.% ZnO and5 wt.% SnO2 co-doped In2O3 [128]) the work function (−5.12 eV) ishigher than for the reference ITO film (−4.8 eV) and leads to a lowerbarrier height between the oxide electrode and the organic layer [21].The band diagrams in Fig. 12 are useful to illustrate the work functionvalues of the various TCOs used in the OLED devices of Table 3.

ITO/Ag/ITOmultilayers are also utilized as a transparent cathode thatexhibits resonant tunneling in transparent OLEDs consisting of glass/ITO-anode/PEDOT:PSS/PFO/LiF/trilayer-cathode, where PEDOT:PSS (poly(styrene sulphonate)-doped poly(3,4-ethylene dioxythiophen)) wasdeposited as a hole transport layer and PFO (poly(9,9-dyoctilfluorene))as an emitting layer, with very thin LiF (0.6 nm) or LiF(0.6 nm)/Al(6 nm)films added as buffer layers [39,42]. For these devices, current injection isdominated by resonant tunneling which causes the OLED to have a peakandavalley in the I–Vcharacteristics between2and6 V. This is due to thedouble potential barriers of ITO and the quantum well of Ag. The Ag2Ophase identified on the surface of the Ag film was quantized by sizeeffects and dominated the quantum tunneling rates [42]. Suchmultilayercathodes have also been applied to small molecule devices in theconfiguration glass/ITO-anode/PEDOT:PSS/NPB/Alq3/LiF/trilayer-cath-ode, which also showed resonant tunneling [39].

4.2. Application of TCO/metal/TCO electrodes in OPV devices

There is increasing interest in the use of organic photovoltaics as arenewable, cost effective, and inexpensive energy source alternative totraditional Si-based solar cells [24,25]. Among the various types of OPVdevices, bulk-heterojunction cells using networks of conjugated poly-mers and fullerene derivatives are themostwidely investigated systemsbecause of their simple fabrication process and simple device structureas well as their ability to flex, roll, and fold for portability [29]. Inparticular, bulk heterojunctions consisting of the semiconductingpolymer poly-3-hexylthiophene (P3HT) as an electron donor and thesoluble fullerene derivative (6,6)-phenyl-C61 butyric acid methylester(PCBM) as an electron acceptor are widely investigated becausetheoretical studies have predicted an external efficiency of 11% [184].In practice, the best results reported for P3HT:PCBM-based organic solarcells are record power conversion efficiency (PCE=Pout /Pin) of 6% andaverage values of 4–5% on glass substrates after thermal annealing[185,186]. PCE below 4% is generally obtained with unheated materials

FF), and power conversion efficiency (PCE) values reported for several OPV devices with

Voc (V) FF PCE (%) Ref.

0.56 0.69 3.26 [45]0.56 0.70 2.78 "0.50 0.46 1.72 "0.50 0.46 2.14 [46]0.48 0.31 1.36 "0.54 0.53 2.84 "0.50 0.39 1.57 "0.56 0.72 3.73 [16]0.54 0.62 3.21 "0.51 0.39 2.00 [47]0.39 0.35 0.48 "0.54 0.69 3.05 [22]0.56 0.68 2.66 "0.54 0.65 3.25 [17]0.55 0.56 2.35 "0.47 0.40 0.57 [44]0.47 0.37 0.52 "

image of Fig.�12


on flexible substrates [187]. TCO/metal/TCO multilayer anodes havebeen applied to P3HT:PCBM-based solar cells, and they provide higherpower conversion efficiencies than single-layer TCO electrodes. Table 4summarizes the OPV device performance achieved by using multilayeranodes in comparison with that of the same device using a single-layerTCO anode. The multilayer electrode characteristics can be found inTable 2.

Transparent electrodes consisting of ITO/Ag/ITO with sheetresistance in the 4–6 Ω/sq range have been applied as anodes in thedevice configuration substrate/anode/PEDOT:PSS/P3HT:PCBM/metal-cathode on rigid glass [17,45] and flexible PES [16] and PPC [47]substrates. Such devices have higher conversion efficiency than thoseemploying a single-layer ITO anode. For devices grown on glasssubstrates, ITO/Ag/ITO and ITO/Cu/ITO electrodes with the same 6 Ω/sq sheet resistance were compared [45] and both showed similar opencircuit voltage (Voc) with fill factor (FF) values higher than thoseobtained with single-layer ITO anodes. However, they differed in theshort circuit current density (Jsc). The power conversion efficiency ofsolar cells is calculated as PCE=Voc Jsc FF/Pin, with the incidentillumination power Pin=100 mW/cm2 for a standard solar spectrum.The observed FF increase is attributed to the lower multilayerelectrode resistance. The FF value of solar cells is critically affectedby both the series resistance and the shunt resistance. The sources ofthe series resistance include the sheet resistances of anode andcathode and the transverse flow of current in the solar cell. Therefore,the sheet resistance of the multilayer electrode to current flowsignificantly affects the fill factor and also the current density ofphotovoltaic devices [17,45]. The short circuit current density washigher for ITO/Ag/ITO than for the ITO/Cu/ITO electrode, despite thatthey had the same sheet resistance value. This was attributed to theoptical transmittance in the wavelength region of 400–600 nm, whichis the absorption region of the P3HT:PSS active layer [47,185]. At suchwavelengths, the average transmittance for the ITO/Ag/ITO anode wasabout 20% higher than for the ITO/Cu/ITO multilayer owing to theoptical scattering of the inserted Cu film [45]. Thus, the higherconversion efficiency achieved with the OPV device fabricated on theITO/Ag/ITO electrode can be explained by the combined effect of hightransmittance adapted to the absorption region of the active layer andvery low sheet resistance resulting from the conductive Ag film.

