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Solution-derived 40 µm vertically aligned ZnO nanowire arrays as photoelectrodes in dye-

sensitized solar cells

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2010 Nanotechnology 21 195602

(http://iopscience.iop.org/0957-4484/21/19/195602)

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Nanotechnology 21 (2010) 195602 (9pp) doi:10.1088/0957-4484/21/19/195602

Solution-derived 40 µm vertically alignedZnO nanowire arrays as photoelectrodesin dye-sensitized solar cellsJijun Qiu1,2, Xiaomin Li1,4, Fuwei Zhuge1, Xiaoyan Gan1,Xiangdong Gao1, Weizhen He3, Se-Jeong Park3, Hyung-Kook Kim3

and Yoon-Hwae Hwang3,4

1 State Key Laboratory of High Performance Ceramics and Superfine Microstructures,Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050,People’s Republic of China2 RCDAMP, Pusan National University, Busan 609-735, Korea3 Department of Nano-Materials Engineering & BK 21 Nano Fusion Technology Division,Pusan National University, Miryang 627-706, Korea

E-mail: jj qiu@163.com, Lixm@mail.sic.ac.cn and yhwang@pusan.ac.kr

Received 19 November 2009, in final form 20 November 2009Published 21 April 2010Online at stacks.iop.org/Nano/21/195602

AbstractWell-aligned ZnO nanowire arrays with a long length of more than 40 μm were preparedsuccessfully by using the polyethylenimine (PEI)-assisted preheating hydrothermal method(PAPHT). Several important synthetic parameters such as PEI content, growth time, preheatingtime and zinc salt concentration were found to determine the growth of ultralong ZnO nanowirearrays, including length, diameter, density and alignment degree. The photoluminescence (PL)spectrum of as-grown ultralong ZnO nanowire arrays revealed a UV emission and a yellowemission, which was attributed to the absorbed hydroxyl group based on the peak shift afterannealing in various atmospheres. The performance of dye-sensitized solar cells (DSSCs)increased with increasing length of ZnO nanowire arrays, which was mainly ascribed to theaggrandized photocurrent and reduced recombination loss according to electrochemicalimpedance spectroscopy (EIS). A maximum efficiency of 1.3% for a cell with a short-circuitcurrent density (Jsc) = 4.26 mA cm2, open-circuit voltage (Voc) = 0.69 V and (fill factor)FF = 0.42 was achieved with a length of 40 μm.

S Online supplementary data available from stacks.iop.org/Nano/21/195602/mmedia

(Some figures in this article are in colour only in the electronic version)

1. Introduction

As a third-generation photovoltaic cell, dye-sensitizedsolar cells (DSSC) are promising devices for inexpensive,stable, large-scale soar energy conversion. Although thesolar power conversion efficiencies of DSSC have beenimproved in the recent two decades, they are still lowerthan 20% for conventional Si-based solar cells. Inprinciple, the improvement in DSSC performance hasbeen made by increasing the light-harvesting capabilityof the photoelectrodes and by reducing the electron–hole

4 Authors to whom any correspondence should be addressed.

recombination rate, which can be achieved in the followingseven ways. First, is to maximize the area of the dye-TiO2

interface by using nanocrystalline-mesoporous architectureto increase the dye loading [1]. Second is to dopenanocrystalline TiO2 with special elements to increase theopen-circuit voltage of the cells [2]. Third is to replacethe nanocrystalline-mesoporous structure by one-dimensionalsingle-crystalline array structures to decrease the electrontraps in the nanocrystalline boundaries [3]. Fourth is tocoat the nanocrystallines or one-dimensional nanostructuresin a conformal metal oxide shell to suppress the carrierrecombination by passivating recombination centers andintroducing an energy barrier [4]. Fifth is to synthesize new

0957-4484/10/195602+09$30.00 © 2010 IOP Publishing Ltd Printed in the UK & the USA1

Nanotechnology 21 (2010) 195602 J Qiu et al

dye sensitizers with a higher molar extinction coefficient andbroader spectral response [5]. Sixth is to maximize the areaof the interface between the transparent conducting substrateand the oxide layer to increase the electron capture [6]. Thelast way is to use a double light-scattering layer TiO2 [7] orZnO [8, 9] film to enhance the light-scattering capability bysignificantly extending the traveling distance of light withinthe photoelectrode films. Among these, replacing TiO2

nanocrystalline-mesoporous networks with one-dimensionalwell-aligned semiconductor arrays has demonstrated a greatpotential to boost the DSSC performance [3, 10–14].

