Nanoscale Energy Transport in Photovoltaic and Thermoelectric Nanomaterials

Kelly M Rickey, Purdue University

Abstract

Semiconductor nanocrystals are promising for photovoltaic and thermoelectric applications due to the size-tuned electrical, thermal, and optical properties. For example, they can be size-tuned to emit and absorb light in a specific wavelength range. Their small size also makes multiple exciton generation possible in certain situations. However, their device performance is usually limited by the poor charge transport. Tellurium-based nanoparticle studies were performed in the form of photovoltaic film research, and later, nanocomposite thermoelectrics have been studied. First, an array of different CdTe and PbSe nanoparticles were synthesized and studied. Next, rapid thermal annealing (RTA) on photovoltaic quantum dot and nanowire films were investigated attempting to improve charge transfer. In rapid thermal annealing, a sample’s temperature is raised to several hundred degrees under an inert atmosphere, then cooled back down again after several seconds. To the best of our knowledge, few such treatments of CdTe nanocrystal (NC) films have been documented. In our experiments, the NW films show only a slight improvement in the electrical conductivity, while the QD films show none at all. To further enhance the electrical transport, we have proposed and carried out laser-peen sintering (LPS) of CdTe NW films, and we have demonstrated that the film quality and charge transfer can be significantly improved while largely maintaining basic particle morphology. During the laser peening phase, a shockwave is used to compress the film. Laser sintering comprises the second step, where a nanosecond pulse laser beam welds the nanowires. Microstructure, morphology, material content, and electrical conductivities of the films are characterized before and after treatment. The morphology results show that laser peening can decrease porosity and bring nanowires into contact, and pulsed laser heating fuses those contacts. The characterization results indicate that solely laser peening or sintering can only moderately improve the thin film quality; however, when coupled together as laser peen sintering (LPS), the electrical conductivity enhancement is dramatic. LPS can decrease resistivity up to a factor of ~10,000, resulting in values on the order of ~10 5 Ω-cm in some cases, which is comparable to CdTe thin films. Bismuth Telluride nanopowders of different sizes were synthesized via wet chemistry. They were then hot-pressed into pellets, and thermal conductivities measured as a function of nanoparticle size. Room-temperature values are 0.185 W/m˙K and 0.23 W/m˙K for sizes of 8.4 nm and 14.0 nm, which show > 80% reduction from bulk. We fit the modified effective media approximation to this experimental data, and obtained interfacial thermal conductances of 45 to 64 W/mm2-K for temperatures of 296 to 401 K. Experiments and simulations show thermal conductivity decreases with decreasing particle size, and that low interfacial thermal conductance has high impact on these results. The effects of ligand levels on the thermoelectric properties of bismuth telluride (Bi2Te3) pellets hot-pressed from nanocrystals are investigated. Bi2Te3 nanopowders were synthesized via wet chemistry methods with different amounts of organic ligands remaining. The nanopowders were then hot-pressed into bulk pellets—first in air, then in an argon environment—and the thermoelectric properties were characterized. The analyses performed consisted of XRD, TEM, XPS, density, specific heat capacity, thermal conductivity, electrical conductivity, and Seebeck coefficient measurements. Overall we found that hydrazine rinsing significantly increases the pellet density by more than 40 percent, decreases thermal conductivity, and increases electrical conductivity by up to greater than 4,000, but does not as greatly affect Seebeck coefficient magnitude (which ranges from –88 to roughly –155 µV/K) and instead simply reverses its trend with temperature. The best figure of merit at room temperature, ZT, was 0.37 achieved on the 5x rinsed sample, pressed in air. It represents more than a factor of 18,000 increase from the unrinsed sample, and over a factor of 6 increase from the 1x-rinsed sample. Finally, we explored effects of various nanoinclusions on Bi2Te 3 pellets. So far, the thermal and electrical conductivity changes are promising, and we discuss this in our future work. The wet-synthesis methods used to create the materials are potentially scalable. These studies also offer useful insights towards how to engineer the transport properties for improved renewable energy applications.

Degree

Ph.D.

Advisors

Ruan, Purdue University.

Subject Area

Mechanical engineering|Nanoscience|Nanotechnology

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