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Aim of this lecture was an analysis of the thermodynamics of thermoelectric phenomena and of thermoelectric generators seen as heat engines. The basic theory of classical irreversible thermodynamics is recalled, and the conversion efficiency of thermoelectric generators is computed thereof under Dirichlet boundary conditions both in the constant--property limit (namely for vanishingly small temperature differences) and by using the concept of thermoelectric compatibility (for large temperature differences).

L'insegnamento delle materie scientifiche nelle università italiane ha subito profondi cambiamenti con l'introduzione della riforma dell'autonomia didattica degli atenei del 1999 e con la conseguente introduzione del credito formativo universitario, che ha costretto i corsi di laurea ad una significativa contrazione del numero di ore di lezione erogabili. Questo ha indotto se non costretto molti docenti ad una semplificazione dei contenuti, riducendo pesantemente le esemplificazioni proponibili e anche, spesso, rinunciando al rigore deduttivo che molte discipline potrebbero richiedere. Questo testo è il risultato del tentativo di presentare i fondamenti della meccanica statistica agli studenti delle lauree magistrali italiane impiegando un approccio assiomatico. Il lettore viene guidato in un percorso di costruzione della teoria che lo aiuta a comprenderne i principali snodi concettuali, stimolandone l’interesse e lo spirito critico. Rifuggendo da eccessi formali, il volume si propone come un compromesso tra la necessità di mantenere il carico didattico dell’insegnamento proporzionato alle ore di lezione impartite e alle competenze matematiche degli studenti, garantendo allo stesso tempo esempi di applicazioni della teoria sufficienti per apprezzarne la rilevanza e le potenzialità predittive.

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.

Thermoelectricity was a cornerstone in near- equilibrium irreversible thermodynamics. Conversely, classical irreversible thermodynamics (CIT) with its local equilibrium hypothesis (LEH) remains the dominating framework in thermoelectricity. In this talk two instances where CIT approach fails will be presented. Defect engineering is used to control materials thermal conductivity κ, and multiple morphological defects were reported to effectively serve the scope, suppressing phonons with various mean free paths. We studied how grain boundaries and dispersed nanovoids reduce Si κ. The co-presence of defects with different scattering lengths were confirmed to reduce κ. However, application of Matthiessen's rule even to non-gray phonon models led to inconsistencies, showing that non-local descriptors are needed to account for κ modulation. Non-instantaneous response takes a major share in time-dependent thermoelectric phenomena. While the topic is receiving growing attention, still it is mostly framed within CIT, neglecting the disputable validity of the LEH for stimuli with characteristic times comparable to system relaxation times. An approach based on extended irreversible thermodynamics (EIT) will be sustained. However, complete EIT constitutive relations are not yet available for thermoelectricity. This state of affairs will be reviewed and the hurdles hindering the writing of evolutionary equations for thermoelectricity will be commented upon along with recent results.

Silicon is known to be a poor thermoelectric material. Despite its high power factor (PF), its large thermal conductivity (≈ 130 W/mK) makes its thermoelectric figure of merit zT as low as 0.01 around room temperature. This notwithstanding, silicon has found applications in integrated devices, where its compatibility with microelectronic technology has prevailed over meager conversion efficiencies. However, in 2008 it was shown how phonon scattering at nanowire walls may reduce Si thermal conductivity without affecting its PF, leading to zT of ≈ 0.6 at 300 K. We will show how selective charge carrier scattering may be used instead to enhance its PF. Energy-selective charge carrier scattering (also referred to as ‘energy filtering’) was demonstrated long ago to provide a route to enhance PF in nanocomposites. In the presence of potential barriers, charge carriers are scattered with an efficiency dependent upon their kinetic energy. Low-energy carriers are scattered more efficiently than high-energy ones, causing the average carrier mobility to increase – while mobile (non-localized) carrier density decreases. This makes the Seebeck coefficient increase while keeping the electrical conductivity about steady. To our knowledge, such an effect was never reported in silicon until 2010, when we observed that very heavily boron-doped nanocrystalline Si films display a remarkable increase of their PF when extensively annealed at temperatures above 800 °C. Transmission electron microscopy revealed that annealing promotes the precipitation of silicon boride around grain boundaries. Computational and theoretical analyses showed that the potential barriers generated at the interphases filter out low-energy holes, enhancing the PF that raises from fractions of mW/mK2 in as-implanted films to ≈ 20 mW/mK2 in fully annealed samples. More recently, we could further elucidate the physical chemistry of the phenomenon – and why such an effect was not previously reported. Boron precipitates only when hydrogen (embedded upon CVD deposition) diffuses out of the sample, since H forms complexes with boron, preventing its precipitation. In wafers this requires prohibitively long high-temperature processing while the process is viable in small chips. Procedures to promote H diffusion out of large-size silicon chips could then be devised, enabling energy filtering also in wafer-scale thin films. Availability of high-zT Si thin films paves then the way to relevant novel applications of thermoelectrics, impacting microharvesting for several miniaturized devices.