In 2005, Baxter [10] and co-workers first fabricatedZnO-nanowire-based DSSC using metal–organic chemicalvapor deposition (MOCVD) and the hydrothermal method,and an overall light conversion efficiency as high as 0.5%was achieved. Law et al [3] also fabricated ZnO-nanowire-based DSSC using a polyethylenimine (PEI)-assistedhydrothermal method, obtaining an overall efficiency ashigh as 1.5%. Subsequently, they further studied core–shell structures combining their fabricated ZnO nanowiresystem with layers of TiO2, resulting in a further increasein the overall light conversion efficiency to 2.25% [4].Henceforth, the one-dimensional ZnO nanowire and itsderivative hierarchical nanostructures, including core–shellnanostructures [4], nanotubes [11], flower-like nanowires [12],dendritic nanowires [13] and branched nanowires [14], havebeen intensively investigated as alternative photoelectrodes toimprove the DSSC performance.

Although there are many available methods to fabricateone-dimensional ZnO nanostructures, it is also difficultto establish an optimal synthesis methodology for DSSCapplication because their performances depend to a greatextent on the preparation conditions. To date, the mostattractive synthesis technique for obtaining well-aligned ZnOnanowires and hierarchical or branched nanowires for DSSCis the hydrothermal method. Compared with other high-temperature physical or chemical vapor deposition methods,including metal–organic vapor deposition (MOCVD), atomiclayer deposition (ALD), sputtering, thermal decomposition,thermal evaporation and condensation, the low-temperaturehydrothermal method offers the potential for much lowercost because of eliminating the expense associated withhigh-temperature manufacturing and vacuum processing. Inaddition, hydrothermal synthesis is easily scalable to largeareas and is compatible with roll-to-roll processing of softplastic substrates. At the same time, DSSC using thehydrothermal-route-synthesized nanowires was found to yielda better efficiency compared to cells prepared using ZnOnanowires fabricated by the MOCVD method.

Although a lot of effort has been made to investigate theeffect of synthesis parameters on the dimension and qualityof ZnO nanowires [3, 15–19], it is still difficult to fabricateultralong ZnO nanowire arrays with a high aspect ratio by thehydrothermal method, which blocks a further rise of DSSCperformance. Recently, it is found that the introduction ofpolyethylenimine (PEI) [3, 15] into the growth solution as anadditive could efficiently increase the length and decrease theaspect ratio of ZnO nanowires. ZnO nanowire arrays as long

as 20 μm were successfully fabricated by the PEI-assistedhydrothermal method (PAHT) at 92 ◦C for 50 h. However,without the additive the length of the nanorods was no morethan 5 μm. In this synthesis process, the growth solutionneeds to be repeatedly refreshed every 2.5 h to complement thedepletion of reagents for a rapid growth rate of 0.5 μm h−1.At the same time, acid must be added to the growth solutionto decrease the pH value in the range from 10.5 to 7.2, whichensures the PEI can be adsorbed on the lateral plane of the ZnOnanorods by electrostatic attraction to achieve a large aspectratio of 100 [20]. However, the introduction of fresh solutionand the acid not only increases the fabrication cost, but alsodecreases the reproducibility, because ZnO seeds are easilydestroyed by fresh growth solution involving hydrogen ions(H+) in the initial growth stage due to low chemical stability.Moreover, ZnO nanowire arrays with more than 30 μm lengthhave not been successfully fabricated by the PAHT method.

Herein, an improved PAHT, called preheating PEI-assistedhydrothermal synthesis (PPAHT), was developed to fabricatelonger ZnO nanowire arrays with high reproducibility. Firstly,a preheating process is employed to avoid adding acid tothe growth solution and the ZnO dissolution, giving rise toa higher reproducibility. At the same time, the populationdensity of ZnO nanowires can be easily tuned by controllingthe preheating time. Secondly, the original growth solutionhas been utilized until the whole growth process is over, whichsignificantly decreases the fabrication cost. In this paper, weinvestigated the effect of the synthetic parameters, involvingPEI content, growth solution concentration and preheatingtime on the length and aspect ratio of ZnO nanowire arraysobtained from the PPAHT method. In addition, we also usedsynthesized ZnO nanowires to assemble DSSC, and to see ifthe further increase of nanowire length can improve the deviceperformance.

2. Experimental details

2.1. Synthesis of ultralong ZnO nanowire arrays

2.1.1. Deposition of ZnO seed layer by sol–gel dip-coating. Firstly, the transparent conducting F:SnO2 (FTO)substrates were seeded with a oriented ZnO thin film byusing the sol–gel dip-coating method. Prior to the seeding,the FTO substrates were ultrasonically cleaned in ethanoland acetone for 15 min, respectively. For the orientedseeds, 0.075 mol Zn(CH3COO)2·2H2O was dissolved in a100 ml 2-methoxyethanol-monoethanolamine solution at roomtemperature. The resultant solution was stirred at 60 ◦C for0.5 h to yield a clear and homogeneous sol. Then cleanedFTO substrates were slowly immersed into the ZnO sol for 10 sand then withdrawn at 3.0 cm min−1. Subsequently they werepreheated at 300 ◦C for 10 min to form a ZnO gel seed layer.The desired thickness of ZnO seed layers was controlled byrepeating the dip-coating and preheating processes. Finally,the preheated substrates were heated from 300 to 550 ◦C at2 ◦C min−1 and held at 550 ◦C for 1 h to obtain the orientedZnO seed layers.

