Compound Semiconductor Nanowire Solar Cells

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Compound Semiconductor Nanowire Solar CellsKe Sun, Student Member, IEEE, Alireza Kargar, Student Member, IEEE, Namsoek Park,

Kristian N. Madsen, Student Member, IEEE, Perry W. Naughton, Timothy Bright,Yi Jing, and Deli Wang

(Invited Paper)

Abstract—There have been many recent developments in com-pound semiconductor nanowire photovoltaic devices. Of these, ad-vances in nanowire synthesis and performance enable nanowires tobe implemented for efficient and low-cost solar-energy-harvestingdevices. On the other hand, many challenges in device fabricationmust be resolved in order for nanowires to assure a role at theforefront of solar cell technology.

Index Terms—Compound semiconductor, nanowires, solar cell.

I. INTRODUCTION

ENERGY production is among the top problems that hu-manity will face over the next century. Consequently, many

organizations from around the world are searching for alterna-tives to fossil fuels that are low cost, sustainable, and clean.For example, as a national initiative, the USA has spent over5 billion dollars on related research related to alternative andcleaner energy. Among these clean energy sources, solar energyis one of the most promising and fastest growing renewable en-ergy sources worldwide. Over the past 10 years, the photovoltaic(PV) industry has seen double-digit growth; in 2008, solar panelinstallation increased by 110% from what it was in 2007. Con-verting solar energy into electricity or hydrogen fuel using PVcells is one of the most attractive solutions to modern energyissues because solar energy is produced energy with almost zerocarbon-emission, which limits carbon-emissions, limits the con-centration of green-house gases in the atmosphere, potentiallyslows the global climate change [1]. Despite this tremendousgrowth, however, solar power still accounts for share less than0.1% of global energy generation because of its high cost ofproduction [2].

A large variety of thin film and nanomaterial technologiesare being actively researched due to their potentially low-costproduction (less materials used) and possibility of higher perfor-mance than current crystal Si technology. These nanostructured

Manuscript received August 28, 2010; revised October 26, 2010; acceptedOctober 26, 2010. This work was supported in part by the Department of Energy(DOE) under Grant DE-FG36-08G018016, and in part by the National ScienceFoundation under Grant ARRA ECCS0901113. The work of D. Wang wassupported by the World Class University (WCU) Program at Sunchon NationalUniversity, Korea, Abgent, Inc., and AEM, Inc.

K. Sun, A. Kargar, N. Park, K. N. Madsen, P. W. Naughton, Y. Jing, andD. Wang are with the University of California at San Diego, La Jolla, CA 92093USA (e-mail: [email protected]).

T. Bright is with the Oral Robert University, Tulsa, OK 74171 USA and waswith the University of California at San Diego, La Jolla, CA 92093 USA.

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/JSTQE.2010.2090342

materials offer very attractive advantages in photon absorptionand carrier confinement. In particular, when compared to con-ventional Si PV devices, semiconductor nanowires (NWs) offerthe advantages of enhanced light absorption, efficient carrierseparation and collection at contact electrodes, and low-costprocessing using fewer materials [3], [4]. Great efforts havebeen made to fabricate 1-D NWs and ordered NW arrays forPV device applications based on top-down or bottom-up tech-niques [5]–[7]. NW solar cells based on a single NW or NW ar-rays have been theoretically and empirically demonstrated usinggroup IV, III–V, or II–VI compound semiconductors [8]–[15],allowing semiconductor NWs to emerge as building blocks forPV devices [16].

In this paper, we aim to review the recent developments ofcompound semiconductor NW PV devices, including a com-parison to Si NW devices. We illustrate advantages in mate-rial synthesis and device performance, along with challengesof device fabrication, and conclude that NW structures providebeneficial properties for efficient solar-energy-harvesting at lowcost. In Section II, we illustrate the structure of NW solar cellsand the advantages in light absorption, photogenerated chargeseparation and collection. In Section III, we review compoundsemiconductor NW solar cells, where Si/Ge NW solar cells aresummarized for comparison purposes. The use of NW arraysfor semiconductor-sensitized solar cells (SCSSCs) and organic-inorganic hybrid solar cells are outlined in Section IV. SectionV concludes and provides perspectives.

II. NANOWIRE SOLAR CELLS

The process in which light is converted to electricity in a solarcell involves four stages: light absorption, charged carrier gen-eration, separation, and collection. Considering the efficiencyin each stage, we can formulate the overall light conversionefficiency in the equation below:

η =∏

i

ηi (1)

where η is the overall efficiency, and i = a, g, s, and c representefficiency for absorption, generation, separation and collection,respectively. NW structures, particularly vertical NW arrays,offer significant advantages in enhancing the efficiency of allthe stages in the light harvesting process, making them attractivefor solar cell devices.

A. Nanowire Solar Cell Structure

NW solar cells can be in the form of either a single NW or anarray of NWs, which are usually in an aligned vertical geometry

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Fig. 1. (a) Schematic of NW array solar cell structures, (b) NW with axialmultiple junctions from semiconductors with different band gaps, (c) NW withradial multiple junctions, (d) NW with axial p/n junction, (e) NW with radialp/n junction, and (f) NW with radial heterojunction.

[see Fig. 1(a)]. p/n junctions in NW solar cells can be formedin radial or axial directions. For axial NW PVs, the electrodecontacts are placed on the p- and n- regions, respectively, whilethe contacts of the core/shell NWs are placed at the bottom andtop of NWs. NW arrays for solar-cell applications are normallyfreestanding NWs with abrupt junctions for charge separation.These NWs have sizes comparable to characteristic scale offundamental solid-state phenomena [16], and offer potential2-D semiquantum confinement of carriers. Vertical NW ar-rays enhance light absorption and improve both charge sepa-ration and collection efficiencies. Fig. 1(b) and (c) shows theformation of axial and radial multijunction NW heterostruc-tures, which enable-enhanced light absorption by solar spec-trum and through the formation of stacked heterojunction solarcell devices. Fig. 1(d)–(f) show the axial p/n junction, radial(core/shell) p/n junction, and type-II heterojunction structures,along with their respective energy band diagrams. Other benefitsNWs offer are their large surface-to-volume ratio for effectivechemical and catalytic reactions and a large number of surfacestates to minimize dark currents [17]. NW array PV devices al-low the use of low-cost materials and substrates, because NWscan be grown epitaxially on lattice mis-matched or amorphoussubstrates. NWs are manufactured via sufficiently controllabletop-down and bottom-up fabrication techniques that can poten-tially lead to economical manufacturing [18].

B. Light Absorption in NW Arrays

Light absorption is critical for efficient energy conversion inPV devices. Broadband suppression of specular and diffusive re-flection using nanostructure arrays over a wide range of incidentangles has been demonstrated [19], [20]. As the theoretical workby Huang et al. demonstrates [20], Si substrates decorated withNW arrays show reduced reflection and are insensitive to thelight polarization and the angle of incidence incident (AOI) overa broad range of wavelengths (250–2000 nm). By comparison,a polished planar Si structure shows significantly higher reflec-tion and incident light polarization dependence, yielding 10%differences in reflectance between s- and p-polarized light. Incontrast, the reflectance from an aperiodic Si NW array demon-

Fig. 2. (a) Gradient refractive-index profile simulation (reproduced with per-mission from [20], copyright Nature Publishing Group), and (b) transmissionspectrum of NW array (reproduced with permission from [19], copyright Amer-ican Chemical Society).

strates minimum sensitivity to the AOI compared to periodicallycoated and nonperiodic porous substrates. This improved lightabsorption in NWs is primarily caused by a gradual reductionof effective refractive index [21] and internal scattering cen-ters [22].

