The energy landscape of reduced-dimensional perovskites (RDPs) can be tailored by adjusting their layer width (n). Recently, two/three-dimensional (2D/3D) heterostructures containing n = 1 and 2 RDPs have produced perovskite solar cells (PSCs) with >25% power conversion efficiency (PCE). Unfortunately, this method does not translate to inverted PSCs due to electron blocking at the 2D/3D interface. In this talk, I will discuss our work on tuning the layer width of RDPs in 2D/3D heterostructures to address this problem. In situ transient absorption spectroscopy during 2D ligand treatment reveals that, in general, larger cations form 2D heterostructures more slowly, resulting in wider RDPs; small modifications to ligand design further affect the preference of n-values of the resultant 2D structure, due to strain relaxation. We discover that 3-fluoro-phenethylammonium (3F-PEAI) induces preferential growth of n ≥ 3 RDPs. Leveraging these insights, we developed efficient inverted PSCs (with a certified quasi-steady-state PCE of 23.91%). Unencapsulated devices operate at room temperature and around 50% relative humidity for over 1,000 h without loss of PCE; and, when subjected to ISOS-L3 accelerated ageing, encapsulated devices retain 92% of initial PCE after 500 h.