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2.1.2. Synthesis of ultralong ZnO nanowire arrays by PPAHT.The growth solutions for the hydrothermal route were preparedby dissolving Zn(NO3)2·6H2O, (CH2)6N4 and PEI (branched,low molecular weight) in distilled water. The equivalentconcentrations of Zn(NO3)2·4H2O and (CH2)6N4 were tunedin the range from 0.01 to 0.1 M to investigate the effect of thereagent concentration on the length and aspect ratio of ZnOnanowires. At the same time, the PEI content was changedfrom 0 to 0.015 M corresponding to the change of reagentconcentration. Then 150 ml growth solution was first sealedin a glass bottle of maximum volume 250 ml and was thenheated to 95 ◦C for 2–12 h, namely a preheating process.After the preheating process, the ZnO-seeded FTO substrateswere quickly immersed in the hot preheated growth solutionand tilted against the wall of the bottle with ZnO seed layersfacing down. Subsequently, the bottle was sealed again andheated to 95 ◦C for various times from 12 to 150 h withoutany stirring. These various synthesis conditions were givenin supporting information (available at stacks.iop.org/Nano/21/195602/mmedia) to study the evolution of the morphology asa function of the preheating time and the growth time. Aftergrowth, the resultant samples were removed from the vials,rinsed thoroughly with ethanol to remove any residual reactantsand dried in air at 80 ◦C.

2.2. Fabrication of ZnO-nanowire-based DSSCs

The cis-bis(isothiocyanato)bis(2,2′-bipyridyl-4,4′-dicarboxy-lato)-ruthenium(II)bistetrabutylammonium (N719) (SolaronixS. A.) was used as a sensitizer. ZnO nanowire arrayphotoelectrodes were prepared by immersing in a 0.0005 Methanolic solution of N719 for 2 h. The counter electrodewas an F:SnO2 substrate with a 20 nm Pt layer depositedby e-beam evaporation. The photoelectrode was sandwichedbetween the counter electrodes separated by a 100 μm hot-meltpolypropylene spacer. Then the electrolyte, which consisted of0.5 M tetrabutylammonium iodide, 0.05 M I2 and 0.5 M 4-tertbutylpyridine in acetonitrile, was introduced between theelectrodes by capillary forces. The active electrode area wastypically 0.27 cm2. Colloidal liquid silver (Ted Pella Inc.,Redding, CA) was placed at the electrical contacts to improvethe contact points and allowed to cure for 30 min at roomtemperature.

2.3. Characterization

2.3.1. Characterization of structures and morphologies ofZnO nanowire arrays. The crystalline structure of thesamples was characterized by using an x-ray diffraction(XRD) system (Bruker D8 ADVANCE) with Cu Kα of1.5406 A. The morphology and chemical composition of thesamples were taken with a field-emission scanning electronmicroscopy (FESEM, Hitachi S-4700). Transmission electronmicroscope (TEM) and high-resolution (HRTEM) imagesand selected-area electron diffraction (SAED) patterns wereobtained on a transmission electron microscope (JEOL JEM-2100F). Photoluminescence spectra were measured by using aphotoluminescence spectrometer (SLM48000DSCF), using anHe–Cd laser (325 nm) as the excitation source.

Figure 1. General transmission line model of ZnO-nanowire-basedsolar cells.

2.3.2. Characterization of ZnO-based DSSC photovoltaicproperties. The photovoltaic properties of the fabricated solarcells under a simulated AM 1.5G illumination with a lightintensity of 100 mW cm−2 (300 W, model 91160B, Oriel)were measured with the aid of a potentiostat (CHI-660B, CHInstruments). The electrochemical impedance spectra (EIS)measurements were carried out by applying bias of the open-circuit voltage under illumination of AM 1.5G 100 mW cm−2

over the frequency range of 10−1–105 Hz with a 10 mVac signal. Zview equivalent circuit modeling software wasused to fit the data by utilizing a built-in extended element(DX type 11-Bisquert #2), which allows for transmission linemodeling (see figure 1). In this equivalent circuit modeling, thefitted parameters denote the electron transport resistance (Rw),recombination resistance (Rk), the charge transfer constant(Keff), capacitance of the electrical double layer (Cp), electrondensity (ns) and the effective diffusion coefficient (Deff).Although experimental error is inevitable in the determinationof the parameters of each sample, it is too small (�5%) to haveany influence on the change tendency of the parameters.

