In situ X-ray Diffraction Study of High Performance Organic Semiconductor PolymorphismZhenan Bao, Stanford University, DMR 1303178

Flexible, transparent and low cost electronic devices based on organic semiconductors are being actively explored. To realize such applications, the charge carrier mobilities of organic semiconductors must be improved. 6,13(bis-triisopropylsilylethynyl)pentacene (TIPS-pentacene), a small molecule organic semiconductor, adopts metastable polymorphs possessing significantly faster charge transport than the equilibrium crystal when deposited using the solution shearing method. We utilize a combination of high-speed polarized optical microscopy, in situ microbeam grazing incidence wide angle X-ray scattering and molecular simulations to understand the mechanism behind formation of metastable TIPS-pentacene polymorphs during solution shearing. We observe that thin film crystallization occurs first at the air-solution interface, and nanoscale vertical spatial confinement of the solution results in formation of metastable polymorphs, a one-dimensional and large-area analogy to crystallization of polymorphs in nanoporous matrices. We demonstrate metastable polymorphism can be tuned with unprecedented control and produced over large areas by varying physical confinement conditions.

Giri, G., Li, R., Smilgies, D.M., Li, E.Q., Chiu, M., Lin, D.W., Allen, R., Diao, Y., Mannsfeld, S.C.B., Thoroddsen, S.T., Bao, Z. and Amassian, A. (2014) Nature Communications. 5, 3573.

a) Setup of the in-situ experiment. b) In-situ, high speed X-ray scattering data shows the equilibrium polymorph forms earlier than the metastable polymorph of TIPS-pentacene. c) Hypothesis of film confinement causing metastable film growth.

In situ X-ray Diffraction Study of High Performance Organic Semiconductor PolymorphismZhenan Bao, Stanford University, DMR1303178

Education:This work was carried out by an interdisciplinary team, including students and postdocs with backgrounds in organic synthetic chemistry, x-ray crystallography, theoretical characterization, physics, electrical engineering, and chemical engineering. We hosted five Stanford undergraduate students throughout the academic year and three REU students in the summer, who worked on sample preparation and characterization, surface preparation and characterization, and transistor device preparation and testing. Graduate students and postdocs in the lab organized a course on organic electronics that was presented to middle and high school students as part of the SPLASH program at Stanford. The goal of this course was to educate young students about electronic materials research, to get them excited about science as a whole, and to answer questions about how to pursue a research career path. Societal Impacts and Technological Output:The research outlined will impact society by making organic electronics cheaper and more reliable for photovoltaics, ID tags and sensors.

Photo of graduate students and postdocs from Bao group discussing science, doing demonstrations and hands-on experiments with local middle school and high school students.