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REVIEW

Energy harvesting based on semiconductingpiezoelectric ZnO nanostructures

Brijesh Kumara, Sang-Woo Kima,b,n

aSchool of Advanced Materials Science and Engineering, Sungkyunkwan University (SKKU), Suwon 440-746, Republic of KoreabSKKU Advanced Institute of Nanotechnology (SAINT), Center for Human Interface Nanotechnology (HINT),Sungkyunkwan University (SKKU), Suwon 440-746, Republic of Korea

Received 23 December 2011; received in revised form 1 February 2012; accepted 1 February 2012Available online 14 February 2012

KEYWORDSZinc oxide;Nanostructures;Energy harvesting;Solar cells;Nanogenerators;Hybrid architecture

ont matter & 2012n.2012.02.001

thor at: School ofngkyunkwan Univeel.: +82 31 290 73

imsw1@skku.edu

AbstractMultifunctional ZnO semiconductor is a potential candidate for electronics and optoelectronicsapplications and can be commercialized owing to its excellent electrical and optical properties,inexpensiveness, relative abundance, chemical stability towards air, and much simpler and widerange of crystal-growth technologies. The semiconducting and piezoelectric properties ofenvironmental friendly ZnO are extremely important for energy harvesting devices. This articlereviews the importance of energy harvesting using ZnO nanostructures, mainly focusing on ZnOnanostructure-based photovoltaics, piezoelectric nanogenerators, and the hybrid approach toenergy harvesting. Several research and design efforts leading to commercial products in thefield of energy harvesting are discussed. This paper discusses the future goals that must beachieved to commercialize these approaches for everyday use.& 2012 Elsevier Ltd. All rights reserved.

Introduction

Environment-friendly, multi-functional ZnO is one of themost important II–VI semiconductor materials with a widedirect band gap of 3.37 eV. The interest in this material isfueled and fanned by its prospects in optoelectronicsapplications, owing to its direct wide band gap and largeexciton binding energy of 60 meV at room temperature. The

Elsevier Ltd. All rights reserved.

Advanced Materials Sciencersity (SKKU), Suwon 440-746,52; fax: +82 31 290 7381.

(S.-W. Kim).

existence of various one-dimensional (1D) and two-dimen-sional (2D) forms of ZnO nanostructures [1,2] has providedopportunities for applications, not only in optoelectronics,but also in energy harvesting including photovoltaics [3,4].This material has been demonstrated to have enormousapplications in electronic, optoelectronic, electrochemical,and electromechanical devices [5–10], such as ultraviolet(UV) lasers [11,12], light-emitting diodes [13], field emissiondevices [14–16], high performance nanosensors [17–19], solarcells [4,20–22], and piezoelectric nanogenerators [23–29], dueto its excellent optical and electrical properties and the abilityto control the synthesis of various ZnO nanostructures such asnanoparticles, nanowires, nanorods, nanobelts, nanotubes, andother complex nanoarchitectures [1,2,30]. It is a potential

Energy harvesting based on semiconducting piezoelectric ZnO nanostructures 343

candidate for commercial purposes, due to its inexpensiveness,relative abundance, chemical stability towards air, and muchsimpler and wide range of crystal-growth technologies.

The semiconducting and piezoelectric properties of ZnOare extremely important in energy harvesting, particularlyin photovoltaics [4,20–22], piezoelectric nanogenerators[23–29], and hybrid energy harvesting devices [31–35], inaddition to hydrogen fuel generation: a source of energythrough water splitting [36]. ZnO has several key advantagesin these areas, being a biologically safe piezoelectricsemiconductor occurring in a wide range of 1D and 2D formsof nanostructures which can be integrated with flexibleorganic substrates for future flexible, stretchable, andportable electronics.

For biomedical applications, developing a novel wirelessnano-scale system, i.e. the integration of nanodevices,functional components and a power source, is of criticalimportance for real-time and implantable bio-sensing[37,38]. Wireless nanosystems require their own powersource despite their small size and low power consumption.There are two ways of achieving wireless nanosystems. Oneis to use a battery. However, even if the battery has a hugecapacitance, it has a limited lifetime and the miniaturiza-tion of devices limits the size of the battery, resulting in ashort battery lifetime. Therefore, the main challenge is toachieve small-sized and lightweight batteries with a longlifetime. In addition, the battery must be rechargedoccasionally. Consequently, the miniaturization of the powerpackage and self-powering of these nanosystems are some ofthe key requirements for their biomedical applications. It isalso important to consider the toxicity of the materialsthat compose the batteries of the power source used innanosystems.

The other way is to generate electrical power throughharvesting the ambient energy. Energy harvesting from theambient for powering a nanosystem is very important forindependent, wireless, and sustainable operation. Piezo-electric nanogenerators fabricated with ZnO nanostructuresare particularly promising for this application. Nanogenera-tors can be used in areas that require a foldable or flexiblepower source, such as biosensors implanted in muscles orjoints, and have the potential to directly convert biome-chanical or hydraulic energy in the human body, such as theflow of body fluid, blood flow, heartbeat, contraction ofthe blood vessels, muscle stretching or eye blinking, intoelectricity to power devices implanted in the body [39–42].Flexible nanogenerators driven by the beating of the heartcan serve as ultrasensitive sensors for the real-timemonitoring of its behavior, which might be applied tomedical diagnostics as sensors and measurement tools andconfirming the feasibility of power conversion inside abiofluid for self-powering implantable and wireless nanode-vices and nanosystems in a biofluid and any other type ofliquid [41].

ZnO nanostructures are good candidates for photovoltaicapplications for three straightforward reasons: they have alow reflectivity that enhances the light absorption, rela-tively high surface to volume ratio that enables interfacialcharge separation, and fast electron transport along thecrystalline 1D nanostructures that improves the chargecollection efficiency. ZnO nanostructures have been em-ployed in both conventional p–n junction solar cells and

excitonic solar cells (including organic, dye-sensitized, andquantum dot-sensitized solar cells). In Si-based tandemstructures of solar cells, ZnO nanostructures have been usedto enhance the light absorption [43].

