Organic semiconductors have attracted significant interest due to their structural tuneability, low cost, ease of processing and desirable optical and electronic properties. The molecular orientation and order of organic semiconductors strongly influence their functional properties and consequently device performance. Among the organic semiconductors, pentacene, a small
organic molecule with a well-defined crystal structure and attractive charge carrier mobility
and singlet fission properties, is investigated as the model system. The relative geometry of pentacene molecules is known to influence its photophysical and electronic properties, which
in hand governs its device applicability. An effective means to control molecular orientation is needed to optimise the functional properties of organic semiconductors and broaden the range of device applications possible.
This thesis identifies structural templating and molecular mixing as strategies for controlling
the molecular orientation and structure (up to a point) of organic small molecules. Copper
iodide is identified as an effective templating layer for planar rod-like molecules and helicenes, which are a class of fused aromatic chiral molecules. Multimodal characterisation, including microscopy, spectroscopy and X-ray diffraction measurements, elucidate changes in molecular structure and uncover the underlying mechanisms of interaction and relaxation. For example, on copper iodide, pentacene adopts a flat-lying orientation, which is attributed to quadrupolar interactions between the pentacene partial charges and negatively charged copper iodide surface. Organic and inorganic templating layers are implemented to control the molecular
orientation that can be easily transferred to a range of different organic molecules and device
architectures.
This thesis provides a methodology of effective molecular orientation control and characterisation that can be extended to a wider range of organic small molecules. This thesis extends our understanding of the role of polarity (substrate and molecule) in the control of molecular orientation and provides design strategies that are applicable to a wider range of aromatic systems.