Mapping the morphology of organic photovoltaic films

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Artem Levitsky




Organic photovoltaics (OPV) recently reached over 17% power-conversion efficiency thereby making this technology promising for clean, sustainable and abundant energy. The ideal morphology of OPV devices comprises a blend of organic electron donor and electron acceptor species in interpenetrated continuous nm-scale networks. Such morphologies are, however, metastable and prone to modulations that lead to poor device stability. Moreover, following the complex evolution of film morphology is challenging because there is no direct method useful for characterizing all-organic, mainly amorphous, nm-scale structured morphology. In this work, we use a new labeling method to probe the complex blend morphology, based on vapor phase infiltration (VPI). VPI infuses inorganic materials into an organic matrix by exposure to gaseous precursors that diffuse into the film and in-situ convert to an inorganic product. Applying this methodology on highly efficient OPV blend as a model system we directly established processing-structure-property relationships in OPV devices and developed a fast screening methodology for various OPV systems. Furthermore, we used the method to study morphology evolution during sequences of isothermal annealing steps. We found that phase evolution includes both mutually exclusive phase transformation mechanisms: spinodal decomposition and nucleation and growth. The presence of both reveals that the morphology evolution of this blend during annealing goes through two distinct and consecutive transformation processes.

Advisor: Prof. Gitti Frey