In the present manuscript, the effect of temperature on crystallisation of perovskite thin films (CH3NH3PbI3-xClx) was analysed (perovskite films are utilized to produce perovskite-based solar cells that appear to be devices with lower production costs and higher efficiency than silicon-based solar cells). To synthesise the perovskite films, four samples with precursor solution were spin coated for 45 s at 2000 rpm. After that, the samples were annealed at three different temperatures. Likewise, samples were characterised by UV/Vis Spectroscopy, Emission Spectroscopy, Fluorescence Microscopy, and ImageJ software were used to analyse crystal size. It was found that the sample annealed at 80 oC presented the highest fluorescence emission with 49539.09 CPS and the best particle size distribution. Band Gap Energy (Eg) was also calculated resulting in 1.60 ± 0.01 eV (An ideal solar cell has a band gap of 1.4 eV to absorb the maximum number of photons from the solar radiation).