Figure 3.

PTIR image ratios close to the absorption band gap in MAPbI_3-xCl_x mirroring the Cl⁻ distribution: AFM height images (first column: a, e, i, m) and PTIR absorption ratio maps (with respect to 556 nm, 2.23 eV absorption) recorded by illuminating the sample at 702 nm (1.77 eV, second column: b, f, j, n), at 678 nm (1.83 eV, third column: c, g, k, o) and at 645 nm (1.92 eV, last column: d, h, l, p). The data are displayed in rows as a function of the annealing: A0, as prepared sample (first row: a-d), A5, after annealing at 95 °C for 5 min (second row: e-h), A20, after additional annealing at 95 °C for 5 min and at 110 °C for 10 min (third row: i-l), and A30, after additional annealing at 110 °C for 10 min (fourth row: m-p). The as prepared sample is highly heterogeneous with Cl-poor (red tones) and Cl-rich (blue tones) regions. The sample becomes progressively more homogenous with annealing time. The intensity of the PTIR maps are plotted with the same scale range and all scale bars are 1.0 μm. Dots in panel a mark the locations where the PTIR spectra in figure 4 were collected.