Energy harvesting based on semiconducting piezoelectric ZnO nanostructures

Transcription

1 Nano Energy (2012) 1, Available online at journal homepage: REVIEW Energy harvesting based on semiconducting piezoelectric ZnO nanostructures Brijesh Kumar a, Sang-Woo Kim a,b,n a School of Advanced Materials Science and Engineering, Sungkyunkwan University (SKKU), Suwon , Republic of Korea b SKKU Advanced Institute of Nanotechnology (SAINT), Center for Human Interface Nanotechnology (HINT), Sungkyunkwan University (SKKU), Suwon , Republic of Korea Received 23 December 2011; received in revised form 1 February 2012; accepted 1 February 2012 Available online 14 February 2012 KEYWORDS Zinc oxide; Nanostructures; Energy harvesting; Solar cells; Nanogenerators; Hybrid architecture Abstract Multifunctional ZnO semiconductor is a potential candidate for electronics and optoelectronics applications and can be commercialized owing to its excellent electrical and optical properties, inexpensiveness, relative abundance, chemical stability towards air, and much simpler and wide range of crystal-growth technologies. The semiconducting and piezoelectric properties of environmental friendly ZnO are extremely important for energy harvesting devices. This article reviews the importance of energy harvesting using ZnO nanostructures, mainly focusing on ZnO nanostructure-based photovoltaics, piezoelectric nanogenerators, and the hybrid approach to energy harvesting. Several research and design efforts leading to commercial products in the field of energy harvesting are discussed. This paper discusses the future goals that must be achieved to commercialize these approaches for everyday use. & 2012 Elsevier Ltd. All rights reserved. Introduction Environment-friendly, multi-functional ZnO is one of the most important II VI semiconductor materials with a wide direct band gap of 3.37 ev. The interest in this material is fueled and fanned by its prospects in optoelectronics applications, owing to its direct wide band gap and large exciton binding energy of 60 mev at room temperature. The n Corresponding author at: School of Advanced Materials Science and Engineering, Sungkyunkwan University (SKKU), Suwon , Republic of Korea. Tel.: ; fax: address: kimsw1@skku.edu (S.-W. Kim). existence of various one-dimensional (1D) and two-dimensional (2D) forms of ZnO nanostructures [1,2] has provided opportunities for applications, not only in optoelectronics, but also in energy harvesting including photovoltaics [3,4]. This material has been demonstrated to have enormous applications in electronic, optoelectronic, electrochemical, and electromechanical devices [5 10], such as ultraviolet (UV) lasers [11,12], light-emitting diodes [13], field emission devices [14 16], high performance nanosensors [17 19], solar cells [4,20 22], and piezoelectric nanogenerators [23 29], due to its excellent optical and electrical properties and the ability to control the synthesis of various ZnO nanostructures such as nanoparticles, nanowires, nanorods, nanobelts, nanotubes, and other complex nanoarchitectures [1,2,30]. It is a potential /$ - see front matter & 2012 Elsevier Ltd. All rights reserved. doi: /j.nanoen

2 Energy harvesting based on semiconducting piezoelectric ZnO nanostructures 343 candidate for commercial purposes, due to its inexpensiveness, relative abundance, chemical stability towards air, and much simpler and wide range of crystal-growth technologies. The semiconducting and piezoelectric properties of ZnO are extremely important in energy harvesting, particularly in photovoltaics [4,20 22], piezoelectric nanogenerators [23 29], and hybrid energy harvesting devices [31 35], in addition to hydrogen fuel generation: a source of energy through water splitting [36]. ZnO has several key advantages in these areas, being a biologically safe piezoelectric semiconductor occurring in a wide range of 1D and 2D forms of nanostructures which can be integrated with flexible organic substrates for future flexible, stretchable, and portable electronics. For biomedical applications, developing a novel wireless nano-scale system, i.e. the integration of nanodevices, functional components and a power source, is of critical importance for real-time and implantable bio-sensing [37,38]. Wireless nanosystems require their own power source despite their small size and low power consumption. There are two ways of achieving wireless nanosystems. One is to use a battery. However, even if the battery has a huge capacitance, it has a limited lifetime and the miniaturization of devices limits the size of the battery, resulting in a short battery lifetime. Therefore, the main challenge is to achieve small-sized and lightweight batteries with a long lifetime. In addition, the battery must be recharged occasionally. Consequently, the miniaturization of the power package and self-powering of these nanosystems are some of the key requirements for their biomedical applications. It is also important to consider the toxicity of the materials that compose the batteries of the power source used in nanosystems. The other way is to generate electrical power through harvesting the ambient energy. Energy harvesting from the ambient for powering a nanosystem is very important for independent, wireless, and sustainable operation. Piezoelectric nanogenerators fabricated with ZnO nanostructures are particularly promising for this application. Nanogenerators can be used in areas that require a foldable or flexible power source, such as biosensors implanted in muscles or joints, and have the potential to directly convert biomechanical or hydraulic energy in the human body, such as the flow of body fluid, blood flow, heartbeat, contraction of the blood vessels, muscle stretching or eye blinking, into electricity to power devices implanted in the body [39 42]. Flexible nanogenerators driven by the beating of the heart can serve as ultrasensitive sensors for the real-time monitoring of its behavior, which might be applied to medical diagnostics as sensors and measurement tools and confirming the feasibility of power conversion inside a biofluid for self-powering implantable and wireless nanodevices and nanosystems in a biofluid and any other type of liquid [41]. ZnO nanostructures are good candidates for photovoltaic applications for three straightforward reasons: they have a low reflectivity that