Complex Morphologies and Molecular Ordering in OPVs: Critical Parameters Determined by Soft X-Ray Scattering
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Interface structure and the quantitative composition of morphology are known to be critical for fullerene based bulk heterojunction solar cells, yet have been very difficult to study previously due to a paucity of characterization methods. Recently developed soft X-ray microscopy and scattering tools provide new avenues and contribute substantially to indentify the number of phases present and to provide a quantitative measurement of their composition fluctuations and size distribution. This led to the realization that mixed domains are prevalent in OPVs and rather than just being detrimental can have important beneficial contributions for charge generation and charge transport. Furthermore, polarized x-ray scattering can reveal preferential orientation of the donor polymer (edge-on or face-on) relative to the fullerene aggregate interface. Such ordering has previously not been observed solar cells and is shown here to be a critical factor for high performance in a number of systems.