There are several renewable energy harvesting methodsfor harvesting the environmental energies, including solarenergy [44], thermal gradient [45], and mechanical energy[23,24]. Many renewable energy systems can generateelectricity based only on each specific mechanism. Some-times, the absence of the energy source, such as theabsence of light in the nighttime, can cause solar cells to beinactive for energy harvesting. The major challenges facedby developers attempting to realize cheap and efficientenergy harvesting devices, which can work all the time withthe expectation of utilizing one or all of the availableenergies, can be solved by combining different energyharvesting approaches in a hybrid approach [34]. Thecombination of the semiconducting and piezoelectricproperties of ZnO is extremely important in energy harvest-ing, particularly in this hybrid approach. Developing anintegrated architecture for the hybrid approach that canharvest multiple types of energies simultaneously is desir-able for efficient energy harvesting in nature, so that theenergy resources can be effectively and complementarilyutilized whenever and wherever one or all of them areavailable.

Solar energy harvesting using ZnOnanostructures

Solar energy is commonly considered to be the ultimatesolution to our need for a clean, abundant, and renewableenergy resource available in nature. It can be converteddirectly into electrical energy by photovoltaic (PV) solarcells [44]. Although, in addition to crystalline silicon (Si) [44]and amorphous Si [46], several other thin-film semiconduc-tor materials such as CdTe [47–51], CIS [52], CIGS [53], GaAs,and InGaP semiconductor multi-junctions have been used insolar cells [54], they still require major breakthroughs tomeet the long-term goal of low production and operatingcosts. In this respect, dye-sensitized solar cells (DSSCs), bulkheterojunction (BHJ) solar cells, as third generation solarcells, have emerged as a promising alternative in recentyears for easy and low cost production, as discussed in thefollowing section.

Dye-sensitized solar cells

DSSCs possess the advantages of a lower cost and easierfabrication compared to traditional silicon solar cells [55]. Infact, at present, it is an undeniable fact that the efficiencyof DSSCs based on ZnO is lower than that of DSSCs based onTiO2 [56,57]. Nevertheless, currently, considerable interestis focused on ZnO-based solar cells, due to significantlyhigher electron mobility and greater flexibility in thesynthesis and morphologies of ZnO in comparison withTiO2. Better electron transport can in principle result inmore efficient electron collection. Therefore, it is expectedthat reduced recombination would be achieved if ZnO wasused as the photoanode in DSSCs instead of TiO2, due to therapid electron transfer and collection. Hence, so far, various

B. Kumar, S.-W. Kim344

ZnO nanostructures have been extensively investigated asthe photoanode for DSSCs [58]. Electrons are photoexcitedwithin the dye and are subsequently injected into the ZnOnanoparticles. These photogenerated electrons diffusethrough the sintered nanoparticle film to the collectionelectrode via a series of interparticle hopping steps.

However, excess electron hopping through the interpar-ticle barriers could result in a long dwell time within theindividual particles and, thus, increase the probability ofcharge recombination between the injected electrons andthe oxidized dye or redox species in the electrolyte. Toreduce the number of interparticle hops and significantlyenhance the electron transport velocity within the photo-anode, ZnO arrays of 1D nanostructures, such as nanowires[20] and nanotubes [59], have been widely utilized, as theyprovide a direct conduction pathway for the rapid collectionof the photogenerated electrons. However, the insufficientinternal surface area of these 1D nanostructure arrays limitsthe power conversion efficiency, owing to deficient dyeloading and light harvesting [60]. In this regard, to achievehigher dye adsorption, branched 1D ZnO nanostructuresconsisting of upstanding nanowires and outstretchedbranches are used to further improve the power conversionefficiency of the DSSCs. In addition, 2D ZnO nanostructureshave also been studied for DSSC applications because theyalso have a large specific surface area. For instance, theDSSCs constructed using upright-standing ZnO nanosheetfilms exhibit a very high conversion efficiency of 3.9% [61].The specific surface area is not the only factor thatdetermines the photovoltaic efficiency of the DSSC. Theefficiency is generally believed to be significantly affectedby the geometrical structure of the photoanode films thatprovide particular properties in terms of the electrontransport and/or light propagation. Therefore, Xu et al.[62] reported the fabrication of a DSSC with a hierarchicalZnO nanowire–nanosheet nanoarchitecture film photoanode,as shown in Fig. 1.

Hierarchical ZnO nanoarchitectures consist of a frame-work of ZnO nanosheet arrays and dense nanowires grown onthe primary ZnO nanosheets. This is based on the considera-tion that the nanosheet arrays alone may not capturethe photons completely, due to the gaps inherent in themorphology. The hierarchical ZnO nanowire–nanosheetarchitectures, however, have nanoscale branches that

Figure 1 Schematic diagram of the DSSC based on thehierarchical ZnO nanowire–nanosheet architectures [62].

stretch to fill these gaps and, therefore, provide both alarger internal surface area and a direct pathway forelectron transport along the channels from the branchednanowires to the nanosheet backbone. It was demonstratedthat using a hierarchical nanowire–nanosheet architecturephotoanode helps to greatly increase the dye loading andlight harvesting, while retaining good electron conductivity,as in the case of the upright-standing nanosheet photo-anode. DSSC based on hierarchical ZnO nanowire–nanosheetarchitectures showed a power conversion efficiency of 4.8%,which is nearly twice as high as that of DSSC constructedusing a photoanode consisting of bare ZnO nanosheet arrays.In addition to 2D DSSCs, Weintraub et al. demonstrated 3DDSSC using grown ZnO nanowires surrounding an optical fibersurface for remote locations, such as under the ground or indeep water [22]. Wei et al. extended this strategy one stepfurther by integrating multiples stacks of plane opticalwaveguides with nanowires [4].

