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Studies of Charge Transport and Energy Level in Solar Cells Based on Polymer/Fullerene Bulk Heterojunction
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]

π-Conjugated polymers have attracted considerable attention since they are potential candidates for various opto-electronic devices such as solar cells, light emitting iodes, photodiodes, and transistors. Electronic de vices based on conjugated polymers can be easily processed at low temperature using inexpensive technologies. This leads to cost reduction, a key-deriving factor for choosing conjugated polymers for various types of applications. In particular, polymer based solar cells are of special interest due to the fact that they can play a major role in generating clean and cheap energy in the future.

The investigations described in thesis are aimed mainly at understanding charge transport and the role of energy le vels in solar cells based on polymer/acceptor bulk heterojunction (BHJ) active films. Best polymer based solar cells, with efficiency 4 to 5%, rely on polymer/fullerene BHJ active films. These solar cells are in an immature state to be used for energy conversion purposes. In order to enhance their performance, it is quite important to understand the efficiency-limiting factors. Solid films of conjugated polymers compose conjugation segments that are randomly distributed in space and energy. Such distributio n gives rise to the localization of charge carriers and hence broadening of electron density of states. Consequently, electronic wave functions have quite poor overlap resulting into absence of continuous band transport. Charge transport in polymers and organic materials, in general, takes place by hopping among the localized states. This makes a bottleneck to the performance of polymer-based solar cells. In this context, the knowledge of charge transport in the solar cell materials is quite important to develop materials and device architectures that boost the efficiency of such solar cells.

Most of the transport studies are based on polyfluorene copolymers and fullerene electron acceptor molecules. Fullerenes are blended with polymers to enhance the dissociation of excited state into free carriers and transport free electrons to the respective electrode. The interaction within the polymer-fullerene complex, therefore, plays a major role in the generation and transport of both electrons and holes. In this thesis, we present and discuss the effect of various polymer/fullerene compositions on hole percolation paths. We mainly focus on hole transport since its mobility is quite small as compared to electron mobility in the fullerenes, leading to creation of spa ce charges within the bulk of the solar cell composite. Changing a polymer band gap may necessitate an appropriate acceptor type in order to fulfill the need for sufficient driving force for dissociation of photogenerated electron-hole pairs. We have observed that different acceptor types give rise to completely different hole mobility in BHJ films. The change of hole transport as a function of acceptor type and concentration is mainly attributed to morphological changes. The effect of the acceptors in connection to hole transport is also discussed. The later is supported by studies of bipolar transport in pure electron acceptor layers. Moreover, the link between charge carrier mobility and photovoltaic parameters has also been studied and presented in this thesis.

The efficiency of polymer/fullerene-based solar cells is also significantly limited by its open-circuit voltage (Voc), a parameter that does not obey the metal-insulator-metal principle due to its complicated characteristics. In this thesis, we address the effect of varying polymer oxidation potential on Voc of the polymer/fullerene BHJ based solar cells. Systematic investigations have been performed on solar cells that comprise several polythiophene polymers blended with a fullerene derivative electron acceptor molecule. The Voc of such solar cells was found to have a strong correlation with the oxidation potential of the polymers. The upper limit to Voc of the aforementioned solar cells is thermodynamically limited by the net internal electric filed generated by the difference in energy levels of the two materials in the blend.

The cost of polymer-based solar cells can be reduced to a great extent through realization of all-plastic and flexible solar cells. This demands the replacement of the metallic components (electrodes) by highly conducting polymer films. While hole conductor polymers are available, low work function polymer electron conductors are rare. In this thesis, prototype solar cells that utilizes a highly conducting polymer, which has a work function of ~ 4.3 eV, as a cathode are demonstrated. Development of this material may eventually lead to fabrication of large area, flexible and cheap solar cells. The transparent nature of the polymer cathode may also facilitate fabrication of multi-layer and tandem solar cells.

In the last chapter of this thesis, we demonstrate generation of red and near infrared polarized light by employing thermally converted thin films of polyfluorene copolymers in light emitting diodes. This study, in particular, aims at fabricating polarized infrared light emitting devices.

