fabrication of cds/hg(1−x)cdxte nanowire heterostructures on conductive glass using templated...

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Materials Science in Semiconductor Processing

Materials Science in Semiconductor Processing ] (]]]]) ]]]–]]]

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Fabrication of CdS/Hg(1�x)CdxTe nanowire heterostructureson conductive glass using templated electrodeposition

Brian A. Ashenfelter, Terry P. Bigioni n

Department of Chemistry and Wright Center for Photovoltaics Innovation and Commercialization, University of Toledo, 2801 W. BancroftSt., Toledo, OH 43606, USA

a r t i c l e i n f o

Keywords:Cadmium sulfideMercury cadmium tellurideNanowireHeterojunctionMultiple exciton generationAnodic aluminum oxideTemplateElectrodepositionGalvanic contact deposition

01/$ - see front matter & 2013 Elsevier Ltd.x.doi.org/10.1016/j.mssp.2013.08.015

esponding author. Tel.: þ1 419 530 4095; faail address: [email protected] (T.P. B

e cite this article as: B.A. Ashenfeltrg/10.1016/j.mssp.2013.08.015i

a b s t r a c t

Single-material and heterojunction nanowires were fabricated from CdS and Hg(1�x)CdxTe(MCT) in anodic aluminum oxide templates grown on transparent conductive oxide coatedglass substrates. Structural and compositional analyses were carried out by electronmicroscopy,elemental analysis and x-ray diffraction. CdS was deposited using potentiostatic electrodeposi-tion, forming CdS nanowires with a 1:1 stoichiometry and wurtzite structure. MCT was alsodeposited using potentiostatic electrodeposition, forming MCT nanowires with a stoichiometryof Hg0.24Cd0.76Te and a zinc blende structure. Annealing of the electrodeposited MCT nanowiresincreased crystallite size from �9 nm to �23 nm in the (1 1 1) direction. Heterojunctionnanowires were prepared by sequential electrodeposition of CdS and MCT with control overthe length and diameter. These materials have a desirable band gap and geometry for multiple-exciton generation studies for nanostructured photovoltaics applications.

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1. Introduction

Increasing the efficiency and decreasing the cost of solarcells is a priority in the development of photovoltaics (PV).Third generation PV has the general promise of uniqueand tunable physical properties and inexpensive syntheticroutes [1]. This will require the development of novel struc-tures, materials, processing methods, and strategies, however,if it is to have a positive impact on device performance.

Nanostructures inherently have the disadvantage of pos-sessing more numerous interfaces compared to bulk or thinfilm materials, so it is imperative that their use enables anadvantageous new strategy. The generation of multipleexcitons from the absorption of a single photon is one suchexample [1–11]. Typically, when the energy of an absorbedphoton exceeds the band gap energy (Eg) of the semicon-ductor, the excess energy is dissipated as heat as the electronthermalizes through phonon scattering. With multiple-exciton generation (MEG), however, if there is sufficient

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er, T.P. Bigioni, Materia

excess energy (hν42Eg), then a second electron has thepotential to be excited by the process of impact ionization. Inthis case, the high-energy electron transfers its extra energythrough a collision with a less energetic electron, exciting itinto the conduction band [4]. If the incident photon energyhν4nEg, then in principle, as many as n electrons may beexcited. Detailed balance calculations have shown thatsingle-junction MEG devices could attain conversion effi-ciencies as high as 44%, with an optimal band gap between0.80 eV and 1.1 eV [4]. This efficiency limit is close to that of amultijunction device [12] but with the simpler constructionand engineering of a single-junction device.

