Vacancy-Rich Silicon as a Flexible Thermoelectric Material

Principal Investigator: Dr Nick Bennett
Co-Investigator: Prof Roger Webb (Surrey)
Research Associate: Dr Peter Szabo
Funding: EPSRC

Over 15 TW of power is continually lost worldwide in the form of waste heat. Thermoelectric generators (TEGs) offer one method of reducing this waste, by harvesting the heat and using it to create electrical power. While the conversion efficiency of TEG devices is often <10%, the sheer abundance of waste heat, offering a free fuel source, makes TEGs appealing for many diverse applications. This proposal is aimed at thin-film TEGs (active thickness, 1-20 micrometres), forecast to be a core market sector in the future, with the advent of flexible/wearable electronics, and with the increased uptake of sensors, all of which require low-power. If TEGs can be produced at low-cost and with increased functionality (e.g. flexible), their potential is significant to act as a power source for future electronic devices that improve our quality of life. As an alternative to generators, the same thin-film technology can also be used in reverse for small-scale heating/cooling applications, with thin-film modules already used for chip-cooling in high-performance electronics (space, military and aerospace applications). Silicon-based technologies underpin the global electronics industry due to their many practical advantages. These same benefits would extend to TEGs were it not for the poor thermoelectric conversion performance of silicon.

This project will undertake pioneering materials work in the area of “vacancy-rich silicon” – essentially silicon with many atoms removed at the atomic level – building on initial work carried-out by us, which has shown vacancy-rich silicon to be competitive with other state-of-the-art thermoelectric materials. The realisation of flexible thin-film TEGs based on vacancy-rich silicon will represent a transformative step applicable to numerous applications, including power generation and heating/cooling within clothing, as targeted specifically by us in co-operation with our industry partners.


Defect-Engineering for Silicon Thermoelectrics

PhD Student: Neil Wight
Supervisor: Dr Nick Bennett
Funding: EPSRC (Doctoral Training Award)

Si is a remarkably useful element. Its abundance, low cost and low toxicity, combined with vast practical know-how means it is a leading material on which to base technologies. However for good reason, certain applications have under-utilised it, with thermoelectrics being one example. Thermoelectric performance is governed by a materials’ thermoelectric power factor and thermal conductivity. Compared to other materials, highly-doped Si has desirably large power factor, but this is negated by high thermal conductivity, meaning that thermoelectric performance is relatively poor for bulk Si, about 100-fold worse than for popular materials such as bismuth telluride.

This study is investigating the introduction of defects in Si as a means to reduce the thermal conductivity in Si nano-films.


Silicon Nanocomposites for Thermoelectrics

PhD Student: Alessandro Calvi
Supervisor: Dr Nick Bennett
Funding: EPSRC (Doctoral Training Award)


Manufacturing Solar Conversion Devices on the Lunar Surface Using In-Situ Resources

PhD Student: Jürgen Schleppi
Supervisor: Dr Nick Bennett
Funding: European Astronaut Centre, ESA, and local support from Heriot-Watt

Future exploration missions outlined in the General Exploration Roadmap (GER), including human missions to the Moon and Mars, are expected to have increasingly demanding payload and situational requirements. Even with advances in the development of heavy lift systems, such missions will have signicantly constrained masses. Resupply flights will be limited, and in some cases impractical, and so most elements needed to safely complete a mission will have to be included within the mass constraints. This is where In Situ Resource Utilisation (ISRU) can make a significant contribution and open up potentially new approaches to mission design. At present, a burden of proof exists on ISRU-related technology and mission designers are conservative in their integration of such technologies and methodologies for future exploration missions. Investigating processes that are available for the extraction of valuable resources for integration into missions, and where possible carrying out physical validation and development of such processes will be beneficial to manufacturing solar cells and other solar conversion devices on the Moon.

This study aims at using existing approaches to ISRU for creating solar conversion devices and validating the manufacturing processes first under standard then under lunar like conditions.


Thin-film Thermoelectrics

PhD Student: Edwin Acosta
Supervisor: Dr Nick Bennett
Funding: Secretaria Nacional de Educación Superior, Ciencia y Tecnología e Innovación, Ecuador

Thin-film thermoelectric generators (TEGs) with an active device thickness of 1-20 um are forecast to find a growing number of applications for powering modern sensors and with the advent of flexible/wearable electronics. Since such components only require low power, energy-harvesting devices can feasibly be employed as their power source. TEGs are one such type of device, powered by the existence of a temperature difference across the module, producing electrical power as a function of the thermoelectric properties of the active material.

This study is investigating various low-cost routes for the realisation of thin-film TEG devices, involving both low-cost materials and low-cost fabrication methods.

