Crystallization and Polymorphism of Organic Semiconductor in Thin Film Induced by Surface Segregated Monolayers

Preparation of highly crystalline organic semiconductor films is vital to achieving high performance in electronic devices. Here we report that surface segregated monolayers (SSMs) on top of phenyl-C61-butyric acid methyl ester (PCBM) thin films induce crystal growth in the bulk, resulting in a dramatic change in the structure to form a new crystal phase. Highly ordered crystalline films with large domain sizes of several hundreds of nanometers are formed with uniaxial orientation of the crystal structure perpendicular to the substrate. The molecular rearrangements in SSMs trigger the nucleation at a lower temperature than that for the spontaneous nucleation in PCBM. The vertical charge mobility in the SSM-induced crystal domains of PCBM is five times higher than in the ordinary polycrystalline domains. Using surface monolayers may be a new strategy for controlling crystal structures and obtaining high-quality organic thin films by post-deposition crystallization.


Synthesis of oligosiloxane fullerene derivatives
The synthetic route to the surface modifiers with oligosiloxane is shown in Scheme S1. The details of the synthetic procedures, and the spectral data from 1 H NMR spectroscopy and MALDI-TOF-MS, are described below.
-0.5 1.5 3.5 5.5 7. 5 9.5 Chemical shift (ppm) -0.5 1.5 3.5 5.5 7. 5 9.5 Chemical shift (ppm) Si:C atomic ratios on the film surfaces measured by XPS Figure S3. Si/C atomic ratios on the film surfaces measured by XPS plotted as a function of the nSi concentrations in the spin-coating solution. The solutions contain a fixed concentration of PCBM (10 g L -1 ). All the films were thermally annealed at 160 °C prior to the measurements.

Static water contact angles
Static water contact angles of the films were measured. The larger contact angles for the surface modified films compared with the PCBM film indicate that the surface energies of the films were lowered by surface segregation of bSi, nSi, and pSi. Table S1. Static water contact angle on PCBM, bSi/PCBM, nSi/PCBM, and pSi/PCBM films. Values in parentheses are standard deviations. The films with SSM were prepared by spin-coating the mixed solution of PCBM (10 g L −1 ) and the surface modifiers. The concentration of bSi, nSi and pSi are 0.88 g L −1 , 1.32 g L −1 and 1.36 g L −1 , respectively. All the films were thermally annealed at 160 °C.  All the films were thermally annealed at 160 °C. Surface etching with an Ar + ion beam (etching rate: 0.25 nm/s).

Angle-resolved XPS
The thickness of the oligosiloxane layer on the surface of the films was estimated by angle-resolved XPS (ARXPS) by using a uniform bilayer model that consists of oligosiloxane and fullerene layers.
According to previous reports, 1 where ISi and IC are the intensities of Si 2p and C 1s peaks, λ is the attenuation length of photoelectrons, θ is the take-off angle of the measurements, and XSi and XC are the local concentrations of silicon and carbon atoms, respectively. We assume that the attenuation lengths of photoelectrons from C 1s and  annealed at 160 °C prior to the measurements. The lines indicate the best fit with eq. 1.

Elemental mapping
Elemental mapping of the PCBM films with bSi, nSi, and pSi was performed by scanning transmission electron microscopy (STEM) as shown in Figure S6. The results indicated that carbon and silicon atoms homogeneously dispersed at the surface of the films.

Incident angle dependence of GIWAXS patterns
The critical angle of Si substrate (density: 2.33 g/cm 3 ) is 0.144°, whereas the critical angle calculated by the density of the PCBM film (1.49 g/cm 3 ) is 0.115°. GIWAXS patterns measured at an incident angle of 0.12° reflects the diffraction from the crystal in the bulk of the thin films. Figure S7 shows the incident angle dependence of GIWAX patterns for the annealed pSi/PCBM film. The patterns with the smaller incident angles showed smaller peak intensities without any change of the pattern, indicating that the diffraction patterns came from the bulk of the film and not only from the surface.