Although MEG has been observed in bulk materials, it isthought to be more efficient in nanostructures since theirdiscrete energy levels suppress phonon scattering [4].While MEG has been measured in PbS, PbSe, PbTe, andCdSe nanocrystals [2], device performance typically suffersdue to poor charge transport across the many interfaces ofthe nanocrystalline thin films [13]. Also, the MEG thresholdenergy for nanocrystals is, in practice, nearly 3Eg, not 2Eg aspredicted by theory [4]. One-dimensional (1D) nanowiresmight offer solutions to both of these problems, however.Recent reports state that a MEG threshold of just 2.2Eg was

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found in nanowires [4]. Further, their axial connectivityought to greatly improve charge transport [4,13]. Semicon-ductor nanowire arrays are therefore attractive for theconstruction of solid-state PV devices that exploit MEG.Here, we establish electrochemical methods of fabricatingaligned arrays of Hg(1�x)CdxTe (MCT) nanowires. The arraysare embedded in an optical quality Al2O3 matrix, enablingspectroscopic study of MEG in 1D nanostructures. With thisarchitecture, contacts could also be made to either end ofthe nanowires such that transport measurements could bemade of MEG-based PV devices.

MCT was chosen because its band gap can be tunedbetween that of CdTe (1.5 eV) [14] and HgTe (�0.03 eV)[14] by varying the stoichiometry (x) of the Hg(1�x)CdxTealloy. With electrodeposition, adjusting the depositionpotential can control the stoichiometry. In this way, theoptimal band gap for MEG can be achieved, wherex¼0.65–0.80 [15–17]. Electrochemical synthesis of MCTthin films has been reported [15,18–21]. In fact, electro-deposited CdS/MCT thin film PV devices with an efficiencyof 10.6% have been demonstrated [16].

Nanoporous anodic aluminum oxide (AAO) was chosenfor the template as electrodeposited semiconductor nano-wire arrays have been previously reported for relatedmaterials [22–26]. Heterostructures, such as p–n junctions,can also be fabricated by sequential deposition of differentsemiconductors into the same template. In this case,nanowire arrays have been electrodeposited into AAO thatwas grown on transparent conductive oxide (TCO) coatedglass [22–31]. The strategy for fabricating MCT nanowiresreported here uses this approach, as depicted in Fig. 1.

2. Experimental

2.1. Anodic aluminum oxide (AAO) templates

Commercially available TCO-coated glass was obtainedfrom Pilkington (Tec-15). The glass substrates were cleaned

Fig. 1. Fabrication strategy for electrodepositing semiconducto

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ultrasonically in a warm 10% microsoap solution for 30 min.This was followed by two ultrasonic rinses in deionizedwater, and an ultrasonic rinse in ethanol. Substrates werethen dried with Ar gas and baked at 80 1C overnight. E-beamevaporation was used to deposit a �10 nm adhesion layer ofTi followed by a 4 μm film of Al onto the cleaned TCO surface.

Al films were anodized at room temperature in 0.3 Moxalic acid using a MPJA model 9305 PS DC power supply anda two-electrode cell with constant stirring. The Al-coated TCOwas used as the working electrode and graphite was used asthe counter electrode. Substrates were anodized at a constantpotential in a two-step process. The first anodization wasconducted at 40 V for 12 min. The resulting disorderedalumina film was etched away in a 0.2 M phosphoric acidand 0.4 M chromic acid solution at 60 1C for 15 min. After thedisordered oxide filmwas completely removed, the patternedAl filmwas anodized for a second time at a constant potentialof 15–40 V for �20min. The volume expansion during theconversion of Al to AAO is 1:1.8, generating about 3 μmof AAOafter both anodization steps. The resulting film was opticallytransparent and completely anodized; no Al metal remained.

The barrier layer was thinned during the final stages ofanodization by stepping the potential down by 1 V/minuntil 0 V was achieved. Following anodization, the barrierlayer was completely opened by a pore-widening step,wherein the substrate was immersed in 10% phosphoricacid for 5–45 min. This enabled DC electrodeposition ofsemiconductor materials directly into the pores of thetemplate and onto the underlying TCO electrode.

