Circuits based on organic semiconductors (OSCs) hold the promise to generate numerous flexible, transparent and low-cost electronics. A critical bottleneck for organic electronics is their limited charge carrier mobilities, the rate of charge carrier transport in the OSC. The OSC molecular packing structure governs the electronic coupling between adjacent molecules and determines the intrinsic charge transport mobility. By using solution shearing, we are able to incrementally tune the molecular packing (polymorph) in thin films of the model OSC 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-pentacene), for the first time, and increase the charge carrier mobility by an order of magnitude.

Using grazing incidence X-ray diffraction (GIXD) resources available at the Stanford Synchrotron Radiation Lightsource (SSRL), we observed that the incremental polymorphism occurred in TIPS-pentacene and by collaborating with scientists at SSRL, we determined the molecular packing of the non-equilibrium polymorph. Charge carrier mobility in TIPS-pentacene transistors increased from 0.8 cm2/Vs for equilibrium polymorphs to a record high mobility of 11 cm2/Vs for a non-equilibrium polymorph. Using GIXD at SSRL, we show that solution shearing is a general method of inducing polymorphism in various classes of OSCs, like polymers and small molecules. Using solution processing to modify molecular packing and controlling crystal texture opens up new avenues for developing high performance, low-cost OSC devices.

We have also combined high speed in situ microbeam GIXD with high speed in situ polarized optical microscopy (POM) to reveal, for the first time, the fundamental processes behind TIPS-pentacene non-equilibrium polymorph formation. We have done so by building a miniature solution shearing instrument compatible with a synchrotron-based GIXD setup and with high speed POM. We find that spatial confinement of thin film growth during solution shearing leads to metastable polymorphs. We also show a new method of controlling polymorph formation by changing the solvent molar volume. This investigation expands the knowledge of how organic semiconductor polymorph selection occurs during solution shearing, and yields new approaches for controlling metastable polymorph selection.