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Highly efficient ultrathin amorphous silicon solar cells on top of imprinted periodic nanodot arrays

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Highly efficient ultrathin-film amorphous silicon solar cells on top of imprinted periodic nanodot arraysWensheng Yan, Zhikuo Tao, Thiam Min Brian Ong, and Min GuCitation: Applied Physics Letters 106, 093902 (2015); doi: 10.1063/1.4914110View online: /10.1063/1.4914110View Table of Contents: /content/aip/journal/apl/106/9?ver=pdfcovPublished by the AIP PublishingArticles you may be interested inThe impact of oxygen incorporation during intrinsic ZnO sputtering on the performance of Cu(In,Ga)Se2 thin film solar cellsAppl. Phys. Lett. 105, 083906 (2014); 10.1063/1.4894214Toward a high Cu2ZnSnS4 solar cell efficiency processed by spray pyrolysis methodJ. Renewable Sustainable Energy 5, 053137 (2013); 10.1063/1.4825253Influence of SnO2:F/ZnO:Al bi-layer as a front electrode on the properties of p-i-n amorphous silicon based thin film solar cellsAppl. Phys. 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Lett. 102, 053901 (2013); 10.1063/1.4789855Highly efficient ultrathin-film amorphous silicon solar cells on top of imprinted periodic nanodot arraysWensheng Y an,1,a)Zhikuo Tao,2Thiam Min Brian Ong,3,4and Min Gu 1,b)1Centre for Micro-Photonics,Faculty of Science,Engineering and Technology,Swinburne University of Technology,Hawthorn,Victoria 3122,Australia 2College of Electronic Science and Engineering,Nanjing University of Posts and Telecommunications,Nanjing 210023,China 3Plasma Sources and Application Center,NIE,Nanyang Technological University,1Nanyang Walk,Singapore 637616,Singapore 4Institute of Materials Research and Engineering,A*STAR (Agency for Science,Technology and Research),3Research Link,Singapore 117602,Singapore(Received 12November 2014;accepted 23February 2015;published online 3March 2015)The addressing of the light absorption and conversion efficiency is critical to the ultrathin-film hydrogenated amorphous silicon (a-Si:H)solar cells.We systematically investigate ultrathin a-Si:H solar cells with a 100nm absorber on top of imprinted hexagonal nanodot arrays.Experimental evi-dences are demonstrated for not only notable silver nanodot arrays but also lower-cost ITO and Al:ZnO nanodot arrays.The measured external quantum efficiency is explained by the simulation results.The J sc values are 12.1,13.0,and 14.3mA/cm 2and efficiencies are 6.6%,7.5%,and 8.3%for ITO,Al:ZnO,and silver nanodot arrays,respectively.Simulated optical absorption distributionshows high light trapping within amorphous silicon layer.VC 2015AIP Publishing LLC .[/10.1063/1.4914110]Compared with conventional hydrogenated amorphous silicon (a-Si:H)thin film solar cells,a-Si:H solar cells with much thinner intrinsic layer are of great interest because of many advantages such as lower cost,higher throughput,and reduced detrimental Staebler-Wronski degradation effect.In that case,both the light trapping and efficiency are of critical challenges as a result of poor light absorption.1–12Currently,most experimental investigations about the light trapping enhancement for a-Si:H thin-film solar cells remain with an active layer thickness of more than 200nm.5–7In contrast,the works on ultrathin-film a-Si:H solar cells with a much thinner absorber (for example,100nm or less)are sig-nificant as a roadmap to maximize the cost-effectiveness.To date,however,for this kind of ultrathin a-Si:H solar cells,most investigations are limited to the design and simula-tion 2,9–11and there are still few experimental reports.8,13Furthermore,it was noted that the short-circuit current density (J sc )values in Ref.8are over high compared with the meas-ured external quantum efficiency (EQE).7Besides Ag nano-particles,it is promising to increase the light trapping within a-Si:H thin-film solar cells with lower-cost materials such as ITO and Al:ZnO nanoparticles.4,8,9,12An experiment has been demonstrated for a thick intrinsic layer of 350nm.12But,the investigations on a-Si:H solar cells on top of ITO and Al:ZnO nanoarrays with an absorber of 100nm are significant accord-ing to the above-mentioned benefits.In this letter,we propose and systematically investigate the light absorption and photovoltaic properties of ultrathin a-Si:H solar cells with a 100nm absorber on the top of imprinted periodic hexagonal nanodot arrays.The three nanodot arrays based on three materials are designed toachieve significant light-trapping enhancement.The three nanodot arrays are made of not only notable silver but also lower-cost ITO and Al:ZnO.In the present work,the peri-odic hexagonal nanodot arrays are imprinted by nanoimprint pared with random nanoparticles by most fabrication methods,periodic nanoparticles with accurate ge-ometry control offer a potential to maximize the light absorption by manipulating and tailoring the