Today's best perovskite solar cells are realized under highly controlled ambient conditions, using N₂ filled glove boxes with only trace levels of oxygen and water. The influence of the atmospheric conditions, and in particular of the relative humidity (RH), during the device fabrication is thus an issue that needs to be understood in order to realize efficient perovskite solar cells without any control of the environmental conditions. This would simplify the upscale of the devices for future mass production. Here we illustrate a method to improve the power conversion efficiency of perovskite (CH₃NH₃PbI₃) solar cells fabricated by two-step procedure in an environment with high relative humidity, based on the addition of HCl in the PbI₂/N,Ndimethylformamide precursor solution. Perovskite solar cells have been realized using various concentrations of HCl under different relative humidity values. The main result is that the addition of 0.5 vol % of HCl in the precursor solution allows a relative efficiency increase of about 70% when the devices are fabricated at very high relative humidity (RH=65%).