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Molecular Doping and Trap Filling in Organic Semiconductor Host-Guest Systems
Linköping University, Department of Physics, Chemistry and Biology, Complex Materials and Devices. Linköping University, Faculty of Science & Engineering.
Chalmers, Sweden.
Linköping University, Department of Physics, Chemistry and Biology, Complex Materials and Devices. Linköping University, Faculty of Science & Engineering.
Linköping University, Department of Physics, Chemistry and Biology, Complex Materials and Devices. Linköping University, Faculty of Science & Engineering.
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2017 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 121, no 14, p. 7767-7775Article in journal (Refereed) Published
Abstract [en]

We investigate conductivity and mobility of different hosts mixed with different electron-withdrawing guests in concentrations ranging from ultralow to high. The effect of the guest material on the mobility and conductivity of the host material varies systematically with the guests LUMO energy relative to the host HOMO, in quantitative agreement with a recently developed model. For guests with a LUMO within similar to 0.5 eV of the host HOMO the dominant process governing transport is the competition between the formation of a deep tail in the host DOS and state filling. In other cases, the interaction with the host is dominated by any polar side groups on the guest and changes in the host morphology. For relatively amorphous hosts the latter interaction can lead to a suppression of deep traps, causing a surprising mobility increase by 1-2 orders of magnitude. In order to analyze our data, we developed a simple method to diagnose both the presence and the filling of traps.

Place, publisher, year, edition, pages
AMER CHEMICAL SOC , 2017. Vol. 121, no 14, p. 7767-7775
National Category
Condensed Matter Physics
Identifiers
URN: urn:nbn:se:liu:diva-137610DOI: 10.1021/acs.jpcc.7b01758ISI: 000399629000022OAI: oai:DiVA.org:liu-137610DiVA, id: diva2:1097359
Note

Funding Agencies|Chinese Scholarship Council (CSC); Knut och Alice Wallenbergs stiftelse (project "Tail of the Sun"); Swedish Research Council; Swedish Research Council Formas; Chalmers Area of Advance Energy

Available from: 2017-05-22 Created: 2017-05-22 Last updated: 2018-05-14
In thesis
1. Doping and Density of States Engineering for Organic Thermoelectrics
Open this publication in new window or tab >>Doping and Density of States Engineering for Organic Thermoelectrics
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
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.  

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2018. p. 67
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1934
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:liu:diva-147778 (URN)10.3384/diss.diva-147778 (DOI)9789176853115 (ISBN)
Public defence
2018-06-04, Planck, Fysikhuset, Campus Valla, Linköping, 10:00 (English)
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Supervisors
Available from: 2018-05-14 Created: 2018-05-14 Last updated: 2018-09-14Bibliographically approved

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