A Universal Method for Thermal Conductivity Measurements on Micro-/Nano- Films With and Without Substrates using Micro-Raman Spectroscopy

Scholarship holder: Neil Wight
Supervisor: Dr Nick Bennett
Partner/Host: Prof Patrick McNally, Dublin City University
Funding: Royal Society of Edinburgh (JM Lessells Scholarship)

The ability to measure intrinsic thermal conductivity via a non-contact, non-destructive process is extremely attractive. Micro-Raman spectroscopy has been demonstrated to enable effective non-contact thermometry with further work providing a non-destructive estimation of values for thermal conductivity on suitable materials. However significant limitations remain for nano- and micro-films. Materials that do not meet dimensional requirements for thickness or that are in-situ on a substrate or supporting structure present significant challenges using existing approaches. For such samples, representative measurements must be obtained using alternative methods that can compromise samples and/or require relative complexity in experimental design and analysis. Here an analytical model is shown allowing thermal conductivity to be measured free of such limitations via a straightforward approach using micro-Raman spectroscopy. Results are then obtained experimentally and values compared with those obtained using a complimentary technique demonstrating an improved accuracy over existing micro-Raman approaches. Furthermore, this model enables the effect of any substrate or supporting structure on measured values to be quantified and estimations for thermal conductivity of the sample itself to then be calculated where an influence is determined. Current estimations determining the threshold of substrate influence are shown to be insufficient and the importance of obtaining values of thermal conductivity for samples themselves under such conditions is demonstrated.

This study formed a vital part of Neil’s PhD project allowing sophisticated characterisation on defect-engineered Si films.


Sustainable Energy Systems for Future Human Space Missions (SEnSe)

Principal Investigator: Dr Nick Bennett
Co-Investigators: Dr John Andresen, Dr Aidan Cowley (ESA)
Funding: Energy Academy
Partner: European Astronaut Centre, ESA


Future exploration missions outlined in the General Exploration Roadmap (GER), include human missions to the Moon and Mars. The establishment of a lunar base for human habitation is certainly not beyond the realms of possibility.

This project was carried out in partnership with the European Astronaut Centre (EAC) – part of the European Space Agency (ESA).
It developed a detailed sustainable space-base framework involving photovoltaic and fuel cell technologies.


Novel Cu-X based materials for transparent electronics (X: halide/delafossite)

Principal Investigator: Dr Nick Bennett
Co-Investigator: Dr Daragh Byrne (DCU)
Funding: Royal Society
Partner: Dublin City University


The potential applications for transparent electronics are diverse. Such materials can be used as transparent electrodes for solar cells, mobile devices or for next generation flexible electronics. The combination of conductivity and transparency is difficult to achieve, usually requiring complex compositions or very high doping of oxide materials. The most frequently used material for current generation transparent electronics is Indium Tin Oxide (ITO), a so-called Transparent Conductive Oxide (TCO). The present scarcity of indium (and thus its high commodity price) has promoted new research into finding a cost-competitive substitute. In addition, the great majority of materials idealised for transparent electronics are n-type.

In this study our project partners at Dublin City University (DCU) developed new deposition techniques for the formation of low-cost p-type Cu-based semiconductor materials based on the halide and delafossite families. We worked jointly with DCU to maximise the performance of these materials and thoroughly tested them for various end-applications.


Nanostructured next-generation silicon based thermoelectric power reclamation (Nextstep)

Principal Investigator: Dr Nick Bennett
Funding: Science Foundation Ireland / Enterprise Ireland

In recent years, research on thermoelectric materials has intensified thanks to the exciting potential of low-dimensional structures such as nanowires. Experiments have shown that nano-structuring materials can greatly reduce their thermal transport properties, significantly enhancing thermoelectric performance. With reduced thermal conductivity, nano-structured silicon – which is plentiful and low-cost – becomes a competitive TE material, but still trails traditional TE materials in overall performance.

In this study we showed that the creation of extended defects within the crystal structure of silicon nanowires can create an additional enhancement in thermoelectric performance. Relative to regular silicon nanowires, extended defects lead to an increased Seebeck coefficient. The effect is a consequence of the creation of dislocations and dislocation-loops, intentionally introduced in the nanowires. These defects create nano-scale potential barriers which theoretical studies have predicted can enhance silicon’s thermopower by energy filtering of low-energy carriers. Although the defects slightly reduce carrier mobility – increasing electrical resistivity in the nanowires – their presence creates an overall two-fold enhancement in the thermoelectric power factor.


Development of a Nano-Material doping characterisation tool by a novel combination of Secondary-Ion mass Spectrometry and Resistivity Measurement (SIMSAR)

Principal Investigator: Prof Patrick McNally
Co-Investigator: Dr Nick Bennett
Funding: Science Foundation Ireland / Enterprise Ireland

Secondary-ion mass spectrometry (SIMS) is an established technique for measuring doping profiles in semiconductors. The method benefits from high sensitivity, wide dynamic range and good depth resolution. These advantages have made it a staple technique for the semiconductor industry for measuring the one-dimensional depth-profile of ion-implanted dopants – such as arsenic – in silicon. One limitation of the SIMS technique is that while it identifies the total concentration of an element as a function of depth, it is unable to distinguish between those atoms that act as donors or acceptors from those that do not, i.e. it cannot distinguish so-called ‘active’ dopants from ‘inactive’ dopants. Several techniques exist for chemical dopant profiling and numerous for electrical profiling. Crucially however, there is no technique which can measure both electrical and chemical profiles in parallel.

In this study a method was found that allowed measurement of the electrically active dopant profile in unison with the chemical profile. The method was not without complications, however potential problems were identified, explained and corrected for. The measurement principle was tested and could feasibly be incorporated in a commercial SIMS tool for use in semiconductor manufacturing metrology.