Solid Interfaces

Phase Transitions and Transport Phenomena in Thin Films at Solid/Air Interfaces

5.) Heterojunction organic photovoltaic cells need nanometer size structures of the donor/acceptor system to avoid exciton recombination prior to charge separation. We use nucleation and self-organized growth to achieve and optimize suitable donor/acceptor structures (Fig. 5a).

Fig. 5a: Self-organized structures (AFM, dip-coating → drying patterns) of Cu-Phtalocyanine on a SiO2 surface. The structures are only 1nm high.

Future Plans:
We will proceed investigating heterogeneous nucleation and the growth of (small) aggregates. We will prepare nanometer size, defined “active sites” (indentations, etc.) on planar solid surfaces and investigate (AFM) their impact on the adsorption, nucleation and growth of aggregates from supersaturated solutions. Future experimental and theoretical work on the phase transition behavior of molecularly thin alkane terraces will also address nucleation and growth phenomena, but within solid/liquid phase transitions. Molten alkanes do not wet their own solid - a unique property. Thus the studies will also focus on melting in general. We will go on using nucleation and self organized pattern growth for the preparation of organic photovoltaic hetero-junction cells. This work aims at a better understanding of the physics of typical wet preparation processes (spray-, spin-, dip-coating). The resulting (drying) structures are influenced by (surface) flow and solvent evaporation, phenomena that will be studied in detail by future drop coalescence experiments.

H. Riegler, J. Berg, F. Ghani, C. Jin, S. Karpitschka, J. Kristen, C. Weber

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