enhances the light absorption, relatively high surface to volume ratio that enables interfacial charge separation, and fast electron transport along the crystalline 1D nanostructures that improves the charge collection efficiency. ZnO nanostructures have been employed in both conventional p n junction solar cells and excitonic solar cells (including organic, dye-sensitized, and quantum dot-sensitized solar cells). In Si-based tandem structures of solar cells, ZnO nanostructures have been used to enhance the light absorption [43]. There are several renewable energy harvesting methods for harvesting the environmental energies, including solar energy [44], thermal gradient [45], and mechanical energy [23,24]. Many renewable energy systems can generate electricity based only on each specific mechanism. Sometimes, the absence of the energy source, such as the absence of light in the nighttime, can cause solar cells to be inactive for energy harvesting. The major challenges faced by developers attempting to realize cheap and efficient energy harvesting devices, which can work all the time with the expectation of utilizing one or all of the available energies, can be solved by combining different energy harvesting approaches in a hybrid approach [34]. The combination of the semiconducting and piezoelectric properties of ZnO is extremely important in energy harvesting, particularly in this hybrid approach. Developing an integrated architecture for the hybrid approach that can harvest multiple types of energies simultaneously is desirable for efficient energy harvesting in nature, so that the energy resources can be effectively and complementarily utilized whenever and wherever one or all of them are available. Solar energy harvesting using ZnO nanostructures Solar energy is commonly considered to be the ultimate solution to our need for a clean, abundant, and renewable energy resource available in nature. It can be converted directly into electrical energy by photovoltaic (PV) solar cells [44]. Although, in addition to crystalline silicon (Si) [44] and amorphous Si [46], several other thin-film semiconductor materials such as CdTe [47 51], CIS [52], CIGS [53], GaAs, and InGaP semiconductor multi-junctions have been used in solar cells [54], they still require major breakthroughs to meet the long-term goal of low production and operating costs. In this respect, dye-sensitized solar cells (DSSCs), bulk heterojunction (BHJ) solar cells, as third generation solar cells, have emerged as a promising alternative in recent years for easy and low cost production, as discussed in the following section. Dye-sensitized solar cells DSSCs possess the advantages of a lower cost and easier fabrication compared to traditional silicon solar cells [55].In fact, at present, it is an undeniable fact that the efficiency of DSSCs based on ZnO is lower than that of DSSCs based on TiO 2 [56,57]. Nevertheless, currently, considerable interest is focused on ZnO-based solar cells, due to significantly higher electron mobility and greater flexibility in the synthesis and morphologies of ZnO in comparison with TiO 2. Better electron transport can in principle result in more efficient electron collection. Therefore, it is expected that reduced recombination would be achieved if ZnO was used as the photoanode in DSSCs instead of TiO 2, due to the rapid electron transfer and collection. Hence, so far, various

3 344 B. Kumar, S.-W. Kim ZnO nanostructures have been extensively investigated as the photoanode for DSSCs [58]. Electrons are photoexcited within the dye and are subsequently injected into the ZnO nanoparticles. These photogenerated electrons diffuse through the sintered nanoparticle film to the collection electrode via a series of interparticle hopping steps. However, excess electron hopping through the interparticle barriers could result in a long dwell time within the individual particles and, thus, increase the probability of charge recombination between the injected electrons and the oxidized dye or redox species in the electrolyte. To reduce the number of interparticle hops and significantly enhance the electron transport velocity within the photoanode, ZnO arrays of 1D nanostructures, such as nanowires [20] and nanotubes [59], have been widely utilized, as they provide a direct conduction pathway for the rapid collection of the photogenerated electrons. However, the insufficient internal surface area of these 1D nanostructure arrays limits the power conversion efficiency, owing to deficient dye loading and light harvesting [60]. In this regard, to achieve higher dye adsorption, branched 1D ZnO nanostructures consisting of upstanding nanowires and outstretched branches are used to further improve the power conversion efficiency of the DSSCs. In addition, 2D ZnO nanostructures have also been studied for DSSC applications because they also have a large specific surface area. For instance, the DSSCs constructed using upright-standing ZnO nanosheet films exhibit a very high conversion efficiency of 3.9% [61]. The specific surface area is not the only factor that determines the photovoltaic efficiency of the DSSC. The efficiency is generally believed to be significantly affected by the geometrical structure of the photoanode films that provide particular properties in terms of the electron transport and/or light propagation. Therefore, Xu et al. [62] reported the fabrication of a DSSC with a hierarchical ZnO nanowire nanosheet nanoarchitecture film photoanode, as shown in Fig. 1. Hierarchical ZnO nanoarchitectures consist of a framework of ZnO nanosheet arrays and dense nanowires grown on the primary ZnO nanosheets. This is based on the consideration that the nanosheet arrays alone may not capture the photons completely, due to the gaps inherent in the morphology. The hierarchical ZnO nanowire nanosheet architectures, however, have nanoscale branches that Figure 1 Schematic diagram of the DSSC based on the hierarchical ZnO nanowire nanosheet architectures [62]. stretch to fill these gaps and, therefore, provide both a larger internal surface area and a direct pathway for electron transport along the channels from the branched nanowires to the nanosheet backbone. It was demonstrated that using a hierarchical nanowire nanosheet architecture photoanode helps to greatly increase the dye loading and light harvesting, while retaining good