2.2. Electrodeposition

Electrodeposition was carried out potentiostatically in athree-electrode cell using a Princeton Applied Research Model263A Galvanostat/Potentiostat. The AAO/TCO template wasused as the working electrode and graphite was used as theauxiliary electrode. Single-material and heterojunction nano-wire arrays were achieved on TCO/glass by electrodepositing

r nanowires into an AAO template on TCO-coated glass.






B.A. Ashenfelter, T.P. Bigioni / Materials Science in Semiconductor Processing ] (]]]]) ]]]–]]] 3

CdS and MCT into the pores of the AAO template using amethod similar to that reported elsewhere [22,24–26].

2.2.1. CdS electrodepositionCadmium sulfide was potentiostatically electrodeposited at

a constant voltage of �1.0 V, using Cd wire as the referenceelectrode. The electrolytic bath consisted of 0.055M CdCl2 and0.19 M elemental sulfur dissolved in dimethyl sulfoxide at110 1C.

2.2.2. MCT electrodepositionMercury cadmium telluride was grown by potentio-

static electrodeposition. All MCT was deposited from an

Fig. 2. (a) Photograph of an AAO template fabricated on TCO-coated glass. ((b)–(fcoated glass, (c) AAO template with irregular pore structure after one anodizatioanodization step, (e) AAO template on TCO-coated glass that has not had the baresistive barrier layer was completely removed.


electrolytic bath consisting of an aqueous solution of 1.0 MCdSO4, 1.5 mM TeO2, and 0.15 mM HgCl2. The pH wasadjusted to �1.5 using concentrated H2SO4 and thedeposition temperature was held at 91 1C. Electrodeposi-tions were carried out at �0.700 V, using an Ag/AgClreference electrode with constant stirring.

2.3. Annealing treatment

CdS nanowires were annealed at 400 1C for 10 min inair. MCT nanowires were annealed in a tube furnace with aflowing mixture of 95% Ar and 5% H2 at 280 1C for 10 min.

)) SEM images showing (b) cross section of a typical AAO template on TCO-n step, (d) AAO template with more regular pore structure after a secondrrier layer thinned or removed, and (f) AAO on TCO-coated glass after the







2.4. Powder x-ray diffraction (pXRD)

Powder x-ray diffraction data were collected using aPANalytical X′pert Pro MPD diffractometer equipped with aCu Kα radiation source (λ¼1.54056 Å). Nanowires weremeasured while oriented in the AAO template. Peaks werefit using the PANalytical X′pert High Score software andcrystallite sizes were calculated using the Scherrer formula.

2.5. Electron microscopy

Scanning electron microscopy (SEM) and transmissionelectron microscopy (TEM) images were acquired using aJEOL JSM-7500F microscope. Nanowires were liberatedfrom the templates for imaging by dipping the substratein 1 M NaOH for at least 30 min to dissolve the AAO. Wireswere imaged on the TCO substrate and dispersed onto Cugrids. High-resolution TEM (HRTEM) images were acquiredusing a Hitachi HD-2300A scanning transmission electronmicroscope (STEM) with 200 keV accelerating voltage.Samples were prepared on 200 mesh lacey carbon or SiOcoated Cu grids.

2.6. Elemental analysis

Energy dispersive x-ray spectroscopy (EDS) spectrawere collected on a JEOL JSM-7500F microscope using aBruker XFlash 6|60 detector with 129 eV resolution.

3. Results

3.1. AAO on TCO/glass substrates

Growing AAO directly on bare TCO can be challengingdue to vigorous gas evolution at the alumina–TCO inter-face and subsequent delamination of the film duringanodization. Electrochemical degradation of the TCO hasalso been reported after direct exposure to anodizationconditions [29], which can significantly reduce the con-ductivity of the TCO film making it unsuitable for electro-deposition. Depositing a thin (�10 nm) adhesion layer ofTi between the TCO and Al films significantly improvesadhesion and makes AAO film growth possible [29], sinceanodization of Al films with Ti adhesion layers proceededwithout gas evolution or delamination.

