Interfacial Interactions in Perovskite Solar Cells Tailored by Amino-Terminated  Self- Assembled Monolayers

Abstract

The use of self-assembled monolayers (SAMs) in hybrid photovoltaic has proven to be an extremely effective way of controlling the physicochemical properties of interfaces in thin film, layered architectures. In particular, they are used to improve perovskite solar cell (PSC) performance and stability. SAMs made up of X–Sp –Y molecules have been tailored to fit specific requirements: strong anchoring to the substrate by a COOH group (X), an effective self-organization by the Sp spacer (Sp= -(CH2)n-PP-(CH2)m; Sp = phenyl, (n,m) = (0,0), (0,1) (1,1)) and chemical affinity with the top layer by a NH or NH3+ group (Y).  SAMs with an amino/ammonium end-group have been deposited onto the metal oxide layer underneath the perovskite to reduce chemical instability at the interface. The amino/ammonium terminal substituent ensures a great affinity with the perovskite layer, leading to efficient anchoring which enhances the apparent filling level (AFL) in well organized and defect-free perovskite thin films. The chemical composition and structure of the molecules in the powder form and when deposited on the metal oxide were analyzed by X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy-energy dispersive X-ray analysis (SEM-EDX). In particular, we have correlated the amount of these Y groups at the outermost surface with the AFL and the overall photovoltaic performance. We compare the influence of such SAMs with different spacers on MAPbI3 or MA0.17FA0.83Pb(I0.83Br0.17)3  hybrid perovskite based either on MA = CH3NH3+ or  FA+  = (NH2-C+H-NH2).

Keywords: self-assembled monolayers, mesoporous TiO2, hybrid perovskite, interface engineering, solar cells