Data files here contain the XY data reported in graphs in the main manuscript and ESI.

The file "Fig1B" contains the raw UV/Vis spectra for a CH3NH3PbI3-xClx film before (pristine) and after (degraded) thermal degradation, as well as PL data (emission) of pristine film. The first column contains the wavelength (X) data in nm.

"Fig2" contains the raw PXRD data for CH3NH3PbI3-xClx films before thermal degradation (pristine) and after thermal degradation (uncoated and coated with the polymers PMMA, PC, EC, PMP). The first column contains the values of 2 θ (X) in degrees.

“Fig3A" contains the raw UV/Vis spectra for a PMP-coated CH3NH3PbI3-xClx films degraded at 80°C for 0, 24, 48, 72, 96 and 144 hours. The first column contains the wavelength (X) data in nm.

“Fig3B" provides the degradation time (X) in hours and difference in absorbance between 750 nm and 800 nm for CH3NH3PbI3-xClx films (uncoated and coated with the polymers PMMA, PC, EC, PMP) degraded at 60°C in air.

“Fig3C" provides the degradation time (X) in hours and difference in absorbance between 750 nm and 800 nm for CH3NH3PbI3-xClx films (uncoated and coated with the polymers PMMA, PC, EC, PMP) degraded at 80°C in air.

“Fig3D" provides the degradation time (X) in hours and difference in absorbance between 750 nm and 800 nm for CH3NH3PbI3-xClx films (uncoated and coated with the polymers PMMA, PC, EC, PMP) degraded at 100°C in air.

“Fig8A" contains the raw data for the I-V curve of a CH3NH3PbI3-xClx film. Column 1 is voltage (V), column 2 is the current density (mA/cm2) for a pristine film and column 3 is for a thermally degraded film (heated at 100°C for 14 hours).

“FigS2" contains the raw PXRD data for CH3NH3PbI3-xClx films (uncoated and coated with the polymers PMMA, PC, EC, PMP) before thermal degradation. The first column contains the values of 2 θ (X) in degrees.

“FigS3A" contains the raw UV/Vis spectra for a CH3NH3PbI3-xClx film. Column 1 is wavelength (X) in nm, column 2 is the absorbance before PMMA coating and column 3 is the absorbance after PMMA coating.

“FigS3B" contains the raw UV/Vis spectra for a CH3NH3PbI3-xClx film. Column 1 is wavelength (X) in nm, column 2 is the absorbance before PC coating and column 3 is the absorbance after PC coating.

“FigS3C" contains the raw UV/Vis spectra for a CH3NH3PbI3-xClx film. Column 1 is wavelength (X) in nm, column 2 is the absorbance before EC coating and column 3 is the absorbance after EC coating.

“FigS3D" contains the raw UV/Vis spectra for a CH3NH3PbI3-xClx film. Column 1 is wavelength (X) in nm, column 2 is the absorbance before PMP coating and column 3 is the absorbance after PMP coating.

“FigS4" contains the raw UV/Vis spectra for polymer thin films, with wavelength (X) in nm and % transmittance for a substrate (blank), and PMMA, PC, EC,PMP thin films.

“FigS5A" contains the raw UV/Vis spectra for uncoated CH3NH3PbI3-xClx films heated at 80°C for 0, 24, 48, 72, 96 and 144 hours. The first column contains the wavelength (X) data in nm.

“FigS5B" contains the raw UV/Vis spectra for PMMA-coated CH3NH3PbI3-xClx films heated at 80°C for 0, 24, 48, 72, 96 and 144 hours. The first column contains the wavelength (X) data in nm.

“FigS5C" contains the raw UV/Vis spectra for PC-coated CH3NH3PbI3-xClx films heated at 80°C for 0, 24, 48, 72, 96 and 144 hours. The first column contains the wavelength (X) data in nm.

“FigS5D" contains the raw UV/Vis spectra for EC-coated CH3NH3PbI3-xClx films heated at 80°C for 0, 24, 48, 72, 96 and 144 hours. The first column contains the wavelength (X) data in nm.