Figure 1 shows the schematic illustration of SnO₂ films fabricated by the chemical bath deposition and PAPT method. Etched FTO glasses were immersed in dilute SnCl₂ aqueous solution with urea, thioglycolic acid, and hydrochloric acid, and then heated at 70 °C for 4 h. For PAPT‐modified substrates, they were subsequently immersed in periodic acid solution with different concentrations for 5 min. After that, the substrates were thoroughly washed with deionized water to remove the residual reagents and annealed at 150 °C for 40 min.

To study the treatment mechanism of periodic acid to SnO₂ films, we conducted X‐ray photoelectron spectroscopy (XPS) measurements. For the Sn 3d XPS spectra (Figure 2a), the two peaks of Sn 3d_5/2 and 3d_3/2 in pristine SnO₂ shifted from 487.2 and 495.6 eV to the higher binding energies of 487.6 and 496.1 eV, with the spin–orbit splitting energy of 8.4 eV.^{[ 18 ]} The higher binding energy of Sn 3d indicates more Sn (II)’s conversion to Sn (IV),^{[ 16} , ^{19 ]} potentially widening the band gap and decreasing the number of oxygen vacancies.^{[ 20} , ^{21 ]} In terms of O 1s spectra (Figure 2b), two peaks of around 532.5 and 531.1 eV refer to the hydroxyl group (–OH) on the SnO₂ surface and the saturated lattice oxygen in SnO₂ film, respectively.^{[ 17} , ^{20 ]} The calculated areas of the –OH in pristine SnO₂ reduced from 82.9% to 35.3% in the PAPT‐modified SnO₂, indicating more chemisorbed non‐lattice oxygen transferred to lattice oxygen and existed in the form of SnO₂. It is known that surface –OH introduces a deep energy level into the bandgap that is near the valence band, bringing the generation of non‐radiative recombination and energy losses in the devices.^{[ 22 ]} As shown in Figure 2c, two typical peaks of I 3d orbitals appeared in the PAPT‐modified SnO₂ sample. In contrast, the pristine SnO₂ sample showed no signal, indicating iodide remained on the PAPT‐modified SnO₂ surface by chemical doping. The two peaks of I 3d_5/2 and 3d_3/2 located at 619.8 and 631.3 eV refer to iodine anion (I⁻), which were consistent with the referenced SnI₂ signals in the previous reports.^{[ 23} , ^{24 ]} According to the XPS results of Sn 3d and O 1s, we believe that the PAPT treatment urges the transformation of Sn(II) to Sn(IV) and the conversion of chemically absorbed hydroxyl to lattice oxygen, which lead to reduced trap states and potentially higher V _OC and FF of the PSCs.

As previously reported,^{[ 17} , ^{22 ]} four kinds of oxygen defects exist on the surface of the CBD‐SnO₂ films, including two kinds of hydroxyl defects and two kinds of oxygen vacancy defects (V _O): 1) terminal hydroxyl (OH_T), which is bonded to a single Sn atom on the surface (Figure S1a, Supporting Information); 2) bridge hydroxyl (OH_B), including a bridge oxygen and an absorbed hydrogen atom (Figure S1b, Supporting Information); 3) bridge‐oxygen vacancy (V _OB), i.e., the missing of the bridge oxygen atom (Figure S1c, Supporting Information); 4) deep oxygen vacancy (V _OD), i.e., the missing of another oxygen atom located at a deeper layer than bridge oxygen (Figure S1d, Supporting Information). To figure out if the iodine atom will remain at the hydroxyl site or fill the V _O, we conducted density functional theory (DFT) calculations. The stoichiometric (110) lattice plane was chosen as the studied surface due to that it is the most thermodynamically stable configuration.^{[ 12} , ^{25 ]} According to the results of DFT calculations, the absorption energies (E _a) of the iodine atom absorbed on terminal Sn (Figure 2d) and bridge O site (Figure 2e) are −1.170 and −2.732 eV. The negative value of E _a indicates that the iodine atom is prone to be absorbed in the same site after the hydroxyl groups were eliminated by periodic acid. Also, the more negative E _a implies iodine atoms are more apt to be absorbed in the bridge oxygen sites. As for V _O, the E _a of the iodine atom substituted the bridge V _OB is exactly the same as previously calculated results, which is −2.732 eV. And the E _a of the iodine atom substituted at V _OD (−1.973 eV) showed a similar result (Figure 2f), indicating that iodine atoms can effectively passivate the V _O defects on the SnO₂ surface. Above all, the schematic image of the PAPT method is shown in Figure 2g.

