Programm Photovoltaik Ausgabe 2009 ... - Bundesamt für Energie BFE
Programm Photovoltaik Ausgabe 2009 ... - Bundesamt für Energie BFE Programm Photovoltaik Ausgabe 2009 ... - Bundesamt für Energie BFE
Eidgenössisches Departement für Umwelt, Verkehr, Energie und Kommunikation UVEK Bundesamt für Energie BFE ORGANIC PHOTOVOLTAIC DEVICES NANOSCALE STRUCTURING OF IONIC DYES IN THIN SEMICONDUCTING BLEND FILMS Annual Report 2008 Author and Co-Authors R. Hany, F.A. Castro, F. Nüesch, J. Heier Institution / Company Empa, Lab for Functional Polymers Address Überlandstr. 129, CH-8600 Dübendorf Telephone, E-mail, Homepage +41 (0) 44 823 40 84, roland.hany@empa.ch, www.empa.ch Project- / Contract Number Duration of the Project (from – to) 2007 - 2010 Date 10.12.2008 ABSTRACT The controlled fabrication of submicron phase-separated morphologies of semiconducting organic materials is attracting considerable interest, for example in emerging thin-film optoelectronic device applications. We use the phenomenon of liquid-liquid dewetting during spin coating to fabricate films of cyanine dye / PCBM blends in a variety of morphologies with tunable dimensions below 100 nm and a large-area interface. The structure formation mechanism proceeds via a transient bilayer, which further spinodally destabilizes because of long-range molecular interactions. We developed a thermodynamic model showing that electrostatic forces induced by the mobile cyanine counter ions act as destabilizing pressure. We study the quantitative aspect of the film rupture mechanism and will combine this unique patterning methodology with the concept of cyanine dye doping for the fabrication of efficient organic solar cells. 131/290
Introduction and objectives Excitonic heterojunction solar cells based on semiconducting organic small molecules and polymers are promising devices for inexpensive, large-scale solar energy conversion. Organic solar cells can be fabricated by simple coating processes from solution, thereby preserving the low-cost potential that these materials offer. Organic components with matched optical and electronic properties are obviously important for high light-to-current conversion. In addition, the nanoscale arrangement of the materials in the thin-film geometry needs to be carefully controlled. Indeed, charge generation within the ~10 nm diffusion length of the primary photoexcitation and loss-free charge transport to the electrodes via percolating paths requires phase-separated domains on multiple length scales. Detailed studies on the organization of phase-separating organic blends, however, are still very limited, and the driving forces are poorly understood. This project specifically relates to the controlled arrangement of ionic cyanine dyes in thin films of twocomponent mixtures. Cyanine dyes have several attractive properties for use in organic photovoltaic devices [1-4]: Cyanine dyes have good film-forming properties, very high extinction coefficients and the light absorption can simply be tuned over the visible to the near-infrared region using structural analogues. As generally true for small molecules, cyanines are more easily to purify than their polymeric counterparts. We recently reported a planar bilayer solar cell with a promising efficiency of 1.2% when using a cyanine dye as electron donor and buckminsterfullerene C60 as acceptor [2]. Increased performance was achieved via light-induced cyanine doping by oxygen and water. From this we conclude that a cyanine dye solar cell with an optimized morphology can potentially reach much higher efficiencies and effectively compete with other materials. Work performed and results obtained We found that thin films coated from a cyanine dye / PCBM (a soluble C60 derivative) mixture show sub-micrometer phase-separated morphologies (Fig. 1) [5,6]. We identified that the mechanism leading to these morphologies occurs by liquid-liquid dewetting. Liquid-liquid dewetting proceeds via a transient bilayer that forms during coating driven by differences in surface tension, and this layer is destabilized by long range interfacial interactions. The destabilization forces do not only act at the liquid-air interface, but also at the liquid-liquid interface, leading to significant interfacial roughening. The dispersive force driving the instability is counterbalanced by the inherent energy cost of having a large interface, leading to thickness fluctuations with a characteristic spinodal wavelength. Fig. 1. Left: Diagram showing how dewetting patterns evolve from destabilizing double layers. Right: The developing morphology depends on the processing parameters [5,6] and can consist, for example, of droplets of one component embedded in the film-forming second component, or of laterally demixed structures. Organic photovoltaic devices, F. Nüesch,Empa 132/290 2/4
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Eidgenössisches Departement <strong>für</strong><br />
Umwelt, Verkehr, <strong>Energie</strong> und Kommunikation UVEK<br />
<strong>Bundesamt</strong> <strong>für</strong> <strong>Energie</strong> <strong>BFE</strong><br />
ORGANIC PHOTOVOLTAIC DEVICES<br />
NANOSCALE STRUCTURING OF IONIC DYES<br />
IN THIN SEMICONDUCTING BLEND FILMS<br />
Annual Report 2008<br />
Author and Co-Authors R. Hany, F.A. Castro, F. Nüesch, J. Heier<br />
Institution / Company Empa, Lab for Functional Polymers<br />
Address Überlandstr. 129, CH-8600 Dübendorf<br />
Telephone, E-mail, Homepage +41 (0) 44 823 40 84, roland.hany@empa.ch, www.empa.ch<br />
Project- / Contract Number<br />
Duration of the Project (from – to) 2007 - 2010<br />
Date 10.12.2008<br />
ABSTRACT<br />
The controlled fabrication of submicron phase-separated morphologies of semiconducting organic<br />
materials is attracting considerable interest, for example in emerging thin-film optoelectronic device<br />
applications. We use the phenomenon of liquid-liquid dewetting during spin coating to fabricate films<br />
of cyanine dye / PCBM blends in a variety of morphologies with tunable dimensions below 100 nm<br />
and a large-area interface. The structure formation mechanism proceeds via a transient bilayer, which<br />
further spinodally destabilizes because of long-range molecular interactions. We developed a thermodynamic<br />
model showing that electrostatic forces induced by the mobile cyanine counter ions act<br />
as destabilizing pressure. We study the quantitative aspect of the film rupture mechanism and will<br />
combine this unique patterning methodology with the concept of cyanine dye doping for the fabrication<br />
of efficient organic solar cells.<br />
131/290
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