In 1879, Lord Rayleigh proposed the principle of reflectionsuppression based on a gradual reduction in effective refractiveindex. He showed that inhomogeneous structures essentiallyremove the sharp interface between the media and substrateproviding a gradual transition in the refractive index [21]. LikeLord Rayleigh’s model, the functionality of an inhomogeneousaperiodic Si NW layer can be modeled as a series of thin filmswith small refractive index difference between each [23], [24].In this model, size and shape of the NW plays an important rolein obtaining the best absorption. For example, Cui’s group fromStanford has demonstrated that nanocone arrays with graduallyvarying radial size help to smooth the transition of effectiverefractive index, and thus, reduces reflectance when comparedto Si NW arrays of fairly uniform radial size [25]. Also, otherrecent work on ZnO-based nanostructures also supports thisphenomenon [26], and based on this physical concept, Javey’sgroup at UC-Berkley has introduced dual-diameter Ge NWsto reduce light reflection and enhance light absorption. This isachieved by using a structure composed of smaller NWs onthe top and large NWs at the bottom, capturing the gradienteffect [27].

Besides shape, NW length is also critical for light absorptionoptimization [see Fig. 2(b)]. Generally, longer NWs are believedto show better performance with short wavelength due to afiner refractive-index gradient at the air-solid interface [20],

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SUN et al.: COMPOUND SEMICONDUCTOR NANOWIRE SOLAR CELLS 3

which also shows dependence on the extinction coefficient ofthe material [4]. However, the effect of NW length on lightabsorption is sophisticated because diffuse scattering becomessignificant for longer NW devices, leading to diffuse reflectance.This is more pronounced, particularly when the wavelength iscomparable to the physical dimensions of the NWs [22]. Theother drawback of longer and thinner NWs is a decrease intheir moment of inertia and spring constant. Longer and thinnerNWs are more flexible, allowing van der Waal forces to collapsethe NWs into disordered clusters, which consequently increasesdiffuse reflectance also and charge recombination as well [28].

Light absorption enhancement in InAs/InP [29] and Si [30]NW arrays has also been studied by simulations conducted us-ing a 2-D wave-guiding approach. Power dissipation along theNW array exhibits dependence on the dispersion characteristics,which include the input light radiation and dielectric propertiesof both the surrounding media and active material. Most impor-tantly, these studies suggest that the absorption characteristics ofNW arrays are not primarily influenced by material composition,but by the field distribution of various modes, mainly dependingon the structure geometry [29]. Moreover, the physical fillingratio of a NW array on a surface has a significant effect on thedevice performance. Intuitively, a smaller filling ratio reducesreflection for a wider light spectrum. However, reduced fillingratio decreases absorption for longer wavelengths and increasesabsorption in shorter wavelengths, which effectively shifts theabsorption spectra [4].

Additional effects include optical leaky-mode resonance(LMR) [31] and polarization with respect to the optical field[32], [33]. Under exposure to normal incident light, azimuthallyrotating samples showed a four-fold pattern of birefringence[32]–[34]. The relative orientation between the NW and thepolarized light vector contributes to this effect, resulting in anazimuth-angle-dependent perpendicular and horizontal ratio oflight absorption (I⊥/I// ). However, this absorption anisotropydiminishes with decreased NW size and increased array den-sity, due to pronounced disorder features in arrays with thinNWs [35]. This is consistent with the contributions from theaperiodic Si NW arrays discussed above. Additionally, scatter-ing can be further suppressed by introducing higher refractiveindex materials in between NWs [36]. In addition to the theoret-ical studies on ordered NW arrays [4], [22], [25], [37], opticalproperties of disordered NW arrays are studied theoreticallyand experimentally [38], [39], and is essentially determined bymultiple diffuse scattering of light [22].

Improvements in understanding NW synthesis, alloying, andengineering at the nanoscale, such as the formation of het-erostructures in both axial and radial directions, allow for bettercontrol in fabricating nanostructures and provides ways to tailorabsorption spectra for specific applications [31].

C. Charge Generation and Separation in NWs

Generally speaking, incident photons with energy greater thanthe semiconductor band gap will be absorbed by the semicon-ductor and will generate carriers (electrons and holes). Thisgeneration process can be hindered by semiconductor imper-fections, relaxations and other effects, which are discussed in

detail in existing textbooks and will not be repeated here. NWgeometry offers many improvements for charge generation andseparation and can be organized according to the internal forcesof separation.

1) Charge Separation Assisted by Electrostatic Force: Thebuilt-in field across the interface of p- and n-doped regionscreates an electrostatic force. The magnitude of this force isproportional to the Fermi level offset and inversely propor-tional to the width of the depletion region. Theoretically, theelectrostatic force assists in the separation of photogeneratedelectrons and holes, influencing the efficiency of the PV device.A p-n junction in a NW can be in the radial or axial direc-tion, as shown in Fig. 1(d) and (e). Unique geometric propertiesof NW structures enhance charge generation and facilitate thetransportation of carriers. For example, in core/shell structures,the geometry of the vertical NW array separates the pathwaysof incident photon absorption (along the axial direction) andcharge separation by the built-in electric field (along the ra-dial direction). In addition, NWs also offer enhanced junctionarea (to volume), which greatly enhances the charge separationefficiency.

Another junction configuration worth mentioning is a radialp-n junction formed at the interface of a semiconductor andelectrolyte. This alternative construction is known as a photo-electrochemical cell (PEC) and has a theoretical potential forhigh-efficiency energy conversion. This is due to the advan-tages of high quality, conformal, and rectifying liquid-solutionjunctions, which can enhance minority carrier generation, sep-aration and collection similar to core/shell junctions. This issupported by findings that show photocurrent density enhance-ment in Si NW photoelectrodes when compared to planar Sidevices [40]–[42].

2) Charge Separation in Heterostructures (NonelectrostaticForce): Nonelectrostatic forces arise from composition varia-tions in compound semiconductors, e.g., the composition (x)of a binary compound semiconductor (AxB1−xC). Effectivemasses of electrons and holes vary between materials, whichgives rise to a quasielectric field for electrons or holes [43].Type-II, or staggered band core/shell, NW solar cell structures[see Fig. 1(f)] are studied and distinguished from other solarcells with junction structures driven by the electrostatic force.The charge separation scheme in these devices suppresses theintrinsic recombination at the junction [44] without the need foradditional doping processes.

