(561b) Polymorph and Morphology Control of Organic Semiconductor Thin Films

Molecular packing strongly influences charge transport in organic semiconductor (OSC) thin films. A slight displacement between adjacent molecules leads to a significant change in the transfer integral and therefore to the charge carrier mobility. Compared with inorganic semiconductors, OSC crystals are characterized by weak intermolecular interactions, allowing one material to assume more than one polymorphic form, which are accessible under typical processing conditions. Despite its importance, structural and thermodynamic information on crystalline phases of an OSC system are often missing due to the challenge in characterizing nanoscopic thin films. Therefore, not surprisingly, there have been few strategies reported for controlling OSC polymorphism so far, not to mention methods for controlling both polymorphism and thin film morphology. 

In this work, we report new strategies for controlling OSC polymorph formation, by taking advantage of the spatial confinement effect and/or by employing polymorph induction layers. We characterize the OSC thin film molecular packing using advanced synchrotron X-ray techniques, combined with surface-enhanced Raman spectroscopy. We further perform crystal structure refinement using in-house developed software based on Simulated Annealing algorithm. By controlling both molecular packing and thin film morphology, we have achieved record high charge carrier mobility for the system investigated.

[1] Diao, Y.; Tee. B.C-K.; Giri, G.; Xu, J.; Kim, D.H.; Becerril, H.A.; Stoltenberg, R.M.; Lee, T.H.; Xue, G.; Mannsfeld, S.C.B.; Bao, Z. “Solution coating of large-area organic semiconductor thin films with aligned single-crystalline domains”. Nature Materials. In print.

[2] Xu, J.*; Diao, Y.*; Zhou D.; Giri, G; Chen, W; Liu, N; Xue G.; Bao, Z. “Probing interfacial molecular packing in TIPS-pentacene organic semiconductors”. Submitted.