After complete anodization, the resulting films weresmooth and optically transparent over their entire areasand the TCO retained its conductivity. A typical AAOtemplate fabricated on TCO-coated glass is shown inFig. 2a. A cross section of the resulting AAO template isshown in Fig. 2b. A two-step anodization process wasessential to creating an ordered through-pore nanoarray [30].When the substrate was completely anodized in a single step,the resulting pore structure in the oxide film was irregular, asshown in Fig. 2c. By adding a pre-anodization step, pores withuniform diameters and more regular packing were achieved,as shown in Fig. 2d.

Control over pore diameter has been previously demon-strated by adjusting the applied potential and the porewidening time [32]. Here, substrates were anodized atpotentials between 15 V and 40 V and pore widened for


5 min to 45 min, producing pores ranging in size from 20 nmto 100 nm in diameter.

To facilitate electrodeposition into the AAO pores, theresistive barrier layer at the bottom of the pores (Fig. 2e) wasremoved. Ramping down the applied potential at the end ofanodization and subsequently widening the pores in 10%phosphoric acid successfully opened the pores to expose theunderlying TCO electrode, as shown in Fig. 2f [29].

3.2. CdS nanowire arrays

CdS nanowires were electrodeposited into the pores ofan AAO film on TCO/glass, liberated from the AAO matrix,and then imaged by SEM, as shown in Fig. 3a. Thenanowires were compact and straight with an averagenanowire diameter of �75 nm, in agreement with thepore structure of the AAO template. The UV–vis absorptionspectrum of the wires shown in Fig. 3b indicated a bandedge near the bulk band gap of 2.42 eV. Powder x-raydiffraction of the CdS nanowires embedded in the AAOmatrix (Fig. 3c) showed that the CdS was polycrystallinewith a wurtzite structure. Annealing the nanowires in airat 400 1C for 10 min led to recrystallization and an increasein grain size, as shown in Fig. 3c. The as-deposited CdScrystallite size was 10.4 nm, which increased to 12.4 nmupon annealing. Crystallite sizes are tabulated in Table 1.

3.3. MCT nanowire arrays

MCT nanowires were grown inside the pores of an AAOfilm on TCO/glass using potentiostatic electrodeposition.Fig. 4a shows an SEM cross-section image of the electro-deposited MCT nanowires, still embedded in the AAOtemplate. Although the nanowires do not appear to becompact and monolithic, their broken structure is due tothe fracturing process. This is better seen in the unbrokennanowires shown in Fig. 4b, which were liberated from theAAO pores by dissolution of the template. Althoughmonolithic, the MCT nanowires were not as compact asthe CdS nanowires.

Atomic resolution HRTEM imaging of as-depositednanowires (see Fig. 4c) shows a lattice spacing of 3.7 Å,which indicates growth in the (1 1 1) direction for thisparticular wire. Analysis by x-ray diffraction found similarcrystallite sizes in all directions, however, revealing thatthere was no preferred growth direction (see Table 1).

The pXRD pattern showed that the MCT nanowireswere polycrystalline with a zinc blend structure, as shownin Fig. 4d. Improvement of the nanowire crystallinity byannealing was not trivial, however. Annealing of the MCTnanowires at 400 1C for 10 min resulted in the completeloss of material due to volatilization of mercury from thelattice. To overcome this instability, the MCT nanowireswere instead annealed in a reducing environment of 95%Arþ5% H2 and at a decreased temperature of 280 1C. Thissuppressed the loss of MCT from the array and promotedgrain growth in the (1 1 1) direction by more than a factorof 2.5.

