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Doping and Density of States Engineering for Organic Thermoelectrics
Linköpings universitet, Institutionen för fysik, kemi och biologi, Komplexa material och system. Linköpings universitet, Tekniska fakulteten.
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: urn:nbn:se:liu:diva-147778DOI: 10.3384/diss.diva-147778ISBN: 9789176853115 (tryckt)OAI: oai:DiVA.org:liu-147778DiVA, id: diva2:1205616
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
Delarbeid
1. Impact of doping on the density of states and the mobility in organic semiconductors
Åpne denne publikasjonen i ny fane eller vindu >>Impact of doping on the density of states and the mobility in organic semiconductors
2016 (engelsk)Inngår i: PHYSICAL REVIEW B, ISSN 2469-9950, Vol. 93, nr 23, s. 235203-Artikkel i tidsskrift (Fagfellevurdert) Published
Abstract [en]

We experimentally investigated conductivity and mobility of poly(3-hexylthiophene) (P3HT) doped with tetrafluorotetracyanoquinodimethane (F(4)TCNQ) for various relative doping concentrations ranging from ultralow (10(-5)) to high (10(-1)) and various active layer thicknesses. Although the measured conductivity monotonously increases with increasing doping concentration, the mobilities decrease, in agreement with previously published work. Additionally, we developed a simple yet quantitative model to rationalize the results on basis of a modification of the density of states (DOS) by the Coulomb potentials of ionized dopants. The DOS was integrated in a three-dimensional (3D) hopping formalism in which parameters such as energetic disorder, intersite distance, energy level difference, and temperature were varied. We compared predictions of our model as well as those of a previously developed model to kinetic Monte Carlo (MC) modeling and found that only the former model accurately reproduces the mobility of MC modeling in a large part of the parameter space. Importantly, both our model and MC simulations are in good agreement with experiments; the crucial ingredient to both is the formation of a deep trap tail in the Gaussian DOS with increasing doping concentration.

sted, utgiver, år, opplag, sider
AMER PHYSICAL SOC, 2016
HSV kategori
Identifikatorer
urn:nbn:se:liu:diva-130276 (URN)10.1103/PhysRevB.93.235203 (DOI)000378813800009 ()
Merknad

Funding Agencies|Chinese Scholarship Council (CSC)

Tilgjengelig fra: 2016-08-01 Laget: 2016-07-28 Sist oppdatert: 2018-08-29
2. Molecular Doping and Trap Filling in Organic Semiconductor Host-Guest Systems
Åpne denne publikasjonen i ny fane eller vindu >>Molecular Doping and Trap Filling in Organic Semiconductor Host-Guest Systems
Vise andre…
2017 (engelsk)Inngår i: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 121, nr 14, s. 7767-7775Artikkel i tidsskrift (Fagfellevurdert) 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.

sted, utgiver, år, opplag, sider
AMER CHEMICAL SOC, 2017
HSV kategori
Identifikatorer
urn:nbn:se:liu:diva-137610 (URN)10.1021/acs.jpcc.7b01758 (DOI)000399629000022 ()
Merknad

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

Tilgjengelig fra: 2017-05-22 Laget: 2017-05-22 Sist oppdatert: 2018-05-14
3. High Seebeck Coefficient in Mixtures of Conjugated Polymers
Åpne denne publikasjonen i ny fane eller vindu >>High Seebeck Coefficient in Mixtures of Conjugated Polymers
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
Emneord
conjugated polymers, doping, kinetic Monte Carlo simulations, organic thermoelectrics, Seebeck coefficients
HSV kategori
Identifikatorer
urn:nbn:se:liu:diva-147779 (URN)10.1002/adfm.201703280 (DOI)000430101100004 ()
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
4. Range and energetics of charge hopping in organic semiconductors
Åpne denne publikasjonen i ny fane eller vindu >>Range and energetics of charge hopping in organic semiconductors
2017 (engelsk)Inngår i: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 96, nr 24, artikkel-id 241202Artikkel i tidsskrift (Fagfellevurdert) Published
Abstract [en]