Studies in thin film solar cells show that graded band-gapmaterials demonstrate a broadened spectral response and there-fore a high conversion efficiency [45]–[49]. Normally, a gradedband-gap solar cell has a wide band gap at the surface, creating awindow effect that significantly reduces surface recombination.Meanwhile, the low gap gradient near the junction avoids theoccurrence of an inverted window effect, which would result inincomplete absorption of photons with energies just greater thanthe energy gap at the junction [50]. To the best of our knowledge,no one has utilized graded band-gap structures with type-II junc-tion NW solar cells. However, some works have demonstratedthe capability of synthesizing spatial composition-graded alloyNWs [51]–[53], which may lead to improved solar power con-version efficiency.

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Recent interest has focused on strain engineering of Si NWs toenhance charge separation through the controllability of the NWmorphology [54]. Technically, in a narrow tapered or strainedSi NW, both the quantum confinement strength and the surfacemorphology vary significantly along the NW axis. This causesdifferent shifts in local near-gap energy levels along the NWaxis [55] and forms an ideal type-II junction. Therefore, tuningthe electronic structure offers another way of controlling intrin-sic properties of nanostructure materials without introducing ahomo- or heterojunction.

D. Charge Collection in NWs

Charge collection, or transportation, of photogenerated car-riers to contacts is plagued by recombination, which can occurat the surface, interface, and at bulk defects. In the case of NWsolar cells, surface recombination remains a major challengebecause of the inherently large surface-to-volume ratio. For ex-ample, in the case of core/shell NWs, charges generated andseparated into the shell or core far from the top or bottom con-tacts have a high chance to be trapped and recombined becauseof the large distance they must travel. Traditional approaches,such as transparent conducting oxide (TCO) deposition and in-terdigitated metal grid are insufficient to solve this problem.Therefore, charge collection unfortunately limits NW solar cellperformance. Proper device design and fabrication based onsimulation is needed to improve NW PV performance. Initia-tive work has already begun demonstrating the advantages ofusing uniform contact in solid-state devices [56]. On the otherhand, dye-sensitized solar cells (DSSCs) and PECs using NWelectrodes have demonstrated better charge collection efficiencydue to the uniform interfaces between semiconductor and elec-trolyte [35].

E. Nanowire Solar Cell Devices

There are distinct advantages regarding the use of radial junc-tions. First, the junction interface extends along the length of theNW maximizing the junction area. Second, since the radial con-figuration yields orthogonalized pathways for light absorptionand carrier collection [3], a carrier collection distance compara-ble to or even smaller than the minority carrier diffusion length(Ln ) is possible. This allows photogenerated carriers to reach thep/n regions or electrodes (if in the surrounding geometry) withhigh efficiency and substantially low bulk recombination, a keylimitation of conventional planar solar cells. Recent theoreticalstudies have supported that core/shell NW structures improvelight absorption and carrier collection efficiencies when com-pared to planar PV devices [4], [19], [57]. Although, generallyspeaking, generation and separation efficiencies are improved,design and optimization issues remain in core/shell NW solarcells. Detailed discussion can be found in Atwater and Lewis’ssimulation study of nondegenerately doped Si and GaAs [3].Their study addresses the effects of different design parameterson the performance of solar cell devices, namely the minor-ity carrier lifetime (τ ), open-circuit voltage (VOC ), short-circuitcurrent density (JSC ), fill factor (FF), and eventually, conver-sion efficiency (η). Like planar structures, optimization consid-

TABLE IDEVICE PERFORMANCES VERSUS PHYSICAL PARAMETERS

IN RADIAL JUNCTION

erations must be made with respect to some of the geometricparameters of NW structures due to the inverse behavior of theJSC and VOC (see Table I). This study offered guidance to laterexperimental studies [8], [13], [58].

The axial junction can be formed by introducing differ-ent doping along the axial direction of NWs [59], [60], bygrowing vertical NW arrays on substrates with opposite dop-ing [5], [61]–[63], or by creating rectifying metal contacts tounintentionally doped NWs [12]. In axial junction NWs, thecarrier transport direction is parallel to the NW growth direc-tion and axial junctions do not have the same advantages incharge separation that radial junctions do.

Heterostructures, such as tandem stacking of multiple p/njunctions and multiple quantum well (MQW) structures, canalso be integrated in series in axial or radial junctions [seeFig. 1(b) and (c)]. The fabrication of these tandem cell struc-tures leads to a maximization of the absorption of solar spectrumby using multiple materials with different band gaps and a min-imization the hot electron effect. Also, quantum wells can actas absorbers of additional photons, thus resulting in increasedJSC and η.

To summarize, vertical NW solar cells offer enhanced energy-conversion efficiency due to enhanced light absorption, im-proved charge separation and improved charge collection. Thevertical NW array geometry with varied size and/or compositionalong the NW axis enhances light absorption due to the waveg-uiding effect, reduces surface reflectance and minimizes angulardependence. Lastly, the formation of heterostructure solar cellsenables potential for further enhancements in overall efficiency.

III. COMPOUND SEMICONDUCTOR NANOWIRE SOLAR CELLS

A. Group IV Semiconductor Nanowire Solar Cells

For the purpose of drawing comparison to compound semi-conductor NW solar cells, we must first recall recent reports onsilicon NW solar cells. To summarize the material, synthesis,structure and performance, a table comparing each kind of com-pound semiconductor NW solar cell can be found in Table II.Si is the dominant semiconducting material in today’s technol-ogy and has been used extensively for PV applications [40], [64],[65] because of its low cost, abundance on earth and tunability of

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TABLE IISUMMARY OF NANOWIRE SOLAR CELL PERFORMANCES

doping and morphology [8], [60]. Si NWs have been applied tosolar-cell applications with different structures (e.g., single NWor an array of NWs). Single coaxial p-i-n silicon NW solar cells,consisting of a p-type core with intrinsic and n-type shells [seeFig. 3(a)], have been fabricated by the vapor–liquid–solid (VLS)method and subsequent thin film deposition using chemical va-

por deposition (CVD) [8], resulting in a. conversion efficiencyof 3.4% which is owed to a high JSC caused by strongly im-proved absorption in the thin shell due to its nanocrystallinefeature and intrinsic layer. Alternatively, Kelzenberg et al. havefabricated single-NW silicon solar cells using VLS method bycreating rectifying contacts [12] [see Fig. 3 (b)]. This cell has an

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Fig. 3. Si NW solar cells: (a) single NW with radial p-i-n junction (repro-duced with permission from [8], copyright Nature Publishing Group), (b) singleNW with axial p-n junction (reproduced with permission from [12] copyrightAmerican Chemical Society), (c) NW array with axial p-n junction (reproducedwith permission from [62] copyright American Chemical Society), and (d) NWarray with radial p-n junction (reproduced with permission from [13], copyrightAmerican Chemical Society).

efficiency of 0.46%. When compared to single radial p-i-n NWsolar cells, the conversion efficiency is low, while the carrierdiffusion lengths are long (>2 μm).

