New thin film materials to suit the needs of future energy technology
Titanium dioxide with small amounts of added niobium, TNO, has recently attracted interest as a transparent conductor and as a thermoelectric material that converts waste heat into electricity.
In his doctoral dissertation, Janne-Petteri Niemelä, doctoral candidate at Aalto University, discovered an optimal atomic layer deposition method for TNO thin films, demonstrating that the films so fabricated exhibited high-quality transparent conductor properties.
‘We discovered that we need to use sufficiently low deposition temperatures in atomic layer deposition. We managed to reduce the grain-boundary scattering that suppresses electron mobility and weakens electric conductivity,’ he explains.
Figure 1. In a layer-structured hybrid thin film, inorganic (e.g. titanium dioxide) and organic layers alternate. Organic layers are fabricated between ALD-based inorganic layers using the molecular layer deposition technique. The thickness of the layers can be controlled to an accuracy of a single atom or molecule layer..
Electricity from heat with hybrid materials
Atomic layer deposition, ALD, is an excellent method for fabricating materials for new energy technologies, as it allows depositing thin film materials for different kinds of nanostructured substrate surfaces according to their geometry.
When the ALD method is combined with the molecular layer deposition (MLD) method, new types of hybrid inorganic-organic materials can be fabricated. In his doctoral dissertation, Niemelä also examined the electric and thermal conductivity properties of ALD/MLD-fabricated titanium dioxide- and zinc oxide-based hybrid materials.
‘In our study, we discovered that organic molecule layers deposited between oxide layers reduced the thermal conductivity of oxide materials considerably, which is promising in view of thermoelectric applications,’ Niemelä explains.
In the future, thermoelectric materials can be used for producing electricity from the heat generated by humans for charging portable electronic devices, for example.
‘Potential applications for transparent conductors, on the other hand, can be found in solar cells and LED lights,’ says Niemelä.
Figure 2. In polycrystalline thin film materials, the grain boundaries suppress electron mobility and weaken electric conductivity. The grain-boundary scattering was successfully reduced for ALD-based TNO thin films as a result of which the materials work better as transparent conductors.
Thin Films of TiO2 and Related Oxides by ALD/MLD: Tailoring of Transport Properties on luettavissa sähköisenä osoitteessa https://aaltodoc.aalto.fi/handle/123456789/19339