We present the UV–Vis–NIR, infrared, and Raman spectra of dedoped neutral PBFDO and redoped PBFDO at various doping levels. The infrared spectrum of a PBFDO film doped at various doping levels showed broad background absorption with many peaks and derivative-like bands. These features were interpreted as the electron–molecular–vibration coupling between broad electronic absorption and discrete vibrational levels and the presence of doping-induced infrared bands associated with the strong Raman bands of neutral PBFDO. The UV–Vis–NIR absorption spectra of doped PBFDO showed two bands at 0.26 and 0.86 eV, which are attributable to negative polarons. No transition from polarons to bipolarons was observed. At the maximum doping level, a polaron network that exhibited metallic properties was likely formed.
Furukawa et al. (Mon,) studied this question.