Nucleation and strain-stabilization during organic semiconductor thin film deposition

The nucleation mechanisms during solution deposition of organic semiconductor thin films determine the grain morphology and may influence the crystalline packing in some cases. Here, in-situ optical spectromicroscopy in reflection mode is used to study the growth mechanisms and thermal stability of 6,13-bis(trisopropylsilylethynyl)-pentacene thin films. The results show that the films form in a supersaturated state before transforming to a solid film. Molecular aggregates corresponding to subcritical nuclei in the crystallization process are inferred from optical spectroscopy measurements of the supersaturated region. Strain-free solid films exhibit a temperature-dependent blue shift of optical absorption peaks due to a continuous thermally driven change of the crystalline packing. As crystalline films are cooled to ambient temperature they become strained although cracking of thicker films is observed, which allows the strain to partially relax. Below a critical thickness, cracking is not observed and grazing incidence X-ray diffraction measurements confirm that the thinnest films are constrained to the lattice constants corresponding to the temperature at which they were deposited. Optical spectroscopy results show that the transition temperature between Form I (room temperature phase) and Form II (high temperature phase) depends on the film thickness, and that Form I can also be strain-stabilized up to 135 °C.


1) Supplementary results for Section: In-situ optical monitoring
of the crystallization process.
a. Optical reflectance peak positions. Table S1. Reflectance features of TIPS-pentacene (8.7 mg/ml in toluene, 25 ˚C, 0.4 mm/s); silicon wafers were used as substrates, Ten reflection spectra have been collected for each region and the standard deviation of each peak position was calculated. Uncertainties for peak shifts include a systematic uncertainty of 6-7 nm due to the possiblility of dispersive effects in reflection mode measurements. Typical reflection spectra for each region can be seen in Fig. 2 S1. Absorption spectrum for a 82 nm thin film of TIPS-Pentacene on UV-ozone treated glass (deposited from a 1.5 mg/ml solution with toluene as the solvent; substrate temperature: 25 ˚C; writing speed: 0.05 mm/s).

c. Real-time X-ray scattering study.
In-situ μGIWAXS is carried out to study TIPS-pentacene thin film crystallization. The capillary is moving for this experiment rather than the substrate. The writing speed is 0.4 mm/s at 25˚C and the concentration is 8.7 mg/ml. As the capillary is moving, the capillary passes the area illuminated by the X-ray beam, followed by meniscus, supersaturated region and finally the solid film, producing a real-time intensity record with a time resolution of 0.1 s, and an effective spatial resolution of ~100 µm. Note that the effective spatial resolution is mainly limited by a small curvature of the contact line where the meniscus meets the substrate surface coupled with the fact that the X-ray footprint is elongated along the contact line due to the grazing incidence of the X-rays. The in-situ X-ray results are shown in Fig. S1. We set the time at which the capillary passes the X-ray beam to be t = 0 s. The change of the integrated Bragg peak intensities versus time is plotted in Fig. S1(a). At t = 0 s, a broad ring from toluene scattering appears, which disappears again at t = 1.3 s. This marks the point at which the X-ray beam illuminates the supersaturated region. We also observe that at t = 1.3 s, the measured (001) peak intensity is only 16% of its final value. We believe that the supersaturated region does not contribute significantly to this intensity and we attribute the small measured intensity to the limited spatial and temporal resolution in the experiment -that is, a small part of the X-ray beam is already illuminates the crystalline part of the film at this moment. Subsequently, after the initial rapid intensity increase, the (001) and (101) intensities continue to increase at a slower rate for more than 10 s. This indicates that the ordering of the crystalline film continues to improve. b. Additional reflectance spectra and peak positions Fig. S4. Examples of reflectance spectra for every temperature shown in Fig. 4(b) of the main text. The reflection spectra were measured at the deposition temperature for each sample, as described in the main text. Peak position versus temperature for three reflection peaks (A 0 , A 1 , A 2 ) are also shown. Mean peak positions and standard deviations are derived from five measurements in different spots on each sample.

3) Supplementary results for Section: Metastable polymorph fabrication and stabilization.
a. Optical microscope and AFM images of thin films prepared with isotropic grain structures.
The images below in Figs. S5-S7 show that the as-deposited grain structure for samples A, B, and D are still intact after the heating and cooling cycles that each sample was subjected to.  Fig. 7(a) of the main text. The sample is listed as sample A in Table II of the main text. After deposition, this film was heated up to 135˚C and then cooled down to 25˚C before these images were recorded. The scale bar in (a) is 50 µm and in (b) is1 µm.  Table II of the main text. The sample was cooled down to 25˚C, heated up to 135˚C and then cooled down to 25˚C again before these images were recorded. The scale bars in (a) and (b) are 50 µm and 1 µm respectively.

b. GIWAXS of Form I sample and in-plane peak positions for Form I and II.
Sample C is a reference sample prepared at 25˚C, and was not subjected to thermal cycling.

Fig. S8
. X-ray scattering of Form I TIPS-pentacene. The film was made at 25˚C on a Si/SiO 2 substrate. This sample is referred to as sample C in Tables II and III of the main  text. For comparison, the X-ray scattering of Form II is shown in Fig. 10 of the main text.  Fig. 10 in the paper. The Q || of (10L), (01L) and (11L) for both thin film forms were calculated by choosing (102), (011) and (112) peaks. The values are used to calculate the a, b and γ lattice constants, which are listed in Table III