Temperature-dependent photoluminescence (PL) spectroscopy measurements have been performed over a range from 9 K to room temperature on polycrystalline methylammonium (MA)/formamidinium (FA) lead iodide (MA1-xFAxPbI3) perovskite thin films. Our low-temperature PL analysis reveals the existence of charge compensating defects in MAPbI3, which may explain the lower net free carrier concentration in MAPbI3 perovskite. More interestingly, we observe the suppression of the PL emission associated with the charged defects by appropriate FA inclusion. Furthermore, FA incorporation into MAPbI3 has been found to slow the phase transformation of MA1-xFAxPbI3 from orthorhombic to tetragonal phase, which occurs with increasing temperature. Our analyses of the FA concentration's impact on defect density and structural phase transformation provide beneficial insights that improve the understanding of the photovoltaic properties and application of organic-inorganic metal halide perovskites.