1：Experimental Design and Method Selection:

The study systematically investigated the role of PbI2 in CH3NH3PbI3 perovskite thin films using scanning electron microscopy (SEM), X-ray diffraction (XRD), time-resolved photoluminescence spectroscopy (TRPL), and photoconductive atomic force microscopy (PC-AFM). Laterally-structured photodetectors were fabricated based on perovskite thin films deposited from precursor solutions with various CH3NH3I:PbI2 ratios (

2：

1,

3：

1,

4：

1, 1:1).

5：1). Sample Selection and Data Sources:

2. Sample Selection and Data Sources: Perovskite precursor solutions were prepared by dissolving CH3NH3I (MAI) and PbI2 in dimethylformamide (DMF) with fixed PbI2 concentration at 1 mol/L and varying MAI:PbI2 mole ratios. Indium tin oxide (ITO) glass substrates were used for device fabrication.

6：List of Experimental Equipment and Materials:

Materials included MAI, PbI2, DMF, ITO glass, polytetrafluoroethylene filter. Equipment included SEM (FEG SEM SU-70, Hitachi), XRD (Rigaku D/MAX-2500), TRPL (FS5 Fluorescence Spectrometer, Edinburgh Instruments), PC-AFM (Bruker Dimension ICON AFM), electrochemical station (CHI 760e, CH Instruments), LED light sources (MCWHF2, Thorlabs), power meters (TES-132, Thorlabs PM100D with S121C sensor), and lasers (532 nm, Changchun New Industries).

7：Experimental Procedures and Operational Workflow:

ITO substrates were cleaned and patterned, then treated with air plasma. Perovskite thin films were spin-coated and annealed. Devices were fabricated in a glove box. Characterization involved SEM for morphology, XRD for composition, PL and TRPL for optical properties, IV and I-t measurements for electrical performance, and PC-AFM for local photocurrent analysis.

8：Data Analysis Methods:

XRD patterns were analyzed for phase identification, PL and TRPL data were fitted for lifetime analysis, IV curves were used to calculate responsivity and detectivity, and PC-AFM images were analyzed with NanoScope-Analysis software for current mapping.