Enhancement of thermoelectric power factor by modulation doping of bulk polycrystalline SnS / thin film PEDOT:PSS bilayer

Silicon‐added and modulation‐doped higher manganese silicide (HMS, MnSi1.7) films have been prepared on glass substrates by magnetron‐sputtering of MnSi1.85, Si, and Al targets. Silicon‐addition and modulation‐doping are used to enhance the Seebeck coefficient and reduce the electrical resistivity, respectively. Raman spectra indicate that the silicon‐added MnSi1.7 film consists of two phases, crystalline MnSi1.7 and crystalline silicon. It is found that the silicon‐added MnSi1.7 film has a larger Seebeck coefficient (S), but a higher electrical resistivity (ρ) as well. Consequently, the thermoelectric power factor (PF = S2/ρ) is not enhanced, 0.320 × 10−3 W/m K2 at 733 K, and about the same as that of a pure MnSi1.7 film. The silicon‐added MnSi1.7 layer in a modulation‐doped structure Si:Al/MnSi1.7/glass, however, has a higher energy barrier height, a larger Seebeck coefficient, and a lower electrical resistivity. As a result, the thermoelectric power factor is greatly enhanced and can reach 0.573 × 10−3 W/m K2 at 733 K.

The analytical properties of macroscopic transport coefficients of two-component composites are first used to discuss the thermoelectric power factor of such a composite. It is found that the macroscopic power factor can sometimes be greater than the power factors of both of the pure components, with the greatest enhancement always achieved in a parallel slabs microstructure with definite volume fractions for the two components. Some interesting examples of actual mixtures are then considered, where the components are a “high quality thermoelectric” and a “benign metal,” leading to the conclusion that considerable enhancement of the power factor is often possible, with but a modest reduction in the thermoelectric figure of merit, compared to those of the high quality thermoelectric component. Two possibilities for fabricating real composites with such improved thermoelectric properties emerge from this study: a parallel slabs microstructure of benign metal and high quality thermoelectric, and a sintered collection of benign metal grains, each of them coated by a thin shell of high quality thermoelectric.

The simultaneous realization of low thermal conductivity and high thermoelectric power factor in materials has long been the goal for the social use of high-performance thermoelectric modules. Nanostructuring approaches have drawn considerable attention because of the success in reducing thermal conductivity. On the contrary, enhancement of the thermoelectric power factor, namely, the simultaneous increase of the Seebeck coefficient and electrical conductivity, has been difficult. We propose a method for the power factor enhancement by introducing coherent homoepitaxial interfaces with controlled dopant concentration, which enables the quasiballistic transmission of high-energy carriers. The wavenumber of the high-energy carriers is nearly conserved through the interfaces, resulting in simultaneous realization of a high Seebeck coefficient and relatively high electrical mobility. Here, we experimentally demonstrate the dopant-controlled epitaxial interface effect for the thermoelectric power factor enhancement using our "embedded-ZnO nanowire structure" having high-quality nanowire interfaces. This presents the methodology for substantial power factor enhancement by interface carrier scattering.

We report the enhancement of thermoelectric power factor in composite of SiGe-CrSi2. P-type SiGe- CrSi2 was synthesized by mechanical alloying and sintering method. In order to achieve nanocrystalline structure composite powder was prepared by high energy ball milling. Prepared powders were sintered at different press conditions to optimize for maximum power factor. The crystal structure and phase formation of SiGe and CrSi2 alloys in the composite were investigated using x- ray diffraction analysis. The electrical conductivity, Seebeck coefficient and thermal conductivity of sintered samples were measured from room temperature to 850 C. The result shows about 50% improvement in thermoelectric power factor of SiGe-CrSi2 compared to SiGe alloy.

Enhancement of thermoelectric power factor of silicon germanium films grown by electrophoresis deposition

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.

Thermoelectric (TE) materials have undergone revolutionary progress over the last 20 years. The thermoelectric figure of merit ZT, which quantifies the ability of a material to convert heat into electricity has more than doubled compared to traditional values of $$ZT\sim 1$$ZT~1, reaching values even beyond $$ZT\sim 2$$ZT~2 in some instances. These improvements are mostly attributed to drastic reductions of the thermal conductivity in nanostructured materials and nanocomposites. However, as thermal conductivities in these structures approach the amorphous limit, any further benefits to ZT must be achieved through the improvement of the thermoelectric power factor. In this work we review two of the most promising avenues to increase the power factor, namely (i) modulation doping and (ii) electron energy filtering, and present a computational framework for analysis of these mechanisms for two example cases: low-dimensional gated Si nanowires (electrostatically achieved doping), and superlattices (energy filtering over potential barriers). In the first case, we show that a material with high charge density, but free of ionized impurities, can provide up to a five-fold thermoelectric power factors increase compared to the power factor of the doped material, which highlights the benefits of modulation doping, or gating of materials. In the second case, we show that optimized construction of energy barriers within a superlattice material geometry can improve the power factor by up to $$\sim 30\,\%$$~30%. This paper is intended to be a review of our main findings with regards to efforts to improve the thermoelectric power factor through modulation doping and energy filtering.

