| 1. |
- Andersson, Mattias, et al.
(författare)
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Mixed C60/C70 based fullerene acceptors in polymer bulk-heterojunction solar cells
- 2012
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Ingår i: Organic Electronics: physics, materials, applications. - : Elsevier BV. - 1566-1199 .- 1878-5530. ; 13:12, s. 2856-2864
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Tidskriftsartikel (refereegranskat)abstract
- Different mixtures of identically substituted C60 and C70 based fullerens have been used as acceptors in three polymer: fullerene systems that strongly express various performance limiting aspects of bulk heterojunction solar cells. Results are correlated with, and discussed in terms of e.g. morphology, charge separation, and charge transport. In these systems, there appears to be no relevant differences in either mobility or energy level positions between the identically substituted C60 and C70 based fullerenes tested. Examples of how fullerene mixtures influence the nano-morphology of the active layer are given. An upper limit to the open circuit voltage that can be obtained with fullerenes is also suggested.
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| 2. |
- Andersson, Mattias, et al.
(författare)
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Mobility and fill factor correlation in geminate recombination limited solar cells
- 2011
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Ingår i: Journal of Applied Physics. - : American Institute of Physics (AIP). - 0021-8979 .- 1089-7550. ; 110:2, s. 024509-
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Tidskriftsartikel (refereegranskat)abstract
- Empirical data for the fill factor as a function of charge carrier mobility for two different polymer: fullerene systems is presented and analyzed. The results indicate that charge extraction depth limitations and space charge effects are inconsistent with the observed behavior, and the decrease in the fill factor is, instead, attributed to the field-dependent charge separation and geminate recombination. A solar cell photocurrent limited by the Onsager-Braun charge transfer exciton dissociation is shown to be able to accommodate the experimental observations. Charge dissociation limited solar cells always benefit from increased mobilities, and the negative contribution from the reduced charge separation is shown to be much more important for the fill factor in these material systems than any adverse effects from charge carrier extraction depth limitations or space charge effects due to unbalanced mobilities. The logarithmic dependence of the fill factor on the mobility for such a process is also shown to imply that simply increasing the mobilities is an impractical way to reach very high fill factors under these conditions since unrealistically high mobilities are required. A more controlled morphology is, instead, argued to be necessary for high performance.
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| 3. |
- Andersson, Mattias, et al.
(författare)
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Unified Study of Recombination in Polymer:Fullerene Solar Cells Using Transient Absorption and Charge-Extraction Measurements
- 2013
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Ingår i: Journal of Physical Chemistry Letters. - : American Chemical Society. - 1948-7185 .- 1948-7185. ; 4:12, s. 2069-2072
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Tidskriftsartikel (refereegranskat)abstract
- Recombination in the well-performing bulk heterojunction solar cell blend between the conjugated polymer TQ-1 and the substituted fullerene PCBM has been investigated with pump-probe transient absorption and charge extraction of photo-generated carriers (photo-CELIV). Both methods are shown to generate identical and overlapping data under appropriate experimental conditions. The dominant type of recombination is bimolecular with a rate constant of 7 x 10(-12) cm(-3) s(-1). This recombination rate is shown to be fully consistent with solar cell performance. Deviations from an ideal bimolecular recombination process, in this material system only observable at high pump fluences, are explained with a time-dependent charge-carrier mobility, and the implications of such a behavior for device development are discussed.
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| 4. |
- Andersson, Viktor
(författare)
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Electron tomography and optical modelling for organic solar cells
- 2012
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- 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.
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| 5. |
- Andersson, Viktor, et al.
(författare)
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Full day modelling of V-shaped organic solar cell
- 2011
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Ingår i: Solar Energy. - : Elsevier Science B.V., Amsterdam.. - 0038-092X .- 1471-1257. ; 85:6, s. 1257-1263
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Tidskriftsartikel (refereegranskat)abstract
- Folded and planar solar cells are examined with optical simulations, with the finite element method. The maximum photocurrent densities during the full day are compared between cells of different geometries and tilting angles. The change of incident angle and spectrum over time are handled in this analysis. The results show that the light trapping effect of the folded cell makes these cells show higher maximum photocurrent densities than the planar cells during all hours of the day. This is the case for both single and tandem cells. The results also indicate that balancing the currents in the tandem cells by adjusting the active layer thickness may be more cumbersome with the folded tandem cells than the stacked planar cells.
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| 6. |
- Andersson, Viktor, et al.
(författare)
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Morphology of organic electronic materials imaged via electron tomography
- 2012
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Ingår i: Journal of Microscopy. - : Wiley. - 0022-2720 .- 1365-2818. ; 247:3, s. 277-287
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Tidskriftsartikel (refereegranskat)abstract
- Several organic materials and blends have been studied with the use of electron tomography. Tomography reconstructions of active layers of organic solar cells, where various preparation techniques have been used, have been analysed and compared to device behaviour. In addition, materials with predefined structures, including contrast enhancing features, have been studied and double tilt data collection has been employed to improve reconstructions. Small changes in preparation procedures may lead to large differences in morphology and device performance, and the results also indicate a complex relation between these.
