Dual‐Function Equol Molecule Suppresses Superoxide‐Mediated Degradation in Perovskite Solar Cells

ABSTRACT

Under light and oxygen operational conditions, perovskite solar cells suffer severe degradation from superoxide radicals (${\mathrm{O}}_2^{-}$•). These radicals are generated when photogenerated electrons reduce adsorbed oxygen, and they rapidly decompose the perovskite by oxidizing iodide ions and deprotonating organic cations. Here, we introduce Equol, a phenolic natural isoflavonoid, into the perovskite precursor solution for two critical functions. First, phenolic oxygen coordinates with undercoordinated Pb²⁺ to passivate deep defects and regulate energy alignment, optimizing charge transport and device efficiency. It suppresses electron transfer to adsorbed oxygen and reduces ${\mathrm{O}}_2^{-}$• generation. Second, Equol acts as a hydrogen donor to scavenge ${\mathrm{O}}_2^{-}$• and restrain ${\mathrm{O}}_2^{-}$•‐induced degradation. Its phenoxyl radical possesses resonance stabilization and low reactivity, avoiding further oxidative chain reactions. Additionally, Equol improves perovskite hydrophobicity and long‐term environmental stability. As a result, the target device achieves a power conversion efficiency of 24.08% and shows excellent stability, retaining 80.18% of its initial efficiency after 2000 h under N₂ and 64.51% after 1000 h under ambient conditions (25°C, 40%–60% relative humidity), far exceeding the performance of control devices. This work establishes Equol as a multifunctional stabilizer against ${\mathrm{O}}_2^{-}$•‐driven degradation, offering a bio‐inspired route to enhanced perovskite stability.

This work introduces Equol, a natural isoflavonoid, into perovskite solar cells to suppress O⁻2•‐triggered degradation. Equol passivates defects and scavenges superoxide radicals, delivering an optimized power conversion efficiency of 24.08% and further improving long‐term ambient stability, offering a bio‐inspired strategy for stabilizing perovskite solar cells.

1. Introduction

Organic‐inorganic hybrid lead halide perovskites have demonstrated exceptional optoelectronic properties, achieving power conversion efficiencies (PCE) exceeding 27%, and thus show great promise for photovoltaic applications [1, 2, 3]. However, their instability under environmental stresses such as light, oxygen, moisture, or heat remains a major barrier to commercialization [4]. The synergistic effect of light and oxygen is a key driver of perovskite degradation. Atmospheric oxygen molecules are first physically adsorbed onto the perovskite surface via van der Waals forces and subsequently capture photogenerated electrons to form superoxide radicals (${\mathrm{O}}_2^{-}$•). ${\mathrm{O}}_2^{-}$• is highly oxidative and can not only directly oxidize I⁻ to generate volatile I₂ or I₃ ⁻, leading to iodine loss and exposure of undercoordinated Pb²⁺, but also abstract protons from organic cations such as MA⁺ or FA⁺, causing their deprotonation and conversion into volatile methylamine (CH₃NH₂) or formamidine (CH(NH₂)₂), which then escape from the lattice. These reactions not only disrupt the stoichiometry of the ABX₃ lattice but also trigger Pb‐I bond cleavage, resulting in crystal structure collapse. In the presence of light, oxygen, and moisture, the perovskite decomposes irreversibly into PbI₂, iodine species, and volatile organic fragments [5, 6, 7, 8, 9, 10].