electron conductivity, as in the case of the upright-standing nanosheet photoanode. DSSC based on hierarchical ZnO nanowire nanosheet architectures showed a power conversion efficiency of 4.8%, which is nearly twice as high as that of DSSC constructed using a photoanode consisting of bare ZnO nanosheet arrays. In addition to 2D DSSCs, Weintraub et al. demonstrated 3D DSSC using grown ZnO nanowires surrounding an optical fiber surface for remote locations, such as under the ground or in deep water [22]. Wei et al. extended this strategy one step further by integrating multiples stacks of plane optical waveguides with nanowires [4]. Bulk hetero-junction organic solar cells Besides DSSCs, BHJ organic solar cells are also a promising alternative to traditional silicon-based solar cells, mainly due to their potential for low cost, facile fabrication with large area printing, and coating technologies on lightweight flexible substrates [63,64]. In the conventional regular structure of BHJ solar cells, indium tin oxide (ITO) modified with poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) is used as the anode [63]. However, PEDOT:PSS is an acidic water-based solution, which causes interface instability in the photoactive layer and corrosion of the ITO [65,66]. To improve the interface stability and prevent device degradation, an alternative is to use an inverted configuration, with ITO serving as the cathode and a high work function metal as the anode [67]. It should be pointed out that only modified ITO can serve as the cathode for electron extraction and, thus, the functional layers employed for modifying ITO mainly focus on metal oxides. ZnO is one of the functional metal oxides which can be used in this application, due to its high electron mobility and high degree of transparency in the visible wavelength range [68]. Moreover, its crystal structure allows it to be grown anisotropically, making possible the production of efficient organic solar cells based on vertically oriented ZnO nanorods for use as continuous electron transport pathways. BHJ organic solar cells with an inverted configuration, which are also known as inverted organic solar cells, as shown in Fig. 2 [69], offer a more promising concept than those with a regular structure in terms of their interface stability and device degradation; air-stable high-work-function metals (e.g., Au, Ag) are used as the anode to collect holes and a metal oxide such as ITO is used as the cathode to collect electrons. In IOSCs, n-type metal oxides are deposited on the ITO electrode to improve the device stability [68]. ZnO is one of the n-type metal oxides that can be used in inverted cells. To increase their power conversion efficiency and reliability, recently, a ZnO nanostructured layer was used as an optical spacer, a hole blocking layer and a direct and ordered path by which the photo-generated electrons can be collected at the cathode in IOSCs [70 73]. The

4 Energy harvesting based on semiconducting piezoelectric ZnO nanostructures 345 various ZnO nanostructures with poly(3-hexylthiophene) (P3HT):(6,6)-phenyl-C 71 -butyric acid methyl ester (PCBM) and poly[2,6-(4,4-bis-(2-ethylhexyl)-4h-cyclopenta[2,1-b;3, 4-b 0 ]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 the charge collection and charge blocking layer in DSSCs and IOSCs, as discussed previously, and the antireflection coating layers which play an important role in enhancing the device efficiency by increasing the light coupled with the active region of the solar cell, ZnO has also been employed in inexpensive inorganic solar cells [74]. While ZnO p n homojunction solar cells have rarely been reported, due to the lack of stable p-zno materials, ZnO heterojunction solar cells have been employed as an alternative. Recently, researchers have demonstrated a number of n-zno/p-cu 2 O heterojunction solar cells [74 77]. The fundamental differences between conventional p n junction solar cells and excitonic solar cells are that electrons and holes are generated in conventional solar cells, while excitons are generated in excitonic solar cells. Another fundamental difference is that the open circuit voltage (V OC ) in conventional cells is limited to less than the magnitude of the band bending, while in excitonic solar cells the V OC is commonly greater than the band bending. Several attempts have been made to fabricate inorganic ZnO/Cu 2 O solar cells. Although the theoretical limit of the power conversion efficiency of Cu 2 O based solar cells is about 18%, the highest efficiency of ZnO/Cu 2 O devices reported is 2% [77]. The inadequate minority carrier transport length has been implicated as an important factor behind this poor performance. In addition to the viable application of ZnO in the solar cells discussed above, in tandem structures of solar cells, ZnO has been incorporated to enhance the light absorption. Briseno et al. recently demonstrated an organic/inorganic hybrid single nanowire solar cell [78]. Also, there is new trend emerging involving the application of ZnO in quantum dot multi-junction solar cells such as ZnO/PbS quantum dot solar cells [79]. Mechanical energy harvesting using ZnO nanostructures-based piezoelectric nanogenerators Figure 2 [69]. Schematic illustration of inverted organic solar cells introduction of a ZnO optical spacer between the cathode and the active layer allows for the increase in the light absorption by the active layer resulting from the redistribution of the light intensity. There have been several attempts to increase the power conversion efficiency of IOSCs using Energy harvesting in our living environment is a feasible approach for powering micro- and nano-devices and mobile electronics due to their small size, lower power consumption, and special working environment. The type of energy harvesting depends on the application. For mobile, implantable, and personal electronics, solar energy may not be the best choice, because it is not available in many cases all the times. Alternatively, mechanical energy, including vibrations, air flow, and human physical motion, which is called random energy with irregular amplitudes and frequencies, is available almost everywhere at all times. Piezoelectricity is a novel approach that has been developed for harvesting these types of mechanical and biomechanical energies using piezoelectric materials [24,42].

