Tuning of titanium dioxide surface energy levels by self-assembled monolayers for optoelectronic applications

Abstract

Tailoring the surface work function and/or surface functional group of oxide active layers in an optoelectronic device is an important means for performance improvement. One way to proceed is to adsorb molecules with varying dipole moment strength and sign. We report on the surface modification of mesoporous TiO₂ using different self-assembled monolayers (SAMs) of acids. The energetics at the interface have been determined using a series of photoelectron spectroscopy techniques (UPS, IPES, XPS). We have shown that the observed changes in work function is correlated to the dipole moments of the respective acids, calculated by density functional theory. This interfacial engineering approach can be used to control the charge extraction from an optoelectronic device. A novel approach is proposed for boosting the performances of self-powered photodetectors. Visible-blind UVA photodetectors have been built by combining a mesoporous TiO₂ layer with a Spiro-OMeTAD layer. SAMs interlayer induces a step in the vacuum energy, the formed dipole field dramatically affects the charge transfer and then the photocurrent/photoresponse of the device. The effect of para-substituted benzoic acids and β-alanine on the functioning of triple cation perovskite solar cells is then developed. We show then a best improvement for 4-chlorobenzoic acid SAMs which is due to the reduction of interfacial states, to the improvement of the quality of the perovskite material and to a better structural continuity.