physical prop-erties of nanoparticles.In fact,nanoimprint lithography stands out as a promising technology for high-throughput,up-scalable nanoscale patterning with the accurate control of nanoparticles at low cost,as is required by photovoltaic applications.14,15The three ultrathin-film a-Si:H solar cells on top of nanodot arrays are investigated experimentally and theoretically.The J sc values of the fabricated solar cells are 12.1,13.0,and 14.3mA/cm 2for ITO,Al:ZnO,and silver nanodot arrays,respectively.Energy conversion efficiencies are 6.6%,7.5%,and 8.3%,respectively.The light absorption is modelled by the finite-difference time-domain (FDTD)simulation.The measured EQE curves are explained by the calculated wavelength-dependent light absorption.Figure 1shows the proposed architecture of ultrathin-film a-Si:H solar cells on top of imprinted periodic hexago-nal nanodot arrays.Figure 1(a)is a three-dimensional (3D)schematic of the ultrathin a-Si:H solar cells on top of nano-dot arrays.The distance between the nanodots shown on the top surface can be tuned by the parameters of the nanodot mould.Figure 1(b)demonstrates SEM image of periodic hexagonal silver nanodot arrays imprinted by nanoimprint li-thography (NX-B200),where the right-top inset is a zoomed image of silver nanodots and the right-bottom inset is an image of the top surface of the device.Figure 1(c)shows a cross-sectional schematic drawing of the model for the pres-ent solar cells on top of nanodot by combining the two SEMa)yws118@ b)mgu@.au0003-6951/2015/106(9)/093902/4/$30.00VC 2015AIP Publishing LLC 106,093902-1APPLIED PHYSICS LETTERS 106,093902(2015)images,well-established conformal deposition principal,and the related references.4,5,7,8,12,16The present solar cell struc-ture consists of ITO (80nm)/n-i-p a-Si:H (20nm-100nm-10nm)/Al:ZnO (100nm)/nanodot arrays/silver back reflector (100nm).A flat a-Si:H solar cell without the nanodot arrays is used as a reference.The fabrication procedure for nanodot arrays is as follow:0.25cm 2square moulds made of silicon are prepared by electron beam lithography.Then,nanoim-print lithography (NX-B200)is used to impress the mould on the substrate covered with a thin layer of resist on the sur-face.A thermal evaporation method is then used to deposit the thin films such as silver,Al:ZnO,and ITO on the sub-strate,separately.The nanodot arrays are prepared after dis-solving the resist in a chemical solution.When the nanodot arrays are prepared,ITO (80nm)/n-i-p a-Si:H (20nm-100nm-10nm)/Al:ZnO (100nm)is deposited on the nanodot arrays,where a-Si:H thin film is deposited by plasma enhanced chemical vapour deposition.ITO and Al:ZnO thin films are prepared by the physical vapour deposition method.The EQE curves are measured for all structural ultrathin-film a-Si:H solar cells in the wavelength range of 300–800nm,as shown in Figure 2(a).It is found that the three ultrathin a-Si:H solar cells with nanodot arrays demon-strate significant broadband light absorption enhancements in the whole wavelength range,compared with the referenceflat cell.Note that that the geometries of the three fabricated nanodots are the same in the investigation.To understand the EQE data obtained,the light absorption in the active layer are modelled and simulated by using the Lumerical’s FDTD commercial software.17The details about the simulation including the dielectric functions of the materials can be obtained from our previous work.9In the present simulation,the lateral diameter and height of nanodots are approxi-mately set as 200nm and 140nm,respectively,which are consistent with the experimental parameters.The distance between the nanodots is 410nm.The simulation results are shown in Figure 2(b).It is seen that the calculated curves show a good consistency with the measured wavelength-dependent EQE curves.In the case of silver nanodot arrays,the enhanced light absorption is attributed to the waveguide modes by the a-Si:H slab as well as the plasmonic effect by silver nanodots.8,12In contrast,the light trapping mechanism for ITO and Al:ZnO nanodot arrays is mainly due to the waveguide modes.8,12It is seen in Figure 2that the light absorption curves as well as EQE curves of ITO and Al:ZnO nanodot arrays are close to that of silver nanodot arrays.It indicates that ITO and Al:ZnO nanodot arrays are promising as a means to enhance the light trapping.It is also seen from Figure 2(b)that,compared with the ITO and Al:ZnO nano-dot arrays,the light absorption improvement for silver nano-dot arrays mainly lies in the long wavelength range of 650–700nm.The feature could be attributed to the additional light-trapping benefit from the plasmonic effect of silver nanodot arrays by an appropriate light manipulation,as the combination of the waveguide modes and the plasmonic effect does not always play a positive role in the light absorption enhancement.9We have compared an integrated J sc between calculations