Perovskite solar cells with record efficiencies are generally fabricated using a solution-based approach for the deposition of the perovskite absorber layer [1-3]. However, for large area processing on industrial scale alternative deposition methods are desired. Vapour-based method  has shown interesting potential, but further investigation on growth mechanism of absorber layer are required for establishing the applicability of these emerging methodologies. Our work aims at improving the understanding of the growth mechanisms, especially the essential factors governing the formation of desired single phase of (Cs,FA)Pb(I,Br)3 perovskite deposited by a sequential two-step vapour process. In this process, an inorganic precursor layer is first deposited by thermal co-evaporation of CsBr and PbI2 at high vacuum. The precursor is subsequently converted into perovskite by the reaction with formamidinium iodide vapour. This step is carried out at a reduced pressure of a few mbar in a tube furnace with separate heating zones for the organic source material and the substrate using nitrogen as carrier gas (see figure 1a).