On many counts, thermoelectricity is still a solution in search of a problem. Beyond its well-known limitations as of their conversion efficiency, thermoelectric devices have not yet found a significant field of application where their unique advantages may prevail over their current restrictions, making thermoelectric generation acceptable to end users while still in their technological infancy. This was instead the case of thermoelectric coolers, which were accepted as the sole viable solution for quiet air conditioning in submarines by the Navy in 1993 [1]. And it was also the case for the use of thermoelectric generators by NASA in deep-space probes [2] – yet a too marginal applicative niche to significantly promote a wide commercial interest for thermoelectric generation. In this talk two instances of applications will be discussed, where thermoelectric generation might meet critical issues that no other energy conversion technology may solve. The first instance is solar energy cogeneration [3]. Over the last years, many ways of pairing photovoltaic and thermoelectric stages have been considered, modeled and, to a lesser extent, experimentally validated. Pairing strategies will be reviewed, showing their points of strength and limitations. A detailed analysis of the energy balance in both photovoltaic-thermoelectric solar generators and in tri-cogeneration systems will be provided. Further to energy profitability [4, 5], special emphasis will be given to criteria of economic convenience, which is required for pairing to be successful. It will be shown how coupling between photovoltaic and thermoelectric stages, while energetically convenient for large-gap photovoltaic materials in non-concentrated solar cells, is instead also economically convenient only when low-concentrated (up to 5-10 suns) solar plants are considered. The second applicative context that will be presented is microgeneration, where thermoelectric generation may complement or compete with conventional batteries as power sources in distributed sensing [6]. Also in this case, both energetic feasibility and economic profitability will be discussed. Results will suggest that microgeneration may be competitive as an alternate powering strategy in the enormously growing fields of the Internet of Thing and of Industry 4.0. In summary, while in general terms thermoelectric generation remains non-competitive with conventional heat conversion approaches, it may be sensible and not too overoptimistic to conclude that thermoelectricity has reached a level of maturity adequate to surface as a key-enabling technology in selected applicative areas. Still, possibly more than higher efficiency, what thermoelectricity keeps missing on the materials side is an extension of device operability to the medium temperature range, where technologically mature materials (and devices) are not yet industrially available.

Photovoltaic (PV) efficiency is intrinsically limited, since a relevant fraction of the available solar power is either transmitted (unconverted) or is partially degraded into heat by carrier relaxation in the PV absorber. However, the significant amount of heat made available thereof might be reused and partially converted into electricity by thermoelectric generators (TEGs), an opportunity made even more attractive by the increasing efficiency of thermoelectric (TE) materials. This talk will focus on the research aimed at developing hybrid photovoltaic−thermoelectric generators (HPVTEGs), namely tandem solar converters wherein a TE stage partially converts into electricity both the heat released by the PV stage and the sub-gap part of the solar spectrum. Strategies being developed to pair PV cells to TEGs will be discussed, addressing the role played by their thermal coupling and by the impact of heat dissipation on the efficiency of the HPVTEG. TEG layout in hybrid cells will be shown to require an ad hoc design critically depending on the PV material. The conclusion will be reached that HPVTEGs could enable the use of low-cost, non-critical PV materials, currently not considered for solar conversion technologies because of their marginal efficiency.

