Phase Separation and Molecular Intermixing in Polymer- Fullerene Bulk Heterojunction Thin Films

(1)

Phase Separation and Molecular Intermixing in Polymer- Fullerene Bulk Heterojunction Thin Films

M. A. Ruderer

¹

, R. Meier

¹

, L. Porcar

²

, R. Cubitt

²

and P. Müller-Buschbaum

¹

1 Technische Universität München, Physik-Department, Lehrstuhl für Funktionelle Materialien, James-Franck-Str. 1, 85748 Garching, Germany

2 Institut Laue Langevin (ILL), 6 Jules Horowitz, 38042 Grenoble, France E-Mail: muellerb@ph.tum.de

The performance of solution processible organic photovoltaic devices improved continuously during the last years. Over the recent years continuous progress was made concerning the efficiency of polymer solar cells reaching meanwhile values above 10%.¹ Major improvements were achieved with the bulk heterojunction concept^2,3, due to the increased inner acceptor-donor interface, and with the development of new semi-conducting small band-gap polymers. However, despite the increase in efficiencies, deep fundamental understanding is still limited.

In the last years the blend out of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61 butyric acid methyl ester (PCBM) established as the best investigated system in organic photovoltaic devices.⁴ While these investigations addressed mainly domain structures or the diffusion and the miscibility of PCBM in the P3HT phase, we focus in the presented work on the detailed structure, i.e. phase sizes, structural length scales, and molecular miscibility of the components, in a P3HT:PCBM bulk heterojunction. Grazing incidence small angle neutron scattering (GISANS) combined with a detailed analysis is used to extract structural information as well as molecular mixing of P3HT and PCBM films.⁵ The analysis is based on the distorted wave Born approximation (DWBA) as shown in figure 1.

Fig. 1: 2d GISANS data (left) and corresponding 2d simulations (right) of P3HT:PCBM BHJ film with 33wt% PCBM. Inset shows the model used in the simulation. Taken from reference [5].

References

[1] Green, M. A.; Emery, K.; Hishikawa, Y.; Warta, W. Prog. Photovolt: Res. Appl., 2011, 19, 84–92

[2] Yu, G.; Heeger, A. J. J. Appl. Phys., 1995, 78, 4510

[3] Halls, J. J. M.; Walsh, C. A. ; Greenham, N. C.; Marseglia, E. A.; Friend, R. H.;

Moratti S. C.; Holmes, A. B. Nature, 1995, 376, 498

[4] Ruderer, M. A.; Müller-Buschbaum, P. Soft Matter, 2011, 7, 5482–5493

[5] Ruderer, M. A.; Meier, R.; Porcar, L.; Cubitt, R.; Müller-Buschbaum, P. J. Phys.

Chem. Lett., 2012, 3, 683-688