The recent upswing in attention for the thermoelectric properties of organic semiconductors (OSCs) adds urgency to the need for a quantitative description of the range and energetics of hopping transport in organic semiconductors under relevant circumstances, i.e., around room temperature (RT). In particular, the degree to which hops beyond the nearest neighbor must be accounted for at RT is still largely unknown. Here, measurements of charge and energy transport in doped OSCs are combined with analytical modeling to reach the univocal conclusion that variable-range hopping is the proper description in a large class of disordered OSC at RT. To obtain quantitative agreement with experiment, one needs to account for the modification of the density of states by ionized dopants. These Coulomb interactions give rise to a deep tail of trap states that is independent of the materials initial energetic disorder. Insertion of this effect into a classical Mott-type variable-range hopping model allows one to give a quantitative description of temperature-dependent conductivity and thermopower measurements on a wide range of disordered OSCs. In particular, the model explains the commonly observed quasiuniversal power-law relation between the Seebeck coefficient and the conductivity.

sted, utgiver, år, opplag, sider
AMER PHYSICAL SOC, 2017
HSV kategori
Identifikatorer
urn:nbn:se:liu:diva-144143 (URN)10.1103/PhysRevB.96.241202 (DOI)000418616700001 ()
Merknad

Funding Agencies|Chinese Scholarship Council (CSC); Knut och Alice Wallenberg stiftelse

Tilgjengelig fra: 2018-01-10 Laget: 2018-01-10 Sist oppdatert: 2018-08-29
5. High Seebeck Coefficient and Power Factor in n-Type Organic Thermoelectrics
Åpne denne publikasjonen i ny fane eller vindu >>High Seebeck Coefficient and Power Factor in n-Type Organic Thermoelectrics
2018 (engelsk)Inngår i: ADVANCED ELECTRONIC MATERIALS, ISSN 2199-160X, Vol. 4, nr 1, artikkel-id 1700501Artikkel i tidsskrift (Fagfellevurdert) Published
Abstract [en]

The n-type thermoelectric properties of [6,6]-phenyl-C-61-butyric acid methyl ester (PCBM) are investigated for different solution-based doping methods. A novel inverse-sequential doping method where the semiconductor (PCBM) is deposited on a previously cast dopant 4-(1,3-dimethyl-2,3-dihydro-1H-benzoimidazol-2-yl)-N,N-diphenylaniline film to achieve a very high power factor PF approximate to 35 mu W m(-1) K-2 with a conductivity sigma approximate to 40 S m(-1) is introduced. It is also shown that n-type organic semiconductors obey the -1/4 power law relation between Seebeck coefficient S and sigma that are previously found for p-type materials. An analytical model on basis of variable range hopping unifies these results. The power law for n-type materials is shifted toward higher conductivities by two orders of magnitude with respect to that of p-type, suggesting strongly that n-type organic semiconductors can eventually become superior to their p-type counterparts. Adding a small fraction lower lowest unoccupied molecular orbital material (core-cyanated naphthalene diimide) into PCBM leads to a higher S for inverse-sequential doping but not for bulk doping due to different morphologies.

sted, utgiver, år, opplag, sider
Wiley-VCH Verlagsgesellschaft, 2018
Emneord
n-type doping; organic thermoelectrics; power factor; Seebeck coefficient
HSV kategori
Identifikatorer
urn:nbn:se:liu:diva-144566 (URN)10.1002/aelm.201700501 (DOI)000419670400026 ()
Merknad

Funding Agencies|Chinese Scholarship Council (CSC)

Tilgjengelig fra: 2018-01-29 Laget: 2018-01-29 Sist oppdatert: 2018-05-14
6. High thermoelectric power factor from multilayer solution-processed organic films
Åpne denne publikasjonen i ny fane eller vindu >>High thermoelectric power factor from multilayer solution-processed organic films
2018 (engelsk)Inngår i: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 112, nr 8, artikkel-id 083303Artikkel i tidsskrift (Fagfellevurdert) Published
Abstract [en]

