Fossil fuel depletion and environmental issues have led to intense study on fuel source, energy efficiency, and associated environmental impact. Alternative energy that is sustainable and environmentally friendly are the most sought-after remedy to avoid further environment deterioration. Thermoelectric (TE) materials have drawn researchers’ attention in recent decades for use as an alternative green energy source, due to their ability of harvesting ubiquitous waste heat using solid-state module that translated into silent during operations with zero-maintenance. However, TE material are uncommon as house-hold power generation because of their low efficiency at low-grade temperature range.

This thesis aims at synthesizing TE materials operating at low-grade temperature range us...[ Read more ]

Fossil fuel depletion and environmental issues have led to intense study on fuel source, energy efficiency, and associated environmental impact. Alternative energy that is sustainable and environmentally friendly are the most sought-after remedy to avoid further environment deterioration. Thermoelectric (TE) materials have drawn researchers’ attention in recent decades for use as an alternative green energy source, due to their ability of harvesting ubiquitous waste heat using solid-state module that translated into silent during operations with zero-maintenance. However, TE material are uncommon as house-hold power generation because of their low efficiency at low-grade temperature range.

This thesis aims at synthesizing TE materials operating at low-grade temperature range using bottom-up strategy, demonstrating improved figures of merit, ZT through composite material. Four main categories of materials were studied, namely: fully inorganic, inorganic nanocomposite, all-polymer, and hybrid polymer composite. For inorganic material system, modulation doping was employed and produced nanocomposites with two electronic effect, i.e., electron charge injection and electron energy filtering that notoriously affecting thermoelectric performance. These two effects were produced by first matching the carrier type of nanodomains/ nanoinclusions and host matrix, i.e., electron or hole, and followed by matching the work function. Such screening of carrier type and matching of work function were experimentally proven to improve thermoelectric performance. For example, a ~2-fold improvement of ZT in Te (p-type, higher work function)/Ag₂Se (n-type) nanocomposite with room temperature ZT=0.79 due to electron filtering effect. On the other hand, a 10% decrease of ZT in Cu (n-type, lower work function)/Ag₂Se (n-type) nanocomposite with room temperature ZT=0.50, ascribed to the electron charge injection.

For hybrid polymer composite, three critical mechanisms in ZT enhancement were investigated, i.e., phonon scattering, energy dependent scattering, and hole phonon interaction. By matching work function of both inorganic nanoparticles (Cu₁₂Sb₄S₁₃) and conducting polymer (PEDOT), a low energy barrier of 0.17 eV was produced. We demonstrated optimized ZT were obtained at around 5 wt.% nanoparticles content, which we attribute to a low hole-phonon interaction, an effective phonon scattering, and mainly to the presence of a proper density of low energy barriers, which selectively scatter low energy carriers.

In summary, the approaches outlined in this thesis provide a general strategy with high versatility, simplicity, and precision up to nanoscale to modulate the carrier concentration. The screening of carrier type and matching of work function to engineer electronic effect at electronic band structure near Fermi level was first reported and experimentally proven to improve thermoelectric performance for both inorganic nanocomposite and hybrid polymer composite. Nonetheless, the size of nanoparticles is expected to play substantial role in electrical and thermal conductivity. The summarized approach when combine with the size effect study is expected to provide a complete methodology in producing high-performance thermoelectric composite materials.