5 346 B. Kumar, S.-W. Kim Several piezoelectric semiconductor materials such as ZnO [24], cadmium sulfide [80,81], zinc sulfide [82], gallium nitride [83,84], and indium nitride [85], and piezoelectric insulator materials such as polyvinylidene fluoride [86], lead zirconate titanate [87,88], and barium titanate [89] have been used in mechanical energy harvesting to generate electricity for self-powered electronics devices. Among them, biologically safe and environment friendly ZnO based piezoelectric nanogenerations are extremely important in mechanical energy harvesting. The main intention for mechanical energy harvesting through nanogenerators is to replace or supplement the current battery systems. Working principle of nanogenerators The working principles of nanogenerators can be explained for alternating current (AC) and direct current (DC) power generation. The mechanism of the power generation behavior of nanogenerators fabricated from piezoelectric semiconductor materials relies on the coupled semiconducting and piezoelectric properties. Power generation from piezoelectric semiconductor nanomaterial-based nanogenerators varies with the direction of the exerted force, viz. perpendicular or parallel to the axis of the nanowire, and can be referred to as AC and DC power generation. The AC and DC power generation behavior of nanogenerators was well described in a previous work [90]. When piezoelectric semiconducting nanowires are subjected to an external force, a piezoelectric potential is generated in the nanowire, owing to the relative displacement of the cations with respect to the anions. If the piezoelectric potential generated in the nanowire is sufficient to drive the piezoelectric induced electrons from the top or bottom electrode to the bottom or top electrode, respectively, through an external circuit, voltage and current pulses can be recorded by applying and releasing the force. Nanogenerators that are driven by the lateral bending of ZnO nanowires using atomic force microscope (AFM) tip scanning and ultrasonic vibration show DC charge generation, due to the coupled semiconducting and piezoelectric properties of ZnO. The key element in such nanogenerators Figure 3 (a) Schematic diagrams showing DC-type output charge generation from T-ZnO nanorods. When the ZnO nanorods are brought into contact with the top ITO electrode by applying an external force, electrons flow from the compressed sides of the ZnO nanorods to the top electrode. (b) T-ZnO-based nanogenerator that shows DC-type charge generation [91]. Figure 4 (a) Proposed mechanism for AC-type charge generation in V-ZnO-based nanogenerators. The electrons flow from the electrode in contact with the sides of the nanorods having a negative potential to the opposite electrode in contact with the sides of the nanorods having a positive potential through the external circuit under a compressive force. (b) V-ZnO-based nanogenerator that presents AC-type charge generation. The switching polarity tests (forward and reverse connections) demonstrate that the output signals are from the nanogenerators rather than the instruments [91].

6 Energy harvesting based on semiconducting piezoelectric ZnO nanostructures 347 is the placement of a Schottky barrier between the ZnO nanowire and an electrode, by which the carriers are accumulated and released. AC power generation from the stretching or bending of laterally packaged ZnO fine microscale wires and from the direct compression of vertically-aligned ZnO nanowires has also been investigated. Recently, it was reported that the AC and DC power generation modes can be controlled by integrating nanogenerators with vertical and tilted ZnO nanorods [91]. This work demonstrates the mode transition of charge generation between DC and AC from transparent flexible piezoelectric nanogenerators, which is dependent solely on the morphology oftheznonanorodswithouttheuseofanac/dcconverter. Figs. 3 and 4 show the typical AC and DC type charge generation mechanisms, as well as the output performance of the DC and AC nanogenerators, respectively. DC-type output charge generation is based on the coupled effects of the semiconducting and piezoelectric properties of ZnO. When tilted ZnO (T-ZnO) nanorods are subjected to an external force, they are bent and generate a piezoelectric potential, due to the charges induced via the polarization created by the ionic charges of the lattice ions along the width of the nanorods. A positive potential is produced on the stretched side of the nanorod and a negative potential is induced on the compressed side, as shown in Fig. 3(a). Since the tilted nanorods are easily bent by an external pushing force (under a load of 0.9 kgf), the piezoelectric potential is formed along the width of the T-ZnO nanorods. Therefore, the piezo-potential induced charges follow the DC-type output behavior of the nanogenerator along the internal and external circuit (see Fig. 3(b)). The AC-type charge generation mechanism and the AC output current generated by the nanogenerator, fabricated with vertical ZnO (V-ZnO) nanorods, are shown in Fig. 4(a) and (b), respectively. The AC-type current behavior is attributed to the direct compression of the ZnO nanorods by the external force (under a load of 0.9 kgf). Considering the geometry of the V-ZnO, the vertically well-aligned nanorods are easily compressed by the external pushing force in the direction of the nanorod length rather than being bent. Hence, a piezoelectric potential is generated in the ZnO nanorod along the c-axis under uniaxial strain. Therefore, when an external force results in the uniaxial strain of the V-ZnO nanorods, one side of the nanorods is subjected to a negative piezoelectric potential and the other side to a positive potential. In order to generate a measurable signal above the noise level from nanogenerators, the presence of a Schottky contact at one end of the nanorods is essential. The Schottky contact at the sides of the nanorods with a negative potential enhances the output signal by preventing the flow of electrons into the ZnO nanorods through the interface. The piezo-potential induced electrons are then moved via the external circuit and are accumulated at the interface between the electrode and the side of the nanorods with a positive potential. When the external force is removed and the compressive strain is released, the piezoelectric potential inside the nanorods instantly disappears and the accumulated 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-based nanogenerators. Designs, fabrication, and applications Wang and Song first introduced piezoelectric nanogeneration by examining the piezoelectric properties of a single ZnO nanowire using AFM in 2006 [23]. To eliminate the use of the AFM tip, as reported in this first nanogenerator, for independent operation and technological applications, there have been various innovation designs for improving the performance and applicability of the nanogenerators. The first independent operation of a nanogenerator was also realized by Wang et al. through the design of a nanogenerator with zigzag trenches as a top electrode to replace the AFM tip, where the zigzag trenches act as an array of aligned AFM tips [24]. This was a DC power nanogenerator and was demonstrated using an ultrasonic wave with a frequency of 41 khz. This