3. Results and discussion

3.1. Structure and morphology

A well-aligned ultralong ZnO nanowire array was synthesizedfrom optimizing the synthetic parameters (given in thesupporting information available at stacks.iop.org/Nano/21/195602/mmedia), and its typical side-view FESEM imagesunder different magnifications were shown in figures 2(a)–(c). Low- and medium-magnification FESEM images shownin figures 2(a) and (b) indicate that well-aligned ZnO nanowirearrays with high density are observed on the surface of thesubstrate. Almost all ZnO nanowires are perpendicular tothe substrate and have a uniform length of 40 μm. Thehigh-magnification FESEM image in figure 2(c) shows thediameter of the nanowires is distributed in the range of 100–150 nm with an average value of 120 nm, and the aspect ratioof nanowires reaches up to around 330. A typical HRTEMimage of a nanowire in figure 2(d) clearly shows the distancesbetween two parallel lattice fringes, which is about 0.52 nmand corresponds to the (0002) planes of the wurtzite hexagonalZnO, indicating that the nanowires are single crystalline with

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Figure 2. Characterizations of a typical ultralong ZnO nanowire array with a length of 40 μm. (a) Low-magnification,(b) medium-magnification and (c) high-magnification side-view SEM images of well-aligned ZnO nanowire arrays, respectively. (d) HRTEMimage of an individual ZnO nanowire and (e) its corresponding SAED pattern. (f) EDS profile of ZnO nanowire arrays; the inset is the XRDpattern.

Figure 3. The length and the diameter of ZnO nanowire arrays wereplotted as a function of the PEI content in the precursor solution.

the wurtzite hexagonal phase and grow preferentially alongthe [0001] (c axis) direction, which was further confirmedby the corresponding SAED dot pattern in figure 2(e). Anenergy dispersive spectroscopy (EDS) image of ZnO nanowirearrays, as shown in figure 2(f), contains only Zn and Oelements, without any other impurity contamination. The XRDpattern in the scan range of 2θ = 20◦–80◦ was illustratedin the inset of figure 2(f). No other diffraction peaks weredetected, excluding peaks assigning to the FTO conductinglayer and ZnO nanowires. All ZnO diffraction peaks are ingood agreement with JCPDS card no. 36-1451 for a typicalwurtzite hexagonal ZnO (P63mc). A significantly higher(0002) diffraction peak intensity indicates that ZnO nanowiresare preferentially oriented along the direction perpendicular tothe (0001) crystallographic face (c-axis direction). It shouldbe noted that ZnO nanowires perpendicular to the substrate

are dominant, but there are still nanowire bundles on thisultralong nanowire array, as indicated by the arrowhead infigure 2(b). The occurrence of nanowire bundles is presumablydue to the following three reasons: (1) tilted nanowiregrowth direction caused by a thin ZnO seed layer, whichexhibits a polycrystalline structure with weak crystallinity;(2) the crowding effect resulting from the collision betweenneighboring nanowires in the solution growth process and(3) the bending and bundling of nanowires caused by surfacetension effects of water evaporation during the drying process.

3.2. Synthetic parameters

A series of experiments indicated that synthetic parametersalso dominate the diameter, length, degree of alignment andgrowth rate of ZnO nanowire arrays, including the PEI content,preheating time, growth time, seed layer thickness and zinc saltconcentration.

3.2.1. PEI content. Based on the reports by Yang [3]and Wu [20], the effect of the PEI content on the length ofnanowires was first investigated. Figure 3 shows the plot ofthe length and diameter of ZnO nanowire arrays versus the PEIcontent in solution. The typical FESEM images of the ZnOnanowire arrays synthesized at 95 ◦C for 24 h with variousPEI contents in the range from 0 to 0.0015 M was shown insupporting information (figure S1 available at stacks.iop.org/Nano/21/195602/mmedia). Without PEI, ZnO nanowire arrayswith a length of 4 μm were obtained after growing for 24 h.A magnified FESEM image shown in the inset of figure S1(available at stacks.iop.org/Nano/21/195602/mmedia) clearlyshows that every nanowire has a syringe-like morphology,exhibiting an abrupt decrease in diameter along the c-axisdirection of ZnO nanorods, which is similar to the results inpapers reported by Sun [19, 21, 22]. With increasing PEI

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Figure 4. The length of ZnO nanowire arrays was plotted as afunction of the growth time. The morphology change of ZnOnanowire arrays was shown in the inset side-view SEM images.

content, the length of nanowires sharply increases and reachesa maximum of 12 μm when the PEI content was up to 0.010 M.As the PEI content increases from 0.010 to 0.012 M, thenanowire length decreases from 10 to 6.5 μm. Moreover, afterthe PEI concentration exceeds 0.012 M, nothing was foundboth on the substrates and the growth solution. At the sametime, the ZnO seed layer was also etched off from the FTOsubstrate. Additionally, the average diameter of the nanowireslinearly decreases with increasing PEI content, as shown infigure 3. Therefore, the optimum PEI content should be locatedin the range from 0.008 to 0.010 M to obtain ZnO nanowirearrays with the maximum length and aspect ratio.