from Figure 2and measured EQE.For example,the integrated J sc value of the flat cell from Figure 2(b)is 8.4mA/cm 2.As seen below,the measured J sc value of the reference flat cell is 7.8mA/cm 2.This calcula-tion value of J sc is slightly greater than that from EQE.The photovoltaic properties of all structural a-Si:H solar cells with a cell area of 0.25cm 2are measured under stand-ard test conditions,i.e.,an AM 1.5illumination with a light intensity of 100mW/cm 2and at room temperature.The J-V characteristics for the three ultrathin a-Si:H solar cells with nanodot arrays as well as the flat cell are shown in Figure 3.The photovoltaic performance parameters for all a-Si:H solar cells are listed in Table I .It is found in Table I that compared with the flat cell,the enhancement of the ultrathina-Si:HFIG.1.(a)3D schematics of the architecture of ultrathin a-Si:H solar cells with nanodot arrays.(b)An SEM image of fabricated silver nanodot arrays with a hexagonal distribution.The right-top inset is an enlarged image of sil-ver nanodot arrays and the right-bottom inset is the morphology for the top surface of the device.(c)Cross-sectional schematic drawing of the model of the ultrathin a-Si:H solar cells on top ofnanodot.FIG.2.(a)The measured wavelength-dependent EQE curves for the flat cell as well as the ultrathin a-Si:H solar cells on top of silver,Al:ZnO,and ITO nanodot arrays.(b)The light absorp-tion calculated as a function of the wavelength for the flat cell and the ultrathin a-Si:H solar cells with nano-dot arrays.solar cells with nanodot arrays mainly comes from the J sc improvement.The observable improvement of the open-circuit voltage(V oc)and thefill factor(FF)is relatively small.It is seen in Table I that the J sc value of theflat cell is 7.8mA/cm2.This small value is due to the poor light absorp-tion of theflat solar cell.In contrast,the J sc values of the a-Si:H cells with nanodot arrays have been significantly enhanced with the values of12.1,13.0,and14.3mA/cm2for ITO,Al:ZnO,and silver nanodot arrays,respectively.The achieved J sc values for the solar cells with ITO and Al:ZnO nanodots outperform the simulation values of a-Si:H thin film solar cells by adopting a200nm absorber.4Compared with the ultrathin a-Si:H solar cells with a90nm absorber, the EQE data are similar to and even slightly higher than the report in Ref.8,where square Ag nanoarrays with a hemiel-lipsoidal shape are used.It can be seen from Table I that the efficiency for theflat cell is4.0%.In contrast,the efficiencies for the ultrathin-film a-Si:H solar cells with ITO,Al:ZnO, and silver nanodot arrays are improved to6.6%,7.5%,and 8.3%.By a comparison,it is found that the efficiency of 8.3%for the ultrathin a-Si:H solar cells with only100nm absorber by using periodic silver nanodot arrays is higher than reported values for a-Si:H solar cells with the thick absorber by using non-periodic silver nanoparticles.5–7 Compared with Ref.5,the improvement of the FF may be attributed to the addressing of the layer contacts for the se-ries resistance as well as the reducing of the parasitic resis-tive losses.It was reported that the Ag particle shape with sharp points is highly absorptive and can lead to a serious parasitic absorption.8,16The results of the present work dem-onstrate that ultrathin-film a-Si:H solar cells with nanodot arrays are promising to achieve highly cost-effective solar cells by the effective design and light management.To further clarify the enhanced light absorption of the ultrathin a-Si:H solar cells with nanodot arrays,the optical absorption distribution is calculated as a function of wave-length.The optical absorption distribution is found to be entirely different between the solar cells with nanodot arrays and theflat solar cell.As a demonstration,the optical absorp-tion profiles for theflat solar cell are shown in Figures 4(a)–4(d)at four wavelength values of400nm,500nm, 650nm,and700nm,respectively.Similarly,the optical absorption distributions for the solar cells with silver nano-dot arrays and Al:ZnO nanodot arrays are shown in Figures 4(e)–4(h)and4(i)–4(l)at four wavelength values,respec-tively.Optical absorption distributions for the solar cells with ITO nanodot arrays are nearly the same with that for Al:ZnO nanodot arrays(not shown here).Figure4shows that when compared with theflat solar cell,the incoming light is significantly trapped within the intrinsic layer of the solar cells on top of nanodot arrays,which leads to the increases of the EQE and J sc values.It is found in Figures 4(e)–4(l)that there is only a small difference of the optical absorption distribution in a-Si:H layer between the cells with Al:ZnO nanodot arrays and the cells with Ag nanodot arrays. It is also noted that there appear to be a concentration of light in the regions of doped layers indicated by the green boxes in Figures4(h)and4(i).FIG.3.The photovoltaic responses for the ultrathin a-Si:H solar cells on top of nanodot arrays as well as the referenceflat cell.TABLE I.The photovoltaic parameters of all structural ultrathin-film a-Si:H solar cells listed(less than4%experimental error estimated).a-Si:H solar cell structure J scðmA=cm2ÞV ocðVÞFF g(%)Flat7.80.830.62 4.0 Silver nanodot arrays14.30.880.668.3 Al:ZnO nanodot arrays13.00.870.667.5 ITO nanodot arrays12.10.850.646.6FIG.4.The calculated optical absorption distribution at the wavelength val-ues of400nm,500nm,650nm,and700nm for theflat cell and the ultrathin a-Si:H solar cells with nanodot arrays.