Meanwhile, several studies have been focused on silicon NW-array-based solar cells [9], [11], [19], [61], [65], [66]. Despitehigh anti-reflecting characteristics of silicon NW arrays, thePV conversion efficiency (9.31%) of the fabricated solar cellsconsisting of n-type Si NW arrays on a p-type Si substratesis not as high as efficiencies of conventional solar cells [61],[67]. We believe that the low conversion efficiency is due tothe limited junction area (axial junction between n-NW and p-substrate) as well as surface states in etched Si NWs. These

effects, compounded by the ultrahigh surface area of the NWarrays, eventually increase the surface recombination velocityand limit the carrier collection efficiency. Other problems whereexperienced by Tsakalakos et al. when they fabricated NW solarcells by depositing a thin n-type amorphous Si layer on a p-typesilicon NW array to form a p-n junction through the VLS method[11]. With this method they significantly reduce the reflectanceof a typical NW cell, but the power conversion efficiency waslow (∼0.1%) which is probably due to high series and low shuntresistances. Sivakov et al. demonstrated higher efficiencies witha solar cell based on Si NW arrays on glass substrates [seeFig. 3(c)]. This multijunction device is formed by applying awet chemical etching process to form p+nn+ polycrystallinesilicon layers on glass, and exhibits a conversion efficiency of4.4% [62].

To improve the minority carrier collection in Si NW-array-based solar cells, NWs with radial junctions are favorable. SiNW arrays consisting of an n-type core and a p-type shell havebeen fabricated and characterized [13] [see Fig. 3(d)]. The NWcell which contains vertically aligned 18 μm long NWs with highpacking density, is synthesized by a low-cost solution methodand gives a measured overall cell efficiency of 0.46%. Garnettand Yang recently demonstrated the strong light-trapping prop-erties of NW arrays, which improve the conversion efficiencyof Si-NW-array solar cells [19]. They reported that their 5-μm-NW-array radial p-n junction solar cells fabricated from 8 μmand 20 μm thin Si absorbing layers generate conversion ef-ficiencies of 4.83% and 5.30%, respectively, under AM 1.5Gillumination. The conversion efficiency of 4.83% for 8 μm ab-sorber Si-NW-array cells is about 20% higher than those on8-μm-thick Si ribbon solar cells (4% higher JSC ). However, thepower conversion efficiency of 5.30% for the 20 μm absorber Si-NW-array solar cells is about 35% lower than the correspondingmicrofilm solar cells that yield an efficiency of 7.2% (14% lowerJSC ) [68]. Their device demonstrated a significant light-trappingeffect, above the theoretical limit for a randomizing system,pointing out that there may be photonic crystal improvement ef-fects. Nevertheless, the overall efficiency of these NW cells doesnot surpass that of planar cells due to increased junction and sur-face recombination. Even in comparison, the efficiency is morethan ten times that of previous reported data [13]. Therefore,such a kind of vertical NW array structure provides a feasiblepath toward high-efficiency and low-cost solar cells by reduc-ing both the quantity and quality of the needed semiconductormaterials.

One of the approaches to enhance the power conversion effi-ciency of Si NW-array-based solar cells is to decorate them withnanoparticles (NPs) or quantum dots (QDs). Materials such asinsulators, metals, and semiconductors are widely used for thispurpose, each contributing a different improvement due to a dif-ferent mechanism. Insulators such as SiO2 [69] and Al2O3 [35]provide efficient light scattering. These NPs disperse the incom-ing light towards the Si NWs in a way that light absorption ismaximized and AOI effect is suppressed. Alternatively, metalNPs, such as Pt [70] and Au [71], can introduce plasmoniceffects besides efficient light scattering [72]. These NPs im-prove light trapping due to the interaction between the surface

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plasmon and metal NPs. For this reason, plasmon-enhanced so-lar cells are recognized as the next generation of solar cells [73].Also, Si NWs decorated with Pt NPs show exceptional catalyticactivity at interfaces with liquids [70]. Semiconductor QDs nor-mally have high refractive index, which is believed to offer betterlight trapping performances. On the other hand, semiconductorNPs (e.g., PbS) provide additional absorption over a broad spec-trum due to their high absorption coefficient [74]. In addition,resonant excitons with high mobility transfer to the adjacent SiNW channel upon light absorption.

B. Group III–V Semiconductor NW Solar Cells

III–V compound semiconductors are believed to be excellentcandidates for solar-cell applications. To date, the highestefficiency solar cells are constructed from multijunction III–Vmaterials, which together with concentrated sunlight reachefficiencies of 40.8% (326 suns) from Spectrolab (USA) [75]and 41.1% (454 suns) from Institute of Solar Energy System(Germany) [76]. III–V semiconductor materials offer outstand-ing electrical properties, including: 1) tunable energy bandgaps and alloys; 2) a larger band gap—than Si—effectivelyoffering lower excess reversed saturation current with increasedVOC [77]; 3) excellent material qualities—potentially defectfree; and 4) high absorption coefficient. In order to clearly illus-trate the potential of compound semiconductor NW solar cells,particularly due to the rational control over NW and heterostruc-ture growth, we briefly summarize the advances in NW synthe-ses, and then focus on recently reported III/V and II/VI NW solarcells.

1) Synthesis III–V NWs: (Catalyst growth): Various metals,such as Au, Pt, Ni, Ag, Al, as well as alloys such as Au/Pd,have been used as agents for VLS NW (IV, III/V, and II/VI)growth [78]. The most commonly used techniques for III–V NWgrowth include metal–organic CVD (MOCVD)/organic–metalvapor phase epitaxy (OMVPE) [49], [79], [80], molecular beamepitaxy (MBE) [81], [82], chemical beam epitaxy (CBE) [83],[84], CVD [63], and laser ablation [85]. During VLS growth,these agents act as sinks for atomic precipitation, resulting insupersaturation that drives NW growth. With the exception ofgroup IV elements, little is known about these multicomponenteutectic phases [86], but interactions between metal catalystand III–V semiconductors have been studied, resulting in an ex-panded understanding of different growth behaviors [87]–[91].Tremendous effort has been made by various research groupsto study the mechanisms of catalyzed InAs NW growth usingMOCVD [92]–[95]. It was discovered that adatom surface dif-fusion rates play a significant role in determining NW growthrate and morphology. A recent review paper has summarizedposition-dependent growth rates affected by adatom diffusionat the catalytic tip, NW sidewall and substrate that are linear,exponential, and parabolic, respectively [96]. Through catalystpatterning [97], [98], NW growth can be confined to specificlocations allowing for the fabrication of arrays, as shown inFig. 4(a) [95]. Recently published data have demonstrated thecapability of growing NWs with square or rectangular crosssections by simply controlling catalyst patterns [99].

Fig. 4. III–V NW growth. (a) Catalyst growth of InAs using dispersed Auparticle (reproduced with permission from [95] copyright American ChemicalSociety), (b) catalyst-free growth of InAs on Si substrate (reproduced with per-mission from [5] copyright American Chemical Society), (c) AlGaN/AlN/GaNcore/shell/shell NW (reproduced with permission from [124] copyrightAmerican Chemical society), (d) InAs/InP NW with radial junction (repro-duced with permission from [123] copyright American Chemical Society), and(e) InAs/InP NW with axial junction (reproduced with permission from [84]copyright American Institute of Physics).