To suppress ${\mathrm{O}}_2^{-}$• induced degradation, researchers have proposed various strategies. For example, fullerene derivatives with radical‐scavenging capability, such as C₆₀HTB [11], C₆₀OPD [10], and C₆₀RTL [12], have been introduced to effectively quench ${\mathrm{O}}_2^{-}$• and passivate interfacial defects, significantly enhancing device stability under illumination and in air. Choi et al. incorporated taurine, a natural antioxidant, at the ETL/perovskite interface. Its sulfonic acid and amino groups synergistically passivated Pb²⁺ defects and suppressed ${\mathrm{O}}_2^{-}$• diffusion through a radical quenching mechanism, thereby achieving simultaneous optimization of efficiency and stability [13]. Su et al. developed a hindered amine light stabilization (HALS) strategy in which a piperidine‐based photostabilizer generated nitroxyl radicals (N‐O•) under illumination and oxygen. These radicals scavenged superoxide (${\mathrm{O}}_2^{-}$•) and protons produced during perovskite decomposition, thereby suppressing photo‐oxidative degradation [14]. While molecular additives have significantly improved the stability of perovskite films, natural small molecules capable of simultaneously passivating interfacial defects and scavenging ${\mathrm{O}}_2^{-}$• remain underexplored.

Phenolic antioxidants can quench highly reactive ${\mathrm{O}}_2^{-}$• to prevent oxidative degradation of organic materials [15, 16, 17]. Inspired by the antioxidant properties of natural plant polyphenols, we introduce Equol, a natural isoflavone derived from soy metabolism, into the perovskite precursor solution. The Equol molecule contains phenolic hydroxyl groups that can donate hydrogen and coordinate with metal ions. On one hand, the phenolic oxygen coordinates with undercoordinated Pb²⁺ ions to passivate deep‐level defects and regulate energy level alignment, optimizing charge transport and photovoltaic efficiency. It also suppresses electron transfer to adsorbed oxygen, reducing ${\mathrm{O}}_2^{-}$• generation. On the other hand, Equol can donate hydrogen atoms to reduce ${\mathrm{O}}_2^{-}$• to hydrogen peroxide (H₂O₂), effectively blocking its oxidation of iodide ions and deprotonation of organic cations such as MA⁺ or FA⁺, thus preventing the formation of volatile I₂, CH₃NH₂, or CH(NH₂)₂. Equol also enhances the hydrophobicity of the perovskite surface, which helps limit the ingress of oxygen and moisture, further reinforcing the long‐term environmental stability of the devices. This multifunctional stabilization strategy enables the devices to achieve a champion PCE of 24.08%, along with excellent photostability and long‐term humidity resilience. After aging for 2000 h under N₂ environment, the target devices retain 80.18% of their initial PCE, whereas the untreated control devices retain only 51.93%. Furthermore, after storage for 1000 h under ambient conditions at 25°C with 40%–60% relative humidity in room light, the PCE of the target devices remains at 64.51% of the initial value, while that of the control devices drops to 28.85%. Building on natural polyphenols, we demonstrate that Equol serves as a multifunctional stabilizer for perovskites, simultaneously enhancing photovoltaic efficiency through defect passivation and ensuring long‐term operational stability via ${\mathrm{O}}_2^{-}$• scavenging and improved hydrophobicity.

2. Results and Discussion

Density functional theory (DFT) calculations were performed to analyze the electrostatic potential (ESP) of the Equol molecule, aiming to investigate its potential interactions with the perovskite surface (Figure 1a). The results show that the regions containing phenolic hydroxyl groups exhibit a significantly negative electrostatic potential (red regions), indicating that these functional groups are electron‐rich and possess a strong binding affinity toward undercoordinated cations. This allows Equol to donate lone‐pair electrons from its phenolic oxygen to undercoordinated Pb²⁺ sites, forming stable coordination bonds that passivate deep‐level interfacial traps [18, 19]. DFT calculations yield a dipole moment (μ = 2.618 D) for Equol (Figure S1). The relatively large dipole moment facilitates its oriented adsorption on the perovskite surface and can enhance the local electric field, promoting charge carrier separation and transport, which further confirms the potential of Equol as a functional additive for improving device performance [20].

FIGURE 1.

(a) Electrostatic potential (ESP) map of Equol. (b,c) SEM images of the control and target perovskite films. (d) XRD patterns of the control and target perovskite films. (e) Water contact angles of the control and target perovskite films. (f,g) In situ PL spectra of the control and target perovskite films during the annealing process. (h,i) Time‐resolved PL peak position and intensity changes of the control and target perovskite films during the annealing process.

