PEDOT:PSS Characterization using Raman Spectroscopy

The characterization of PEDOT:PSS is essential for determining the relationship between the materials structure, morphology and resulting properties. This is particularly important for their application in electronic devices where their electronic and optical properties are crucial in determining device performance. Some of the key characterization techniques include:

Raman Spectroscopy
X-ray Photoelectron Spectroscopy
Fourier Transform Infrared Spectroscopy
X-ray Diffraction

Understanding the how to characterize PEDOT:PSS using these techniques allows us to determine the effect of processing techniques and post-treatments.

PEDOT:PSS Raman Spectroscopy

Raman spectroscopy is used to detect how light interactions with PEDOT:PSS in specific ways. This technique is used to determine a materials structure based its vibrational modes. It is particularly useful to determine the effect of doping on the overall structure of the polymer blend. Raman scattering is mostly sensitive to PEDOT vibrations.

Resonance Raman Effect

The Raman spectra of PEDOT:PSS varies depending on which laser excitation wavelength is used. This is especially the case when using common lasers in the near-infrared (NIR) and visible range due to the resonance Raman effect. When the laser excitation wavelength matches or is near an electronic absorption band of the material, specific molecular vibrations are selectively enhanced.

PEDOT:PSS absorbs light in a way that allows different laser wavelengths to interact with different electronic transitions, making certain vibrational modes stand out more in the spectra. This is particularly the case for the Raman bands which are associated with PEDOT’s conjugated π-bonding system where electrons experience delocalization. Changes in wavelength causes specific segments of the polymer to have preferential resonant enhancement as the laser interacts with the polymer. This results in changes in the spectra including variation in intensity of certain vibrational modes or shifts in peak positions.

Shorter wavelengths (visible) might excite higher energy electronic transitions, while longer wavelengths (NIR) interact with lower energy transitions. Longer wavelengths also selectively enhance segments with longer effective conjugation lengths.

What to Look Out For...

Raman spectrum of PEDOT:PSS — Example Raman Spectrum of PEDOT:PSS

For PEDOT:PSS the vibrational modes most sensitive to change are above 1200 cm^-1. The most intense band is at ~ 1444 cm^-1 and this is the one used to determine the impact of processing and/or doping. This peak is assumed to contain two components. These components are thought to be the benzoid and quinoid tautomers of the PEDOT chains. The width of the band can be linked to the distribution of conjugation lengths. A narrow band can indicate a homogeneous distribution of conjugated domains and is usually red-shifted.

	Quinoid	Benzoid
Raman Signal	Lower Frequency	Higher Frequency
Conjugation Length	Longer	Shorter
Electron Delocalization	More Delocalized	Less Delocalized
Vibrational Mode	Lower Energy	Higher Energy

The higher-frequency (blue shifted) character is thought to arise from the benzoid structure. This is due to it's shorter conjugation lengths where electrons are less delocalized and therefore have higher energy vibrational modes. The lower-frequency (red shifted) character is from the quinoid where the opposite is true. This tautomer has longer conjugation lengths so electrons have more delocalization and lower energy vibrational modes.

What Causes Main Peak at ~1444 cm^-1 to Shift?

In general the main Raman band for PEDOT:PSS at 1444 cm^-1 is affected by two main factors:

Dedoping / removal of PSS = Causes Red shift
Reduction of grain size / changes in morphology = Causes Blue shift

In a higher PSS content film, where the grains are already small, further treatment with DMSO is likely to induce a red shift.

In a lower PSS content film that has larger aggregates, further treatment with DMSO will allow for smaller PEDOT:PSS domains with lower chain aggregation and/or lower overall alignment of grains.