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Electronic Transport in Polymeric Solar Cells and Transistors
Linköping University, Department of Physics, Chemistry and Biology, Biomolecular and Organic Electronics. Linköping University, The Institute of Technology.
2007 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The main topic of this dissertation is electronic charge transport in polymeric and molecular organic materials and material blends intended for solar cell applications. Charge transport in polymers is a strange beast and carrier mobility is rarely a well-defined number. Measurements on different sample geometries and under different conditions tend to give different results and when everything is to be related to solar cell performance it is imperative that there is a way to correlate the results from different measurements. Polymer solar cells utilize composite materials for their function. This puts an additional twist on charge transport studies, as there will also be interaction between the different phases to take into account.

Several measurement techniques have been used and their interrelationships as well as information on their relevance for solar cells have been investigated. Field effect transistors (FET) with an organic active layer have proved to be one of the more versatile measurement geometries and are also an interesting topic in itself. FETs are discussed both as a route for material characterization and as components. A main result correlates bias stress in organic field effect transistors with the electronic structure of the material.

Power conversion efficiency in solar cells is discussed with respect to electrical properties. The interaction of different blend materials and the impact of stoichiometry on transport properties in the active layer have been investigated. Results indicate that charge transport properties frequently are a key determining factor for which material combinations and ratios that works best.

Some work on the conductive properties of nano-fibers coated with semiconducting polymers has also been done and is briefly discussed. The conductive properties of nano-fibers have been studied through potential imaging.

Place, publisher, year, edition, pages
Institutionen för fysik, kemi och biologi , 2007. , 70 p.
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1142
National Category
Physical Sciences
Identifiers
URN: urn:nbn:se:liu:diva-10380ISBN: 978-91-85895-50-2 (print)OAI: oai:DiVA.org:liu-10380DiVA: diva2:17130
Public defence
2007-11-20, Planck, Fysikhuset, Campus Valla, Linköpings Universitet, Linköping, 10:15 (English)
Opponent
Supervisors
Available from: 2007-12-05 Created: 2007-12-05 Last updated: 2017-01-11Bibliographically approved
List of papers
1. Non-equilibrium effects on electronic transport in organic field effect transistors
Open this publication in new window or tab >>Non-equilibrium effects on electronic transport in organic field effect transistors
2007 (English)In: Organic electronics, ISSN 1566-1199, E-ISSN 1878-5530, Vol. 8, no 4, 423-430 p.Article in journal (Refereed) Published
Abstract [en]

Non-ideal behavior in organic field effect transistors, in particular threshold voltage drift and light sensitivity, is argued to be due to intrinsic carrier dynamics. The discussion is based on the theory for hopping transport within a Gaussian density of states. Carrier concentration is shown to be of fundamental importance, and the time required to reach equilibrium at different bias is responsible for device behavior, with implications for mobility evaluation. Experimental results from various conjugated polymers in a field effect transistor illustrate the theory.

Keyword
Organic field-effect transistors (OFETs), Conjugated polymer, Hysteresis, Charge transport, Gaussian disorder model, Bias stress
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-12822 (URN)10.1016/j.orgel.2007.02.001 (DOI)
Available from: 2007-12-05 Created: 2007-12-05 Last updated: 2017-12-14
2. Bipolar transport observed through extraction currents on organic photovoltaic blend materials
Open this publication in new window or tab >>Bipolar transport observed through extraction currents on organic photovoltaic blend materials
2006 (English)In: Applied Physics Letters, ISSN 0003-6951, Vol. 89, 142111- p.Article in journal (Refereed) Published
Abstract [en]

Both electron and hole mobilities have been simultaneously measured through charge extraction by linearly increasing voltage on polymer heterojunction solar cells with varying stoichiometry of polymer and acceptor. The polymer is a low band gap copolymer of fluorene, thiophene, and electron accepting groups named APFO-Green 5, and the acceptor is [6,6]-phenyl-C61-butyric acid methylester. Results are correlated to field effect transistor measurements on the same material system. A monotonous increase in mobility for both carrier types is observed with increased acceptor loading.

