Ex and In Situ Confocal Raman Studies of Organic Thin Film and Its On-Working Transistors

Ex and in situ confocal Raman techniques including Raman mapping, Raman line scanning and in situ Raman were used to collectively study poly (3,3 ''-dialkylquarterthiophene) (PQT) and poly(2,6-bis(3-alkylthiophen-2-yl)dithieno[3,2-b;2',3'-d]thiophene) (PBTDT) thin film and its transistors under operation. Raman spectra of the two semiconducting polymers are measured and theoretically assigned. Spatial distribution and phase homogeneity of the thin films made by a solution process of the two polymers with different post treatment conditions were evaluated by Raman photon mapping of different sized device channels. The organic semiconductor/gold electrode interface and the effective channel length were physicochemically determined by Raman line scanning. In situ Raman spectroscopy was carried out on working organic thin film transistors to monitor the injection of charge carriers. The results reveal that the semiconductor/electrode interface has a stronger Raman signal than the inside channel to determine the effective channel length. With sweeping the gate voltage at a source-drain voltage, a hysteresis was observed for the Raman intensity, possibly indicating a competing process between localization and delocalization of the excited electrons in the polymer backbone. This work demonstrates that confocal Raman spectroscopy could provide a convenient toot to monitor phase homogeneity or physical distribution and analyze microstructure and electronic behavior of polymer thin films.