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The chemical analysis of solar cell materials is an excellent application of Vis-PiFM. In a recent publication, our customers analyzed a ternary cation halide Cs0.05FA0.81MA0.14PbI2.55Br0.45 (CsFAMA)-based perovskite visual sensor that exhibits full-visible-spectra photovoltaic behavior and reconfigurable responsivity for adaptive image sensing and in-sensor machine vision. When this sensor is biased, accumulation of ionic species at the electrode/perovskite interfaces changes, which alters the potential profile inside the photovoltaic matrix, which in turn leads to changes in light absorption profile.
To observe the effect of ion migration, a PiFM tip was used as the electrode to apply the stress voltage to the perovskite sensor while mapping the strength of absorption at 650 nm. When the film is biased at −1V (2nd image in the sequence shown below), a significant increase in absorption (inferred by the strength of the PiFM signal) is observed indicating that the organic cations are attracted by the external electric field from the interior section of the CsFAMA film to the surface area (Figure (a)). As the stress voltage is decreased from −1 to 0V in 0.2V steps, the electric field that holds the positively charged ions gets weaker and becomes less effective to maintain the surface cationic species. Consequently, some ions drift back into the deeper region of the CsFAMA layer under concentration gradient, leading to a decrease in both the maximum PiFM signal intensity and absorption areas. When the polarity of the stress voltage is reversed (Figure (b)), the organic cations are driven away from the perovskite surface area by the electric field, which in turn further reduces the absorption intensity. The last image at 0V is similar to the original image at 0V, suggesting that the electric field-induced ion migration in CsFAMA perovskite film is almost fully reversible.
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