Process-structure relationships for solution-processed organic photovoltaics

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Organic photovoltaics is a promising thin-film solar cell technology that could have a significant impact on the photovoltaics market in the near future. One important advantage when using organic materials is the solution-based fabrication process allowing low-cost and fast production as well as new applications including flexible and semi-transparent solar cells. However, improvement of the efficiency and stability in parallel still remains a major challenge, especially for large-area solar cells. Organic bulk-heterojunction active layers form sophisticated structures during the drying of the wetdeposited solution, because of complex physical processes such as crystallization and/or phase separation. This structure highly impacts the photovoltaic performance. In order to reach significant improvements, a better understanding of the physics driving the active layer formation is strongly needed. We propose to build a simulation framework based on continuum mechanics taking into account all physical processes involved (evaporation, crystal nucleation and growth, phase separation, hydrodynamic coarsening, viscosity increase) to determine the dry film structure. Simulations will then be performed for real photovoltaic systems, whereby the input parameters will be obtained by careful measurements on the corresponding raw materials. The simulation results will be compared with in-situ experimental characterization on the drying structure as well as with measurements on the dry film. The objective of the project is to finally use the validated simulation tool to elucidate physics-based design rules for ink formulation and process parameters in order to gain a better control on the photoactive layer structure.