In organic solar cells, the power generation process is usually divided into basic steps: light absorption, exciton migration, exciton splitting and charge migration to the contacts. Thanks to fundamental and engineering advances, the performance of organic solar cells, as measured by their power conversion efficiency has been steadily increasing, up to ~10% for single junctions and &gt;12% for tandems. There is a broad consensus in the organic photovoltaic community that all the steps involved in generating photocurrent are depedent on the microstructure of the materials. Ultimately then the microstructure is directly coupled to the typical device paramers used to measure power conversion efficiency: JSC, VOC and FF. In this presentation I will discuss a few examples of such coupling. In particular, I will show that all charge generation proceeds via a ground-state charge-transfer (CT) state at the donor-acceptor interface. The energy of this CT state is directly related to the VOC and is affected by molecular conformations and aggregation at the interface. Finally, I will discuss how crystalline disorder in the pure phases induces electronic traps, which are responsible for poor transport and a degradation of FF. Ultimately, the promise of organic electronics is the ability to design materials to perform predetermined functions. The example of organic photovoltaics demonstrates that this promise can be fulfilled only if we understand how the microstructure of these materials, as analyzed at several length-scales, affects their function.

Organic photovoltaics: Materials microstructure and its effects on device parameters

249th National Meeting (2015)

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