From the above analysis, we found that periodic acid has the following three functions: 1) H⁺ can decrease the surface hydroxyl groups; 2) The strong oxidation of IO65− can oxidize Sn²⁺ into Sn⁴⁺; 3) The reduced I⁻ can also reduce the surface –OH and fill in the oxygen vacancies. To figure out which one plays a more critical role, we treated SnO₂ films with sulfuric acid (H₂SO₄) and hydroiodic acid (HI), respectively. It should be noted that the concentrations of H₂SO₄ and HI are 5 and 2 mm, respectively, which are consistent with the H⁺ and I⁻ concentrations in the optimal concentration of periodic acid (2 mm, which will be discussed later).

XPS spectra of SnO₂ films treated by H₂SO₄ and HI are shown in Figure S2 (Supporting Information). The double peaks of the Sn 3d of both samples are located at 487.2 and 495.6 eV (Figure S2a, Supporting Information), which is consistent with the position of the control sample (Figure 2a), indicating that a small amount of Sn²⁺ may exist in the SnO₂ films. It is because dilute H₂SO₄ and HI cannot oxidize Sn²⁺ into Sn⁴⁺. As shown in Figure S2b (Supporting Information), the O 1s peak can be divided into two peaks corresponding to –OH and lattice oxygen, respectively. It can be seen that the –OH peak area of H₂SO₄‐modified SnO₂ samples reduces from 82.9% to 40.1%, while that of HI‐modified samples only reduces it to 53.2%, indicating that H⁺ is more effective for eliminating the surface –OH than I⁻. Finally, we observed weak I 3d double peaks on the surface of the HI‐treated sample (Figure S2c, Supporting Information), the intensity of which is much lower than that of the H₅IO₆‐treated sample, suggesting that more iodine atoms in periodic acid stay on the SnO₂ films. Above all, periodic acid has a stronger ability to eliminate surface oxygen defects than H₂SO₄ and HI. Furthermore, we carried out XPS depth analysis measurements to study the oxygen defects inside the SnO₂ film. The etching depth is around 10 nm. It can be seen that the peak of O 1s is located around 531.1 eV (Figure S3, Supporting Information), corresponding to the position of lattice oxygen, while no –OH peaks are observed. In addition, I 3d double peaks are observed in PAPT‐modified SnO₂ samples, indicating that only the periodic acid can fill the oxygen vacancies inside the SnO₂ films.

Ultraviolet photoelectron spectroscopy (UPS) measurements were performed to investigate the influence of energy level by periodic acid. The valence band maximum (VBM) values of samples can be calculated by the formula of VBM = 21.22 eV − E _cutoff + E _onset.^{[ 26 ]} As shown in Figure 3a, the corresponding VBM values of pristine SnO₂ and PAPT‐modified SnO₂ are 8.20 and 8.12 eV, respectively. In combination with the optical bandgap values of samples (Figure 3b), the conduction band minimum (CBM) values of pristine SnO₂ and PAPT‐modified SnO₂ are 4.30 and 4.19 eV, respectively. Similarly, the perovskite's CBM and optical bandgap can be calculated as 4.15 eV (Figure S4, Supporting Information) and 1.56 eV (Figure S5, Supporting Information), respectively. Therefore, the schematic energy level diagram of FTO, the pristine SnO₂, PAPT‐modified SnO₂, and perovskite is shown in Figure 3c. A better energy band alignment between the PAPT‐modified SnO₂ and perovskite layers was achieved due to the upshift of the conduction band and Fermi level of SnO₂. As a result, a reduced energy barrier and faster electron extraction at the SnO₂ and perovskite interface could be realized.^{[ 27 ]}