(a)–(d)Theflat solar cell.(e)–(l)The ultrathin a-Si:H solar cells on top of silver nanodot arrays and Al:ZnO nano-dot arrays,respectively.The dotted lines are added to assist to differentiate the boundaries between these layers.In conclusion,we have fabricated the prototypes of highly efficient ultrathin a-Si:H solar cells with a100nm absorber on top of ITO,Al:ZnO,and silver nanodot arrays,respectively. High increase of J sc is achieved by the implementation of the light trapping design in the real devices.The J sc and efficiency of the reference cell are7.8mA/cm2and4.0%,respectively. The J sc values of the ultrathin a-Si:H solar cells on top of ITO, Al:ZnO,and silver nanodot arrays are increased to12.1,13.0, and14.3mA/cm2.The achieved efficiencies are6.6%,7.5%, and8.3%,respectively.The work presents a proof that the ultrathin-film a-Si:H solar cells on top of nanodot arrays ena-ble to achieve high material cost-effectiveness.Wensheng Yan thanks for the support fund from Australian Renewable Energy Agency.Min Gu acknowledges the support from the Science and Industry Endowment Fund. The authors acknowledge the technical support from the Victoria-Suntech Advanced Solar Facility established under the Victoria Science Agenda of the Victorian Government.1C.Battaglia, C.-M.Hsu,K.So€derstro€m,J.Escarre0, F.-J.Haug,M. Charrie‘re,M.Boccard,M.Despeisse,D.T.L.Alexander,M.Cantoni, Y.Cui,and C.Ballif,ACS Nano6,2790(2012).2J.Grandidier,D.Callahan,J.Munday,and H.Atwater,Adv.Mater.23, 1272(2011).3G.Kang,H.Park,D.Shin,S.Baek,M.Choi,D.Yu,K.Kim,and W. Padilla,Adv.Mater.25,2617(2013).4M.G.Deceglie,V.E.Ferry,A.P.Alivisatos,and H.A.Atwater,Nano Lett.12,2894(2012).5J.Zhu,C.-M.Hsu,Z.F.Yu,S.H.Fan,and Y.Cui,Nano Lett.10,1979 (2010).6X.Chen,B.H.Jia,J.K.Saha,B.Y.Cai,N.Stokes,Q.Qiao,Y.Q.Wang, Z.R.Shi,and M.Gu,Nano Lett.12,2187(2012).7H.R.Tan,R.Santbergen,A.Smets,and M.Zeman,Nano Lett.12,4070 (2012).8V.E.Ferry,M.A.Verschuuren,C.van Lare,R.J.Walters,R.E.I. Schropp,H.A.Atwater,and A.Polman,Nano Lett.11,4239(2011).9W.Yan and M.Gu,J.Opt.16,045003(2014).10Y.Wang,T.Y.Sun,T.Paudel,Y.Zhang,Z.F.Ren,and K.Kempa,Nano Lett.12,440(2012).11L.Yang,L.Mo,Y.Okuno,and S.He,Prog.Photovoltaics:Res.Appl.21, 1077(2013).12M.van Lare,F.Lenzmann,and A.Polman,Opt.Express21,20738 (2013).13S.Misra,L.Yu,M.Foldyna,and P.Roca i Cabarrocas,Sol.Energy Mater.Sol.Cells118,90(2013).14S.H.Ahn and L.J.Guo,ACS Nano3,2304(2009).15C.Battaglia,J.Escarre,K.S.Derstrom,L.Erni,L.Ding,G.Bugnon,A. Billet,M.Boccard,L.Barraud,S.De Wolf,F.-J.Haug,M.Despeisse,and C.Ballif,Nano Lett.11,661(2011).16V.E.Ferry,A.Polman,and H.A.Atwater,ACS Nano5,10055(2011). 17See for FDTD solutions.。

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The Properties and Applications of Ultrathin Films

The Properties and Applications of Ultrathin Films

The Properties and Applications ofUltrathin FilmsUltrathin films, also known as nanofilms or thin films, are coatings or layers of materials that are just a few nanometers to a few micrometers thick. These films have unique properties that make them useful in a wide range of applications, from electronics and optics to biomedical devices and energy storage.Properties of Ultrathin FilmsOne of the most important properties of ultrathin films is their high surface-to-volume ratio. Because of their small thickness, these films have a high surface area relative to their volume. This means that they can interact with their environment in unique ways, allowing them to be used in a wide range of applications.Another property of ultrathin films is their tunability. By adjusting the thickness and composition of the film, it is possible to control its properties, such as its electrical conductivity, optical properties, and mechanical strength. This allows these films to be used in a wide range of applications where their properties can be tailored to meet specific requirements.Applications of Ultrathin FilmsElectronics - One of the most important applications of ultrathin films is in the field of electronics. These films are used to create thin-film transistors, which are used in electronic devices such as flat-panel displays and touch screens. They are also used in memory devices such as flash drives and hard drives.Optics - Ultrathin films are also used in the field of optics, where they are used to create optical coatings that can be used to manipulate light. These