www.advmat.de REVIEW ZnO Nanostructures for Dye-Sensitized Solar Cells

By Qifeng Zhang,* Christopher S. Dandeneau, Xiaoyuan Zhou, and Guozhong Cao*

crystalline silicon semiconductor can reach This Review focuses on recent developments in the use of ZnO 92% of the theoretical attainable energy nanostructures for dye-sensitized solar cell (DSC) applications. It is shown conversion, with 20% conversion efficiency in commercial designs. However, because that carefully designed and fabricated nanostructured ZnO films are of the considerably high material costs, advantageous for use as a DSC photoelectrode as they offer larger surface thin-film solar cells have been developed to areas than bulk film material, direct electron pathways, or effective address the product costs.[8–10] Amorphous light-scattering centers, and, when combined with TiO2, produce a core–shell silicon (a-Si) is a candidate for thin-film structure that reduces the combination rate. The limitations of ZnO-based solar cells because its defect energy level can be controlled by hydrogenation and the DSCs are also discussed and several possible methods are proposed so as to band gap can be reduced so that the expand the knowledge of ZnO to TiO2, motivating further improvement in the light-absorption efficiency is much higher power-conversion efficiency of DSCs. than crystalline silicon.[11,12] The problem is that amorphous silicon tends to be unstable and can lose up to 50% of its efficiency within the first hundred hours. Today, commercial roof products 1. Introduction are available that operate at 15% efficiency. Bridging the gap between single-crystalline silicon and amorphous silicon is the The increasing demand for fossil fuels and the environmental polycrystalline-silicon film, for which a conversion efficiency of impact of their use are continuing to exert pressure on an around 18% is obtained.[13–15] Compound semiconductors, such already-stretched world energy infrastructure. Significant pro- as gallium arsenide (GaAs), cadmium telluride (CdTe), and gress has been made in the development of renewable-energy copper indium gallium selenide (CIGS), receive much attention technologies, such as solar cells, fuel cells, and biofuels. However, because they present direct energy gaps, can be doped to either although these alternative energy sources have been marginalized p-type or n-type, have band gaps matching the solar spectrum, in the past, it is expected that new technology could make them and have high optical absorbance.[16–18] These devices have more practical and price competitive with fossil fuels, thus demonstrated single-junction conversion efficiencies of enabling an eventual transition away from fossil fuels as our 16–32%.[19] Although those photovoltaic devices built on silicon primary energy sources. Solar energy is considered to be the or compound semiconductors have been achieving high ultimate solution to the energy and environmental challenge as a efficiency for practical use, they still require major breakthroughs carbon-neutral energy source. to meet the long-term goal of very-low cost (US$0.40 kWh1).[1] The conversion from solar energy to electricity is fulfilled by To aim at further lowering the production costs, dye-sensitized solar-cell devices based on the photovoltaic effect. Many solar cells (DSCs) based on oxide semiconductors and organic photovoltaic devices have already been developed over the past dyes or metallorganic-complex dyes have recently emerged as [1–3] five decades. However, wide-spread use is still limited by two promising approach to efficient solar-energy conversion. The [4–6] significant challenges, namely conversion efficiency and cost. DSCs are a photoelectrochemical system, which incorporate a One of the traditional photovoltaic devices is the single-crystalline porous-structured oxide film with adsorbed dye molecules as the silicon solar cell, which was invented more than 50 years ago and photosensitized anode. A platinized fluorine-doped tin oxide [7] currently makes up 94% of the market. Single-crystalline silicon (FTO) glass acts as the counter electrode (i.e., cathode), and a solar cells operate on the principle of p–n junctions formed by liquid electrolyte that traditionally contains I /I3 redox couples joining p-type and n-type semiconductors. The electrons and serves as a conductor to electrically connect the two electro- holes are photogenerated at the interface of p–n junctions, des.[20–25] During operation, photons captured by the dye separated by the electrical field across the p–n junction, and monolayer create excitons that are rapidly split at the nanocrys- collected through external circuits. In principle, the single- tallite surface of the oxide film. Electrons are injected into the oxide film and holes are released by the redox couples in the liquid [*] Dr. Q. F. Zhang, C. S. Dandeneau, Dr. X. Y. Zhou, Prof. G. Z. Cao electrolyte (Fig. 1a). Compared with the conventional single- Department of Materials Science and Engineering crystal silicon-based or compound-semiconductor thin-film solar University of Washington, Seattle, WA 98195 (USA) cells, DSCs are thought to be advantageous as a photovoltaic E-mail: [email protected]; [email protected] device possessing both practicable high efficiency and cost DOI: 10.1002/adma.200803827 effectiveness. To date, the most successful DSC was obtained on

Adv. Mater. 2009, 21, 4087–4108 ß 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 4087 4088 REVIEW TiO ( eo ope() h atr()i omlyblee ob h rdmnn osmcaim lcrntapn ntenncytlie 5 sas a also is (5) nanocrystallites the in trapping Electron of Elsevier. orbital mechanism. o molecular 2004, (2) occupied loss Copyright highest cation predominant the [30]. dye and the from the orbital and permission molecular be electrons unoccupied with injected to lowest Reproduced the the believed respectively. represent between HOMO place normally dye, and take the LUMO is may loss. recombination (4) energy a causes latter and that ‘‘ mechanism (3), The as electrolyte (denoted (4). the band conduction couple in the couple redox to redox dye a the from from transfer injected electron are electrons photoexcited which in nrylvlo h y)alwfrteefcieijcinof injection effective the semiconductor. for the to allow molecules dye) dye the the from electrons of level energy dopin enhl,tesial eaieeeg eesat the of levels TiO position of energy edge the dye-molecule conduction-band (i.e., relative for interface available suitable semiconductor–sensitizer area the the surface Meanwhile, large the adsorption. to due DSCs oosntr fnncytlieTiO nanocrystalline of nature porous eie rmtelc fadpeinlyro h TiO the on layer depletion a of lack the from derived .–.%otie o n r uhlwrta hto 1 for for 11% of of material that than efficiencies ZnO lower much conversion TiO of are the ZnO use for Although obtained the 0.4–5.8% DSCs. on in with reported studies Many application transport, been DSCs. electron in already used for have when favorable loss recombination be reduced would that mobility eins st bobfrmr fteicdn light. incident the of more far near-infrared absorb the to into as extended so was region dye complex the of response ovrinefiinyo 04 a civdo a on achieved was 10.4% of TiO efficiency conversion ewe lcrn n ihrteoiie y oeue or charge- molecules the dye during electrolyte process. oxidized transport the the in either species recombination conversion electron-accepting to and due in loss electrons energy increase between by limited further been of has cost a efficiency compound high or However, silicon the on challenge semiconcuctors. based to cells DSCs solar available of commercially ability the in confidence erdcdwt emsinfo 2] oyih 01 amla aaie t.b E mg fa xd (TiO oxide an of image SEM b) Ltd. Magazines Macmillan 2001, Copyright [20]. from permission with Reproduced 1. Figure of variety wide a electro- in photocatalysis. for or produced properties optics, unique nics, be These presenting to growth. thus anisotropic nanostructures, ZnO and crystallization allow of properties ease its to due properties TiO physical of and those To structure to increased. energy-band similar is an been that semiconductor has possesses film wide-band-gap ZnO a on is photoelectrode ZnO based extensively. technology explored DSC the issue, this of understand when thickness serious significantly the becomes and surface, nanocrystallite oyyiiecmlxde sfis eotdb ’ea and O’Regan by reported first as Gra dye, complex polypyridine 0n ndaee) erdcdwt emsinfo 2] oyih 07 h oa oit.c lcrntasoti aorsaln xd electro oxide nanocrystalline in transport Electron c) Society. Royal The 2007, Copyright [29]. from permission with Reproduced diameter). in nm 20 h civmn facpal ovrinefiinypt great puts efficiency conversion acceptable of achievement The ¨ zli 1991. in tzel 2 2 2 n ssiltogto sadsigihdatraiet TiO to alternative distinguished a as of thought still is ZnO , –RuL’(NCS) aorsaln l Fg b obndwt ruthenium– a with combined 1b) (Fig. film nanocrystalline y-estzdslrcl ae na lcrceia ytm )Shmtco h osrcinadoeainlpicpeo h device. the of principle operational and construction the of Schematic a) system. electrochemical an on based cell solar dye-sensitized A 3 [26] [31–33] ‘bakde’ ytm nwihtespectral the which in system, dye’’) (‘‘black olwn hsie,acrie overall certified a idea, this Following uharcmiaini predominately is recombination a Such 2 Tbe1,bthshge electronic higher has but 1), (Table 2 [34–41] en oe hnteexcited-state the than lower being npriua,rcn studies recent particular, In 2 lsdie hi s in use their drives films ß 09WLYVHVra mH&C.Ka,Weinheim KGaA, Co. & GmbH Verlag WILEY-VCH 2009 [20,27–29] [30] The 2 2 aotutrswt ag pcfi ufc ra tmyalso the may It provides area. all, surface of specific first large This, a scale. with nanometer nanostructures the on units ptedvlpeto Ssta r soitdwt TiO with associated are that photoelectro- DSCs speed of would of turn, development understanding in the This, up better conversion. energy new a based many chemically to delivered leading have concepts, DSCs ZnO-nanostructure-based on atre,spraaios n yrgnsoaematerials. hydrogen-storage lithium-ion and cells, supercapacitors, solar batteries, energy including for storage, nanomaterials and on conversion mainly focused is research gas and (FETs), sensors. transistors field-effect (LEDs), diodes n ftedfiigfaue fnnsrcue sterbasic their is nanostructures of features defining the of One c.b ’)o h aorsalt 1,tedei eeeae by regenerated is dye the (1), nanocrystallite the of .’’) aeil’.Hscurrent His materials’’. Nano- and including ‘‘Nanostructures books 4 edited and authored and papers, 200 refereed over published has He Washington. of University the at Engineering Mechanical and Chemical of Professor Adjunct and Engineering and Science Materials of Professor Steiner Cao Guozhong light-emitting UV cells, solar including devices electrical on materials nanostructured of applications engineering involve interests research His Professor-Temp. Acting Assistant an as Washington of University in is working He currently 2001. in (China) Uni- versity Peking from degree D. Zhang Qifeng 2 lcrd l ihnanocrystallites with film electrode ) d.Mater. Adv. eevdhsPh. his received sBoeing- is 2009 www.advmat.de , 21, 4087–4108 2 . des, r REVIEW 4089 is is (1) (2) max I ,can SC I h [47,48] [67,68] Reference and For use as C. For DSC max 8 V [69–71] ), and 4 2 Cand400 –10 8 8 ) in a mixed solvent with M 10 2 The resulting sol contains ZnO ), tzel-type solar-cell technology, in 2 ¨ (0.03 2 [64–66] is the open-circuit voltage (mV), (nanoparticulate film) OC ) is defined as 0.5 (bulk TiO -based Gra V )andI . It can be somewhat regarded as a straight 2 2 M FF SC ), I 2 nanoparticles are replaced by ZnO nanoparticles SC max I 2 I in OC P V is the input power density (i.e., the intensity of incident OC in max V P V FF ) behavior when the cell is irradiated under AM 1.5-type ¼ V ¼ – 4 I nanocrystalline TiO simulated sunlight. The overall power conversion efficiency, be calculated according to which the TiO FF where h and the fill factor ( implantation of TiO while the dye sensitizer and the electrolyte remain2.1.1. unchanged. Sol–Gel Derived Nanocrystalline Films A traditionalachieved method by the of preparation of ZnOhomogeneous synthesizing sols ethanolic in ZnO the solutions liquidhydroxide phase nanoparticles and from with zinc is acetate. precursors of lithium applications, the film is sensitizedpolypyridine-complex dyes by (so-called N3, immersion N719, or in black dye ruthenium– commercially available) that for is a given amount of time. nanoparticles with an average diameter ranging from ten totens several of nanometers. A furtheryield process film by is spin coating performed or on dip coating, the duringconverted which sol from the system liquid to is sol into solidheat wet treatment, gel. the Following drying gel and isstructured film thermalized so on the as substrate. to The generate doctor-blade method a porous light, in mW cm working electrode, the as-sensitized filmdevice by assembling is it with constructed a counter to electrode consistingplatinum of a a layer thin cell deposited oncontaining FTO KI glass. (0.5 An electrolyte solution ethylene carbonate and acetonitrile (60%:40% byintroduced to the volume) interval is of the then two electrodes. Thethe performance of solar cell( is characterized by recording the current–voltage also a frequently adoptednanocrystalline film, approach during which for Triton X-100 (1% the byconventionally volume) preparation added is of to ZnO theResidual sol organics are to removed facilitatetypically by the at a film temperatures subsequent formation. heat between treatment, 300 the short-circuit current density (mA cm 10 [56] ZnO TiO . 