The strategy of using aliovalent substitution in A2B2O6 double perovskites remained the popular choice to enhance the charge carrier concentration in order to increase their electrical conductivity. In the present investigation, we have shown that the isovalent substitution in A-site can facilitate in manipulating the oxidation states of B-site transition metal cations in double perovskites, which in turn helps in increasing the carrier concentration. Further, using the strategy of manipulating valence states of B-site cations, we could enhance the thermoelectric (TE) power factor of double perovskites. Ceramic samples of Ba x Sr2−x CrMoO6 (x = 0.0, 0.1, 0.2, and 0.3) double perovskites have been synthesized via solid-state reaction route. The phase constitution and morphological study have been carried out via x-ray diffraction (XRD) and field scanning electron microscope (FESEM). Rietveld refinement of XRD data confirms the polycrystalline cubic structure with Pm 3ˉ m space group. Negative values of Seebeck coefficient have been observed for these oxides in the temperature range from room temperature to 1100 K, confirming electrons as the majority charge carriers. The electrical conductivity of Sr2CrMoO6 double perovskite is found to be increased by more than an order of magnitude due to isovalent Ba2+ doping in place of Sr2+. As a result, 5 times enhancement of TE power factor has been attained in Ba x Sr2−x CrMoO6. Charge transport mechanism of these double perovskites is found to be governed by the small polaron hopping conduction model. x-ray photoelectron spectroscopy spectra validate the presence of multivalent cations of Mo5+, Mo6+, Cr3+, and Cr6+ in these double perovskites. Furthermore, the detailed defect chemistry analysis suggests that owing to Ba substitution, Cr is oxidized from Cr3+ to Cr6+ oxidation states, which enhances the electron concentration and reduces the low mobility oxygen vacancies leading to dramatically improved electrical conductivity.

The thermoelectric properties of gate-all-around silicon nanowires (Si NWs) are calculated to determine the potential for significant power factor enhancement. The Boltzmann transport equation and relaxation time approximation are employed to develop an electron transport model used to determine the field-effect mobility, electrical conductivity, Seebeck coefficient, and power factor for Si NWs with cross-sectional areas between 4 nm × 4 nm and 12 nm × 12 nm and a range of gate biases. Electrical conductivity for the gated Si NWs was much higher than that of doped Si due to the lack of ionized impurities and correspondingly greater carrier mobility. A significant increase in electrical conductivity with decreasing Si NW cross-sectional area was also observed due to a large increase in the average carrier density. For all Si NWs, the Seebeck coefficient was lower than that of doped bulk Si due to the different energy dependence between ionized impurity and phonon-mediated scattering processes. This decrease was also confirmed with Seebeck coefficient measurements of multigated Si NWs and n-type Si thin-films. Quantum confinement was also found to increase the Seebeck coefficient for <8 nm × 8 nm Si NWs and also at high charge densities. A maximum power factor of 6.8 × 10−3 W m−1 K−2 was calculated for the 6 nm × 6 nm Si NWs with typical Si/SiO2 interface roughness, which is 2–3 × those obtained experimentally for bulk Si. The power factor was also found to greatly depend on surface roughness, with a root-mean-square roughness of <0.8 nm necessary for power factor enhancement. An increase in ZT may also be possible if a low thermal conductivity can be obtained with minimal surface roughness.

Toward drastic enhancement of thermoelectric power factor, quantum confinement effect proposed by Hicks and Dresselhaus has intrigued a lot of researchers. There has been much effort to increase power factor using step-like density-of-states in two-dimensional electron gas (2DEG) system. Here, we pay attention to another effect caused by confining electrons spatially along one-dimensional direction: multiplied 2DEG effect, where multiple discrete subbands contribute to electrical conduction, resulting in high Seebeck coefficient. The power factor of multiple 2DEG in GaAs reaches the ultrahigh value of ~100 μWcm-1 K-2 at 300 K. We evaluate the enhancement rate defined as power factor of 2DEG divided by that of three-dimensional bulk. The experimental enhancement rate relative to the theoretical one of conventional 2DEG reaches anomalously high (~4) in multiple 2DEG compared with those in various conventional 2DEG systems (~1). This proposed methodology for power factor enhancement opens the next era of thermoelectric research.

