1：Experimental Design and Method Selection

A low-temperature solution method was adopted to deposit a TiO2/SnO2 bilayer ETL. The methodology includes the synthesis of TiO2 nanoparticle solution and the preparation of SnO2 precursor, followed by spin-coating and annealing processes.

2：Sample Selection and Data Sources

The samples included ITO glass substrates, SnO2 colloid precursor, formamidinium iodide (FAI), methylammonium iodide (MAI), spiro-OMeTAD, PbI2, PbBr2, and other reagents. Data were sourced from systematic measurements of optical and electronic properties.

3：List of Experimental Equipment and Materials

Equipment included a solar simulator (Oriel, Sol3A), scanning probe microscope (SPM, Veeco, Dimension 3100), scanning electron microscope (SEM, Thermo Scientific, Verios G4 UC), Ultraviolet-visible spectroscopy (UV/Vis, Perkin-Elmer, Lambda 950), HORIBA FL3-111 fluorescence spectrometer, ellipsometry (Alpha-SE), and ultraviolet photoelectron spectroscopy (UPS, Axis Ultra DLD, Kratos Analytical Ltd, UK). Materials included ITO glass substrates, SnO2 colloid precursor, FAI, MAI, spiro-OMeTAD, PbI2, PbBr2, and other reagents.

4：Experimental Procedures and Operational Workflow

The TiO2 and SnO2 ETLs were prepared by spin-coating with their nanoparticle precursor solutions on the ITO glass at 5000 rpm for 30 s in ambient air, and then annealed on a hot plate at 140 and 150°C for 30 min, respectively. The perovskite precursor was spin-coated at 4500 rpm for 40 s, followed by baking at 140°C for 10 min for crystallization. The HTL solution was spin-coated on the perovskite layer at 4500 rpm for 30 s. Finally, an 80 nm silver electrode was evaporated on the spiro-OMeTAD layer.

5：Data Analysis Methods

The photovoltaic performance of PSCs was determined using a solar simulator. The external quantum efficiency (EQE) spectra were recorded with a commercial apparatus. The morphology images of the ETL films were obtained by SPM and SEM. The transmittance spectra were acquired by UV/Vis spectroscopy. PL and TRPL spectroscopies were investigated for charge transfer properties. The thickness of the TiO2 and SnO2 layer was analyzed via ellipsometry. UPS was used to measure the work function of different ETLs.