The stability of organic-inorganic halide perovskite material is the major issue hindering the fast industry transfer of laboratory technology already approaching 23% certified efficiency. Accurate characterization methods are necessary to understand and describe complex heterogeneous material properties. CH₃NH₃PbI₃ perovskite often contains inclusions of PbI₂ as a result of either a two-step preparation process or degradation in the presence of moisture, light illumination, and external voltage bias. Recently, we have described how such coexistence of these two phases can be detected by absorptance spectroscopy[1], Raman spectroscopy[2], photothermal deflection spectroscopy (PDS), and Fourier-transform photocurrent spectroscopy (FTPS)[3]. We have observed relatively higher photosensitivity in the PbI₂ phase compared to the CH₃NH₃PbI₃ phase in degraded material. This has been explained by a photoinduced modulation doping [3].  

In this work, the light induced degradation of CH­₃NH₃PbI₃ perovskite films in ambient conditions, as well as different dosing of methylammonium iodide during a single step or two step processes, is used to prepare materials with different concentrations of PbI₂ inclusions. Optical absorptance and photocurrent spectroscopy are used to track this concentration. Kelvin Probe and Photoluminescence techniques are used to gain new insight into the CH₃NH₃PbI₃ / PbI₂ interface effects. Using Kelvin Probe techniques, we investigate the effects of the PbI₂ phase on the work function in the dark and under illumination. It is observed that the work function of the CH­₃NH₃PbI₃ film gradually increases during the light induced degradation steps until the film is completely degraded. The gradual increase in work function can be attributed to modulation doping of the CH₃NH₃PbI₃ by PbI₂ phase, as suggested earlier. Using illumination, photovoltage is also observed at the CH­₃NH₃PbI₃ / FTO interface thanks to electron extraction by the FTO. Steady-state and time-resolved PL measurements confirm this extraction, as well as a drop in charge carrier lifetimes as the localized defects and vacancies are increased during the degradation.

References 1. De Wolf, S., et al., The journal of physical chemistry letters, 2014. 5(6): p. 1035-1039. 2. Ledinský, M., et al., The journal of physical chemistry letters, 2015. 6(3): p. 401-406. 3. Holovsky, J., et al., J Phys Chem Lett, 2017. 8(4): p. 838-843.