Zinc oxide as a defect-dominated material in thin films for photovoltaic applications – experimental determination of defect levels, quantification of composition, and construction of band diagram

In the present work, thin ZnO layers were synthesized by the sol-gel method with subsequent spin-coating on Si(100). We show that the detailed analysis of lab-recorded photoemission spectra in combination with Kelvin probe data yielded the work function, ionization energy, and valence band - Fermi level separation - and hence enabled the construction of band diagrams of the examined layers. With small modifications in preparation, very different films can be obtained. One set shows a homogeneous depth-dependent n carrier distribution, and another a significant carrier concentration gradient from n-type conductivity to almost metal-like n(+) character. Likewise, the surface morphology can be tuned from a uniform, compact surface with spherical single-nm sized grain-like features to a structured surface with 5-10 nm tall crystallites with (002) dominating crystal orientation. Based on the band-bending and the energy levels observed, defects of contradictory nature, i.e. acceptor-donor-trap (ADT) properties, were identified. These defects may be groups of point defects, with opposite character. The ADT states affect the energy levels of the oxide layers and due to their nature cannot be considered in the photoemission experiment as mutually independent. The versatile nature of the synthesis provides us with the opportunity to tune the properties with a high degree of freedom, at low processing costs, yielding layers with an exotic electronic structure. Such layers are interesting candidates for applications in photovoltaic and nanoelectronic devices.