Tuning Polymorphism and Orientation in Organic Semiconductor Thin Films via Post-deposition Processing

Though both the crystal
structure and molecular orientation of
organic semiconductors are known to impact charge transport in thin-film
devices, separately accessing different polymorphs and varying the
out-of-plane molecular orientation is challenging, typically requiring
stringent control over film deposition conditions, film thickness,
and substrate chemistry. Here we demonstrate independent tuning of
the crystalline polymorph and molecular orientation in thin films
of contorted hexabenzocoronene, c-HBC, during post-deposition processing
without the need to adjust deposition conditions. Three polymorphs
are observed, two of which have not been previously reported. Using
our ability to independently tune the crystal structure and out-of-plane
molecular orientation in thin films of c-HBC, we have decoupled and
evaluated the effects that molecular packing and orientation have
on device performance in thin-film transistors (TFTs). In the case
of TFTs comprising c-HBC, polymorphism and molecular orientation are
equally important; independently changing either one affects the field-effect
mobility by an order of magnitude.