Abstract: Publication date: 2018Source: Semiconductors and Semimetals, Volume 98Author(s): Olivia D. Hentz, Jayce J. Cheng, Paul H. Rekemeyer, Nina Andrejevic, Silvija Gradečak In contrast to the gradual development of mature wafer-scale silicon photovoltaic (PV) technologies, more recent development of nanostructured PV devices has experienced an explosive growth. These materials are solution-processable and as such can be deposited on flexible substrates, in a form of semitransparent devices, and potentially at low cost. In this chapter, we review our present understanding of the basic mechanisms that govern operation of nanowire-based bulk heterojunction PV devices by focusing on current limitations and future opportunities. In particular, colloidal quantum dots (QDs) have been studied as promising candidates for single-junction and tandem solar cell applications due to their direct and tunable band gap in the visible and near-IR spectral regions. However, a mismatch between the optical absorption length and the carrier collection length prevents these devices from achieving optimal photocurrent generation. To enhance charge collection, it is possible to combine (1) hydrothermally grown ZnO nanowire arrays to form an ordered bulk heterojunction and (2) a band alignment engineered PbS QD film that utilizes inorganic and organic ligands to generate cascaded energy-level offsets. Furthermore, the hydrothermal method enables the growth of high-quality ZnO nanowires on graphene electrodes resulting in flexible semitransparent devices.
Abstract: Publication date: 2018Source: Semiconductors and Semimetals, Volume 98Author(s): Yonatan Calahorra, Canlin Ou, Chess Boughey, Sohini Kar-Narayan Piezoelectric semiconducting nanowires have generated much interest due to the interplay of their mechanical, electrical, and optical properties, which paves the way for potential applications in mechanical energy harvesting as well as sensing. The nature of piezoelectricity in these nanowires is governed by the crystalline phases present, which in turn can be controlled during the nanowire growth process. This chapter provides insight into the manifestation of piezoelectricity in semiconducting nanowires, the effect of growth on their piezoelectric properties, and importantly, how piezoelectricity is characterized at the nanoscale in these materials. Energy-related applications of semiconducting piezoelectric nanowires are described in detail, including their incorporation into nanogenerators for energy harvesting, as well as in piezotronic and photo-piezotronics devices based on the electromechanical and opto-electromechanical interactions taking place in piezoelectric semiconductor-nanowire junction-based devices. Advances in nanofabrication, nanoscale characterization, and device engineering, coupled with a greater understanding and control of piezoelectricity in semiconducting nanowires, will ultimately help unlock the full potential of these fascinating nanomaterials.
Abstract: Publication date: 2018Source: Semiconductors and Semimetals, Volume 98Author(s): Francesco Rossella, Giovanni Pennelli, Stefano Roddaro The advent of nanotechnology and nanomaterials is opening new perspectives for the achievement of efficient solid-state heat converters. After decades of slow progress, in recent years innovative ideas have been put forward to improve the thermodynamic conversion efficiency and, as a consequence, new thermoelectric nanomaterials have been developed. A key and challenging ingredient for the progress of this research ambit is today the refinement of precise methods for the measurement of the thermoelectric parameters of nanostructures.
Abstract: Publication date: 2018Source: Semiconductors and Semimetals, Volume 98Author(s): Gerard Gadea, Alex Morata, Albert Tarancon Semiconductor nanowires present outstanding properties for implementation of the thermoelectric effect, i.e., the direct conversion of thermal to electrical energy, which can be exploited within thermoelectric generators (TEGs) in order to power electronic devices. In nanowires, thermoelectric transport properties—Seebeck coefficient, electrical and thermal conductivity—are modified with respect to bulk leading to a higher figure of merit ZT, indicative of TEG performance. Determination of these properties in low-dimensional systems is a difficult but necessary task in order to be able to improve and optimize the nanowires toward their intended application. Moreover, in order to be able to exploit nanowire properties, one must come up with fabrication routes that allow massive growth and integration in devices. Besides, one must optimize—apart from nanowire thermoelectric properties—design parameters such as diameter, length, areal density, and electrical and thermal external elements such as heat sinks or connected devices. This chapter presents a comprehensive description of all the relevant topics for the application of nanowires in thermoelectric generation. The first section briefly introduces thermoelectricity in semiconductors and explains the working principle and applications of TEGs and micro-TEGs (μTEGs). Afterward, a survey of the size effects modifying nanowire transport properties as well as design considerations affecting TEGs with small thermoelectric elements is presented. The available methods for the fabrication and integration of nanowires in devices together with the currently employed measurement techniques for the determination of nanowire thermoelectric properties are described in subsequent sections. Finally, an updated review of nanowire-based μTEGs and final conclusions and future outlook of the topic are given.