In order to use MCT nanowires to study MEG, the bandgap and therefore the stoichiometry needs to be controlled.Changing the applied deposition potential was successfully






Table 1Electrodeposited nanowire crystallite sizes as determined by powder x-ray diffraction.

hkl

CdS nanowires (1 0 0) (1 0 1) (1 1 0) (1 0 3)As deposited 103.65 Å 83.86 Å 43.03 Å 99.08 ÅAnnealed 123.97 Å 113.31 Å 61.79 Å 88.72 Å

Potentiostatic MCT nanowires (1 1 1) (2 2 0) (3 1 1)As deposited 88.11 Å 71.89 Å 61.54 ÅAnnealed 208.24 Å 190.79 Å 200.23 Å

Fig. 3. (a) Side view SEM image of CdS nanowires on TCO/glass. (b) UV–vis absorption spectrum of CdS nanowires embedded in an AAO template on TCO/glass. (c) pXRD pattern for CdS nanowires embedded in an AAO template before and after annealing. TCO peak positions are indicated by n.


used to vary the stoichiometry of the nanowires. Composi-tions suitable for obtaining band gaps in the MEG windowwere achieved at �700 mV vs. Ag/AgCl. This correspondedto a stoichiometry near x¼0.76, as measured by EDS.

3.4. CdS/MCT heterojunction nanowires

Heterostructures were fabricated by sequential electrode-position of CdS and MCT into the AAO template. Fig. 5a showsa top-view SEM image of an oriented array of CdS/MCTnanowires after the AAO matrix was dissolved. The hetero-junctions were imaged using TEM and low-angle backscat-tered electron (LABe) imaging, both of which show a clearZ-contrast between the CdS and MCT regions of the nano-wires (see Fig. 5b and c, respectively). The nanowires werewell formed and straight. Diffraction measurements verifiedthat the heterojunction nanowires contained polycrystalline


CdS and MCT, as shown in Fig. 5d, with grain sizes similar tothose of the single-material nanowires. The stoichiometry ofeach segment of the heterostructures was analyzed using EDS,as shown in Fig. 6. The nanowires were confirmed to becomposed of MCT, with the composition Hg0.24Cd0.76Te, aswell as stoichiometric CdS. The MCT composition correspondsto a band gap of about 1.0 eV, which lies within the MEGwindow.

4. Discussion

It was possible to control the stoichiometry of the MCTnanowires with potentiostatic electrodeposition. The com-position of MCT could be adjusted from Hg-rich to Cd-rich byapplying more negative potentials. The potentials for whichMCT deposition was stoichiometric ranged from �450 mVto �750 mV vs. Ag/AgCl, as judged by the current-limited






Fig. 4. (a) SEM cross-section image of potentiostatically deposited MCT nanowires embedded in the AAO template grown on TCO-coated glass. (b) MCTnanowires liberated from the AAO pores by dissolution of the template. (c) Atomic resolution HRTEM image of MCT nanowire indicating (1 1 1) orientationalong the wire axis. (d) pXRD pattern of potentiostatically electrodeposited MCT nanowires. Wires showed crystallite size improvement of 2.5 times uponannealing at 270 1C for 10 min in 95% Arþ5% H2.


plateau in a linear sweep voltammetry scan. Within thiswindow, stoichiometries were determined to range fromx¼0.25 for �450 mV and x¼0.76 for �700 mV. For thepresent study, MCT was most often deposited at �700 mV


vs. Ag/AgCl, which produced material with a composition ofx¼0.76. This corresponds to a band gap of 1.0 eV, which iswithin the MEG window. In contrast, growth of CdS was notsensitive to the deposition potential. Deposition of CdS was






MCT

CdS CdS

MCT

200nm 200nm200nm

MCT/CdSannealedCdSannealed

CdSas deposited

MCT(111)

CdS(002)

*

20

25

30*

MCT(220)

MCT(311)

CdS(100)

CdS(101) *

CdSCdS10

15

(110)

* (103)

24 26 28 30 32 34 36 38 40 42 44 46 48

5

0=SnO2(TCO)*

Intensity/100

2θ (degrees)

Fig. 5. (a) Top view secondary electron SEM image of CdS/MCT heterojunction nanowires. (b) TEM image of CdS/MCT heterojunction nanowires grown bypotentiostatic electrodeposition. (c) Low-angle backscattered electron image of CdS/MCT heterojunction nanowires grown by potentiostatic electro-deposition. (d) pXRD pattern following the progression of forming a CdS/MCT heterojunction on the same array of nanowires.


generally done at an over-potential and always resulted in a1:1 stoichiometry.