work formed the basic platform for optimizing and improving the performance of the nanogenerators by integrating them into layered structures. Since then, several vertical nanowire-integrated nanogenerators and lateral nanowire-integrated nanogenerators using ZnO have been fabricated by integrating them into layered structures to improve their performance [25,40,92,93]. There have been continuing efforts to improve the design and fabrication of nanogenerators for several technological applications with better performance. Intensive research into innovative designs has been carried out for the purpose of improving the performance and applicability of the nanogenerators. This has resulted in the development of a fiber-based flexible nanogenerator with ZnO nanowires [94], an integrated transparent flexible nanogenerator with ZnO nanorods [27], a fully rollable graphene-based transparent nanogenerator with ZnO nanorods [29], an integrated sounddriven nanogenerator with ZnO nanowires [26], and a flexible high-output nanogenerator based on lateral ZnO nanowire arrays with output voltages of up to 2.03 V and a peak power density of 11 mw cm 3, which was used successfully to light up a commercial light-emitting diode (LED) [95]. This work was a landmark study toward building self-powered devices by harvesting the energy from the environment. Since then, several other nanogenerators have been fabricated, as described below. Recently, Kim et al. reported the fabrication of foldable and thermally stable paper-based nanogenerators to overcome the problem of the unstable electrical output from plastic based nanogenerators due to thermal induced-stress [96]. To complete the integrated paper nanogenerator, ZnO nanorods were synthesized on a metal-coated cellulose paper substrate using an aqueous solution method, as illustrated in Fig. 5(a). The average diameter and height of the nanorods were approximately 80 nm and 2 mm, respectively. The Aucoated cellulose top electrode was placed above the ZnO nanorod arrays for the formation of a Schottky contact between the Au and ZnO nanorods. The integrated nanodevice was fully foldable (see Fig. 5(b)). Fig. 5(c) is a photographic image of a foldable paper nanogenerator. Mechanical bending tests of the paper nanogenerator were also performed, as shown in Fig. 5(d), and the mechanical durability was discussed and investigated. The mechanical durability of the paper nanogenerators was investigated under mechanical bending. Fig. 5(e) and (f) shows the thermal stabilities of the paper-based and polyethylene naphthalate (PEN)-based nanogenerators, respectively.

7 348 B. Kumar, S.-W. Kim Figure 5 Foldable and thermally stable paper nanogenerator with a cellulose paper substrate and piezoelectrically active ZnO nanorods. (a) A tilted field emission scanning electron microscopy image revealing the morphology of the ZnO nanorods grown on a cellulose paper with Al and Au thin layers via an aqueous solution method. (b) Schematic diagram of an integrated paper nanogenerator 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) PENbased nanogenerator. The PEN-based nanogenerator experiences a severe shape change after alcohol lamp heating for 1 min, while the paper-based nanogenerator was quite thermally stable [96]. The PEN nanogenerator experienced severe shape changes under alcohol lamp heating, while the paper-based nanogenerator was thermally stable. This demonstrates that the paper nanogenerator can be operated even under thermally harsh conditions. Below the glass transition temperature of PEN, the output current density of the PEN-based nanogenerator was approximately two times greater than that of the paperbased nanogenerator, which might be due to the more uniform distribution of the nanorods on PEN than that on paper. However, PEN becomes soft and crooked at temperatures greater than 150 1C, resulting in a drastic reduction of the output current from the PEN-based nanogenerators. Conversely, the current output from the paper-based nanogenerators was extremely stable up to 200 1C. At 250 1C, the paper-based nanogenerators had a greater current output compared with the PEN-based nanogenerators (see Fig. 6). The voltage output from the paper-based nanogenerator was measured in the same way and was not significantly changed up to 200 1C. The measured current and voltage confirm the changes in the electrical transport characteristics of the devices with increasing temperature. This work demonstrates that the paper based nanogenerator exhibits superior charge scavenging performance under thermally harsh conditions. Mechanical energy sources include the vibration of bridges, friction in mechanical transmission systems, deformation in the tires of moving automobiles, etc., all of which are normally wasted. For bicycles, cars, trucks, and even airplanes, a selfpowered monitoring system for measuring the inner tire pressure is not only important for the safe operation of the transportation means, but also for saving energy.

8 Energy harvesting based on semiconducting piezoelectric ZnO nanostructures 349 Figure 6 Thermal stability of the paper-based and PEN-based nanogenerators after thermal annealing at 100, 150, 200, and 250 1C; the inset image shows the PEN-based and paper-based nanogenerators on a hot template at 250 1C [96]. Figure 8 Performance of the nanogenerator attached to the inner surface of the tire, which was triggered by the deformation of the tire. The inset of (a) shows an LCD screen that was lit by the nanogenerator [97]. Figure 7 (a) Shape change of the tire during the vehicle s movement. (b) Experiment setup. The tire was pressed between two boards to simulate its deformation at the position where it comes into or loses contact with the road surface. (c) Sketch map of the nanogenerator construction, which is a cantilever structure with five layers. (d) A photograph showing that the nanogenerator was fixed on the inner surface of the tire using adhesive tape [97]. In a previous work showing a promising application of nanogenerators, one such device was integrated onto the inner surface of a bicycle tire to demonstrate the possibility of using it for harvesting energy from the motion of automobiles [97]. In general, tires turn and are compressed during their rotation. The shape change rate of the tires at the position where they come into or lose contact with the road surface is very large and can be regarded as a good mechanical trigger to quickly introduce or withdraw bending, as shown in Fig. 7(a). Two rigid boards were placed on either side of the tire of a bicycle. One of the boards was fixed; the other one connected to a linear motor and could be moved back and forth (see Fig. 7(b)). The nanogenerator used in this experiment was designed with a free-cantilever beam structure, as shown in Fig. 7(c). Due to its good flexibility, the nanogenerator adheres tightly to the inner surface of the tire. The tire was squeezed and released periodically to simulate the conditions that occur at the position where it comes into or loses contact with the road surface. Each time the nanogenerator was bent, an electric pulse was generated (see Fig. 8). The measured output voltage approached 1.5 V and the measured output current was around 25 na when the travel distance of the board was 12 mm with an acceleration of 30 ms 2.