2 physical or The produc- (nanoparticulate film) nanotubes, [54] 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim [34,38] or electrochemical [54,55] ß [60] The abundant forms of low-temperature aqueous [63] Those nanostructural forms of and nanotips. [55,57] ] 5.2 (bulk ZnO), 1.7 1 [49–51] via a core–shell structure for reduced s 2 [54,58] 2 ] 205–300 (bulk ZnO), 1000 (single nanowire) 0.1–4 [42,43,45] ] 0.26 9 [44] 1 e nanowires (or nanorods), In this article, recent developments in ZnO m Vs are presented to motivate further improvement 4087–4108 2 2 21, nanosheets, chemical bath deposition, [52,53] hydrothermal/solvothermal growth, , [58,61,62] [57] [53] 2009 A comparison of physical properties of ZnO and TiO [56,59] in the conversion efficiency of DSCs. Refractive indexElectron effective mass [ 2.0 2.5 [46] 2. Nanostructures Offering LargeSurface Specific Area A large internal surfacephotoelectrode area film is in the DSCs, so foremost thatbe requirement sufficient of adsorbed dye molecules the to can actphotons. as Nanomaterials an can satisfy ‘‘antenna’’formation this for requirement the of due capture a tospecific of the surface porous incident area may interconnected bewhen network increased compared by with in more bulk than which materials. 1000 times the nanobelts, result in many particular behaviorspropagation in in electron view transport of or theeffect, light or surface photon effect, localization. quantum-confinement Table 1. www.advmat.de Crystal structureEnergy band gap [eV]Electron mobility [cm rocksalt, zinc blende, and wurtzite 3.2–3.3 rutile, anatase, and brookite [42,43] 3.0–3.2 [42–44] ZnO developed during thenanoparticles, past several decades mainly include Adv. Mater. Relative dielectric constantElectron diffusion coefficient [cm 8.5 170 [44] chemical vapor deposition, ZnO films with nanoparticles forextensively studied, application partially in due to DSCs the have directstructures been availability of with porous assembledsynthesis nanoparticles of nanoparticles and via chemically the basedDue solution methods. simplicity to of the mechanismsemiconductor, the of strategy charge for transfer studying DSCslate between with the nanoparticu- dye ZnO and films is almost the same as that adopted for 2.1. ZnO Nanoparticulate Films ZnO nanostructures provideobtain a great highcontribute deal to dye surface-area-to-volume of adsorption, as opportunities well ratios, as light to harvesting, in which DSCs. in turn growth, combination rates. The limitationsdiscussed. of In ZnO-based DSCs the are finalconcepts also section, to several TiO attempts to expand ZnO nanostructures, particularlyreviewed. for It willtructured application show ZnO can that in significantlyby photoelectrode enhance offering a solar-cell large DSCs, films surface performance area for withpathways dye are adsorption, nanos- direct for transport photoexcitedcenters for enhanced electrons, light-harvesting efficiency. In addition, andmay ZnO be combined efficient with TiO scattering deposition. tion ofsynthesis, these structures can be achieved through sol–gel 4090 REVIEW Ssaefil o,wt ausgnrlyaround generally values with low, fairly 0.4–2.22%. output, are of efficiencies power conversion DSCs reported maximum the films, at ZnO-nanoparticle current and voltage respectively. the are uaeZOfim arctdb sn neetottcspray electrostatic coating an for al. et using Jansen by by developed nanoparti- technique (ESD) for fabricated deposition obtained films also be culate-ZnO can conversion efficient Highly Nanoparticulate for Technique Films Deposition Spray Electrostatic 2.1.2. of harvest. sizes light with the to light nanoparticles contribution the additional ZnO to promoting that reason likely significant is a it as harvesting, addressed effect light-scattering not the more is although a ZnO work, the this in to molecules In result dye semiconductor. the could from electrons kinetics of injection interfacial efficient that increased method such demonstrated been compression that had It the process. heat-treatment in a involve didn’t inherent increased the kinetics to due interfacial was performance solar-cell in improvement h bandfimwsue naDCudrilmnto ihan with illumination under cm DSC mW 10 a of in intensity used was film obtained the ovrinefiinisatie o n lswt 150-nm doctor-blade with aforementioned compression, films without the method ZnO using for prepared nanoparticles attained efficiencies conversion h aoatcepwe ne rsueo 00k cm compressing kg by 1000 of prepared pressure a was under powder film nanoparticle the the particular, in and, route, of size average s5 a achieved. was 5% as arcto oaheeahgl cieZOsraefrdye for surface ZnO active highly a achieve adsorption. the film to the for method and compression fabrication a reported al. et technique, Keis DSCs, ZnO film-fabrication the the photoelectrode. nanoparticles, many the film, the of to of post-treatment sensitive of shape and is size porosity performance the solar-cell including factors, the that implies opeso ehd erdcdwt emsinfo 7] Copyright [73]. from Elsevier. permission 2002, with Reproduced method. compression 2. Figure 5 mpa oei h eeaino ih cteig leading scattering, light of generation the in role a play nm 150 tteerydvlpetlsae sn sol–gel-derived using stage, developmental early the At ihrgr oteipoeeto ovrinefiinyin efficiency conversion of improvement the to regard With E mg faZOnnpril lcrd rprdwt a with prepared electrode ZnO-nanoparticle a of image SEM [66,67,72] [73,74] nti rcs,ZOnnprilswt an with nanoparticles ZnO process, this In 5 m(i.2 eesnhszdvaasol–gel a via synthesized were 2) (Fig. nm 150 h ctee itiuino efficiencies of distribution scattered The [73–75] 2 noealcneso fcec shigh as efficiency conversion overall an , oprn hsrsl ihte2–2.1% the with result this Comparing [76,77] h uhr xlie htthe that explained authors the ß 09WLYVHVra mH&C.Ka,Weinheim KGaA, Co. & GmbH Verlag WILEY-VCH 2009 2 When . h pcfi ufc rao h lscnb otoldby controlled surface specific been be maximum m has a 12.55 yield can It of may PVA. 1.8 area films of to ratio nanoparticles the a that of reported of ratio weight area the and adjusting surface porosity the specific both fabrication, the high-temperature this the to With employed during process. was ZnO calcination PVA of film. agglomeration ZnO the of subsequently prevent deposition nanoparticle- was ESD The the obtained was for water. thus used that and solution (PVA) ethanol precursor of alcohol containing mixture polyvinyl a with in mixed dissolved an then of presence and the in amine salts zinc of hydrolysis the through prepared h S technique, ESD the charged the substrate. directs the field to electric droplets of high the the variety since (3) efficiency, and the great coatings, deposition controlling deposited the of the of (1) possibility composition the chemical are (2) technique morphologies, film ESD tailored the of advantages fteiogncpouti eto h usrt surface. substrate the on left is product inorganic their the consisting lose layer of thin they a evaporation, where solvent substrate, complete the After heated charge. Consequently, the difference. on potential impinge applied droplets the of substrate heated result and a grounded as the generated by The attracted this droplets. are charged droplets highly substrate, spray of and (typically spray shape grounded a conical applied form a to into a is out transforms fans nozzle voltage and the of high tip nozzle the a at droplet the if but between at nozzle, formed kV) 6–15 the is droplet of pumped spherical tip a is the Usually liquid nozzle. metal precursor a aerosol the an through liquid such droplets, or obtain to particles micrometer-sized order solid In of of ambient. gaseous dispersion a a or in as droplets inorganic voltage. defined high is containing a aerosol of solvents An influence the organic under using precursors organometallic aerosol an of rud5%. around eety hstcnqewsas xlie ntesnhssof synthesis the in exploited also TiO was technique this Recently, opooy n hcns ftefim a emodestly be can films orientation, the growth of the that thickness is and method this morphology, area. of surface advantage large a The the possessing for films route nanoporous low-cost of and preparation simple a is deposition Electrochemical Fabrication Deposition Electrochemical the resistance 2.2.1. series cell. between the the of both intervals rate in recombination the decrease and a through in resulting diffusion nanowalls, of favorable electrolyte point also, and, the DSCs for from in electrode transport collection is electron the to structure for generation a favorable Such be substrate. to the thought on with grown films vertically nanoporous nanowalls show forming deposition to advantageous bath characteristics chemical and deposition electrochemical fnnpru n films. ZnO high nanoporous their preparation to the of for due reported DSCs been have in techniques Many photoelectrodes porosity. as studied also films ZnO were structured nanoporous films, nanoparticulate Besides Films ZnO Structured Nanoporous 2.2. lcrd films. electrode sfratpclpeaaino aoatclt n lswith films ZnO nanoparticulate of preparation typical a for As 2 aobr,aheiga vrl ovrinefiinyof efficiency conversion overall an achieving nanofibers, [81,82] [78,79] 2 g 1 h ai rnil fEDi h generation the is ESD of principle basic The [80] n ekpoooti fcec f3.4%. of efficiency photovoltaic peak a and olia ouino rcro was precursor of solution colloidal a [61,62,83,84] d.Mater. Adv. mn hs methods, these Among 2009 www.advmat.de , 21, 4087–4108 [78] The REVIEW 4091 . C 2 8 /dye þ O, by 2 2 2H mol cm 10%) sensi- 7 2 illumination, obtained for a ¼ m-thick nano- 2 10 h 2 m Films synthesized COO) 3 [88] . The DSCs with ZnO 1 (CH 12 nm, and surface areas Further improvement to 8 mol cm C through heterogeneous 8 7 [88] photoanode ( (OH) 10 5 2 A typical process for this fabrication However, the excellence of the solar-cell systematically studied the DSC perfor- [86,87] Studies in greater detail pointed out that m-thick ZnO films were sensitized by N719 [90] m [90,91] [89] . In another study, a high photovoltaic efficiency –1 [88,89,92] g [73] 2 /dye complex. This complex is believed to be inactive and þ 2 The enhancement in solar-cell performance was attributed m-thick nanocrystalline TiO m-thick ZnO film prepared via CBD was used for the DSC and [92] m m Hosono et al. 11 nm ZnO crystallites, pore sizes of reduced to 20–40 nm. However, nothe ZnO concentration film was could larger be than formed if 6 g L may hinder electronsemiconductor. injection from the dye molecules to the so as to transform themethod LBZA was film also into reported crystalline for ZnO.films via the A layered hydroxide formation similar zinc carbonate of (LHZC) using nanoporous aof ZnO solution zinc nitrate hexahydrate and urea in water. nucleation, and (3) a heat treatment at a temperature above 150 mance of nanoporous structured ZnO films fabricated bytechnique. the They CBD achieved an overall conversionwhen efficiency as-prepared of 10- 3.9% dye with an immersion time of 2 h. performance was ascribed to the remarkablyas-fabricated improved ZnO stability of films in acidic20- dye. According to thisthe report, film a was sensitized withfor N719. The a results showed that, prolongeddye even dye-loading molecules adsorbed time on (6 the h), ZnO only surface a and no monolayer Zn of through this method showstructure (Fig. a 3b). nest-like morphology and porous films synthesized byconversion efficiency this of 5.08% method under 53 exhibit mW cm an extremely high can be described in the following steps: (1)basic the formation of layered zinc acetate (LBZA), Zn This was comparable to the 1.3 complex was formed. This scenario isof very different from ZnO the case filmselsewhere and has sensitized resulted in withefficiency. a It relatively ruthenium-based high was overall dye conversion alsoloading reported amount demonstrated was that, estimated for to be this about cell, 1.4 the dye- to the use of D149 dyeperpendicular pores. and This allowed a for nanoporous a structure rapid that adsorptionwith contained of a the dye shorter immersion time andof a thus Zn prevented the formation 10- films fabricated by CBD were composed of sheet-like grains with of about 25 m of up toproduced by 4.1% the CBD. was also obtained for nanoporous ZnO films hydrolysis of zinc acetate dihydrate in methanol, (2)of the the deposition LBZA film on a substrate at 60 tized with N3 dye. In this study, although approximately equal dye 4.27% in theHosono et al. conversion when the dye efficiency oforganic N719 dye was was named replaced D149 with reported and a the metal-free 1h. immersion recently time was by reduced to while the photoelectrode consists ofdouble-layer electrochemically ZnO deposited films,porous typically film containing on 8- a 200-nm-thick compact nanocrystalline film. 2.2.2. Chemical Bath Deposition Fabrication Chemical bath deposition (CBD), developed for producing ceramic films from aqueous solutions atlayered low metal temperatures hydroxides, by has also pyrolysis been of usednanoporous for ZnO the films. fabrication of It [85] In this m at 0.7 V [61] m zinc nitrate and 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim M ß , the grain size was m can be obtained at 1 m m at 0.7 V for 20 min, and 3.4 m 4087–4108 21, m (Fig. 3a). Other applied voltages produced films , m C in a water bath with a bias voltage of 0.5–0.7 V. It was 2009 8 SEM images of nanoporous ZnO films. a) Electrochemically hexamethylenteramine (HMT) in a solvent formed by a M C would result in a ZnO film with a compact structure, 8 In electrochemical deposition, the nucleation rate can demonstrated that, in the casethe of obtained an films applied presented voltage a ofhigh sheet-like 0.5 V morphology only, scanning and, under electronfilms microscopy were (SEM) observedparticles magnification, to of the comprise aboutThese of particles 250 nm were interconnected pellet-like with in pore polycrystalline 500 sizes diameter nm ranging from to (20–30 2 nm grain size). mixture of distilled water and ethanol withHMT a volume was ratio of employed 1 to 1. pH in value the of electrolyte aroundacted as 6. as a The a wetting additional agent buffersurface to ethanol coverage. to improve The in the deposition obtain the of thickness porous