MCT thin film analogs electrodeposited on TCO-coatedglass tended to have larger grains than the MCT nanowires.This is not surprising since the constraints imposed on thegrowth by the template ought to limit grain growth. It is alsopossible that the pore walls could serve as nucleation sitesand promote the growth of many small grains, although theinsulating nature of the oxide ought to suppress this.Annealing the MCT nanowires promoted grain growth upto �20 nm in size, a dimension that matches the porediameter of the template (see Table 1). Electrodepositionoften leads to oriented crystal growth as the substrate canact as a template during the initial stages of growth. The factthat the grains are essentially uniform in size, based on thex-ray diffraction analysis, and not larger than the templatepore diameter supports the conclusion that these nanowiresdo not exhibit oriented growth. No oriented growth alongthe pore axis was observed.

In contrast, we found that MCT thin films tended togrow preferentially in the (1 1 1) direction perpendicularto the substrate. This generated large columnar grains thatspan the thickness of the film, which may facilitate chargetransport across the material. This continuity across thefilm may be one of the reasons that an electrodepositedCdS/MCT thin film solar cell was able to achieve an


efficiency of 10.6% [16]. Oriented growth of MCT nanowirescould potentially lead to single-domain nanowires thatspan the template, which would be ideal materials for PVapplications. This remains a technical challenge for thesematerials.

Galvanic contact deposition was also investigated inorder to contrast the results with potentiostatic electro-deposition. For galvanic contact deposition, the workingelectrode (template or TCO/glass) was immersed in theMCT electrodeposition bath and connected to a strip of Alfoil, which set the potential in place of the potentiostat.This galvanic cell seemed to deposit a more uniform MCTwith a consistent stoichiometry, perhaps because of theability of the system to readjust the potential based onthe local conditions at the working electrode surface. Theinternal galvanic potential was about �705 mV, which wasclose to that applied during potentiostatic deposition andtherefore produced the desired stoichiometry. Like poten-tiostatic deposition, galvanic contact deposition was notfound to produce oriented growth of MCT nanowires.Nevertheless, galvanic contact deposition was a simpleand reliable deposition technique that did not require anexternal power source and consistently produced uniformmaterial with a suitable stoichiometry.

The mechanical stability of CdS/CdTe thin film hetero-junctions is not affected by the large (�10%) lattice






CdSMCT

150nm

Cu12

14O12

Cu

Si

8

10Cu

Cd

S

SiCu

6

8

10

CuTe

CdHg

2

4

6

Cu2

4Inte

nsity

/100

0

Energy (keV)

02 4 6 8 10 12 14

Energy (keV)1 2 3 4 5 6 7 8 9

0

Fig. 6. (a) Low-angle backscattered electron image of a CdS/MCT heterojunction nanowire. (b) EDS spectrum of the CdS segment, with a 1:1 stoichiometry.(c) EDS spectrum of the MCT segment, with a stoichiometry of x¼0.76. Cu, O, and Si peaks are from the SiO coated copper TEM grid.


mismatch between the two phases [33], due to their largecontact area. It is somewhat surprising, however, that CdS/CdTe or CdS/MCT heterojunction nanowires share thismechanical stability. It is clear from electron microscopyimages, such as those in Figs. 5 and 6, that the hetero-junction nanowires are mechanically robust since samplehandling, including liberation from the template, did notresult in separation of the two phases. While mechanicalstability does not guarantee good charge transport acrossthe interface, poor mechanical stability implies poor inter-facial charge transport.

