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Recently, lead based organic inorganic hybrid perovskites (OIHP) have emerged in the photovoltaic field, achieving performance comparable to silicon-based technologies in few years of investigation.1,2 Despite the prodigious performance, there is a lack of information about many aspects related to the structure, homogeneity and stability of OIHP films. Synchrotron infrared nanospectroscopy (nano-FTIR) is a technique that uses an atomic force microscope (AFM) equipped with external optics which allow the focus of the synchrotron light onto the metallic AFM tip. The tip acts as an antenna and then we can obtain infrared spectra with resolution of the tip size (~20nm).3 Here, for the first time, the nano-FTIR technique was used to map the surface of the FAMA and CsFAMA perovskite films deposited onto Si/Au substrates before and after degradation in a hot plate in ambient atmosphere. The nano-FTIR images revealed a spatial heterogeneity in nanoscale in both samples. The infrared spectra collected in regions with small scattering nano-FTIR signal presented the main absorption of the formamidinium (FA) molecule at 1712 cm-1 that corresponds to symmetric C-N stretching. The regions with high scattering signal did not show any absorption related to FA, which indicated that these regions are rich in the inorganic elements/compounds. After degradation, we attributed the high scattering regions to lead iodide (PbI2), and this data were corroborated with X-ray diffraction analysis. Our results show that the perovskite films present high compositional heterogeneity even before degradation and that the nano-FTIR can be a powerful tool to monitor the perovskite composition at nanoscale.
Acknowledgments: The authors would like to thank FAPESP, INEO, LNLS and CNPq for financial supports.
References
1. W. S. Yang et al., “High-performance photovoltaic perovskite layers fabricated through intramolecular exchange,” Science 348(6240), 1234–1237 (2015) [doi:10.1126/science.aaa9272].
2. W. S. Yang et al., “Iodide management in formamidinium-lead-halide–based perovskite layers for efficient solar cells,” Science 356(6345), 1376–1379 (2017) [doi:10.1126/science.aan2301].
3. B. Pollard et al., “Infrared Vibrational Nanospectroscopy by Self-Referenced Interferometry,” Nano Lett. 16(1), 55–61 (2016) [doi:10.1021/acs.nanolett.5b02730].
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