(Invited) Attenuated Total Reflectance UV-Vis Spectroscopy Applied for Organic Semiconductor Films
Authors: Tanabe Ichiro , Tanaka Yuki 
Published: December 2024

Poly(3-hexylthiophene) (P3HT) is a p-type organic semiconductor extensively utilized in various electrochemical devices like solar cells and organic field-effect transistors owing to its outstanding solubility in a wide array of solvents and its high field-effect mobility. The packing structure and crystallinity of P3HT have a significant impact on device performance. The electronic state of P3HT is one of important properties, playing a crucial role in both fundamental understanding and practical applications. Ultraviolet-visible spectroscopy is one of the powerful techniques for investigating the electronic states within materials.
In this study, ultraviolet-visible absorption spectra of P3HT films deposited on sapphire substrates were examined utilizing two spectroscopic techniques: transmission and attenuated total reflection (ATR) spectroscopies. Furthermore, a comprehensive comparative analysis was conducted between two deposition methodologies, namely casting and spin-coating methods.
The ATR spectra were measured by the custom-made spectrometer, which consisted of the UV–vis–NIR spectrophotometer (V-770, JASCO) and an original ATR unit [1]. The incident light passed through a monochromator and irradiated into the sapphire ATR prism. The reflected light was detected by a photo-multiplier. The incident angle was set to 70°, and the penetration depth was approximately 50 nm or less in the measured wavelength region (200–800 nm). The original ATR unit and the commercial transmission unit could be easily exchanged, and the transmission spectra of the P3HT film fabricated on the sapphire ATR prism could be meaasured.
In both the transmission spectrum (Figure 1a) and ATR spectrum (Figure 1b), the spectra of the P3HT thin films fabricated via the casting method (red lines) exhibited absorptions at longer wavelengths in contrast to the film prepared using the spin-coating method (blue lines), thus indicating that the casting method yields a more ordered structure. The spectral shapes differed between the transmission and ATR methods. To compare them, it was necessary to apply Kramers-Kronig transformation to the ATR spectra. The details will be discussed in the presentation.
The ATR spectrum of the film prepared through the casting method (Figure 1b, red line) displayed distinctive absorption peaks around the 690 nm region. In the ATR spectrum of the film prepared by the spin-coating method (Figure 1b, blue line), the small absorption at the same wavelength was also observed. However, such absorption were not observed in the transmission spectra (Figure 1a). It has been reported that the absorption at around 690 nm is attributed to the high internal structural order within the single-crystalline P3HT [2]. It should be noted here that the ATR spectra strongly reflect the region near the surface of the ATR prism. Therefore, it can be assumed that the highly ordered structures of P3HT are formed near the sapphire substrate, as detected by ATR measurements.
Consequently, this study revealed the structural variations in the films depending on the fabrication method using ultraviolet-visible spectroscopy. Particularly, the adoption of the ATR method enabled measurements that strongly reflected the film-substrate interface.
[1] I. Tanabe, I. Imoto, D. Okaue, M. Imai, S. Kumagai, T. Makita, M. Mitani, T. Okamoto, J. Takeya and K. Fukui, Commun. Chem., 4, 88 (2021).
[2] K. Rahimi, I. Botiz, J. O. Agumba, S. Motamen, N. Stingelin and G. Reiter, RSC Adv., 4, 11121 (2014).

Figure 1