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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.

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.

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.

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.

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.

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 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.

In previous studies it was shown that heavily boron-doped nanocrystalline silicon submitted to thermal treatments at temperatures ≥800 °C is characterized by an anomalously high thermoelectric power factor. Its enhanced performances were ascribed to the formation of SiBx precipitates at grain boundary, leading to the formation of potential barriers that filter out low-energy carriers, then causing a simultaneous enhancement of the Seebeck coefficient and of the electrical conductivity. To further investigate the effect of thermal treatment on boron-doped nanocrystalline silicon, samples were submitted to a host of annealing processes or of sequences of them at temperatures between 900 and 1000 °C and for various amounts of time. Electrical conductivity and Hall effect measurements were carried out after each thermal treatment over the temperature range 20–300 K. They provided evidence of the formation of an impurity band, and of hopping conduction at very low temperatures. Hall resistivity data versus temperature provided therefore important insights in the electronic structure of the system, which will enable a more complete understanding of the factors ruling energy filtering in this class of materials.

The performances of two hybrid thermoelectric photovoltaic systems are compared. In the first instance, a photovoltaic (PV) device and a thermoelectric generator (TEG) are optically coupled using a vacuumâ sealed compound parabolic concentrator (CPC). As an alternative, PV and TEG devices are thermally coupled putting them directly in contact to each other. Singleâ junction aâ Si and heterojunction Cu2ZnSnS4 (CZTS) have been considered as PV systems. The two systems are studied by varying the heat transfer coefficient of the cooling system between the TEG cold side and the ambient, the TEG device fill factor, and the optical concentration. Hybridization, in both configurations, always enhances the efficiencies, up toÂâ Â57% for single-junction a-Si and up toÂâ Â35% for the heterojunction CZTS. It will be shown that while direct thermal contact enables larger efficiencies, optical coupling grants lower temperatures at the PV side, enhancing reliability and lifetime. Further advantages and limitations of both configurations will be discussed.

Concerns about sufficient availability of fossil sources in the near future to satisfy the increasing demand for energy, along with an increasing consciousness about global warming and emission-related climate change, have further pushed research and technology development of the so-called renewable energy sources. Solar energy is by far the most widely used of such sources. Photovoltaic (PV) cells have actually undergone an impressive efficiency enhancement over the last decades, stepping up from ~13% in 1977 (single-junction, nonconcentrated single-crystalline solar cells) to 140the current record efficiency of 44% in concentrated, three-junction lattice-matched solar cells [1]. At the same time, solar energy costs have dropped from 77 USD/W (1977) to 0.30 USD/W (2015) [2]. Currently, PV research is addressing two major avenues, namely that of further enhancing PV efficiency and that of developing cells based on noncritical raw materials. On the former side, spectral matching is pursued. Actually, as will be shown in greater detail in Section 7.4, optimal photon energy conversion is achieved only for photons with energy very close to the PV material energy bandgap E g. Thus, enhanced efficiency may be obtained either by stacking materials with different E g in multijunction cells or by reshaping the solar spectrum so as to increase the spectral fraction that may be more efficiently converted into electric energy by a given set of PV materials. On the latter side, instead, new PV materials have surfaced over the last years, including solid solutions such as the CIGS (CuIn x Ga1-x Se2) [3] and the CZTS (Cu2ZnSnS4-x Se x) family [4].

Exploitation of solar energy conversion has become a fundamental aspect of satisfying a growing demand for energy. Thus, improvement of the efficiency of conversion in photovoltaic (PV) devices is highly desirable to further promote this source. Because it is well known that the most relevant efficiency constraint, especially for single-junction solar cells, is unused heat within the device, hybrid thermo-photovoltaic systems seem promising . Among several hybrid solutions proposed in the literature, coupling of thermoelectric and PV devices seems one of the most interesting. Taking full advantage of this technology requires proper definition and analysis of the thermal losses occurring in PV cells. In this communication we propose a novel analysis of such losses, decoupling source-dependent and absorber-dependent losses. This analysis enables an evaluation of the actual recoverable amount of energy, depending on the absorber used in the PV cell. It shows that for incoming solar irradiation of $$1000\,\hbox {W}/\hbox {m}^{2}$$ , and depending on the choice of material, the maximum available thermal power ranges from $$380\,\hbox {W}/\hbox {m}^{2}$$ (for single-crystal silicon) to $$130\,\hbox {W}/\hbox {m}^{2}$$ (for amorphous silicon).