We added Equol to the perovskite precursor solution and fabricated perovskite films under identical conditions. Scanning electron microscopy (SEM) and X‐ray diffraction (XRD) were employed to investigate the influence of Equol on the morphology and crystallinity of perovskite films. As shown in Figure 1b,c, the pristine perovskite film exhibits randomly distributed small grains with a rough surface and numerous grain boundaries. In contrast, the Equol‐treated perovskite film displays larger and more uniform grains, a smoother surface, and significantly reduced grain boundaries. After Equol treatment, the average grain size of the perovskite film increases markedly from 275.00 to 316.15 nm (Figure S2), indicating that Equol significantly promotes the crystallization process of perovskite. This morphological improvement enhances the overall film quality and reduces non‐radiative recombination defects [21]. Atomic force microscopy (AFM) images (Figure S3) further confirm the enlarged grain size, and the root‐mean‐square surface roughness (S_q) of the target perovskite film decreases from 15.5 to 13.8 nm. Such a smooth and homogeneous surface helps minimize charge traps, lower interfacial resistance, and improve contact between the perovskite and the electron transport layer, thereby effectively suppressing non‐radiative recombination [22, 23].

XRD analysis (Figure 1d) further reveals that the target perovskite film exhibits significantly enhanced crystallinity along the (100) crystal plane and a reduced amount of residual PbI₂. Compared with the control film, the target film shows markedly increased XRD diffraction peak intensity, which is consistent with the SEM observations and demonstrates that Equol, as an additive, can substantially improve perovskite crystallinity. The enhanced crystallinity and efficient conversion of PbI₂ reduce internal defects, leading to larger grain sizes and fewer defect sites, thereby significantly suppressing defect‐induced non‐radiative recombination [24]. The water contact angle (WCA) of the perovskite films was further measured (Figure 1e). Upon Equol modification, the WCA of the perovskite films increases from 40.1° to 52.7°, suggesting that the modified surface can mitigate the detrimental effects of moisture and oxygen ingress, thus improving device long‐term stability [25].

To gain deeper insight into the influence of Equol on the crystallization kinetics of perovskite, we employed in situ photoluminescence (in situ PL) spectroscopy to monitor in real time the nucleation and growth behavior of the film during the annealing process (Figure 1f,g). The PL intensity evolves over time in a three‐stage profile: it initially increases, then decreases, and finally increases again, corresponding respectively to precursor crystallization, intermediate phase dissolution, and final perovskite recrystallization [26, 27]. Notably, upon introduction of Equol, the kinetic window of the recrystallization stage is significantly prolonged, extending from 24.65 to 64.85 s (Figure 1h,i). This retardation effect arises from the strong interactions between Equol and perovskite components, which effectively modulate the grain growth rate, promote the preferential dissolution of small grains, and facilitate their re‐deposition into larger, more complete crystallites, thereby reducing grain boundaries and bulk defects [28, 29]. The target sample exhibits a higher photoluminescence (PL) intensity than the control sample throughout the entire annealing process, indicating fewer non‐radiative recombination centers and improved crystal quality.

Density functional theory (DFT) calculations further elucidate the passivation mechanism, revealing that Equol forms a stable O‐Pb coordination bond between the oxygen atom of its phenolic hydroxyl group and undercoordinated Pb²⁺ ions (Figure 2a). The differential charge density map and its line‐integrated profile (Figure S4a,b) clearly demonstrate strong charge transfer between Equol and the perovskite. This synergistic interaction not only reduces the interfacial defect state density but also indicates that Equol can effectively diminish the number of sites available for ${\mathrm{O}}_2^{-}$• generation and the active centers for subsequent oxidative reactions [30]. Unpassivated Pb²⁺ and iodide vacancies can promote ${\mathrm{O}}_2^{-}$• formation by leaking photogenerated electrons to oxygen. Equol passivates these traps, suppresses ${\mathrm{O}}_2^{-}$• generation at its source, and thereby inhibit photo‐oxidative degradation in air.