Bulk hetero-junction organic solar cells

Besides DSSCs, BHJ organic solar cells are also a promisingalternative to traditional silicon-based solar cells, mainlydue to their potential for low cost, facile fabrication withlarge area printing, and coating technologies on lightweightflexible substrates [63,64]. In the conventional regularstructure of BHJ solar cells, indium tin oxide (ITO) modifiedwith poly(3,4-ethylene dioxythiophene):poly(styrene sulfo-nate) (PEDOT:PSS) is used as the anode [63]. However,PEDOT:PSS is an acidic water-based solution, which causesinterface instability in the photoactive layer and corrosionof the ITO [65,66].

To improve the interface stability and prevent devicedegradation, an alternative is to use an inverted configura-tion, with ITO serving as the cathode and a high workfunction metal as the anode [67]. It should be pointed outthat only modified ITO can serve as the cathode for electronextraction and, thus, the functional layers employed formodifying ITO mainly focus on metal oxides. ZnO is one ofthe functional metal oxides which can be used in thisapplication, due to its high electron mobility and highdegree of transparency in the visible wavelength range [68].Moreover, its crystal structure allows it to be grownanisotropically, making possible the production of efficientorganic solar cells based on vertically oriented ZnO nanorodsfor use as continuous electron transport pathways. BHJorganic solar cells with an inverted configuration, which arealso known as inverted organic solar cells, as shown in Fig. 2[69], offer a more promising concept than those with aregular structure in terms of their interface stability anddevice degradation; air-stable high-work-function metals(e.g., Au, Ag) are used as the anode to collect holes and ametal oxide such as ITO is used as the cathode to collectelectrons. In IOSCs, n-type metal oxides are deposited onthe ITO electrode to improve the device stability [68].

ZnO is one of the n-type metal oxides that can be used ininverted cells. To increase their power conversion efficiencyand reliability, recently, a ZnO nanostructured layer wasused as an optical spacer, a hole blocking layer and a directand ordered path by which the photo-generated electronscan be collected at the cathode in IOSCs [70–73]. The

Figure 2 Schematic illustration of inverted organic solar cells[69].

Energy harvesting based on semiconducting piezoelectric ZnO nanostructures 345

introduction of a ZnO optical spacer between the cathodeand the active layer allows for the increase in the lightabsorption by the active layer resulting from the redistribu-tion of the light intensity. There have been several attemptsto increase the power conversion efficiency of IOSCs using

various ZnO nanostructures with poly(3-hexylthiophene)(P3HT):(6,6)-phenyl-C71-butyric acid methyl ester (PCBM)and poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b0]dithiophene)-alt-4,7(2,1,3-benzothiadiazole)] (PCDBT):PCBM up to 6% [68].

Inorganic p–n junction solar cells

In addition to the ZnO nanostructures used to improve thecharge collection and charge blocking layer in DSSCs andIOSCs, as discussed previously, and the antireflection coatinglayers which play an important role in enhancing the deviceefficiency by increasing the light coupled with the activeregion of the solar cell, ZnO has also been employed ininexpensive inorganic solar cells [74]. While ZnO p–nhomojunction solar cells have rarely been reported, due tothe lack of stable p-ZnO materials, ZnO heterojunction solarcells have been employed as an alternative.

Recently, researchers have demonstrated a number ofn-ZnO/p-Cu2O heterojunction solar cells [74–77]. Thefundamental differences between conventional p–n junctionsolar cells and excitonic solar cells are that electrons andholes are generated in conventional solar cells, whileexcitons are generated in excitonic solar cells. Anotherfundamental difference is that the open circuit voltage(VOC) in conventional cells is limited to less than themagnitude of the band bending, while in excitonic solarcells the VOC is commonly greater than the band bending.Several attempts have been made to fabricate inorganicZnO/Cu2O solar cells. Although the theoretical limit of thepower conversion efficiency of Cu2O based solar cells isabout 18%, the highest efficiency of ZnO/Cu2O devicesreported is 2% [77]. The inadequate minority carriertransport length has been implicated as an important factorbehind this poor performance. In addition to the viableapplication of ZnO in the solar cells discussed above, intandem structures of solar cells, ZnO has been incorporatedto enhance the light absorption. Briseno et al. recentlydemonstrated an organic/inorganic hybrid single nanowiresolar cell [78]. Also, there is new trend emerging involvingthe application of ZnO in quantum dot multi-junction solarcells such as ZnO/PbS quantum dot solar cells [79].

Mechanical energy harvesting using ZnOnanostructures-based piezoelectricnanogenerators

Energy harvesting in our living environment is a feasibleapproach for powering micro- and nano-devices and mobileelectronics due to their small size, lower power consump-tion, and special working environment. The type of energyharvesting depends on the application. For mobile, implan-table, and personal electronics, solar energy may not be thebest choice, because it is not available in many cases all thetimes. Alternatively, mechanical energy, including vibra-tions, air flow, and human physical motion, which is calledrandom energy with irregular amplitudes and frequencies, isavailable almost everywhere at all times. Piezoelectricity isa novel approach that has been developed for harvestingthese types of mechanical and biomechanical energies usingpiezoelectric materials [24,42].

B. Kumar, S.-W. Kim346

Several piezoelectric semiconductor materials such asZnO [24], cadmium sulfide [80,81], zinc sulfide [82], galliumnitride [83,84], and indium nitride [85], and piezoelectricinsulator materials such as polyvinylidene fluoride [86], leadzirconate titanate [87,88], and barium titanate [89] havebeen used in mechanical energy harvesting to generateelectricity for self-powered electronics devices. Amongthem, biologically safe and environment friendly ZnO basedpiezoelectric nanogenerations are extremely important inmechanical energy harvesting. The main intention formechanical energy harvesting through nanogenerators is toreplace or supplement the current battery systems.

