Sang Mock Lee

Peierls distortion as a route to high thermoelectric performance in In4Se3―δ crystals

Thermoelectric energy harvesting—the transformation of waste heat into useful electricity—is of great interest for energy sustainability. The main obstacle is the low thermoelectric efficiency of materials for converting heat to electricity, quantified by the thermoelectric figure of merit, ZT. The best available n-type materials for use in mid-temperature (500–900 K) thermoelectric generators have a relatively low ZT of 1 or less, and so there is much interest in finding avenues for increasing this figure of merit. Here we report a binary crystalline n-type material, In4Se3-δ, which achieves the ZT value of 1.48 at 705 K—very high for a bulk material. Using high-resolution transmission electron microscopy, electron diffraction, and first-principles calculations, we demonstrate that this material supports a charge density wave instability which is responsible for the large anisotropy observed in the electric and thermal transport. The high ZT value is the result of the high Seebeck coefficient and the low thermal conductivity in the plane of the charge density wave. Our results suggest a new direction in the search for high-performance thermoelectric materials, exploiting intrinsic nanostructural bulk properties induced by charge density waves.

Surfactant‐Free Scalable Synthesis of Bi2Te3 and Bi2Se3 Nanoflakes and Enhanced Thermoelectric Properties of Their Nanocomposites

Surfactant-free nanoflakes of n-type Bi2 Te3 and Bi2 Se3 are synthesized in high yields. Their suspensions are mixed to create nanocomposites with heterostructured nanograins. A maximum ZT (0.7 at 400 K) is achieved with a broad content of 10-15% Bi2 Se3 in the nanocomposites.

Thermoelectric properties and anisotropic electronic band structure on the In4Se3−x compounds

We report the high thermoelectric figure-of-merit (ZT) on the Se-deficient polycrystalline compounds of In4Se3−x (0.02≤x≤0.5) and the anisotropic electronic band structure. The Se-deficiency (x) has the effect of decreasing the semiconducting band gap and increasing the power factor. The band structure calculation for In4Se3−x (x=0.25) exhibits localized hole bands at the Γ-point and Y-S symmetry line, whereas the significant electronic band dispersion is observed along the c-axis. Here, we propose that the high ZT values on those compounds are originated from the anisotropic electronic band structure as well as Peierls distortion.

Thermoelectric properties of bipolar diffusion effect on In4Se3−xTex compounds

We present thermoelectric properties and electronic structure of the series compounds of In4Se3−xTex (0.0≤x≤3.0). Even if the Te-doping is an isoelectronic substitution, we found that the electron dominated carrier transport in Se-rich region (x≤0.2) evolves into the electron-hole bipolar transport properties in Te-rich region (x≥2.5) from the temperature-dependent thermal conductivity κ(T), Seebeck coefficient S(T), and Hall coefficient RH(T) measurements. The electronic band structures of In4Se3−xTex (x=0.0, 2.75, and 3.0) are not changed significantly with respect to Te-substitution concentrations. From the Boltzmann transport calculation, the electron-hole bipolar effect on thermoelectric transport properties in Te-rich region can be understood by lowering the chemical potential to the valence band maximum in the Te-rich compounds.

Enhancement of Seebeck Coefficient in Bi0.5Sb1.5Te3 with High-Density Tellurium Nanoinclusions

Bi0.5Sb1.5Te3 films with homogeneously dispersed ~15 nm Te nanoparticles were prepared by the alternate deposition of Bi0.5Sb1.5Te3 layers and Te nanoparticles. As the amount of Te nanoinclusions increased to 15 vol %, the Seebeck coefficient increased from 169 to 248 µV/K. The authors concluded that the high-density Te nanoinclusions result in a carrier energy filtering effect in Bi0.5Sb1.5Te3. Consequently, the thermoelectric power factor was enhanced by 30% despite a reduction in electrical conductivity. The improvement of the power factor implies the enhancement of the thermoelectric figure of merit ZT, providing the possibility of further ZT improvement by embedding Te nanoinclusions in Bi0.5Sb1.5Te3 bulk materials.