FIGURE 2.

(a) Charge density difference plot illustrating the adsorption of Equol on the perovskite surface. (b,c) XPS spectra of Pb 4f and I 3d for the control and target perovskite samples. (d,e) KPFM images of the control and target perovskite films. (f,g) CPD scan lines extracted from the dashed positions in the KPFM images of both control and target perovskite films. (h,i) Schematic diagrams depicting the energy level alignment for the control and target perovskite films.

X‐ray photoelectron spectroscopy (XPS) was employed to investigate the modulation effect of Equol on the surface chemical states of perovskite. As shown in Figure 2b, upon Equol modification, both the Pb 4f_7/2 and Pb 4f_5/2 peaks shift to lower binding energies. This shift originates from the coordination bond formed between the lone‐pair electrons of oxygen atoms in the phenolic hydroxyl groups of Equol and undercoordinated Pb²⁺ ions on the perovskite surface. The electron‐donating effect of Equol increases the electron density around Pb atoms, thereby reducing the binding energy of the Pb 4f peaks [31]. Similarly, the I 3d_5/2 and I 3d_3/2 peaks also exhibit a decrease in binding energy (Figure 2c), indicating an increased electron density around I⁻ ions. This enhancement arises from the coordination interaction between the lone‐pair electrons of the phenolic oxygen of Equol and undercoordinated Pb²⁺, which effectively passivates deep‐level trap states and suppresses non‐radiative recombination [32]. Concurrently, the O 1s peak shifts to higher binding energy (Figure S5a), signifying a loss of electron density from the oxygen atoms of Equol during coordination and directly confirming electron transfer from Equol to the perovskite surface. Additionally, the N 1s signal shows a shift toward lower binding energy (Figure S5b), reflecting the suppression of oxidative deprotonation degradation of organic cations, which is attributed to the dual function of Equol in surface defect passivation and superoxide radical scavenging [33, 34]. Collectively, the coordinated shifts in O 1s and N 1s signals reflect an optimized interfacial electronic environment. Acting as an electron donor, Equol stabilizes the inorganic framework while simultaneously protecting the organic components.

Dipolar molecules can form an interfacial dipole layer on the perovskite surface, modulating the surface electronic structure through a local vacuum level shift, thereby significantly influencing the film's surface energy and band alignment. To investigate the impact of Equol modification on the surface potential of the perovskite film, we performed Kelvin probe force microscopy (KPFM) to map the surface potential distribution (Figure 2d–g). The results show that the Equol‐treated perovskite film exhibits a distinctly higher surface potential compared to the control sample, with the contact potential difference (CPD) increasing from −329.25 to −282.51 mV, accompanied by a more spatially uniform distribution (Figure 2f,g). The narrower CPD histogram further confirms the improved homogeneity of the surface potential (Figure S6) [35, 36]. Since CPD is inversely correlated with the surface work function (W_F), this shift indicates that Equol modification renders the perovskite surface more electron‐rich, which facilitates better energy‐level alignment with the [6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (PCBM) electron transport layer (ETL), thereby promoting electron extraction and suppressing interfacial recombination. Moreover, the incorporation of Equol enhances the light absorption of the perovskite film (Figure S7a), while Tauc plot analysis reveals that the optical bandgap remains unchanged at 1.55 eV (Figure S7b), indicating that Equol does not alter the intrinsic bandgap structure of the perovskite.

