A real time study on the processing and morphology of polymer:fullerene solar cells

Rene A. J. Janssen^a, Martijn M. Wienk^a, Jacobus Johannes van Franeker^a^,^b

^aMolecular Materials and Nanosystems, Eindhoven University of Technology, Netherlands, Netherlands

^bDutch Polymer Institute (DPI), ), P.O. Box 902, 5600 AX Eindhoven, Netherlands

International Conference on Hybrid and Organic Photovoltaics
Proceedings of International Conference on Hybrid and Organic Photovoltaics 2015 (HOPV15)

Roma, Italy, 2015 May 11th - 13th

Organizer: Filippo De Angelis

Poster, Jacobus Johannes van Franeker, 006
Publication date: 5th February 2015

Dominant length scales of the morphology of bulk-heterojunction polymer:fullerene solar cells are defined during drying of the photoactive film. These length scales are crucially important for the efficiency of these solar cells. To study the kinetics of film and morphology formation time-resolved studies during drying are necessary, in this case during the solvent evaporation stage while spin coating. Here a diketopyrrolopyrrole-based polymer is used with a fullerene derivative, deposited from solvents with and without co-solvent. Using in-situ time-resolved optical techniques, combined with quantitative analysis of ex-situ transmission electron microscopy, we are able to determine the parameters that influence the dominant length scales in the phase separated films [1]. Without co-solvents, liquid-liquid phase separation results in large fullerene domains. We show that the size of these domains is determined by the drying rate normalized to the final thickness. On other hand, when co-solvents are used, the dominant length scale is not determined by liquid-liquid demixing, but by polymer aggregation [2]. The nature of the co-solvent mixture can be adjusted to control the length scales of the polymer fibers that are formed by the aggregation.
The upper panel indicates the effect of co-solvents on the morphology of the photoactive film of polymer:fullerene solar cells, as studied by transmission electron microscopy. The scalebar is 600 nm. As co-solvent 1,8-diiodooctane (DIO) is used. The lower panel shows a schematic of the real time measurements which are used to study wet layer thickness, liquid-liquid phase separation and polymer aggregation during spin coating.
[1] van Franeker, J. J.; Westhoff, D.; Turbiez, M.; Wienk, M. M.; Schmidt, V.; and Janssen, R. A. J. Controlling the Dominant Length Scale of Liquid–Liquid Phase Separation in Spin-coated Organic Semiconductor Films. Adv. Funct. Mater. Early View. doi: 10.1002/adfm.201403392 [2] van Franeker, J. J.; Turbiez, M.; Li, W.; Wienk, M. M.; Janssen, R. A. J. A real-time study of the benefits of co-solvents in polymer solar cell processing. Nature Communications (under revision)

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