uniformity electrolyte a ZnOout and films at was 40 carried Adv. Mater. Figure 3. deposited film without using[61]. reagent. Reproduced with Copyright permissionbath-deposited from nest-like 2008, ZnO[89]. The Copyright film. 2003, Reproduced Elsevier. Electrochemical with permission Society. from b) Chemical- controlled by adjustingvoltage, the current density, deposition temperature, parametersprocess etc.). for A (deposition typical the fabrication electrochemicalfilms deposition at of low nanoporous temperature ZnO was described by Xi et al. www.advmat.de fabrication, a two-electrode systemsheet was as used, the anode consisting and of glass coatedthe a with Zn indium cathode. tin oxide The (ITO) as electrolyte contained 0.04 0.04 for 30 min. Theinfluence temperature on of the the film70 water structure. bath Increasing also theshowing temperature a shows much to an lowerfilm due photovoltaic to efficiency the than decreased the internal porous surface area. significantly affect the filmporosity. structure, especially Through innucleation, terms the ZnO of films the use withdeveloped a of for DSC more application. Poly porous a (vinyl pyrrolidone) structurereported (PVP) as was have surfactant a been highly to efficientwith surfactant the mediate electrochemical that deposition had method been the ous to combined ZnO produce films nanopor- with very impressive photovoltaic efficiencies. with a morethickness irregular is correlated porous to structure. both thetime. The bias Typically, voltage as-deposited a and the thickness film deposition of about 2.6 0.5 V for 30 min, 2.4 was demonstrated that as-obtainedcomprised of films interconnected possessed crystal athus grains. structure the The grain surface size,influenced morphology and by of the ZnOconcentration films, PVP would could concentration. result be Thatwhen in strongly is, the a increased PVP reduced PVP concentration grain was size. 4 g Typically, L 4092 REVIEW odn sotie o n lswt obe thickness doubled with films ZnO for obtained ( is loading nenlsraeae,rsligi ovrinefiinyo pto up in of enhancement efficiency conversion significant 2.61%. a a in have resulting area, may surface agent internal capping the as oxalic nanosheet- an using acid treatment ZnO hydrothermal to by that prepared due 4b) seems (Fig. possibly spheres It 1.55%, area. surface efficiency, internal conversion possess insufficient to low shown been relatively has DSC a a in used 4a) (Fig. nanosheets ZnO hrfr,lwrssac ihrgrst lcrntransport. electron to regards and, with crystallinity of resistance degree high low a therefore, from benefit to believed also is cell arctdb eyrtemlgot rcs fpreviously of process growth rehydrothermal nanoparticles. ZnO a grown hydrothermally by be fabricated can that quasi-two-dimensional structures are nanosheets ZnO Nanosheets and/or 2.3.1. transport electron the of propagation. terms light the in by properties provides affected which Solar-cell particular films, significantly photoelectrode DSC. be the these of to the structure geometrical believed for of also However, efficiency is performance area. photovoltaic that they factor surface the only that the not determines specific fact is as area been the surface large specific the of nanostructures, a also such account have on have morphologies, applications also nanotetrapods, DSC for other and studied nanobelts, with nanosheets, nanostructures ZnO Films ZnO Nanostructured Other 2.3. in ZnO as-prepared of stability unusual dyes. the ruthenium-based of view in exciting oyih 08 mrcnIsiueo hsc.e ewre l ihitronce ZnO interconnected with Elsevier. film 2009, Copyright Networked [97]. e) from with Physics. permission with Reproduced of [96]. Reproduced from Institute array. tetrapods. permission with American Nanobelt Reproduced core. 2008, common three-dimensional c) a a Copyright from in Elsevier. extending formed arms tetrapod four ZnO 2008, with A structure d) Elsevier. Copyright 2007, Copyright [94]. [95]. Reproduced from permission from spheres. Nanosheet-assembled permission b) Elsevier. with 2007, Copyright [93]. from permission 4. Figure 20 m [94] )i oprsnto comparison in m) E mgso aotutrdZOfim:a ipre aohes erdcdwith Reproduced nanosheets. Dispersed a) films: ZnO nanostructured of images SEM sfrnnsetshrs h efrac ftesolar the of performance the nanosheet-spheres, for As 10 m mofTiO [93] ß 2 l ihdispersed with film A 09WLYVHVra mH&C.Ka,Weinheim KGaA, Co. & GmbH Verlag WILEY-VCH 2009 h eut r very are results the , ovrinefiiniso .0 3.27%. 1.20– overall achieved of have efficiencies DSCs 4e). in conversion (Fig. used area tetrapods surface ZnO with specific films large The a to with as network so other porous each a to form connected tetrapods in result can deposition hcns f5n,atpclsraeae f7 m 70 of area surface nanobelt typical a a with 4c) nm, 5 (Fig. of structure stripe thickness porous obtained highly array a nanobelt ZnO shows The surfactant. a as electrolyte the added in was cetylether polyoxyethylene nanobelts, these fabricating ossigo oram xedn rmacmo core common a structure from three-dimensional extending arms a 4d). four possesses (Fig. of tetrapod consisting ZnO A Tetrapods 2.3.3. 2.6%. as high as efficiency photovoltaic xgnprilpesr uigvprdeposition. vapor during pressure partial oxygen 2 to nm 1–20 100 of range the edn osgicn mrvmn nteslrcl perfor- solar-cell the further in be improvement could significant mance. films, films these to with tetrapod nanoparticles leading ZnO of incorporating area by increased surface internal the enpooe oocrete yasre fhpigevents hopping of series a by either has occur films nanoparticulate to on proposed based been DSCs in transport Electron for Transport Pathways Electron Direct with Nanostructures 3. n lswt aoetary rprdtruha electro- an applications. DSC through for prepared studied also arrays were method nanobelt deposition with films ZnO Nanobelts 2.3.2. [96] [96,97] m ycagn h usrt eprtr and temperature substrate the changing by m h egho h rscnb dutdwithin adjusted be can arms the of length The iainbtenteeetosadthe and electro- the in electrons mediator lyte. redox or the dye oxidized between in recom- increase transport bination also-increased the an to due with Electron photocurrent difficult more deficiencies. becomes possess do oxides nanoparticulate the films, crystalline enahee ndesniie TiO dye-sensitized on achieved been igecytlo us igecytladcan, and crystal single quasi or usually crystal single are nanostructures one-dimensional 10 ewe rpsae nnihoigparticles neighboring on states trap between h ute nraeo ovrineffi- conversion of increase ciency. a further be limits to the and loss found energy causes that been factor crucial has nanoparticles, recombination of form the the in film a with DSCs rmteasneo elto ae tthe at layer depletion a of TiO absence the from lwddw ytrapping/detrapping by down events. states slowed extended within transport diffusive by or h eobnto aei DSCs. in rate reduce recombination to as the so developed been differ- have oxides on ent based nanostructures dimensional lcrd film. electrode m ,wietedaee a etndfrom tuned be can diameter the while m, 3 nteps e er,avreyo one- of variety a years, few past the In –10 2 [100] [101] aoatceeetoyeitrae In interface. nanoparticle–electrolyte 6 [21,30] atce hntaeigi h photo- the in traveling when particles eobnto,itiscly arises intrinsically, Recombination, neeto setmtdt cross to estimated is electron An [99] hl n1%efiinyhas efficiency 11% an While d.Mater. Adv. [96,97] twsrpre that reported was It [98] 2009 www.advmat.de Multiple-layer 2 , 21, g [102] 1 4087–4108 2 n a and , These nano- [95] [99] In REVIEW 4093 e M and 2 )Ata . Conse- 1 [47,112] s for TiO , open-circuit 2 1 1 s V 2 2 cm 4 , the highest-surface-area polyethylenimine (PEI) at 2 zinc nitrate hydrate, 25 m M –10 7 M 10 3mWcm for ZnO. This, to some extent, demonstrates 1 for a single nanowire. This value is several cm, with an electron concentration of and a mobility of 1–5 cm s 1 2 3 s V cm 2 3 cm 18 m with a nanowire diameter that varied from 130 to –10 m 10 5 C for 2.5 h. After this period, the substrates were repeatedly 8 The superiority of ZnO nanowires as a direct pathway for 10 Nanowires were grown byaqueous immersing solutions containing the 25 seeded m substrates in hexamethylenetetramine, and 5–7 m the very good electrical conductivitybe of noted, ZnO however, nanowires. that (Itdiffusion the must coefficient electron reflect diffusivity different andin charge the transport electron individual processes nanowiresMoreover, these and two nanoparticledifferent parameters methods, films, are that usually respectively. is,with obtained by through the employing thediffusivity measured Einstein for equation Hall individualintensity-modulated mobility photocurrent nanowires to experiments andcollection to calculate using, time measure for the the ofelectron example, diffusion electron electron coefficient for diffusion nanoparticle and films. then calculate the devices withshort-circuit ZnO current densities of nanowire 5.3–5.85 mA cm arrays were characterized by hundred times larger than the highestcoefficients reported electron for diffusion nanoparticle films inoperating a conditions, DSC that configuration is, under full sun intensity of 100 quently, the0.05–0.5 cm electron diffusivity could be calculated as 92 introduced to fresh solutiongrowth baths until in the order desired to filmPEI, thickness obtain a was continued cationic reached. Thefabrication, polyelectrolyte, use is as of particularly itnanowires. important serves As in to a this enhance result,possessed the nanowires aspect synthesized anisotropic ratios by35 growth billion in this wires of per excess method square centimeter. The20–25 of longest arrays reached 125 and200 nm. densities These up arrayslarge featured to a as surface a areacross-section up nanoparticle image of to film. an one-fifth arraythat Figure as of the 5b ZnO resistivity nanowires. shows It values0.3 was a of found individual typical to nanowires1–5 SEM ranged from 2.0 voltages of 610–710 mV,conversion efficiencies fill of factors 1.2–1.5%. of 0.36–0.38,electron and overall transport isshort-circuit illustrated current by densities Figure as 5c, a in function which of the the internal [55] [103,104] 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ß and chemical-spray [107,108] 10–15 nm) was deposited on the ¼ chemical-solution synthesis at low [104,105] electrodeposition, 4087–4108 A schematic of the construction of a nanowire 21, In general, the vapor-phase or physical-deposition , [106] [109] 2009 ZnO-nanowire dye-sensitized solar cells. Reproduced with permission from [110]. Copyright 2005, Nature Publishing Group. a) Schematic [110,111] 3–4 nm, film thickness ¼ substrates via dip coating in a concentrated ethanol solution. ZnO nanowire arrays were first used inwith DSCs by the Law et intention al. of inwith 2005 replacing the a traditional nanoparticlelength. consideration film of increasing the electron diffusion 3.1. ZnO Nanowires methods possess an inherent advantage inlong forming both thin one-dimensional and nanostructurescrystal quality with while affording agrowth the ability high to rate. precisely degreecapability control Chemical-solution-based of the for large-scale methodsand, mass therefore, production possess a at flexible the low substrate can temperatures be used. performance of nanowire and nanoparticle cells. diagram of the cell with a photoelectrode comprised of the ZnO-nanowire array. b) Cross-sectional SEM image of the ZnO-nanowire array. c) Comparativ Adv. Mater. Figure 5. metal–organic chemicalthermal vapor evaporation, deposition (MOCVD), therefore, serve toproviding a direct increase pathway for electron the transportelectron from electron the injection point to of diffusion theother substrate length words, of the the by transport collection ofcontinuous electrons electrode. occurs crystal In in and, the interiorsuffer consequently, of any the a grain-boundary electrons scattering.case This would is of not different nanoparticulate fromelectrons, the during films their in traversal,nanoparticles take a a and DSC random walk undergo configuration,boundary among the or where numerous the collisions semiconductor/electrolytethe interface. at oxides Among with all the one-dimensionalthe nanostructures, grain ZnO most has reported been growth due and, especially, to the the developmenttechniques of that ease allow physical of for or a chemical (for controllable flexible tailoring anisotropic example, of ZnO shape,one-dimensional morphology ZnO diameter, nanostructures length,types used of density, for nanowires/nanorods, etc.).derivatives, DSCs nanotubes, synthesized Typical through vapor–liquid–solid nanotips, involve processes, and other www.advmat.de DSC is shownsynthesized in in Figure 5a.process. an This Arrays aqueous method of employedsubstrates solution fluorine-doped ZnO tin that nanowires using oxide (FTO) were sonication. were a A thoroughly seeded-growth thinter cleaned film by of acetone/ethanol ZnO quantum dots (dot diame- temperature, pyrolysis. 