Catalyst-free growth: Catalysts in VLS growth such as goldprovide preferential growth along a single axis even on lattice-mismatched substrates and are capable of accommodating strainin two dimensions [1 0 0]. However, catalytic growth has pre-sented some serious limitations. For example, it is believed thatgold atoms provide deep impurity levels in Si [1 0 1]; there-fore, catalyst-free VLS growth on lattice mismatched substrateis desired for electronic and optoelectronic applications. Mech-anisms of catalyst-free NW growth include self-catalyzed VLSthrough group-III elements [102], oxide-assisted growth [103],[104], formation of facets [80], ligand-aided solution–solidgrowth or Si-assisted growth [5], and selective-area MOVPE[105], [106]. It is believed that catalyst-free growth is nota single-mechanism-governed process, but is multiple-factor-controlled with nucleation and growth at the same condition.Factors such as, growth temperature [80], III–V ratio [107], pre-cursor flux rate [82], partial pressure [102], and substrate orien-tation [108], [109] have been extensively studied in order to de-termine their effects on NW crystallinity, orientation, and mor-phology. Of particular interest is the large area hetero-epitaxyof III–V NWs on Si substrates by MOCVD [5], [103], [110],[111] [see Fig. 4(b)], which enables the fabrication of low-cost high-efficiency III–V solar cells. Many challenges remainincluding, large-scale, low cost, reproducible/controllable syn-thesis of high quality, uniform, single crystal NWs, and het-erostructure arrays on Si substrates and other cheap and/or flex-ible substrates.

2) Core/Shell NW Growth: As discussed in previous sec-tions, the core/shell NW geometry offers enhanced separationand collection efficiency of photogenerated carriers. Moreover,the effective relaxation of lateral strain in coaxial heterostruc-tured NWs can enable high-quality integration of materialswith large lattice mismatches [112]. Controllable doping andcomposition modulation in NWs is critical, yet challenging

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for PV applications, especially in the case of GaAs and itsalloys, which have a large optical absorption coefficient andan almost ideal band gap, but include a drawback, high surfacerecombination velocities. Synthesis of p-i-n GaAs NWs withSi-doped p-core and n-shell synthesized on SiO2-coated (1 1 1)GaAs substrate using MBE system has been demonstratedby Morral et al. [113]. By switching dopants during growth,LaPierre’s group has reported the growth of GaAs core/shellNWs on GaAs (1 1 1)B substrates using Te instead of n-typedopants [58]. In-situ doping on other material systems has alsobeen studied [114]–[117], along with other methods such as ionimplantation [118], [119]. However, nanoscale doping is stillchallenging. The formation of compound heterostructures, suchas InP/InAs [105] or GaAs/GaP [120], InAs/GaAs [121] andAlN/GaN [122]–[124] have been widely studied, leading to afundamental understanding that defect/stacking fault formationin NW growth is due to a difference in supersaturation andinterfacial energy between III and V elements [125]–[129].Fig. 4(c) and (d) shows the cross section of an AlGaN/AlN/GaNcore/shell/shell NW and high-resolution TEM image of anInAs/InP core/shell NW [84], [123]. For NW solar cells,core/multishell InP/InAs/InP grown using catalyst-freeselective-area MOVPE has been reported by Fukui andcoworkers [105], and synthesis of n-i-p InAs/GaAs/InGaP/GaPcore-multishell NWs on Si using MOCVD has been demon-strated by our group [121]. The successful control of radialversus axial growth [130] demonstrates the potential forbuilding NW array solar cells with radial multijunction/MQWfoundations [105], [106]. However, due to the low surfacerecombination velocity and high hole mobility, III–V NW sys-tems suffer from high sensitivity to surface defects, bulk defects,and arsenic-antisite defects [131], all of which lead to degradedquantum confinement, discontinuous electron wave functions,reduced carrier lifetimes, and thus, poor electronic proper-ties [132]. It is for this reason that controlling the crystallinestructures and defect density in III–V NWs is of paramountimportance to improving their performance in solar-cell appli-cations. A number of approaches have been demonstrated, suchas tuning their crystalline structure by varying the diameter andgrowth temperature [126] and instituting control over twinningdefects, stacking faults [133] and kinks [134] by tuning thegrowth parameters, changing growth direction from <111> to<100> or <110> [117], and modifying the doping level [135].

3) Axial Junction NW Growth: Although less attractive toPV applications, the NW axial junction solar cell structure, withvertically stacked, varied band-gap semiconductors has drawninterest in wideband solar-energy harvesting [see Fig. 4(e)],in particular multijunction III–V NWs, graded composition,and MQW structures. The capability to form composition-ally abrupt and structurally perfect junctions [136] by con-trolling the switching process via VLS catalytic growth isfairly hard, due to the NW/catalyst interfacial energy differ-ence [137]. Experimental work has been demonstrated in thegrowth of InAs/InP/InAs [84] (using CBE), GaAs/InAs [137]and InAs/GaAs (with suppressed change in growth orientation),GaP/InP [138], GaSb/GaAs [139], and InAs/GaInAs/InAs axialheterojunction NWs [140], etc., or by the catalyst-free growth

of GaAs/InxGa1−xAs/GaAs heterostructure from Ga/In alloydroplets using MBE [141] and GaAs/GaAsSb/GaAs axial het-erostructure NW on Si substrates also using MBE [142]. Never-theless, further issues include, the synthesis of axial III–V het-erostructures with perfect crystalline segments, interface sharp-ness between segments, and minimization of defects/stackingfaults, all of which remain technically challenging.

C. III–V Semiconductor Nanowire Solar Cells

There is significant interest in using III–V compounds forPV applications [5], [59], [60], [68], [124], [125]. III-nitridesare very attractive due to large energy band tunability, fromultraviolet (UV) to infrared (IR). GaN, in particular, hasseveral advantages including high carrier mobility, p- or n-typeselectivity [143], high stability, and a broad band gap for widespectrum coverage [144]. In addition, the dislocation defectdensity due to lattice mismatch between NW and substrateis potentially lower when compared to thin films. Verticallyaligned GaN nanorod arrays on Si substrates are of particularinterest for solar-cell applications, because GaN nanorod arrayscan reduce minority carrier recombination, rate increase opticalabsorption at high frequencies, and decrease loss at visiblefrequencies due to its antireflection property. The fabricationof p-type GaN nanorod arrays on n-type Si substrates havebeen reported [49], and good rectification characteristics, smallreverse currents and a maximum conversion efficiency of 2.73%were achieved. Dong et al. [112] experimentally demonstratedthe synthesis of n-GaN/i-InxGa1−xN/p-GaN core/shell/shellNW solar cells using MOCVD. The variation of indium molefraction was used to control the active layer band gap andlight absorption. Electroluminescent (EL) measurement furtherdemonstrates band-gap tunability of the InxGa1−xN activelayer (2.25–3.34 eV). The single NW device simulated withAM 1.5G illumination yielded a maximum efficiency of 0.19%with an Indium composition of 27%.

