Gabriele Salvinelli

Band offsets and density of Ti3+ states probed by x-ray photoemission o LaAlO3/SrTiO3 heterointerfaces and their LaAlO3 and SrTiO3 bulk precursors

A set of LaAlO3/SrTiO3 (LAO-STO) interfaces has been probed by x-ray photoemission spectroscopy in order to contrast and compare the effects of LAO overlayer thickness and of the growth conditions on the electronic properties of these heterostructures. These effects are tracked by considering the band offset and the density of Ti+3 states, respectively. It is shown that the dominant effects on the local electronic properties are determined by the O2 partial pressure during the growth. In particular, a low PO2 yields Ti+3 states with higher density and lower binding energy compared to the sample grown at high PO2 or to the bare STO reference sample. Band-offset effects are all below about 0.7 eV, but a careful analysis of Ti 2p and Sr 3d peaks shows that valence-band offsets can be at the origin of the observed peak width. In particular, the largest offset is shown by the conducting sample, which displays the largest Ti 2p and Sr 3d peak widths

An ultrathin TiO2 blocking layer on Cd stannate as highly efficient front contact for dye-sensitized solar cells

An engineered multilayer structure of platinum-cadmium stannate-titanium oxide (Pt-CTO-TO), with different TO layer thickness (in the range 1-5 nm), has been grown at 400 °C on glass substrates by RF magnetron sputtering, following a 2-step procedure without breaking vacuum. To produce an alternative and reliable front contact for dye sensitized solar cells (DSCs), morphology and composition of a TO blocking layer have been studied, paying particular attention to the oxide-oxide (CTO-TO) interface characteristics. The influence of the metallic mesh on the transparent conductive oxide sheet resistance has also been considered. A sputtered CTO layer shows a high average transmittance, over 90%. The Pt mesh yields a drastic reduction in the series resistance, almost one order, without affecting the optical properties. The ultrathin blocking layer of Ti oxide prevents charge recombination, improving the overall performance of the solar cells: +86% in efficiency, +50% in short circuit current, with respect to bare CTO.

Layer-Resolved Cation Diffusion and Stoichiometry at the LaAlO3/SrTiO3 Heterointerface Probed by X-ray Photoemission Experiments and Site Occupancy Modeling

The layer-resolved cation occupancy for different conducting and insulating interfaces of LaAlO3 (LAO) thin films on SrTiO3 (STO) has been determined by angle-resoled X-ray photoelectron spectroscopy (AR-XPS). Three STO interfaces with LAO have been considered, namely, a conducting interface with a 5 unit cell (u.c.) LAO layer, an insulating interface with a 5 u.c. LAO layer, and an insulating interface with a 3 u.c. LAO layer. Considering inelastic and elastic scattering processes in the transport approximation, the core-level signal attenuation has been modeled on the basis of Monte Carlo calculations of the electron trajectories across the heterostructures. Different effects involving cation stoichiometry and diffusion through the interface have been considered to interpret data. Beyond a mere abrupt interface modeling, the LaAlO3/SrTiO3 heterojunction is shown to host cation diffusion processes within 3-4 unit cells in the bulk layer, along with a clear Sr substoichiometry, an issue so far virtually neglected in the analysis of these systems. The present results show the capability of the AR-XPS modeling to explore element-sensitive properties at the oxide interfaces, matching and completing the information that can be provided by probes based on electron microscopy or X-ray scattering.

Stoichiometry Gradient, Cation Interdiffusion, and Band Alignment between a Nanosized TiO2 Blocking Layer and a Transparent Conductive Oxide in Dye-Sensitized Solar Cell Front Contacts

An angle-resolved photoemission spectroscopy study allowed us to identify cation interdiffusion and stoichiometry gradients at the interface between a nanosized TiO2 blocking layer and a transparent conductive Cd-Sn oxide substrate. A stoichiometry gradient for the Sn cations is already found in the bare Cd-Sn oxide layer. When TiO2 ultrathin layers are deposited by RF sputtering on the Cd-Sn oxide layer, Ti is found to partially replace Sn, resulting in a Cd-Sn-Ti mixed oxide layer with a thickness ranging from 0.85 to 3.3 nm. The band gap profile across the junction has been reconstructed for three TiO2 layers, resulting in a valence band offset decrease (and a conduction band offset increase) with the blocking layer thickness. The results are related to the cell efficiencies in terms of charge injection and recombination processes.