Suppressing crystallization in solution-processed thin films of organic semiconductors
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olymers/Soft Matter Research Letter
Suppressing crystallization in solution-processed thin films of organic semiconductors Jes B. Sherman, Department of Chemistry and Biochemistry, University of California Santa Barbara, Santa Barbara, CA 93106, USA Chien-Yang Chiu, and Ryan Fagenson, Materials Department, University of California Santa Barbara, Santa Barbara, CA 93106, USA Guang Wu, Department of Chemistry and Biochemistry, University of California Santa Barbara, Santa Barbara, CA 93106, USA Craig J. Hawker, Department of Chemistry and Biochemistry, University of California Santa Barbara, Santa Barbara, CA 93106, USA; Materials Department, University of California Santa Barbara, Santa Barbara, CA 93106, USA Michael L. Chabinyc, Materials Department, University of California Santa Barbara, Santa Barbara, CA 93106, USA Address all correspondence to Michael L. Chabinyc at [email protected] (Received 5 June 2015; accepted 27 July 2015)
Abstract Glassy organic semiconductors provide a convenient host for dispersing guest molecules, such as dopants or light-emitting chromophores. However, many glass-forming compounds will crystallize over time leading to changes in performance and stability in devices. Methods to stabilize amorphous molecular solids are therefore desirable. We demonstrate that solution-processable glasses can be formed from a mixture of 8,8′ -biindeno[2,1-b]thiophenylene (BTP) atropisomers. While the trans isomer of methylated BTP, (E)-MeBTP crystallizes in spin-cast films, the addition of (Z)-MeBTP slows the growth of the spherulites. X-ray scattering and optical microscopy indicate that films containing 40% (Z)-MeBTP do not crystallize, even with the addition of nucleation agents and aging for several months.
Introduction Producing amorphous organic solids, and preventing their crystallization, presents a challenge in organic electronics, pharmaceutical science, and energetic materials.[1,2] Amorphous materials are required when an application requires features such as maintenance of particular domains sizes over time, a host matrix for molecular species, or a stable dissolution rate of a solid. A difficulty in maintaining a glassy form of an organic solid is crystallization where small crystallites, which are thermodynamically disfavored because of their high relative surface areas, grow over time. This transformation leads to a decline in materials performance for applications that require small-sized particles or phase separated structures with small domain sizes. This need is particularly relevant to organic light emitting diodes (OLEDs). Many high performance OLEDs are formed with an emissive guest in a host material; crystallization of the emitting material can lead to dark areas within a device.[3] Futhermore, organic photovoltaics (OPVs) often suffer deterioration in device performance over time as one or more components in a bulk heterojunction device crystallize, leading to larger aggregates.[4] To prevent the situation, a common solution is to use molecular materials with relative
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