Another attractive candidate for PV applications of group III–V material is GaAs due to its large light absorption coefficientand ideal band gap for solar-cell applications. LaPierre’s grouphas fabricated GaAs NW radial p-n junction solar cells usingMBE [58], where Te and Be were used for n-type shell andp-type core doping. The fabricated NWs show different mor-phologies due to the inclusion of dopants during NWs. growth.Although the cell exhibits rectifying characteristics, the leakagecurrent is high, which can be explained based on the low break-down voltage and possible tunneling effects. This cell showsa low efficiency of 0.83%, which were partly due to the for-mation of nonuniform core/shell p-n junctions. The conversionefficiency reduces reduced as the Te-doped growth duration in-creases, which may lead to lower shunt resistance. In anotherstudy, Colombo et al. reported the fabrication of GaAs core/shellNW p-i-n junction solar cells, consisting of a p-type core, intrin-sic and n-type shell using MBE [15]. The achieved conversionefficiency for this single NW solar cell structure is 4.5%, which,to the best of our knowledge is the highest reported efficiency forIII–V NW-based solar cells. The intrinsic layer between n- and

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SUN et al.: COMPOUND SEMICONDUCTOR NANOWIRE SOLAR CELLS 9

p-type materials in the p-i-n junction cell [15] increases its cellconversion efficiency over conventional p-n junction cells [58].

Goto et al. [145] reported high-quality core/shell InP NWarrays for PV applications grown by a selective-area MOCVD,with an overall conversion efficiency of 3.37%. This is attributedto the higher optical absorption coefficient and the band-gap en-ergy of InP, which is more optimally matched to the solar spec-trum than Si [146]. Previous work from our group demonstratesheteroeptaxial growth of vertical InAs NW arrays on Si usingcatalyst-free MOCVD [5]. This type of solar cell with multiple-band-gap absorber materials, InAs and Si, can efficiently harvestsolar energy due to enhanced broad-band light absorption.

The capability of forming heterostructures comprised ofIII–V semiconductors is another attractive feature, allowingfor the formation of multijunction solar cells, such as tandemstacked multiple p/n junctions and axial or radial MQW struc-tures. Similar to multi junction structures in planar solar cells,tandem cell and MQW structured NW solar cells are believedto offer much improved power conversion efficiency.

IV. GROUP II–VI NANOWIRE SOLAR CELLS

II–VI compound semiconductor NWs, such as CdS[147]–[149], CdSe [148], [150], CdTe [150], ZnSe [151], ZnTe[152], ZnO, and PbSe [153] can be produced by low-cost-solution-based methods, thereby reducing the costly thermalexpenditures necessary to the production of group IV and III–V compound devices. This has increased the allure of devel-oping II–VI compound semiconductor NW-based PVs. More-over, these materials carry the intrinsic benefits of inorganicnanomaterials, i.e., high carrier mobility, robust material sta-bility, and high interfacial area. Research effort is needed toexplore the use of II–VI semiconductor NWs in conjunctionwith semiconductor dyes and organic polymers in hybrid solarcells to substantially increase their efficiency, and viability. Un-fortunately, II–VI materials are limited by their native defects.Controlled doping in II–VI semiconductors is still technicallychallenging [154], [155], particularly for II–VI NWs grown bylow-temperature synthesis methods.

A. Core/Shell Nanowire Solar Cell

ZnO NWs are the most researched metal oxide semiconduc-tor and have received a great deal of attention due to their easeof growth, rich optical/electrical/piezoelectric properties and apromising variety of applications. ZnO’s band-gap (3.37 eV)limits absorption to the UV portion of solar spectrum and thematerials itself does not promise high-efficiency solar cells [77].However, ZnO NWs are intrinsically n-type and highly conduc-tive. The NW geometry allows for large junction/contact areafor electron collection [44], [156]–[158].

Many techniques are employed using smaller band-gap ma-terials to expand the absorbance, which in turn increase the lightharvesting and overall energy-conversion efficiency of the solarcell. One of the most popular methods is to fabricate core/shellstructure by coating ZnO NWs with uniform thin film shell(s)of II–VI group materials [158], [159], as shown in Fig. 5(a) and(b). The core/shell type-II heterojunction between the ZnO NW

Fig. 5. II–VI NW solar cells. (a) Core–shell ZnO/CdS NW array (reproducedwith permission from [157] copyright Royal Society of Chemistry), (b) CdSeQDs decorated ZnO NW solar cell (reproduced with permission from [158]copyright American Chemical Society), (c) template free synthesis of CdS NWarray (reproduced with permission from [146] copyright American ChemicalSociety), and (d) CdSe NWs in P3HT polymer (reproduced with permissionfrom [159] copyright American Association for the Advancement of Science).

and the semiconductor surrounding lead to effective charge sep-aration, with the shell acting as an absorber/a generation site andthe ZnO NW core as an electron transporter [159], [160]. Type-II core/shell heterojunction solar cells provide advantages of1) broadening the absorbed solar spectrum through weak in-terfacial transitions [44], 2) increasing the carrier lifetime dueto the slow electron–hole recombination in the spatially chargeseparated region [161] and minimizing radiative recombinationlosses [162]. On the other hand, for effective transport of elec-trons in ZnO core and holes in shell, it is important to be able tofinely adjust the thickness of the cell to maximize the generationof charge pairs while maintaining effective transport.

Theoretical work of effective charge separation is demon-strated by Zhang et al. [57], followed by experimental demon-strations using ZnO/ZnS and ZnO/ZnSe core/shell NWs [45],[145]. Experimentally, Tak et al. are able to synthesize aZnO/CdS core/shell NW array, and moreover, tune the lightabsorption spectrum by altering the thickness of the CdS shell[158]. The thickness of the CdS is controlled by successive ion

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10 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS

layer absorption and reaction. This technique is effective at pro-viding a uniform and adjustable shell, enabling the optimizationof light absorption and carrier generation. The resulting per-formance boosts the JSC of the ZnO array from 0.37 mA/cm2

without a CdS shell to 7.23 mA/cm2 with a CdS shell and amaximum power conversion efficiency of 3.53% is achieved.ZnO/ZnSe core/shell NWs are also synthesized, although nosolar cell devices are characterized so far [44] [see Fig. 5(c)].Fan et al. have demonstrated the growth of CdS NWs in anodicaluminum oxide (AAO) templates using a VLS and embeddedin a CdTe thin film [163]. Further work by the same group hasdemonstrated that CdS/CdTe NW heterostructures are capableof realizing efficiencies >20% due to low recombination veloc-ities at the interface [164].

B. SCSSCs and QDs/NW Solar Cells

The development in colloidal synthesis of 3-dimensionalconfined QDs has boosted PV techniques [165], [166], dueits unique properties. These advantages include tunable bandgap [146]–[148], hot electron utilization [167], and multiple ex-citon generation [156], [168]–[172]. QDs such as CdS [143],CdSe [173], CdTe [174], Sb2S3 [175], PbS [168], [176], [177],PbSe [167], InAs [171], InP [178], and Si [179], etc., extendthe absorption spectrum to VIS-IR range. Electrodes in NWarray configurations improve the transportation of electronsrapidly, directly, and efficiently to the contact, compared to theparticle-to-particle hopping effect normally seen in NP config-urations [159]. NWs also provide large surface-to-volume ratiosuggesting a large coverage of QD sensitizers, and thus, a highquantum yield of phonon-to-electron efficiency.

