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Lateral Phase Separation Gradients in Spin-Coated Thin Films of High-Performance Polymer: Fullerene Photovoltaic Blends
Linköpings universitet, Institutionen för fysik, kemi och biologi, Biomolekylär och Organisk Elektronik. Linköpings universitet, Tekniska högskolan.
Chalmers.
Linköpings universitet, Institutionen för fysik, kemi och biologi, Biomolekylär och Organisk Elektronik. Linköpings universitet, Tekniska fakulteten.
Linköpings universitet, Institutionen för fysik, kemi och biologi, Biomolekylär och Organisk Elektronik. Linköpings universitet, Tekniska fakulteten.
Vise andre og tillknytning
2011 (engelsk)Inngår i: Advanced Functional Materials, ISSN 1616-301X, E-ISSN 1616-3028, Vol. 21, nr 16, s. 3169-3175Artikkel i tidsskrift (Fagfellevurdert) Published
Abstract [en]

In this study, it is demonstrated that a finer nanostructure produced under a rapid rate of solvent removal significantly improves charge separation in a high-performance polymer: fullerene bulk-heterojunction blend. During spin-coating, variations in solvent evaporation rate give rise to lateral phase separation gradients with the degree of coarseness decreasing away from the center of rotation. As a result, across spin-coated thin films the photocurrent at the first interference maximum varies as much as 25%, which is much larger than any optical effect. This is investigated by combining information on the surface morphology of the active layer imaged by atomic force microscopy, the 3D nanostructure imaged by electron tomography, film formation during the spin coating process imaged by optical interference and photocurrent generation distribution in devices imaged by a scanning light pulse technique. The observation that the nanostructure of organic photovoltaic blends can strongly vary across spin-coated thin films will aid the design of solvent mixtures suitable for high molecular-weight polymers and of coating techniques amenable to large area processing.

sted, utgiver, år, opplag, sider
Wiley-VCH Verlag Berlin , 2011. Vol. 21, nr 16, s. 3169-3175
HSV kategori
Identifikatorer
URN: urn:nbn:se:liu:diva-70526DOI: 10.1002/adfm.201100566ISI: 000294166200019OAI: oai:DiVA.org:liu-70526DiVA, id: diva2:440096
Merknad

Funding Agencies|Swedish Energy Agency||Spanish Ministerio de Ciencia e Innovacion||

Tilgjengelig fra: 2011-09-12 Laget: 2011-09-12 Sist oppdatert: 2017-12-08bibliografisk kontrollert
Inngår i avhandling
1. Electron tomography and optical modelling for organic solar cells
Åpne denne publikasjonen i ny fane eller vindu >>Electron tomography and optical modelling for organic solar cells
2012 (engelsk)Doktoravhandling, med artikler (Annet vitenskapelig)
Abstract [en]

Organic solar cells using carbon based materials have the potential to deliver cheap solar electricity. The aim is to be able to produce solar cells with common printing techniques on flexible substrates, and as organic materials can be made soluble in various solvents, they are well adapted to such techniques. There is a large variation of organic materials produced for solar cells, both small molecules and polymers. Alterations of the molecular structure induce changes of the electrical and optical properties, such as band gap, mobility and light absorption. During the development of organic solar cells, the step of mixing of an electron donor and an electron acceptor caused a leap in power conversion efficiency improvement, due to an enhanced exciton dissociation rate. Top performing organic solar cells now exhibit a power conversion efficiency of over 10%. Currently, a mix of a conjugated polymer, or smaller molecule, and a fullerene derivative are commonly used as electron donor and acceptor. Here, the blend morphology plays an important role. Excitons formed in either of the donor or acceptor phase need to diffuse to the vicinity of the donor-acceptor interface to efficiently dissociate. Exciton diffusion lengths in organic materials are usually in the order of 5-10 nm, so the phases should not be much larger than this, for good exciton quenching. These charges must also be extracted, which implies that a network connected to the electrodes is needed. Consequently, a balance of these demands is important for the production of efficient organic solar cells.

Morphology has been found to have a significant impact on the solar cell behaviour and has thus been widely studied. The aim of this work has been to visualize the morphology of active layers of organic solar cells in three dimensions by the use of electron tomography. The technique has been applied to materials consisting of conjugated polymers blended with fullerene derivatives. Though the contrast in these blends is poor, three-dimensional reconstructions have been produced, showing the phase formation in three dimensions at the scale of a few nanometres. Several material systems have been investigated and preparation techniques compared.

