Raman Scattering Methods for Monitoring the Electric Properties of the Postannealed Bulk Heterojunction

In the case of bulk heterojunctions (BHJs) of regioregular poly­(3-hexylthiophene) (P3HT) and the soluble fullerene derivative [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM), electrical properties have been studied based on the Raman spectroscopy. Significantly, a well enough noteworthy phenomenon is the ″return-back″ shift of the Raman signal at the ∼1450 cm^–1 with increasing annealing temperature, which was first observed using the normal Raman spectroscopy of the BHJ system. Based on the Herzberg–Teller coupling term in the organic–organic system, the ″return-back″ shift may be due to the charge transfer (CT), resulting in the significant change of resistivity (ρ). More importantly, the low annealing temperature-dependent frequency shift (ν) is carefully investigated with ρ, and ρ ∝ (−ν)². In addition, the connection between Raman intensity and ρ is also established. This study is unprecedented to establish a clear connection between frequency shift and electrical properties, indicating that the Raman technique opens an avenue to analyze the electrical properties and characterize the performance of the battery.