To push the Shockley-Queisser limit for photovoltaic cell PCE, tunable perovskite-based top cells for triple-junctions have been investigated. The tunability of the bandgap of CsPbI2Br, in combination with its photo-physical properties and thermal stability, makes the material ideal for this application, but it needs to be engineered and deeply characterised. Furthermore, being fully evaporated, the cell can avoid the annealing step usually needed, preventing changes in the underlying tandem induced by high temperatures. The objective of this thesis is to investigate the air- and photo-stability of co-evaporated, high-bandgap CsPbI2Br-based solar cells, and to analyse and engineer this cell for triple-junction applicability.