It is well known that ZnO has a polar hexagonal wurtzitestructure consisting of two polar {0001} planes and sixcrystallographic non-polar {0110} planes. The positivelycharged Zn-terminated (0001) plane has higher surface energy,resulting in the faster growth rate along the [0001] direction.PEI is a non-polar polymer with a large amount of side aminogroups (–NH2), which can be protonated over a wide rangeof pH values from 3 to 11, resulting in the increase of thesolution’s pH value and positively charged PEI molecules [22].The initial pH value of the precursor solution of Zn(NO3)2

and HMT is about 6.2, falling in the range of protonation ofPEI. Therefore, after introducing PEI, the pH value increasesand is proportional to the PEI content. After the pH valueof the growth solution is higher than the isoelectric point(IEP) of ZnO [23], the six lateral facets of the ZnO nanowireswill be negatively charged due to the dissolution of ZnO into[Zn(OH)4]2−. Subsequently, the protonated positively chargedPEI will be adsorbed onto the lateral facets of ZnO nanorodsdue to the electrostatic affinity, restraining the nanowire growthin the radial direction. At the same time, the dissolutionof ZnO, especially ZnO particle precipitation in solution,can complement the depletion of Zn2+, maintaining a highZn2+ concentration and a high c-axis growth rate during thewhole growth duration. However, as the pH value is higherthan 13, the dissolution rate is much larger than the growth rate,resulting in the decrease of length and diameter with increasingPEI concentration. The dissolution can be confirmed by the

Figure 5. The length of ZnO nanowire arrays was plotted as afunction of the preheating time. The morphology change of ZnOnanowire arrays was shown in the inset side-view SEM images.

reduced ZnO particle precipitants in solution with the increasein PEI content.

3.2.2. Growth time. Figure 4 shows the plot of the lengthof ZnO nanowire arrays versus the growth times. Clearly,under the optimum PEI content of 0.009 M, the growth timeis the key parameter to dominate the length of ZnO nanowirearrays. On extending the growth time, the color of thegrowth solution changes from colorless transparent to turbidyellow, then to light brown and finally to dark red. Thelength linearly increases from 10 to 42 μm with an additionof about 0.3 μm h−1. The steady and continuous growth ismainly derived from a continuous reactive ion supply, resultingfrom the accelerated dissolution of ZnO particle precipitationin the solution due to the introduction of PEI. At the sametime, the diameter of every ZnO nanowire shows a slightlydecreasing tendency along the c axis with increasing growthtime, which is due to the decrease of the concentrations ofthe supplied reactive ions resulting from the depletion ofthe reactive mass. It can also be found from the side-viewSEM image (inset in figure 4) that the alignment of ZnOnanowire arrays gets worse with increasing length, and variousnumbers of neighboring nanowires are slightly bent and clingto others, forming pyramid-like bundles after growing for 48 h.The bending and bundling of nanowires is a result of thebalance between the mechanical flexibility and the capillaryinteraction, derived from the increased length (or larger aspectratio) of nanowires and the larger surface tension of waterduring the drying process, respectively.

3.2.3. Preheating time. Figure 5 shows the plot of thelength of ZnO nanowire arrays versus the preheating time.Clearly, ZnO nanowires synthesized from different preheatingtimes exhibit obvious differences in length, diameter, densityand alignment degree, as shown in the inset side-view SEMimages. Firstly, the length linearly decreases from 20 to 7 μmas the preheating time increases from 1 to 12 h, as shownin figure 5. At the same time, the average diameter anddensity of ZnO nanowires decreases with increasing preheating

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Figure 6. The length of ZnO nanowire arrays was plotted as afunction of the thickness of the ZnO seed layer. The morphologychange of ZnO nanowire arrays was shown in the inset side-viewSEM images.

time. Following the decrease in diameter, the alignment degreebecomes poor, and some neighboring nanowires in an area witha diameter of 1–3 μm also bend towards the center, formingpyramid-like bundles at their tips, as shown in the inset SEMimages.