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| 7. |
- Asplund, Maria, et al.
(författare)
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Electroactive polymers for neural interfaces
- 2010
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Ingår i: Polymer chemistry. - : Royal Society of Chemistry (RSC). - 1759-9954 .- 1759-9962. ; 1:9, s. 1374-1391
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Forskningsöversikt (refereegranskat)abstract
- Development of electroactive conjugated polymers, for the purpose of recording and eliciting signals in the neural systems in humans, can be used to fashion the interfaces between the two signalling systems of electronics and neural systems. The design of desirable chemical, mechanical and electrical properties in the electroactive polymer electrodes, and the means of integration of these into biological systems, are here reviewed.
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| 8. |
- Bergqvist, Jonas, et al.
(författare)
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Determination of optical constants and phase transition temperatures in polymer fullerene thin films for polymer solar cells
- 2012
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Plastic photovoltaics combining semiconducting polymers with fullerene derivatives have the potentialto become the first cost efficient solar cells able to compete with fossil fuels. The maximum powerconversion efficiency is already 8.3%[1] , and new polymers arrive frequently in the search for efficienciesof 10%. As a first step in the screening of candidate materials, the optical constants of the purepolymer as well as the polymer blend with fullerenes are determined from Variable Angle SpectroscopicEllipsometry (VASE), using Tauc-Lorentz oscillator models, throughout the solar spectrum. Thesemodels are then used to predict the upper limits to photocurrent generation in devices, in transfermatrix simulations of the multilayer thin film photovoltaic devices. This forms an essential step in thechoice of materials for optimization in devices.Materials optics measurements are also used to deduce the phase diagram of polymer and polymerblend films. The glass transition temperature is very important for plastic solar cells and mustbe higher than the 80C a device can reach to avoid degradation during operation. Temperaturedependent ellipsometric measurements has proven to be a feasible way to determine phase transitionsin polymer thin films[2] . These transitions are displayed as a sudden change of the volumetricexpansion coefficient, and are manifested by an abrupt increase of thickness at the phase transitiontemperature. For thickness determination a Cauchy model is applied to the transparent infrared partof the spectra.References1. Z. He, C. Zhong, X. Huang, W-Y. Wong, H. Wu, L. Chen, S. Su, Y Cao, Advanced Materials 23, 4636(2011)2. M. Campoy-Quiles, P.G. Etchegoin, D.D.C. Bradley, Synthetic Metals 155, 279(2005)
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| 9. |
- Bergqvist, Jonas, et al.
(författare)
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In situ reflectance imaging of organic thin film formation from solution
- 2012
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- The rapid progress of organic photovoltaic devices during the last decade, with power conversion efficiencies now exceeding 8%, has brought the technology close to an industrial breakthrough. For polymer solar cells, roll to roll printing is desired to gain the production advantage. The formation of the photoactive material from solutions needs to be controlled and optimized. Therefore a suitable method to monitor the deposition process is needed as deviations of drying times1 and drying rates2 during the coating process have proven to generate morphology variations causing variations in photocurrent generation.Here we demonstrate how reflectance imaging can be used to monitor the drying process, both for spin coating and blade coating deposition. A blue LED is used as light source to generate specular reflections imaged by a CMOS camera. The thinning of the wet film can then be observed by thin film interference, and can be recorded for each pixel. This enables an estimation of the evaporation rate for each pixel mapped over the substrate. For spin coating the evaporation rate is shown to increase with the distance from the rotation center, whereas the air flow is the determining parameter during blade coating. By mapping the times when interference ceases, lateral variations in drying time are visualized. Furthermore the quenching of polymer photoluminescence during the drying process can be visualized, thus creating a possibility to estimate morphological variations. Moreover lateral thickness variations of the dry film can be visualized by scanning ellipsometry. After depositing a top electrode photocurrent images can be generated by a laser scanning method. This allows for a direct comparison of drying conditions and photocurrent generation. The possibility to monitor the thin film formation as well as lateral variations in thickness in-situ by a non-invasive method, is an important step for future large scale applications where stable high performing generating morphologies have to be formed over large areas.1Schmidt-Hansberg, B.; Sanyal, M.; Klein, M.F.G.; Pfaff, M.; Schnabel, N.; Jaiser, S.; Vorobiev, A.; Müller, E.; Colsmann, A.; Scharfer, P.; Gerthsen, D.; Lemmer, U.; Barrena, E.; and Schabel, W., ACS Nano 5 , 2011, 8579-85902 Hou, L.; Wang, E.; Bergqvist, J.; Andersson, V.B.; Wang, Z.; Müller, C.; Campoy-Quiles, M.; Andersson, M.R.; Zhang, F.; Inganäs, O.,Adv. Func. Mat. 21 , 2011, 3169–3175
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| 10. |
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