Optical absorption spectra of perovskite thin films for defect estimation by photothermal bending spectroscopy

The measurement of the optical absorption coefficient spectra of perovskite thin films of CH3NH3PbI3 by resonant photothermal bending spectroscopy (resonant-PBS) and Fourier-transform photocurrent spectroscopy (FTPS) for the defect estimation of the material has been attempted. A fundamental absorption edge was observed in the photon energy ħω region of 1.5–1.6 eV by both techniques. In addition, extra absorption of about 102 cm−1 was detected at ħω < 1.5 eV only in the spectra measured by resonant-PBS. This extra absorption seems to indicate the existence of localized states in the bandgap of CH3NH3PbI3 films. The difference between resonant-PBS and FTPS profiles below the absorption edge is caused by the difference in measurement mechanism, that is, resonant-PBS utilizes a temperature rise of the sample by the nonradiative recombination of photoexcited carriers, while FTPS measures photocurrent arriving at electrodes by the electric field after the photoexcitation. These results seem to indicate that resonant-PBS is a useful tool for the defect estimation of CH3NH3PbI3.