The spectacular growth of networks of intercommunicating sensing nodes has generated a request for alternate, renewable power sources. Thermoelectric generators (TEGs), either conventional or integrated, are possible candidates. This paper analyzes the usability of TEGs as alternate power sources for wireless sensor network. It is shown how TEGs meet power requirements of low-power sensing nodes and how they outperform batteries as of the installation costs. Factors still hampering TEG wider use are also reviewed and commented upon, and an outlook at specific applications where TEGs might be rapidly deployed is provided.

In this work we perform a theoretical analysis of the thermoelectric performance of polycrystalline Si nanowires (NWs) by considering both electron and phonon transport. The simulations are calibrated with experimental data from monocrystalline and polycrystalline structures. We show that heavily doped polycrystalline NW structures with grain size below 100 nm might offer an alternative approach to achieve simultaneous thermal conductivity reduction and power factor improvements through improvements in the Seebeck coefficient. We find that deviations from the homogeneity of the channel and/or reduction in the diameter may provide strong reduction in the thermal conductivity. Interestingly, our calculations show that the Seebeck coefficient and consequently the power factor can be improved significantly once the polycrystalline geometry is properly optimized, while avoiding strong reduction in the electrical conductivity. In such a way, ZT values even higher than the ones reported for monocrystalline Si NWs can be achieved.

Organic materials have recently gained attention as a valuable choice for electronic applications, such as organic field-effect transistors (OFETs)1, organic light-emitting diodes (OLEDs)2, memories3, photovoltaic cells4 and sensors5. Also thermoelectric applications have been proposed, and a variety of studies about this topic have been developed in the last few years6,7. Particularly, the use of the conductive polymer poly(3,4-ethylendioxithiophene), or PEDOT, appeared attractive due to its high electrical conductivity, its low thermal conductivity, its good environmental stability and its easy processability. Now a days the thermoelectric performance of this polymer is still poor, in comparison with inorganic benchmarks, though several attempts have been made to increase its efficiency8,9. In order to obtain a PEDOT-based material with improved features, we decided to embed inorganic nanoparticles in the polymeric matrix. This will generate an energy filtering effect in the hybrid material, which can lead to an increase of the Seebeck coefficient and, at the same time, an improvement of the carrier average mobility through a selection of the more energetic holes. The expected total result is the enhancement of the nanocomposite power factor10. For such purpose, we developed a chemical path to allow an interaction between the polymer and the inorganic nanoparticles, which involves the grafting of the monomer on the nanoparticle surface (Figure 1), and defined a protocol to obtain samples of hybrid material that can be thermoelectrically characterized. In order to understand if it is possible to exploit the energy filtering effect in such samples, several parameters have to be modified, such as the oxidation level of the polymer and the size and the concentration of the nanoparticles, and the resulting variations in the features of the material are currently being studied. In the present work we report the investigation of the variation of the nanoparticle concentration in the hybrid material. The results obtained for hybrid films made with nanoparticles of Mn3O4 (diameter 120±20 nm) are shown in Figure 2. The variation of the power factor due to the nanoparticles concentration implies the existence of an interaction between the polymer and the nanoparticles, but, as reported, there is a worsening of the performance as nanoparticle concentration increases. This is probably owed to the nanoparticle large size: since the Mn3O4 charge carrier mobility is low (5.3-8.1.10-3cm2/Vs)11, the hole free mean path inside this material is far shorter than the diameter of the nanoparticles used. An outlook of the possible modifications to overcome this problem and reach the desired result will be given. Figure 1: Schematic representation of the monomer (Succinyl-EDOT) grafted nanoparticle Figure 2: Power factor measured for hybrid films plotted versus nanoparticle concentration References: (1) Dodabalapur, A.; Torsi, L.; Katz, H. E. Sci. 1995, 268 (5208 ), 270–271. (2) Burroughes, J. H.; Bradley, D. D. C.; Brown, A. R.; Marks, R. N.; Mackay, K.; Friend, R. H.; Burns, P. L.; Holmes, A. B. Nature 1990, 347 (6293), 539–541. (3) Cavallini, M.; Biscarini, F.; Zerbetto, F.; Bottari, G.; Leigh, D. A. Science 2003, 299 (5606), 531. (4) Reddy, V. S.; Karak, S.; Ray, S. K.; Dhar, A. Org. Electr. 2009, 10, 138. (5) Street, R. A.; Mulato, M.; Lau, R.; Ho, J.; Graham, J.; Popovic, Z.; Hor, J. Appl. Phys. Lett. 2001, 78, 4193. (6) Bubnova, O.; Crispin, X. Energy Environ. Sci. 2012, 5, 9345–9362. (7) He, M.; Qiu, F.; Lin, Z. Energy Environ. Sci. 2013, 6 (5), 1352. (8) Bubnova, O.; Khan, Z. U.; Malti, A.; Braun, S.; Fahlman, M.; Berggren, M.; Crispin, X. Nat. Mater. 2011, 10 (6), 429–433. (9) Wei, Q.; Mukaida, M.; Kirihara, K.; Naitoh, Y.; Ishida, T. Materials (Basel). 2015, 8 (2), 732–750. (10) Neophytou, N.; Zianni, X.; Kosina, H.; Frabboni, S.; Lorenzi, B.; Narducci, D. Nanotechnology 2013, 24, 205402. (11) Metselaar, R. Journal of Solid State Chemistry 1981, 341, 335–341.