In previous publications it was shown that the precipitation of silicon boride around grain boundaries may lead to an increase of the power factor in nanocrystalline silicon. Such an effect was further explained by computational analyses showing that the formation of an interphase at the grain boundaries along with high boron densities can actually lead to a concurrent increase of the electrical conductivity sigma and of the Seebeck coefficient S. In this communication we report recent evidence of the key elements ruling such an unexpected effect. Nanocrystalline silicon films deposited onto a variety of substrates were doped to nominal boron densities in excess of 1020cm-3 and were annealed up to 1000 degrees C to promote boride precipitation. Thermoelectric properties were measured and compared with their microstructure. A concurrent increase of sigma and S with the carrier density was found only upon formation of an interphase. Its dependency on the film microstructure and on the deposition and processing conditions will be discussed.

23rd European Photovoltaic Solar Energy Conference and Exhibition, 1-5 September 2008, Valencia, Spain; 269-271, A significant increase of the total power delivered by a crystalline silicon commercial solar cell has been obtained in Air Mass 1.5 conditions (simulating terrestrial applications) by the deposition of an active coating consisting of poly-vinylacetate doped with tris(dibenzoylmethane)(monophenanthroline) europium(III). This enhancement of the solar energy conversion has been obtained using a very low concentration of the Eu3+ complex, allowing a significant gain in the Watt peak price.

The electrical properties of dislocations and grain boundaries (GB) in directionally solidified polycrystalline silicon were extensively studied as a function of the carbon, oxygen, and nitrogen content, as well as of the relative grain orientation. As a first result of this study we obtained the experimental evidence that the oxygen and carbon content are not independent variables of the material. Therefore, the density and the electrical activity of dislocations are shown to be strongly dependent on the amount of oxygen-carbon compensation. As a second result, experimental evidence was also achieved which demonstrated that grain boundaries are strongly passivated and that recombination losses at GB do not present any relevant relationship with mutual crystallographic orientations of the grains. It appears, therefore, that a careful choice of the growth and postgrowth conditions yields a material which behaves like crucible grown single-crystal silicon. © 1988, The Electrochemical Society, Inc. All rights reserved.

In this work two alternative approaches are introduced for automatically integrating large numbers of several μms long Si NWs in lateral thermoelectric devices. Both cases being planar require a thermally isolated platform to be defined by silicon micromachining. This opens the path to all-silicon thermal harvesters, which can be built in large volumes offering a cost-effective energy harvesting solution where thermal gradients are present.

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.

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.

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.

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.

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.

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.

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.

A simultaneous increase of the Seebeck coefficient and the electrical conductivity resulting in a remarkable enhancement of the thermoelectric power factor was observed in heavily boron doped nanocrystalline silicon films. To understand the underlying mechanisms for this behavior, we explored the thermoelectric transport properties with a theoretical model based on Boltzmann transport equation in the relaxation time approximation. Energy filtering has been included in the model assuming an energy threshold in the transmission coefficient of holes. The relevant scattering mechanisms for holes in bulk silicon have been taken into account. It has been found that the simultaneous increase of S and σ can be interpreted by the synergy of two processes that take place upon annealing: the redistribution of ionized scatterers and the formation of energy barriers. The redistribution of ionized scatterers forms energy barriers at the grain boundaries and increases Seebeck coefficient. Moreover, it decreases the density of ionized scatterers in the grains and increases the mobility. The increase of the mobility is such that over-compensates the decrease of the conductivity due to energy filtering of carriers by the energy barriers. Hence, the conductivity increases simultaneously with the Seebeck coefficient as the annealing process progresses

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 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 Bi$_2$Te$_3$ for low-temperature heat harvesting, also due to its greater geo-availability and lower cost.