To further quantify the interfacial energy‐level changes, we measured W_F of the perovskite films before and after Equol modification using ultraviolet photoelectron spectroscopy (UPS) (Figure S8). The results show that the W_F values are 4.70 eV for the control sample and 4.49 eV for the target sample. Since the W_F is defined as the energy difference between the vacuum level and the Fermi level (E_F), the reduced W_F implies an upward shift of E_F toward the conduction band, reflecting an increased electron concentration at the interface and a more pronounced N‐type character. Based on these energy‐level parameters, we constructed a band alignment model for the perovskite/PCBM interface (Figure 2h,i). Here, ΔE_e is defined as the energy difference between the perovskite conduction band minimum (CBM) and the ETL Fermi level, and ΔE_h as the difference between the valence band maximum (VBM) and the ETL Fermi level. For the control device, ΔE_e and ΔE_h are 0.85 and 1.45 eV, respectively. Upon Equol modification, ΔE_e decreases to 0.64 eV, while ΔE_h increases to 1.66 eV. The reduced ΔE_e facilitates efficient electron injection from the perovskite into the ETL, whereas the enlarged ΔE_h establishes a higher hole‐blocking barrier, effectively suppressing undesired back‐transfer of holes [37]. This synergistically optimized band alignment significantly reduces interfacial charge accumulation and non‐radiative recombination losses, thereby enhancing both the PCE and operational stability of the device.

Steady‐state photoluminescence (PL) measurements were performed on perovskite films deposited on both quartz and ITO/SAM substrates (Figure S9). On quartz, the target sample exhibits higher PL intensity than the control, indicating reduced non‐radiative recombination and improved film quality. In contrast, on ITO/SAM, the PL intensity of the target sample is lower than that of the control, suggesting more efficient extraction of photogenerated carriers to the electrode and reflecting enhanced interfacial charge transport efficiency. Together, these observations confirm that Equol not only passivates defect states but also optimizes carrier transport [13, 38]. Time‐resolved photoluminescence (TRPL) measurements further corroborate this conclusion (Figure S10 and Table S1), showing that the carrier lifetime increases from 246.375 ns for the control sample to 307.610 ns after Equol modification. This prolonged lifetime is consistent with the enhanced PL intensity and demonstrates that Equol effectively reduces the density of deep‐level trap states.

We employed density functional theory (DFT) calculations to investigate the influence of Equol on the formation energy of iodide vacancies. As illustrated in Figure 3a–c, in the absence of Equol, the formation energy of an iodide vacancy is 1.107 eV. Upon Equol incorporation, this value increases to 1.753 eV. Notably, in the presence of ${\mathrm{O}}_2^{-}$• (Figure 3d–f), the iodide vacancy formation energy rises dramatically from 1.543 to 2.719 eV with the introduction of Equol. We attribute this obvious improvement to the strong coordination interaction between the phenolic oxygen of Equol and undercoordinated Pb²⁺ sites in the perovskite lattice. The robust bonding interaction greatly stabilizes the perovskite lattice from a thermodynamic perspective, rendering the spontaneous formation of iodide vacancy defects much more difficult. By stabilizing the perovskite lattice and suppressing defect generation, Equol reduces the number of catalytic sites available for ${\mathrm{O}}_2^{-}$• production, thereby cooperatively blocking the initiation step of photo‐oxidative degradation [25, 39].

FIGURE 3.

(a,b) Optimized atomic configurations of iodide vacancy defects in perovskite, calculated by DFT, in the absence and presence of Equol, respectively. (c) Formation energy of the iodide vacancy defect. (d,e) DFT calculated atomic configurations of iodide vacancy defects in perovskite in a superoxide‐rich environment, without and with Equol, respectively. (f) Formation energy of the iodide vacancy defect under superoxide radical conditions. (g,h) UV–vis absorption spectra of FAI solutions with and without Equol during aging. (i) Schematic illustration of the general mechanism for ${\mathrm{O}}_2^{-}$• scavenging by phenolic antioxidants. (j,k) Schematic illustrations of the degradation mechanisms of control and target perovskite under superoxide radical attack.

