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High Seebeck Coefficient in Mixtures of Conjugated Polymers
Linköpings universitet, Institutionen för fysik, kemi och biologi, Komplexa material och system. Linköpings universitet, Tekniska fakulteten.
Linköpings universitet, Institutionen för fysik, kemi och biologi, Ytors Fysik och Kemi. Linköpings universitet, Tekniska fakulteten.
Linköpings universitet, Institutionen för fysik, kemi och biologi, Ytors Fysik och Kemi. Linköpings universitet, Tekniska fakulteten.ORCID-id: 0000-0001-9879-3915
Linköpings universitet, Institutionen för fysik, kemi och biologi, Komplexa material och system. Linköpings universitet, Tekniska fakulteten.ORCID-id: 0000-0002-7104-7127
2018 (engelsk)Inngår i: Advanced Functional Materials, ISSN 1616-301X, E-ISSN 1616-3028, Vol. 28, nr 15, artikkel-id 1703280Artikkel i tidsskrift (Fagfellevurdert) Published
Abstract [en]

A universal method to obtain record?high electronic Seebeck coefficients is demonstrated while preserving reasonable conductivities in doped blends of organic semiconductors through rational design of the density of states (DOSs). A polymer semiconductor with a shallow highest occupied molecular orbital (HOMO) level?poly(3?hexylthiophene) (P3HT) is mixed with materials with a deeper HOMO (PTB7, TQ1) to form binary blends of the type P3HTx:B1?x (0 ≤ x ≤ 1) that is p?type doped by F4TCNQ. For B = PTB7, a Seebeck coefficient S = 1100 µV K?1 with conductivity σ = 0.3 S m?1 at x = 0.10 is achieved, while for B = TQ1, S = 2000 µV K?1 and σ = 0.03 S m?1 at x = 0.05 is found. Kinetic Monte Carlo simulations with parameters based on experiments show good agreement with the experimental results, confirming the intended mechanism. The simulations are used to derive a design rule for parameter tuning. These results can become relevant for low?power, low?cost applications like (providing power to) autonomous sensors, in which a high Seebeck coefficient translates directly to a proportionally reduced number of legs in the thermogenerator, and hence in reduced fabrication cost and complexity.

sted, utgiver, år, opplag, sider
Wiley-Blackwell, 2018. Vol. 28, nr 15, artikkel-id 1703280
Emneord [en]
conjugated polymers, doping, kinetic Monte Carlo simulations, organic thermoelectrics, Seebeck coefficients
HSV kategori
Identifikatorer
URN: urn:nbn:se:liu:diva-147779DOI: 10.1002/adfm.201703280ISI: 000430101100004OAI: oai:DiVA.org:liu-147779DiVA, id: diva2:1205579
Konferanse
2018/05/14
Merknad

Funding Agencies: Chinese Scholarship Council (CSC)

Tilgjengelig fra: 2018-05-14 Laget: 2018-05-14 Sist oppdatert: 2018-05-31bibliografisk kontrollert
Inngår i avhandling
1. Doping and Density of States Engineering for Organic Thermoelectrics
Åpne denne publikasjonen i ny fane eller vindu >>Doping and Density of States Engineering for Organic Thermoelectrics
2018 (engelsk)Doktoravhandling, med artikler (Annet vitenskapelig)
Abstract [en]

Thermoelectric materials can turn temperature differences directly into electricity. To use this to harvest e.g. waste heat with an efficiency that approaches the Carnot efficiency requires a figure of merit ZT larger than 1. Compared with their inorganic counterparts, organic thermoelectrics (OTE) have numerous advantages, such as low cost, large-area compatibility, flexibility, material abundance and an inherently low thermal conductivity. Therefore, organic thermoelectrics are considered by many to be a promising candidate material system to be used in lower cost and higher efficiency thermoelectric energy conversion, despite record ZT values for OTE currently lying around 0.25.

A complete organic thermoelectric generator (TEG) normally needs both p-type and n-type materials to form its electric circuit. Molecular doping is an effective way to achieve p- and ntype materials using different dopants, and it is necessary to fundamentally understand the doping mechanism. We developed a simple yet quantitative analytical model and compare it with numerical kinetic Monte Carlo simulations to reveal the nature of the doping effect. The results show the formation of a deep tail in the Gaussian density of states (DOS) resulting from the Coulomb potentials of ionized dopants. It is this deep trap tail that negatively influences the charge carrier mobility with increasing doping concentration. The trends in mobilities and conductivities observed from experiments are in good agreement with the modeling results, for a large range of materials and doping concentrations.

Having a high power factor PF is necessary for efficient TEG. We demonstrate that the doping method can heavily impact the thermoelectric properties of OTE. In comparison to conventional bulk doping, sequential doping can achieve higher conductivity by preserving the morphology, such that the power factor can improve over 100 times. To achieve TEG with high output power, not only a high PF is needed, but also having a significant active layer thickness is very important. We demonstrate a simple way to fabricate multi-layer devices by sequential doping without significantly sacrificing PF.

In addition to the application discussed above, harvesting large amounts of heat at maximum efficiency, organic thermoelectrics may also find use in low-power applications like autonomous sensors where voltage is more important than power. A large output voltage requires a high Seebeck coefficient. We demonstrate that density of states (DOS) engineering is an effective tool to increase the Seebeck coefficient by tailoring the positions of the Fermi energy and the transport energy in n- and p-type doped blends of conjugated polymers and small molecules.

In general, morphology heavily impacts the performance of organic electronic devices based on mixtures of two (or more) materials, and organic thermoelectrics are no exception. We experimentally find that the charge and energy transport is distinctly different in well-mixed and phase separated morphologies, which we interpreted in terms of a variable range hopping model. The experimentally observed trends in conductivity and Seebeck coefficient are reproduced by kinetic Monte Carlo simulations in which the morphology is accounted for.  

sted, utgiver, år, opplag, sider
Linköping: Linköping University Electronic Press, 2018. s. 67
Serie
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1934
HSV kategori
Identifikatorer
urn:nbn:se:liu:diva-147778 (URN)10.3384/diss.diva-147778 (DOI)9789176853115 (ISBN)
Disputas
2018-06-04, Planck, Fysikhuset, Campus Valla, Linköping, 10:00 (engelsk)
Opponent
Veileder
Tilgjengelig fra: 2018-05-14 Laget: 2018-05-14 Sist oppdatert: 2018-09-14bibliografisk kontrollert

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