Working principle of nanogenerators

The working principles of nanogenerators can be explained foralternating current (AC) and direct current (DC) powergeneration. The mechanism of the power generation behaviorof nanogenerators fabricated from piezoelectric semiconduc-

Figure 3 (a) Schematic diagrams showing DC-type output chargebrought into contact with the top ITO electrode by applying an extenanorods to the top electrode. (b) T-ZnO-based nanogenerator that

Figure 4 (a) Proposed mechanism for AC-type charge generationelectrode in contact with the sides of the nanorods having a negativethe nanorods having a positive potential through the external circuitpresents AC-type charge generation. The switching polarity tests (signals are from the nanogenerators rather than the instruments [9

tor materials relies on the coupled semiconducting andpiezoelectric properties. Power generation from piezoelectricsemiconductor nanomaterial-based nanogenerators varies withthe direction of the exerted force, viz. perpendicular orparallel to the axis of the nanowire, and can be referred to asAC and DC power generation. The AC and DC power generationbehavior of nanogenerators was well described in a previouswork [90]. When piezoelectric semiconducting nanowires aresubjected to an external force, a piezoelectric potential isgenerated in the nanowire, owing to the relative displacementof the cations with respect to the anions. If the piezoelectricpotential generated in the nanowire is sufficient to drive thepiezoelectric induced electrons from the top or bottomelectrode to the bottom or top electrode, respectively,through an external circuit, voltage and current pulses canbe recorded by applying and releasing the force.

Nanogenerators that are driven by the lateral bending ofZnO nanowires using atomic force microscope (AFM) tipscanning and ultrasonic vibration show DC charge genera-tion, due to the coupled semiconducting and piezoelectricproperties of ZnO. The key element in such nanogenerators

generation from T-ZnO nanorods. When the ZnO nanorods arernal force, electrons flow from the compressed sides of the ZnOshows DC-type charge generation [91].

in V-ZnO-based nanogenerators. The electrons flow from thepotential to the opposite electrode in contact with the sides ofunder a compressive force. (b) V-ZnO-based nanogenerator thatforward and reverse connections) demonstrate that the output1].

Energy harvesting based on semiconducting piezoelectric ZnO nanostructures 347

is the placement of a Schottky barrier between the ZnOnanowire and an electrode, by which the carriers areaccumulated and released. AC power generation from thestretching or bending of laterally packaged ZnO finemicroscale wires and from the direct compression ofvertically-aligned ZnO nanowires has also been investigated.

Recently, it was reported that the AC and DC powergeneration modes can be controlled by integrating nanogen-erators with vertical and tilted ZnO nanorods [91]. This workdemonstrates the mode transition of charge generationbetween DC and AC from transparent flexible piezoelectricnanogenerators, which is dependent solely on the morphologyof the ZnO nanorods without the use of an AC/DC converter.Figs. 3 and 4 show the typical AC and DC type chargegeneration mechanisms, as well as the output performance ofthe DC and AC nanogenerators, respectively. DC-type outputcharge generation is based on the coupled effects of thesemiconducting and piezoelectric properties of ZnO. Whentilted ZnO (T-ZnO) nanorods are subjected to an externalforce, they are bent and generate a piezoelectric potential,due to the charges induced via the polarization created by theionic charges of the lattice ions along the width of thenanorods. A positive potential is produced on the stretchedside of the nanorod and a negative potential is induced on thecompressed side, as shown in Fig. 3(a). Since the tiltednanorods are easily bent by an external pushing force (under aload of 0.9 kgf), the piezoelectric potential is formed along thewidth of the T-ZnO nanorods. Therefore, the piezo-potentialinduced charges follow the DC-type output behavior of thenanogenerator along the internal and external circuit (seeFig. 3(b)).

The AC-type charge generation mechanism and the ACoutput current generated by the nanogenerator, fabricatedwith vertical ZnO (V-ZnO) nanorods, are shown inFig. 4(a) and (b), respectively. The AC-type current behavioris attributed to the direct compression of the ZnO nanorodsby the external force (under a load of 0.9 kgf). Consideringthe geometry of the V-ZnO, the vertically well-alignednanorods are easily compressed by the external pushingforce in the direction of the nanorod length rather thanbeing bent. Hence, a piezoelectric potential is generated inthe ZnO nanorod along the c-axis under uniaxial strain.Therefore, when an external force results in the uniaxialstrain of the V-ZnO nanorods, one side of the nanorods issubjected to a negative piezoelectric potential and theother side to a positive potential.

In order to generate a measurable signal above the noiselevel from nanogenerators, the presence of a Schottkycontact at one end of the nanorods is essential. The Schottkycontact at the sides of the nanorods with a negativepotential enhances the output signal by preventing the flowof electrons into the ZnO nanorods through the interface.The piezo-potential induced electrons are then moved viathe external circuit and are accumulated at the interfacebetween the electrode and the side of the nanorods with apositive potential. When the external force is removedand the compressive strain is released, the piezoelectricpotential inside the nanorods instantly disappears and theaccumulated electrons flow back via the external circuit,creating a negative electric pulse and, consequently,allowing the current to flow in AC mode from V-ZnO-basednanogenerators.

Designs, fabrication, and applications

Wang and Song first introduced piezoelectric nanogenera-tion by examining the piezoelectric properties of a singleZnO nanowire using AFM in 2006 [23]. To eliminate the use ofthe AFM tip, as reported in this first nanogenerator, forindependent operation and technological applications,there have been various innovation designs for improvingthe performance and applicability of the nanogenerators.The first independent operation of a nanogenerator was alsorealized by Wang et al. through the design of a nanogen-erator with zigzag trenches as a top electrode to replace theAFM tip, where the zigzag trenches act as an array of alignedAFM tips [24]. This was a DC power nanogenerator and wasdemonstrated using an ultrasonic wave with a frequency of41 kHz. This work formed the basic platform for optimizingand improving the performance of the nanogenerators byintegrating them into layered structures. Since then, severalvertical nanowire-integrated nanogenerators and lateralnanowire-integrated nanogenerators using ZnO have beenfabricated by integrating them into layered structures toimprove their performance [25,40,92,93].

