First Evidence of CH3NH3PbI3 Optical Constants Improvement in a N2 Environment in the Range 40–80 °C

We study the optical response of CH3NH3PbI3 layers to light solicitation under different environmental gas and temperature conditions. The measurements were performed in nonreactive (Ar or N2) and reactive (O2 or humid air) gas in the range 40–80 °C crossing the tetragonal–cubic transition (∼50 °C). With respect to truly inert Ar, the use of N2 not only assures the reversibility of the optical constants during thermal cycles but also improves the optical response of the material. While in N2 and Ar atmospheres the optical parameters of the material can be recovered at the end of the cycle, in contrast, the presence of humidity in the air causes the absorption coefficient to monotonically and inexorably decrease in the whole visible range, especially after the lattice has moved to cubic. The use of N2 thus represents an effective strategy to improve the absorption under thermal operation conditions.