In this work, a facile, versatile approach to control the grain size and layer porosity of perovskite films is introduced. By gradually tuning the amount of additive (hypophosphorous acid, HPA) in the methylammonium lead triiodide (MAPbI3) perovskite precursor solution, the perovskite grain size can be enhanced by over one order of magnitude regardless of the choice of the underlying layer or substrate. A further increase in the HPA amount results in the formation of a porous perovskite layer with large grains. When incorporated in perovskite solar cells with an inverted architecture, the increase in grain size results in devices with a far higher open-circuit voltage (up to 1.16 V), corresponding to a power conversion efficiency of 20.1%. This work shows that HPA is superior to many other additives in its ability to tune the film microstructure and further demonstrates that unlike other additives, it can also be used to form high-quality porous perovskite films of great interest for application in light-emitting diodes.