For devices grown onto flexible PES or PPS substrates, thesignificant improvement in conversion efficiency obtained by usingITO/Ag/ITO in place of ITO electrodes has also been attributed [16,47]to the low sheet resistance of the multilayer anode in addition to thegood fit of its optical characteristics with the absorption region of theorganic layer as mentioned above. Due to the small active area ofthese solar cells (typically about 0.05 cm2 [16,17,45]), it is difficult toevaluate the real advantage of trilayer electrodes relative to single-layer TCOs. It is thought that the effect of the lower sheet resistancefor multilayer electrodes on the performance of flexible OPVs will beeven more important for larger area devices. The influence of themetal interlayer has also been analyzed by comparing IZO/Ag/IZO andIZO/Au/IZO anodes with similar sheet resistances in solar cellscomposed of P3HT:PSS active layer [22]. Because of the same sheetresistance and device structure, similar Voc and FF values wereachieved. However, higher current density and conversion efficiencywere obtained with the IZO/Ag/IZO anode owing to its higher opticaltransmittance. In the wavelength region 400–600 nm, which corre-sponds to P3HT:PSS absorption, the average transmittance for theIZO/Ag/IZO electrode was about 10% higher than for the IZO/Au/IZOanode, thus providing a higher current density [22].

Indium-free TCOs have been tested in multilayer structures withoptimized sheet resistance and visible transmittance parameters,AZO/Ag/AZO (7 Ω/sq, 82%) and GZO/Ag/GZO (6 Ω/sq, 87%), and also inannealed single-layers of AZO (74 Ω/sq, 93%) and GZO (30 Ω/sq, 94%),which were applied and compared as alternative anodes in the deviceconfiguration glass/anode/PEDOT:PSS/P3HT:PCBM/metal-cathode

[46]. The solar cell fabricated with the GZO/Ag/GZO electrode showedthe highest conversion efficiency value of 2.84%, followed by the cellgrown on the AZO/Ag/AZO anode with 2.14%. The devices fabricatedon the single-layer GZO and AZO electrodes gave efficiency values of1.57% and 1.36%, respectively. Although the conversion efficiency ofthe solar cell fabricated with the GZO/Ag/GZO electrode is slightlylower than obtained with ITO-based multilayer anodes in the samedevice structure (see Table 4), it is much higher than reported forother cells with single-layer GZO electrodes [188,189] for which thelow efficiency has been attributed to the small work function of GZO.The higher efficiency attained with multilayer electrodes has beenattributed to lower sheet resistance, resulting from the insertion ofthe Ag layer, which improved the fill factor and the current density ofthe OPV devices.

Whenplanarheterojunctionsbased on theCuPc–C60donor-acceptorcouple were used to compare ITO and ITO/Ag/ITO anodes in theconfiguration glass/anode/PEDOT:PSS/CuPc/C60/BCP/Al-cathode, thesimultaneous measurement of the anode transmission and the photo-generation spectra showed that the best contribution to photocurrentproduction was in the 600–750 nmwavelengths region, correspondingto the maximum phthalocyanine absorption, where the multilayeranode transmission decreased from its maximum value achieved atabout 550 nm [44]. Owing to this fact, the application of the multilayerelectrode for CuPc–C60 based devices resulted in low photocurrent andlow conversion efficiency, although the FFwas highdue to the lowsheetresistance [44]. In order to obtain OPV devices with high conversionefficiency, the matching between the most transparent range of theanode and the absorption region of the active organic material is veryimportant. Thus, the anode layers must be optimized for differentorganic solar cells.

5. Conclusions

TCO/metal/TCO structures offer the best chance for achieving lowresistance andhigh transmittance valueswith reducedfilm thicknesses;this means superior performance compared to single-layer electrodes.In the TCO/metal/TCOsystem, the key element for the sheet resistance isthe metal layer; higher conductivity allows the layer to be thin enoughto provide good transmittance. However, the overall transmittance ofthe stack is increased by embedding the metal between very thin TCOfilms that suppress the reflection from the metal layer in the visibleregion and provide higher selective transparency.

Based upon electrical resistivity values for bulk metals, Ag is thefirst choice, followed by Cu and Au. For the various TCO/Ag/TCOstructures tested, it is observed that sheet resistance valuesdecreasing from 15 to 3 Ω/sq are generally obtained with Ag filmthicknesses increasing from 8 to 20 nm, although the specific sheetresistance value depends on film deposition conditions. Sheetresistances below 10Ω/sq have also been obtained by using Cu andAu interlayers in the 8–16 nm thickness range, although sometransmission decrease is detected with respect to Ag films. Thecomparison of Cu and Au interlayers with different carrier densitieshas established that the absorption increases and moves towardshigher wavelengths in the visible region when the carrier densitydecreases.

