HYTEC · Hybrid Organic Thermoelectrics: an Insight into Charge Transport Physics towards High-Performance Organic Thermoelctric Generators

Hybrid Organic Thermoelectrics: an Insight into Charge Transport Physics towards High-Performance Organic Thermoelctric Generators

1. Importance for society Thermoelectric generators (TEGs) are solid-state devices that directly convert heat to electricity. TEGs could be a valuable contributor to the world’s increasing demand for renewable energy, particularly when considering the amount of energy loss in terms of heat during our daily life. For instance, around 67% of the energy generated from hydrocarbon sources is lost as waste heat. In a silicon solar cell, approximately 50% of incident solar energy is lost by thermalisation and absorption. What is more, if we consider human body as a heat engine, it releases about 100W of thermal energy under basal metabolic condition, in which the energy loss is far sufficient to power most of microelectronic devices. In this case, TEGs provide a solution to tackle such a challenge of harvesting electrical energy from this significant amount of waste heat. One key advantage of TEGs is their scalability. TEGs are able to harness waste heat through various scales of sources: from something as big as industrial or geothermal sources, medium size such as home water heater, to even micro scale for instance microchips and human body. Thus, TEGs offer unlimited potential in various applications, such as co-generation, spacecraft, automotive, electronic skins, and Internet of Things (IoT). 2. Problem being addressed Although demonstration of a bright future in wide applications, TEGs have long been too inefficient to be cost-effective in most applications. Fundamental to the thermoelectric field is the need to optimize a variety of conflicting properties of thermoelectric materials (TEMs). For TEMs, the efficiency of the heat-electricity conversion is dictated by a dimensionless figure of merit zT=P/κ, where P (P=α2×σ) is power factor and T is temperature. In order to fully implement the capacity of a TEG, its materials need to equip with high electrical conductivity (σ), large thermopower (α) and low thermal conductivity (κ). As these transport characteristics depend on interrelated material properties, a number of parameters such as carrier concentration, effective mass, and thermal conductivity, need to be optimized. Thermoelectrics require a rather unusual material: a ‘phonon-glass electron-crystal’. In order to achieve above behaviour, fundamental understanding of transport physics in TEMs is extremely important for both materials and process design. However, the transport processes has not yet been well understood since it is complicated by experimental evidence that clearly does not follow a single transport mechanism. 3. Overall objectives with Conclusions of the action Three Objectives out of four were achieved: Objective (i): to understand the charge transport of TEMs Outcome: two models have been developed to understand the charge transport of TEMs, one has been published in Advanced Functional Materials ( https://doi.org/10.1002/adfm.201910079). Another has been submitted and currently is under review in Nature Energy. Objective (ii) to optimize their thermoelectric performance Outcome: A grain boundary engineering approach has been developed to optimise the thermoelectric performance. The manuscript containing the relative results is under review in Nature Energy Objective (iii) to fabricate the hybrid TEMs with optimised thermoelectric performance Outcome: the fabricated TEM with maximum zT of 1.7, which meet original goal (zT>1.5). The manuscript containing the relative results is under review in Nature Energy Objective (iv) to fabricate a prototype hybrid TEGs and demonstrate its application Outcome: this objective has been severely disturbed by the Convid-19 pandemic. Both the labs in host organisation and the collaboration organisation were closed

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