Size of the crystalline domains estimated by Scherrer equation
Mean size of the crystalline domains (τ) can be calculated by the following Scherrer equation:

=
Where K is the shape factor (typical value: 0.9), λ is the X-ray wavelength (0.1 nm), β is the spectrum broadening at half the maximum intensity (FWHM) of diffraction peaks, and θ is the Bragg angle. The plot profile of the GIWAXS patterns of the annealed pSi/PCBM film are shown in Figure S10. FWHM of 110 peak at 0.0217 rad is 0.000487 rad, therefore τ of 110 direction is calculated to be 185 nm. This value should be regarded as the lower limit for the crystal size because the grazing incident angle makes a large longitudinal footprint of the X-ray and causes the significant broadening of the peaks.
Nevertheless, the value is in the same order with the observed domain sizes in the TEM image. ( Figure   S9).

Crystal structure analysis of PCBM in SSM-induced phase
To determine the crystal system, the cell constant and the space group for the crystal structure, molecules in the unit cell. Therefore, we investigate these two space groups for the following analysis.
After the correction of the distortion due to the use of the planar image detector, the GIWAXS patterns were converted to a series of lateral 1D profiles by using the plot profile function of ImageJ (NIH).
The peaks in the 1D profiles are fitted by pseudo-Voigt functions by using PDXL software (Rigaku) and the peak intensities are extracted by the integrations of the peak area. In-plane data (hk0) was omitted from the analysis to avoid the effects of the Yoneda wings. 4 The intensity data were corrected with Lorentz and polarization factors considering the measurement geometry. Diffraction data of outof-plane geometry, which cannot be measured in GIWAXS geometry, were separately collected with a 2θ/ω scan technique performed on an X-ray diffractometer (SmartLab, Rigaku) by using a parallel X-ray beam (Cu Kα1 radiation at 45 kV and 200 mA). The peaks were integrated, and the appropriate Lorentz and polarization corrections were applied to obtain the 00l-I dataset.
The structure was first solved by direct-space method with a simulated annealing algorithm implemented in SIR2014 software 5 using the hkl-I dataset from GIWAXS to explore the positions of the three independent PCBM molecules in the unit cell. Owing to the low resolution of the diffraction data, intermolecular anti-bumping constraints were imposed in the simulated annealing exploration runs. The runs are repeated until a solution with the lowest figure of merit (R-value) was obtained.
Since the assumption of I4cm space group resulted in unrealistic porous structures with relatively high R-values, I4 c2 space group was adopted. The optimized structure was further refined by using SHELXLL-2016/6 software. 6 The GIWAXS and the out-of-plane data were scaled by batch scalefactor refinements. The C60, phenyl and ester moieties were treated as rigid bodies, and the bond distances and angles of the other moieties were strongly constrained with those of the reported singlecrystal PCBM structure. GIWAXS pattern was simulated by using a program made by T. Koganezawa from the structure with the lowest figure of merit (R = 0.3858 for 136 reflections with I > 2σ (I), attached CIF). The solution can reproduce the observed pattern well (Figure 4a and b).
The mosaicity of the crystal in the out-of-plane direction has been estimated to be 0.95 (2) F:C atomic ratio at the surface of FC 8 /PCBM films Figure S13. F:C atomic ratio on the surfaces of the FC8/PCBM films measured by XPS plotted as a function of the thermal annealing temperature. The FC8/PCBM films were prepared by spin-coating the mixed solution of PCBM (10 g L −1 ) and FC8. The concentration of FC8 is 1.00 g L −1 , respectively.

Nano I-V curve and space charge-limited current model analysis in c-AFM
I-V curves were measured at 10 different points by c-AFM. Figure S14 shows all the I-V curves and measurement points. The electron mobility was calculated by using the modified SCLC model for c-AFM in the case of electron injection from the substrate. 7 The modified SCLC model is expressed by where J is the current density, ε0 is the vacuum dielectric constant, εr is the relative dielectric constant (PCBM: 3.9), 8 μ is the electron mobility, V is applied voltage, and L is the film thickness (40 nm).
Current density was calculated from the measured current and tip-sample contact area. The radius of the tip-sample contact (A) is given by Hertzian mechanics as 9 where R is the radius of the tip (25 nm), F is the contact force of the tip on the sample surface (2