coatings are used in a wide range of applications, including cameras, telescopes, and photovoltaic cells.Biomedical Devices - Ultrathin films are also used in the field of biomedical devices. They can be used to create coatings that are biocompatible and that can be used to improve the performance of medical devices such as implants and prosthetics. They can also be used to create biosensors that can detect diseases and other medical conditions.Energy Storage - Ultrathin films are also being used to create new types of energy storage devices such as batteries and supercapacitors. These devices are being developed for use in a wide range of applications including electric vehicles, renewable energy systems, and consumer electronics.ConclusionUltrathin films are a rapidly growing area of research and are being used in a wide range of applications. These films have unique properties that allow them to be used in a wide range of applications, from electronics and optics to biomedical devices and energy storage. As research in this field continues, it is likely that new applications for ultrathin films will be discovered, making them even more important in the years to come.。

6-Ultrathin, transparent, and flexible graphene films,APL,2009

6-Ultrathin, transparent, and flexible graphene films,APL,2009

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The reduction process was repeated for another cycle to remove residual oxygen groups. The ultrathin films were prepared using a vacuum filtration method as described in literature.7 A well dispersed, 0.2 mg/mL solution of graphene in water was prepared via ultrasonication. An appropriate volume of graphene solution required to fabricate films of 25, 50, 75, and 100 nm thick, was chosen to filtered through an alumina Anodisc membrane ͑47 mm in diameter, Whatman͒, which results in a film of 40 mm in diameter. The solution was filtered, and the graphene film on the alumina membrane was dried at 90 ° C for about 30 min. To transfer the graphene film to a glass slide, PET substrate or glassy carbon electrode ͑0.2 cm2͒, a 0.05 M NaOH solution bath was employed to etch away the alumina membrane.7 Figure 1͑a͒ presents a photograph of the 25, 50, 75, and 100 nm films and illustrates the homogeneity of the over a large area. This is attributed to the small filter pore size, excellent dispersion of the graphene solution and van der Waal attraction among the graphene sheets. Figure 1͑b͒ shows a typical transmission electron microscopy ͑TEM͒ image of the graphene collected from the graphene suspension prior to filtration. The slightly wrinkled morphology of the sheet is a result of buckling caused by epoxy groups formed
12-Amorphous Silicon-based Solar Cells

12-Amorphous Silicon-based Solar Cells


In Dundee, Scotland, Walter Spear and Peter LeComber discovered around 1973 that amorphous silicon prepared using a “glow discharge” in silane (SiH4 ) gas had unusually good electronic properties; they were building on earlier work by Chittick, Sterling, and Alexander [3]. Glow discharges are the basis for the familiar “neon” light; under certain conditions, an electric voltage applied across a gas can induce a significant electrical current through the gas, and the molecules of the gas often emit light when excited by the current. Amorphous silicon was deposited as a thin film on substrates inserted into the silane gas discharge.1 Spear and LeComber reported in 1975 [4] that amorphous silicon’s conductivity could be increased enormously either by mixing some phosphine (PH3 ) gas or some diborane (B2 H6 ) gas with the silane. Just as for crystal silicon, the phosphorus doping of the amorphous silicon had induced a conductivity associated with mobile electrons (the material was “n-type”), and the boron doping had induced a conductivity associated with mobile holes (the material was “p -type”). In 1974, at the Radio Corporation of America (RCA) Research Laboratory in Princeton, David Carlson discovered that he could make fairly efficient solar cells using a silane glow discharge to deposit films. In 1976, he and Christopher Wronski reported a solar cell based on amorphous silicon [5] with a solar conversion efficiency of about 2.4% (for historical discussion see Reference [6, 7]). Carlson and Wronski’s report of the current density versus output voltage is presented in Figure 12.1 (along with the curve from a far more efficient cell reported in 1997 [8]). As these scientists had discovered, the optoelectronic properties of amorphous silicon made by glow discharge (or “plasma deposition”) are very much superior to the amorphous silicon thin films prepared, for example, by simply evaporating silicon.
Highly transparent and conductive double-layer oxide thin films as anodes for o

Highly transparent and conductive double-layer oxide thin films as anodes for o

