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The application of nanotechnology in developing novel thermoelectric materials has yielded remarkable advancements in material efficiency. In many instances, dimensional constraints have resulted in a beneficial decoupling of thermal conductivity and power factor, leading to large increases in the achievable thermoelectric figure of merit ((Formula presented.)). For instance, the (Formula presented.) of silicon increases by nearly two orders of magnitude when transitioning from bulk single crystals to nanowires. Metal-assisted chemical etching offers a viable, low-cost route for preparing silicon nanopillars for use in thermoelectric devices. The aim of this paper is to review strategies for obtaining high-density forests of Si nanopillars and achieving high-quality contacts on them. We will discuss how electroplating can be used for this aim. As an alternative, nanopillars can be embedded into appropriate electrical and thermal insulators, with contacts made by metal evaporation on uncapped nanopillar tips. In both cases, it will be shown how achieving control over surface termination and defectivity is of paramount importance, demonstrating how a judicious control of defectivity enhances contact quality.

Silicon (Si) nanowires have garnered significant interest for their potential applications in Si-based thermoelectrics, primarily due to their low thermal conductivity. While there are several methods to obtain Si nanowires, their density has been a limiting factor, resulting in a low power density that can be achieved by thermoelectric generators. To address this limitation, metal-assisted chemical etching (MACE) has been developed, enabling the creation of high-density nanopillar “forests”. This technique overcomes the previous density constraints. However, Si nanopillars protrude from a bulk Si wafer, which adds its thermal and electric resistivity to those of nanopillars, ultimately reducing the overall power density that can be attained. In this paper, we demonstrate how precise control of pre- and post-MACE processing allows for the creation of fully self-sustained quasi-1D Si nanostructures. We additionally demonstrate that pre-MACE Si processing does not control nanopillar bundling; rather, it primarily determines their shape. This outcome is attributed to the formation of gas nanobubbles during the initial steps of MACE.

Magnetic materials may be engineered to produce thermoelectric materials using spin-related effects. However, clear understanding of localized magnetic moments (µI), free carriers, and Seebeck coefficient (S) interrelations is mandatory for efficient material design. In this work, we investigate µI influence on the spin-dependent S of model ferromagnetic thin films, allowing µI thermal fluctuations, ordering, and density variation influence to be independently investigated. µI influence on free carrier polarization is found to be of highest importance on S: efficient coupling of free carrier spin and localized magnetic moment promotes the increase of S, while spin-dependent relaxation time difference between the two spin-dependent conduction channels leads to S decrease. Our observations support new routes for thermoelectric material design based on spin-related effects in ferromagnetic materials.

The possibility of recovering heat released by photovoltaic (PV) solar cells into electric power by using thermoelectric generators has attracted a remarkable research effort over the last two decades. Views and results are not fully converging, however, ranging from overoptimistic estimates of power gains to fully negative opinions about the convenience of hybridization. The aim of this paper is to review both energetic and economic profitability of photovoltaic-thermoelectric hybridization, as both issues are to be considered in the design of hybrid solar harvesters. It will be shown how the PV material rules the total system efficiency and its economic competitiveness compared to standard PV modules. Marginal opportunities are reported for roof-top concentrated solar harvesters. Yet, hybridization may leverage novel PV materials, currently not considered due to their lower efficiency compared to polycrystalline silicon. Much more significant is instead the window of opportunity when thermoelectric generators are coupled to perovskite solar cells.

Silicon is the most widely used functional material, as it is geo-abundant and atoxic. Unfortunately, its efficiency as a thermoelectric material is very poor. In this paper, we present and discuss advances of research on silicon and related materials for thermoelectric applications, mostly focusing on the comparison between the two strategies deployed to increase its performance, namely either reducing its thermal conductivity or, in polycrystalline materials, increasing its power factor. Special attention will be paid to recent results concerning silicon thin films. The enhancement of Si performances has motivated efforts to develop integrated heat microharvesters operating around room temperature, which will be reviewed also in view of their applications to power wireless sensors for the Internet of Things.