4094 REVIEW rjce ufc ra r oprdfrclswt ZnO with cells for compared are TiO area) the nanowires, to area surface surface actual of projected ratio the as (defined factor roughness n ihacrircnetaino 10 of concentration carrier a with ZnO vnwt h taneto ihrcneso efficien- conversion higher of attainment in the DSCs in cies. collection elsewhere nanowires with the ZnO found of to even be application electrons the can the regarding literature investigations sweep the to Similar Moreover, serve electrode. the nanowires. may reduce the injected field to of of axial as interior diffusion the so the towards electrons accelerate be electrons injected (2) the and can to corral rate, thought recombination (1) is field field to electric electric able internal be the internal surface, nanowires inner the in the an bending around band upward that the to Owing so established. Debye length the than larger screening much is dimension nanowires the ZnO that as-discussed important the is of It nm. 4 about is length screening n-hr ftetikeso h pc-hrelayer. space-charge roughly the be of to thickness estimated significant the been which of has over one-third occur, distance screening can the Debye separation as charge the defined is semiconductors, which oxide length, In the around surface. nanowires the in inner layer space-charge a of formation and the interface existence semiconductor–electrolyte the the to at due an diffusion is charge of of nanowires the establishment within The field separating electric electrode. internal collection by the collection towards sweeping and carrier them electrolyte surrounding assist the from can electrons injected that within nanowires field electric the internal the an both and nanowires to of attributed crystallinity been high has mechanism The compared when nanoparticles. transport to confirmation electron a better also a offer is nanowires This at the accessible 5c). that Fig. higher in the shown 55–75% as over 200, (approximately of roughness cells factors roughness nanoparticle of ZnO range the In than current rate. higher densities considerably recombination generate cells decreased nanowire the with addition, nanowires in electrons as thick fcec hncmae oarnol retdnanorod photovoltaic oriented the randomly in a increase to ZnO ten-fold compared film. a a that when to found efficiency led study one array example, in nanorod electron For convincingly demonstrated for research. been pathway has previous This direct of vertically DSCs. the one in as provide with transport addressed that also arrays is aspects substrate nanowire/nanorod important FTO the of on growth feature aligned the ods, etc.). light-source film, method photoelectrode composition, of measuring (pattern, area active the electrolyte array and intensity, and and process, etc.), nanowires (sensitization density, parameters, the length, growth of diameter, treatment, the structure on substrate geometrical specifically method, conditions, experimental synthesis the on dependent hr-ici urn est a eosre nteclsbuilt cells the the on in observed decline TiO 12-nm be subsequent either with can a density and current saturation short-circuit rapid a that shows erylna nraei h hr-ici est htmaps that density short-circuit TiO a the the show onto in directly films almost increase nanowire to linear the due of thickness nearly However, efficiency film certain recombination. transport a above critical the off falls that films nanoparticle confirms This nanoparticles. eie h n-iesoa tutr fnanowires/nanor- of structure one-dimensional the Besides [113–118] [119] oee,teisfcetsraeae fnanowire/ of area surface insufficient the However, 25 oee,i em htterslsaequite are results the that seems it However, m 2 ,idctn ihyefiin rnpr of transport efficient highly a indicating m, aoatce,adZOnnprils h plot The nanoparticles. ZnO and nanoparticles, 2 aoatce r3-mad20n ZnO 200-nm and 30-nm or nanoparticles 2 aaee huhtefim r as are films the though even data ß 18 09WLYVHVra mH&C.Ka,Weinheim KGaA, Co. & GmbH Verlag WILEY-VCH 2009 cm 3 h Debye the , [110] sfor As otemds ogns atro omrilmembranes. commercial of factor roughness modest due primarily the 1.6%, to of efficiency oxide conversion low aluminum relatively it a However, anodic yields (ALD). deposition coating layer atomic by via prepared membranes (AAO) also be can arrays mlydamnu yrxd ocag h supersaturation Zn the of change degree to hydroxide ammonium employed fcec f23 a enrpre o Sswt ZnO with DSCs for 5.4 a reported and nm of 500 been of density diameter nanotube has a possessing arrays 2.3% nanotube of efficiency omdb mesn h n aoatcepwe na in and powder isopropoxide titanium nanoparticle 2% ZnO 0.02 containing the solution immersing methanol per- by pretreatment a formed films, nanowire–nanoparticle-composite of aoatce nonnwr surfaces. nanowire onto nanoparticles rvd atrcnuto aha o lcrntasot This transport. electron for pathway conduction faster nanotips a the provide that respect the in transport electron nontrap-limited hni h aeo TiO of case the in intensities a light than present higher 3.2- nanotips at efficiency ZnO for conversion that overall reported obtained maximum overall been has An was It film. nanotips. 0.55% the ZnO photoelectrode to of the due efficiency nanotips of ZnO conversion area the surface of in length increase the with increased the of cells efficiency studies. energy-conversion the previous that of confirmed in results performance investigated The been DSC has The nanotips synthesized. these be can with arrays lengths nanotip different ZnO methods, processing MOCVD using By Nanotips ZnO 3.3. be of can that of arrays than growth aqueous temperatures. nanotube area the low for at nanowires method ZnO surface ZnO modified a larger of using by a synthesis achieved high offer The possesses a nanotubes may nanowires. have of and typically array they An porosity that structure. in cavity nanowires hollow from differ Nanotubes Nanotubes ZnO 3.2. length). o setrto aot50n ndaee n 10 and diameter in nm 500 (about relatively ratios with arrays aspect nanorod almost ZnO low is for which obtained 1.7%, that of times efficiency three conversion DSC overall an 100–200 to 14 about the of of length ratio regulate a aspect with to The an nanowires. served possessed individual nanowires This the resulting array. of ratio nanowire aspect ZnO length-to-diameter a of growth rm08%t .%we n aoatce ihdaeesof nanowires. diameters ZnO with to added nanoparticles were efficiency ZnO nm a 5–30 when conversion reported 2.2% DSC al. to overall et 0.84% Ku from the area. in surface promotion the significant optimizing in effective to proved be also been have films nanowire–nanoparticle-composite aoie/aoos hsicesn h est fthe the of of size density diameter the the reduce increasing to array. thus problem. is this solve approach nanowires/nanorods, to made such the of been efficiency limits have One conversion attempts that the Many as factor cells. well primary the as adsorption the dye be of to amount seems arrays nanorod M [113,116,117] ctcai a rvdt efvrbefrteatcmn of attachment the for favorable be to proved has acid acetic [120] ybedn h aoie ihnnprils ZnO nanoparticles, with nanowires the blending By 2 þ rcrosdrn h rcs fsolution-based of process the during precursors 10 a ta.rpre ipemto that method simple a reported al. et Gao m 6 n imtro 2–5 m hslead This nm. 120–150 of diameter a and m e qaecentimeter. square per 2 aoatce.Ti mle a implies This nanoparticles. [56,123] [122] d.Mater. Adv. [121] noealconversion overall An sfrtefabrication the for As [124] 2009 www.advmat.de n nanotube ZnO , 21, m 4087–4108 m-long m [126,127] min [125] REVIEW 4095 m thickness displayed an m grew 100-nm-diameter ZnO20-nm nanowires with secondarynucleated nanowire and grew brancheswire from backbone. It the was that indicated primary that eachnanowires nano- of the was crystallinearies with separating secondary grain nanowires from bound- the primary nanowire. Theprimary substrate nanowires with andbranches both secondary was nanowire nanowire then growth, used calledtion’’ growth. ‘‘secondary to During genera- the continuegeneration growth of the of ananowire second nanowires, branches act as the newfor nucleation sites nanowire outstretched growth. Thecontinued growth for can third and also fourth be generations for The dendritic ZnO nanowires are intentionally developed to the attainment of a(Fig. dendrite-like 7). branched ZnO DSCdensity nanostructure increased characterization with increasing showedlarger growth surface generation that area, due which the to indye molecules. the turn A short-circuit led total to improvement increaseddensity of and adsorption over over 250 of 400 times times inthe in current efficiency film has been morphology observedbranched was when second-generation nanowires changed (Fig. 7b). from Theobtained efficiencies smooth using fourth-generation nanowires dendritic nanowire to filmsbranched with a nanostructure andoverall a conversion 10- efficiencyimprovement of over 0.5%,nanoparticles smooth within a the film nanowires. more ofsurface dendritic nanowires, By than area the was specific 600-fold integrating increased,efficiency leading of ZnO to 1.1% for an these improved cells. conversion increase the specific surfacemultiple-generation area growth of the ofphotovoltaic photoelectrode ZnO film efficiency, nanowires. by nanowires However, 0.5%, is the obtained lowerarrays achieved than for by the Lawnanowire density dendritic et 1.5% and al. the value insufficient This ZnO ratio) nanowire is for of length dendritic (or possibly ZnO ZnO aspect due nanowires.difference Another nanowire to in possible the the cause crystallinity lower is of the fabrication nanowires produced by methods, different nanowires that and seed-assisted is,ZnO solution-based nanowire epitaxy MOCVD arrays. growth for for dendritic ZnO Dendritic ZnO nanowires for DSC application. a) Cross-sectional SEM image of a film [127] Figure 7. with dendritic ZnO nanowires (after twocurrent generations of (filled growth). b) Semi-log squares)with plot of insets and short-circuit showing efficiencypermission images from of (open [103]. nanowire Copyright circles) 2005, morphology Elsevier. versus for generations nanowire 0–2. growth Reproduced generation, with 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ß They first [103] , and 0.36 for films of nanorod arrays 2 4087–4108 , and a fill factor of 0.53. These values are higher These as-synthesized nanoflowers, as shown in 2 21, , zinc chloride aqueous solution with a small amount of [128] 2009 M Flower-like ZnO nanostructures. a) A schematic to indicate the difference in light [129] Adv. Mater. Dendritic ZnO nanowires,fractal structure which more complicated possess than thatnanoflowers, a of are formed bybone a with outstretched nanowire branches, on back- whichgrowth the of smaller-sizedand nanowire backbones branches isdescribed a reduplicated. MOCVD fabrication BaxterZnO for nanowires et dendritic by al. usingmultiple-generation a route growth. of so-called 3.5. Dendritic ZnO Nanowires The useupstanding of nanowires ZnO andalso outstretched reported films for branchesconsideration with application have that ‘‘nanoflowers’’ in been thephotons DSCs. consisting completely nanowires due This alone to of isthe the may morphology. existence Nanoflower based structures, of not however, intervals on havebranches inherent nanoscale capture that the in stretch the to fill theseboth intervals and, a therefore, provide largertransport surface along the area channelsnanowire and from backbone the (Fig. a 6). branched Nanoflower direct ‘‘petals’’hydrothermal films to method can pathway the be at grown for low bying temperatures, a electron a 5 typically m by employ- 3.4. ZnO ‘‘Nanoflowers’’ feature allows for themore use stable of and ZnO efficient nanotipsalso DSCs in under been the fabrication high demonstrated of illumination.could be that It increased has to the 0.77% by overalla combining the Ga-doped ZnO conversion nanotips ZnO with efficiency film as a transparent conducting layer. Figure 6. harvest for films withReproduced nanowires with and permission nanoflowers. from b) [128]. Copyright SEM 2007, images American of Institute a of ZnO-nanoflower Physics. film. www.advmat.de Figure 6b, have dimensionssolar-cell of performance about of 200 ZnOized nm by nanoflower in an films overall diameter. conversion was The efficiencyof of character- 5.5 1.9%, mA a cm current density with comparable diameters and array densities that were also fabricatedmethod. by the hydrothermal ammonia. than the 1.0%, 4.5 mA cm 4096 REVIEW g,SrTiO MgO, hudices.I eodapoc,annpru TiO nanoporous a approach, second a In increase. should ihaselo nte ea xd rsl Fg 8). (Fig. salt or oxide metal another of shell a with rnpr hog h TiO the electron through for allowing transport other, each to directly connected are particles st banafilm. a obtain so to substrate as conducting a onto application then and nanoparticles lcrnafiiy ihiolcrcpit n oenegative more and low TiO its point, to of compared view isoelectric when in edge high conduction-band attention most affinity, the electron attracting been has ZnO hnta ftecr eiodco (TiO semiconductor negative core more the be of should that shell than the of potential conduction-band efrac ypoiiga3%ices nteoverall the in increase 35% a efficiency. providing conversion by performance hr aebe eea eot ntefbiaino n shells ZnO TiO of fabrication the the on for reports several been have There Shell of Influence Thickness and Structures Core–Shell of Fabrication 4.1. Ss h n hl rvdsa nrybrira h nefc between interface the at barrier energy an TiO provides the shell ZnO The DSCs. ihteoiie y rrdxmdao nteelectrolyte. the in mediator redox or dye core the oxidized in electrons the of with reaction the hinders which barrier energy oeselsrcue a ecasfidit w ae.One cases. two TiO into structured core–shell classified of synthesis be initial the can involves structures core–shell ehdi h omto fa nrybrirntol tthe at individual only the not between barrier energy also TiO an but of interface formation electrode–electrolyte the is method are o ufc ioelyr nTiO on layer) dipole surface a (or barrier rnpr fpoonetdeetostruhteTiO the through electrons photoinjected of transport otn fati n hl layer. shell for ZnO matrix a thin as a serves then of film coating This fabricated. is film electrode Al ZnO, as such materials shell Several electrode TiO nanoporous nanostructured a core–shell of consists A electrode/ the usually at interface. rate recombination the electrolyte reduce electrode to for DSCs designed in configuration films a are structures Core–shell for Rate Shell Recombination ZnO Reduced with Structures Core–Shell 4. 8. Figure erdcdwt emsinfo 10.Cprgt20,American 2004, Copyright [130]. from electrolyte. Society. the permission Chemical in species with accepting those Reproduced or molecules dye oxidized with 2 atce.Tu,a es ocpuly eitnet the to resistance conceptually, least at Thus, particles. 