Late stage crystallization and healing during spin-coating enhance carrier transport in small-molecule organic semiconductors
AuthorsChou, Kang Wei
Khan, Hadayat Ullah
Niazi, Muhammad Rizwan
Payne, Marcia M.
Anthony, John Edward
Smilgies, Detlef Matthias
KAUST DepartmentPhysical Sciences and Engineering (PSE) Division
KAUST Solar Center (KSC)
Materials Science and Engineering Program
Organic Electronics and Photovoltaics Group
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AbstractSpin-coating is currently the most widely used solution processing method in organic electronics. Here, we report, for the first time, a direct investigation of the formation process of the small-molecule organic semiconductor (OSC) 6,13-bis(triisopropylsilylethynyl) (TIPS)-pentacene during spin-coating in the context of an organic thin film transistor (OTFT) application. The solution thinning and thin film formation were monitored in situ by optical reflectometry and grazing incidence wide angle X-ray scattering, respectively, both of which were performed during spin-coating. We find that OSC thin film formation is akin to a quenching process, marked by a deposition rate of ∼100 nm s-1, nearly three orders of magnitude faster than drop-casting. This is then followed by a more gradual crystallization and healing step which depends upon the spinning speed. We associate this to further crystallization and healing of defects by residency of the residual solvent trapped inside the kinetically trapped film. The residency time of the trapped solvent is extended to several seconds by slowing the rotational speed of the substrate and is credited with improving the carrier mobility by nearly two orders of magnitude. Based on this insight, we deliberately slow down the solvent evaporation further and increase the carrier mobility by an additional order of magnitude. These results demonstrate how spin-coating conditions can be used as a handle over the crystallinity of organic semiconductors otherwise quenched during initial formation only to recrystallize and heal during extended interaction with the trapped solvent. This journal is © the Partner Organisations 2014.
SponsorsThe authors thank the Frechet group at UC Berkeley for providing access to a fume hood during synchrotron experiments and for help with profilometry measurements and the Thoroddsen group at KAUST for assistance with high speed optical imaging of the spin-coating process. We'd like to thank Eunhee Lim of the Advanced Light Source for technical assistance. Research reported in this publication was supported by the King Abdullah University of Science and Technology (KAUST) and by KAUST's Office of Competitive Research Funds under award number FIC/2010/04. The authors acknowledge use of the D1 beam line at the Cornell High Energy Synchrotron Source supported by the National Science Foundation and NIH-NIGMS via NSF grant DMR-1332208.
PublisherRoyal Society of Chemistry (RSC)
JournalJournal of Materials Chemistry C