Highly transparent and conductive double-layer oxide thinfilms as anodes for organic light-emitting diodesYu Yang,Lian Wang,He Yan,Shu Jin,and Tobin J.Marks a͒Department of Chemistry and the Materials Research Center,Northwestern University,Evanston,Illinois60208-3113Shuyou LiDepartment of Materials Science and Engineering,Northwestern University,Evanston,Illinois60208-3108͑Received7February2006;accepted9June2006;published online4August2006͒Double-layer transparent conducting oxide thinfilm structures containing In-doped CdO͑CIO͒andSn-doped In2O3͑ITO͒layers were grown on glass by metal-organic chemical vapor deposition andion-assisted deposition͑IAD͒,respectively,and used as anodes for polymer light-emitting diodes͑PLEDs͒.Thesefilms have a very low overall In content of16at.%.For180-nm-thick CIO/ITOfilms,the sheet resistance is5.6⍀/ᮀ,and the average optical transmittance is87.1%in the400–700nm region.The overallfigure of merit͑⌽=T10/R sheet͒of the double-layer CIO/ITOfilmsis significantly greater than that of single-layer CIO,IAD-ITO,and commercial ITOfilms.CIO/ITO-based PLEDs exhibit comparable or superior device performance versus ITO-based controldevices.CIO/ITO materials have a much lower sheet resistance than ITO,rendering them promisinglow In content electrode materials for large-area optoelectronic devices.©2006American Instituteof Physics.͓DOI:10.1063/1.2240110͔Transparent conducting oxides͑TCOs͒are critical com-ponents as transparent electrodes inflat-panel displays,solar cells,window deicers,gas sensors,and smart windows.1,2Most TCOs are n-type conductors composed of pure ordoped CdO,ZnO,In2O3,SnO2,or ternary oxides.Amongthese materials,Sn-doped In2O3͑ITO͒with a conductivity of 3000–5000S/cm and transparency of85%–90%in the vis-ible is extensively used in manufacture.However,the risingcost of In and less than optimum ITO conductivity raise thequestion of whether it will be useful in future large-area op-toelectronic devices.Therefore,extensive efforts are beingdedicated tofinding superior low/non-In TCOs which areinexpensive but have higher conductivity and a broadertransparency window than ITO.3–10Such TCOs would sig-nificantly benefit a variety of next-generation optoelectronictechnologies.To achieve greater conductivity with greater transpar-ency and multifunctionality,TCOs have been incorporated inmultilayer stacks.For example,ITO/͑metallic Ag͒/ITOtrilayers exhibit greater conductivity than ITO and can beused as transparent electrodes.4,8,9Kawashima et al.reportedthat F-doped SnO2-coated ITO shows improved thermal sta-bility versus ITO with conductivity retained during solar cellprocessing at400–600°C.3Martin et al.reported pulsed-laser-deposition-derived TCO double layers consisting of Sn-doped CdO,CdIn2O4,and Cd2SnO4which show higher con-ductivity than ITO with comparable optical transparency.5Thus,multilayer TCO structures have the potential toachieve greater functionality than single-layer TCOs by judi-cious layer structure and component materials properties de-sign.Here we report a simple/efficient double-layer strategyfor ITO/͑In-doped CdO͒thinfilm growth.The attractions ofsuch double-layer materials include͑1͒lower sheet resis-tance,͑2͒greater optical transparency,͑3͒smoother surface morphology and greater resistance to corrosion,͑4͒tunable work function,͑5͒encapsulation of potentially nonbenign materials,͑6͒greater environment stability,͑7͒lower mate-rials cost,and͑8͒simplefilm growth process applicable to other double-layer TCO structures.CdO TCOs are of interest due to their large carrier mo-bilities,near-metallic conductivity,low Cd cost,and applica-bility as photovoltaic electrodes.11Previous studies showed that metal-organic chemical vapor deposition͑MOCVD͒-derived,In-doped CdO͑CIO͒thinfilms having conductivi-ties ofϳ17000S/cm are some of the most conductive TCOs grown to date on amorphous substrates.12CdO thin films have also been successfully implemented in organic light-emitting diodes͑OLEDs͒,10and it was demonstrated that CdOfilms effectively inject holes into such devices. However,this highly conductive CdO-based device exhibits less than optimum performance compared to ITO-based con-trols,due to the relatively low CdO work function,vulner-ability to corrosive hole injection materials,and rough mor-phology.We report here the growth,electrical,and optical properties of double-layer ITO/CIO thinfilms.Thus,a thin ITO layer is overcoated on CIO thinfilms by ion-assisted deposition to engineer the CIO surface and tune the work function while exploiting superior CIO electrical properties. These TCOfilms are then implemented in polymer light-emitting diodes͑PLEDs͒.It is found that ITO/CIO thinfilms, with a very low In content,significantly lower sheet resis-tance,and higherfigure of merit͑⌽=T10/R sheet,where T =average transmittance from400to700nm͒than that of ITO,yield comparable or enhanced PLED performance ver-sus ITO-based devices.CIO thinfilm growth was carried out in a previously described MOCVD reactor13using