In this work we study the effect of different synthetic conditions on thermal transport properties of poly(3,4-ethylenedioxythiophene) (PEDOT) by focusing in particular on the role of proton scavengers. To this aim, different PEDOT samples were generated in silico using a novel computational algorithm based on a combination of first-principles density functional theory and classical molecular dynamics simulations. The corresponding thermal conductivities were then estimated using the approach to equilibrium molecular dynamics methodology. The results show that the initial synthetic conditions strongly affect the corresponding thermal conductivities, which display variations up to a factor of $\ensuremath{\sim}2$ depending on the proton scavenger. By decomposing the thermal conductivity tensor along the direction of maximum chain alignment and the corresponding perpendicular directions, we attribute the thermal conductivity differences to the variations in the average polymer chain length ${\ensuremath{\lambda}}_{\mathrm{ave}}$. A dependence of the thermal conductivity with the polydispersity index was finally observed, suggesting a possible role of intercrystallite chains in enhancing thermal transport properties. By means of the Green-Kubo modal analysis, we eventually characterize the vibrational modes involved in PEDOT thermal transport and investigate how they are related to the thermal conductivity values of the samples.

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.

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.

The field of thermoelectric materials has undergone a revolutionary transformation over the last couple of decades as a result of the ability to nanostructure and synthesize myriads of materials and their alloys. The ZT figure of merit, which quantifies the performance of a thermoelectric material has more than doubled after decades of inactivity, reaching values larger than two, consistently across materials and temperatures. Central to this ZT improvement is the drastic reduction in the material thermal conductivity due to the scattering of phonons on the numerous interfaces, boundaries, dislocations, point defects, phases, etc., which are purposely included. In these new generation of nanostructured materials, phonon scattering centers of different sizes and geometrical configurations (atomic, nano- and macro-scale) are formed, which are able to scatter phonons of mean-free-paths across the spectrum. Beyond thermal conductivity reductions, ideas are beginning to emerge on how to use similar hierarchical nanostructuring to achieve power factor improvements. Ways that relax the adverse interdependence of the electrical conductivity and Seebeck coefficient are targeted, which allows power factor improvements. For this, elegant designs are required, that utilize for instance non-uniformities in the underlying nanostructured geometry, non-uniformities in the dopant distribution, or potential barriers that form at boundaries between materials. A few recent reports, both theoretical and experimental, indicate that extremely high power factor values can be achieved, even for the same geometries that also provide ultra-low thermal conductivities. Despite the experimental complications that can arise in having the required control in nanostructure realization, in this colloquium, we aim to demonstrate, mostly theoretically, that it is a very promising path worth exploring., 72 pages, 13 figures

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 nanowires, whose thermal conductivity is strongly reduced with respect to that of the bulk silicon, are very promising for high-efficient thermoelectric conversion. This work focuses on the development of a technique for the fabrication of thermoelectric generators which are based on vertical silicon nanowire forests, achieved through a metal-assisted chemical etch. As heavily doped nanowires are essential in thermoelectric applications, this chemical process has been applied both on lightly and on highly doped (> 1019 cm−3) silicon substrates. A comparison of the results shows that the etch behaves in a completely distinct way when applied to the differently doped substrates. The results of this comparison and a preliminary insight into the diverse behavior occurred are reported. The different initial nucleation of silver, which determines the hole injection, essential to the etching of silicon, seems to be the key point of this different behavior.

PdGe contact fabrication on Ge(001) wafers doped with Ga is investigated using conventional complementary metal-oxide-semiconductor processes. Despite a p-type doping level of ~1.4 × 1020 cm−3, the resistivity of the PdGe contact is found to be twice higher than that of undoped Ge. Ga doping has no influence on the Pd reaction with Ge. However, the doping process and the Salicide process led to the formation of Ga-Pd defects in both sides of the PdGe/Ge interface, resulting from Ga and Pd co-segregation on Ge dislocation loops.

