Min-Wook Oh

Lossless hybridization between photovoltaic and thermoelectric devices

The optimal hybridization of photovoltaic (PV) and thermoelectric (TE) devices has long been considered ideal for the efficient harnessing solar energy. Our hybrid approach uses full spectrum solar energy via lossless coupling between PV and TE devices while collecting waste energy from thermalization and transmission losses from PV devices. Achieving lossless coupling makes the power output from the hybrid device equal to the sum of the maximum power outputs produced separately from individual PV and TE devices. TE devices need to have low internal resistances enough to convey photo-generated currents without sacrificing the PV fill factor. Concomitantly, a large number of p-n legs are preferred to drive a high Seebeck voltage in TE. Our simple method of attaching a TE device to a PV device has greatly improved the conversion efficiency and power output of the PV device (~30% at a 15°C temperature gradient across a TE device).

Abnormal drop in electrical resistivity with impurity doping of single-crystal Ag

Resistivity is an intrinsic feature that specifies the electrical properties of a material and depends on electron-phonon scattering near room temperature. Reducing the resistivity of a metal to its potentially lowest value requires eliminating grain boundaries and impurities, but to date few studies have focused on reducing the intrinsic resistivity of a pure metal itself. We could reduce the intrinsic resistivity of single-crystal Ag, which has an almost perfect structure, by impurity doping it with Cu. This paper presents our results: resistivity was reduced to 1.35 μΩ·cm at room temperature after 3 mol% Cu-doping of single-crystal Ag. Various mechanisms were examined in an attempt to explain the abnormal behavior.

Fabrication of high-quality single-crystal Cu thin films using radio-frequency sputtering

Copper (Cu) thin films have been widely used as electrodes and interconnection wires in integrated electronic circuits, and more recently as substrates for the synthesis of graphene. However, the ultra-high vacuum processes required for high-quality Cu film fabrication, such as molecular beam epitaxy (MBE), restricts mass production with low cost. In this work, we demonstrated high-quality Cu thin films using a single-crystal Cu target and radio-frequency (RF) sputtering technique; the resulting film quality was comparable to that produced using MBE, even under unfavorable conditions for pure Cu film growth. The Cu thin film was epitaxially grown on an Al2O3 (sapphire) (0001) substrate, and had high crystalline orientation along the (111) direction. Despite the 10−3 Pa vacuum conditions, the resulting thin film was oxygen free due to the high chemical stability of the sputtered specimen from a single-crystal target; moreover, the deposited film had >5× higher adhesion force than that produced using a polycrystalline target. This fabrication method enabled Cu films to be obtained using a simple, manufacturing-friendly process on a large-area substrate, making our findings relevant for industrial applications.

Importance of crystal chemistry with interstitial site determining thermoelectric transport properties in pavonite homologue Cu–Bi–S compounds

The crystal chemistry of complex structured pavonite homologue Cux+yBi5−yS8 (1.2 ≤ x ≤ 1.4, 0.4 ≤ y ≤ 0.55) compounds with various crystallographic atomic sites was investigated in the context of their thermoelectric properties. We clarified the origins of the electronic and thermal transport properties of Cux+yBi5−yS8 compounds based on the change in the composition, which is strongly correlated with the occupancy of each atomic site. Ab initio calculations revealed that the narrow gap n-type semiconducting nature of Cux+yBi5−yS8 compounds originates from the presence of interstitial Cu ions. Structural refinements combined with transport measurements revealed that asymmetrical disorders of interstitial Cu ions have a large anisotropic thermal displacement factor, leading to an intrinsically low value (∼0.49 W m−1 K−1) and temperature-independent behavior of lattice thermal conductivity. Comprehensive structural analysis provided an elemental doping strategy focusing on interstitial sites. Thermoelectric properties were significantly enhanced by the simultaneous increase of power factor and decrease of lattice thermal conductivity. It is noted that structural factors, such as occupancy and thermal displacement parameter, of interstitial sites among the various crystallographic sites should be considered as primary characteristics in the crystal chemistry of complex structured crystals. Correspondingly, a peak ZT for the system was obtained in Cu1.576Zn0.024Bi4.6S8, which showed ∼30% enhancement over that of the pristine Cux+yBi5−yS8 compound.