Energy filtering by energy barriers has been proposed to interpret observations on large thermoelectric power factor (TPF) enhancement in highly doped nanocrystalline Si (nc-Si). Previous Boltzmann transport equation (BTE) modeling indicated that high TPFs could be explained as the result of the presence of energy barriers at the grain boundaries, the high Fermi energy due to the high doping level, and the formation of a low thermal conductivity second phase. To test the assumptions of the BTE modeling and provide more realistic simulations, we have performed Monte Carlo (MC) simulations on the transport properties of composite nc-Si structures. Here, we report on (i) the effect of an energy barrier, and (ii) the effect of multiple barriers on the conductivity and the Seebeck coefficient. In short structures, a TPF enhancement was found and it has been attributed to energy filtering by the energy barrier. The MC indicated that the TE performance can be improved by multiple barriers in close separation. It has been shown that TPF enhancement is possible even when the condition for thermal conductivity non-uniformity across the composite structure is not-fulfilled.

This paper discusses enhancements of solar energy conversion by photovoltaic (PV) cells using thermoelectric (TE) generators harvesting the heat released by the PV cell. To optimize the TE–PV coupling, PV efficiency losses are analyzed by decoupling source- and absorber-dependent losses. This shows that the implementation of a tandem PV–TE device leads to a substantial increase of available power densities depending on the PV material. We further show that a major increase of the conversion efficiency can be achieved by adding an intermediate layer between the PV and the TE stage capable of absorbing the under-the-gap fraction of the solar spectrum. This increase of efficiency is discussed and commented upon also in view of the constraints PV–TE coupling imposes to the TE generator layout and to its material characteristics. The critical importance of effective heat dissipation will also be addressed. The conclusion is reached that not only could tandem PV–TE cells improve the conversion rate of existing solar cells but also that they could enable the use of lower cost PV materials, currently not considered because of their marginal PV efficiency.

Silicon is the reference material of microelectronics, is readily available, relatively unexpensive, and its use may take profit of a fantastic technology. This may explain why a substantial effort has focused on improving its thermoelectric efficiency, either by top-down nanostructuring or through suitable processing. In this paper we report an analysis of the electronic transport properties of heavily boron-doped nanocrystalline silicon films. High-temperature thermal treatments are confirmed to remarkably increase its thermoelectric power factor. Electrical conductivity and Hall effect measurements were carried out over the temperature range 20–300 K along with Seebeck coefficient measurements. We provide evidence of the occurrence of low-temperature hopping conduction between impurity subbands. Dopant ionization was studied as a function of temperature. Freeze-out temperature was found to correlate with the Seebeck coefficient in agreement with Pisarenko equation. This brings to the conclusion that, while untreated samples are weakly degenerate, the thermal processing reverts them into non-degenerate semiconductors, in spite of the high doping level.