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 development and commercialization of Photovoltaic (PV) cells with good cost-efficiency trade-off not using critical raw materials (CRMs) is one of the strategies chosen by the European Community (EC) to address the Energy Roadmap 2050. In this context Cu2ZnSnS4 (CZTS) solar cells are attracting a major interest since they have the potential to combine low price with relatively high conversion efficiencies. Although a ≈9% lab scale efficiency has already been reported for CZTS this technology is still far from being competitive in terms of cost per peak-power (€/Wp) with other common materials. One possible near-future solution to increase the CZTS competiveness comes from thermoelectrics. Actually it has already been shown that Hybrid Thermoelectric-Photovoltaic Systems (HTEPVs) based on CIGS, another kesterite very similar to CZTS, can lead to a significant efficiency improvement. However it has been also clarified how the optimal hybridization strategy cannot come from the simple coupling of solar cells with commercial TEGs, but special layouts have to be implemented. Furthermore, since solar cell performances are well known to decrease with temperature, thermal decoupling strategies of the PV and TEG sections have to be taken. To address these issues, we developed a model for two different HTEPV solutions, both coupled with CZTS solar cells. In the first case we considered a Thermally-Coupled HTEPV device (TC-HTEPV) in which the TEG is placed underneath the solar cell and in thermal contact with it. The second system consists instead of an Optically-Coupled but thermally decoupled device (OC-HTEPV) in which part of the solar spectrum is focused by a non-imaging optical concentrator on the TEG hot side. For both solutions the model returns conversion efficiencies higher than that of the CZTS solar cell alone. Specifically, increases of ≈30% are predicted for both kind of systems considered. Copyright © 2017 American Scientific Publishers All rights reserved.

Breaking the conventional decrease of the Seebeck coefficient with increasing conductivity would be a significant advancement towards large thermoelectric power factor enhancement and high thermoelectric efficiency. We report on a mechanism identified in hyper-doped nanocrystalline Si films that can lead to this task: a transition from dominant ionized impurity scattering to dominant phonon scattering upon thermal annealing at a high annealing temperature T a that takes place to fulfill charge neutrality. We show that the synergy between charge neutrality and energy filtering activated by thermal annealing of the originally defective nanocrystalline sample leads to high mobility, simultaneous increase of the conductivity and the Seebeck coefficient and large enhancement of the thermoelectric power factor. This is demonstrated by means of advanced theoretical modeling and excellent quantitative agreement with the experiment. Our work provides interpretation of so far not understood observations in nanocrystalline Si and indicates a new route for engineering Si as well as other nanostructured materials for high thermoelectric efficiency.

Micromorphology of conjugated polymers is expected to play a crucial role in both heat and charge transport properties. In this perspective, the details of the polymerization mechanism acquire a fundamental relevance, providing the link between the basic chemical reaction paths and the resulting molecular structure and arrangement. For PEDOT, the role played by the Bronsted bases (proton scavengers) and their impact on the distribution of polymer chain lengths are still a matter of debate. In the present work, we have systematically analyzed several reaction paths leading to PEDOT polymerization. By means of atomistic simulations, we identified the thermodynamically preferred reaction path, proving that tosylate anions rule proton scavenging. PEDOT chain length was computed to be ∼12-13 monomeric units. We could also demonstrate how the proton scavengers set at once the chain lengths and the sample crystallinity. Furthermore, we found that tosylate gives rise to a sharper multimodal distribution of chain length, a feature that supports hypotheses regarding the occurrence of a percolative transport regime mediated by tie chains bridging paracrystalline regions.