Flexible, lightweight, and low-cost thermoelectric thin films are promising for self-powered wearable electronics and sensors. In this work, we report on flexible Te nanostructures/PEDOT:PSS composite thin films with high power factor and their application as flexible temperature sensors. Te nanostructures with high crystallinity and high aspect ratios were synthesized through an environmentally friendly method without using highly toxic chemicals. Individual Te nanostructures achieve a thermoelectric figure of merit (ZT) of 0.13 at 300 K, indicating good potential as inorganic fillers for nanostructures/polymer hybrid materials. Based on the synthesized Te nanostructures, flexible p-type Te/PEDOT:PSS thin films were fabricated through a simple dilution and vacuum filtration method. The power factor of the as-prepared composite thin film outperforms that of either a Te or DMSO-treated PEDOT:PSS thin film, and importantly, it can be further enhanced to 149 μW m-1 K-2 by hot pressing, which is nearly threefold enhancement compared to the values reported for the vacuum-filtered flexible Te/PEDOT:PSS thin films in the literature. The hot-pressed composite thin film shows high flexibility with the electrical conductivity remaining almost unchanged after 1000 bending cycles under a bending radius of 5 mm. Flexible temperature sensors were fabricated based on the hot-pressed Te/PEDOT:PSS thin film, which exhibited high sensitivity in detecting temperature stimuli. The developed temperature sensors were applied onto a two-finger flexible mechanical claw for identifying hot/cold objects in robotic grasping. This work demonstrates an effective approach to enhance the thermoelectric power factor of flexible Te nanostructures/polymer composites and their promising application in flexible thermal sensing.

A significant enhancement in thermoelectric power factor of In2Te3 thin films by Se doping and Te composition tuning

Modulation doping is one of the strategies to improve thermoelectric power factors of nanocomposites and thin‐film bilayered heterostructures by effectively increasing electrical conductivity. Here, it is reported that thin‐film heterostructures of heavily doped p‐type organic conducting polymer, poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and undoped thin‐film Ge can enhance thermoelectric power factor by modulation doping. The maximum power factor and Seebeck coefficient of the bilayered heterostructures are 154 µW m−1 K−2 and 398 µV K−1, respectively, corresponding to 47‐fold and 41‐fold increases compared to those of bulk PEDOT:PSS and 64‐fold increase compared to power factor of undoped Ge. The enhancements in power factor and Seebeck coefficient are quantitatively described by the hole transfer from PEDOT:PSS to Ge, which takes into account the band alignment at the interface detected by Kraut's method. Agreement between the simulation and experiment results also implies predictability of thermoelectric performances of nanoscale bilayered heterostructures in general, when band offset, Fermi level, and individual electronic properties are available. This work can be further extended to predict performance of other nanoscale combinations of thermoelectric and other electronic materials in general.

Bi85Sb15−xPbx(x=0, 0.5, 1, 2, 3, 4) alloys with partial substitution of Pb for Sb were synthesized by mechanical alloying followed by high-press sintering under a pressure of 5 GPa for 30 min at 523 K. The electrical conductivity, Hall coefficient and Seebeck coefficient were measured in the temperature range of 77–300 K. The results show that the doped sample is n-type for x=0.5, while the doped samples are p-type for x>0.5. The power factor reaches a maximum value of 2.97×10−3 W/mK2 at 225 K for the sample of Bi85Sb14.5Pb0.5, which is nearly two times larger than that of the undoped sample. It is confirmed that a better low temperature thermoelectric power factor can be attained by optimizing the Pb content in Bi85Sb15−xPbx alloys prepared by high-pressure sintering.

Strontium Titanate (SrTiO3) nanoparticles were synthesised by varying the hydrothermal growth period as 12, 24 and 48 h. The crystal structure, morphology, functional groups and elemental composition of the prepared SrTiO3 nanoparticles were studied using XRD, FESEM, Raman and XPS, respectively. XRD analysis shows that the intensity of the diffraction peaks of SrTiO3 increased with growth period due to high crystallinity of the hydrothermally grown samples. From the FESEM images, it was observed that the morphology of SrTiO3 was changed from spherical to cubic when the hydrothermal growth period increased from 12 to 24 h. The different modes of vibration of samples were studied using Raman spectroscopy. XPS substantiate the composition and binding states of each element in the sample. The Seebeck coefficient and electrical resistivity of the prepared SrTiO3 nanostructures were measured at various temperatures by pelletizing the samples. The Seebeck coefficient of the sample gradually increased with hydrothermal growth period. The electrical resistivity of the sample relatively decreased with growth period. The power factor of the samples was calculated from the obtained Seebeck coefficient and electrical resistivity. A power factor of the sample prepared at 24 h of hydrothermal growth (2.191 × 10−4 W.m−1.K−2 at 550 K) was two order higher than that of as prepared sample (0.012 × 10−4 W.m−1.K−2 at 550 K). The experimental results revealed that the increase in hydrothermal growth period has a potential effect on the morphology. The cubic morphology with high crystalline nature facilitated the electron transport thereby thermoelectric power factor was enhanced in SrTiO3 nanostructures.