2 n h y reetoyet euetercmiaino electrons of recombination the reduce to electrolyte or dye the and ceai fatpclTiO typical a of Schematic 2 3 n CaCO and , ufc.I eea,tefbiainapoc of approach fabrication the general, In surface. [135,144] [132–143] 3 2 aebe eotdt oma energy an form to reported been have h osqec fti fabrication this of consequence The network. mn l hs aeil studied, materials those all Among 2 n oeselsrcueue in used structure core–shell ZnO [145,146] 2 2 nacn h solar-cell the enhancing , 2 O arxta scovered is that matrix 2 ß 3 .Ti salse an establishes This ). sarsl,tecore the result, a As ,SiO 09WLYVHVra mH&C.Ka,Weinheim KGaA, Co. & GmbH Verlag WILEY-VCH 2009 2 . 2 ,Nb 2 2 O [130] network 5 2 ,WO ZnO The [131] 3 2 , ceaial niae h ucino h n hl na in that shell diagram ZnO the the energy-level in of an function species presents TiO the accepting indicates 9 those schematically or Fig. with electrons molecules of electrolyte. recombination dye the reduce oxidized to as so electrolyte, rcpttd olwdb h rcptto fZn of precipitation the by followed precipitated, rnmsineeto irsoy(E)iae eeldthe revealed images (TEM) TiO microscopy electron Transmission ouinadajse h Hvlet ihKH After KOH. with 5 to value 450 at pH 170 at the growth hydrothermal adjusted and solution eosrtdta h ovrinefiinyo TiO of efficiency conversion the that demonstrated xlddadi a nerdta h Ti the that inferred was it and excluded n ae yteie yti ehdwsmaue obe to measured transport. was particle-to-particle of method process this by synthesized layer ZnO 0n,prilybcueo h ml fetv aso electrons of mass effective than small larger the be ( of to because out partially turned nm, thickness 30 shell the when whichdecreased density, short-circuit the affect significantly could shell ZnO eaae n hs o te hs)wsdtce.Therefore, detected. was Zn phase) of other possibility the (or phase ZnO separated odcinbn ( band conduction ae,teoealcneso fcec ece .1,wiei was TiO it bare while 4.51%, for reached 3.31% efficiency conversion overall the layer, htgnrtdeetosi h y oeue ihhg kinetic high with molecules dye the in electrons photogenerated eerhr ie . o ZnCl % mol 0.5 mixed researchers aaee a vlae ihrgrst S efrac.I was It performance. DSC TiO for to that, found regards with evaluated was parameter tao.Ti upninslto a hnsrydit liquid into sprayed then was N solution suspension This ethanol. otn ae nteTiO the on ZnO a of layer deposition the coating for reported also was method sputtering n hl a ototnbe eosrtdt rvd an provide to TiO the demonstrated between been interface the often the at of barrier most role energy has the applications, shell DSC in ZnO structures core–shell for As Shell ZnO the of Role The 4.2. modification. ZnO the to due 6.55% to 4.76% from improved ehns,Kme l lorpre iperuet bana obtain to route TiO simple on a reported shell also ZnO al. et Kim mechanism, rcs,floe ysneiga 500 at sintering by followed process, odr(eus,P5 na1 m 10 a in P25) (Degussa, powder Zn The surface. the TiO that indicated ZnO-covered analysis (XRD) diffraction X-ray nanoparticles. aeasre fsseai tde ntecntuto fZnO of construction thicknesses. the varying on studies with systematic shells of series a made al. et Han layer-by-layer technique, a using shell ZnO the depositing oeiglyro n a oiflec ntegot fTiO of growth the on influence no had ZnO of layer covering fZOaelwrta h oetuocpe oeua orbital molecular unoccupied lowest the than (LUMO, lower are ZnO of (HOMO, fTiO of hntoeof those than 2 .0n,wihwsti nuhfreeto unln nthe in tunneling electron for enough thin was which nm, 0.50 0.3 yfis rprn h TiO the preparing first By odrsml a nlyotie fe freeze-drying a after obtained finally was sample powder A . age l eotdahdohra ehdfrteformation the for method hydrothermal a reported al. et Wang 2 2 aoatce ob bu 0n ndaee n htthe that and diameter in nm 10 about be to nanoparticles 8 2 n oeselstructure. core–shell ZnO m ,04 o n a on nteTiO the in found was ZnO % mol 0.46 C, e n aoatce ihacr–hl structure. core–shell a with nanoparticles ZnO nZnO. in ) . V n h ihs cuidmlclrorbital molecular occupied highest the and eV) 3.8 . V nrylvl ftede n r lohigher also are and dye, the of levels energy eV) 5.4 2 . Vand eV 4.2 þ 2 2 om n fe neln.Bsdo similar a on Based annealing. after ZnO forms 2 hteetoe otdwt 0n-hc ZnO 30-nm-thick a with coated photoelectrodes 2 2 a tl ntepr nts hs n no and phase anatase pure the in still was [148,149] . V n h o fvlnebn ( band valence of top the and eV) 4.0 þ aoatce ydrcl okn h TiO the soaking directly by nanoparticles ihu oicto.Tetikeso the of thickness The modification. without usiuigteltiepsto fTi of position lattice the substituting ai rqec R) magnetron (RF)- frequency radio A 2 8 . VfrTiO for eV 7.4 n neln fteprecipitates the of annealing and C aorsaln films. nanocrystalline 2 M [145,147] aorsaln l n then and film nanocrystalline [147] ouino ZnCl of solution 8 2 C. d.Mater. Adv. hti,tebto of bottom the is, That ih2 with [144] h nuneo this of influence The 2 h hcns fthe of thickness The epciey Those respectively. , 4 þ M 2 2009 oswr first were ions www.advmat.de 2 TiCl 2 n h y or dye the and þ 2 , 2 nteTiO the on [150] nproduct. Zn 21, nabsolute in 4 Sswas DSCs 4087–4108 aqueous [135] twas It . eV) 6.8 4 þ The was 2 2 2 REVIEW 4097 , 2 l k [166] [159] have [161–165] [156] particles. particles as the 2 2 The scattering reaches a nanocrystalline films are ,orTiO 3 films could be promoted 2 and Rothenberger et al. O [160] 2 2 [158] surface is still under discussion. ,Al 2 2 is the wavelength. Experimentally, it also succeeded in enhancing the l [167] The approaches outlined above serve mostly [31,32] Ferber and Luther, surface or a low electron-injection efficiency due to 2 [157] to minimize the recombination rate. Besides those is a constant and 2 k Usami, simultaneously cause adensity, ultimately decrease resulting in inThis a has the been reduced explained by conversion short-circuit either pooron efficiency. dye current adsorption the to TiO zinc sites where has beensignificantly verified improved when that the TiO the performance of DSCs can be theoretically demonstrateddye-sensitized that nanocrystalline TiO the optical absorption of By couplingnanocrystalline a films for photonic-crystaland light layer scattering, Halaoui Nishimura to et et conventional al. al. TiO 5. Light Scattering Enhancement Effect In DSCs, the dynamicrecombination competition of between photoexcited the generation carriersbottleneck and has restricting been found theefficiencies. to That development is, be the a film ofthan thickness the was higher light-absorption expected length to conversion Furthermore, so be the larger as film to capture thicknessthan more was photons. the constrained to electron-diffusionrecombination. be length smaller so as to avoid or reduce to overcome the recombination bynanostructures either using one-dimensional thattransport provide or using aon core–shell structures direct TiO with pathway an oxide for coating electron approaches, a series ofphotoexcited methods carriers that by addressoptical combining the effects generation nanostructured (light of films scattering with or optical confinement) light-harvesting capability ofintroduction the of large-sized photoelectrode. particles into nanocrystalline However, filmsthe the has unavoidable effect of lowering thephotoelectrode internal film. surface This area of serves the effect to counteract of the light enhancement scatteringincorporation on the of opticalundesirable absorption, a whereas increase the photonic-crystalconsequently, in increase the layer recombination the rate ofcarriers. may photogenerated electron A recently lead transport reportedZnO hierarchically aggregates, length to structured which provides film and, large an the with photoelectrode surface with area both and a efficient light-scattering centers, can, to The difference in the observed effectsdue of to the ZnO the shell dye is adsorption perhaps are and both the sensitive electron to injectionAlso, the process surface that the or interface statusfabrication of of method semiconductors. as the well ZnO as other shell experimental is variables. quite dependent on the the buildup of arole thin of insulating the ZnO ZnO shell surface on the layer. TiO As such, the by additionally admixing large-sized TiO combined with large-sized SiO light-scattering centers. The light-scatteringshown to efficiency correlate has with both been thethe size wavelength of the of scattering the centers incident and light. maximum when the size of the scattering centers is about also beenlight-harvesting demonstrated capability to ofimprove the DSC be performance. photoelectrode effective film so in as enhancing to the 2 with a , Zaban 2 2 , and thus 2 based on an . They observed 2 2 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ß [154] -based DSCs by forming 2 believe that this enhancement [151] In addition, the ZnO shell has been conduction band is moved towards a . Hagfeldt et al. studied charge-transport 2 2 [152,153] . 2 It is thought that the optical absorbance is 4087–4108 21, , and Bandara et al. [135] These viewpoints originate from experimental phe- 2009 . However, the transport of electrons in the reverse Schematic diagram of the energy levels of a core–shell-structured 2 ZnO) photoelectrode in DSCs. Reproduced with permission from alone. conduction band is attributed to the existence of a dipole [131,137] . Thus, the TiO 2 2 2 2 [130,155] However, opinions can also be found in literature that deny the Although the energy-barrier model can sufficiently explain the proportional toconduction the band concentration ofcharacterized the of by semiconductor using free while aque variable-potential developed the electrons by spectroscopy electrode Rothenberger techni- in is et al. the improvement in efficiency of TiO direction may be blocked due toprovided the presence by of the an ZnO energy barrier rate shell, of thereby photogenerated reducing electrons. the recombination enhanced DSC performance by ZnO-coated TiO nomena showing that ZnO modification of nanocrystalline TiO Adv. Mater. may indeed increase the open-circuit voltage, but may also energy can readily tunnel throughthe the TiO ZnO shell and inject into Figure 9. (TiO www.advmat.de outer-shell structures ofals. insulator or semiconductor materi- layer at the electrode–electrolytebetween interface due the to shell the andisoelectric differences core point materials and with lower respect electronTiO to affinity of the ZnO higher compared to [147]. Copyright 2006, American Institute of Physics. relatively negative direction,voltage resulting for in the a DSCs.ment Some higher of arguments the open-circuit ZnO insist shell thatresult with the regards of enhance- to the the DSC prolongedZnO performance lifetime over is of a that photogenerated in electrons TiO in that the electrons existed longer in ZnO than in TiO proposed toconduction increase the band free-electron with concentration respect in to the pure TiO results from the conduction-band-shiftthe mechanism, formation rather of an than energyTiO barrier at the surface. The shift of the et al. experimental observation that the electrode of TiO properties inde–electrolyte system aphotocurrents and with compared with nanostructured results time-resolved on TiO ZnO laser-flash-induced thin-film electro- the electron losses toZnO than acceptors for in TiO the electrolyte were less for ZnO coating presents anTiO optical absorbance larger than that for 4098 REVIEW nacmn nteoealcneso fcec was efficiency conversion impressive overall very a the DSCs, observed. for in of type used this enhancement was When inconsistency. film an nanostructured such resolve extent, some n h tutr fteageae.I a ese httefimis film the that seen be film can ZnO It aggregates. structured the of hierarchically structure a the and of morphology the shows imtr n grgtswt ihramndses or monodisperse prepared. a be either can distribution with in size nm aggregates 5 polydisperse of ZnO nanoparticles ZnO containing diameter, solution stock a using ieadsz itiuino grgtsi y-estzdZOslrcls erdcdwt permission with Reproduced cells. solar Wiley-VCH. ZnO 2008, dye-sensitized Copyright in film [169,170]. aggregates a the from of within on distribution efficiency localization conversion photon size overall and and the scattering of size Dependence light g) of aggregates. Schematic submicrometer-sized the of f) e) consisting illustrating (d). nanocrystallites. of for diagram aggregation spectra of Schematic degree absorption the c) Optical in behavior differences aggregate. Photovoltaic with d) samples ZnO ZnO-film nanocrystallites. N3-dye-adsorbed individual packed of closely an of comprised of ZnO aggregated image of microstructure SEM Magnified b) film. 10. Figure at medium polyol a in salt zinc of be can 160 hydrolysis aggregates ZnO by of synthesis achieved submicrometer- The aggregates. of ZnO consist sized films ZnO structured Hierarchically Aggregates ZnO 5.1. 