Molecular Ordering in Functional Blends of Organic Semiconductors
Organic semiconductors offer a convenient reason to study how blending materials affects molecular packing. For example, organic solar cells rely on phase separation between dissimilar compounds to produce the "bulk heterojunction" morphology requisite for efficient devices. X-ray scattering and atomic force microscopy reveal that functionalized pentacene acceptors, which are highly ordered in neat films, lose long-range correlation when blended with a donor. These same acceptors can form substitutionally disordered single crystals (molecular alloys) when blended with other functionalized pentacenes, which share a high degree of structural similarity. Single crystal diffraction, UV/vis spectroscopy and thin film x-ray scattering show that substitutional disorder does not lead to loss of long-range order, even with the presence of multiple polymorphs. Finally we show that blends of organic materials can also lead to stable molecular glasses. We show that these stable molecular glasses can be formed from solution casting rather than from the more commonly used vapor deposition.