There have been continuing efforts to improve the designand fabrication of nanogenerators for several technologicalapplications with better performance. Intensive researchinto innovative designs has been carried out for the purposeof improving the performance and applicability of thenanogenerators. This has resulted in the development of afiber-based flexible nanogenerator with ZnO nanowires [94],an integrated transparent flexible nanogenerator with ZnOnanorods [27], a fully rollable graphene-based transparentnanogenerator with ZnO nanorods [29], an integrated sound-driven nanogenerator with ZnO nanowires [26], and aflexible high-output nanogenerator based on lateral ZnOnanowire arrays with output voltages of up to 2.03 V and apeak power density of 11 mW cm�3, which was usedsuccessfully to light up a commercial light-emitting diode(LED) [95]. This work was a landmark study toward buildingself-powered devices by harvesting the energy from theenvironment. Since then, several other nanogenerators havebeen fabricated, as described below.

Recently, Kim et al. reported the fabrication of foldableand thermally stable paper-based nanogenerators to over-come the problem of the unstable electrical output fromplastic based nanogenerators due to thermal induced-stress[96]. To complete the integrated paper nanogenerator, ZnOnanorods were synthesized on a metal-coated cellulose papersubstrate using an aqueous solution method, as illustrated inFig. 5(a). The average diameter and height of the nanorodswere approximately 80 nm and 2 mm, respectively. The Au-coated cellulose top electrode was placed above the ZnOnanorod arrays for the formation of a Schottky contactbetween the Au and ZnO nanorods. The integrated nanode-vice was fully foldable (see Fig. 5(b)). Fig. 5(c) is aphotographic image of a foldable paper nanogenerator.Mechanical bending tests of the paper nanogenerator werealso performed, as shown in Fig. 5(d), and the mechanicaldurability was discussed and investigated. The mechanicaldurability of the paper nanogenerators was investigatedunder mechanical bending. Fig. 5(e) and (f) shows thethermal stabilities of the paper-based and polyethylenenaphthalate (PEN)-based nanogenerators, respectively.

Figure 5 Foldable and thermally stable paper nanogenerator with a cellulose paper substrate and piezoelectrically active ZnOnanorods. (a) A tilted field emission scanning electron microscopy image revealing the morphology of the ZnO nanorods grown on acellulose paper with Al and Au thin layers via an aqueous solution method. (b) Schematic diagram of an integrated papernanogenerator with a ZnO nanorod array on a foldable cellulose paper. (c) Photographic image of a foldable paper nanogenerator.(d) Mechanical bending tests of the paper nanogenerator. The thermal stability of the (e) paper-based nanogenerator and (f) PEN-based nanogenerator. The PEN-based nanogenerator experiences a severe shape change after alcohol lamp heating for 1 min, whilethe paper-based nanogenerator was quite thermally stable [96].

B. Kumar, S.-W. Kim348

The PEN nanogenerator experienced severe shape changesunder alcohol lamp heating, while the paper-based nanogen-erator was thermally stable. This demonstrates that thepaper nanogenerator can be operated even under thermallyharsh conditions.

Below the glass transition temperature of PEN, the outputcurrent density of the PEN-based nanogenerator wasapproximately two times greater than that of the paper-based nanogenerator, which might be due to the moreuniform distribution of the nanorods on PEN than that onpaper. However, PEN becomes soft and crooked at tempera-tures greater than 150 1C, resulting in a drastic reduction ofthe output current from the PEN-based nanogenerators.Conversely, the current output from the paper-basednanogenerators was extremely stable up to 200 1C. At250 1C, the paper-based nanogenerators had a greater

current output compared with the PEN-based nanogenera-tors (see Fig. 6). The voltage output from the paper-basednanogenerator was measured in the same way and was notsignificantly changed up to 200 1C. The measured currentand voltage confirm the changes in the electrical transportcharacteristics of the devices with increasing temperature.This work demonstrates that the paper based nanogeneratorexhibits superior charge scavenging performance underthermally harsh conditions.

Mechanical energy sources include the vibration of bridges,friction in mechanical transmission systems, deformation in thetires of moving automobiles, etc., all of which are normallywasted. For bicycles, cars, trucks, and even airplanes, a self-powered monitoring system for measuring the inner tirepressure is not only important for the safe operation of thetransportation means, but also for saving energy.

Figure 8 Performance of the nanogenerator attached to theinner surface of the tire, which was triggered by thedeformation of the tire. The inset of (a) shows an LCD screenthat was lit by the nanogenerator [97].

Figure 7 (a) Shape change of the tire during the vehicle’smovement. (b) Experiment setup. The tire was pressed betweentwo boards to simulate its deformation at the position where itcomes into or loses contact with the road surface. (c) Sketchmap of the nanogenerator construction, which is a cantileverstructure with five layers. (d) A photograph showing that thenanogenerator was fixed on the inner surface of the tire usingadhesive tape [97].

Figure 6 Thermal stability of the paper-based and PEN-basednanogenerators after thermal annealing at 100, 150, 200, and250 1C; the inset image shows the PEN-based and paper-basednanogenerators on a hot template at 250 1C [96].

Energy harvesting based on semiconducting piezoelectric ZnO nanostructures 349

In a previous work showing a promising application ofnanogenerators, one such device was integrated onto theinner surface of a bicycle tire to demonstrate the possibility

of using it for harvesting energy from the motion ofautomobiles [97]. In general, tires turn and are compressedduring their rotation. The shape change rate of the tires atthe position where they come into or lose contact with theroad surface is very large and can be regarded as a goodmechanical trigger to quickly introduce or withdraw bend-ing, as shown in Fig. 7(a). Two rigid boards were placed oneither side of the tire of a bicycle. One of the boards wasfixed; the other one connected to a linear motor and couldbe moved back and forth (see Fig. 7(b)). The nanogeneratorused in this experiment was designed with a free-cantileverbeam structure, as shown in Fig. 7(c). Due to its goodflexibility, the nanogenerator adheres tightly to the innersurface of the tire. The tire was squeezed and releasedperiodically to simulate the conditions that occur at theposition where it comes into or loses contact with the roadsurface. Each time the nanogenerator was bent, an electricpulse was generated (see Fig. 8). The measured outputvoltage approached 1.5 V and the measured output currentwas around 25 nA when the travel distance of the board was12 mm with an acceleration of 30 ms�2.