the precursors Cd͑hfa͒2͑N,N-DE-NЈ,NЈ-DMEDA͒͑hfa=hexafluoroacetylacetonate, N,N-DE-NЈ,NЈ-DMEDA=N,N-diethyl-NЈ,NЈ-dimethyleth-a͒Author to whom correspondence should be addressed;electronic mail:t-marks@APPLIED PHYSICS LETTERS89,051116͑2006͒0003-6951/2006/89͑5͒/051116/3/$23.00©2006American Institute of Physics89,051116-1ylenediamine ͒14and In ͑dpm ͒3͑dpm=dipivaloymethanate ͒.15The reactor precursor reservoir temperatures/Ar carrier gasflow rates were ͓Cd ͑hfa ͒2͑N ,N -DE-N Ј,N Ј-DMEDA ͔͒85°C/16SCCM ͑SCCM denotes cm 3per minute STP ͒and ͓In ͑dpm ͒3͔94°C/9SCCM.O 2oxidizing gas was intro-duced upstream at 400SCCM after bubbling through dis-tilled water.A system pressure of 4.2±0.1Torr and a sub-strate temperature of 410°C were maintained duringdeposition.Corning 1737F glass substrates were cleanedwith acetone prior to the film deposition.CIO thin films witha thickness of ϳ160nm were deposited on glass at a growthrate of ϳ1.3nm/min for 2h.The In doping level is4.3±0.5at.%as determined by inductively coupled plasmaatomic emission spectrometry ͑ICP-AES ͒,close to the opti-mal In doping level for maximum CIO conductivity and op-tical transparency.12The as-deposited CIO thin films werethen transferred to an ion-assisted deposition ͑IAD ͒chamberfor ITO overcoating using an ITO ͑In 2O 3:SnO 2=9:1͒target.Prior to ITO film deposition,the CIO film surface wascleaned with the assisted ion beam ͑Ar ion energy =100eV ͒for ϳ1min.The ITO growth process was describedpreviously.16The ITO thickness was monitored using aquartz crystal balance calibrated with a Tencor profilometer.Film composition analyses,carried out using ICP-AES,reveal a very low overall In content of 16at.%in optimizedCIO/ITO double-layer thin films.This is significantly smallerthan the In content in commercial ITO ͑ϳ90%͒and IAD-derived ITO ͑94%͒.Transmission electron microscopic ͑TEM ͒images were obtained with a Hitachi HF-2000micro-scope.Figure 1shows optical transmittance spectra of as-grownCIO ͑167nm ͒,bilayer CIO/ITO-1͑CIO=157nm,ITO =23nm ͒,and bilayer CIO/ITO-2͑CIO=157nm,ITO =37nm ͒films.CIO/ITO-1and CIO/ITO-2exhibit compa-rable or slightly greater transmittance in the blue region than CIO.The average optical transmittance of the CIO/ITO-1and CIO/ITO-2films in the visible ͑400–700nm ͒,87.1%and 88.0%,respectively,are slightly greater than that of as-grown CIO films,86.4%.Optical transmittance,sheet resis-tance,and the figures of merit ͑⌽=T 10/R sheet ͒are summa-rized in Table I.Note that the CIO/ITO double-layer films exhibit significantly greater figures of merit than any of the components or commercial ITO films.A TEM cross-sectional image of a CIO/ITO ͑CIO =100nm,ITO=37nm ͒film ͑Fig.2͒shows that the CIO/ITO interface is distinct and sharp.The CIO layer ͑ϳ100nm thick ͒is polycrystalline with a columnar,grained structure,while the ITO layer ͑ϳ40nm ͒is essentially amorphous.The root-mean-square ͑rms ͒roughness of the CIO/ITO surface is ϳ4nm,by contact mode AFM,less than that of CIO,5–8nm.The as-deposited CIO/ITO-1thin films were next imple-mented in PLED fabrication using the structure,TCO/PEDOT-PSS/͑TPDSi 2+TFB ͒/BT/Ca/Al.17Here the PEDOT-PSS ͑a doped polythiophene polystyrene sulfonate ͒hole-transport layer ͑HTL ͒was first spin coated onto the TCO anodes and cured at 200°C overnight.Next,a cross-linkable TPDSi 2͑4,4Ј-bis ͓͑p -trichlorosilylpropyl-phenyl ͒phenylamino ͔biphenyl ͒+TFB ͑poly ͓9,9-dioctyl-fluorene-co-N-͑4-butylphenyl ͔͒diphenylamine ͒HTL/electron-blocking layer ͑EBL ͒͑molecular weight ϳ50000g/mol ͒blend ͑1:1mass ratio ͒was spin coated onto the anodes,followed by cross-linking to afford a robust,in-soluble film.A third layer of the emissive polymer BT ͓poly ͑9,9-dioctylfluorene-co-benzothiadiazole ͔͒wasthen FIG. 1.͑Color online ͒Optical transmittance spectra of CIO ͑167nm ͒,CIO ͑157nm ͒/ITO ͑23nm ͒,and CIO ͑157nm ͒/ITO ͑37nm ͒films.TABLE I.Properties of single-and double-layered TCO thin films and operating characteristics of PLED devices having the structure TCO/PEDOT:PSS/͑TPDSi 2+TFB ͒/BT/Ca/Al.Sample Sheetresistance͑⍀/Ǣ͒Average transmittance in 400–700nm ͑%͒Figure of merit ⌽=T 10/R sheet ͑10−3⍀−1͒Turn-on voltage ͑V ͒Lightoutput ͑cd/m 2͒Maximum current efficiency ͑cd/A ͓͒voltage ͑V ͔͒Bias drop across anode a ͑mV ͒CIO 5.686.441.4¯b ¯b ¯b ¯b CIO-ITO-1 5.687.144.9 3.615900͑8.0͒15.0͑6.0͒1CIO-ITO-2 6.188.046.0¯c ¯c ¯c ¯c IAD-ITO 27.785.47.4 4.115400͑9.5͒14.2͑7.0͒4Commercial ITO18.095.133.6 4.014900͑8.5͒14.1͑8.0͒2aBias drop across anode was measured when each PLED was operated under 5.5V applied bias.b Not available due to the significant PEDOT corrosion of In-doped CdO thin films.c Not implemented into the PLED fabrication.FIG.2.Cross-sectional TEM image of an CIO/ITO double-layer film.spin coated onto the HTL/EBL to form a multilayer hetero-structure.Finally,inside a glovebox,Ca and Al were sequen-tially thermally evaporated onto the emitting layer at Ͻ10−6Torr,using a shadow mask to define the 10mm 