9 350 B. Kumar, S.-W. Kim A small liquid-crystal display (LCD) screen was lit directly using a nanogenerator that scavenges mechanical energy from the deformation of the tire during its motion. The effective working area of the nanogenerator was about 1.5 cm 0.5 cm and the maximum output power density approached 70 mw cm 3. This work demonstrates the potential of the nanogenerator for energy harvesting from the motion of automobiles and for self-powered tirepressure sensors and speed detectors. Hybrid energy harvesting using ZnO nanostructures Over the years, energy harvesting technologies such as photovoltaics, thermoelectrics, and piezoelectrics for converting solar, heat, and mechanical energies into electricity have been intensively developed. However, because of the completely different mechanisms utilized for harvesting different types of energy, each type of harvesting technology can only generate electricity based on its own specific mechanism. The absence of the particular energy source puts the device out of action; for example the absence of light in the nighttime makes the photovoltaic device inactive, the absence of heat makes a thermoelectric device inactive, and the absence of mechanical energy makes a piezoelectric device inactive. Therefore, an innovative hybrid approach has to be developed for the conjunctional harvesting of multiple types of energy using an integrated structure/material, so that one or all of the available energy resources can be effectively and complementarily utilized to generate electricity all the time. Recently, harvesting multiple-type energies using a single device has been one of the most important research issues in energy harvesting technologies. The combination of the semiconducting and piezoelectric properties of ZnO is extremely important in energy harvesting in this hybrid approach. This approach has great potential for the full utilization of the energy in the environment under which the devices will be operating. In the following sections, we describe several approaches that have been developed using ZnO nanomaterials for simultaneously harvesting solar, and mechanical energies. approach has been developed for the conjunctional harvesting of solar and mechanical energies using an integrated structure/material, so that at least one or both energies can be effectively and complementarily utilized to generate electricity for viable applications. Solar and mechanical energy harvesting hybrid devices can complement the weaknesses of each individual device. The design and development of devices that can harvest multiple types of energy without crosstalk and with synergetic effects are critically necessary for the effective exploitation of the energies available in nature. As a breakthrough concept, Choi et al. demonstrated a flexible hybrid cell. This device can be used as both a solar energy harvester and a touch-sensitive piezoelectric power generator on a single platform [35]. The basic structure was designed from an inverted organic solar cell with a ZnO nanostructured buffer layer on a plastic substrate, where ZnO serves as both the electron transport layer for the solar cell and the active layer for the formation of a piezoelectric potential. Recently, Choi et al. demonstrated a multi-type energy scavenger that converts individually or simultaneously lowfrequency mechanical energy and photon energy into electricity using piezoelectric ZnO in conjunction with organic solar cell. Since the multi-type energy scavenger is based on the coupled piezoelectric and semiconducting properties of ZnO, it has an intrinsically hybrid architecture without crosstalk and an additional assembling process to fabricate it [34]. Fig. 9 (inset) is a schematic illustration of the hybrid device based on ZnO nanostructures. This naturally hybrid architecture is based on the integration of a piezoelectric generator and an organic solar cell. To achieve a fully flexible power generating device, the authors prepared an ITO-coated polyethersulfone (PES) substrate as a cathode window for a solar cell. A ZnO thin film layer was first sputtered to a thickness of 50 nm on the ITO/PES substrate. ZnO nanorods were then formed on the sputtered ZnO film by an aqueous solution method. A P3HT:PCBM polymer blend was spin-coated to a thickness of about 250 nm, and a few nm thick molybdenum oxide (MoO x ) layer and an Au anode with a thickness of about 70 nm were then deposited. ZnO was chosen as the Solar and mechanical energy harvesting Traditionally, it has been believed that harvesting solar energy is sufficient because it has a high efficiency. Such a conclusion was made based on the hypothesis that all of the operations are under full sun illumination (100 mw cm 2 ). In reality, however, many mobile electronics devices are operated indoors and possibly in a hidden area with very dim light. In such cases, the power that can be harvested from available light drops by 2 3 orders of magnitude in comparison to that under full sun illumination. Thus, a solar cell generates electricity effectively only in an area with appreciable light illumination and the absence of light makes the device inactive; the same is true in the case of a piezoelectric nanogenerator. A nanogenerator generates electricity from its surrounding mechanical energies and the absence of mechanical energies makes the nanogenerator device inactive as well. Therefore, an innovative hybrid Figure 9 Naturally hybrid architecture of piezoelectric and photovoltaic power generators. J V characteristics under AM 1.5 G illumination. The inset shows a schematic illustration of the hybrid device based on the nanostructured ZnO layer [34].

