Nanostructured thin film thermoelectric composite materials using conductive polymer PEDOT:PSS

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2013.Cataloged from PDF version of thesis.Includes bibliographical references (p. 65).Thermoelectric materials have the ability to convert heat directly into electricity. This clean energy technology has advantages over other renewable technologies in that it requires no sunlight, has no moving parts, and is easily scalable. With the majority of the unused energy in the United States being wasted in the form of heat and the recent mandates to reduce greenhouse gas emissions, thermoelectric devices could play an important role in our energy future by recovering this wasted heat and increasing the efficiency of energy production. However, low conversion efficiencies and the high cost of crystalline thermoelectric materials have restricted their implementation into modem society. To combat these issues, composite materials that use conductive polymers have been under investigation due to their low cost, manufacturability, and malleability. These new composite materials could lead to cheaper thermoelectric devices and even introduce the technology to new application areas. Unfortunately, polymer composites have been plagued by low operating efficiencies due to their low Seebeck coefficient. In this research, we show an enhanced Seebeck coefficient at the interface of poly(3,4- ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) spin coated onto silicon substrates. The maximum Seebeck coefficient achieved was 473 uV/K with a PEDOT:PSS thickness of 7.75 nm. Furthermore, the power factor of this interface was optimized with a 15.25 nm PEDOT:PSS thickness to a value of 1.24 uV/K2-cm, which is an order of magnitude larger than PEDOT:PSS itself. The effect of PEDOT:PSS thickness and silicon thickness on the thermoelectric properties is also discussed. Continuing research into this area will attempt to enhance the power factor even further by investigating better sample preparation techniques that avoid silicon surface oxidation, as well as creating a flexible composite material of PEDOT:PSS with silicon nanowires..by Chris A. Kuryak.S.M

Disordered stoichiometric nanorods and ordered off-stoichiometric nanoparticles in n-type thermoelectric Bi₂Te₂.₇Se₀.₃

N-type Bi₂Te₂.₇Se₀.₃ bulk thermoelectric materials with peak ZT values up to ∼1 were examined by transmission electron microscopy and electron diffraction. Two nanostructural features were found: (i) a structural modulation of ∼10 nm, which consisted of nanorods with crystalline and nearly amorphous regions, having the rod axes normal to (0,1,5)-type planes, and wave vector normal to (1,0,10)-type planes and (ii) non-stoichiometric ordered Bi-rich nanoparticles. The presence of the structural modulation was not influenced by the ion milling energy or temperature in this study while the non-stoichiometric ordered nanoparticles were only observed when ion milling at low temperatures and low energy was used. It is proposed that both the structural modulation of ∼10 nm and the presence of non-stoichiometric nanoparticles are responsible for the low lattice thermal conductivity (∼0.6 W/mK) of the Bi₂Te₂.₇Se₀.₃ bulk thermoelectric materials studied.United States. Dept. of Energy. Office of Science (Solar-Thermal Energy Conversion Center, Award No. DE-FG02-09ER46577

Quantitative analyses of enhanced thermoelectric properties of modulation-doped PEDOT:PSS/undoped Si (001) nanoscale heterostructures

Poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) has high electrical conductivity (∼10³ S cm⁻¹) but it exhibits a low Seebeck coefficient (<15 μV K⁻¹), resulting in a low power factor. Mixing PEDOT:PSS with nanostructured semiconductors can enhance the Seebeck coefficient and achieve an improved thermoelectric power factor. However, underlying mechanisms for those composite thermoelectric systems are scarcely understood so far. In this study, quantitative analyses on the electrical conductivity and Seebeck coefficient for the heterostructures of nanometer-thick PEDOT:PSS on single-crystal Si (001) on sapphire (SOS) are reported. The heterostructures have larger Seebeck coefficients up to 7.3 fold and power factors up to 17.5 fold relative to PEDOT:PSS. The electrical conductivity increased with decreasing combined thicknesses of PEDOT:PSS and Si, and the Seebeck coefficient increased with decreasing PEDOT:PSS thickness, which can be attributed to modulation doping caused by diffusion of holes from PEDOT:PSS into undoped Si. This hypothesis is supported by simulation per band alignment. The valence band offset between Si and PEDOT:PSS dominantly controls the electrical conductivity and Seebeck coefficient of the heterostructures. This study not only suggests mechanistic insights to increase the power factors of PEDOT:PSS-based composites but also opens the door for new strategies to enhance the thermoelectric efficiencies of heterostructured nanocomposite materials

Quantitative analyses of enhanced thermoelectric properties of modulation-doped PEDOT:PSS/undoped Si (001) nanoscale heterostructures

Poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) has high electrical conductivity (∼10³ S cm⁻¹) but it exhibits a low Seebeck coefficient (<15 μV K⁻¹), resulting in a low power factor. Mixing PEDOT:PSS with nanostructured semiconductors can enhance the Seebeck coefficient and achieve an improved thermoelectric power factor. However, underlying mechanisms for those composite thermoelectric systems are scarcely understood so far. In this study, quantitative analyses on the electrical conductivity and Seebeck coefficient for the heterostructures of nanometer-thick PEDOT:PSS on single-crystal Si (001) on sapphire (SOS) are reported. The heterostructures have larger Seebeck coefficients up to 7.3 fold and power factors up to 17.5 fold relative to PEDOT:PSS. The electrical conductivity increased with decreasing combined thicknesses of PEDOT:PSS and Si, and the Seebeck coefficient increased with decreasing PEDOT:PSS thickness, which can be attributed to modulation doping caused by diffusion of holes from PEDOT:PSS into undoped Si. This hypothesis is supported by simulation per band alignment. The valence band offset between Si and PEDOT:PSS dominantly controls the electrical conductivity and Seebeck coefficient of the heterostructures. This study not only suggests mechanistic insights to increase the power factors of PEDOT:PSS-based composites but also opens the door for new strategies to enhance the thermoelectric efficiencies of heterostructured nanocomposite materials