Distinct interfaces of poly (9,9-dioctylfluorene-co-benzothiadiazole) with cesium and calcium as observed by photoemission spectroscopy

Abstract

We report that the green-emitting polymer—poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT), a copolymer of poly(9,9-dioctylfluorene) (F8), formed two distinct interfaces with calcium and cesium, respectively, as observed in photoemission measurements. Ultraviolet photoemission spectroscopy showed that Ca formed a stable interface with F8BT without significantly changing the electronic structure of F8BT, which is in contrast to the Cs/F8BT interface where bipolaron states occurred in the forbidden gap as a result of charge transfer processes. X-ray photoemission spectroscopy revealed that Ca covalently bonded with the sulfur atoms whereas Cs preferably interacted with the nitrogen in the F8BT. The results are useful to account for the undesirable device performance using CsF/Al as a cathode in the F8BT-based polymer light-emitting device. The exposure of Cs/F8BT interface to residual gases at a pressure of ${\text{2.0×10}}^{\text{−9}}\hspace{0.17em}\mathrm{mbar}$ for 2 and 12 h, respectively, slightly and largely eliminated the gap states. Deposition of a thin layer of Ag onto the Cs/F8BT interface protected the films from the influence of residual gases and hence retained the affected valence-band structures.