One of the QD SCSSCs that has been tested to increase the PVefficiency is CdSe. CdSe-decorated ZnO NWs show increasedabsorbance. Without QDs, device produces very little photocur-rent in the 400–600 nm range. On the contrary, with CdSe QDsthere was a substantial photocurrent in this range [159]. CdSeQDs form a type-II heterojunction with ZnO NWs allowing theelectron to be injected into ZnO NWs after being generated inless than a nanosecond. Likewise, holes are able to leave theQDs via an electrolyte. Unfortunately, the use of electrolytebreaks down QDs, which will contaminate the electrolyte anddevices would degrade as well. Therefore, future technologywill probably have to find an alternative hole transport mediumrather than an electrolyte solution [156], [159]. The other exam-ple is using freestanding ZnO NWs with successive layers CdSe(Eg = 1.7 eV) and CuSCN (Eg = 3.4 eV) composite nanostruc-tures. This is synthesized through electrochemical and chemicalbath deposition techniques [160]. The substantially increasedsurface area of the nanostructured interface allows for a reducedthickness of the absorber layer. Meanwhile, devices exhibit aneffective absorption of 89% in the 400–800 nm range [160].

C. Nanowire Hybrid Solar Cell

NW–polymer hybrid solar cells capitalize on the high elec-tron affinity of inorganic semiconductors, and the low ionization

energy of organic polymers to generate fast Foster charge trans-fer [180]. Such hybrid solar cells utilize large absorption coef-ficients of polymers and large electron mobilities in inorganicII–VI semiconductors. A high energy-conversion efficiency isempowered by the fast charge separation between the donor(polymer)/host (NWs).

High electron affinity of inorganic semiconductors and thelow ionization energy of organic polymers promise a fast chargetransfer rate [180]. Hybrid systems based on metal oxide, suchas ZnO [181]–[184] and TiO2 [185], [186] nanostructures arewidely investigated. TiO2 /ZnO core–shell [187] and CdS/ZnOcore–shell [188] heterostructures are also proposed to improvedevice performances.

The other kind of compound semiconductor studied in hybridsystem is Cd-VI compounds, such as CdSe, CdS, and CdTe. Ver-tically aligned NW structures based on these materials can befabricated using various methods, such as electrodeposition inAAO templates [189], [190], electroless deposition based onsolution–liquid–solid (SLS) mechanism [147], [191], and gas-phase VLS growth [192]. Hybrid solar cell is then finished byspin-coating photoactive polymers onto the surface of NWs. De-vices containing the vertically aligned II–VI NWs show a drasticimprovement in energy-conversion efficiency over a polymer orNW only device [190], [192].

By combining solution-phase-synthesized colloidal CdSenanorods with poly(3-hexylthiophene) (P3HT), this blendsolar cell shows improved absorption in the spectrum of300–720 nm and with tunable absorption spectra by controllingthe diameter [191]. The effect of aspect ratio on the perfor-mance is also characterized [180]. Devices containing nanorodswith higher aspect ratio demonstrated greater efficiencies, dueto improved charge mobility in longer nanorods. Moreover,interface in hybrid system is critical for overall efficiency. Itwas demonstrated that the efficiency of a polymer/CdS solarcell can be increased through the use of pyridine as a solventwhen spin coating the active layer of the device [189]. Sampleprocessed using pyridine demonstrates a more even dispersionof the nanotetrapods, which contributes to the increased effi-ciency [189]. Samples processed with chlorobenzene demon-strated power conversion efficiencies of 0.14% while those pro-cessed with pyridine showed efficiencies of 0.89%, which isfurther increased to 1.17% through thermal annealing [189].The solvent effect and the thermal treatment are believed to cre-ate phase separation and conducting polymer networks, whichleads to higher hole mobilities and consequently lower seriesresistance [189].

For hybrid solar cells, vertical NW array offers: 1) improvedlight absorption, 2) direct conducting pathway to electrode con-tact, and 3) potential guidance for aligned polymer morphology,which leads to higher hole mobilities.

Although as for now efficiencies of the II–VI semiconductor-based devices are low compared to other classes of solarcell, benefits of using group II–VI materials include low costof production, general abundance of materials, as well asenvironmental-friendly. Through the research mentioned above,it is clear that because of their economic feasibility, II–VIcompound semiconductors have good potential to replace or

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SUN et al.: COMPOUND SEMICONDUCTOR NANOWIRE SOLAR CELLS 11

combine with some of the existing technologies to further im-prove solar cell performance.

V. CONCLUSION AND PERSPECTIVES

NW PVs have drawn great interests for research and devel-opment globally in both academia and industry. To conclude,semiconductor NWs, particularly vertical NW arrays, offer ad-vantages for PV applications, such as enhanced light absorptionand improved separation and collection efficiencies of photo-generated carriers. Furthermore, the capability of doping, alloy-ing, and heterostructure formation at nanoscale enables bandgap and strain engineering along the nanowire’s axial and ra-dial directions. Further improvements to light absorption andcharge separation/collection are realized by constructing tan-dem, MQW or superlattice NW solar cells. Moreover, NWarray PV devices decrease fabrication costs by reducing theamount of required material, allowing the use of various sub-strates, through the capability of growth on lattice mis-matchedor amorphous substrates, and through the use of low-cost fab-rication methods. Despite these advantages, the best reportedefficiency for NW solar cells is less than 10%, much lowerthan the 41.1% energy-conversion efficiency achieved by thin-film devices. We believe there is plenty of room for improve-ment of device design/modeling, rational growth, and the re-producible/controllable fabrication of NW devices. Also, thereexists great opportunities for NW-stacked multijunction arraysfor high-efficiency solar cells, light weight, flexible and print-able PVs, low-cost and environmentally green processing, andreduced material consumption.

Ultrahigh efficiency: Maximum efficiencies of 30% for singlejunction solar cell [193], 72% for ideal 36-multijunction solarcell [194], 68% for infinite stack of p-n junction solar cell withgraded band gap [195], [196], and 70% of intermediate bandsolar cell [197], under 1 sun illumination is theoretically pre-dicted based on the thermodynamic limits. All these efficiencieswere calculated based on thin film solar cells. With the promiseof engineering for near unity light absorption, efficient chargeseparation and collection, compound semiconductor NW solarcells have the potential to lead a new generation of the PVs withsuperior conversion efficiency.

Light weight/portable, flexible, printable PVs: Lightweight/portable, flexible/foldable/stretchable solar cells are par-ticularly interesting for various medical, military and civil appli-cations, such as smart PV clothing, tents, beach umbrellas, cur-tains, etc. Both flexible organic and inorganic thin film solar celldevices produced by various methods have been reported [198],[199]. One possible approach is to embed micro- or nanoscalesolar cell devices into flexible polymers and transfer them fromthe source substrate to the flexible carrier substrate. The mostwidely used polymer is polydimethylsiloxane (PDMS). Si mi-crowires from VLS growth are embedded into PDMS and peeledoff from source Si substrate. Lewis and Atwater have success-fully demonstrated the application of this flexible device in PECsolar cells [35], [200], shown in Fig. 6(a) and (b). The poly-mer supported, flexible, and inexpensive Si wire film showsnondegradation in energy conversion performance comparing

Fig. 6. Current development in flexible NW solar cell devices: (a) SEM im-age of VLS silicon wire array, (b) SEM image of Si wires embedded in PDMSpolymer demonstrating flexible feature (insets show optical diffraction patterns)(reproduced with permission from [35] copyright Wiley Inter-Science), (c) CdSNW array embedded in AAO tempalate, and (d) devices with CdTe film embed-ded in PDMS demonstrating flexible feature (reproduced with permission from[163] copyright Nature Publishing Group).

to the original unpeeled source substrate. Javey’s group hasdemonstrated similar devices, where n-CdS NWs are grownvia CVD using an AAO template, which is then coated with alayer of p-CdTe film, and finally embedded in PDMS elastomer.This flexible device is shown to sustain large bending withoutstructural and device performance degradation [163]. Along thesame line, researchers around the world are developing flexibletransparent electrode contact for the replacement of a layer oftraditionally used TCO film, which is brittle and unstretchable,by using networks of metal NWs [201]–[205], carbon nanotubes(CNTs) [206]–[209] and graphene [210], [211].