Place, publisher, year, edition, pages
Institutionen för fysik, kemi och biologi , 2006.
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1056
Keyword [en]
Bulk heterojunction, Charge transport, Space charge limited, Bipolar transport, Origin of open circuit voltage, Polymer-fullerene blend, Soft contact lamination, Polarized infrared emission
National Category
Physical Sciences
Identifiers
URN: urn:nbn:se:liu:diva-8047ISBN: 91-85643-51-3 (print)OAI: oai:DiVA.org:liu-8047DiVA: diva2:22936
Public defence
2006-11-22, Planck, Fysikhuset, Campus Valla, Linköpings universitet, Linköping, 11:15 (English)
Opponent
Supervisors
Note
On the day of the defence day the status of article III was In press and article VI was Manuscript.Available from: 2007-01-15 Created: 2007-01-15 Last updated: 2009-02-20
List of papers
1. Correlation between oxidation potential and open-circuit voltage of composite solar cells based on blends of polythiophenes/fullerene derivative
Open this publication in new window or tab >>Correlation between oxidation potential and open-circuit voltage of composite solar cells based on blends of polythiophenes/fullerene derivative
2004 (English)In: Applied Physics Letters, ISSN 0003-6951, Vol. 84, no 9, 1609-1611 p.Article in journal (Refereed) Published
Abstract [en]

The photovoltaic parameters of donor/acceptor blend organic solar cells are highly influenced by several parameters, such as the strength of the acceptor species, the morphology of the film due to the solvent, and the mobility of the free charge carriers. In this work, the open-circuit voltage (Voc) of solar cells based on series of conjugated polythiophene polymers were measured and compared. In every cell, the donor polymer was blended with an electron acceptor fullerene molecule. The devices were constructed in a sandwich structure with indium tin oxide (ITO)/metallic polymer (PEDOT:PSS) acting as an anode and Al or LiF/Al acting as a cathode. Comparing the Voc of all the cells shows that this important photovoltaic parameter is systematically varying with the polymer. The variation of photovoltage is attributed to the variation of the oxidation potential of the donor conjugated polymers after due consideration of the different injection conditions in the varying polymers.

National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-14219 (URN)10.1063/1.1650878 (DOI)
Available from: 2007-01-15 Created: 2007-01-15
2. Stoichiometry dependence of charge transport in polymer/methanofullerene and polymer/C70 derivative based solar cells
Open this publication in new window or tab >>Stoichiometry dependence of charge transport in polymer/methanofullerene and polymer/C70 derivative based solar cells
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2006 (English)In: Organic electronics, ISSN 1566-1199, Vol. 7, no 4, 195-204 p.Article in journal (Refereed) Published
Abstract [en]

Charge transport in a near infrared absorbing polyfluorene copolymer (APFO-Green1) and its blends with methanofullerene [6,6]-phenyl C61-butyric acid methyl ester (PCBM), and 3′-(3,5-bis-trifluoromethylphenyl)-1′-(4-nitrophenyl)pyrazolino[70]fullerene (BTPF70) is reported. PCBM and BTPF70 are electron acceptor and transporting molecules in polymer based solar cells. The BTPF70 has emerged as a new electron acceptor molecule that provides adequate exciton dissociation when blended with the low band gap polyfluorene copolymer APFO-Green1. Electron transport in both net PCBM and BTPF70 films are subjected to positional and energetic disorder, with the degree of disorder being more pronounced in BTPF70. On the other hand, mixing PCBM with conjugated polymers usually leads to increased hole mobility. We have investigated and compared the acceptor concentration dependence of charge transport in APFO-Green1/PCBM and APFO-Green1/BTPF70 blend films. For better understanding of the charge transport in the heterojunction films, the field and temperature dependence of hole transport in pure APFO-Green1 films has also been studied. It is observed that the behavior of hole mobility in the blend layer is sensitive to the acceptor type. For APFO-Green1/PCBM hole only devices, the hole mobility attains a local maximum at 67 wt.% of PCBM, while on the contrary mixing any amount of BTPF70 with APFO-Green1 results into degradation of hole transport. Electron transport in both blends, however, increases monotonically as a function of acceptor loading.

Keyword
Solar cells; Acceptor concentration; Charge transport; Space charge limited current
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-14220 (URN)10.1016/j.orgel.2006.01.003 (DOI)
Available from: 2007-01-15 Created: 2007-01-15 Last updated: 2009-10-05
3. Improvements of fill factor in solar cells based on blends of polyfluorene copolymers as electron donors
Open this publication in new window or tab >>Improvements of fill factor in solar cells based on blends of polyfluorene copolymers as electron donors
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2007 (English)In: Thin Solid Films, ISSN 0040-6090, Vol. 515, no 5, 3126-3131 p.Article in journal (Refereed) Published
Abstract [en]

