MSE5: Interface dynamics for two-layer liquid films in the context of organic solar cells

Background

Organic solar cells are a promising technology for replacing current photovoltaic devices. A common method for producing organic solar cells is by spin coating a mixture of two organic polymers that have been blended in a volatile solvent. The efficiency of the resulting device depends on the morphology of the interface that forms between these two polymers as the solvent evaporates. Understanding how the polymer–polymer interface evolves during fabrication is crucial for maximising the efficiency of the device.

Techniques and Challenges

Experiments on spin coating organic polymer blends suggest that gradients in the surface tension at the polymer–polymer interface can induce a Marangoni instability. Moreover, they suggest that the subsequent evolution of the interface and the resulting morphology are linked to the dynamics of this instability. Traditionally, the Marangoni instability is studied using linear stability analysis in the context of films heated from below. In spin coating, however, the instability is ultimately driven by evaporation, which causes gradients in the solvent concentration. Evaporation complicates the analysis because the evaporative mass loss causes the basic solution to be a function of time. However, if the evaporation rate is slow compared to the time scale of the instability evolution, then asymptotic methods can be used to carry out the analysis.

Results

This approach has been used to study Marangoni instabilities in a single layer of fluid composed of a single polymer blended in a volatile solvent. The air–liquid interface is assumed to be non-deformable and spinning is neglected. An asymptotic solution to the linear stability problem is constructed based on the method of multiple scales. A comparison with a numerical approximation to the solution shows that the asymptotic solution is accurate.

The Future

In the future, the assumption of a non-deformable interface will be removed and a second polymer will be introduced into the stationary system. We will also focus our attention on the stability of a polymer–bilayer interface. When the dynamics of these systems are understood, spinning will be added into the model.

References

[11/54] Hennessy M.G., Münch A.: A multiple scales approach to evaporation induced Marangoni convection

[10/59] Münch A., Please C.P., Wagner B.: Spin coating of an evaporating polymer solution, Physics of Fluids

Heriot S.Y., Jones R.A.L.: An interfacial instability in a transient wetting layer leads to lateral phase separation in thin spin-cast polymer-blend films. Nature Materials, 2005

Moons E.: Conjugated polymer blends: linking film morphology to performance of light emitting diodes and photodiodes. J. Phys.: Condens. Matter, 2002