With the prolongation of preheating, the initial reagentconcentration continuously decreases due to the formation ofZnO particle precipitation in the growth solution, resulting notonly in the decrease of nucleation density on the seed layer,and consequently decreased nanowire density and diameter,but also in lowering the reagent supplying concentration andtransport rate, and consequently a decreased growth rate ofnanowires. At the same time, the mechanical flexibility ofnanowires increases with the decrease in diameter, and slendernanowires facilitate bending and aggregation under the effectof centripetal surface tension induced by the rapid evaporationof water drops on the top of nanowires in the drying process.

3.2.4. ZnO seed layer thickness. Figure 6 shows the plot ofthe length of ZnO nanowire arrays versus the ZnO seed layerthickness. The side-view SEM images of the ZnO nanowireson seed layers with thicknesses of 30, 60 and 90 nm wereshown in the inset of figure 6. Clearly, on increasing theseed layer thickness, the nanowire density increases, the degreeof vertical alignment becomes better and the length sharplydecreases. Although the nanowires with the longest lengthof 30 μm were obtained on the thinnest seed layer of 30 nmthickness, they exhibit the lowest density and the worst degreeof vertical alignment. Vertically well-aligned nanowire arrayswith a relative high density were formed on a seed layer with50 nm thickness. However, the nanowire length shows adecrease of 10 μm compared with the length of nanowiresgrown from the thinner seed layer. No significant density oralignment changes were found after further increasing the seedlayer thickness, except for a further decreased length.

The reason for the morphology change can be explainedby the diversity of surface roughness, crystal quality andreactive ion concentration resulting from variation of the seed

Figure 7. The length of ZnO nanowire arrays was plotted as afunction of the concentration of zinc salt. The morphology change ofZnO nanowire arrays was shown in the inset side-view SEM images.

layer thickness. Firstly, the surface roughness of the seedlayer increases with increasing thickness, which increasesthe active nuclei site density and restricts the migration andcoalescence of ZnO nuclei formed in the initial growth stage,finally resulting in an increase of nanowire density. Becauseof the polycrystalline feature of seeds, the crystallinity andc-axis orientation of ZnO seeds on the layer surface willbe improved due to the decrease of lattice mismatch withthe FTO substrate on increasing thickness, resulting in theimprovement of the alignment of the nanowire arrays. Atthe same time, the increased nanowire density accelerates theeffect of space confinement and selective competition betweennanowires, further enhancing the alignment degree. This isbecause the increased nanowire density pronouncedly reducesthe free growth space of an individual nanowire both in radialand axial directions. Therefore, the inclined nanowires grownfrom the seeds with tilted c axis to the substrate will collidewith neighboring nanowires and stop growing, whereas,onlythe vertical nanowires grown from the nucleation seeds alongthe c axis perpendicular to the substrate can continuouslygrow unimpeded indefinitely in the axis direction, and self-assemble into vertical well-aligned nanowire arrays. Theinverse relationship between the nanowire length and thedensity is attributed to the faster precursor consumption overdense areas, leading to lower local reactive concentration andgrowth rate. The thicker the seed layer, the higher the nanowiredensity. Therefore the greater nanowire surface area will leadto a larger precursor depletion near the surface and hence alower surface reactive concentration, consequently resulting indecreasing the growth rate.

3.2.5. Zinc salt concentration. Figure 7 shows the plotof the length of ZnO nanowire arrays versus the zinc saltconcentration in the growth solution and their correspondingside-view SEM images (inset). The ZnO nanowires werefabricated from various zinc salt concentrations ([Zn2+]) of0.01, 0.025 and 0.06 M, which all have a fixed concentrationratio of Zn(NO3)2 to HMT and PEI ([Zn2+]/[HMT] = 1,[Zn2+]/[PEI] = 4.5). Clearly, the length, the density and

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the corresponding alignment degree of nanowires increasewith increasing [Zn2+]. For a relatively low [Zn2+], ZnOnanowires randomly grow from seed-layer-coated substrateswith low density, not exhibiting an oriented growth featureperpendicular to the substrates. The length is about 30 μmafter growing for 140 h. Although no evident improvement inthe degree of alignment was found after increasing the amountof [Zn2+] to 0.025 M, the corresponding length of nanowiresincreases to 35 μm. As the amount of [Zn2+] increasedto 0.06 M, vertically well-aligned nanowire arrays with alength of 44 μm were obtained. Obviously, the improvementin the degree of alignment can only be attributed to thefurther increased nanowire density, because there is no obviousdiameter change with increasing [Zn2+]. It should be noticedthat a larger concentration of [Zn2+] was not investigatedin our case, because the plentiful emulsion precipitation ofZnO particles emerges during the preparation procedure of theprecursor solution at room temperature.