Cost effective and environmental friendly manufacturing pro-cess: NW solar cells utilize less material than thin film cells withcomparable absorption and conversion efficiency [18]. Gener-ally, less material means cheaper solar cell devices. However,a cost-effective manufacturing process has to be considered aswell. This cost is limited by the fact that NWs are easy to processon high quality source substrates. Controlled growth of verti-cally aligned NWs on low-cost amorphous source substrates ischallenging. Therefore, research has focused on different routesof reducing substrate cost. Novel techniques for handling NWstructures efficiently have been proposed. Releasable nanostruc-ture through sacrificial etching [212], dry transfer [213], [214],and soft transfer [163], [200] on to carrier processing substrateshave been widely used for NW-based devices, which could po-tentially offer a new scheme of fabrication using reusable sourcesubstrates.

Self-cleaning: Retaining physical geometry and chemicalcomposition in nanostructured solar cells is critical for sta-ble and uniform performances. Particles accumulated on so-lar panel coverings scatter light and reduce light absorption.Routine maintenance to remove dirt, grime, bird droppings, or

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12 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS

even snow is required to maximize energy production capac-ity. Self-cleaning features for the solar panels, particularly inthe unreachable area, is desired [25]. The photocatalytic [215],[216] and superhydrophobic properties [217]–[219] offered byvertical nanostructure arrays lead to unique and very interestingself-cleaning capabilities.

Stability: Long-term stability and robustness is primarily dis-cussed in polymer and hybrid solar cells [220] and DSSCs [221]but not the NW array solar cells. Retaining physical geometryand chemical composition, and thus related optical and elec-trical properties, in harsh environments are critical for stableand uniform device performances. Long-term robustness andstability of NW solar cells are needed and still require furtherevaluation.

Device design/modeling, contact for effective chargecollection: Transportation of photogenerated electrons and holesto ohmic contacts after charge separation is critical for the contri-bution to external current (output power), as discussed in earliersection. Computer-assisted design, modeling of NW solar celldevices and contacts, along with the fabrication, will enableoptimization of carrier collection and efficiency.

ACKNOWLEDGMENT

K.M. and P.N. would like to thank UCSD Cal-IT2 summerundergraduate fellowship as part of the support of this work.T.B. wants to appreciate UCSD Summer Training Academy forResearch in the Sciences (STARS) Fellowship Program for thesupport.

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Ke Sun (S’07) received the B.S. degree from BeijingInstitute of Technology, Beijing, China, in 2003, andthe M.S degree from the University of South Florida,Tampa, in 2009. Since 2009, he has been workingtoward the Doctoral degree in ECE, UC, San Diego.

His current research interests include renewablesolar-energy-related topics using III–V nanowires(NWs) synthesized from metal–organic chemi-cal vapor deposition (MOCVD) and ZnO-basedheterostructures.

Mr. Sun is currently an active Student Member ofthe Society of Photo-Optical Instrumentation Engineers (SPIE), Electrochemi-cal Society (ECS), and The Minerals, Metals and Materials Society (TMS).

Alireza Kargar (S’08–M’08) received the B.S. de-gree from Shiraz University, Shiraz, Iran, in 2008.He is currently working toward the M.S. degree inthe Department of ECE, University of California atSan Diego (UCSD), La Jolla.

His research interests include nanomaterial-basedphotovoltaics, nanoelectronics, nanofabrication, andnanophotonics.

Mr. Kargar is a Student Member the Optical So-ciety of America (OSA) and the Society of Photo-Optical Instrumentation Engineers (SPIE).

Namsoek Park received the B.S. degree in electri-cal engineering in 2004 from Inha University, In-cheon, Korea. He is currently working toward theM.S. degree in the Department of ECE, University ofCalifornia at San Diego (UCSD), La Jolla.

He was with the ARMY for two years as anAdministrative Specialist. His current research inter-ests include the synthesis of transparent oxide semi-conductors such as ZnO, NiO, and Cu2 O, and theirapplications in optoelectronics.

Kristian N. Madsen (S’10) is currently with the Uni-versity of California at San Diego (UCSD), La Jolla,where he is completing his senior year as an under-graduate in the Department of ECE.

He is currently a Volunteer in Dr. Wang’s lab atUCSD. He was engaged in several fields of engineer-ing including civil, mechanical and electrical, playinga primary role in design development. His current re-search interests include nanoengineering, materialsscience, and development of low-cost semiconduc-tors for alternative energy.

Perry W. Naughton is currently working toward theB.Sc. degree in the Department of ECE, Universityof California at San Diego (UCSD), La Jolla.

He completed a position as a Summer Intern atthe World Minerals, and is currently completing aposition as a Summer Research Scholar through theUCSD’s Calit2 program. His current research inter-ests include fabrication of low-cost solar cells usingthe spray pyrolysis deposition method.

Mr. Naughton is currently a member in The Min-erals, Metals and Materials Society TMS.

Timothy Bright received the B.S. degree in electricalengineering from Oral Roberts University, Tulsa, OK,in 2010.

He was a Summer Undergraduate Researcher inDr. Wang’s group at the University of California atSan Diego (UCSD), La Jolla, during the summer of2010. His current research interests include flexiblenanoscale sensors, and robotic prosthesis.

Yi Jing received the B.S. degree in physics fromPeking University, Beijing, China, in 2005, and theM.S. degree in electrical engineering in 2007 fromthe University of California at San Diego (UCSD),La Jolla, where he is currently working toward thePh.D. degree.

His current research interest include Si and III–Vsemiconductor nanowires for applications of photo-voltaics and photodetectors.

Mr. Jing is a Student Member of the Society ofPhoto-Optical Instrumentation Engineers (SPIE).

Deli Wang received the Ph.D. degree in materialssciences from University of California (UC) at SantaBarbara in 2001.

He was a Postdoctoral Fellow at the HarvardUniversity. In 2004, he joined the Jacobs School ofEngineering, University of California at San Diego(UCSD), La Jolla, where he is currently an AssociateProfessor in electrical and computer engineering.His current research interests include nanomaterials,nanofabrication, electronics, optoelectronics, bionan-otechnology, nanomedicine, and renewable energy.

He has authored or coauthored more than 45 scientific articles. He holds fiveU.S. or pending patents.