Replacement of oxide glass with metallic glass for Ag screen printing metallization on Si emitter

Cu–Zr-based metallic glass (MG) has been applied as a binding agent of Ag paste for front contact formation in Si solar cell by screen printing process. Use of electroconductive MG binder significantly improves the quality of the contact by the formation of highly dense 10–50 nm size Ag crystallites and the noncorrugation of the emitter surface with a very shallow Ag crystallite penetration depth of 10–30 nm. Nanoscale Ag crystallites form on the emitter surface by local Si–Cu–Ag eutectic melting, leading to the formation of pyramidal pits on the Si emitter surface, followed by precipitation of Ag crystallites during cooling.

Effect of cationic substitution on the thermoelectric properties of In4−xMxSe2.95 compounds (M = Na, Ca, Zn, Ga, Sn, Pb; x = 0.1)

We report an electrical and thermal transport study for polycrystalline samples of In4Se3−x substituted with various metals. Our experimental and theoretical investigation revealed that the Na, Ca, and Pb substitutions at the In4 site in the 4:3 indium selenide lattice are more effective for increase in electron concentration than the Zn, Ga, and Sn substitutions at other sites, and the Peierls distortion in cationic substituted compounds may not be as strong as In4Se3−x based on higher lattice thermal conductivities of the former than the latter. It is found that the cationic substitutions may weaken the effect of Peierls distortion on thermal properties for In4Se3−x while they maintain the same trend for electrical properties of In4Se3−x with different electron concentrations.

Exploiting metallic glasses for 19.6% efficient back contact solar cell

An interdigitated back contact silicon solar cell with conversion efficiency of 19.6% was fabricated by screen-printing the Ag paste. In the Ag paste, oxide glass frits were totally replaced by Al85Ni5Y8Co2, Al-based metallic glass (MG) ones. The thermoplastic forming of the MG in the super cooled liquid region led to large contact area at the interface between Ag electrodes and Si layers and thus to specific contact resistance (ρc) as low as 0.86 mΩ cm2. The specific contact resistance was a function of both contact area and thickness of the interlayer formed at the interface working as a tunneling barrier.

Improvement in the thermoelectric performance of the crystals of halogen-substituted In4Se3−xH0.03 (H = F, Cl, Br, I): Effect of halogen-substitution on the thermoelectric properties in In4Se3−x

We explored the thermoelectric properties of the crystals of halogen-substituted In4Se3−xH0.03 in an effort to understand the significant effects of halogen-substitution on both electrical and thermal transport properties of In4Se3−x crystals as well as the origin of the high thermoelectric performance over a wide temperature range in the chlorine-substitued crystal. The X-ray diffraction patterns and typical infrared absorption spectra of the crystals of In4Se3−xH0.03 exhibit preferred oriented ac- or bc-planes of crystals with energy band gaps between 0.62 and 0.63 eV. The chlorine, bromine, and iodine-substituted In4Se3−xH0.03 crystals exhibit significantly higher room temperature electrical conductivities than the unsubstituted and fluorine-substituted crystals. Except for fluorine, the other halogen-substituted in the In4Se3−xH0.03 crystals show electron concentrations as comparable as the unsubstituted crystals. Thus, the substantial increase in electrical conductivity of the halogen-substituted crystals should result from a remarkable increase in Hall mobility. It is quite notable that the room temperature power factors of the halogen-substituted (such as Cl, Br, and I) crystals are significantly higher than that of the unsubstituted and F-substituted crystals, which is mainly due to the substantial increase in room temperature electrical conductivity. Finally, a relatively low lattice thermal conductivity combined with a high power factor results in a high ZT of ∼1.0 at ∼660 K for the crystal of In4Se2.32I0.03.

Capillary flow of amorphous metal for high performance electrode

Metallic glass (MG) assists electrical contact of screen-printed silver electrodes and leads to comparable electrode performance to that of electroplated electrodes. For high electrode performance, MG needs to be infiltrated into nanometer-scale cavities between Ag particles and reacts with them. Here, we show that the MG in the supercooled state can fill the gap between Ag particles within a remarkably short time due to capillary effect. The flow behavior of the MG is revealed by computational fluid dynamics and density funtional theory simulation. Also, we suggest the formation mechanism of the Ag electrodes, and demonstrate the criteria of MG for higher electrode performance. Consequently, when Al85Ni5Y8Co2 MG is added in the Ag electrodes, cell efficiency is enhanced up to 20.30% which is the highest efficiency reported so far for screen-printed interdigitated back contact solar cells. These results show the possibility for the replacement of electroplating process to screen-printing process.