zno-tio2 nanocomposite core-shell

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  • ZnOTiO2 Nanocomposite Films for High Light Harvesting Eciencyand Fast Electron Transport in Dye-Sensitized Solar CellsVenkata Manthina,, Juan Pablo Correa Baena,, Guangliang Liu,, and Alexander G. Agrios*,,

    Department of Civil & Environmental Engineering, University of Connecticut, Unit 3037, 261 Glenbrook Rd, Storrs, Connecticut06269, United StatesDepartment of Chemical, Materials & Biomolecular Engineering, University of Connecticut, Unit 3222, 191 Auditorium Rd, Storrs,Connecticut 06269, United StatesCenter for Clean Energy Engineering, University of Connecticut, 44 Weaver Rd, Storrs, Connecticut 06269, United States

    ABSTRACT: Electron transport and recombination are theessential processes that determine the charge collectioneciency in dye-sensitized solar cells (DSSC). While nearly100% of charges are collected in well-built ordinary DSSCs,this value can be sharply reduced by the use of redox couplesother than iodide/triiodide due to fast electron recombination.To compensate, structures capable of fast electron transportare needed. Nanorod arrays that have this attribute tend tosuer from low surface area, resulting in low dye loading andreduced light harvesting. We have therefore developed a novelnanocomposite structure consisting of zinc oxide (ZnO)nanorods coated with titanium dioxide (TiO2) nanoparticles using an electrostatic layer-by-layer (LbL) deposition technique.The titanium dioxide nanoparticle coating can add an order of magnitude of surface area and is compatible with known high-performance dyes. This composite nanostructure has been designed to take advantage of the improved electron transport alongthe nanorods and surface area provided by the nanoparticles, yielding good charge collection and light harvesting. Transientmeasurements indicate that the composite lm can transport electrons at least 100 times faster than a nanoparticulate TiO2 lm.In tests using ferrocene/ferrocenium as a model alternative redox couple with fast recombination, currentvoltage measurementsindicate that the ZnOTiO2 hybrid lms generate much higher currents than conventional TiO2 nanoparticulate lms. However,not all charges successfully transfer from TiO2 to ZnO due to an energy barrier between the materials.

    INTRODUCTIONDye-sensitized solar cells (DSSCs)1 are low-cost alternatives tosilicon photovoltaics. The conventional DSSC consists of twosandwiched pieces of conducting glass, one of them coated witha mesoporous layer of nanoparticulate TiO2 with a self-assembled monolayer of chemisorbed dye molecules, lled withan electrolyte for dye regeneration. The dye is a transition-metal complex or organic chromophore that harvests sunlightby absorbing strongly in the visible region of the solarspectrum. The principal photovoltaic losses in the DSSC aredue to incomplete light harvesting, recombination of thephotoinjected electrons with the electrolyte, and the over-potential required for dye regeneration. Ruthenium complexdyes like N719, N3, and black dye exhibit high eciency withthe I/I3

    redox couple, but there is little room forimprovement in light harvesting in the visible range.In DSSCs, the I/I3

    redox couple limits the overalleciency due to the high overpotential (ca. 0.5 V) for dyeregeneration by I. This is believed to be due to the complexmultielectron mechanism of the I/I3

    redox couple involvingthe radicals diiodide (I2

    ) and atomic iodine (I).2 The eectis that the dye HOMO level must be about 0.5 V more positivethan the I/I3

    redox level. Since the voltage output of the cell

    is the dierence between the redox level and the quasi-Fermilevel in the semiconductor, this 0.5 V is lost. An alternativeredox electrolyte that could regenerate the sensitizer from apotential closer to its HOMO would result in higher cellvoltage and eciency, if all else were held equal. Unfortunately,alternatives (such as iron or cobalt complexes) tend torecombine rapidly with conduction-band electrons, reducingthe quasi-Fermi level and resulting in no benet.There have been important recent advances in using

    specialized dyes to slow recombination with electrolytesbased on ferrocene or cobalt bipyridyl complexes.35 Ourapproach is to utilize novel semiconductor structures to attainfast electron transport in order to improve charge collectioneciency. The combination of these approaches will maximizesolar cell performance. In particular, by providing sometolerance of accelerated recombination, structures for fastelectron transport enable the use of standard dyes such asN719, which are more facile products than the specialized dyes

    Received: May 12, 2012Revised: September 22, 2012Published: October 2, 2012

    Article

    pubs.acs.org/JPCC

    2012 American Chemical Society 23864 dx.doi.org/10.1021/jp304622d | J. Phys. Chem. C 2012, 116, 2386423870

  • mentioned above, which tend to be large and dicult tosynthesize.Electron transport in DSSCs is slow, due to trapping of the

    electrons in the grain boundaries and the relatively long andtortuous path of the electron to the uorine-doped tin oxide(FTO).6,7 In a 10 m thick lm an electron visits about 106

    nanoparticles on average before reaching the FTO surface.8

    The resulting slow transport is adequate in the presence ofiodide/triiodide, since its recombination kinetics are slow, butinadequate when using an alternative redox couple with fasterrecombination.Fast electron transport can be achieved by developing 1-D

    nanostructures like nanotubes, nanorods, and nanowires ofmetal oxides. ZnO is a highly favorable material for applicationin DSSCs, since it can be grown in monocrystalline nanorodsusing facile methods, its electron mobility is high,9 and its bandedge energies are very close to those of TiO2.

