Electronic structures and chemical reactions at the interface between Li and regioregular poly (3-hexylthiophene)

Interfaces between metals and π-conjugated polymers play an important role in the organic electronic and optoelectronic devices such as polymer-based light-emitting diodes (PLEDs) and photovoltaic devices. In this study, synchrotron radiation photoemission spectroscopy (SRPES) and X-ray photoelectron spectroscopy (XPS) have been applied to in situ investigate the chemical reactions and electronic structure during the interface formation of Li on the regioregular poly(3-hexylthiophene) (rr-P3HT) thin films. Upon Li adsorption onto P3HT at 300 K, Li dopes electrons into P3HT, inducing the occurrence of the P3HT band bending. Moreover, Li can diffuse into the subsurface and react with both S and C atoms in the thiophene rings, leading to the formation of Li₂S and Li–C complex. Compared to the interface of Ca/P3HT, the diffusion/reaction depth of Li is much larger at the Li/P3HT interface. Through the investigation of the evolution of core level and valence band spectra together with secondary electron cutoff an energy level alignment diagram at the Li/rr-P3HT interface is derived.