Solvent Vapor Annealing
in the Molecular Regime Drastically Improves Carrier Transport in
Small-Molecule Thin-Film Transistors
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Abstract
We demonstrate a new way to investigate and control the
solvent vapor annealing of solution-cast organic semiconductor thin
films. Solvent vapor annealing of spin-cast films of 6,13-bis(triisopropylsilylethynyl)
pentacene (TIPS-Pn) is investigated in situ using quartz crystal microbalance
with dissipation (QCM-D) capability, allowing us to monitor both solvent
mass uptake and changes in the mechanical rigidity of the film. Using
time-resolved grazing incidence wide angle X-ray scattering (GIWAXS)
and complementary static atomic force microscopy (AFM), we demonstrate
that solvent vapor annealing in the molecular regime can cause significant
performance improvements in organic thin film transistors (OTFTs),
whereas allowing the solvent to percolate and form a liquid phase
results in catastrophic reorganization and dewetting of the film,
making the process counterproductive. Using these lessons we devise
processing conditions which prevent percolation of the adsorbed solvent
vapor molecules for extended periods, thus extending the benefits
of solvent vapor annealing and improving carrier mobility by nearly
two orders of magnitude. Ultimately, it is demonstrated that QCM-D
is a very powerful sensor of the state of the adsorbed solvent as
well as the thin film, thus making it suitable for process development
as well as in-line process monitoring both in laboratory and in future
manufacturing settings