(a) Schematic of the experimental geometry used for SHG measurements on MEH-PPV films. (b) Angle dependence of the intensity of the second-harmonic light from MEH-PPV films that were spin cast at different spin speeds from chlorobenzene solutions to produce films with different thicknesses; for the data presented, both the 800 nm incident and 400 nm exigent beams were -polarized. The blue curves denote SHG data taken from MEH-PPV films cast from a 1% w/v solution in chlorobenzene, the green curves correspond to data taken from films cast from a 0.5% w/v solution, and the red curves show the data collected from films cast from a 0.25% w/v solution. Dotted curves show data for films spun at 1400 rpm, dashed curves correspond to films spun at 3000 rpm, and solid curves correspond to films spun at 6000 rpm.
Angularly integrated SHG data for MEH-PPV films spin cast from solutions with three different concentrations as a function of film thickness; the data are plotted on logarithmic scales on both axes. The orange triangles denote data for films cast from 0.25% w/v MEH-PPV solutions in chlorobenzene, the green diamonds are for films cast from 0.5% w/v solutions, and the blue circles are films cast from 1.0% w/v solutions. Although each set of data shows a general increase in signal intensity with increasing film thickness, the data for the thickest films cast from each solution show a decrease in SHG intensity with increasing film thickness.
(a) Angularly integrated SHG data for MEH-PPV films (same data and symbols as Fig. 2) plotted vs spin speed instead of film thickness. (b) The same data as in panel a, with the overall thickness dependence normalized out by scaling the data sets to minimize the squared deviations between them. It is clear that the SHG intensity has a similar trend with spin speed for films cast from different concentration solutions, with the SHG intensity reaching a maximum around 1400 rpm.
Schematic model for the local packing of MEH-PPV chains looking down (a) the -axis and (b) the -axis. The characteristic lengths are represented by , the interchain stacking distance, , the interchain spacing within a monolayer, and , the monomer-to-monomer repeat unit spacing. In panel a, a single MEH-PPV monomer is highlighted in red. In panel b, the side chains have been omitted for clarity.
2D grazing incidence XRD of an MEH-PPV film spin-coated at 1400 rpm from a 1% w/v chlorobenzene solution. The inset shows the full two dimensional diffraction pattern, with cuts at various angles indicated by the colored lines. The -axis is normal to the plane of the film, and the peaks labeled , , and correspond to the crystallographic axes illustrated in Fig. 4. The peak marked with the asterisk is due to thin film reflection and is not a diffraction peak.
(a) Integrated intensity of the -axis XRD peak in the direction (blue circles) and SHG intensity (red squares) for MEH-PPV films spin cast from 1% w/v chlorobenzene solutions as a function of spin speed. Because the ordinate for both data sets have arbitrary units, the SHG data was scaled to minimize the sum of the squared deviation between the two. The correlation between the degree of crystallinity as measured by the integrated diffraction and the SHG intensity is essentially perfect. (b) Integrated intensity of the -axis XRD peak in the direction (green squares) for annealed MEH-PPV films on the same vertical scale used in panel a. Upon annealing, the spin speed dependence of the crystallinity disappears.
2D grazing incidence XRD of an MEH-PPV film prepared identically to that studied in Fig. 5 but following thermal annealing at for 12 h. The inset shows the full 2D scattering pattern, and the colored curves show cuts in different directions (same as in Fig. 5). The peak marked with an asterisk is due to thin film reflection and is not a diffraction peak.
Integrated SHG for annealed films cast from three different solution concentrations shown on a linear-linear plot. Unlike the case with unannealed films (cf. Fig. 2), the SHG signal from the films made from the three different solution concentrations completely overlap, verifying that there is no longer a spin-speed dependent component to the signal.
Comparison of ED and EQ SHG models for annealed MEH-PPV films. Along the top, from left to right, are the full 3D model fit for the ED model (panel a), the raw SHG data (panel b), and the model fit for the EQ model (panel c). Black lines indicate the location of the slices in the lower plots of panels a and c. The ED fits in panel a do a better job of fitting the data at smaller thicknesses, but do poorly for large thicknesses; the EQ fits in panel b do a better overall job of fitting the data, particularly at larger thicknesses.
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