Micro and nanotechnologies are called to play a key role in the fabrication of small and low cost sensors with excellent performance enabling new continuous monitoring scenarios and distributed intelligence paradigms (Internet of Things, Trillion Sensors). Harvesting devices providing energy autonomy to those large numbers of microsensors will be essential. In those scenarios where waste heat sources are present, thermoelectricity will be the obvious choice. However, miniaturization of state of the art thermoelectric modules is not easy with the current technologies used for their fabrication. Micro and nanotechnologies offer an interesting alternative considering that silicon in nanowire form is a material with a promising thermoelectric figure of merit. This paper presents two approaches for the integration of large numbers of silicon nanowires in a cost-effective and practical way using only micromachining and thin-film processes compatible with silicon technologies. Both approaches lead to automated physical and electrical integration of medium-high density stacked arrays of crystalline or polycrystalline silicon nanowires with arbitrary length (tens to hundreds microns) and diameters below 100 nm.

Over the last years interest on thermoelectricity for microharvesting or for prospective large-scale conversion of heat into electric power has revamped. This was mostly due to the tremendous impact of nanotechnology onto the conversion rate achievable by thermoelectric generators [1], grown up by almost a factor three over the last two decades. Novel classes of thermoelectric materials have appeared – but also old materials have rejuvenated. Nanowires and nanolayers, along with multilayered structures, have imparted new improved characteristics to many age-old materials [2–4]. Two avenues were actually explored, namely reducing the thermal conductivity κ and increasing the power factor σα (where σ is the electrical conductivity and α is the Seebeck coefficient). Both approaches lead to an increase of the thermoelectric figure of merit ZT = σαT/κ (where T is the absolute temperature) – and thus of the thermoelectric efficiency with which heat is converted into electric energy [5]. As thermoelectric performances have remarkably enhanced, much attention is now dedicated to selecting materials and technologies that could be scaled up and brought to bulk production. On this avenue, geo-abundance and low material costs are key issues. Thus, it is not surprising that thermoelectric properties of silicon, silicon-germanium alloys, and silicides had been the subject of more than 700 publications over the last five years. This Special Issue is devoted to cover the most important advances in this field through some mini-review papers and a selected number of research contributions, spanning from theoretical and experimental research on silicon-based nanostructured materials to the technology enabling their implementation in actual thermoelectric generators. Possibly, a key date in the history of nanostructured thermoelectric silicon may be set at 2008, when Nature published back-to-back two seminal papers by Hochbaum et al. [6] and by Boukai et al. [7] on nanowires. Both

The thermoelectric properties of doped polycrystalline silicon nanowires have been investigated using doping techniques that impact grain growth in different ways during the doping process. In particular, As- and P-doped nanowires were fabricated using a process flow which enables the manufacturing of surface micromachined nanowires contacted by Al/Si pads in a four-terminal configuration for thermal conductivity measurement. Also, dedicated structures for the measurement of the Seebeck coefficient and electrical resistivity were prepared. In this way, the thermoelectric figure of merit of the nanowires could be evaluated. The As-doped nanowires were heavily doped by thermal doping from spin-on-dopant sources, whereas predeposition from POCl3 was utilized for the P-doped nanowires. The thermal conductivity measured on the nanowires appeared to depend on the doping type. The P-doped nanowires showed, for comparable cross-sections, higher thermal conductivity values than As-doped nanowires, most probably because of their finer grain texture, resulting from the inhibition effect that such doping elements have on grain growth during high-temperature annealing.

Second-phase precipitation in nanocrystalline silicon may lead to improved thermoelectric power factors (up to ≈15 mW K−2 m−1 at room temperature) and high figures of merit. In this review, energy filtering is shown to provide an explanatory framework for these findings, helping predict strategies to further improve nanocrystalline silicon efficiency in thermal harvesters.