For the TCO constituents, film thicknesses in the 30–60 nm rangeare commonly used to obtain maximum transmission around 550 nm.Doped metal oxides ITO and AZO are extensively utilized as single-layer TCO electrodes and also for multilayer structures. Mixed IZO orIZTO compounds and the corresponding binaries In2O3, ZnO, and SnO2

without extrinsic doping usually present higher resistivity, but theycan be readily used for multilayer structures for which the TCOconductivity requirement is lower than for the single-layer electrodes.By using a SnO2 film as substitute for other more conductive ITOlayers, an increase in the transmittance at 800 nm is obtained,whereas the substitution of GIO layers for ITO provides an increase in


optical transmission below 400 nm. It should be noted that the shapeof the transmittance spectrum depends on the thickness and theoxygen content of the metal oxide layer, and some transmittanceenhancement at a specific wavelength range can also be achieved byadjusting these parameters. IZO and IZTO films have been utilized as agood alternative for ITO anode contacts. They have higher workfunction values that can favor hole injection into the adjacent activematerials when the electrodes are applied in OLEDs.

Experimental data have shown that the same transmittance andsheet resistance values can be obtained with TCO/metal/TCO trilayerstructures composed of different TCO and metal combinations. Inaddition, high temperature deposition and annealing are not requiredto achieve good electrical conductivity. Thus, TCO/metal/TCO struc-tures have been grown with the same or similar properties on glassand thermoplastic substrates at room temperature. Formanufacturingflexible electronic devices, continuous electrode deposition processesonto unheated plastic substrates by roll-to-roll techniques areadvantageous in terms of processing cost, processing speed, andthermal budget. However, this requires that the electrodes are stableduring bending and unbending cycles. In this respect, TCO/metal/TCOelectrodes have shown better mechanical reliability than analogoussingle-layer TCOs, as has been verified by standard bending tests. Theresults obtained for PET/ITO/Ag/ITO samples as a function of the Aginterlayer thickness show that a minimum metal thickness of 8 nmenhances the bending endurance since the ductility of the Ag layerprovides effective electrical conductivity even after the ITO is beyondits failure strain.

Although high quality TCO/metal/TCO electrodes can be depositedat room temperature, thus allowing low cost manufacturing oninexpensive plastics, post-deposition thermal treatments have beenshown to improve their thermal stability on glass substrates forapplications that require higher temperatures. If the electricalconductivity of the TCO/metal/TCO structures decreases upon anneal-ing, there are two primary reasons. One is metal layer contaminationdue to oxygen in-diffusion from the annealing ambient and/oradjacent TCOs. The other is metal layer agglomeration that breaksfilm continuity. TCO deposition conditions have been found to play arole in determiningmetal layer stability. Optimization requires tuningthe oxygen partial pressure during TCO film growth. For structureswith Ag interlayers, the addition of small concentrations of Pd or Cu iseffective for increasing the onset temperature and the activationenergy for agglomeration, resulting in a significant improvement ofthe overall thermal stability of trilayer electrodes. Ag layer agglom-eration in trilayer structures is also responsible for propertydegradation due to humidity. Agglomeration is not only governedby temperature and humidity and their effects on Ag surfacediffusivity, but it is also controlled by the surrounding layers via theeffect of contamination on the interface free energy and transport ofsilver.

So far, TCO/metal/TCO electrodes have primarily been applied inOLED and OPV devices, where the trilayer structure is commonly usedas an anode and the active organic layers are stacked between theupper TCO layer and a metallic film used as cathode. When themultilayer electrodes are applied to OLEDs, the current density andthe luminescence are higher than obtained with single-layer TCOanodes at same voltage. The magnitude of improvement depends onthe work function of the TCO layer, which can lead to a lower energybarrier between the electrode and the adjacent organic layer. Thereduction in ohmic losses at the anode is especially advantageous forhigh brightness and passive matrix displays where higher momentarybrightness is necessary and anode transmission lines are long andnarrow. In addition, the application of trilayer electrodes for flexibleOLEDs reduces the drive voltage and heat generation in the devices aswell as increases the device efficiency and operation lifetime. Whenthe multilayer electrodes are applied in OPVs, higher powerconversion efficiencies are also achieved compared with devices

fabricated on single-layer TCO anodes. This is due to the lower sheetresistance of the multilayer electrode to current flow which, in turn,enhances the fill factor and current density of photovoltaic devices.The improvement in the conversion efficiency depends on thegeometry of the solar cell and on matching the high transmittancespectral range of the electrode with the absorption region of thephotoactive organic layer. In addition to the application of TCO/metal/TCO structures to OPV and OLED devices, with significant impact inlarge-area and flexible photovoltaics and electronics, the applicationof trilayer structures can be extended to other markets including solarcell, flat panel display, and solid state lighting areas that also requireimproved transparent and conductive electrodes.

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TCO/metal/TCO structures for energy and flexible electronics