Electronic structures of pristine and potassium-doped rubrene thin films

The photoemission spectra of pristine and potassium-doped rubrene thin films are reported and investigated with density functional simulations in this work. Rubrene (5,6,11,12-tetraphenyltetracene) is an organic hydrocarbon with a tetracene-like backbone and four phenyl side groups. For the pristine rubrene, the spectral features closer to the valence band edge are dominated by the contributions from the backbone carbons, while the remaining part of the spectrum at higher binding energy from the phenyl carbons. When doped with potassium, the doping electron is found to be localized among the backbone carbons and has negligible effects on the electronic structures of the phenyl side groups. This localization of the doping electron within the backbone carbons accounts for the observation that the spectral shapes at higher binding energy are not much changed by doping, and further leads to the formation of the states at the formerly forbidden gap of the pristine material. The doping electron causes splitting of the former highest occupied molecular orbital (HOMO) level into two due to the exchange effects. Overwhelmed by the broadening effects of the solid-states, the split levels appear as one peak in the photoemission spectra. This peak and the new HOMO level occupied by the doping electron constitute the doubly-peaked gap states observed in the photoemission experiments.

When rubrene is doped with potassium, the doping electron is found to be localized among the backbone carbons. This localization of the doping electron accounts for the spectral features of photoemission and leads to formation of the states at the formerly forbidden gap of the pristine material.Figure optionsDownload as PowerPoint slideHighlights
► Photoemission spectra of pristine and potassium-doped rubrene thin films are reported.
► Backbone and phenyl contributions to the spectral features of pristine rubrene are identified.
► Spectral features of n-doped rubrene are explicated by localization of doping electrons.
► Gap states formation mechanism is elucidated for n-doped organic semiconducting materials.