2electrode areas.Details of device fabrication are described elsewhere.17In parallel,devices with an identical structure were fabricated using CIO,IAD-derived ITO,and commer-cial ITO films on glass for comparison.Device steady-state light outputs and J -V characteristics of the PLED devices were measured under ambient atmosphere.17It was not pos-sible to fabricate a PLED device on CIO due to its relatively poor acid resistance to PEDOT solutions.Figure 3shows steady-state light output and I -V ͑J -V ͒characteristics of PLEDs based on CIO/ITO-1,IAD-ITO,and a control with commercial ITO/glass.Differences between CIO/ITO-1,IAD-ITO,and ITO devices are attributed to differences in anode characteristics since all devices were fabricated simul-taneously in a parallel fashion.As seen in Fig.3,CIO/ITO-1anodes PLEDs exhibit a turn-on voltage of 3.6V,somewhat lower than that of the IAD-ITO and commercial ITO-based devices,4.1and 4.0V,respectively.A maximum luminance of 15900cd/m 2at 8V with a maximum current efficiency of 15cd/A at 5.8V is achieved with the CIO/ITO-1-based PLEDs—comparable or superior to devices with IAD-ITO and commercial ITO anodes.Note that CIO/ITO-1has a sheet resistance of only 5.6⍀/Ǣ,significantly lower than that of 27.7and 18.0⍀/Ǣfor IAD-ITO and commercial ITO,respectively.To investigate other anode effects on turn-on voltage,the bias drops across the anodes were mea-sured over 1.0cm when all PLEDs were operating under5.5V applied bias ͑Table I ͒.Clearly,the bias drops across all anodes here are negligible ͑ϳmV ͒,and sheet resistance does not have a major influence on PLED performance ͑e.g.,turn-on voltage ͒,although this should not be the case in larger-area pared to IAD-ITO,CIO/ITO-1should have comparable surface chemistry,but the surface roughness,ϳ4nm,is slightly greater than that of IAD-ITO,ϳ1nm,which may enhance interfacial contact and electric fields,hence assist charge carrier injection,leading to lower operating voltages 8and higher current efficiency.18Double-layer transparent conducting CIO/ITO films were used as transparent anodes for PLED fabrication.The overall figure of merit of these films is significantly greater than any of the component materials.Also,CIO/ITO-based PLEDs exhibit comparable or superior PLED performance to that of a commercial ITO control.Note that CIO/ITO mate-rials have a much lower In content and sheet resistance than ITO,rendering them promising for large-area optoelectron-ics.In addition,high work function ITO alternative TCOs,such as ZITO,7,19or other thin film materials could be em-ployed as top layers to yield functionalized,multilayer TCOs.This research was supported by NREL ͑XAT-5-33636-02͒.Characterization facilities were provided by the North-western University NSF-MRSEC ͑DMR-00760097͒.1MRS Bull.25͑2005͒,special issue on transparent conduction oxides,edited by D.S.Ginley and C.Bright,and references therein.2H.Hosono,H.Ohta,M.Orita,K.Ueda,and M.Hirano,Vacuum 66,419͑2002͒.3T.Kawashima,T.Ezure,K.Okada,H.Matsui,K.Goto,and N.Tanabe,J.Photochem.Photobiol.,A 164,199͑2004͒.4E.Kusano,J.Kawaguchi,and K.Enjouji,J.Vac.Sci.Technol.A 4,2907͑1986͒.5E.J.J.Martin,M.Yan,ne,J.Ireland,C.R.Kannewurf,and R.P.H.Chang,Thin Solid Films 461,309͑2004͒.6J.Ni,L.Wang,Y .Yang,H.Yan,S.Jin,T.J.Marks,J.R.Ireland,and C.R.Kannewurf,Inorg.Chem.44,6071͑2005͒.7J.Ni,H.Yan,A.C.Wang,Y .Yang,C.L.Stern,A.W.Metz,S.Jin,L.Wang,T.J.Marks,J.R.Ireland,and C.R.Kannewurf,J.Am.Chem.Soc.127,5613͑2005͒.8M.Sawada,M.Higuchi,S.Kondo,and H.Saka,Jpn.J.Appl.Phys.,Part 140,3332͑2001͒.9J.Szczyrbowski,A.Dietrich,and K.Hartig,Sol.Energy Mater.19,43͑1989͒.10Y .Yang,Q.Huang,A.W.Metz,S.Jin,J.Ni,L.Wang,and T.J.Marks,J.Soc.Inf.Disp.13,383͑2005͒.11T.J.Coutts,T.O.Mason,J. D.Perkins,and D.S.Ginley,Proc.-Electrochem.Soc.99-11,274͑1999͒.12A.Wang,J.R.Babcock,N.L.Edleman,A.W.Metz,ne,R.Asahi,V .P.Dravid,C.R.Kannewurf,A.J.Freeman,and T.J.Marks,Proc.Natl.Acad.Sci.U.S.A.98,7113͑2001͒.13B.J.Hinds,R.J.McNeely,D.B.Studebaker,T.J.Marks,T.P.Hogan,J.L.Schindler,C.R.Kannewurf,X.F.Zhang,and ler,J.Mater.Res.12,1214͑1997͒.14A.W.Metz,J.R.Ireland,J.G.Zheng,R.Lobo,Y .Yang,J.Ni,C.L.Stern,V .P.Dravid,N.Bontemps,C.R.Kannewurf,K.R.Poeppelmeier,and T.J.Marks,J.Am.Chem.Soc.126,8477͑2004͒.15Z.Jablonski,I.Rychlowskahimmel,and M.Dyrek,Spectrochim.Acta,Part A 35,1297͑1979͒.16Y .Yang,Q.L.Huang,A.W.Metz,J.Ni,S.Jin,T.J.Marks,M.E.Madsen,A.DiVenere,and S.T.Ho,Adv.Mater.͑Weinheim,Ger.͒16,321͑2004͒.17H.Yan,B.J.Scott,Q.L.Huang,and T.J.Marks,Adv.Mater.͑Weinheim,Ger.͒16,1948͑2004͒.18Y .Yang,E.Westerweele,C.Zhang,P.Smith,and A.J.Heeger,J.Appl.Phys.77,694͑1995͒.19J.Cui,A.Wang,N.L.Edleman,J.Ni,P.Lee,N.R.Armstrong,and T.J.Marks,Adv.Mater.͑Weinheim,Ger.͒13,1476͑2001͒.FIG.3.͑Color online ͒Steady-state measurements of ͑a ͒light output,͑b ͒current density,and ͑c ͒current efficiency as a function of bias for PLEDs having the structure TCO/PEDOT:PSS/TPDSi 2+TFB/BT/Ca/Al.Lines through the data points are drawn as a guide to the eye.。