  We have produced solution-processed thin films of
6,13-bis(triisopropyl-silylethynyl) pentacene with grain sizes from a few
micrometers up to millimeter scale by lateral crystallization from a
rectangular stylus. Grains are oriented along the crystallization direction,
and the grain size transverse to the crystallization direction depends
inversely on the writing speed, hence forming a regular array of oriented grain
boundaries with controllable spacing. We utilize these controllable arrays to
systematically study the role of large-angle grain boundaries in carrier
transport and charge trapping in thin film transistors. The effective mobility
scales with the grain size, leading to an estimate of the potential drop at
individual large-angle grain boundaries of more than one volt. This result
indicates that the structure of grain boundaries is not molecularly abrupt,
which may be a general feature of solution processed small molecule organic
semiconductor thin films where relatively high energy grain boundaries are
typically formed. This may be due to the crystal Transient measurements after
switching from positive to negative gate bias or between large and small
negative gate bias reveal reversible charge trapping with time constants on the
order of 10 s, and trap densities that are correlated with grain boundary
density. We suggest that charge diffusion along grain boundaries and other
defects is the rate determining mechanism of the reversible trapping.
