|Ph.D Student||Levitsky Artem|
|Subject||Using Small Molecule Infiltration to Directly Map the|
Morphology of Molecular Organic Photovoltaic
|Department||Department of Materials Science and Engineering||Supervisor||Professor Gitti Frey|
In recent years a global demand for renewable energy has encouraged worldwide scientific and industrial efforts to develop a new power-harnessing technology to reduce the fossil fuels consumption. To date, one of the most utilized renewable energy sources is the Si-based solar-cell technology. Organic solar cells (OSC) is a rapidly emerging photovoltaic technology thanks to the abundance and wide choice of materials, lightweight, flexibility and compatibility with printing and roll-to-roll production techniques. Recently, OSCs reached over 18% power-conversion efficiency thereby making this technology a promising alternative for Si-based solar-cells. The state of the art OSC devices are based on the bulk heterojunction (BHJ) structure, composed of intricate continuous networks of electron donor and electron acceptor species. The length scale of phase separation, purity and microstructure of each phase, and their continuity govern the overall OSC device performance and hence, huge efforts are invested in controlling the produced BHJ morphology. Yet, the resulting morphology is metastable and prone to modulations, leading to poor OSC device stability and limiting their potential for application on the industrial scale. Moreover, the obtained BHJs are highly complex including amorphous and ordered, pure and mixed domains, making the characterization a difficult task.
In this work we use a new labeling method to probe the complex BHJ 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. Using this labelling approach to spatially map OSC BHJ is in concept similar to the staining approach used to image low contrast biological specimens in electron microscopy. First, we studied the feasibility of VPI as an effective methodology for BHJ characterization by means of different OSC blends, composed of conjugated polymers and different derivatives of fullerene or non-fullerene acceptors. Then, applying this methodology on a highly efficient polymer:fullerene blend model system we directly established processing-structure-property relationships in OSC devices. We demonstrated that by means of straight-forward thermal analysis, VPI and transient-absorption spectroscopy, we can rapidly screen various blend compositions and processing methodologies. Finally, we studied the BHJ morphology evolution during sequences of isothermal annealing steps and found that fullerene phase segregation, initially, goes through a spinodal decomposition mechanism, while the latter stage is governed by the nucleation and growth of fullerene polycrystalline aggregates. Separately, both mechanism were previously observed and reported in the literature, while their coexistence is unusual. To resolve this unexpected observation we introduced a metastable monotectic phase diagram. The proposed mechanism was successfully implemented to follow organic solar cells degradation process during thermal annealing.