To verify its potential influence on iodide oxidation, we added Equol to a formamidinium iodide (FAI) solution and monitored its photodecomposition behavior under UV illumination. As shown in Figure 3g,h, the control sample exhibits a distinct absorption peak at 365 nm, which is attributed to the formation of I₂/I₃ ⁻ species. In contrast, this absorption signal is weakened upon Equol addition. This observation suggests that Equol may alleviate the oxidation of I⁻ to I₂/I₃ ⁻ by scavenging ${\mathrm{O}}_2^{-}$•, which can be beneficial for improving the chemical stability of both the precursor solution and the resulting perovskite film [14, 40, 41]. To further evaluate the level of ${\mathrm{O}}_2^{-}$• generation, we performed electron paramagnetic resonance (EPR) spectroscopy using the spin trap agent DMPO (5,5‐dimethyl‐1‐pyrroline N‐oxide). Both control and target perovskite films were scraped into powders, dispersed in dimethyl sulfoxide (DMSO), and illuminated under ambient air before EPR signal acquisition (Figure S11). The results show that the characteristic EPR signal of ${\mathrm{O}}_2^{-}$• is significantly weaker in the target sample compared to the control, confirming that Equol effectively suppresses ${\mathrm{O}}_2^{-}$• generation and reduces its concentration in the system [10, 42].

Long‐term UV excitation generates ${\mathrm{O}}_2^{-}$•, which triggers photooxidation, structural degradation, and abundant non‐radiative recombination centers in perovskite. Accordingly, we investigated the ${\mathrm{O}}_2^{-}$• scavenging capability of Equol under UV irradiation. We carried out UV–vis absorption measurements on control and Equol‐modified perovskite films after 30 h of continuous UV exposure (Figure S12). The control sample shows obvious attenuation in absorption intensity, whereas the Equol‐modified film maintains nearly identical absorption features. This demonstrates that Equol efficiently scavenges ${\mathrm{O}}_2^{-}$• and suppresses UV‐induced optical degradation. We further employed PL spectroscopy to monitor defect evolution during UV aging (Figure S13). After 30 h of irradiation, the control film exhibits dramatic PL quenching, while only a mild drop is observed for the Equol‐modified sample. These results verify that Equol eliminates ${\mathrm{O}}_2^{-}$•, restrains defect generation, and enhances the photostability of perovskite.

Perovskites are prone to photo‐oxidative degradation under illumination in ambient air. Photogenerated electrons can reduce adsorbed oxygen molecules (O₂) to superoxide radicals (${\mathrm{O}}_2^{-}$•). These reactive oxygen species can further attack organic cations (such as MA⁺ or FA⁺) and iodide ions (I⁻), triggering irreversible decomposition and compromising the structural integrity of the film during prolonged aging. Polyphenolic compounds are well‐known to scavenge reactive oxygen species via a hydrogen atom transfer (HAT) pathway, as illustrated by the general reaction scheme in Figure 3i [43, 44]. We speculate that Equol, a natural isoflavone metabolite, follows this established mechanism to neutralize ${\mathrm{O}}_2^{-}$•. Specifically, Equol donates a hydrogen atom from its phenolic hydroxyl group to ${\mathrm{O}}_2^{-}$•, converting the radical into less harmful H₂O₂ and forming a resonance‐stabilized phenoxyl radical intermediate [17]. Notably, the Equol‐derived phenoxyl radical exhibits favorable resonance stabilization, which endows it with low chemical reactivity and restrains the continuous propagation of oxidative chain reactions [45]. Schematic illustrations (Figure 3j,k) further clarify the mechanism: under coexisting illumination and oxygen, photogenerated electrons in the perovskite transfer to O₂ to generate ${\mathrm{O}}_2^{-}$•. These radicals then oxidize I⁻ to form I₂/I₃ ⁻ and induce deprotonation of organic cations, ultimately leading to perovskite phase decomposition and the formation of PbI₂, I₂, and volatile organic fragments [46]. Incorporation of Equol effectively suppresses this degradation pathway by scavenging ${\mathrm{O}}_2^{-}$•. In addition, Equol enhances the hydrophobicity of the perovskite film, which reduces the interaction between the perovskite surface and ambient moisture and oxygen. This effect works synergistically with the defect passivation to retard environmental degradation, thereby further enhancing the overall stability.

