Electroluminescence at the n ‐ ZnO / Electrolyte Interface: A Comparative Study of the Emission Processes under Cathodic and Anodic Pulsed Polarization

Abstract

Electroluminescence (EL) of polycrystalline electrodes in aqueous electrolytes is studied (i) under cathodic polarization during reduction of the ions, and (ii) under strong anodic polarization in a supporting electrolyte. The pulsed technique used here avoids a too rapid electrochemical decomposition of the electrodes. Under cathodic conditions (i), the EL onset vs. SCE (pH 3.0) is only 0.3V negative to the flatband potential and indicates a hole injection into . The two‐step potential dependence of both emission and current intensities between and −4.0V vs. SCE evidences the direct relationship between the interfacial electrochemical events and the radiative recombinations e¯/h+ pairs inside the material. The narrow UV band‐to‐band emission with a structure at 384 and 394 nm shows that the injection occurs via the valence band; a broad visible EL band (400–750 nm) is also observed, which presents a 80 nm blue‐shift with increasing excitation, suggesting that emission arises from a donor‐acceptor (D‐A)mechanism. The El relaxation time is roughly 60 μs, whatever the wavelength is. Cobalt doping (1.0 atom percent (a/o)) decreases the emission intensity but induces a new EL band at 705 nm. Under anodic polarization (ii), intense breakdown currents appear at 9.7V vs. SCE and are attributed to an electron injection into the conduction band by surface oxygen atoms . The quasi‐simultaneous light emission arises from an impact‐ionization mechanism, according to the dependence found here. Anodic El spectra of both pure and Co‐doped electrodes show a visible emission and an intense near‐IR band , the latter being due to recombinations on a native midgap EL center. The long‐lived IR emission put forward the role of traps in the EL mechanism. The excitation mode, either forward (i) or reverse (ii) polarization, is determinant for the EL characteristics of the junction.