8 C. [168] [168–170] n-grgt y-estzdslrcls )CosscinlSMiaeo ZnO-aggregate a of image SEM Cross-sectional a) cells. solar dye-sensitized ZnO-aggregate yajsigtehaigrt uigsnhssand synthesis during rate heating the adjusting By ß 09WLYVHVra mH&C.Ka,Weinheim KGaA, Co. & GmbH Verlag WILEY-VCH 2009 [169,170] i.10 Fig. infiatdfeec ntesotcrutcretdensity, current short-circuit the in difference significant a aecytliesz faot1 madsmlrseicsurface specific similar and nm 15 the of about area approximately of present been size samples crystallite has same these It all of alone. that nanocrystallites parts demonstrated dispersed includes but with 3 slight constructed aggregates is no sample with 4 sample shape, aggregates and nanocrystallites, spherical and of aggregates DSC the consists their of 2 distortion of sample comparison polydisperse aggregates, well-packed of a ZnO comprised is of for 1 degree Sample the performance. prepared in differing were films, ZnO aggregation, of kinds Four of light. wavelengths visible the a with of comparable possession numerous size c). their geometrical are and and porosity aggregates 10b by the from (Fig. of nanometers features ranging formed structural of The hundreds sizes several are to have tens several aggregates that nanocrystallites spherical interconnected the stacking, disordered highly a while with aggregates ZnO by packed well 0m 80 2 g 1 oee,terpoooti eair exhibit behaviors photovoltaic their However, . rnfr(LT nN dye, N3 in (MLCT) transfer iil t visible the sample to corresponds This only nm. 520 centered around peak 10e, absorption an presents Figure spectra 4 the in minimum In and 4. shown sample 3, for and intensity 2 samples highest intensity lower for a 1, the sample varies for intensity nm revealing 400 above significantly, wavelengths the at absorption to However, electron band band. valence conduction the with from transfer semiconductor the by absorption caused ZnO intrinsic nm, 390 similar below an intensity with exhibit the absorption All samples dye. N3 ZnO with sensitized after being kinds discussed previously four films ZnO the of of spectra l-absorption performance. high with DSC a achiev- ZnOing for of favorable is aggregation nanocrystallites the In words, efficiency destroyed. other gradually is spherical conversion the aggregation of degree the as overall decreases that the is trend obvious An 10d). (Fig. 3 2 and samples for for found are efficiencies observed and densities are current Intermediate 4. sample respectively 2.4% n.Alteohrsmlsso a absorption show the samples in other increase monotonic the All ZnO. and N3 between coupling electronic to the to shifted the due blue-shift) (a slightly N3, wavelengths shorter is pure peak of absorption that with compared iiu auso 0m cm mA 10 of values while minimum 1, sample for observed are 5.4% of mmsotcrutcretdniyof cm density mA 19 overall current short-circuit of max- imum difference a Typically, efficiency. a conversion in resulting iue1esosteoptica- the shows 10e Figure 2 ! 2 p n ovrinefficiency conversion and d.Mater. Adv. * ea-olgn charge metal-to-ligand 2009 www.advmat.de , 21, 4087–4108 [69] 2 and but REVIEW 4099 . 2 /dye complex. þ 2 -based systems. The . Furthermore, ZnO 2 2 The films were typically /dye complex seems to be þ -based DSCs. The molecules 2 [171] 2 /dye agglomerates) and the slow þ 2 verified that a short sensitization time /Dye Complex [174] /dye-complex layer on ZnO nanoparticle þ 2 þ 2 They found that such a complex layer could always be and Chou et al. [172] [173] dimensional ZnO nanostructures. 6. Limitation on ZnO-Based DSCs It has been demonstrated thatband ZnO has gap approximately the and same band position as TiO observed if the immersionimmersion time time of was 10 min, longer noet than complex al. was 3 formed. h. Westermark Below an Commercially available dyes suchderived as from N3, N719,widely ruthenium–polypyridine or used as ‘‘black’’ complexes a dye sensitizer have for TiO been surface. limited performance in ZnO-basedthe DSCs instability may of ZnO because in the explained dissolution acidic of by Zn dye atoms at (i.e.,formation ZnO protons surface, of resulting from in the excessive the dyes Zn fabricated using10-nm-diameter ZnO nanoparticles dispersed aprepared in 1-butanol as were spray ananostructures precursor. (nanorods, deposition Based nanoflakes, onproduced or such technique, nanobelts) by a hadwhich an been precursor, where could various electric-field-induced causeFilms dipole–dipole self-assembly fabricated interactions through process, of thisareas. nanoparticles. technique Moreover, displayed it large was surface structures speculated contributed that to thesephotoelectrodes highly the by disordered light-harvesting causing random efficiencywell multiple of as light possible scattering, the photon as optical localization traps. because of Typically, an thewas overall formation achieved of conversion on efficiency films of with 4.7% randomly oriented nanorods. However, direct use of thesethe surface dyes structure with of ZnO thethey ZnO is are crystals difficult soaked may in because an be acidic destroyedfor dye when solution containing an a Ru complex extendedZnO-nanoparticle films period and comparing ofand the fluorescence transient spectra time. absorption of those films Byfor immersed in different preparing dye solution durations N3-adsorbed formation of of the time, Zn Horiuchi et al. studied the may be favorable to avoid the formation of Zn very sensitive to the experimental conditions, resulting in a possesses a highgood electron crystallization mobility, into low anefforts abundance combination of rate, nanostructures. have and Many nanocrystalline been films or madetures films that on consisting are highly of efficient ZnO-based incapture. various electron However, transport nanostruc- DSCs so and/or far, photon theZnO with results solar cells obtained either have for stillefficiencies dye-sensitized shown relatively when low overall conversion compared with TiO electron-injection kinetics from dye to ZnO. 6.1. Instability of ZnO in Acidic Dyes 6.1.1. Formation of Zn However, the formation of the Zn of these dyes have carboxyl groups that connect with TiO 2 the method particles or 2 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ß [162,166,167] 4087–4108 21, , The films with polydisperse aggregates, which result in 2009 [170] Compared with those using large-sized TiO Further studies reinforce the light-scattering mechanism. It a more disordered structure and achievehigher better packing, conversion establish efficienciesaggregates. The than enhancement effect becomes those morethe intense with when maximum size monodisperse of theaverage aggregates size in of polydisperse the films, aggregates or in monodispersebe the films, as increases to large as,(Fig. or 10g). comparable to, These thesolar-cell performance results wavelength arising of from confirm light visible scattering,hierarchically the light generated by structured rationality ZnO ofcapturing films enhanced ability of and the promoting photoelectrode. the light- Adv. Mater. Recently, the enhancementperformance effect was of also light scattering observed on in DSC films consisting of one- 5.2. One-Dimensional ZnO NanostructuresScattering for Light particles with solid cores thator consume the the internal photonic-crystal surfacescattering area but layer with that increaseddiffusion film length. is thickness added as well to as electron generate light intensity as the wavelength variesportion from of visible the to absorption ultraviolet. is This contributedare by the adsorbed dye molecules on that absorption the intensity ZnO impliesrelated. surface, It has that and been suggested that the the theof difference in difference samples absorption the 1–3 absorption in arises isthat from the cause structure the the submicrometer-sized light aggregates scattering. Thisthe weakens the films transmittance of andperformance of causes the solar pseudo celllight absorption is scattering improved in in due hierarchically to structuredthe the the ZnO traveling presence films, distance spectra. of by of The light which be within significantly extended the (Fig. photoelectrode 10f). film Asincident can such, the opportunities photons for increased. to The be difference inkinds captured the of by optical film absorptionaggregation the implies of of nanocrystallites dye that the would induce four scattering molecules an more in effective improvement light the are visible in region.also occur the A on photon-localization these degree effect films may that due of to confines their the highly disordered light structure scattering in closed loops. has been demonstratedhierarchically structured that ZnO films theby can be performance either significantly of affected thegates. DSCs average with size or the size distribution of aggre- www.advmat.de using controlled ZnOpresents aggregation obvious advantages of in nanocrystallites the generationwithout in of causing light DSCs detrimental scattering, lossthe in photoelectrode film. the That internal is, the surfacea size area submicrometer of these of scale aggregates is comparablelight, on to so the wavelength thatbe of established efficient within visible the photoelectrode scattering filmtraveling and distance of thus of extend photons, the leading the to increasedthe incident opportunity photons for to be light absorbed by would dyewith molecules. Meanwhile, aggregates the film consisting oflites interconnected provides a nanosized highly crystal- poroussurface structure, ensuring area a large for specific dye adsorption, unlike the large-sized TiO photonic-crystal layers reported elsewhere, 4100 REVIEW osdrbedfeec nteotmzto ftesensitization the Zn of The optimization time. the in difference considerable r eaae rmteZOsmcnutrb h ae factive of layer the are by semiconductor dyes. ZnO agglomerates the from separated These agglomerates are the in crystals. electrons photogenerated those since micrometer-sized inactive form to ihti oe,amnlyro y oeue sformed the of is exterior the Zn molecules On of agglomeration injection. monolayer, electron dye to contribute of are and dyes active These monolayer surface. ZnO-nanoparticle a the on homogeneously model, this With n n h itiuino Zn on adsorption of dye distribution of the structure and the a exhibit Accordingly, ZnO to injection. proposed electron is for model inactive are agglomerates xie y nonncytlie a erflce ytransient by systems. reflected photochemical studying for heterogeneous be technique developed can newly a microscopy, nanocrystallites absorption into dye an excited from process electron-injection the and surface semiconductor insufficient performance in poor DSCs. resulting thus of stability, and injection its electron by and adsorption limited dye is ZnO dyes of time acidic sensitization in the that for means inactive This therefore injection. is electron and monolayer, a of instead layer covering n l,idctn httefloecneeiso originates emission be fluorescence Zn excited only N3/ the the from diluted that can of indicating instead emission film, film, ZnO fluorescence N3/ZnO concentrated (3) dye for the obtained to agglomerates, due also the but surface, N3 in ZnO to the solely on due adsorbed not directly is dye films, absorption the ground-state the across that homogeneous revealing not are absorption ground-state ntesrae hra ogtm meso 1 )lasto leads h) (12 immersion Zn long-time the of a agglomeration serious of molecules whereas dye surface, of short monolayer the extremely study adsorbed an on an for obtain observa- dye the can in min) experimental (3 immersed time the films for ZnO on (1) based that microscopy tions is films absorption ZnO N3-adsorbed transient using y oeue na taoi ouin h ntblt fZnO of instability The solution. ethanolic the an from in released protons molecules the dye by atoms Zn surface of dissolution ooeeul nteZOfim nohrwrs hs Zn those words, other In film. ZnO the on homogeneously ein hti,i pt ftehtrgniyo h itiuinof film distribution the the of in heterogeneity homogeneous the Zn of be spite in to is, shown That region. is injection, efficiency the electron to proportional of is intensity transient the of which image in the absorption, (4) and image, heterogeneous observed the oyih 06 mrcnCeia Society. Chemical American 2006, Copyright Zn2 11. Figure h omto fZn of formation The h pta mgso ohtedsrbto fdeasre on adsorbed dye of distribution the both of images spatial The 2 þ þ deagoeae.Rpoue ihpriso rm[183]. from permission with Reproduced agglomerates. /dye deagoeae,teeeto neto sdistributed is injection electron the agglomerates, /dye rpsdsrcueo 3desniie n l with film ZnO N3-dye-sensitized a of structure Proposed 2 þ decmlxcnagoeaet omathick a form to agglomerate can complex /dye 2 þ deagoeae,wihi ossetwith consistent is which agglomerates, /dye 2 þ decmlxhsbe trbtdt the to attributed been has complex /dye 2 2 þ þ 2 decmlx 2 h mgsof images the (2) complex, /dye þ decmlxi vnyyielded evenly is complex /dye deagoeae Fg 11). (Fig. agglomerates /dye [175] ß yia xml of example typical A 09WLYVHVra mH&C.Ka,Weinheim KGaA, Co. & GmbH Verlag WILEY-VCH 2009 2 þ /dye 0.005 h ufc fnnsrcue ZnO. nanostructured of surface the nodrt vi h omto fZn of formation the avoid to order Stability In Improved for ZnO on Coating 6.1.2. the dye on of adsorbed dye occurrence of amount the the ZnO. reduce increase and effectively aggregation can temperature O 2 euetercmiainstsa n surface. ZnO at and sites surface, recombination electrode the the reduce on (2) adsorption dye adequate establish otoldo h re f1n.Fgr 2sostpclTEM typical shows precisely ZnO 12 be Figure can nm. of thickness 1 shell of images the order as the on well controlled as rate growth the 1 upestefraino Zn of formation the suppress (1) lcrceia eoiini ouincnann 0.15 containing solution a in deposition electrochemical iperuefrcaigaTiO a coating for route simple grgto fdeo h ufc fZnO. of and surface adsorption the the on on effect dye an of have aggregation may temperature that achieved. 