Abstract: Publication date: 2018Source: Semiconductors and Semimetals, Volume 98Author(s): Valerio Piazza, Lorenzo Mancini, Hung-Ling Chen, Stéphane Collin, Maria Tchernycheva Semiconductor nanowires are key materials for next-generation solar cells and light-emitting diodes (LEDs), which can improve the performance and bring new functionalities. However, the performance of nanowire devices today is still below theoretical predictions. Standard macroscopic characterizations averaged over millions of nanowires do not provide all the information necessary to optimize the device efficiency. Nanoscale analyses of individual nanowire properties are required in order to evaluate the material quality, to assess the wire-to-wire homogeneity and to detect eventual electrical or optical failures. In this chapter, nanoscale characterizations of nanowire LEDs and solar cells are reviewed. Electron beam-induced current microscopy, photo- and cathodoluminescence, and atomic probe tomography are applied to single nanowires and to macroscopic nanowire devices to address the compositional homogeneity, to analyze defects, to estimate the doping, to measure the minority carrier diffusion lengths and to assess the nonradiative recombination.
Abstract: Publication date: 2018Source: Semiconductors and Semimetals, Volume 98Author(s): Samik Mukherjee, Simone Assali, Oussama Moutanabbir This chapter presents the recent progress in growth and characterization of group IV nanowires and discusses their relevance for energy conversion applications. In a broader sense, the chapter elaborates on two main approaches to enhance the capabilities of silicon-compatible nanowires and expand their applications. The first approach focuses on phonon-engineering processes in nanowire-based structures and their impact on nanoscale heat transport and thermoelectric applications. A review of the current theoretical and experimental understanding of the interplay between nanowire properties and thermal conductivity is provided and recent progress in characterization of nanowire thermoelectric figure of merit is outlined. Moreover, special emphasis is made on nanowires with tailor-made isotopic composition as a rich playground to elucidate subtle but important phenomena governing heat transport in nanowires. The second approach elaborated in this chapter consists of bandgap engineering in nonequilibrium tin-containing group IV nanowires and quantum-sized structures based on silicon–germanium–tin (SiGeSn) nanowires. This emerging family of semiconductors provides two degrees of freedom to tune the band structure, namely, alloying and strain. The ability to independently manipulate strain and lattice parameter in this emerging class of group IV semiconductors is central to engineer low-dimensional systems and heterostructures in a similar fashion to the more mature III–V semiconductors. The chapter outlines the state of the art in the development of these semiconductors and provides a comprehensive overview of the progress in the growth and characterization of GeSn nanowires and nanowire heterostructures. A critical discussion is included to evaluate their potential use to implement high-performance and/or cost-effective optoelectronic devices with emphasis on light absorption engineering and its implication for nanowire-based photovoltaics.
Abstract: Publication date: 2018Source: Semiconductors and Semimetals, Volume 98Author(s): Amit Solanki, Handon Um This chapter provides an overview of fabrication of silicon nanowires by top-down etching methods. The focus will be on wet and dry etching methods to achieve vertically aligned silicon nanowires. We will start by building an understanding of the dry etching process. Then we will delve deeper into the mechanisms, benefits, and drawbacks of these dry etching processes. This is followed by wet etching methods, primarily metal-assisted chemical etching methods. Finally, we will provide a guideline to fabricate vertically aligned Si nanowire arrays with both dry and wet chemical processes.
Abstract: Publication date: 2018Source: Semiconductors and Semimetals, Volume 98Author(s): Tim Ludwig, Christoph Bohr, Albert Queraltó, Robert Frohnhoven, Thomas Fischer, Sanjay Mathur Electrospinning is a promising technique for producing ultrafine fibers of a large variety of one-dimensional materials that can be assembled into nonwoven architectures useful for various smart technologies and lightweight applications such as wearable and flexible electronics, lightweight batteries, and high-surface area substrates for catalysis and sensing. Self-integration of electrospun fibers into webs and yarns not only enhances their functionality but also opens new innovative directions ranging from energy and environment applications to regenerative medicine. This chapter presents a comprehensive account on the processing of single-phase and composite nanofibers and their manifold applications in batteries, supercapacitors, transparent conducting materials, photovoltaics, as well as solar fuels. Using multinozzle approaches in electrospinning, heterojunctions and sophisticated 3D architectures at the nanoscale can be achieved in a single process step by engineering the spinnerets and fiber collectors. Given its modular nature and variability of precursor chemistry, electrospinning enables scaled-up production of micro- and nanofibers at reasonable cost and represents a promising fabrication method for integrating functional nanomaterials into devices. Nanofibrous meshes of carbon, metal, metal oxide, and their mixtures have been obtained in different geometries such as core–shell, Janus-type fibers, and yarns to demonstrate the possibility of net shaping at the nanoscale and possible conversion of interconnected 1D networks into 3D structures. The insufficient strength of nanofiber meshes can be reinforced through infiltration of a secondary phase that also decreases the intrinsic porosity of electrospun mats thereby offering new experimental space to create multimaterial junctions and bulk heterostructures. The chapter also alludes to the future trends and existing challenges of shape control and retention of flexible and fibrous structure after heat treatment that is often mandatory to obtain crystalline materials.
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