Single-crystalline silicon is widely recognized as a suboptimal thermoelectric material. When heavily doped, it can achieve high power factors (approximately 10 mW/mK2). However, its high thermal conductivity (approximately 140 W/mK at 300 K) severely diminishes its figure of merit, resulting in a modest 0.01 at room temperature. Nevertheless, the prospect of transforming silicon into an effective thermoelectric material holds significant implications for expanding the utilization of thermoelectrics in both heat harvesting and cooling. As a result, research has zeroed in on leveraging nanotechnology to reduce its thermal conductivity. Boukai et al.1 and Hochbaum et al.2 demonstrated in 2008 that rough, single-crystalline silicon nanowires (NWs) exhibit a thermal conductivity of ≈ 5 W/mK due to inelastic scattering of phonons at wire boundaries, with their figure of merit at 300 K reaching approximately 1. While low dimensionality has proven to be a potent strategy for enhancing thermoelectric efficiency, it also results in reduced heat absorption, as a significant portion of the available heat flux is redirected through the material template when NWs are grown parallel to the substrate. Conversely, when NWs are aligned perpendicular to it, the temperature difference primarily occurs on the substrate. Consequently, higher efficiencies come at the expense of lower converted heat flux, leading to lower electric power densities. To circumvent these limitations, dense, unsupported nanowire arrays are required. During this presentation, recent advancements in the fabrication of Si NWs through metal-assisted chemical etching (MACE)3 will be elucidated. Our exploration began with an analysis of the physical chemistry governing the MACE process. We demonstrated that the competing 2- and 4-electron etching processes are contingent on oxide etching kinetics, accounting for the comparable etching rates observed in p- and n-type silicon. Additionally, we provided evidence for th

The increasing energy demand and the ever more pressing need for clean technologies of energy conversion pose one of the most urgent and complicated issues of our age. Thermoelectricity, namely the direct conversion of waste heat into electricity, is a promising technique based on a long-standing physical phenomenon, which still has not fully developed its potential, mainly due to the low efficiency of the process. In order to improve the thermoelectric performance, a huge effort is being made by physicists, materials scientists and engineers, with the primary aims of better understanding the fundamental issues ruling the improvement of the thermoelectric figure of merit, and finally building the most efficient thermoelectric devices. In this Roadmap an overview is given about the most recent experimental and computational results obtained within the Italian research community on the optimization of composition and morphology of some thermoelectric materials, as well as on the design of thermoelectric and hybrid thermoelectric/photovoltaic devices.

Over the last few years, a growing interest has surfaced about the possibility of enhancing solar harvester efficiency by coupling photovoltaic (PV) cells with thermoelectric generators (TEGs). To be effective solutions, hybrid thermoelectric-photovoltaic (HTEPV) solar harvesters must not only increase the solar conversion efficiency but should also be economically competitive. The aim of this paper is to estimate the profitability of HTEPV solar harvesters with no reference to specific materials, relating it instead to their physical properties only and thus providing a tool to address research effort toward classes of HTEPV systems able to compete with current PV technologies. An economic convenience index is defined and used to assess the economic sustainability of hybridization. It is found that, although hybridization often leads to enhanced solar power conversion, power costs (USD/W) may not always justify HTEPV deployment at the current stage of technology. An analysis of the cost structure shows that profitability requires largely enhanced thermoelectric stages, concentrated solar cells, or PV materials with favorable temperature efficiency coefficients, such as perovskite solar cells.

The decrease of solar cell efficiency with temperature is a known problem for photovoltaics (PV). Temperature sensitivity can lead to a considerable amount of energy losses over the lifetime of solar panels. In this perspective Hybrid Thermoelectric-Photovoltaic (HTEPV) systems, which recover solar cell heat losses to produce an additional power output, can be a suitable option. However only hybridization of wide-gap solar cells is convenient in terms of efficiency gains and deserves investigation to evaluate HTEPV devices effectiveness. In this work we report the modeling and the development of customized bismuth telluride thermoelectric generators, optimized to be hybridized with amorphous silicon (aSi), Gallium Indium Phosphide (GaInP) or Perovskites solar cells. The model results showed in all three cases efficiency gains with a maximum of +3.1% for Perovskites (from 16.4% to 19.5%). These enhancements were then experimentally validated for the case of Perovskites solar cells, for which maximum gains were found to occur at typical operating temperatures of conventional PVs. This experimental evaluation demonstrated in an accurate fashion the real potential of thermoelectric hybridization of solar cells.

The field of thermoelectricity has continued to develop rapidly in recent years and remains one of the most exciting areas of research for a materials physicist. The need for sustainable energy has added a technological momentum to the challenge of devising materials with exceptional properties such as low thermal conductivity, high electrical conductivity and a large Seebeck coefficient, and has triggered a global, interdisciplinary effort. More recently, research on thermoelectric materials has promoted and motivated a major research endeavor to clarify the factors affecting thermal conductivity in nanostructures as part of a more general effort to apply nanotechnology to enhance the performance of thermoelectric materials for use in thermoelectric generators and coolers. This book contains the lectures presented as Course 207 of the International School of Physics Enrico Fermi, Advances in Thermoelectricity: Foundational Issues, Materials, and Nanotechnology, held in Varenna, Italy from 15 - 20 July 2019. This comprehensive course aimed to provide students with a modern vision of the physics of thermoelectric phenomena, starting from the thermodynamics of thermoelectricity and from the physics of transport processes and demonstrating how material structure and nanostructure, together with defects, have been used to tailor the physical properties of advanced thermoelectrics. Special attention was also given to areas of current research - from spin-caloritronics to charge transport in polymers - and to a selected number of applications for heat recovery. Encompassing the full complexity of modern thermoelectricity and covering the most cogent themes relevant to current research, the book will be of interest to all those working in the field.

