New Approach for Analyzing the Vertical Structure of Polymer Thin Films Based on Surface-Enhanced Raman Scattering

Abstract

We report on a new approach for measuring the chemical composition of the 20 nanometers at the top or bottom of a polymer film. This approach is based on a variation of the surface enhanced Raman scattering effect with laser illumination through a thin gold layer (∼4 nm). We show that the introduction of the thin gold layer has little or no effect on the morphology of the film that is spin coated on top of it. We demonstrate that this technique has better than 20 nanometer vertical resolution by studying bilayers of polyfluorines with varying thicknesses and by showing the existence of top and bottom wetting layers in a polymer blend of the same polymers. We also show that the top wetting layer is thinner than the bottom one. The difference in thicknesses explains how a solar cell with an electron blocking layer at the cathode works. ■ INTRODUCTIONIn recent years, organic optoelectronic devices based on polymers and small molecules are becoming more and more popular, ranging from organic light emitting diodes (OLED) 1−7 through organic solar cells, 8−11 organic thin film transistors (OTFT) 12,13 and more. The attractive properties of these devices that make them so popular are large versatility of the optoelectronic properties of the materials, ease of manufacturing, excellent mechanical properties and ultrathin thickness. A commonly used architecture in many devices, such as OLED and organic photovoltaic (OPV), is vertical and hence, their functionality is largely determined by the chemical structure and composition in the vertical direction. For the study of the influence of the chemical structure and composition on the optoelectronic properties, characterization techniques with better than 50 nm vertical resolution need to be developed, because the common thicknesses of these devices is around a hundred nanometer Analysis of the vertical structure is usually performed by using either environmental scanning electron microscopy (ESEM) on a cleaved sample 14 or X-ray photoelectron spectroscopy (XPS). 15 The drawbacks of these two techniques are that they are destructive in nature and therefore, cannot be applied in situ to a working device and they both lack spatial resolution in the horizontal direction. A method that has high spatial resolution in all three directions is nanotomography. 16 This technique gives three-dimensional images with high spatial resolution of the topography of the films. However, this technique cannot give an indication on the chemical composition, unless a specific topography feature can be associated with a specific chemical structure. Moreover, because it is based on transmission electron microscope (TEM) it cannot be applied to working devices. A different approach that is short-range and sensitive to the chemical composition is the surface enhanced Raman scattering technique (SERS). The SERS effect 17−23 is a variant of the Raman effect where the Raman cross-section is considerably enhanced in the presence of plasmons in metals. The SERS effect is used to detect low quantities of ma...

Interfacial Chemical Composition and Molecular Order in Organic Photovoltaic Blend Thin Films Probed by Surface-Enhanced Raman Spectroscopy

“…A more accurate estimation, based on linear fitting of EF values for 11 samples with different spacer thicknesses (see Figure ), found the decay length of enhancement δ = 7.6 ± 0.2 nm. This compares well to values reported in the literature for general SERS applications and represents a significantly more accurate estimation than has been previously reported for SERS of conjugated polymer systems (<20 nm) …”

“…This compares well to values reported in the literature for general SERS applications and represents a significantly more accurate estimation than has been previously reported for SERS of conjugated polymer systems (<20 nm). 20 We noted that between Raman and SERS spectra, there was a shift of the CC peak toward higher wavenumbers, a shift that was only apparent when the separation was small enough for surface enhancement of P3HT (see insets of Figure 2c). For the first sample, where the spacer layer thickness was 0 nm, the peak was up-shifted from 1454 to 1460 cm −1 under SERS conditions, as the spacer thickness increased up to δ = 7.6 nm, the upshift reduced to zero and a consistent CC peak position of 1452 cm −1 .…”

“…The calculated EF int value represents the average EF (EF app ) that would be obtained from the ratio I SERS /I RS for a film of thickness δ but neatly avoids any impact on morphology that may result from intentionally depositing such ultrathin films. It is important to note that this model only works for films that are thicker than δ, but as reported values of δ vary from 5 to 20 nm, , the majority of neat films and blends relevant to organic devices are sufficiently thick that this model should be representative.…”

“…However, by combining the short-range of the enhancement effect with the sensitivity of conventional Raman spectroscopy to relevant material properties such as chemical structure and conformation, ,, it is possible to use SERS to probe the composition and morphology of metal/organic interfaces within ∼100 nm thick organic thin films. The application of SERS to study organic semiconductors and thin films is relatively novel − and has been limited in part by the following problems: (1) disruption of thin film morphology by the introduction of metallic nanostructures, (2) difficulty in estimating the <10 nm decay length of the enhancement effect, and (3) that the SERS enhancement factor (EF) can only be calculated when the number/density of molecules is known.…”

Raman spectroscopy as an advanced structural nanoprobe for conjugated molecular semiconductors

“…The length scale of the enhancement effect, corresponding with the vertical resolution of the measurement, was measured at around 20 nm by using bilayer reference samples. [172] The use of SERS in the study of organic semiconductors to date is limited, perhaps because of the complexity of the measurements and the difficulty in ensuring that any observations are representative. One such difficulty is that the nanostructured metal interacts with the organic material and so can interfere with the morphology being probed.…”