Even if excitons are readily dissociated and paths for charge extraction exist, the low charge mobilities of many materials put a limit on film thickness. Although more light could be absorbed by increased film thickness, performance is hampered due to increased charge recombination. A large amount of light is thus reflected and not used for energy conversion. Much work has been put into increasing the light absorption without hampering the solar cell performance. Aside from improved material properties, various light trapping techniques have been studied. The aim is here to increase the optical path length in the active layer, and in this way improve the absorption without enhanced extinction coefficient.

At much larger dimensions, light trapping in solar cells with folded configuration has been studied by the use of optical modelling. An advantage of these V-cells is that two materials with complementing optical properties may be used together to form a tandem solar cell, which may be connected in either serial or parallel configuration, with maintained light trapping feature. In this work optical absorption in V-cells has been modelled and compared to that of planar ones.

sted, utgiver, år, opplag, sider
Linköping: Linköping University Electronic Press, 2012. s. 63
Serie
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1414
Emneord
Organic solar cells, Electron tomography, Optical modelling
HSV kategori
Identifikatorer
urn:nbn:se:liu:diva-72903 (URN)978-91-7393-007-9 (ISBN)
Disputas
2012-02-03, Planck, Fysikhuset, Campus Valla, Linköpings universitet, Linköping, 10:15 (engelsk)
Opponent
Veileder
Tilgjengelig fra: 2011-12-13 Laget: 2011-12-09 Sist oppdatert: 2012-01-03bibliografisk kontrollert
2. Optoelectrical Imaging Methods for Organic Photovoltaic Materials and Moduls
Åpne denne publikasjonen i ny fane eller vindu >>Optoelectrical Imaging Methods for Organic Photovoltaic Materials and Moduls
2015 (engelsk)Doktoravhandling, med artikler (Annet vitenskapelig)
Abstract [en]

To achieve a high living standard for all people on Earth access to low cost energy is essential. The massive burning of fossil fuels must be drastically reduced if we are to avoid large changes of our climate. Solar cells are both technologically mature and have the potential to meet the huge demand for renewable energy in many countries. The prices for silicon solar cells have decreased rapidly during the course of this thesis and are now in grid parity in many countries.

However, the potential for even lower energy costs has driven the research on polymer solar cells, a class of thin film solar cells. Polymer solar cells can be produced by roll to roll printing which potentially enables truly low cost solar cells. However, much research and development remain to reach that target.

Polymer solar cells consist of a semiconducting composite material sandwiched between two electrodes, of which one is transparent, to let the light energy in to the semiconductor where it is converted to electric energy. The semiconductor comprise an intimate blend of polymer and fullerenes, where the nanostructure of this blend is crucial for the photo current extraction.

To reach higher solar cell performance the dominating strategy is development and fine tuning of new polymers. To estimate their potential as solar cell materials their optical response have been determined by spectroscopic ellipsometry. Furthermore, optical simulations have been performed where the direction dependency of the optical response of the transparent electrode material PEDOT:PSS have been accounted for. The simulations show reduced electrode losses for light incident at large oblique angles.

Moreover, we have shown that a gentle annealing of the active layer induces a local conformational changes of an amorphous polymer that is beneficial for solar cell performance. The active layer is deposited from solution where the drying kinetics determine the final nanostructure. We have shown that using in-situ photoluminescence phase separation can be detected during the drying process while a reflectance method have been developed to image lateral variations of solvent evaporation rate.

Imaging methods are important tools to detect performance variations over the solar cell area. For this purpose an intermodulation based photo current imaging method have been developed to qualitatively differentiate the major photo current loss mechanisms. In addition, a 1D LED-array photo current imaging method have been developed and verified for high speed in-line characterization of printed organic solar modules.

sted, utgiver, år, opplag, sider
Linköping: Linköping University Electronic Press, 2015. s. 60
Serie
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1712
HSV kategori
Identifikatorer
urn:nbn:se:liu:diva-123035 (URN)10.3384/diss.diva-123035 (DOI)978-91-7685-923-0 (ISBN)
Disputas
2015-12-17, Planck, Fysikhuset, Campus Valla, Linköping, 13:15 (engelsk)
Opponent
Veileder
Merknad

The corrections in the published errata list are implemented in the electronic version.

Tilgjengelig fra: 2015-12-03 Laget: 2015-12-02 Sist oppdatert: 2016-01-14bibliografisk kontrollert

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