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Improvements of fill factor in solar cells based on blends of polyfluorene copolymers as electron donors
Linköping University, Department of Physics, Chemistry and Biology, Biomolecular and Organic Electronics . Linköping University, The Institute of Technology.
Linköping University, Department of Physics, Chemistry and Biology, Biomolecular and Organic Electronics . Linköping University, The Institute of Technology.
Department of Electrical Engineering, Indian Institute of Technology Kanpur, Kanpur, India.
Department of Organic Chemistry and Polymer Technology, Chalmers University of Technology, Göteborg, Sweden.
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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.

Place, publisher, year, edition, pages
2007. Vol. 515, no 5, 3126-3131 p.
Keyword [en]
Solar cells, Photovoltaic characteristics, Charge transport
National Category
Physical Sciences
URN: urn:nbn:se:liu:diva-14221DOI: 10.1016/j.tsf.2006.08.043OAI: diva2:22932
Available from: 2007-01-15 Created: 2007-01-15 Last updated: 2009-04-28
In thesis
1. Studies of Charge Transport and Energy Level in Solar Cells Based on Polymer/Fullerene Bulk Heterojunction
Open this publication in new window or tab >>Studies of Charge Transport and Energy Level in Solar Cells Based on Polymer/Fullerene Bulk Heterojunction
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
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1056
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
urn:nbn:se:liu:diva-8047 (URN)91-85643-51-3 (ISBN)
Public defence
2006-11-22, Planck, Fysikhuset, Campus Valla, Linköpings universitet, Linköping, 11:15 (English)
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

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