Grazing incidence X‐ray diffraction (GIXRD) was conducted to investigate the change in the crystal structure of the SnO₂ films. As shown in Figure 4a, several peaks of the PAPT‐modified SnO₂ film are slightly higher than that of the pristine sample, indicating a better crystallinity was obtained after PAPT treatment. We carried out electron conductivity and space‐charge‐limited current (SCLC) measurements of devices based on the FTO/SnO₂/Au structure, as shown in Figure 4b and Figure S6 (Supporting Information). The PAPT‐modified SnO₂ film exhibit higher conductivity and electron mobility than those of the pristine SnO₂ film, suggesting the PAPT treatment could improve the electron transport of the SnO₂ films. As shown in Figure 4c, Hall effect measurements demonstrate a similar result. The PAPT‐modified SnO₂ film showed a higher hall mobility µ _H (3.07 × 10⁻³ m² V⁻¹ s⁻¹) and larger carrier concentration n (5.63 × 10²⁷ m⁻³) than those of the pristine SnO₂ film (µ _H = 2.88 × 10⁻³ m² V⁻¹ s⁻¹ and n = 5.36 × 10²⁷ m⁻³).

According to the UV–visible light transmittance spectra of SnO₂ films (Figure S7, Supporting Information), the PAPT treatment would not influence the high visible light transmission of SnO₂ films. Atomic force microscopy (AFM) measurements were carried out to investigate the surface morphology of the chemical‐bath deposited SnO₂ layers (Figure S8, Supporting Information). The PAPT‐modified SnO₂ film showed a relatively lower roughness with a root mean square (RMS) of 9.91 nm compared to the pristine SnO₂ film (RMS = 11.08 nm), indicating the PAPT treatment would not impair the crystal growth process of the upper‐level perovskite layers. Scanning electron microscope (SEM) of perovskite films (Figure S9, Supporting Information) deposited on the surface of the pristine and PAPT‐modified SnO₂ films could also prove this point of view.

Steady‐state photoluminescence (PL) and time‐resolved PL (TRPL) measurements were carried out to study the charge transfer dynamics at the SnO₂/perovskite interfaces. As shown in Figure 4d, the perovskite film on the PAPT‐modified SnO₂ film exhibited only 23.1% PL intensity of that of the perovskite on the pristine SnO₂ film. It indicates more efficient charge extraction of PAPT‐modified SnO₂ film, owing to the better energy level match. For the TRPL spectra, both samples showed bi‐exponential decay curves (Figure 4e). The average carrier lifetime (τ _avg) was fitted and listed in Table S1 (Supporting Information). The perovskite film deposited on the PAPT‐modified SnO₂ film exhibited the τ _avg of 50.55 ns, which is about 60% shorter than that of the perovskite film on the pristine SnO₂ film, 82.70 ns. It also suggests faster electron collection and transportation.

We also estimated the trap density by measuring the dark current density–voltage (J–V) curves based on electron‐only devices (FTO/SnO₂/perovskite/PCBM/Ag). The typical dark J–V curves can be divided into three regions: ohmic region, trap‐filling limited (TFL) region, and trap‐free space charge limited current region. The electron defect density (N _t) can be estimated by the trap‐filling limit voltage (V _TFL).^{[ 25} , ^{28 ]} The electron‐only devices based on the PAPT‐modified SnO₂ film obtained a much lower V _TFL (0.334 V) and N _t (3.51 × 10¹⁵ cm⁻³) compared to those based on the pristine SnO₂ film (V _TFL = 0.574 V and N _t = 6.04 × 10¹⁵ cm⁻³), as shown in Figure 4f. The fitting parameters (R ²) were listed in Table S2 (Supporting Information). Obviously, the PAPT treatment helps reduce the electron defect density and facilitates the charge transport between the SnO₂ and perovskite films.

To investigate the photovoltaic performances of PSCs with the PAPT‐modified SnO₂ films, we fabricated devices with a structure of FTO/SnO₂/perovskite/OAI/Spiro‐OMeTAD/Au, as shown in Figure 5a,b. After careful optimization of the periodic acid concentration (1–4 mm) (Table S3, Supporting Information), an optimal concentration of the PAPT treatment (2 mm) led to a champion PCE of 22.25% (Figure 5c), with a J _SC, V _OC, and FF of 25.02 mA cm⁻², 1.09 V, and 81.55%, respectively. Meanwhile, PSCs based on the pristine SnO₂ layer only achieved a champion PCE of 19.77% with a J _SC, V _OC, and FF of 25.09 mA cm⁻², 1.06 V, and 74.28%, respectively. The external quantum efficiency (EQE) spectrum of PSCs based on the PAPT‐modified SnO₂ film led to an integrated J _SC of 22.45 mA cm⁻², as shown in Figure 5d. The steady‐state PCE of the champion PSC is measured to be 22.10% under a constant bias voltage of 0.949 V (Figure 5e). Statistic distributions of the PCE, FF, and V _OC of the PSCs based on different concentrations of periodic acid are shown in Figure 5f,g and Figure S10 (Supporting Information).

