Formation of aligned periodic patterns during the crystallization of organic semiconductor thin films

Abstract

Self-organizing patterns with micrometre-scale features are promising for the large-area fabrication of photonic devices and scattering layers in optoelectronics. Pattern formation would ideally occur in the active semiconductor to avoid the need for further processing steps. Here, we report an approach to form periodic patterns in single layers of organic semiconductors by a simple annealing process. When heated, a crystallization front propagates across the film, producing a sinusoidal surface structure with wavelengths comparable to that of near-infrared light. These surface features initially form in the amorphous region within a micrometre of the crystal growth front, probably due to competition between crystal growth and surface mass transport. The pattern wavelength can be tuned from 800 nm to 2,400 nm by varying the film thickness and annealing temperature, and millimetre-scale domain sizes are obtained. This phenomenon could be exploited for the self-assembly of microstructured organic optoelectronic devices.

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Fig. 1: Periodic pattern formation in TPBi films and dependence on temperature.
Fig. 2: Characterization of pattern tunability and surface topography.
Fig. 3: In situ optical microscopy of periodic pattern formation during annealing.
Fig. 4: Topography and mechanics near the crystal growth front.
Fig. 5: Pattern formation in other organic semiconductors and on common electrode surfaces.

Data availability

The data that support the findings of this study are available from the corresponding author on reasonable request.

Code availability

The code used to analyse pattern periodicity and pattern quality is available at https://github.com/jsbangsund/pattern-image-analysis. The code and interactive graphical user interface used to extract crystal growth rates is provided at https://github.com/jsbangsund/crystal-growth-rate-analysis.

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Acknowledgements

Funding for this work was provided by the NSF Program in Solid-State and Materials Chemistry under grant numbers DMR-1307066 and DMR-1708177. J.S.B. acknowledges support from the NSF Graduate Research Fellowship under grant no. 00039202. The authors acknowledge helpful discussions with C. Teresi, as well as the groups of P. Dauenhauer, C. D. Frisbie and R. L. Penn for the use of their high-speed camera, atomic force microscope and X-ray diffractometer, respectively. Parts of this work were carried out in the Characterization Facility, University of Minnesota, which receives partial support from the NSF through the MRSEC programme.

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J.S.B. designed the experiments and analysed the data. T.R.F. made the initial observation and characterization of the pattern formation phenomenon. J.S.B., T.R.F., T.J.S. and J.R.V.S. prepared, annealed and imaged samples. K.S. performed tapping mode AFM measurements. J.S.B. performed peak force AFM measurements. C.P.C. performed XRD measurements. J.S.B., T.R.F. and R.J.H. interpreted the results and wrote the manuscript. All authors discussed and reviewed the manuscript. R.J.H. supervised the work.

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Correspondence to Russell J. Holmes.

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Supplementary Information

Supplementary Figs. 1–18, Supplementary Tables 1 and 2

Supplementary Video 1

Propagation of crystal growth front

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Bangsund, J.S., Fielitz, T.R., Steiner, T.J. et al. Formation of aligned periodic patterns during the crystallization of organic semiconductor thin films. Nat. Mater. 18, 725–731 (2019). https://doi.org/10.1038/s41563-019-0379-3

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