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Time-resolved morphology formation of solution cast polymer: fullerene blends revealed by in-situ photoluminescence spectroscopy
Linköping University, Department of Physics, Chemistry and Biology, Biomolecular and Organic Electronics. Linköping University, Faculty of Science & Engineering.
Linköping University, Department of Physics, Chemistry and Biology, Biomolecular and Organic Electronics. Linköping University, Faculty of Science & Engineering.
Linköping University, Department of Physics, Chemistry and Biology, Chemical and Optical Sensor Systems. Linköping University, Faculty of Science & Engineering.
INTERACT, Department of Engineering and Physics, Karlstad University, Karlstad, Sweden.
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2015 (English)Manuscript (preprint) (Other academic)
Abstract [en]

The nanoscale morphology of the photo-active layer in organic solar cells is critical for device efficiency. The photoactive layer is cast from solution and during drying both the polymer and the fullerene self-assemble to form a blend. Here, we introduce in-situ spectroscopic photoluminescence (PL) combined with laser reflectometry to monitor the drying process of an amorphous polymer:fullerene blend. When casting only the pristine components (polymer or PCBM only), the strength of PL emission is proportional to the solid content of the drying solution, and both kinetics reveal a rapid aggregation onset at the final stage of film drying. On the contrary, when casting polymer:fullerene blends, the strength of PL emission is proportional to the wet film thickness and reveals polymer/fullerene charge transfer (CT) already at the earliest stages of film drying, i.e. in dilute solutions. The proposed method allows to detect polymer/fullerene phase separation during film casting – from a reduction in the PL quenching rate as the film dries. Poor solvents lead to phase separation already at early stages of film drying (low solid content), resulting in a coarse final morphology as confirmed by atomic force microscopy (AFM). We therefore anticipate that the proposed method will be an important tool in the future development of processing inks, not only for solution-cast polymer:fullerene solar cells but also for organic heterojunctions in general.

Place, publisher, year, edition, pages
2015.
National Category
Physical Sciences Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
URN: urn:nbn:se:liu:diva-123032OAI: oai:DiVA.org:liu-123032DiVA: diva2:876138
Available from: 2015-12-02 Created: 2015-12-02 Last updated: 2015-12-03
In thesis
1. Optoelectrical Imaging Methods for Organic Photovoltaic Materials and Moduls
Open this publication in new window or tab >>Optoelectrical Imaging Methods for Organic Photovoltaic Materials and Moduls
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
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.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2015. 60 p.
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1712
National Category
Physical Sciences
Identifiers
urn:nbn:se:liu:diva-123035 (URN)10.3384/diss.diva-123035 (DOI)978-91-7685-923-0 (print) (ISBN)
Public defence
2015-12-17, Planck, Fysikhuset, Campus Valla, Linköping, 13:15 (English)
Opponent
Supervisors
Note

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

Available from: 2015-12-03 Created: 2015-12-02 Last updated: 2016-01-14Bibliographically approved

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