To quantitatively assess the defect passivation efficacy, we measured the trap state density using the space‐charge‐limited current (SCLC) method. Single‐hole devices were fabricated and characterized (Figure S14). The results show that the trap‐filled limit voltage (V_TFL) decreases from 0.314 to 0.228 V upon Equol modification. The reduction in V_TFL directly corresponds to a decrease in trap state density [47]. This result is attributed to the coordination bond formed between the oxygen atoms of the phenolic hydroxyl groups of Equol and undercoordinated Pb²⁺ ions, which effectively passivates deep‐level traps and thereby enhances both the crystalline quality and optoelectronic performance of the perovskite film.

We fabricated inverted PSCs with a device architecture of glass/ITO/Ph‐4PACZ/ FA_0.9Cs_0.1PbI₃ (with or without Equol)/PEABr/PCBM/BCP/Ag to evaluate the impact of Equol on device performance (Figure 4a). As shown in Figure 4b, the champion target device achieves a PCE of 24.08% under reverse scan, significantly outperforming the control device (22.95%). The detailed photovoltaic parameters are as follows: the Equol‐based device exhibits a short‐circuit current density (J_sc ) of 25.66 mA/cm², an open‐circuit voltage (V_oc ) of 1.16 V, and a fill factor (FF) of 80.90%, whereas the control device shows J_sc = 25.42 mA/cm², V_oc = 1.14 V, and FF = 79.20%. The concurrent improvement in device parameters, particularly the enhanced V_OC and FF, primarily indicates that Equol reduces both interfacial and bulk defect densities and optimizes interfacial energy level alignment, thereby boosting the power conversion efficiency [48]. Moreover, the hysteresis index of the target device is 3.86%, markedly lower than that of the control device (4.67%) (Figure S15 and Table S2), further confirming that defect passivation suppresses ion migration and charge accumulation. This reduction originates from the multifunctional effects of Equol: On one hand, Equol passivates undercoordinated Pb²⁺ defects through coordination interactions, which effectively reduces trap‐assisted charge trapping and inhibits ion migration within the perovskite lattice. On the other hand, Equol optimizes the energy level alignment at the perovskite/ETL interface and enables efficient charge extraction and transport, which alleviates interfacial charge accumulation. These synergistic contributions jointly mitigate J–V hysteresis and improve the operational reliability of the device. The statistical distribution of device efficiencies also demonstrates excellent reproducibility (Figure S16), validating the robustness of this modification strategy. The integrated J_sc derived from external quantum efficiency (EQE) spectra are 24.14 mA/cm² for the control and 24.73 mA/cm² for the target device (Figure 4c), in close agreement with the J–V measurements and confirming the reliability of the photocurrent data. To assess operational stability, we performed maximum power point (V_mpp ) tracking under continuous AM 1.5G illumination for 300 s. The target device maintains a steady‐state PCE (PCE_mpp ) of 23.77%, compared to only 22.52% for the control device (Figure 4d), consistent with the trend observed in J–V scans.

FIGURE 4.

(a) Schematic of the device structure. (b) J–V curves of champion devices for the control and target perovskite solar cells. (c) EQE spectra. (d) Steady‐state power output of the respective devices at their maximum power points. (e) Dark J–V characteristics of the control and target devices. (f) Nyquist plots of the control and target devices (inset: equivalent circuit model). (g) PCE for unencapsulated PSCs stored in a N₂ atmosphere. (h) PCE evolution of perovskite solar cells under ambient conditions at room temperature with 40%–60% relative humidity in room light.

