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International audience; The p-type Ge-doped (0, 1, 4 and 7 at.%) Mg

Recently we reported on a unique, chip based measurement platform for the complete characterization of the in-plane thermoelectric properties of thin film samples. For the validation of the platform, good thermoelectric materials with a low thermal conductivity and large Seebeck coefficient have already been characterized. In order to check the capability of the platform, with regard to new material classes and applications, we performed measurements of the electrical conductivity σ, thermal conductivity λ and Seebeck coefficient S of two 100 nm Au and Ti thin film samples with considerably larger thermal and electrical transport values. The measured thermoelectric properties of both films show clear size effects, with a remarkably reduced electrical and thermal conductivity as well as Seebeck coefficient, compared to the bulk material, respectively. The Lorenz number of the thin films, which are calculated from the measured electrical and thermal conductivities, are equal to the bulk value, predicted by the Wiedemann–Franz law.

Low-cost, non-toxic, abundant organic thermoelectric materials are currently under investigation for use as potential alternatives for the production of electricity from waste heat. While organic conductors reach electrical conductivities as high as their inorganic counterparts, they suffer from an overall low thermoelectric figure of merit (ZT) due to their small Seebeck coefficient. Moreover, the lack of efficient n-type organic materials still represents a major challenge when trying to fabricate efficient organic thermoelectric modules. Here, a novel strategy is proposed both to increase the Seebeck coefficient and achieve the highest thermoelectric efficiency for n-type organic thermoelectrics to date. An organic mixed ion-electron n-type conductor based on highly crystalline and reduced perylene bisimide is developed. Quasi-frozen ionic carriers yield a large ionic Seebeck coefficient of -3021 μV K-1 , while the electronic carriers dominate the electrical conductivity which is as high as 0.18 S cm-1 at 60% relative humidity. The overall power factor is remarkably high (165 μW m-1 K-2 ), with a ZT = 0.23 at room temperature. The resulting single leg thermoelectric generators display a high quasi-constant power output. This work paves the way for the design and development of efficient organic thermoelectrics by the rational control of the mobility of the electronic and ionic carriers.

An analytical study has been performed on the measurement capabilities of a 100-nm thin suspended membrane setup for the in-plane thermal conductivity measurements of thin film samples using the 3ω measurement technique, utilizing a COSMOL Multiphysics simulation. The maximum measurement range under observance of given boundary conditions has been studied. Three different exemplary sample materials, with a thickness from the nanometer to the micrometer range and a thermal conductivity from 0.4 W/mK up to 100 W/mK have been investigated as showcase studies. The results of the simulations have been compared to a previously published evaluation model, in order to determine the deviation between both and thereby the measurement limit. As thermal transport properties are temperature dependent, all calculations refer to constant room temperature conditions.

National audience; Impact of Cu doping and Mg-deficiency on Mg2Sn thin films thermoelectric properties

The characterization of nanostructured samples with at least one restricted dimension like thin films or nanowires is challenging but important to understand their structure and transport mechanism and to improve current industrial products and production processes. We report on the development of a chip-based platform to simultaneously measure the in-plane electrical and thermal conductivity, the Seebeck coefficient as well as the Hall constant of a thin film in the temperature range from 120 K up to 450 K and in a magnetic field of up to 1 T. Due to the design of the setup, time consuming preparation steps can be omitted and a nearly simultaneous measurement of the sample properties is achieved. Typical errors caused by different sample compositions, varying sample geometries, and different heat profiles are avoided with the presented measurement method. As a showcase study displaying the validity and accuracy of our system, we present measurements of the thermoelectric properties of a 110 nm Bi87Sb13 thin film in the temperature range from 120 K up to 450 K.

Tin selenide (SnSe) has attracted great attention in thermoelectrics (TE) thanks to its record high figure of merit (ZT) discovered in single crystal (SC). However, the practical application of SnSe SC is limited due to its extreme growth condition. With the development of flexible integrated electronics, using flexible polycrystalline TE films to substitute rigid SC becomes a main trend. Nevertheless, large resistances of TE films restrict their application as power devices. Here, self-powered SnSe-based photothermoelectric (PTE) detectors are demonstrated. The PTE detectors are sensitive to wide-spectrum and exhibit higher responsivity and respnse speed comparing with SC, due to the small heating volume, small conductive heat flux and large dissipation content of thin films. High-quality (400)-textured SnSe polycrystalline thin films are one-step synthesized by chemical vapor deposition. In-situ in-plane TE measurement shows that the obtained SnSe films possess a dimensionless ZT of 0.15 and a power factor of 0.322 mW m−1 K−2 at 550 K, showing SC level TE performance at low temperatures.

International audience; Mg-Sn thin films (21 ≤ at. % Sn ≤ 42.5) were deposited by magnetron sputtering in the argon atmosphere. The structure and morphology of the films were characterized as a function of the composition. Mg2Sn structure was changed from stable face-centered cubic to metastable orthorhombic structure while the content of Sn in the films increased. The influence of this structural modification on thermoelectric properties was discussed in a wide range of temperatures (30–200 °C). The film carrier concentration and mobility were measured to explain the electronic transport behavior as a function of the film structural modifications. The maximum figure of merit ZT ≈ 0.26 at 200 °C was reached for the film with 36 at. % Sn while a mixture of cubic and orthorhombic Mg2Sn structures coexisted. An annealing treatment was performed under vacuum (∼10−4 Pa) at different temperatures (up to 600 °C) to determine the limit of structural and morphological stability of this film.