13- glmrtsdet h togitrcinbtenSi between interaction strong the to due agglomerates fZOi cdcdeslto ycaigabfe ae nteZnO the on layer buffer a SiO coating by surface. solution dye acidic in stability ZnO the of improve to developed been have structures core–shell etteteta 400 at treatment heat aeilo n,peetn h eeaino Zn of generation the preventing ZnO, on material esrcue TiO be-structured ydrcl okn rdpcaigZOfimi ouino 0m 10 of solution a in film ZnO coating dip or soaking directly by hce ls eie h rtcinefc fteTiO the of for effect anatase protection polycrystalline the and Besides nm 5 films. than thicker less completely for being thickness, amorphous its on dependent be to demonstrated carbonate. ufc oicto fZOt rvn h ufc natoms Zn Zn forming surface and the dissolved prevent to being ZnO from of modification surface oto.ADi el eeoe ehiu o thin-film for technique TiO developed coating for reported newly also been a has and to is fabrication hard is thickness ALD film the that control. fact the of account on drawbacks anmn a hw htdsouino n olisoccurs colloids ZnO 7.4. of pH dissolution below that shown has Bahnemann h rsa hs ftedpstdTiO deposited the of phase crystal The (Ti(OBu) alkoxide titanium tm,adtu eknteZn the atoms. Zn weaken surface of dissolution thus and atoms, otdwt SiO a with coated niaeta dopino omcai HOH nteZnO the on (HCOOH) acid formic (10 of adsorption that indicate en htteZOsraei oiieycagd hs the ZnO. the Thus, dissolve charged. will surface positively ZnO is ( the ZnO surface on of adsorbed ZnO charge protons zero the of that of point the means concentrations than lower the (pH the pH much For the which is dye, equal. Ru-complex at with are process groups pH sensitization ZnO surface the deprotonated metal as and negatively of protonated and charge defined zero charge of is zero point positively the of oxides while point predominantly value, the this is above below charged oxide pH a the a at of in charged general, surface In dyes. the acidic in solution, properties surface its from results 2 TiO m 0 ufc a nraetebn eghbtenZ n O and Zn between length bond the increase may surface 10) os yial,we -mdaee n aoatce were nanoparticles ZnO 5-nm-diameter when Typically, ions. -hc l,a vrl ovrinefiinyo .%was 5.2% of efficiency conversion overall an film, m-thick M 2 sas ersnaiemtra htmyb sdfrthe for used be may that material representative a also is Zn(NO [179] [181] 2 [176] a endmntae ob eyefcieshell effective very a be to demonstrated been has h oeo h SiO the of role The oee,teemtosaentwtottheir without not are methods these However, 3 ) 2 2 hoeia netgtosb eso ta.also al. et Persson by investigations Theoretical 2 ae mlrrtoo it naon .) o a for 0.2), around Zn to Si of ratio (molar layer TiO yrt,ad0.05 and hydrate, hlspeae yteADmethod. ALD the by prepared shells 2 8 C. oeselnnwrsadnanotu- and nanowires core–shell 4 isle n2poao,floe by followed 2-propanol, in dissolved ) [180] 2 ae nZOcnb implemented be can ZnO on layer [177] tcnas eahee by achieved be also can It 2 [182] nte eetsuyreports study recent Another hl a eosrtdto demonstrated was shell od eutn nthe in resulting bond, O 2 þ d.Mater. Adv. 2 ihteADtechnique, ALD the With deagoeae and agglomerates /dye M þ 2 þ decmlxs many complexes, /dye deagoeae.A agglomerates. /dye ZnCl 2 hl a been has shell 2009 2 www.advmat.de [178] npropylene in , 21, 2 higher A aeson layers M 4087–4108 ) That 9). 4 LiNO 2 2 þ þ shell /dye ¼ and [183] [73] 5) 3 M , REVIEW 4101 L- [69] The ¼ ) How- is the l ( C [190] . h [190] þ 4 IPCE Such biphasic it is dominated 2 conduction band is 2 film, revealing that the [184–189] 2 shell. 2 ) is the light-harvesting effi- l ( layer on ZnO nanowires. These or TiO or using the ALD technique for the 3 , while the TiO 2 2 , the injection of electrons from þ O 2 LHE 2 2 , and the incident photon-to-current inj and using ultrafast infrared transient or Al w 2 /dye-complex agglomerates, resulting in a low , where C þ [186] 2 h inj is the wavelength of incident light, and w The different injection dynamics most likely originate l ) l [184] . ( 2 For either ZnO or TiO above, these methods,include which can soaking be ZnO(TMOS) greatly for nanoparticles the different, coating of in typically SiO tetramethylorthosilicate The electron-injection efficiency is, above all,electronic coupling determined between by the the dye and semiconductor,also and it is judgedsemiconductor, by the theelectrons relative residential in energy lifetime then-accepting levels states dye in of of the molecules, semiconductor.sensitized In photogenerated the and particular, with for dye the Ru-complex ZnO strated density acidic and that the dyes, of injection process itformation electro- of may Zn has also be been influenced by demon- the deposition of a TiO 6.2. Low Electron-Injection Efficiency Electron-injection efficiency describes thegenerated probability electrons of photo- transferringsemiconductor. In from the terminjection dye efficiency, of molecules DSC to performance, the electron LHE conversion efficiency (IPCE) are related by absorption spectroscopy, a quantitativedescribe study the has been electron-injectioncomplexes given to dynamics to nanocrystalline ZnO from or TiO Ru–polypyridyl Ru-based dyes toinclude a semiconductor a showscomponents fast similar on kinetics component a that picosecond of time less scale. than 100 fs and slower by fast100 components, times leading ininjection the to injection a model rate constant. difference Based of on more a two-state than from the conduction-band structures of the semiconductors.is, That the ZnOempty s and conduction p orbitals bands of Zn are largely derived from the processes are likely tolayer result in in different qualities termsroughness, of and the of, the coating coverage for density.ment Furthermore, effect the example, is enhance- also thought the tonanostructures. For example, be in crystallinity, related the case to the of the ZnOthe mentioned morphology surface above, of nanoparticles the and nanowiresurface arrays areas possess and different porosities specific and,adsorption therefore, the dynamics dye-diffusion and would bedifference in different. the This degree wouldafter of being lead coated photovoltaic-efficiency to enhancement with a a SiO ever, for ZnOdominated by with slow components, whereas Ru-based for TiO dyes, the electron injection is electron-injection efficiency. ciency, difference in band structure results inand, a different possibly, density of different states electronic coupling strengths with the injection time scalesstates from to ZnO are unthermalized estimated to and beof 1.5 and relaxed 150 which ps, excited respectively, both areTiO one order of magnitude slower than those of kinetics areultrafast caused electron by injection and competition molecular relaxation. processes between the comprised primarily of empty 3d orbitals from Ti collecting efficiency for injected electrons at the back contact. 2 2 ,as was 3 shell O 3 2 SC coating J O 2 2 aqueous HCl. M ,andAl 2 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim /dye agglomerates), ß ,TiO þ 2 2 In a related set of ions and the cations (i.e., core–shell-structured nano- 2 2 [183] TiO shell was proven to act as an was also studied as a coating shell prepared by ALD. Reproduced was attributed to the Al 3 3 2 O O -coated ZnO nanowire arrays have 2 OC 2 2 V the Al ) with increasing shell thickness and, shell could simultaneously lower or block SC ), which may lead to a difference in interface J 3 [179] , þ 4087–4108 2 O 3 or the result reported for ZnO nanoparticles 2 21, , ) of the cell but a larger decrease in short-circuit [183] 2 OC Characterization of a ZnO ,orAl shell is also believed to play a role in suppressing the 2009 V þ 2 4 ,Ti þ Although the use of oxides, such as SiO 4 ultimately, a decrease insolar the cells. power-conversion The increase efficiency in of the Adv. Mater. (which may improve the stabilityand, of ZnO therefore, nanowires avoid in acidic the dye formation of Zn shell, where the ZnO-nanowire core was removed using 1 shell. b) Selected-area electrontured diffraction nanowire. pattern c) of Energy-dispersive spectroscopy the (EDS)along core–shell elemental the struc- profile dashed line in (a). d) TEM image of a nanotube-structured TiO Figure 12. www.advmat.de with permission from [183]a) Copyright TEM image 2006, of American a Chemical core–shell structure Society. with ZnO-nanowire core and TiO electron injection at the semiconductor–electrolyte interface. recombination rate bythe passivating ZnO surface the and recombination bythe injected forming sites electrons an from energy on approaching barrier thea nanowire that surface. result, prevents As solar cells with TiO current density ( states and thus affectmethods the used electron-injection for process, and the (2) formation the of oxide shells. As mentioned coated with SiO material on ZnOusing nanowires. However, TiO in contrast to the case insulating barrier, leading to avoltage slight increase ( in the open-circuit wire and nanotube-structured TiO demonstrated significant enhancements in bothvoltage the and open-circuit fill factor,overall conversion leading efficiencies. to Typically, it asoverall was much conversion reported efficiency that as was the doublyZnO increased improved nanowire from array 0.85% for to bare 1.7–2.1% with a crystalline TiO layer (10–35 nm thick) on ZnO. Si because the Al shells on ZnO is basedsurface on stability a of basic consideration ZnO of inphotovoltaic improving efficiency the acidic by dye, these the materials is resultantseems obviously effect that different. on better It insight the intoZnO how to and select achieve theefficiency shell an material is optimal for still enhancementgeneral, of in it is the need believed that photovoltaic of the surface-modificationto effect further (1) is the related investigation. chemical bonding However, between O in experiments, amorphous Al the TiO having the capacity tosurface passivate of the recombination ZnO sites nanowires, on whereas the the decrease in 4102 REVIEW adsae ertebn dei smc stoodr of orders two as much as is edge band TiO in higher the magnitude near conduction- of states density the band that estimated al. et Enright adsorbate. ( h odcinbn lcrni TiO in electron conduction-band the re fmgiuehge hni h aeo TiO of case the in than higher magnitude an of are that order photocurrents generate could ZnO for photosensitizer a as chloride) (1,6diamino-10-methylacridinium acriflavine of use n-ae Ss ih11%cneso fcec o nano- for efficiency conversion films. 1.11% crystalline with DSCs, ZnO-based (C eobnto u otehge ufc-rpdniyi the electron in interfacial density of surface-trap higher degree the ZnO. higher N3-dye-adsorbed to a due possess was recombination device latter to the where dye, believed N3 significantly with factor obtained fill that a than with larger DSCs ZnO provide could sensitizer ysasre nZOfrasrto ntered/near-infrared of the aim in absorption the region, for with (IR) ZnO developed on adsorbed heptamethine-cyanine been include dyes already Examples criteria. New have these range. fulfilling be wavelength dyes broad and of a efficiency, in injection types absorption high light a for with effective semiconductor, transferable ZnO the charge to bonded be chemically be to photosensitizers expected These subject are investigated. a widely is DSCs been development ZnO already the in has use dyes, that for acidic photosensitizers in of types ZnO new of of instability the of view In ZnO for Photosensitizers of Types and New states 6.3. intermediate efficiency. of injection low existence a the in results to to therefore considered due is transfer slowly electron surface the The occur surface. localized to ZnO and ascribed the dye on been states photoexcited has the states between intermediate interaction the of origin The epciey n e and respectively, ysprsigeeto aktransport. back electron photocurrent suppressing and photovoltage by increase which acid, deoxycholic ac h oa pcrmbte eas hi absorption- their because to better ability spectrum the pure present solar even dots the or quantum metallorganic match Moreover, the dyes. over stability organic of of form offer advantage therefore, the and, the take semiconductors dots compound Quantum of in nanocrystals (SSSCs). semiconductor- application cells called for solar sometimes photosensitizers sensitized accordingly as are which studied DSCs, been also have injection electron the of by result described as a states, be intermediate via to stepwise proposed proceeding also is dyes hr N3 where N l,oealcneso fcece f2024 aebeen have 2.0–2.4% of efficiencies conversion obtained. overall film, iha 9 t50n n noealcneso fcec of efficiency conversion overall an and as nm IPCE 510 an 2.5%. at offering ZnO, 69% for as suitable most high is date, to that, tizers 3 20 0.3 sa lentv oogncde,smcnutrqatmdots quantum semiconductor dyes, organic to alternative an As Ru-based with ZnO for dynamics electron-injection slower The þ H m [195,196] 8 ZnO Br e ). [198,199] 2 [149] * HgNa ! n N3 and [192,193] ! hv 150 twsas eotdta ecrcrm photo- mercurochrome that reported also was It ps [66] 2 N eety eeiahee l eotdta the that reported al. et Senevirathne Recently, )i n ftenwydvlpdphotosensi- developed newly the of one is O) N 3 CB hnesnYi obndwt nanoporous a with combined is Y eosin When þ 3 n nymtia qaan yswith dyes squaraine unsymmetrical and – þ þ ersn h xie n xdzdstates, oxidized and excited the represent [197] niae odcigeeto nZnO. in electron conducting a indicates þ ZnO 2 e yrao ftelre fetv asof mass effective larger the of reason by CB oi sas eyefiin y for dye efficient very a also is Y Eosin < ! 100 fs intermediate 2 (5–10 ß [194] m 09WLYVHVra mH&C.Ka,Weinheim KGaA, Co. & GmbH Verlag WILEY-VCH 2009 e hnta nZnO in that than ) Mercurochrome 2 . [200] [191] l ssilepce ob agrta h ih-bopinlength light-absorption a the than though larger be even ( to nanocrystalline expected that, the the still of is is However, thickness film the configuration light. used, is a of layer light-scattering such harvesting of the disadvantage increasing in effective 2–5- around layer ehdi opaealyro 0-mTiO 400-nm of layer a place to is reported method frequently more A layer. photonic-crystal a incorporating o,tehgetoealcneso fcec fTiO of efficiency conversion overall highest the now, iuto svr ifrn o h irrhclysrcue films structured hierarchically the for different very is situation aorsaln l ihlreszdTiO large-sized with the admixing film by scattering nanocrystalline light introduce methods mentioned eeto oso nietlgt yial,tephotoelectrode the Typically, light. 