A 4 × non-imaging optically concentrated solar thermoelectric generator (STEG) was simulated and its layout was optimized depending on materials characteristics. The performances of seven state-of-the-art thermoelectric materials were realistically compared considering direct normal irradiances (DNI) between 400 and 900 W/m2 and temperature dependence of the thermoelectric parameters. The model was tested with experimental data from literature and leg aspect ratios, fill factor (or thermal concentration), and leg number per STEG unit area were also used as variables. Due to the high values of thermal concentrations at maximum efficiency, different materials filling the gap among STEG legs were also considered. Maximum efficiency weakly decreases for filler thermal conductivities typical of common insulating materials, opening novel opportunities for STEGs not requiring vacuum. Results of the analysis show that skutterudites, lead telluride and bismuth telluride exhibit the highest efficiencies (≈7%) in the studied range of thermal concentrations and for a DNI equal to 900 W/m2. However, skutterudites and lead telluride were found to be very sensitive on the DNI level, differently from bismuth telluride, which therefore qualifies as the best solution for energy conversion. Moreover, optimal layouts for STEGs based on bismuth telluride more easily meet manufacturing constraints.

Thermoelectric generators, which convert heat directly into electrical power, have great potentialities in the energy harvesting field. The exploitation of these potentialities is limited by the materials currently used, characterized by good thermoelectric properties, but also by several drawbacks. This work presents a silicon-based thermoelectric generator, made of a large collection of heavily p-doped silicon nanostructures. This macroscopic device (area of several mm2) collects together the good thermoelectric features of silicon, in terms of high power factor, and a very reduced thermal conductivity, which resulted in being exceptionally low (1.8 W/(m K), close to the amorphous limit). The generated electrical power density is remarkably high for a Si-based thermoelectric generator, and it is suitable for scavenging applications which can exploit small temperature differences. A full characterization of the device (Seebeck coefficient, thermal conductivity, maximum power output) is reported and discussed.

Silicon is the most widely used functional material and, as such, it enjoys a fantastic technology. It is also geo-abundant and non-toxic. Unfortunately, its efficiency as a thermoelectric material is known to be very poor, unless strategies are deployed to decrease its thermal conductivity. Alloying with Ge and nanostructuration went into this direction, making it the reference material for radioisotope thermoelectric generators. Lately, silicon has also emerged as a candidate for the low-temperature range, where its usability as thin film fully harnesses its easy integrability and the available body of technological know-how. In this talk, recent advances of research on silicon and related materials for low-temperature thermoelectric applications will be presented, comparing the two current strategies to increase its performances, namely reducing its thermal conductivity and, in polycrystalline materials, increasing its power factor. It will be then shown how Si-based thermoelectric generators and coolers are excellent runners-up for microharvesting and localized thermal management around room temperature –a key enabling technology for applications ranging from the Internet of Things to healthcare support and monitoring.

Single-crystalline silicon is well known to be a poor thermoelectric material due to its high thermal conductivity. Most excellent research has focused on ways to decrease its thermal conductivity while retaining acceptably large power factors (PFs). Less effort has been spent to enhance the PF in poly-and nanocrystalline silicon, instead. Here, we show that in boron-hyperdoped nanocrystalline thin films, the PF may be increased up to 33 mW K-2 m-1 at 300 K when hydrogen embedded in the film during deposition is removed. The result makes nanocrystalline Si a realistic competitor of Bi2Te3 for low-Temperature heat harvesting, also due to its greater geo-Availability and lower cost.

Hybrid thermoelectric-photovoltaic (HTEPV) generators can be seen as an evolution of STEGs. In their structure the opto-thermal converter is constituted by a solar cells. The intention is to implement thermoelectric materials to recover the high fraction of heat losses occurring in solar cell, adding an extra output power to the system. HTEPV have been already shown to be able to enhance the efficiency of solar cells, even if researchers are still trying to make this approach good enough to become reality. In this paper we will analyse the layout and the achievable efficiency of this hybrid devices, highlighting the challenges and the strong points of this approach.

Solar thermoelectric generators (STEGs) are two steps energy harvesting systems that convert solar power into electricity. Even if the first generation of this kind of systems was developed in the first half of the twentieth century the interest around this technology has been intermittent. Only in recent years the progress of the thermoelectric material efficiency stimulated a renewed interest on STEGs as a viable alternative to harness solar energy. In this paper the basic aspects of STEG technology, along with the analysis of its efficiency, and the state of the art of this field are discussed.

