Role of TiO2 as a cathode buffer layer for minimizing electron injection barrier at the Co/Alq3 interface

Abstract

This work highlights the important role of a titanium dioxide (TiO2) cathode buffer layer in enhancing charge injection at the cobalt (Co)/tris(8-hydroxyquinolinato) aluminum (Alq3) interface—an important junction in spin organic light-emitting diodes (S-OLEDs) and organic spin valves. We systematically investigate the interfacial electronic structure of the TiO2 layer within the [Co/TiO2/Alq3/Si substrate] thin film architecture using ultraviolet photoelectron spectroscopy (UPS) and x-ray photoelectron spectroscopy (XPS). Four thin film structures—[Alq3/Si], [TiO2/Alq3/Si], [Co/TiO2/Alq3/Si], and [Co/Alq3/Si]—were fabricated via electron beam evaporation under ultra-high vacuum conditions. The insertion of the TiO2 layer induces significant interface dipoles of 1.6 eV at the Co/TiO2 interface and 0.9 eV at the TiO2/Alq3 interface, as revealed by UPS, effectively lowering the electron injection barrier. XPS further confirms the presence of Ti3+ states and oxygen vacancies, which contribute to improved electronic structure and reduced conductivity mismatch. These insights demonstrate that TiO2 acts as an efficient cathode buffer layer, offering a compelling approach to optimize charge transport and device performance in organic spintronic applications.