We investigate the suitability of the "sequential doping" method of organic semiconductors for thermoelectric applications. The method consists of depositing a dopant (F4TCNQ) containing solution on a previously cast semiconductor (P3HT) thin film to achieve high conductivity, while preserving the morphology. For very thin films (similar to 25 nm), we achieve a high power factor around 8 mu W/mK(-2) with a conductivity over 500 S/m. For the increasing film thickness, conductivity and power factor show a decreasing trend, which we attribute to the inability to dope the deeper parts of the film. Since thick films are required to extract significant power from thermoelectric generators, we developed a simple additive technique that allows the deposition of an arbitrary number of layers without significant loss in conductivity or power factor that, for 5 subsequent layers, remain at similar to 300 S/m and similar to 5 mu W/mK(-2), respectively, whereas the power output increases almost one order of magnitude as compared to a single layer. The efficient doping in multilayers is further confirmed by an increased intensity of (bi)polaronic features in the UV-Vis spectra. Published by AIP Publishing.

sted, utgiver, år, opplag, sider
AMER INST PHYSICS, 2018
HSV kategori
Identifikatorer
urn:nbn:se:liu:diva-145764 (URN)10.1063/1.5016908 (DOI)000425977500021 ()
Merknad

Funding Agencies|China Scholarship Council (CSC); Knut och Alice Wallenbergs stiftelse (Project "Tail of the Sun")

Tilgjengelig fra: 2018-03-22 Laget: 2018-03-22 Sist oppdatert: 2018-05-14
7. Morphology Determines Conductivity and Seebeck Coefficient in Conjugated Polymer Blends
Åpne denne publikasjonen i ny fane eller vindu >>Morphology Determines Conductivity and Seebeck Coefficient in Conjugated Polymer Blends
2018 (engelsk)Inngår i: ACS Applied Materials and Interfaces, ISSN 1944-8244, E-ISSN 1944-8252, Vol. 10, nr 11, s. 9638-9644Artikkel i tidsskrift (Fagfellevurdert) Published
Abstract [en]

The impact of nanoscale morphology on conductivity and Seebeck coefficient in p-type doped all-polymer blend systems is investigated. For a strongly phase separated system (P3HT:PTB7), we achieve a Seebeck coefficient that peaks at S similar to 1100 mu V/K with conductivity sigma similar to 3 x 10(-3) S/cm for 90% PTB7. In marked contrast, for well-mixed systems (P3HT:PTB7 with 5% diiodooctane (DIO), P3HT:PCPDTBT), we find an almost constant S similar to 140 mu V/K and sigma similar to 1 S/cm despite the energy levels being (virtually) identical in both cases. The results are interpreted in terms of a variable range hopping (VRH) model where a peak in S and a minimum in a arise when the percolation pathway contains both host and guest sites, in which the latter acts as energetic trap. For well-mixed blends of the investigated compositions, VRH enables percolation pathways that only involve isolated guest sites, whereas the large distance between guest clusters in phase separated blends enforces (energetically unfavorable) hops via the host. The experimentally observed trends are in good agreement with the results of atomistic kinetic Monte Carlo simulations accounting for the differences in nanoscale morphology.

sted, utgiver, år, opplag, sider
AMER CHEMICAL SOC, 2018
Emneord
Seebeck coefficient; morphology; charge transport; conjugated polymers; kinetic Monte Carlo simulations
HSV kategori
Identifikatorer
urn:nbn:se:liu:diva-147581 (URN)10.1021/acsami.8b00122 (DOI)000428356800052 ()29488380 (PubMedID)
Merknad

Funding Agencies|China Scholarship Council (CSC)

Tilgjengelig fra: 2018-04-26 Laget: 2018-04-26 Sist oppdatert: 2018-05-18

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