Figure 9 Naturally hybrid architecture of piezoelectric andphotovoltaic power generators. J–V characteristics under AM1.5 G illumination. The inset shows a schematic illustration ofthe hybrid device based on the nanostructured ZnO layer [34].

B. Kumar, S.-W. Kim350

A small liquid-crystal display (LCD) screen was lit directlyusing a nanogenerator that scavenges mechanical energyfrom the deformation of the tire during its motion. Theeffective working area of the nanogenerator was about1.5 cm� 0.5 cm and the maximum output power densityapproached 70 mW cm�3. This work demonstrates thepotential of the nanogenerator for energy harvesting fromthe motion of automobiles and for self-powered tire-pressure sensors and speed detectors.

Hybrid energy harvesting using ZnOnanostructures

Over the years, energy harvesting technologies such asphotovoltaics, thermoelectrics, and piezoelectrics for con-verting solar, heat, and mechanical energies into electricityhave been intensively developed. However, because of thecompletely different mechanisms utilized for harvestingdifferent types of energy, each type of harvesting technol-ogy can only generate electricity based on its own specificmechanism. The absence of the particular energy sourceputs the device out of action; for example the absence oflight in the nighttime makes the photovoltaic deviceinactive, the absence of heat makes a thermoelectric deviceinactive, and the absence of mechanical energy makes apiezoelectric device inactive. Therefore, an innovativehybrid approach has to be developed for the conjunctionalharvesting of multiple types of energy using an integratedstructure/material, so that one or all of the available energyresources can be effectively and complementarily utilizedto generate electricity all the time.

Recently, harvesting multiple-type energies using a singledevice has been one of the most important research issues inenergy harvesting technologies. The combination of thesemiconducting and piezoelectric properties of ZnO isextremely important in energy harvesting in this hybridapproach. This approach has great potential for the fullutilization of the energy in the environment under which thedevices will be operating. In the following sections, wedescribe several approaches that have been developed usingZnO nanomaterials for simultaneously harvesting solar, andmechanical energies.

Solar and mechanical energy harvesting

Traditionally, it has been believed that harvesting solarenergy is sufficient because it has a high efficiency. Such aconclusion was made based on the hypothesis that all of theoperations are under full sun illumination (100 mW cm�2). Inreality, however, many mobile electronics devices areoperated indoors and possibly in a hidden area with verydim light. In such cases, the power that can be harvestedfrom available light drops by 2–3 orders of magnitude incomparison to that under full sun illumination. Thus, a solarcell generates electricity effectively only in an area withappreciable light illumination and the absence of lightmakes the device inactive; the same is true in the case of apiezoelectric nanogenerator. A nanogenerator generateselectricity from its surrounding mechanical energies andthe absence of mechanical energies makes the nanogenera-tor device inactive as well. Therefore, an innovative ‘‘hybrid

approach’’ has been developed for the conjunctionalharvesting of solar and mechanical energies using anintegrated structure/material, so that at least one or bothenergies can be effectively and complementarily utilized togenerate electricity for viable applications. Solar andmechanical energy harvesting hybrid devices can comple-ment the weaknesses of each individual device.

The design and development of devices that can harvestmultiple types of energy without crosstalk and withsynergetic effects are critically necessary for the effectiveexploitation of the energies available in nature. As abreakthrough concept, Choi et al. demonstrated a flexiblehybrid cell. This device can be used as both a solar energyharvester and a touch-sensitive piezoelectric power gen-erator on a single platform [35]. The basic structure wasdesigned from an inverted organic solar cell with a ZnOnanostructured buffer layer on a plastic substrate, whereZnO serves as both the electron transport layer for the solarcell and the active layer for the formation of a piezoelectricpotential.

Recently, Choi et al. demonstrated a multi-type energyscavenger that converts individually or simultaneously low-frequency mechanical energy and photon energy intoelectricity using piezoelectric ZnO in conjunction withorganic solar cell. Since the multi-type energy scavenger isbased on the coupled piezoelectric and semiconductingproperties of ZnO, it has an intrinsically hybrid architecturewithout crosstalk and an additional assembling process tofabricate it [34]. Fig. 9 (inset) is a schematic illustration ofthe hybrid device based on ZnO nanostructures. Thisnaturally hybrid architecture is based on the integration ofa piezoelectric generator and an organic solar cell.

To achieve a fully flexible power generating device, theauthors prepared an ITO-coated polyethersulfone (PES)substrate as a cathode window for a solar cell. A ZnO thinfilm layer was first sputtered to a thickness of 50 nm on theITO/PES substrate. ZnO nanorods were then formed on thesputtered ZnO film by an aqueous solution method. AP3HT:PCBM polymer blend was spin-coated to a thicknessof about 250 nm, and a few nm thick molybdenum oxide(MoOx) layer and an Au anode with a thickness of about70 nm were then deposited. ZnO was chosen as the

Energy harvesting based on semiconducting piezoelectric ZnO nanostructures 351

piezoelectric material for the mechanical energy converteras well as the electron transport layer in the solar cell. Thecurrent density–voltage (J–V) curve of the hybrid device isshown in Fig. 9. Its power conversion efficiency (PCE)is about 1.5% on average with a VOC of 0.55 V and ashort circuit current density (JSC) of 9.2 mA cm�2 understandard air mass (AM) 1.5 global (G) illumination conditions(100 mW cm�2) without any mechanical strain (i.e. inde-pendent solar cell performance).

