methylammonium 
 iodide ( ch3nh3i ) was prepared by dissolving 
 methylamine ( sigma - aldrich 534102 ) in absolute ethanol and reacting 
 with hydroiodic acid ( sigma - aldrich 210021 ) in an ice bath . then , 
 12.4 ml of methylamine and 6 ml of hydroiodic acid were mixed with 
 50 ml of ethanol . the solvent was removed by rotary evaporation , and 
 the obtained white powder was washed with anhydrous diethyl ether . ch3nh3i and pbcl2 ( sigma - aldrich 
 268690 ) were dissolved at a final concentration of 40 wt % in anhydrous n , n - dimethylformamide ( dmf ) ( the molar 
 ratio between ch3nh3i and pbcl2 was 
 3:1 ) . mapbi3 films were grown onto clear white glass 
 substrates ( proscitech ) previously cleaned sequentially in ethanol , 
 acetone , 2-propanol , and finally treated oxygen plasma for 300 s. 
 ch3nh3pbi3 precursor solution ( 100 
 l ) was deposited ( substrate area  2 cm  2 cm ) 
 by spin - coating at 5000 rpm . after 6 s , while the spinning process 
 took place , 200 l of anhydrous chlorobenzene was dropped onto 
 the center of the substrate , as previously reported . the resulting 
 films were then annealed at 100 c for 3 h , until a pure ch3nh3pbi3 layer was obtained . optical characterization . total transmittance ( t ) and total reflectance ( r ) spectra of the ch3nh3pbi3 films were measured using an 
 integrating sphere attached to a uv  vis spectrophotometer ( shimadzu 
 uv-2101 pc ) . the spectral dependence of the absorptance ( a ) of the ch3nh3pbi3 films was determined 
 by making use of the formula a = 1  r  t. ple measurements were 
 performed in a commercial fluorometer ( fluorolog 3 from horiba ) . the 
 samples were excited with a spot of 1  1 cm with 
 a wavelength variable between 450 and 850 nm ( fwhm = 5 nm ) , and detection 
 was carried out at an angle of  = 22.5 at the maximum 
 of pl (  = 775 nm ) . pl measurements were carried out by 
 optically pumping the samples with a tunable picosecond laser source 
 ( fianium sc400 ) delivering low power ( sub - mw ) , 900 ps long pulses 
 with a 1 khz repetition rate . the pump beam was focused down to a 
 100 m spot with an achromatic lens ( f = 10 
 cm ) , which also collected the emission . spectra were collected at 
 0.5 s intervals with a fiber - coupled spectrometer ( usb2000 from ocean 
 optics ) .
the photophysical properties of films 
 of organic  inorganic lead halide perovskites under different 
 ambient conditions are herein reported . 
 we demonstrate that their 
 luminescent properties are determined by the interplay between photoinduced 
 activation and darkening processes , which strongly depend on the atmosphere 
 surrounding the samples . 
 we have isolated oxygen and moisture as the 
 key elements in each process , activation and darkening , both of which 
 involve the interaction with photogenerated carriers . 
 these findings 
 show that environmental factors play a key role in the performance 
 of lead halide perovskites as efficient luminescent materials .