Characterization of solution processed TiOx buffer layers in inverted organic photovoltaics by XPS and DFT studies

TL;DR

This study investigates the chemical composition and electronic structure of solution-processed TiOx buffer layers in inverted organic photovoltaics using XPS and DFT, revealing Ti^{3+} states and residual carbon effects.

Contribution

It provides a detailed analysis of TiOx buffer layers' chemical states and electronic properties, highlighting the presence of Ti^{3+} and residual carbon through combined experimental and theoretical methods.

Findings

Ti^{3+} states are present in TiOx films.

Residual carbon influences the TiOx electronic structure.

DFT confirms oxygen deficiency effects.

Abstract

We present the results of XPS measurements (core levels and valence bands) of solution processed TiOx thin films prepared from titanium butoxide (C16H36O4Ti) diluted in isopropanol which is a common sol-gel route for fabricating TiOx electron selective contacts for ITO/TiOx inverted organic photovoltaics bottom electrodes. XPS Ti 2p and valence band spectra show the presence of additional features which are absent in spectra of titanium butoxide deposited on Si and are attributed to appearance of Ti^{3+} valence states in ITO/TiOx. This conclusion is confirmed by density functional theory electronic structure calculations of stoichiometric TiO2 and oxygen deficient TiO_{2-1/8}. XPS C 1s measurements show the formation of C-O and O-C=O bonds which evidence the presence of residual carbon which can draw oxygen from the film network and induce the formation of fraction of Ti3+ states in TiOx films.