The working principle of the hybrid device for hybridoperation involving both solar and mechanical energies is asfollows. When the device is under light illumination alone, itgenerates continuous electrical output according to theusual mechanism of solar power generation in BHJ solarcells. When dynamic mechanical strain is applied to thedevice together with photon energy, the sharp piezoelectricoutput signal is added to the overall output signal as a resultof the instantaneous high piezoelectric field created in thenanostructured ZnO layer. Fig. 10 shows the hybrid opera-tion of the device under solar and mechanical energy withcontrolled mechanical straining processes. The authorsobserved the dependency of the piezoelectric output onthe strain and straining rate. Depending on the factorscontrolling the strain and straining rate, the piezoelectricoutput voltage ranged from several tens of mV up to 150 mVand the output current was several hundreds of nA. Further,they demonstrated that the piezoelectric output could bechanged from the alternating current type to the directcurrent type by tailoring the mechanical straining processesboth in the dark and under light illumination. This work wasa successful demonstration of a dual-mode scavengingenergy generator that employs both solar and mechanicalenergies.

This flexible hybrid cell converts individually or simulta-neously low-frequency mechanical energy and photonenergy into electricity using ZnO with coupled piezoelectricand n-type conductive properties. This work establishes amethodology to harvest solar energy and low-frequencymechanical energies such as a light, wind, and bodymovements, making it possible to produce a promisingpower generator that could be embedded in flexiblearchitectures such as the ‘‘flag/shirt/bag/curtain’’ one.

Figure 10 Hybrid operation by solar energy (SE) and mechan-ical energy with controlled mechanical straining processes.Under the ‘solar’ energy provided by indoor light, the overalloutput voltage was controlled by the mechanical strainingprocesses; FB: fast bending and FR: fast releasing [34].

This is also of critical importance for its future applicationsin defense technology, environmental monitoring, andpersonal electronics. Therefore, such a hybrid energygenerator is expected to be a novel multi-functional powersupply that could provide electricity at anytime andanywhere.

Recently, Xu and Wang demonstrated an approach tomaking a compact hybrid cell (CHC) that convolutes a solidstate DSSC and an ultrasonic wave driven piezoelectricnanogenerator into a single compacted structure forconcurrently harvesting solar and mechanical energies[32]. The structure was fabricated based on vertical ZnOnanowire arrays with the introduction of a solid electrolyteand metal coating. Under standard AM 1.5 G illuminationconditions, the optimum power was enhanced by 6% afterincorporating the contribution made by the nanogenerator.The ‘‘convolution’’ of the DSSC and nanogenerator in seriesto form a CHC is presented in Fig. 11. ITO serves as thecathode, while Ag paste in contact with GaN serves as theanode in this configuration. After connecting it to the outputwires, the entire CHC was sealed and packaged by epoxyresin to prevent the infiltration of any liquid except thewindow of the DSSC.

The working principle of the CHC is presented in the formof its electron energy band diagram (see Fig. 12). Theelectrons are promoted by the piezoelectric potential andphotovoltaic potential consecutively through the twodevices. The maximum output voltage achievable is thedifference between the Fermi level of the ZnO nanowires inthe DSSC and that of the ZnO nanowires in the nanogen-erator, i.e. it is the summation of the output voltages of thenanogenerator and DSSC. In the nanogenerator section, thegap between the Fermi level of the ZnO nanowires and thatof the Au determines the maximum output voltage of thenanogenerator. The Au–ZnO junction is a Schottky contactwhich serves as a ‘‘gate’’ that blocks the back flow ofelectrons. When the Au electrode slowly pushes thenanowire like an AFM tip, a strain field is created acrossthe nanowire width, with the outer surface being undertensile strain and the inner surface being under compressivestrain. The piezoelectric potential on the compressive side

Figure 11 Design of the CHC structure composed of the DSSCand nanogenerator. Schematic illustration of the CHC, which isilluminated by sunlight from the top and excited by ultrasonicwaves from the bottom. The ITO layer on the DSSC part and GaNsubstrate are defined as the cathode and anode of the CHC,respectively [32].

B. Kumar, S.-W. Kim352

of the nanowire causes the Schottky contact to be forwardbiased and drives the electrons across the Au–ZnO junction.Through an electron-transfer process, these charge carriersare continuously transported through the solid stateelectrolyte into the DSSC.

Figure 13 Performance of the CHC. (a) Comparison of the J–Vcharacteristics of the CHC when illuminated by simulated sunlightwith (red curve) and without (blue curve) the ultrasonic waveexcitation. The inset shows the expanded output of the VOC pointsaround the axial cross point, showing the increment of VOC by�19 mV after turning on the ultrasonic waves. (b) J–V character-istics of the nanogenerator when subjected to excitation byultrasonic waves, but with the sunlight off. (c) Comparison of thepower output J–V characteristics of the CHC. The rectangular areais the optimal power output of the CHC [32]. (For interpretation ofthe references to color in this figure legend, the reader is referredto the web version of this article.)

Figure 12 Electron energy band diagram of the CHC, showingthat the maximum output voltage is the sum of those produced bythe DSSC and nanogenerator. The abbreviations are as follows:sensitized solar cell (SSC), nanogenerator (NG), conduction band(CB), valence band (VB), and Fermi level (EF) [32].

To demonstrate the technological feasibility of the CHCfor the simultaneous harvesting of solar and mechanicalenergies, the authors measured the J–V curve of the CHCunder different conditions. When the full sunlight sourcewas on and the ultrasonic wave source was off, the CHCexhibited a VOC of 0.415 V and JSC of 252 mA cm�2 (see bluecurve in Fig. 13(a)). When both the ultrasonic wave andsunlight were turned on, the VOC reached 0.433 V, while theJSC remained at 252 mA cm�2 (see red curve in Fig. 13(a)).The output voltage of the CHC showed a 19 mV differencewhen turning on and off the ultrasonic wave, as shownby the expanded plot of VOC in the right-hand inset ofFig. 13(a), which corresponds to the output voltage of thenanogenerator when the sunlight was off (see Fig. 13(b)).Therefore, in addition to the open circuit voltage, the CHCsuccessfully cumulated the total power outputs from boththe DSSC and nanogenerator.

