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Surface Energy Patterning and Optoelectronic Devices Based on Conjugated Polymers
Linköping University, Department of Physics, Chemistry and Biology, Biomolecular and Organic Electronics . Linköping University, The Institute of Technology.
2006 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The work presented in this thesis concerns surface energy modification and patterning of the surfaces of conjugated polymers. Goniometry and Wilhelmy Balance techniques were used to evaluate the surface energy or wettability of a polymer’s surface; infrared reflectionabsorption spectroscopy (IRAS) was used to analyse the residuals on the surface as modified by a bare elastomeric stamp poly(dimethylsiloxane) (PDMS). The stamp was found to be capable of modifying a polymer surface. Patterning of a single and/or double layer of conjugated polymers on the surface can be achieved by surface energy controlled dewetting. Modification of a conjugated polymer film can also be carried out when a sample is subjected to electrochemical doping in an aqueous electrolyte. The dynamic surface energy changes during the process were monitored in-situ using the Wilhelmy balance method.

This thesis also concerns studies of conjugated polymer-based optoelectronics, including light-emitting diodes (PLEDs), that generate light by injecting charge into the active polymer layer, and solar cells (PSCs), that create electrical power by absorbing and then converting solar photons into electron/hole pairs. A phosphorescent metal complex was doped into polythiophene to fabricate PLEDs. The energy transfer from the host polymer to the guest phosphorescent metal (iridium and platinum) complex was studied using photoluminescence and electroluminescence measurements performed at room temperature and at liquid nitrogen temperature. PSCs were prepared using low-bandgap polyfluorene copolymers as an electron donor blended with several fullerene derivatives acting as electron acceptors. Energetic match is the main issue affecting efficient charge transfer at the interface between the polymers and the fullerene derivatives, and therefore the performance of the PSCs. Photoluminescence, luminescence quenching and the lowest unoccupied molecular orbital (LUMO) together with the highest occupied molecular orbital (HOMO) of the active materials in the devices were studied. A newly synthesized fullerene, that could match the low-bandgap polymers, was selected and used as electron acceptor in the PSCs. Photovoltaic properties of these PSCs were characterised, demonstrating one of the most efficient polymer:fullerene SCs that generate photocurrent at 1 μm.

Place, publisher, year, edition, pages
Institutionen för fysik, kemi och biologi , 2006.
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 996
Keyword [en]
Surface energy modification, Patterning, Dewetting, Conjugated polymer, plastic solar cell, Low bandgap, Electron acceptors and donors
National Category
Physical Sciences
Identifiers
URN: urn:nbn:se:liu:diva-7065ISBN: 91-85497-00-2 (print)OAI: oai:DiVA.org:liu-7065DiVA: diva2:22133
Public defence
2006-03-10, Planck, Fysikhuset, Campus Valla, Linköpings universitet, Linköping, 10:15 (English)
Opponent
Supervisors
Note
On the day of the defence the status of article number III was Manuscript and article VII was Accepted.Available from: 2006-07-07 Created: 2006-07-07 Last updated: 2009-10-05
List of papers
1. PEDOT surface energy pattern controls fluorescent polymer deposition by dewetting
Open this publication in new window or tab >>PEDOT surface energy pattern controls fluorescent polymer deposition by dewetting
2004 (English)In: Thin Solid Films, ISSN 0040-6090, E-ISSN 1879-2731, Vol. 449, no 1-2, 125-132 p.Article in journal (Refereed) Published
Abstract [en]

An elastomeric stamp of poly(dimethylsiloxane) (PDMS) can modify the surface energy of some surfaces when brought into conformal contact with these for some time. The substrates under investigation are a conducting polymer poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT-PSS) and a polyelectrolyte poly(sodium 4-styrenesulfonate) (NaPSS). The changes in surface wetting are characterized by contact angle measurement. Changes are due to the PDMS stamp, which leaves low molecular weight residues on the surface, as shown by infrared reflection absorption spectroscopy. This process may also be operating when other inks are transferred in microcontact printing. Patterning of fluorescent polymer film with feature size of 10–100 μm range is done by confining polymer solutions on the modified surface, by means of spin- or dip-coating. The profile of the patterned film and factors that influence the profile are discussed. This technique is a convenient way to build polymer microstructures for application in organic and biomolecular electronics and photonics.

