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Microfabricated thermoelectric generators (μTEGs) can harvest modest temperature differences to provide reliable solid-state electricity for low-power electronics, sensors in distributed networks, and biomedical devices. While past work on μTEGs has focused on fabrication and demonstration, here we derive and explore comprehensive design guidelines for optimizing power output. A new closed-form thermoelectric device model agrees well with the traditional iterative approach. When thermoelectric leg length is limited by thin-film fabrication techniques, a very low (< 10%) active thermoelectric fill fraction is required to optimize device power output, requiring careful selection of filler material. Parasitic resistance due to electrical interconnects is significant when a small number of thermocouples is used, and this loss can be reduced by increasing the number of thermocouples while decreasing the cross-sectional area of the legs to maintain the same fill fraction. Finally, a discussion of the "incompleteness of ZT" shows that different combinations of thermal conductivity, electrical conductivity, and Seebeck coefficient resulting in the same ZT will result in different device performance and optimization decisions. For μTEGs, we show it is best to increase Seebeck coefficient, followed by decreasing thermal conductivity for short leg lengths and increasing electrical conductivity for long leg lengths.

Polymer‐based materials hold great potential for use in thermoelectric applications but are limited by their poor electrical properties. Through a combination of solution‐shearing deposition and directionally applied solvent treatments, poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) thin films with metallic‐like conductivities can be obtained with high power factors in excess of 800 µW m−1 K−2. X‐ray scattering and absorption data indicate that structural alignment of PEDOT chains and larger‐sized domains are responsible for the enhanced electrical conductivity. It is expected that further enhancements to the power factor can be obtained through device geometry and postdeposition solvent shearing optimization.

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. Published by AIP Publishing. [http://dx.

Microfabricated thermoelectric generators (µTEGs) are excellent candidates for sustainable power delivery for the next generation of smart sensors and wearable devices through harvesting of waste heat. However, the assembly process and inherently small contact areas for thermal and electrical transport introduce losses which can significantly reduce the effective figure of merit ZT. Further, the form factor of µTEGs makes these losses extremely challenging to quantify. The relative contributions of the thermoelectric film and interfaces greatly impact the choice of materials, device geometry, and maximum power point operation. A comprehensive study of µTEG devices including microfabrication, detailed modeling and optimization, and electrical, structural, and thermal characterization of modules and their constituent films is presented. Using a combination of novel infrared microscopy and thin‐film characterization techniques, the average thermoelectric material properties and the power output as a function of the true temperature difference across the device are isolated. Power outputs as high as 1 mW for a µTEG with 13.8 mm2 footprint and device ΔT of 7.3 K are measured. An order of magnitude reduction in figure of merit for the devices (ZT ≈ 0.03) compared to the constituent thermoelectric films (zT ≈ 0.3), with implications for the selection of maximum power point operation, is demonstrated.

Ideal thermoelectric materials should possess low thermal conductivity j along with high electrical conductivity r. 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 j 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.

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The optical performance of a novel solar concentrator consisting of a 400 spherical heliostat array and a linked two-axis tracking system is analyzed using the Monte Carlo ray-tracing technique. The optical efficiency and concentration ratio are compared for four different heliostat linkage configurations, including linkages of 1 × 1, 1 × 2, 2 × 2, 4 × 4, and 5 × 5 heliostats for 7-hour operation and the selected months of June and December. The optical performance of the concentrator decreases with the increasing number of heliostats in the individual groups due to increasing optical inaccuracies. In June, the best-performing linked configuration, in which 1 heliostat in the east-west direction and 2 heliostats in the north-south direction are linked, provides a monthly-averaged 7-hour optical efficiency and average concentration ratio of 79% and 511 suns, respectively. In December, the optical efficiency and the average concentration ratio decreases to 61% and 315 suns, respectively.