Thermoelectricity offers a sustainable path to recover and convert waste heat into readily available electric energy, and has been studied for more than two centuries. From the controversy between Galvani and Volta on the Animal Electricity, dating back to the end of the XVIII century and anticipating Seebeck’s observations, the understanding of the physical mechanisms evolved along with the development of the technology. In the XIX century Ørsted clarified some of the earliest observations of the thermoelectric phenomenon and proposed the first thermoelectric pile, while it was only after the studies on thermodynamics by Thomson, and Rayleigh’s suggestion to exploit the Seebeck effect for power generation, that a diverse set of thermoelectric generators was developed. From such pioneering endeavors, technology evolved from massive, and sometimes unreliable, thermopiles to very reliable devices for sophisticated niche applications in the XX century, when Radioisotope Thermoelectric Generators for space missions and nuclear batteries for cardiac pacemakers were introduced. While some of the materials adopted to realize the first thermoelectric generators are still investigated nowadays, novel concepts and improved understanding of materials growth, processing, and characterization developed during the last 30 years have provided new avenues for the enhancement of the thermoelectric conversion efficiency, for example through nanostructuration, and favored the development of new classes of thermoelectric materials. With increasing demand for sustainable energy conversion technologies, the latter aspect has become crucial for developing thermoelectrics based on abundant and non-toxic materials, which can be processed at economically viable scales, tailored for different ranges of temperature. This includes high temperature applications where a substantial amount of waste energy can be retrieved, as well as room temperature applications where small and local temperature differences offer the possibility of energy scavenging, as in micro harvesters meant for distributed electronics such as sensor networks. While large scale applications have yet to make it to the market, the richness of available and emerging thermoelectric technologies presents a scenario where thermoelectrics is poised to contribute to a future of sustainable future energy harvesting and management. This work reviews the broad field of thermoelectrics. Progress in thermoelectrics and milestones that led to the current state-of-the-art are presented by adopting an historical footprint. The review begins with an historical excursus on the major steps in the history of thermoelectrics, from the very early discovery to present technology. A panel on the theory of thermoelectric transport in the solid state reviews the transport theory in complex crystal structures and nanostructured materials. Then, the most promising thermoelectric material classes are discussed one by one in dedicated sections and subsections, carefully highlighting the technological solutions on materials growth that have represented a turning point in the research on thermoelectrics. Finally, perspectives and the future of the technology are discussed in the framework of sustainability and environmental compatibility.

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.

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.

In this paper we analyze the thermodynamic efficiency expected for a fully dissipative thermoelectric generator (TEG) operating under stationary conditions at a finite rate. Although a finite-time thermodynamic analysis of TEGs has been aimed at since long time, no complete theory is available yet. The state of the art of theory is reviewed, and a simple expression for the maximum achievable efficiency of TEGs operating under fully irreversible conditions is obtained. This also sets a reference efficiency for forthcoming studies of nonstationary operation of TEGs.

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.

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.

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 dedicated to present the state of the art of hybrid photovoltaic–thermoelectric generators based on either organic or inorganic photovoltaic cells. Present challenges and future perspectives of this approach to energy harvesting will be discussed with a special emphasis on materials issues. It will be seen that both classes of PV materials deserve attention in view of applications in hybridized converters, although absorber stability and degradation of its PV efficiency with increasing temperatures sets limitations to currently achievable efficiencies, also in view of the still low efficiency of thermoelectric stages.

Ideal thermoelectric materials should possess low thermal conductivity $$\kappa $$ along with high electrical conductivity $$\sigma $$ . Thus, strategies are needed to impede the propagation of phonons mostly responsible for thermal conduction while only marginally affecting charge carrier diffusion. Defect engineering may provide tools to fulfill this aim, provided that one can achieve an adequate understanding of the role played by multiple morphological defects in scattering thermal energy carriers. In this paper, we study how various morphological defects such as grain boundaries and dispersed nanovoids reduce the thermal conductivity of silicon. A blended approach has been adopted, using data from both simulations and experiments in order to cover a wide range of defect densities. We show that the co-presence of morphological defects with different characteristic scattering length scales is effective in reducing the thermal conductivity. We also point out that non-gray models (i.e. models with spectral resolution) are required to improve the accuracy of predictive models explaining the dependence of $$\kappa $$ on the density of morphological defects. Finally, the application of spectral models to Matthiessen’s rule is critically addressed with the aim of arriving at a compact model of phonon scattering in highly defective materials showing that non-local descriptors would be needed to account for lattice distortion due to nanometric voids.