Strong correlation between the crystal structure and the thermoelectric properties of pavonite homologue Cux+yBi5−yCh8 (Ch = S or Se) compounds

Complex structured semiconducting compounds are promising thermoelectric materials due to their inherently low thermal conductivities. It has been demonstrated that the limited phonon mean free path with structural complexities such as a large unit cell, disorders, or a variety of atom types resulted in low lattice thermal conductivity. However, there is still a missing piece to elucidate which structural component manipulates the transport properties effectively. Herein we review our recent progress on the thermoelectric properties of the intrinsically disordered system, pavonite homologue Cux+yBi5−yCh8 (Ch = S or Se). Through the controlled tuning of composition and occupancy for each atomic site based on the comprehensive structural analysis, we found that the structural factor (occupancy of interstitial Cu) could be a critical basis for determining both electronic and thermal transport properties. Also, we present a short overview of common structural traits inhibiting phonon propagation in complex structured thermoelectric materials. This consideration outlines a strategy to search for new high performance complex structured thermoelectric materials in relation with the possibilities of their chemical and structural modifications.

Enhanced thermoelectric properties of AgSbTe 2 obtained by controlling heterophases with Ce doping

We report the enhanced thermoelectric properties of Ce-doped AgSbTe2 (AgSb1−xCexTe2) compounds. As the Ce contents increased, the proportion of heterophase Ag2Te in the AgSbTe2 gradually decreased, along with the size of the crystals. The electrical resistivity and Seebeck coefficient were dramatically affected by Ce doping and the lattice thermal conductivity was reduced. The presence of nanostructured Ag2Te heterophases resulted in a greatly enhanced dimensionless figure of merit, ZT of 1.5 at 673 K. These findings highlight the importance of the heterophase and doping control, which determines both electrical and thermal properties.

Enhanced thermoelectric properties of screen-printed Bi0.5Sb1.5Te3 and Bi2Te2.7Se0.3 thick films using a post annealing process with mechanical pressure

A cost-effective and large-scale thermoelectric (TE) energy harvester is becoming increasingly important for energy recovery systems such as self-powered electronics and renewable power generation. Here, we report on a TE device composed of p-type Bi0.5Sb1.5Te3 and n-type Bi2Te2.7Se0.3 TE materials prepared using a screen-printing process, which has the advantages of low cost, scalability to large areas and the ability to form a flexible TE generator. The TE properties of the screen-printed TE thick films were optimized via subsequent annealing with mechanical pressure. It was found that thermal annealing with the application of mechanical pressure plays a key role in controlling the carrier concentration and improving the density of the TE thick films. Under optimized annealing conditions, the Bi0.5Sb1.5Te3 (p-type) thick film had a ZT of 0.89 and a density of 5.67 g cm−3 while the Bi2Te2.7Se0.3 (n-type) thick film had a ZT of 0.57 and a density of 5.68 g cm−3 at room temperature. TE generators composed of 72 and 200 couples were fabricated with these thick films. The output power of the device composed of 72 couples was 0.1 W for a temperature difference (ΔT) of 28 K. Another device with 200 couples generated 0.31 W of electric power for the same ΔT.

Thermal conductivity reduction in three dimensional graphene-based nanofoam

This work investigates the thermoelectric properties of a three dimensional nanofoam of few layer graphene (3D-NFG) decorated with holes having diameter of several tens of nanometer. The nanoporous 3D graphene structures were fabricated by a chemical vapor deposition method to ensure high electrical conductivity required for potential applications as thermoelectric materials. The thermal conductivity of the suspended 3D-NFG samples was measured by an optothermal method and found to be 10.8 W m−1 K−1. The substantially reduced values of thermal conductivity were attributed to the small diameter of the pores and their inhomogeneous distribution. Suppression of heat conduction with preserved electrical conductivity is beneficial for the proposed thermoelectric applications.

Anisotropic Thermal Characteristics of Graphene-Embedded Polyimide Composite Sheets

Polyimide-graphene (PIG) composites were prepared, and their thermal properties were characterized as films or sheets. Graphene loadings were controlled up to 8 wt.% or 12 wt.% in their composites. The graphenes used in this work were prepared by the chemical treatment of graphite. The as-prepared polyimide-graphene composites were thermally characterized in films or sheets. The thermal conductivities of the PIG sheets were separately analyzed in both in-plane and out-of-plane directions. The thermal expansion coefficients (TECs) of the PIG composites slightly changed with the amount of graphene added to the composites. The in-plane thermal conductivities were higher than the out-of-plane thermal conductivities, and the thermal conductivities of both directions proportionally increased with the graphene loadings in the PIG composites. The temperature-dependent stresses of the composite films monotonically decreased with the graphene loadings. The experimentally obtained thermal characteristics of the polyimide-graphene composites were compared and discussed on the basis of the composite morphologies.