We present a parametric modeling of the thermoelectric transport coefficients based on a model previously used to interpret experimental measurements on the conductivity, σ, and Seebeck coefficient, S, in highly Boron-doped polycrystalline Si, where a very significant thermoelectric power factor (TPF) enhancement was observed. We have derived analytical formalism for the transport coefficients in the presence of an energy barrier assuming thermionic emission over the barrier for (i) non-degenerate and (ii) degenerate one-band semiconductor. Simple generic parametric equations are found that are in agreement with the exact Boltzmann transport formalism in a wide range of parameters. Moreover, we explore the effect of energy barriers in 1-d composite semiconductors in the presence of two phases: (a) the bulk-like phase and (b) the barrier phase. It is pointed out that significant TPF enhancement can be achieved in the composite structure of two phases with different thermal conductivities. The TPF enhancement is estimated as a function of temperature, the Fermi energy position, the type of scattering, and the barrier height. The derived modeling provides guidance for experiments and device design.

We have investigated the behavior of one B impurity in the presence of dislocations, nanovoids and grain boundaries (GBs) in Si. Our study has been motivated by recent experiments on highly B-doped polycrystalline Si where a very significant thermoelectric power factor enhancement was observed. We study the influence of nanovoids on B diffusivity and activation in Si. We compare with the grain-boundary of pure tilt Si[001](120) poly-Si, where the presence of dislocations is dominant, similarly as in the case of nanovoids in c-Si. We performed DFT calculations to investigate minimum energy configurations, energy barriers and activation or deactivation of one B impurity in the proximity of the above structures. We found that, B preferably occupies substitutional lattice sites at the edge of the cavities. Thus the presence of small voids reduces its diffusivity and mobility, but the electrical activation of B is not significantly influenced. In the case of Si[001](120) GB, B is found to form Boron interstitials clusters in the close vicinity of a GB. The formation energy of those clusters is lower when the cluster is closer to center of the GB. Our results are in agreement with previously reported experimental data. It is provided insight to the effect of structural inhomogeneity on the dopant properties in Si structures that are currently of interest for thermoelectric applications.

A possible approach to raise the efficiency of single-junction solar cells is to couple them with thermoelectric generators (TEGs). It was shown that TEG contribution to the output power is basically ruled by the characteristics of the photovoltaic (PV) material. In this study, we present a quantitative model that correlates the efficiency of the hybrid thermoelectric–photovoltaic (HTEPV) device with the energy gap and the working temperature of the solar cell. Two HTEPV structures are discussed, one capable only to recover the heat released by relaxation of hot electron–hole pairs; and a second one also capturing the low–energy part of the solar spectrum. We show that in the second case the increase of the conversion efficiency could justify the effort needed to add a TEG stage to the PV device. HTEPV constructions are also shown to enable the use of wide-gap materials that are not currently considered in PV applications.

The ability to tune the thermal conductivity of semiconductor materials is of interest for thermoelectric applications, in particular, for doped silicon, which can be readily integrated in electronic microstructures and have a high thermoelectric power factor. Here, we examine the impact of nanovoids on the thermal conductivity of highly doped, high-power factor polysilicon thin films using time-domain thermoreflectance. Voids are formed through ion implantation and annealing, evolving from many small (∼4 nm mean diameter) voids after 500 °C anneal to fewer, larger (∼29 nm mean diameter) voids with a constant total volume fraction after staged thermal annealing to 1000 °C. The thermal conductivity is reduced to 65% of the non-implanted reference film conductivity after implantation and 500 °C anneal, increasing with anneal temperature until fully restored after 800 °C anneal. The void size distributions are determined experimentally using small-angle and wide-angle X-ray scattering. While we believe multiple physical mechanisms are at play, we are able to corroborate the positive correlation between measurements of thermal conductivity and void size with Monte Carlo calculations and a scattering probability based on Matthiessen's rule. The data suggest an opportunity for thermal conductivity suppression combined with the high power factor for increased material zT and efficiency of nanostructured polysilicon as a thermoelectric material.