    10 ZnO is themetal oxide with the second highest eciency achieved inDSSCs.1113 ZnO nanorods can be synthesized on varioussubstrates in situ by procedures such as chemical bathdeposition,14,15 electrodeposition,1618 and chemical vapordeposition,19,20 and in DSSCs they can transport electronsmore than an order of magnitude faster than a TiO2nanoparticulate lm.21 However, the main disadvantage ofnanorods is their lower surface area than a nanoparticle lm foradsorption of light-harvesting molecules. In addition, dyespartially dissolve the ZnO, forming a deleterious Zn2+dyesurface complex.22

    Previous eorts to circumvent these shortcomings havetended to either boost the surface area of the ZnO, e.g. byadding ZnO nanoparticles23 or secondary nanorods,24 or toimprove dye performance via, for example, a protectiveconformal TiO2 encapsulation of ZnO.

    25 To create a structurewith both fast electron transport and high surface area, wedeveloped a hybrid photoanode consisting of ZnO nanorodscoated with TiO2 nanoparticles using facile wet-chemicalmethods. ZnO nanorods were grown by chemical bathdeposition (CBD), a low-temperature catalyst-free processsuitable for industrial applications. TiO2 nanoparticles werecoated uniformly over the nanorods by electrostatic layer-by-layer (LbL) deposition. The LbL technique is based onexposing a substrate sequentially to cationic and anionicsubstances, which form successive bilayers by electrostaticattraction. The TiO2 nanoparticles greatly increase the totalsurface area and provide a superior surface for dye attachmentcompared to ZnO, while the nanorods can provide fasttransport of electrons from TiO2 to the conducting substrate.In addition, the structure includes wide open channels that arehelpful for mass transport of redox species and for lling withsolid hole-transporting materials in solid-state DSSC devices.Despite the simplicity and attractiveness of this structure andfabrication method, it has not been done previously, althoughWang et al. added a thin layer of 5-nm TiO2 particles to ZnOnanorods by sputtering.26

    In this work, we compared hybrid lms to ZnO nanorod-only lms and TiO2 nanoparticle-only lms in terms of dyeloading and device performance. For the latter measure wecompared DSSC devices using two dierent redox couples:iodide/triiodide (I/I3

    ) and ferrocene/ferrocenium (Fc/Fc+).We have chosen the latter as a model alternative redox couplewith fast recombination kinetics. Our tests with Fc/Fc+ showthat these hybrid photoanodes can collect more injected

    electrons than a conventional TiO2 nanoparticle lm withequivalent dye loading.

    EXPERIMENTAL METHODSReagents and Materials. Except where noted, all

    chemicals were purchased from Sigma-Aldrich and were ACSgrade or better. N719 was purchased from Dyesol. SnO2:F glass(FTO, transmission >80% in the visible spectrum; sheetresistance 8 /) was purchased from Hartford Glass Co.Electrode Fabrication. ZnO nanorods were synthesized by

    a two-step chemical bath deposition (CBD) technique.27 Thenanorods were optimized for subsequent TiO2 deposition usinga modied seed layer technique that will be the subject of afuture publication. Slides of borosilicate glass with a conductingSnO2:F (FTO) layer were cut into 25 50 mm pieces andcleaned by 10 min sonication in a detergent solution (5% RBS-25 in water) followed by ethanol. The clean FTO substrateswere coated with the seed layer and then heated at 350 C on atitanium hot plate for 30 min. CBD was followed by immersionin an aqueous solution of 50 mM zinc nitrate hexahydrate, 50mM hexamethylenetetramine, and 6 mM polyethyleneimine.The seeded substrates were placed at an angle of 60 fromhorizontal in a 100-mL glass bottle with the seeds facing thebottom of the bottle and held in an oven at 90 C for 24 h.TiO2 nanoparticles were synthesized by hydrolysis of

    titanium tetraisopropoxide as previously described.28,29 Electro-static layer-by-layer (LbL) deposition was accomplished byimmersing a substrate alternately in a cationic 1 g/L polymersolution (PDAC, polydiallyldimethyl ammonium chloride, 70kDa) in water and

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