Challenges and opportunities

Although ZnO is being extensively investigated for use insolar cells, there is good potential for this material to beused in nanogenerators and hybrid devices to furtherimprove the output performance through scientific break-throughs, such as the neutralization of the piezoelectricpotential screening effect due to the presence of freecarriers in the semiconductor nanowires and the optimiza-tion and localization of the free carriers in the nanowires,which affect the piezoelectric signals of nanogenerators.The carrier density, conductivity, and Schottky contact playa major role in maximizing and optimizing the outputperformance of the nanogenerators. Recently, Wang dis-cussed several fundamental issues in order to optimizing theoutput performance of the nanogenerators [98]. Addition-ally, the doping of various ferroelectric materials into theZnO nanowires can enhance the piezoelectric signal of thenanogenerators and efficiency of the hybrid devices, due tothe resulting increase in the polarization and built-ininternal electric field in the ZnO nanowires.

In addition to the various successful demonstrations ofphotovoltaics, piezoelectricity, and hybrid devices forenergy harvesting using ZnO nanostructures and the applica-tion of ZnO in hydrogen fuel generation, a source of energythrough water splitting, the multi-functionality of thismaterial has the potential to modulate the light throughthe piezophototronic effect in optoelectronic devices. TheWang group demonstrated this concept through LEDs, wherethe polarization of the output light was modulated by thepiezophototronic effect [99]. They controlled the perfor-mance of the LED by the piezoelectric effect by introducinga piezopotential in ZnO by inducing a strain which controlsthe charge transport process at the ZnO–GaN interface. Theemission intensity and injection current at a fixed appliedvoltage were enhanced by factors of 17 and 4 after applyinga compressive strain of 0.093%, respectively, and thecorresponding conversion efficiency was improved by afactor of 4.25 in reference to that without applying anystrain. Also, an external efficiency of 7.82% was achieved.The authors suggested that this hugely improved perfor-mance is due not only to the increase of the injectioncurrent by the modification of the band profile, but also to

Energy harvesting based on semiconducting piezoelectric ZnO nanostructures 353

the more elegant effect of the creation of a trappingchannel for holes near the heterojunction interface, whichgreatly enhances the external efficiency. Moreover, Shi et al.demonstrated a piezophototronic effect in a photolectro-chemical (PEC) water splitting process to enhance the PECefficiency [100].

Concluding remarks

There have been successful demonstrations of photovoltaic,piezoelectric, and hybrid devices for energy harvesting usingZnO nanostructures. The controlled morphologies of variousZnO nanostructures in nanocrystals, nanowires, nanobelts, andother complex nanoarchitectures, and their high electronmobility enable interfacial charge separation and fast electrontransport which improve the charge collection efficiency insolar cells. In the absence of light, the main intention formechanical energy harvesting using nanogenerators is toreplace or supplement the current battery systems and createa more efficient source of power for the self-power charging ofmobile, implantable, and personal electronics. The mechanismof power generation behavior of nanogenerators fabricatedfrom ZnO nanostructures relies on their coupled semiconduct-ing and piezoelectric properties. A solar cell generateselectricity effectively only in an area with appreciablelight illumination and the absence of light makes the solarcell device inactive; the same is true in the case of ananogenerator. A nanogenerator generates electricity from itssurrounding mechanical energies and the absence of mechanicalenergies makes the nanogenerator device inactive as well.Therefore, an innovative ‘‘hybrid approach’’ was developed forthe conjunctional harvesting of solar and mechanical energiesusing an integrated structure/material, so that at least one orboth energies can be effectively and complementarily utilizedto generate electricity for viable applications. The combinationof the semiconducting and piezoelectric properties of ZnO isimportant for such hybrid devices to control their outputperformance.

Acknowledgments

This research was supported by the International Research andDevelopment Program of the National Research Foundation ofKorea (NRF) funded by the Ministry of Education, Science andTechnology (MEST) (2010-00297), the Energy InternationalCollaboration Research and Development Program of the KoreaInstitute of Energy Technology Evaluation and Planning (KETEP)funded by the Ministry of Knowledge Economy (MKE) (2011-8520010050), and Basic Science Research Program through theNRF funded by the MEST (2010-0015035).

Appendix A. Supporting information

Supplementary data associated with this article can be found inthe online version at doi:10.1016/j.nanoen.2012.02.001.

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Brijesh Kumar received his Ph.D. degreefrom Indian institute of Technology, Delhi,in 2009 under the supervision of Prof.R.K. Soni. Presently, he is working withProfessor Sang-Woo Kim as a ResearchProfessor at School of Advanced MaterialsScience and Engineering, SungkyunkwanUniversity (SKKU), S. Korea. His currentresearch areas are fabrication of energyharvesting nanoelectronics devices such as

solar cells, nanogenerators, hybrid devices, and graphene-baseddevices.

Sang-Woo Kim is an Associate Professor inSchool of Advanced Materials Science andEngineering at Sungkyunkwan University(SKKU). He received his Ph.D. from KyotoUniversity in Department of ElectronicScience and Engineering in 2004. Afterworking as a postdoctoral researcher atKyoto University and University of Cam-bridge, he spent 4 years as an assistantprofessor at Kumoh National Institute of

Technology. He joined the School of Advanced Materials Science and

Engineering, SKKU Advanced Institute of Nanotechnology (SAINT) atSKKU in 2009. His recent research interest is focused on piezo-electric nanogenerators, photovoltaics, and two-dimensionalnanomaterials including graphene and hexagonal boron nitridenanosheets.