12–14- incident a includes of loss the reducing reflection and scattering light motivating film, nanocrystalline ftelgtsatrn fetfrTiO for effect light-scattering the of of efficiency conversion maximum 11%. the around achieved already have i h ot fageae.A uh twudb niiae that anticipated TiO be nanocrystalline would for work it also such, can As approach this aggregates. improved of further route be can the films via nanoparticulate on a based performance DSCs the is that of This indicating ZnO, nanocrystallites. in ZnO found scenario dispersed desirable for double obtained than 2.4% more is efficient the the which possess 5.4%, with of aggregates area efficiency ZnO conversion surface highest result, a large As relatively centers. a light-scattering combine thus crystallites nanosized and of aggregation aggre- with structure ZnO a feature nanostructures, gates these sum- on Among based 2 DSCs nanostructures. for Table ZnO obtained results recent rates. of majority recombination the marizes reduced for demonstrated ihqatmdt saot2.8%. about is dots quantum with oprbet h eut bandfrsmlrZOnanowires ZnO similar dye. 50–60%, for Ru-complex with obtained of sensitized results efficiency the quantum to internal which comparable cell, an photoelectrochemical demonstrated a nanowires for has ZnO dots combine quantum to is CdSe example with One ZnO. on made been tutrs yial ihaZOselo TiO a on shell Core–shell ZnO film. a photoelectrode with the traveling typically within the structures, light extend of that dye electron effects distance for for light-scattering pathways areas and direct material, transport, surface film specific bulk than larger adsorption offer these review, To can DSCs. advanta- in film nanostructures be photoelectrode to the as demonstrated use for been geous have ZnO of Nanostructures Conclusions 7. ope nteeetoyeaon h TiO the around electrolyte redox the of concentration in the dye. in oxidized reduction couples possible the the of to regeneration due is the This affect and to film nanocrystalline likely the is between electrolyte located presence layer is the scattering Moreover, thickness a length. film of diffusion the electron since the serious than is very larger recombination be case, may this and In unavoidable captured. be can photons incident ral eeca ntrso htvlacdvcspossibly devices photovoltaic efficiencies. conversion of to high terms able extremely were in attaining dots beneficial be quantum More would which greatly the photon, quantization. per pairs that size electron-hole multiple reported by generate was tailored it be recently, can range wavelength a culy hr r led ut e netgtoso h use the on investigations few a quite already are there Actually, 1 , a steasrto ofcet.Ti ss htms fthe of most that so is This coefficient). absorption the is m m -hc aorsaln l n scattering a and film nanocrystalline m-thick hc.Tesatrn ae hudbe should layer scattering The thick. m [205] [203,204] 2 bsdDC.Teafore- The DSCs. -based d.Mater. Adv. e tepshv also have attempts few A 2 2 atce bv the above particles 2 aoatce.The nanoparticles. atce rby or particles 2009 www.advmat.de [201,202] 2 , ls which films, 2 2 21, sensitized oe are core, 4087–4108 pto Up REVIEW 4103 [144] [183] and tita- [135,146,147] [168–170,174] [211,212] is a task that can , many titanium ), 2 2 4 # titanium isopropoxide 4.4%, 5.4% 4.51%, 9.8% flexible substrate) [208] can be adopted as the ), 4 [213] ), N3 4.3%, 4 N719 0.704% [181] N719 1.02% [61] N719 2.25% [183] N719N719 1.78% 5.2% [180] [179] N719 surface for dye adsorption. Many 2 ALD is actually a self-limiting process [207] ) 2 ) titanium ethoxide (Ti(OEt) ) ) ) 3 2 2 2 Aggregates TiO As for the deposition of TiO O 2 2 ZnO) ZnO) TiO SiO TiO Al 2 2 [183] [209,210] ), (ZnO (ZnO (TiO (TiO (ZnO (ZnO 4 films (ZnO Pr) i 7.1. Surface Modification ofMethod ZnO for Aggregates TiO – An Indirect The modification of ZnO aggregates with TiO (Ti(O of vapor–solid deposition, in which film formationcyclic takes place manner in a throughbetween a the series adsorbed ofsurface, saturated precursor and, moreover, surface and multilayerexcluded. reactions the adsorption In is, species this bycontrolled left method, definition, by the on adjusting film the growth thickness the rate. can number be of precisely deposition cycles and salts, such as titanium chloride (TiCl physical- or chemical-depositionpurpose. methods can However, bedistinguishes used the itself for by this recently offeringlarge-area atomic-level uniformity. thickness developed as well ALD as technique titanium-oxide precursor dueand to also present the a moderate fact vapor pressure, that which is they convenient are liquids be achieved bystructure simply of utilizing ZnO the tosame already-achieved serve as aggregate time, light attain scattering centers a and, at TiO the nium methoxide (Ti(OMe) 2 it [72,77,115] Nanotips N719 0.55%, 0.77% [126,127] [67,76,80,93] Nanobelts N719 2.6% [95] [206] [110,115,116,118,119] Core–shell nanowires 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ß 1.5% [194] N3 1.55% [93] 0.16%, 0.67 [192,193] Nanosheets N719 2.61%, 3.3% [91,94] 1.5%, 1.54% (0.56 sun), 3.4% (0.06 sun), 2.22% , (2) hydrothermal growth of 2 . As an outlook, we would like to , it is hampered by the availability 2 2 N3 0.4%, 0.75%, 2% D149 4.27% [92] N719 6.55% [150] N719 0.9% ;(flexible substrate) [122] N719 0.23%(hybrid ZnO/N719) [62] N719 5% (0.1 sun) [73,74] Nanotubes N719 1.6%, 2.3% [124,125] N719 0.3%, 0.6%, 0.9%, N719 3.9%, 4.1% [88,90] Core–shell nanoparticles eosin-Y 2.0%, 2.4% [198,199] N3 1.21% (0.2 sun, eosin-Y 3.31% (0.1 sun) [198] Core–shell nanoporous eosin-Y 1.11% [66] Nanotetrapods N719 1.20%, 3.27% [96,97] cyanine squaraine acriflavine 0.588% [200] Nanoflowers N719 1.9% [128] QDs (CdSe) 0.4% [205] Core–shell nanotubes heptamethine unsymmetrical mercurochrome 2.5% [195,196] Core–shell nanoparticles Mercurochrome 2.2% [121] aggregates. These films have the capability to aggregates, including (1) surface modification 2 4087–4108 2 , and (5) synthesis of porous-structured TiO 21, 2 , 2009 nanostructures, (4) electrostatic spray deposition using Summary of DSCs based on ZnO nanostructures. 2 -nanoparticle aggregates, (3) emulsion-assisted aggregation 2 However, when a similar light-management strategy to the of TiO colloidal TiO Adv. Mater. Dendritic nanowires N719 0.5%, 1.1% [103,129] Aggregates N3 3.51%, Nanowire/nanoparticle composite films spheres. with ZnO or TiO Table 2. StructureNanoparticles Photosensitizer N719 Efficiency 0.44%, 2.1% Ref. Structure Photosensitizer Efficiency Ref. www.advmat.de Nanowires N3 0.73%, 2.1%, 2.4%, 4.7% [113,117,171,197] Core–shell nanowires Nanoporous films N3 5.08% (0.53 sun) [85] Core–shell nanoparticles generate light scattering withinenhance the the photoelectrode light-harvesting film efficiency. In so thisto as case, it to use is possible alight-absorption length photoelectrode may remain unchanged, film ordue even increase, with to alight-scattering the reduced actions of extended thickness. thedecreased The aggregates. light-traveling recombination This would rate distance result sodensity in caused as as a to well by as benefit the the power-conversion the photocurrent efficiency of aZnO solar case is cell. employed with TiO of ZnO aggregates using TiO propose several possiblesynthesis of methods TiO that are intended for the TiO of synthesis methodstructures. in Although the the mechanism of production growthhas in of ZnO been aggregates appropriate proposed nanos- tonanocrystallites be in a a result of supersaturation the colloidal dipole nature solution, of ZnO seems to be unsuitable for TiO 4104 REVIEW ohnl.I a enrpre htbt h rsa tutr and structure crystal the TiO both of that reported phase been has It handle. to hs aoatce r ooeosyasre tteactive the TiO of at surface adsorbed the around homogenously sites are nanoparticles These odtosi h rsneo Cl of presence the in conditions iaimpeusr TiO precursor, titanium )SMiaeo peia grgtswt naeaedaee of diameter average an with aggregates nanoparticles. needle-like spherical of Elsevier. 2008, image Copyright [215]. SEM from a) permission with Reproduced tures. tions. ieiohoynt) erdcdwt emsinfo 27.Copyright [217]. from Wiley-VCh. permission 2008, with Reproduced isothiocyanate). mine temperature. a etoe htamjrpooto fteas-synthesized the of proportion TiO major rutile-phase It were a center. nanostructures the that from growth mentioned radial was in resulting hydrolysis, of stage eetsuyrpre ntesnhsso oe-ieTiO flower-like of synthesis the on reported study recent A mrhu,weestoedpstda 250 phase. at anatase deposited with polycrystalline those whereas amorphous, muto edelk aoatce fTi(OC of nanoparticles needle-like of amount peia TiO spherical 14. Figure tetrabut- titanium (Ti(OBu) of oxide treatment hydrothermal by nanostructures TiO of Growth Hydrothermal 7.2. 13. Figure edelk TiO needle-like fflwrlk aotutrshsbe srbdt h hydrolysis the Ti(OBu) to ascribed of been has to formation nanostructures appear The flower-like DSCs. structures for of These scatterers 13). light efficient (Fig. as nm promising 300 be about of size a with 0 m )TMiaervaigta h grgtsta oss of consist that aggregates the that revealing image TEM b) nm. 300 [215] ceai fteeuso-sitdmto o h ytei of synthesis the for method emulsion-assisted the of Schematic TiO flower-like of structure and morphology The hs oe-iennsrcue eeageae by aggregated were nanostructures flower-like These 4 2 [213] grgtswt aoatce.(TRITC nanoparticles. with aggregates 2 ihhg ocnrto,wihlast large a to leads which concentration, high with 4 r rdmntl eemndb h deposition the by determined predominately are 2 qeu ouinudrsrnl cdccondi- acidic strongly under solution aqueous ) aoatce n osse peia shape spherical a possessed and nanoparticles o xml,we sn Ti(OMe) using when example, For 2 lstpclydpstda 200 at deposited typically films 2 ulignrtda h primary the at generated nuclei 2 . [216] [214] aoatceAggregates Nanoparticle 2 u otesrnl acidic strongly the to due ß 09WLYVHVra mH&C.Ka,Weinheim KGaA, Co. & GmbH Verlag WILEY-VCH 2009 ¼ 8 raoeare above or C tetramethylrhoda- 4 H 9 2 ) 4– nanostruc- 4 n (OH) sthe as 8 are C n 2 . imtro h irppte and micropipette, the of diameter oosysrcue TiO structured Porously ftedolt a eajse nawd ag rmafew of a fabrication the from a for range therefore, employed is, wide be TiO It may a micrometers. that in method of promising adjusted hundreds to be up can size nanometers the droplets and droplets, the into of nanoparticles of packaging the allows mlie h upnino hnigtewih rcinof fraction preparation weight the TiO illustrated of the al. et that Kim changing suspension. micropipette in or the nanoparticles of suspension size the tip the emulsifies tuning by controlled readily hntewtri eoe rmtedolt ydyn,the drying, by 14). droplets (Fig. the spherical aggregates form are from to assemble removed spontaneously nanoparticles nanoparticles emulsion is colloidal colloidal water water-in-oil geometries. the confined the provide When of that technique, droplets water means in this encapsulated by In aggregates droplets. colloidal creation the towards of route easy an is synthesis Emulsion-assisted Aggregation nbigefiin ih cteigadlresraeaest be to thus areas TiO aggregates, surface porous-structured of large synthesis nanocrystallite The and simultaneously. of attained scattering that light efficient to enabling similar structure a eetmtdb h relation the by estimated be n edt necag fratns iimlini stabilized is miniemulsion a reactants, place of exchange take an processes to lead diffusion and which in emulsion, conventional eim ntennmtrsae nasal anr The the manner. for viability stable its TiO a other is of method in aggregation spherical this scale, of nanometer importance technological liquid the the a of in on properties dispersed are the medium, they towards that requirement provided materials any constituent have not does that advantage it the possesses method emulsion-assisted the gates, suspension. in nanoparticles microspheres, the of size the that demonstrated was It TiO of Synthesis Emulsion-Assisted 7.3. aoeco o h naslto fclodlTiO a colloidal as of acts encapsulation droplet the each for condensation, titania surfactants. nanoreactor and of containing material phase hydrolysis organic precursor During continuous a miniemulsion a of with inverse consists mixed dispersed a that a by the phase generated via are aqueous with method droplets method, template-free this combined In a technique. using process achieved sol–gel be can spheres 2.1.2.). Section for (see treatment heat evaporation with (containing electrohydrodynamic substrate the charged solvent an on solution formed into is by film colloidal a needle atomized and force capillary a is a which through solvent) droplets in and binder, process nanoparticles, a is ESD TiO Aggregates of Fabrication Electrostatic-Spray-Deposition 7.4. efficient highly for potential huge a DSCs. exhibit which nanotubes, ..Snhsso oosSrcue TiO Porous-Structured of Synthesis 7.5. oprdwt te ehd fpeaigclodlaggre- colloidal preparing of methods other with Compared 2 grgtscnitn fnanocrystallites. of consisting aggregates 2 [222–224] irshrsb sn h mlinasse method. emulsion-assisted the using by microspheres [217–221] 2 pee ihsbirmtrszshave sizes submicrometer with spheres h ieo h grgtscnbe can aggregates the of size The d m / w 2 w d aoaeil,sc sTiO as such nanomaterials, stewih rcino TiO of fraction weight the is t ’ w 1 d.Mater. 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Washington Intel EnerG2. of Science Corporation, Technology Energy Center, Foundation Acknowledgements spheres is typically about 200 nmspheres in diameter. are Impressively, these formed bysize interconnected of several nanocrystallites nanometers, leading with toof the a more high specific than surface 300 area m against diffusion and coalescence so that bothsize the droplet distribution size and can be well controlled. www.advmat.de reported on the fabrication of porous-structured TiO glycol-modified titanate (bis(2-hydroxyethyl)titanate,the EGMT) precursor material and as an amphiphilicB- block copolymer (P(E/ 4106 REVIEW 12 .H u .H Chen, H. J. Yu, H. K. [102] 11 .J nih .J ol,C li,K eroz .H red .Gratzel, M. Friend, H. R. Meerholz, K. Klein, C. Moule, J. A. Snaith, J. H. [101] 10 .Hged,M Gratzel, M. Hagfeldt, A. [100] 7]Nt htteepoe luiainitniyi hswr s1 Wcm mW 10 is work this in intensity Hagfeldt, illumination employed A. the that Lindquist, Note E. [75] S. Lindstrom, H. 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