Keyword
Surface energy, Surface modification by PDMS stamp, Contact angle analysis, IRA spectrum, Polymer patterning
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-13884 (URN)10.1016/j.tsf.2003.10.153 (DOI)
Available from: 2006-07-07 Created: 2006-07-07 Last updated: 2017-12-13
2. Single and bilayer submicron arrays of fluorescent polymer on conducting polymer surface with surface energy controlled dewetting
Open this publication in new window or tab >>Single and bilayer submicron arrays of fluorescent polymer on conducting polymer surface with surface energy controlled dewetting
2005 (English)In: Nanotechnology, ISSN 0957-4484, Vol. 16, 437-443 p.Article in journal (Refereed) Published
Abstract [en]

Construction of luminescent single- and bilayer polymer arrays in micron and submicron scales through dewetting on a heterogeneous conducting polymer surface is demonstrated. We study the influence of the pattern geometry and film thickness of polymer dewetting upon annealing, and the morphology of created polymer arrays on the heterogeneous surface. The materials used for patterning are an insulating poly(methyl methacrylate) (PMMA) or a conjugated fluorescent polymer, poly(dioctylphenylthiophene) (PDOPT). The substrate used is the conducting polymer poly(3,4-ethylenedioxythiophene)–poly(styrenesulfonate) (PEDOT–PSS), with modified heterogeneous surface energy obtained by application of a bare polydimethylsiloxane (PDMS) stamp.

National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-13885 (URN)10.1088/0957-4484/16/4/018 (DOI)
Available from: 2006-07-07 Created: 2006-07-07 Last updated: 2009-10-05
3. In-situ Wilhelmy balance surface energy determination of poly(3-hexylthiophene) and poly(3,4-ethylenedioxythiophene) during electrochemical doping-dedoping
Open this publication in new window or tab >>In-situ Wilhelmy balance surface energy determination of poly(3-hexylthiophene) and poly(3,4-ethylenedioxythiophene) during electrochemical doping-dedoping
2006 (English)In: Langmuir, ISSN 0743-7463, E-ISSN 1520-5827, Vol. 22, no 22, 9287-9294 p.Article in journal (Refereed) Published
Abstract [en]

Changes in the contact angle between conjugated polymers surface poly(3-hexylthiophene) [P3HT] and poly(3,4-ethylenedioxythiophene) (PEDOT) upon electrochemical doping−dedoping in aqueous electrolyte were determined in situ using a Wilhelmy plate tensiometer in an electrochemical cell. The hydrophobic P3HT was less hydrophobic in the oxidized state than in the neutral state; the more hydrophilic PEDOT was less hydrophilic in the oxidized state than when neutral. The tensiometry results were in good agreement with those measured by contact angle goniometry, and further corroborated by the capillary rise upon doping in a fluid cell with two parallel polymer coated plates, another in situ dynamic determination method. The contact angle changes depend on doping potential, electrolyte type, and concentration. We also deconvoluted the surface energy into components of van der Waals and acid−base interactions, using three probe liquids on the polymer surfaces, ex situ the electrochemical cell. The methods and the obtained results are relevant for the science and technology areas of printed electronics and electrochemical devices and for the understanding of surface energy modification by electrochemical doping.

National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-13886 (URN)10.1021/la061606p (DOI)
Available from: 2006-07-07 Created: 2006-07-07 Last updated: 2017-12-13
4. Electrophosphorescence from substituted poly(thiophene) doped with iridium or platinum complex
Open this publication in new window or tab >>Electrophosphorescence from substituted poly(thiophene) doped with iridium or platinum complex
2004 (English)In: Thin Solid Films, ISSN 0040-6090, E-ISSN 1879-2731, Vol. 468, no 1-2, 226-233 p.Article in journal (Refereed) Published
Abstract [en]

Electrophosphorescence has been observed in doped polythiophene light-emitting diodes (LEDs) with poly(3-methyl-4-octylthiophene) [PMOT] as host and the phosphorescent compounds bis(2-phenylbenzothiazole) iridium acetylacetonate (BTIr) or platinum(II) 2,8,12,17-tetraethyl-3,7,13,18-tramethyl porphyrin (PtOX) as guest. The photoluminescence (PL) and electroluminescence (EL) of host–phosphorescent guest blends PMOT:BTIr (or PMOT:PtOX) showed the existence of energy transfer from host to guest, which were guest concentration-dependent. At a certain guest concentration, emission from host PMOT was completely quenched in both blends based LEDs, and this gave rise to electrophosphorescence. The PL from host PMOT in the PMOT:BTIr blend film could not be quenched completely but was totally quenched in PMOT:PtOX. This implies a more efficient energy transfer from PMOT to PtOX than that from PMOT to BTIr under optical excitation. Comparison of PL and EL showed that the mechanism of exciton formation at the guest site under electrical excitation was not identical for these two systems. Energy transfer was a dominating route for exciton formation in PMOT:PtOX-based LEDs; charge trapping effect additionally contributed to the formation of exciton at BTIr in PMOT:BTIr-based LEDs. This study demonstrates a new direction in which polythiophene can be a candidate as a host to realize electrophosphorescence in polymer light-emitting diodes (PLEDs). Authors further indicate that to optimize the performance of the polythiophe/phosphorescent complexes, LEDs proper polythiophenes with large bang gap are needed.