The PCE enhancement is mainly attributed to the increment of FF and V _OC, which benefited from the interface modification by the periodic acid treatment. In addition, we investigated the role of periodic acid on the transport properties in PSCs by dark I‐V characterization. As shown in Figure 5h, PAPT‐modified devices exhibited a significantly lower dark current, indicating the reduction of leakage current between the SnO₂/perovskite interface, which could be attributed to the higher compactness and uniformity of the PAPT‐modified SnO₂ films.^{[ 25 ]} Moreover, we conducted electrochemical impedance spectroscopy (EIS) to investigate the charge transfer and recombination processes in the PSCs. Figure 5i shows the Nyquist plots of devices based on the pristine SnO₂ and PAPT‐modified SnO₂ films, measured at a bias potential of −1.0 V under dark conditions. It is worth mentioning that the semicircle at the high‐frequency region representing the charge transfer resistance (R _ct) was too small to be identified.^{[ 29 ]} Therefore, only one semicircle at the low‐frequency region was observed from the Nyquist plots, reflecting the interfacial recombination resistance (R _rec) between the SnO₂ layer and the perovskite layer. According to the fitting results (Table S4, Supporting Information) employing the equivalent circuit (inset of Figure 5h), the R _rec value increased from 240.6 to 399.7 Ω after the PAPT modification. The higher R _rec indicates a lower charge recombination rate,^{[ 29 ]} thus contributing to the improved FF and V _OC. In addition, the fitted series resistance (R _S) values are 29.49 and 23.96 Ω for PSCs based on the pristine SnO₂ and PAPT‐modified SnO₂ films, respectively. The smaller R _S is ascribed to increased conductivity of the SnO₂ layer, consistent with the previous measurement. We also performed the electric capacity–voltage (C–V) measurement to analyze the built‐in potential (V _bi) by the Mott–Schottky equation. As shown in Figure S11 (Supporting Information), the PAPT‐modified device displayed a higher V _bi (0.94 V) than the control device (0.77 V), indicating more efficient charge separation and collection and, thus, higher V _OC.^{[ 30 ]} Drive‐level capacitance profiling (DLCP) measurement was conducted to obtain spatial distributions of trap densities in SnO₂ films.^{[ 31} , ^{32 ]} As shown in Figure S12 (Supporting Information), the trap density in PAPT‐SnO₂ is relatively lower than that of the control‐SnO₂ film, indicating PAPT modification is beneficial for reducing the defect density in the SnO₂ layer. All these characterization results suggest the PAPT modification optimizes the interfacial charge transportation and inhibits the interfacial recombination between the ETL and perovskite layers, leading to better photovoltaic performances.

Finally, we fabricated the laser‐etched perovskite solar mini‐modules (PSMs) with an area of 3 × 3 cm² using a slot‐die coating method, as shown in Figure 6a. The champion PSM based on the PAPT‐modified SnO₂ film achieved an excellent PCE of 18.10% (J _SC = 7.16 mA cm⁻², V _OC = 3.45 V, and FF = 73.42%) with an aperture area of 2.29 cm², while the control one only achieved a PCE of 16.13% (J _SC = 7.08 mA cm⁻², V _OC = 3.35 V, and FF = 68.02%). These results reveal that the CBD process with the periodic acid treatment is quite suitable for large‐area production of PSCs and PSMs. Next, we evaluated the stabilities of PSCs according to the International Summit on Organic PV Stability (ISOS) protocols.^{[ 33 ]} As shown in Figure 6b, the unencapsulated device based on the PAPT‐modified SnO₂ film remained at 93.32% of its initial PCE after 3000 h of exposure to a relative humidity of 20–25% at 20 °C in an ambient atmosphere (conform to ISOS‐D‐1 procedure). The control device only kept 29.79% of its original PCE under the same conditions. The excellent stability of PAPT‐modified devices could be ascribed to the more compact and smoother interface between the SnO₂ and perovskite films, with fewer defects and channels for moisture.