This chapter is devoted to an analysis of the physics behind the conversion efficiency of thermoelectric generators. After recalling the basic theory of linear irreversible thermodynamics of thermoelectricity, we will focus on the materials and device factors ruling the conversion efficiency of thermoelectric generators. Moving from the well–known Ioffe–Altenkirch formula, the efficiency in the constant–property limit will be comparatively analyzed under Dirichlet and Neumann boundary conditions. Efficiency will be then reconsidered when large temperatire differences are applied, using both Snyder’s concept of compatibility and Ren’s engineering figure of merit. Perfect thermoelectric generators as instances of exo– and endo–reversible engines will also be briefly reviewed along with the yet widely unsolved problem of thermoelectric efficiency under transient conditions.

The most common and efficient way to covert solar power into useful work is by photovoltaic generation. Photovoltaic cells are devices that convert radiative energy into electric energy. This chapter outlines the mechanism of photovoltaic conversion. The physical principles are introduced and described, and their implementation in real devices (cells and modules) is discussed with reference to the so called three solar cell generations, namely bulk cells, thin film cells, and cells based on dye sensitization. The role played by materials in each cell generation is also examined.

The main topics covered in this book will be introduced. An overview of the historical trend of energy consumption over the last one hundred years will show the crucial need for renewable sources progressively replacing fossil and nuclear power supply. Among renewables, solar harvesting is surely the most promising technology, already playing a significant role in the global power landscape. Demand for higher efficiencies and lower power costs may open yet partially unexplored paths where PV modules are paired to ancillary harvesters to improve the usability of solar power, which will be the main focus of this book.

A summary of the main issues covered in the previous chapters will serve a comparative analysis of the current and perspective possibilities that the hybridization of thermoelectric and photovoltaic generators provides. Materials demand, technological open questions, and market-related issues will be discussed. Also concerning the competition with alternate hybridization strategies, an analysis of HTEPV cost-effectiveness will be outlined. It will be shown that HTEPV may have a key role in the development of renewable energy sources, provided that a careful selection of photovoltaic materials is made. The importance of rethinking the layout of thermoelectric generators will be stressed, along with the merits of hybridization in concentrated solar generators. As an overall conclusion, pairing thermoelectric generators to photovoltaic cells will be proved to be profitable for third-generation PV materials, where hybridization might support the differentiation of the solar module market, currently pinned to silicon-based technology.

This chapter is devoted to provide the general theory describing the hybridization of solar cells with thermoelectric generators. Moving from a description of the system, its main components will be introduced and analysed. Their characteristics and their impact on the final system efficiency will be scrutinised. Specifically, the heat generation within solar cells will be detailed considering the main losses occurring in a PV cell. This will bring to an evaluation of the temperature sensitivity of solar cells, which is one of the most important parameter to be considered when pairing PV cells and TEGs. In addition, we will introduce the concept of fully hybridized systems, where the thermoelectric and PV devices are both thermally and electrically connected to each other.

In this chapter we will present the full-thermal approach to thermoelectric solar harvesting. Analysing the state of the art of this field we will report on its historical development, showing its advantages. Technical and technological issues solved and yet to be solved will be addressed as well. Starting from a description of the main system components we will analyse the literature and the strategies reported so far. Then we will discuss how a solar thermoelectric genenerator (STEG) may be modeled, quantitatively predicting their final efficiency. This analysis will show which are the main parameters influencing STEG performances, suggesting which are the best solutions to achieve efficiencies competitive with other solar strategies.

In this chapter, we will describe triple cogeneration technologies for solar conversion. The costs of solar conversion technologies are determined by the efficiency of power conversion, the lifetime and reliability of its components, the cost of the raw materials, potentially including storage, and any fabrication or construction required. Recently, photovoltaics and solar thermal have emerged as viable candidates for low cost power production; they each have losses that vary across the solar spectrum, with realized and theoretical efficiencies that are well below fundamental thermodynamic limits. Thus, it is desirable to split the solar spectrum to utilize both technologies in parallel over their respective optimal wavelength ranges. This chapter will present promising triple co-generation solutions that have been developed and implemented to provide electric power generation by a combination of photovoltaic and thermal generation. In particular, we show that splitting the solar spectrum, and then using high-energy solar photons for photovoltaics and medium-energy solar photons for thermoelectrics with a bottoming Rankine cycle has potential to achieve 50% solar-to-electricity conversion using existing materials. Also, over 50% of the harvested energy goes to thermal storage for generation after sunset, which could enable highly efficient baseload solar electricity and heat generation at all hours of the day.