The characterization of nanostructured samples with at least one restricted dimension like thin films or nanowires is challenging, but important to understand their structure and transport mechanism, and to improve current industrial products and production processes. We report on the 2nd generation of a measurement chip, which allows for a simplified sample preparation process, and the measurement of samples deposited from the liquid phase using techniques like spin coating and drop casting. The new design enables us to apply much higher temperature gradients for the Seebeck coefficient measurement in a shorter time, without influencing the sample holder's temperature distribution. Furthermore, a two membrane correction method for the 3ω thermal conductivity measurement will be presented, which takes the heat loss due to radiation into account and increases the accuracy of the measurement results significantly. Errors caused by different sample compositions, varying sample geometries, and different heat profiles are avoided with the presented measurement method. As a showcase study displaying the validity and accuracy of our platform, we present temperature-dependent measurements of the thermoelectric properties of an 84 nm Bi

In this work, a novel Si-based Lab-on-a-Chip test platform is introduced to perform in-plane ZT measurements of various classes of thermoelectric films with a complete set of thermoelectric test structures. The electrical conductivity σ, the Seebeck coefficient S, the thermal conductivity κ, and the power factor PF and the figure of merit ZT values can be experimentally determined in a single simultaneous measurement run as a function of temperature. It is shown that this Lab-on-a-Chip test system can be applied to classic inorganic thermoelectric films such as PbTe and PbSe and also to hybrid inorganic-organic films such as SURMOF films and as well as to conducting organic PEDOT:PSS films. Generally, the thermoelectric film can be deposited in the exposed test area of the chip by using a shadow mask. All the parameters can be measured in the temperature range of 300 K to 370 K with a 1 T magnet field. Applications are demonstrated for all three film material categories. Here, PbTe and PbSe thin films and double-layer PbTe/PbSe films were synthesized by Atomic Layer Deposition (ALD) technology directly on the pre-structured Si chip. A thin Al2O3 protection and diffusion barrier layer was applied to avoid oxidation and degradation of PbTe/PbSe films. The Seebeck coefficient, electrical conductivity and thermal conductivity for PbTe, PbSe and double-layer PbTe/PbSe film were measured simultaneously by applying in-pane ZT chips. Furthermore, this Lab-on-a-Chip test system can be extended to Surface Anchored Metal-organic-framework (SURMOF) films, which consist of quasi-liquid phase epitaxially grown highly ordered crystalline films. MOF films are designed scaffold-like compounds that consist of metal ions connected by organic ligands. Furthermore, the example of a complete thermoelectric characterization of a conductive organic polymer thin film material will be discussed.

SURMOF based "Designer Solids" and Organic thin film materials will be presented as non-volatile RRAM Devices. Additionally a fully thermoelectric characterization have been performed on ZT Test Cips (from Linseins Messgeraete GmbH). Thermoeletric Properties and Seebeck measurements of pristine and loaded SURMOF thin films will be presented.

The temperature and thickness dependent thermoelectric properties of Bi87Sb13 nano-films with a thickness from 84 nm to 282 nm have been studied in a temperature range from 110 K up to 450 K. The films have been prepared by thermal evaporation of the raw material from an Al2O3 coated tungsten boat under vacuum conditions of at least 10−6 mbar. The measurements have been performed using a novel measurement platform, which allows the nearly simultaneous characterization of the thermal conductivity, electrical conductivity, the Seebeck coefficient and the Hall coefficient. All properties are measured in the in-plane direction at the same sample within one measurement run, avoiding many sources of uncertainties and allowing the calculation of the direction dependent, in-plane thermoelectric figure of Merit ZT with high precision. The maximum ZT value of 0.28 has been obtained for the thickest sample at a temperature of 265 K. All comparative measurements have been performed after an initial thermal annealing step, as the heat treatment shows a strong impact on the thermoelectric performance of the films.

The conversion efficiency of the thermoelectric (TE) device is determined by the dimensionless figure of merit ZT, which is expressed as ZT = S2σT/κ, where S is the Seebeck coefficient, σ is the electrical conductivity, κ is the thermal conductivity, and T is the absolute temperature. The key strategy to enhance ZT involves the reduction in thermal conductivity k of TE materials. This led to the synthesis of low-dimensional structured materials such as 2-D superlattice structure, 1-D nanowire, and 0-D quantum dots. The phonon transport was suppressed if the dimension of the film approaching the mean free path of the phonons, resulting in a reduction in thermal conductivity and consequently improves ZT. The direction of the measurement, either in-plane or vertical to the surface, plays an important role in determining TE properties of the anisotropic samples, such as superlattices and quantum well structures. It is essential to measure all the parameters in the same direction and simultaneously to obtain accurate ZT value. In this work, a novel Si-based test chip was utilized to perform in-plane ZT measurements of PbTe/PbSe superlattice films. Figure 1 (a) shows the pre-fabricated circuit on the chip for measuring simultaneously the Seebeck coefficient, electrical conductivity, and thermal conductivity [1]. The thermoelectric film was deposited in the red square area by using a shadow mask. All the parameters were measured in the temperature range of 120 K to 450 K with a 1 T magnet field. In our work, PbTe and PbSe thin films and superlattice layers were synthesized by Atomic Layer Deposition (ALD) technology directly on the pre-structured Si chip. Figure 1 (b) shows an SEM micrograph of the 30 nm thick PbSe film grown on the chip. The PbTe/PbSe superlattice was then deposited on the PbSe buffer layer using ALD by alternatively depositing PbTe and PbSe layers. A thin Al2O3protection and diffusion barrier layer was applied to avoid oxidation and degradation of PbTe/PbSe films. The period of superlattice structure was varied from 2 nm to 10 nm to observe optimized period for achieving highest ZT value. Reference: [1]. Linseis, V., Völklein, F., Reith, H., Woias, P., & Nielsch, K. Platform for in-plane ZT measurement and Hall coefficient determination of thin films in a temperature range from 120 K up to 450 K. Journal of Materials Research, 31(20), 3196-3204, (2016). Figure 1