Keyword
polymer blends, organic semiconductors, fullerene compounds, solar cells, electron mobility, hole mobility, stoichiometry, carrier density
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-12823 (URN)10.1063/1.2360199 (DOI)
Available from: 2007-12-05 Created: 2007-12-05
3. Stoichiometry, mobility, and performance in bulk heterojunction solar cells
Open this publication in new window or tab >>Stoichiometry, mobility, and performance in bulk heterojunction solar cells
2007 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 91, no 7, 071108- p.Article in journal (Refereed) Published
Abstract [en]

Bipolar transport in blends of a copolymer of fluorene, thiophene and electron accepting groups, and the substituted fullerene [6,6]-phenyl-C61-butyric acid methylester have been studied through charge extraction by linearly increasing voltage on solar cells and with field effect transistors. Between 10% and 90% polymer has been used and the results show a clear correlation to solar cell performance. Optimal solar cells comprise 20% polymer and have a power conversion efficiency of 3.5%. The electron mobility is increasing strongly with fullerene content, but is always lower than the hole mobility, thus explaining the low amount of polymer in optimized devices.

Keyword
electron mobility, field effect transistors, fullerenes, hole mobility, polymer blends, solar cells, stoichiometry
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-12824 (URN)10.1063/1.2771524 (DOI)
Available from: 2007-12-05 Created: 2007-12-05 Last updated: 2017-12-14
4. Acceptor influence on hole mobility in fullerene blends with alternating copolymers of fluorene
Open this publication in new window or tab >>Acceptor influence on hole mobility in fullerene blends with alternating copolymers of fluorene
2006 (English)In: Applied Physics Letters, ISSN 0003-6951, Vol. 88, 082103- p.Article in journal (Refereed) Published
Abstract [en]

Hole mobility in polyfluorene/fullerene blends has been studied with field effect transistors. Two different C60 derivatives and one C70 derivative have been investigated together with two different polyfluorenes. Mobility is presented as a function of acceptor loading at ratios suitable for photovoltaics and varies between 10–3 and 10–5  cm2  V–1  s–1 depending on the polymer/acceptor combination. The hole mobility is increased in blends with the commonly used acceptor [6-6]-phenyl-C61-butyric acid methylester (PCBM). With related C60 and C70 derivatives the hole mobility is decreased under the same circumstances.

Keyword
fullerenes, polymer blends, hole mobility, impurities, field effect transistors
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-12825 (URN)10.1063/1.2177663 (DOI)
Available from: 2007-12-05 Created: 2007-12-05
5. Intrinsic and extrinsic influences on the temperature dependence of mobility in conjugated polymers
Open this publication in new window or tab >>Intrinsic and extrinsic influences on the temperature dependence of mobility in conjugated polymers
Show others...
2008 (English)In: Organic electronics, ISSN 1566-1199, E-ISSN 1878-5530, Vol. 9, no 5, 569-574 p.Article in journal (Refereed) Published
Abstract [en]

The temperature dependence of charge carrier mobility in conjugated polymers and their blends with fullerenes is investigated with different electrical methods, through field effect transistor (FET), space charge limited current (SCLC) and charge extraction (CELIV) measurements. Simple models, such as the Gaussian disorder model (GDM), are shown to accurately predict the temperature behavior, and a good correlation between the different measurement methods is obtained. Inconsistent charge carrier concentrations in the modeling are explained through intrinsic non-equilibrium effects, and are responsible for the limited applicability of existing numerical models. A severe extrinsic influence from water in FETs with a hydrophilic insulator interface is also demonstrated. The presence of water leads to a significant overestimate of the disorder in the materials from measurements close to room temperature and erratic behavior in the 150-350 K range. To circumvent this problem it is shown to be necessary to measure under ultra high vacuum (UHV) conditions. © 2008 Elsevier B.V. All rights reserved.

Place, publisher, year, edition, pages
Elsevier, 2008
Keyword
Electrical transport, Field effect transistor, Fullerene, Mobility, Polymer, Temperature dependence
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-50043 (URN)10.1016/j.orgel.2008.03.002 (DOI)
Note

The previous status of this article was Manuscript.

Available from: 2009-10-11 Created: 2009-10-11 Last updated: 2017-12-12Bibliographically approved

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Andersson, Lars Mattias

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