Title: WS$_2$ Monolayer Integration in a FAPbI$_3$-based Heterostructure

Abstract: Incorporating a monolayer of WS$_2$ via interface engineering enhances the overall physical properties of a FAPbI3 perovskite based heterostructure. FAPbI$_3$/WS$_2$/TiO$_2$/ITO and FAPbI$_3$/TiO$_2$/ITO heterostructures were analyzed by UV-Vis spectroscopy, X-ray diffraction, scanning electron microscopy and atomic force microscopy. The configuration with WS$_2$ interlayer presents higher absorption in the visible region with a bandgap of {\aprox} 1.44 eV. WS$_2$ also enhances the deposition process of FAPbI$_3$, resulting in the formation of pure photoactive $\alpha$-phase without the non-photoactive $\delta$-phase nor residual plumbates. The incorporation of the monolayer improves the crystalline structure of the FAPbI$_3$, promoting a preferential growth in the [100] direction. The smooth surface of WS$_2$ favors a homogeneous morphology and an increase of the grain size to ~4.5 $\mu$m, the largest reported for similar structures. Furthermore, the work function obtained lets us propose an enhance an adequate energy band alignment between FAPbI$_3$ and the n-type layers for the electron flux to the cathode. These findings strongly suggest that the interfacial coupling of FAPbI$_3$/WS$_2$ could be a promising candidate in photovoltaic applications.