Control over the diameter of the nanowires wasachieved by tuning the diameter of the pores with theanodization voltage [27–30]. Post-anodization pore widen-ing was also used to increase and fine tune the size of thenanowires, such that diameters between 20 nm and100 nm were synthesized. The 20 nm pores were preparedby applying 15 V and pore widening for just 5 min. Apply-ing 40 V and pore widening for 45 min resulted in the100 nm pores.

It is worth considering the effect of nanowire diameter onthe band gap. The Bohr exciton radii of CdS, CdTe and HgTeare 2.8 nm [34], 7.5 nm [35], and 40 nm [36], respectively.Given the large diameters of the nanowires discussed here,properties of the CdS were not affected by quantum con-finement. This was confirmed by extrapolating the band gapof the CdS nanowires from their UV–vis absorption (seeFig. 3b). For MCT nanowires, however, quantum size effectscould become important below diameters of 40 nm. Based


on the Cd-rich composition, it is expected that the band gapwould be closer to that of CdTe than HgTe, however. While itwould seem reasonable that the nanowire diameter wouldnot affect the MCT band gap for the diameter rangeconsidered here, absorption measurements were not ableto confirm a clear band edge location.

Recombination due to the presence of defects and trapstates leads to a reduced performance in PV devices andhas been shown to be the dominant factor limitingminority carrier diffusion lengths in single-crystal siliconnanowires [37]. In particular, surface recombination leadsto a 60 mV decrease in open-circuit voltage for every orderof magnitude increase in surface area [37]. Polycrystallinenanowires, such as those presented here, contain manyinterfaces and have large total surface areas, thereforeresolving or passivating these interfaces will be key tooptimizing PV performance. To that end, both annealing toeliminate defects and using common surface-passivationtechniques have been shown to dramatically reduce thetrapping of charges [37].

The optical density of these nanostructured thin films isalso important when considering use of this nanowire archi-tecture for PV applications. Without pore widening, approxi-mately 25% of the cross-sectional area of the film is made upof semiconductor material, which effectively reduces theabsorptivity of the films by 75% compared to a continuousthin film of the same thickness. Increasing the diameter of thenanowires by pore widening can improve light absorption forphotovoltaic applications; however, a balance may be needed







to optimize MEG efficiency. In this case, it is possible toincrease the optical density of the films by increasing thelength of the nanowires (note: gaps between nanowires aremuch smaller than thewavelength of incident solar radiation).By increasing the growth time, the lengths of the nanowirescould be adjusted from a few hundred nanometers up toseveral microns depending on absorptivity requirements.

5. Conclusions

The ability to fabricate large-area, optically-transparentAAO templates on TCO/glass has been shown. These porousoxide templates have tunable pore diameters and are ideal foroptoelectronic device construction. The underlying TCO ofthese templates retained good conductivity after completeanodization of the Al film such that nanowire arrays could beelectrodeposited across the entire electrode. CdS/MCT p–nheterojunction nanowires were fabricated by sequentialdeposition of CdS and MCT into the template and were foundto have well-defined junctions between the materials. CdSnanowires were found to have polycrystalline wurtzite struc-tures and MCT nanowires were found to have polycrystallinezinc blende structures, whether deposited as single-materialnanowires or as heterostructures. Low temperature annealingof potentiostatically deposited MCT nanowires promotedgrain growth of up to 2.5 times. The stoichiometry of theMCT was controlled by the deposition potential by potentio-static deposition, however galvanic contact deposition wasfound to be a simpler deposition technique that consistentlyproduced uniform material with a composition of x¼0.76,which is a suitable stoichiometry for MEG studies.

Acknowledgements

We thank the Air Force Research Laboratory, the WrightCenter for Photovoltaics Innovation and Commercializa-tion, the School of Solar and Advanced Renewable Energyat the University of Toledo, and the National ScienceFoundation (Grant no. 0840474) for support.

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