10 Energy harvesting based on semiconducting piezoelectric ZnO nanostructures 351 piezoelectric material for the mechanical energy converter as well as the electron transport layer in the solar cell. The current density voltage (J V) curve of the hybrid device is shown in Fig. 9. Its power conversion efficiency (PCE) is about 1.5% on average with a V OC of 0.55 V and a short circuit current density (J SC ) of 9.2 ma cm 2 under standard air mass (AM) 1.5 global (G) illumination conditions (100 mw cm 2 ) without any mechanical strain (i.e. independent solar cell performance). The working principle of the hybrid device for hybrid operation involving both solar and mechanical energies is as follows. When the device is under light illumination alone, it generates continuous electrical output according to the usual mechanism of solar power generation in BHJ solar cells. When dynamic mechanical strain is applied to the device together with photon energy, the sharp piezoelectric output signal is added to the overall output signal as a result of the instantaneous high piezoelectric field created in the nanostructured ZnO layer. Fig. 10 shows the hybrid operation of the device under solar and mechanical energy with controlled mechanical straining processes. The authors observed the dependency of the piezoelectric output on the strain and straining rate. Depending on the factors controlling the strain and straining rate, the piezoelectric output voltage ranged from several tens of mv up to 150 mv and the output current was several hundreds of na. Further, they demonstrated that the piezoelectric output could be changed from the alternating current type to the direct current type by tailoring the mechanical straining processes both in the dark and under light illumination. This work was a successful demonstration of a dual-mode scavenging energy generator that employs both solar and mechanical energies. This flexible hybrid cell converts individually or simultaneously low-frequency mechanical energy and photon energy into electricity using ZnO with coupled piezoelectric and n-type conductive properties. This work establishes a methodology to harvest solar energy and low-frequency mechanical energies such as a light, wind, and body movements, making it possible to produce a promising power generator that could be embedded in flexible architectures such as the flag/shirt/bag/curtain one. This is also of critical importance for its future applications in defense technology, environmental monitoring, and personal electronics. Therefore, such a hybrid energy generator is expected to be a novel multi-functional power supply that could provide electricity at anytime and anywhere. Recently, Xu and Wang demonstrated an approach to making a compact hybrid cell (CHC) that convolutes a solid state DSSC and an ultrasonic wave driven piezoelectric nanogenerator into a single compacted structure for concurrently harvesting solar and mechanical energies [32]. The structure was fabricated based on vertical ZnO nanowire arrays with the introduction of a solid electrolyte and metal coating. Under standard AM 1.5 G illumination conditions, the optimum power was enhanced by 6% after incorporating the contribution made by the nanogenerator. The convolution of the DSSC and nanogenerator in series to form a CHC is presented in Fig. 11. ITO serves as the cathode, while Ag paste in contact with GaN serves as the anode in this configuration. After connecting it to the output wires, the entire CHC was sealed and packaged by epoxy resin to prevent the infiltration of any liquid except the window of the DSSC. The working principle of the CHC is presented in the form of its electron energy band diagram (see Fig. 12). The electrons are promoted by the piezoelectric potential and photovoltaic potential consecutively through the two devices. The maximum output voltage achievable is the difference between the Fermi level of the ZnO nanowires in the DSSC and that of the ZnO nanowires in the nanogenerator, i.e. it is the summation of the output voltages of the nanogenerator and DSSC. In the nanogenerator section, the gap between the Fermi level of the ZnO nanowires and that of the Au determines the maximum output voltage of the nanogenerator. The Au ZnO junction is a Schottky contact which serves as a gate that blocks the back flow of electrons. When the Au electrode slowly pushes the nanowire like an AFM tip, a strain field is created across the nanowire width, with the outer surface being under tensile strain and the inner surface being under compressive strain. The piezoelectric potential on the compressive side Figure 10 Hybrid operation by solar energy (SE) and mechanical energy with controlled mechanical straining processes. Under the solar energy provided by indoor light, the overall output voltage was controlled by the mechanical straining processes; FB: fast bending and FR: fast releasing [34]. Figure 11 Design of the CHC structure composed of the DSSC and nanogenerator. Schematic illustration of the CHC, which is illuminated by sunlight from the top and excited by ultrasonic waves from the bottom. The ITO layer on the DSSC part and GaN substrate are defined as the cathode and anode of the CHC, respectively [32].

11 352 B. Kumar, S.-W. Kim of the nanowire causes the Schottky contact to be forward biased and drives the electrons across the Au ZnO junction. Through an electron-transfer process, these charge carriers are continuously transported through the solid state electrolyte into the DSSC. Figure 12 Electron energy band diagram of the CHC, showing that the maximum output voltage is the sum of those produced by the 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]. Figure 13 Performance of the CHC. (a) Comparison of the J V characteristics of the CHC when illuminated by simulated sunlight with (red curve) and without (blue curve) the ultrasonic wave excitation. The inset shows the expanded output of the V OC points around the axial cross point, showing the increment of V OC by 19 mv after turning on the ultrasonic waves. (b) J V characteristics of the nanogenerator when subjected to excitation by ultrasonic waves, but with the sunlight off. (c) Comparison of the power output J V characteristics of the CHC. The rectangular area is the optimal power output of the CHC [32]. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) To demonstrate the technological feasibility of the CHC for the simultaneous harvesting of solar and mechanical energies, the authors measured the J V curve of the CHC under different conditions. When the full sunlight source was on and the ultrasonic wave source was off, the CHC exhibited a V OC of V and J SC of 252 ma cm 2 (see blue curve in Fig. 13(a)). When both the ultrasonic wave and sunlight were turned on, the V OC reached V, while the J SC remained at 252 ma cm 2 (see red curve in Fig. 13(a)). The output voltage of the CHC showed a 19 mv difference when turning on and off the ultrasonic wave, as shown by the expanded plot of V OC in the right-hand inset of Fig. 13(a), which corresponds to the output voltage of the nanogenerator when the sunlight was off (see Fig. 13(b)). Therefore, in addition to the open circuit voltage, the CHC successfully cumulated the total power outputs from both the DSSC and nanogenerator. Challenges and opportunities Although ZnO is being extensively investigated for use in solar cells, there is good potential for this material to be used in nanogenerators and hybrid devices to further improve the output performance through scientific breakthroughs, such as the neutralization of the piezoelectric potential screening effect due to the presence of free carriers in the semiconductor nanowires and the optimization and localization of the free carriers in the nanowires, which affect the piezoelectric signals of nanogenerators. The carrier density, conductivity, and Schottky contact play a major role in maximizing and optimizing the output performance of the nanogenerators. Recently, Wang discussed several fundamental issues in order to optimizing the output performance of the nanogenerators [98]. Additionally, the doping of various ferroelectric materials into the ZnO nanowires can enhance the piezoelectric signal of the nanogenerators and efficiency of the hybrid devices, due to the resulting increase in the polarization and built-in internal electric field in the ZnO nanowires. In addition to the various successful demonstrations of photovoltaics, piezoelectricity, and hybrid devices for energy harvesting using ZnO nanostructures and the application of ZnO in hydrogen fuel generation, a source of energy through water splitting, the multi-functionality of this material has the potential to modulate the light through the piezophototronic effect in optoelectronic devices. The Wang group demonstrated this concept through LEDs, where the polarization of the output light was modulated by the piezophototronic effect [99]. They controlled the performance of the LED by the piezoelectric effect by introducing a piezopotential in ZnO by inducing a strain which controls the charge transport process at the ZnO GaN interface. The emission intensity and injection current at a fixed applied voltage were enhanced by factors of 17 and 4 after applying a compressive strain of 0.093%, respectively, and the corresponding conversion efficiency was improved by a factor of 4.25 in reference to that without applying any strain. Also, an external efficiency of 7.82% was achieved. The authors suggested that this hugely improved performance is due not only to the increase of the injection current by the modification of the band profile, but also to

12 Energy harvesting based on semiconducting piezoelectric ZnO nanostructures 353 the more elegant effect of the creation of a trapping channel for holes near the heterojunction interface, which greatly enhances the external efficiency. Moreover, Shi et al. demonstrated a piezophototronic effect in a photolectrochemical (PEC) water splitting process to enhance the PEC efficiency [100]. Concluding remarks There have been successful demonstrations of photovoltaic, piezoelectric, and hybrid devices for energy harvesting using ZnO nanostructures. The controlled morphologies of various ZnO nanostructures in nanocrystals, nanowires, nanobelts, and other complex nanoarchitectures, and their high electron mobility enable interfacial charge separation and fast electron transport which improve the charge collection efficiency in solarcells.intheabsenceoflight,themainintentionfor mechanical energy harvesting using nanogenerators is to replace or supplement the current battery systems and create a more efficient source of power for the self-power charging of mobile, implantable, and personal electronics. The mechanism of power generation behavior of nanogenerators fabricated from ZnO nanostructures relies on their coupled semiconducting and piezoelectric properties. A solar cell generates electricity effectively only in an area with appreciable light illumination and the absence of light makes the solar cell device inactive; the same is true in the case of a nanogenerator. A nanogenerator generates electricity from its surrounding mechanical energies and the absence of mechanical energies makes the nanogenerator device inactive as well. Therefore, an innovative hybrid approach was developed for the conjunctional harvesting of solar and mechanical energies using an integrated structure/material, so that at least one or both energies can be effectively and complementarily utilized to generate electricity for viable applications. The combination of the semiconducting and piezoelectric properties of ZnO is important for such hybrid devices to control their output performance. Acknowledgments This research was supported by the International Research and Development Program of the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (MEST) ( ), the Energy International Collaboration Research and Development Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) funded by the Ministry of Knowledge Economy (MKE) ( ), and Basic Science Research Program through the NRF funded by the MEST ( ). Appendix A. Supporting information Supplementary data associated with this article can be found in theonlineversionatdoi: /j.nanoen References [1] S. Xu, Z.L. Wang, Nano Research 4 (2011) [2] Z.L. Wang, Journal of Materials Chemistry 15 (2005) [3] M.-T. Chen, M.-P. Lu, Y.-J. Wu, J. Song, C.-Y. Lee, M.-Y. Lu, Y.-C. Chang, L.-J. Chou, Z.L. Wang, L.-J. Chen, Nano Letters 10 (2010) [4] Y. Wei, C. Xu, S. Xu, C. Li, W. Wu, Z.L. Wang, Nano Letters 10 (2010) [5] Y.W. Heo, D.P. Norton, L.C. Tien, Y. 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14 Energy harvesting based on semiconducting piezoelectric ZnO nanostructures 355 [99] Q. Yang, W. Wang, S. Xu, Z.L. Wang, Nano Letters 11 (2011) [100] J. Shi, M.B. Starr, H. Xiang, Y. Hara, M.A. Anderson, J.-H. Seo, Z. Ma, X. Wang, Nano Letters 11 (2011) Brijesh Kumar received his Ph.D. degree from Indian institute of Technology, Delhi, in 2009 under the supervision of Prof. R.K. Soni. Presently, he is working with Professor Sang-Woo Kim as a Research Professor at School of Advanced Materials Science and Engineering, Sungkyunkwan University (SKKU), S. Korea. His current research areas are fabrication of energy harvesting nanoelectronics devices such as solar cells, nanogenerators, hybrid devices, and graphene-based devices. Sang-Woo Kim is an Associate Professor in School of Advanced Materials Science and Engineering at Sungkyunkwan University (SKKU). He received his Ph.D. from Kyoto University in Department of Electronic Science and Engineering in After working as a postdoctoral researcher at Kyoto University and University of Cambridge, he spent 4 years as an assistant professor at Kumoh National Institute of Technology. He joined the School of Advanced Materials Science and Engineering, SKKU Advanced Institute of Nanotechnology (SAINT) at SKKU in His recent research interest is focused on piezoelectric nanogenerators, photovoltaics, and two-dimensional nanomaterials including graphene and hexagonal boron nitride nanosheets.