Keyword
Polymer LED, Electrophosphorescence, Energy transfer, Quenching of luminescence, Polythiophene, Phosphorescent complex
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-13887 (URN)10.1016/j.tsf.2004.05.095 (DOI)
Available from: 2006-07-07 Created: 2006-07-07 Last updated: 2017-12-13
5. Infrared photocurrent spectral response from plastic solar cell with low-bandgap polyfluorene and fullerene derivative
Open this publication in new window or tab >>Infrared photocurrent spectral response from plastic solar cell with low-bandgap polyfluorene and fullerene derivative
Show others...
2004 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 85, no 21, 5081-5083 p.Article in journal (Refereed) Published
Abstract [en]

Plastic solar cells were fabricated using a low-band-gap alternating copolymer of fluorene and a donor–acceptor–donor moiety (APFO-Green1), blended with [6,6]-phenyl-C61-butyric acid methylester or 3-(3,5-Bis-trifluoromethylphenyl)-1-(4-nitrophenyl)pyrazolino[60]fullerene as electron acceptors. The polymer shows optical absorption in two wavelength ranges from 300<<500  nm and 650<<1000  nm. Devices based on APFO-Green1 blended with the later fullerene exhibit an outstanding photovoltaic behavior at the infrared range, where the external quantum efficiency is as high as 8.4% at 840  nm and 7% at 900  nm, while the onset of photogeneration is found at 1  µm. A photocurrent density of 1.76  mA/cm2, open-circuit voltage of 0.54  V, and power conversion efficiency of 0.3% are achieved under the illumination of AM1.5 (1000  W/m2) from a solar simulator.

National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-13888 (URN)10.1063/1.1825070 (DOI)
Available from: 2006-07-07 Created: 2006-07-07 Last updated: 2017-12-13
6. Enhanced photocurrent spectral response in low-bandgap polyfluorene and C70-Derivative-Based Solar Cells
Open this publication in new window or tab >>Enhanced photocurrent spectral response in low-bandgap polyfluorene and C70-Derivative-Based Solar Cells
Show others...
2005 (English)In: Advanced Functional Materials, ISSN 1616-301X, Vol. 15, no 10, 1665-1670 p.Article in journal (Refereed) Published
Abstract [en]

Plastic solar cells have been fabricated using a low-bandgap alternating copolymer of fluorene and a donor-acceptor-donor moiety (APFO-Green1), blended with 3-(3,5-bis-trifluoromethylphenyl)-1-(4-nitrophenyl)pyrazolino[70]fullerene (BTPF70) as electron acceptor. The polymer shows optical absorption in two wavelength ranges, < 500 nm and 600 <  < 1000 nm. The BTPF70 absorbs light at < 700 nm. A broad photocurrent spectral response in the wavelength range 300 <  < 1000 nm is obtained in solar cells. A photocurrent density of 3.4 mA cm-2, open-circuit voltage of 0.58 V, and power-conversion efficiency of 0.7 % are achieved under illumination of AM1.5 (1000 W m-2) from a solar simulator. Synthesis of BTPF70 is presented. Photoluminescence quenching and electrochemical studies are used to discuss photoinduced charge transfer.

Keyword
Fullerenes, Polyfluorenes, Solar cells - organic
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-13889 (URN)10.1002/adfm.200500114 (DOI)
Available from: 2006-07-07 Created: 2006-07-07 Last updated: 2009-10-05
7. Polymer solar cells with low-bandgap polymers blended with C70-derivative give photocurrent at 1 μm
Open this publication in new window or tab >>Polymer solar cells with low-bandgap polymers blended with C70-derivative give photocurrent at 1 μm
Show others...
2006 (English)In: Thin Solid Films, ISSN 0040-6090, Vol. 511-512, 576-580 p.Article in journal (Refereed) Published
Abstract [en]

A new series of low-bandgap alternating polyfluorenes with different donor–acceptor–donor moieties have been synthesized. Electrochemical and optical absorption measurement show that onset bandgaps of these polymers range from 1.2 to 1.5 eV. These polymers, blended with a C70-derivative as acceptor, are used for solar cell fabrication. Devices show promising photovoltaic properties, and the spectral response of photocurrent covers all visible and near-infrared wavelength regions with its onset extended to 1 μm. The best data gives a photocurrent density of 3.4 mA/cm2, open circuit voltage of 0.58 V and power conversion efficiency of 0.7% under illumination of AM1.5 (1000 W/m2) from a solar simulator.

Keyword
Low-bandgap; Polyfluorene; Fullerene C70-derivative; Solar cell
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-13890 (URN)10.1016/j.tsf.2005.12.013 (DOI)
Available from: 2006-07-07 Created: 2006-07-07 Last updated: 2009-10-05

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