The photovoltaic characteristics of solar cells based on alternating polyfluorene copolymers, poly(2,7-(9,9-dioctyl-fluorene)-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)) (APFO-3), and poly(2,7-(9,9-didodecyl-fluorene)-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)) (APFO-4), blended with an electron acceptor fullerene molecule [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), have been investigated and compared. The two copolymers have the same aromatic backbone structure but differ by the length of their alkyl side chain. The overall photovoltaic performance of the solar cells is comparable irrespective of the copolymer used in the active layer. However, the fill factor (FF) values of the devices are strongly affected by the copolymer type. Higher FF values were realized in solar cells with APFO-4 (with longer alkyl side chain)/PCBM bulk heterojunction active layer. On the other hand, devices with blends of APFO-3/APFO-4/PCBM were found to render fill factor values that are intermediate between the values obtained in solar cells with APFO-3/PCBM and APFO-4/PCBM active film. Upon using APFO-3/APFO-4 blends as electron donors, the cell efficiency can be enhanced by about 16% as compared to cells with either APFO-3 or APFO-4. The transport of holes in each polymer obeys the model of hopping transport in disordered media. However, the degree of energetic barrier against hopping was found to be larger in APFO-3. The tuning of the photovoltaic parameters will be discussed based on studies of hole transport in the pure polymer films, and morphology of blend layers. The effect of bipolar transport in PCBM will also be discussed.

Keyword
Solar cells, Photovoltaic characteristics, Charge transport
National Category
Physical Sciences
Identifiers
urn:nbn:se:liu:diva-14221 (URN)10.1016/j.tsf.2006.08.043 (DOI)
Available from: 2007-01-15 Created: 2007-01-15 Last updated: 2009-04-28
4. Bipolar transport and infrared light emission in C60 and C70 derivative electron acceptors
Open this publication in new window or tab >>Bipolar transport and infrared light emission in C60 and C70 derivative electron acceptors
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2006 (English)In: Applied Physics Letters, ISSN 0003-6951Article in journal (Refereed) Submitted
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-14222 (URN)
Available from: 2007-01-15 Created: 2007-01-15
5. Transparent polymer cathode for organic photovoltaic devices
Open this publication in new window or tab >>Transparent polymer cathode for organic photovoltaic devices
Show others...
2006 (English)In: Synthetic metals, ISSN 0379-6779, Vol. 156, no 16-17, 1102-1107 p.Article in journal (Refereed) Published
Abstract [en]

We demonstrate a prototype solar cell with a transparent polymer cathode, and indium-tin-oxide (ITO)/poly (3, 4-ethylene dioxythiophene)-poly (styrene sulphonate) (PEDOT:PSS) anode. As an active layer, thin film of a bulk heterojunction of polyfluorene copolymer poly[2,7-(9,9-dioctyl-fluorene)-alt-5,5-(4′,7′-di-2thienyl-2′,1′3′-benzothiadiazole)] (APFO-3) and an electron acceptor molecule [6] and [6]-phenyl-C61-butyric acid methyl ester (PCBM) (1:4 wt.) was sandwiched between the two transparent polymer electrodes. The cathode is another form of PEDOT formed by vapor phase polymerised PEDOT (VPP PEDOT) of conductivity 102–103 S/cm. The cathode is supported on an elastomeric substrate, and forms a conformal contact to the APFO-3/PCBM blend. Transparent solar cells are useful for building multilayer and tandem solar cells.

Keyword
PEDOT; Vapor phase polymerization; VPP–PEDOT; Soft contact lamination; All-plastic solar cell; Ultraviolet photoelectron spectroscopy
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-14223 (URN)10.1016/j.synthmet.2006.07.006 (DOI)
Available from: 2007-01-15 Created: 2007-01-15 Last updated: 2013-09-12Bibliographically approved
6. Red and near infrared polarized light emission from polyfluorene copolymer based light emitting diodes
Open this publication in new window or tab >>Red and near infrared polarized light emission from polyfluorene copolymer based light emitting diodes
2007 (English)In: Applied Physics Letters, ISSN 0003-6951, Vol. 90, no 11, 113510- p.Article in journal (Refereed) Published
Abstract [en]

The authors report polarized red, electroluminescence peak at 705  nm and near infrared, electroluminescence peak at 950  nm, light emission from light emitting diodes based on two polyfluorene copolymers. The copolymers are synthesized from a fluorene monomer combined with donor-acceptor-donor comonomers and designed to have a low band gap and form birefringent liquid crystalline phases. Emission occurs from aligned thin films of polymer layers. The emissive layers are aligned by spin coating on a layer of rubbed conducting polymer poly(3,4-ethylene dioxythiophene)-poly(styrene sulphonate) and thermally converted into glassy nematic liquid crystalline state.

National Category
Physical Sciences
Identifiers
urn:nbn:se:liu:diva-14224 (URN)10.1063/1.2713139 (DOI)
Available from: 2007-01-15 Created: 2007-01-15 Last updated: 2009-04-28

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