Morphology Characterization of Bulk Heterojunction Solar Cells

Revealing the nanostructure of bulk-heterojunction (BHJ) photovoltaic blends is a critical task in the field of organic photovoltaics. The complicated morphology, ranging from binary blends to ternary mixtures, shows quite varied structural details that need to be fully characterized in terms of correlating them with device performance. The nonequilibrium nature of the BHJ thin-film morphology makes the structure optimization difficult, and fundamental principles that relate to chemical structure, processing methods, and device performances have not yet been fully understood. On the other hand, various high-powered techniques ranging from in-house instrumentation to synchrotron techniques have been applied to this tremendously difficult research topic. More structural details, including length scale of phase separation, structural ordering, chain/molecular orientation, surface topology, etc., can be obtained in a quantitative manner, which finally allows the structure-property relationships to be determined. Here, the most important characterization techniques in understanding the morphology of BHJ solar cells are summarized, targeting chemists and materials scientists.