Energy filtering has been widely considered as a suitable tool to increase the thermoelectric performances of several classes of materials. In its essence, energy filtering provides a way to increase the Seebeck coefficient by introducing a strongly energy-dependent scattering mechanism. Under certain conditions, however, potential barriers may lead to carrier localization, that may also affect the thermoelectric properties of a material. A model is proposed, actually showing that randomly distributed potential barriers (as those found, e.g., in polycrystalline films) may lead to the simultaneous occurrence of energy filtering and carrier localization. Localization is shown to cause a decrease of the actual carrier density that, along with the quantum tunneling of carriers, may result in an unexpected increase of the power factor with the doping level. The model is corroborated toward experimental data gathered by several authors on degenerate polycrystalline silicon and lead telluride.

An experimental investigation about the thermoelectric properties of heavily doped p ad n-type nanocrystalline silicon nanowires (NWs) is described. The NWs are produced with low cost CMOS compatible processes, highly customizable in terms of cross-section and placement, which enables the fabrication of both stacked NWs in nearly vertical arrays within nanostructured templates built with SiO2/Si3N4 thin films and individual, freestanding NWs suited for thermal conductivity measurements. The cross-section dimensions of the investigated NWs range between 30 and 120 nm in size and up to about 2 cm in length. The structure of the NWs, as shown by SEM/TEM observations, is nanocrystalline with average size of the nanocrystals in one dimension that is comparable with the nanowire diameter. On the NWs, Seebeck coefficient, electrical resistivity and thermal conductivity have been measured, yielding thermoelectric figure of merit (ZT) values of 0.2 at 300 K for the best case.

Nanostructuring has opened new ways to increase the thermoelectric performance of a host of materials, mainly by decreasing their thermal conductivity κ while preserving the Seebeck coefficient S and electrical conductivity σ. The thermoelectric properties of degenerated polycrystalline silicon films with nanocavities (NCs) have been studied as a function of annealing temperature upon isochronous annealings in argon carried out every 50°C in the range 500 – 1000°C which were used to modify the shape of the NCs. We found that presence of the NCs had no negative effect on the electronic properties of the system. The measured values of S and σ were close to those previously reported for the blank polycrystalline silicon films with the same doping level. The thermal conductivity was also found to be close to the value measured on the blank sample, about half of the reported value in polycrystals. This led to a power factor of 15.2 mWm-1K-2 and a figure of merit of 0.18 at 300 K.

We report the study of the thermoelectric properties of degenerate, boron-doped polycrystalline silicon on insulator structures. The occurrence of a regime where both the Seebeck coefficient and the conductivity increase is confirmed. This results in a power factor P of 13 mW K-2 m-1. We propose that such high values of P may be determined by adiabatic energy filtering occurring at grain boundaries decorated by segregated boron.

This paper will show that, while ZT is an appropriate performance index when optimizing the heat conversion rate, it may mislead research in view of applications aiming at large electric power production. This is of special relevance when related to the surge of research in the area of low-dimensionality semiconductors where ZT is increased by lowering the thermal conductivity . It will be shown that, when operating between sources at fixed temperature, the highest power output can be obtained by increasing , not decreasing it, the larger electric power output economically enabling thermoelectric generators for massive electric power production. © 2011 American Institute of Physics.

It is well known that the major constraints to the efficiency of photovoltaic devices come from the generation of heat. In this context, thermoelectric generators have been proposed as a viable heat recovery solution, leading to an increase of the overall efficiency. Within this kind of hybrid solution, the photovoltaic and thermoelectric parts can be either electrically separated or connected in the same circuit. In the latter case, the presence of the thermoelectric generator in series to the solar cell may lead to electrical losses. In this work, we analyze the effect of several parameters on the output power of electrically hybridized thermoelectric-photovoltaic systems. Both electrical measurements and simulations are used. The results show that while an electrical lossless condition exists (as also reported in previous works), it does not necessarily lead to significant power gains compared to the sole photovoltaic case. In addition, the strong temperature sensitivity of the lossless condition makes electrical hybridization difficult to implement. Since solar irradiation varies over time, such sensitivity would make the system work mostly in a suboptimal regime. Therefore, this study provides clues on the actual applicability of electrically hybridized devices.