Engineering the chemical composition of inorganic-organic hybrid perovskite materials is an effective strategy to boost the performance and operational stability of perovskite solar cells (PSCs). Among the diverse family of ABX(3) perovskites, methylammonium-free mixed A-site cation Cs(x)FA(1-x)PbI(3) perovskites appear as attractive light-absorber materials because of their optimum band gap, superior optoelectronic property, and good thermal stability. Here, we develop a simple and very effective one-step solution method for the preparation of high-quality (Cs)(x)(FA)(1-x)PbI3 perovskite films upon the addition of excess CsCl to the FAPbI(3) precursor solution. It is found that the addition of CsCl as a source of Cs cation instead of relevant addition of CsI to the parent perovskite solution increases effectively the grain size and film quality leading to improved charge mobility, reduced carrier recombination, and long carrier lifetime. The resultant mesoscopic perovskite devices exhibit a maximum efficiency of 20.60% with a stabilized power conversion efficiency of 19.85% and lower hysteresis compared to the reference device. This performance is among the highest reported for PSC devices incorporating mixed cation (Cs)(x)(FA)(1-x)PbI3 perovskites.