Dark J–V measurements (Figure 4e) show that the target device exhibits lower leakage current and higher rectification ratios at ± 1 V bias, indicating significantly enhanced diode characteristics. This improvement stems from reduced interface defects and optimized energy level alignment, which effectively suppress non‐radiative recombination and lower carrier extraction barriers. Typically, reverse leakage current is mainly caused by micro‐short circuits due to interface defects or ion migration, both of which are effectively blocked by the introduction of Equol, thereby reducing non‐radiative recombination and improving device performance [49]. Electrochemical impedance spectroscopy (EIS) further reveals improvements in interfacial charge dynamics. The Nyquist plot (Figure 4f) and equivalent circuit fitting results (Table S3) indicate that after Equol modification, the series resistance (R_s ) decreases significantly from 21.43 to 11.56 Ω, while the recombination resistance (R_rec ) increases from 102.0 to 337.9 Ω. The reduction in R_s suggests more efficient charge transport, while the increase in R_rec directly reflects the effective suppression of non‐radiative recombination, which is attributed to the passivation of undercoordinated Pb²⁺ defects on the perovskite surface by Equol [50].

Transient photovoltage (TPV) measurements were employed to further investigate the charge extraction and recombination processes at the interface. The TPV decay lifetime of the target device is 245.08 µs (Figure S17a and Table S4), significantly longer than the 138.45 µs observed for the control device, indicating an extended carrier lifetime and effective suppression of non‐radiative recombination at the interface. Additionally, transient photocurrent (TPC) curves (Figure S17b and Table S4) show that the decay time of the target device decreases from 74.46 to 55.21 ns, indicating improved charge extraction and transport efficiency, which further enhances the device's photovoltaic performance [51]. These findings are consistent with PL and TRPL analyses, confirming the effectiveness of Equol in defect passivation and energy level modulation.

We investigated the impact of Equol on device stability by evaluating the long‐term performance of control and target devices stored in a N₂ atmosphere. As shown in Figure 4g, after approximately 2000 h, the target device retains 80.18% of its initial PCE, whereas the control device retains only 51.93%. This significant enhancement in stability is attributed to the effective passivation of vacancy defects within the perovskite film by Equol, thereby inhibiting the structural degradation of the perovskite lattice under prolonged operation. We also monitored the long‐term stability of unencapsulated devices under ambient conditions at 25°C with 40%–60% relative humidity in room light. The target device maintains 64.51% of its initial PCE after 1000 h, while the control device retains merely 28.85% (Figure 4h). This remarkable enhancement in air stability is attributed to the dual functionality of Equol. It not only effectively passivates vacancy defects but also acts as a scavenger for ${\mathrm{O}}_2^{-}$•. By restraining defect‐triggered structural deterioration and suppressing reactive oxygen species‐mediated photo‐oxidative degradation, Equol collectively fortifies the perovskite film against environmental stressors. Furthermore, the hydrophobic nature of Equol enhances the film's tolerance to ambient moisture and oxygen, thereby mitigating environmental degradation. These combined effects synergistically retard performance decay under illumination and environmental stress, thereby substantially improving the environmental and light‐soaking durability of perovskite devices.

3. Conclusion

In summary, the natural isoflavone Equol is introduced into the perovskite system to simultaneously boost photovoltaic performance and device stability. The phenolic hydroxyl groups of Equol effectively passivate undercoordinated Pb²⁺ defects, reduce trap density, and optimize interfacial energy level alignment, which suppresses non‐radiative recombination and improves charge transport. Meanwhile, Equol inhibits the generation of ${\mathrm{O}}_2^{-}$• and directly scavenges the generated ${\mathrm{O}}_2^{-}$•. Additionally, Equol enhances the hydrophobicity of the perovskite film, thereby improving its long‐term environmental stability. Benefiting from the synergistic effects of defect passivation, energy level modulation and ${\mathrm{O}}_2^{-}$• scavenging, the power conversion efficiency is increased from 22.95% to 24.08%. In addition, long‐term durability is greatly enhanced. The modified device maintains 80.18% of the initial efficiency after 2000 h of storage in N₂ and retains 64.51% of the original PCE after 1000 h under ambient and light conditions. This work demonstrates that natural small‐molecule additives provide a feasible strategy for fabricating high‐efficiency and stable perovskite solar cells.

Conflicts of Interest

The authors declare no conflicts of interest.

Supporting information

Supporting File: smll74320‐sup‐0001‐SuppMat.docx.

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.