The increased nanowire density and the improvement ofthe degree of alignment are related to the increase of nucleationdensity in the initial growth stage, which results from increased[Zn2+]. The higher the precursor concentration, the larger thesupersaturation. The more the nucleation density, the higherthe nanowire density, and the better the alignment degree.The increase of the reagent concentration not only increasesthe transport rate of active ions, resulting in a relatively highnanowire growth rate, but also extends the virtual feedingtime of active ions, maintaining the relatively high growth ratethrough the whole growth duration.

3.3. Optical properties

Figure 8 shows the room temperature (RM) PL spectra ofultralong ZnO nanowire arrays with a length of 42 μm. ThePL spectrum of the as-grown sample is shown by a weakUV peak at 378 nm (3.28 eV) and a strong and broad yellowemission centered at 550 nm (2.25 eV). The PL intensity ratioof the UV emission to the yellow emission is very low (0.07),indicating the presence of high density point defects in theultralong nanowire arrays grown at low temperatures. The UVemission originates the free excitonic recombination, whichcan be observed at room temperature due to the large excitonbinding energy of ZnO (about 60 meV). It has been reportedthat thermal energy at room temperature may be enough torelease bound excitons because the binding energy of boundexcitons is only a few millielectronvolts [24]. The yellowemission is commonly reported in the PL spectrum of ZnOnanostructures and represents a common feature in samplesprepared from aqueous solutions of Zn(NO3)2 and HMT [25].This emission is typically attributed to oxygen interstitials,although Li impurity can be another possible candidate [26].Recently, hydroxyl groups are also proposed to explain theorigin of the yellow emission in ZnO nanorods fabricated bythe hydrothermal method [25].

In order to address the origin of the yellow emissions,post-annealing in various atmospheres was carried out and theresultant PL spectra were also shown in figure 8. An obviousredshift from the yellow emission to red emission centered at600 nm (2.06 eV) was observed after annealing in air at 550 ◦C

Figure 8. Room temperature PL spectra of ultralong ZnO nanowirearrays after annealing in different atmospheres.

for 1 h. At the same time, the ratio of UV to red emissionfurther decreased to 0.05. The reason for the redshift of theyellow emission and the decreased intensity ratio of the UVto red emission is due to an excessively oxidized layer formedon the surface of ZnO nanorods by annealing in oxygen-richconditions at high temperatures, resulting in increasing thedefect concentrations related to excess oxygen [27]. Therefore,it is believed that the origin of the yellow emission is notrelated to excess oxygen defects. Otherwise, an increasedintensity, rather than a redshift, could be observed in PLspectra after annealing in air due to the increase of oxygendefects. However, after annealing in vacuum at 550 ◦C for 1 h,a blueshift from yellow emission to green emission centeredat 515 nm (2.40 eV) was observed from the correspondingPL spectra shown in figure 8. The green emission is oftenattributed to the recombination of electrons and holes in singlyionized oxygen vacancies, and could be quenched or redshiftedafter annealing in an oxygen-rich atmosphere. In addition,the corresponding intensity ratio of UV to green emissionsharply increased to 1.35, indicating a decrease of the pointdefect concentration and an improvement of crystallinity. Thisresult indicates that the yellow emission in as-grown ZnOnanowire arrays is attributed to OH groups, instead of thecommonly assumed interstitial oxygen defect. The reason forthe blueshift and the enhancement of UV emission is due to thedesorption of hydroxyl groups (150 ◦C) and hydrogen (420 ◦C)on the hydrothermally grown ZnO nanowires. Therefore,vacuum annealing at 550 ◦C is an effective way to improve theoptical properties of ultralong ZnO nanowire arrays resultingfrom desorbing the hydroxyl group absorbed from the growthprecursor solution, whereas annealing in air will result incrystal degradation resulting from increasing oxygen defects.

3.4. ZnO nanowire dye-sensitized solar cells

Figure 9 shows the photocurrent–voltage (J–V ) characteristicsof ZnO nanowire dye-sensitized solar cells with variouslengths. With increasing length of the nanowire array to40 μm, the efficiency of the solar cell increases gradually. Amaximum efficiency of 1.31% was achieved in the cell with a

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Table 1. Performances and electron transport properties of N719-sensitized ZnO nanowire DSSCs with various thicknesses.

Length (μm) Jsc (mA cm−2) Voc (V) FF η (%) Rw (�) Rk (�) Kerr (s−1) C p (F) ns (cm−3) Deff (cm2 s−1)

10 2.95 0.71 0.35 0.73 15.6 199 11.9 1.2 × 10−4 3.69 × 1017 1.02 × 10−4

20 3.49 0.72 0.38 0.97 14.6 180 11.9 1.4 × 10−4 2.04 × 1017 3.98 × 10−4

30 4.15 0.70 0.38 1.13 12.7 138 8.07 1.3 × 10−4 1.21 × 1017 1.16 × 10−3

40 4.26 0.69 0.42 1.31 2.44 90 8.07 1.8 × 10−4 1.38 × 1017 7.02 × 10−3

Figure 9. J –V curves of the ZnO nanowire DSSCs fabricated fromdifferent lengths.

