ReviewEnergy harvesting based on semiconducting piezoelectric ZnO nanostructures
Graphical abstract
This paper reviews the importance of multi-functional ZnO nanostructures in energy harvesting including solar, mechanical, and multi-type energies. Intensive research and design efforts to commercialize energy harvesting using multi-dimensional ZnO nanostructures are discussed.
Highlights
► Multifunctional ZnO nanostructures have potential in efficient energy harvesting. ► Research efforts of solar, mechanical, and multi-type energies harvesting are discussed. ► Further challenge and opportunities of ZnO nanostructures in energy harvesting are highlighted.
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 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], [6], [7], [8], [9], [10], such as ultraviolet (UV) lasers [11], [12], light-emitting diodes [13], field emission devices [14], [15], [16], high performance nanosensors [17], [18], [19], solar cells [4], [20], [21], [22], and piezoelectric nanogenerators [23], [24], [25], [26], [27], [28], [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 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], [21], [22], piezoelectric nanogenerators [23], [24], [25], [26], [27], [28], [29], and hybrid energy harvesting devices [31], [32], [33], [34], [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], [40], [41], [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.
Section snippets
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], [48], [49], [50], [51], CIS [52], CIGS [53], GaAs, and InGaP semiconductor multi-junctions have been used in
Mechanical energy harvesting using ZnO nanostructures-based piezoelectric nanogenerators
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
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
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
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 solar cells. In the absence of light, the main intention for mechanical
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) (2010-00297), 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) (2011-8520010050), and Basic Science Research Program through the NRF funded by the MEST
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.
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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 2004. 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 2009. His recent research interest is focused on piezoelectric nanogenerators, photovoltaics, and two-dimensional nanomaterials including graphene and hexagonal boron nitride nanosheets.