Thermoelectricity may play a major role in waste heat recovery of fossil fuel consuming devices. Unfortunately thermoelectric generators to date only have poor conversion efficiencies (5 %). One way to improve the efficiency is to improve the performance of the active thermoelectric material. For this the figure of merit Z is given by Z=(S^2 sigma)/kappa,where S denotes the Seebeck coefficient, sigma the electrical conductivity, and kappa; the thermal conductivity. Z can be improved by either increasing the numerator S^2 sigma; (the so called power factor) or decreasing the denominator.The typical and best understood thermoelectric materials so far are based on Te, such as Bi2Te3 or PbTe. Unfortunately, for a mass application of thermoelectric devices, estimations show that the tellurium resources will be consumed very quickly. Hence it is worth trying to develop novel thermoelectric materials which are more sustainable and green. Exemplarily the thermoelectric properties of ZnO as an ideal model system were investigated in the framework of this thesis. Main goal of the work was to get a better understanding of the influence of effects on the microscopic length scale (e.g. due to thin-films, grain boundaries, artificial structuring) on the macroscopic behavior of the sample. In this context the following results were found:Investigations of degenerately doped thin ZnO:Al films and subsequent annealing in air showed that at very high carrier concentrations, where the samples have metallic character, a sign reversal of S may occur. Although the sample is clearly n-type, small positive Seebeck coefficients can be measured, changing their sign with decreasing temperature. This is due to changes of the density of states at the Fermi-energy in a degenerately doped semiconductor.The energy filtering effect due to grain boundaries, e.g. the increase of the power factor with increasing carrier concentration only works to a certain extend: If the carrier concentration n exceeds a certain value, screening effects diminish the barrier height and width leading to a decrease of the power factor.Concerning the investigation of interfaces first measurements on a multilayer sample series of alternating ZnO/ZnS layers in in-plane geometry gave hints for the formation of interface layers of very high electrical conductivity between ZnO and ZnS, dominating the transport behaviour at large layer thicknesses (d > 100 nm). At smaller d, where d becomes comparable to the typical fluctuation length of the interface roughness, the transport path and hence the thermoelectric properties are strongly determined by the surface fluctuations. These results could be approved qualitatively by simulations within a Network Model (NeMo).Stronger impact on the thermoelectric parameters, especially on the thermal conductivity, were found in cross plane direction, i.e. perpendicular to the interfaces. Unfortunately measurements of multilayers in cross-plane direction are very difficult to perform. To overcome this problem lateral structuring of thin-films offers attractive possibilities. To realize bar structures of alternating materials the method of self-aligned pattern transfer was developed and employed.Measurements perpendicular to the interfaces show that the number of interfaces as well as their shape (i.e. length) and morphology has a strong influence on the power factor. Supported by numerous NeMo simulations the results indicated that the thermoelectric properties across the sample are dominated by the shortest path of electrical conductance. The transport path is strongly influenced by assuming space-charge regions of different width and conductivity. Best agreement between experiment and simulations has been achieved by replacing a certain fraction of the lowly conducting material with a highly conducting space-charge region. However, the origin of this highly conducting surface region requires further clarifications. The findings of this work suggest that due to its high Seebeck coefficients and the possibility to tune the electrical conductivity by doping, ZnO is a promising candidate for an environmentally friendly and sustainable n-type thermoelectric material. The fact that its thermal conductivity is quite high may be overcome by a combination with ZnS. However this back door shown by theory still needs to be approved by experiment.
展开▼