40 μm ZnO nanowire array, with short-circuit current density(Jsc) = 4.26 mA cm2, open-circuit voltage (Voc) = 0.69 Vand (fill factor) FF = 0.42. The improvement of the conversionefficiency comes from the increased short-circuit current, sincethe open-circuit voltage decreases slightly in the lengthenednanowire array. The enlarged surface area in the lengthenednanowire array results in improved dye loading: besides, themulti-scattering effect between the nanowires enhances theoptical path of the incident light in the nanowire array. Bothof these effects contribute to the improved energy conversionof solar cells with longer nanowire arrays.

The electron transfer processes in the nanowire arrayare further investigated by electrochemical impedance spec-troscopy at the open-circuit condition in the frequency rangeof 10−1–105 Hz. The results are fitted with the equivalent cir-cuit in figure 1 and some basic parameters of rw , rk , Ck areobtained. The Nyquist plots of both the measured data pointsand fitted curves are shown in figure 10. The electron densityns and diffusion coefficient Deff in the ZnO nanowire are cal-culated according to the process proposed by [28]. The overallcharacteristics of the ZnO nanowire solar cells are summarizedin table 1. The largest Helmholtz capacitance Cp, which cor-responds to the capacitance of the electrical double layer, isobserved for the nanowire array of 40 μm, which is incon-sistent with the enlarged surface area and dye loading capa-bility. On the other hand, the recombination resistance Rk atthe photoanode/electrolyte interface decreases gradually withincreasing length of the nanowire array, indicating more re-combination loss in the solar cells with longer nanowire ar-rays. However, by considering the enlarged surface area by

Figure 10. Nyquist plots of the impedance data of the ZnO nanowireDSSCs constructed from different lengths. The solid lines are thefitting results based on the equivalent circuit model shown infigure 1.

rk = Rk/area, the recombination process is indeed slowed inthe solar cells with longer nanowire arrays, as indicated bythe recombination coefficient Keff = 1/τeff, in which τeff isthe effective electron lifetime. Therefore, the enhanced over-all efficiency in the DSSC with long ZnO nanowire arrays istherefore mainly caused by the aggrandized surface area andslowed recombination process, as is verified by the increasedfill factor. Surprisingly, decreased electron transport resistancein the nanowire was obtained with decreasing electron density,which was in contrast with the trap/detrap transport model, inwhich electron transport was limited by the detrapping processof trapped electrons in the defects. This phenomenon couldbe attributed to locally concentrated electrons, probably at thebottom of the nanowire, or mutual scattering transfer of elec-trons in the nanowire, which are beyond the discussions of thecurrent work.

Consistently, the efficiency of the DSSC composed ofZnO nanowire arrays increases with increasing length of thenanowires, probably due to the enlarged dye loading andslowed recombination process. However, it should be noticedthat the efficiency of ZnO nanowire solar cells is still muchlower than that of porous TiO2 nanocrystalline solar cells, dueto the unfavorable formation of the Zn2+/dye-complex layer onthe ZnO nanowire surface, which inhibited electron injectionfrom dye molecules into the ZnO nanowire [29–31]. Theefficiency of the dye-sensitized ZnO nanowire solar cells couldbe further improved by better engineering the ZnO nanowireand the interface of dye molecules and the nanowire, e.g. bymodifying the ZnO nanowire with surface coatings (SiO2,Al2O3 and TiO2) to form various core/shell structures.

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Nanotechnology 21 (2010) 195602 J Qiu et al

4. Conclusion

Vertically aligned ZnO nanowire arrays with a long lengthand high aspect ratio were prepared successfully by usingthe polyethylenimine (PEI)-assisted preheating hydrothermalmethod. Several important synthetic parameters were foundto determine the growth of ultralong ZnO nanowire arrays.The lengths of ZnO nanowire arrays are mainly determinedby PEI content and growth time, whereas the preheating time,the thickness of the ZnO seed layer and the concentrationof zinc salt have a large influence on the density andthe alignment degree of ZnO nanowire arrays. The roomtemperature photoluminescence (PL) spectrum of as-grownultralong ZnO nanowire arrays reveals a UV emission anda yellow emission. The yellow emission was attributed tothe absorbed hydroxyl group based on the peak shift afterannealing at various atmospheres: a redshift after annealing inair and a blueshift after annealing in vacuum. The performanceof dye-sensitized solar cells (DSSCs) increased with increasinglength of the ZnO nanowire arrays, indicating that the ultralongZnO nanowire arrays have great potential in improving theperformance of dye-sensitized solar cells.

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