Kilogram-scale one-pot synthesis of multicomponent fullerene composites for efficient inverted perovskite solar cells

Domain: biology
Published: 2026-03-12
Editorial outcome: minor_revision

Abstract
The high cost and detrimental self-aggregation of conventional single-component fullerenes such as C60 and PCBM constitute a major obstacle to the commercialization of inverted perovskite solar cells (IPSCs). We report a kilogram-scale, one-pot synthesis of a multi-component fullerene composite (FC), comprising C60, a bis((3-methyloxetan-3-yl)methyl) malonate-C60 mono-adduct (BCM), and its bis-adduct (BCB). FC is obtained in 96% yields without complex column chromatography, significantly reducing production costs. Upon thermal annealing, BCM and BCB undergo cross-linking to form a robust encapsulation network that homogeneously incorporates C60, enhancing film stability and electron mobility. IPSCs incorporating cross-linked fullerene composite (CFC) demonstrate an impressive efficiency of 26.55%, surpassing that of PCBM-based devices (24.82%). Additionally, CFC-based devices maintain 96.0% and 95.1% of their initial efficiency after 1,000 hours under ISOS-L-1 and ISOS-D-2 protocols, respectively. Notably, CFC demonstrates excellent performance across a range of device configurations, including wide-bandgap (1.68 eV and 1.77 eV) cells, large-area devices (1 cm2), and mini-modules (14.4 cm2).

Introduction
Perovskite materials have garnered significant attention in the field of solar cells due to their high absorption coefficient,1-3 long carrier lifetimes,4,5 and excellent defect tolerance.6 Inverted perovskite solar cells (IPSCs) employing fullerene-based electron transport layers (ETLs) offer distinct advantages such as negligible hysteresis,7 high power conversion efficiency,8-13 continuously improving long-term operational stability, 14-18 and compatibility with tandem architectures,19-22 making them one of the most promising perovskite photovoltaic technologies for industrialization. A critical challenge limiting their commercial-scale deployment, as universally acknowledged by both academia and industry, lies in the instability of conventional electron transport layers (ETLs).23 Fullerenes, particularly C60 and (6,6)-phenyl-C61-butyric acid methyl ester (PCBM), have been extensively employed as ETLs in IPSCs due to their excellent electron extraction capabilities and suitable energy level alignment with perovskite absorbers. However, these single-component fullerenes suffer from severe self-aggregation under continuous illumination, which has been conclusively linked to their poor long-term stability and identified as a primary barrier to commercial-scale implementation, as demonstrated by recent collaborative studies between academic laboratories and industrial manufacturers.24,25 The aggregation-induced interfacial defects facilitate ion migration from the perovskite layer, accelerating device degradation and compromising operational reliability.26-28 Therefore, dense and stable fullerene-based ETLs are essential for ensuring the overall stability of IPSCs.

Cross-linking is widely recognized as a key strategy for designing robust and stable films across various research fields.29,30 In IPSCs, several cross-linkable fullerene derivatives have been reported as ETLs or electron extraction layers to improve device stability. For instance, some cross-linkable fullerenes, MPMIC60, [6, 6]-phenyl-C61-butyric styryl dendron ester (PCBSD) and phenyl-C61-butyric acid benzocyclobutene ester (PCBCB), as replacements for PCBM and C60, resulting in improved device stability.31-33 However, these cross-linkable fullerenes typically require high temperatures of at least 150 °C to form cross-linked films, which can damage the perovskite (PVSK) layer, limiting their application in IPSCs. Our recent study addressed this issue by discovering that C60 functionalized with bis(3-methyloxazolidin-3-yl)methyl malonate can undergo cross-linking at a significantly lower temperature of 100 °C.34 This not only prevents damage to the PVSK layer but also enables functional groups to chemically interact with the PVSK, further enhancing device stability. Integrating these multiple functions into a single molecule typically requires sophisticated functional‑group design, laborious purification steps, and high synthesis costs, making such an “all‑ in‑one” architecture considerably challenging. Meanwhile, researchers have also explored blending various fullerene derivatives to cooperatively enhance performance.35,36 However, the synthesis and purification costs of the relevant derivatives remain prohibitively high, which hinders their potential for scalable application.

In this study, we move beyond the pursuit of a single “perfect molecule” and instead develop a fullerene composite (FC) prepared via a one‑pot, kilogram‑scale process for use as the ETL in IPSCs. The one‑pot synthesis eliminates the need for costly separation and purification steps, delivers a yield as high as 96%, and significantly reduces material production costs. Unlike conventional single-component fullerenes, FC consists of C60, bis((3-methyloxetan-3-yl)methyl)malonate-C60 monoadduct (BCM), and bis((3-methyloxetan-3-yl)methyl)malonate-C60 bisadduct (BCB). Thermal annealing at 100 °C triggers cross-linking of BCM and BCB to form a robust network, while C60 is confined within this network. The three components act synergistically, enabling efficient electron transport while markedly enhancing the long‑term stability of the film. The resulting cross-linked multi-component fullerene composite (CFC) exhibits favorable energy level alignment with perovskite absorbers, high electron mobility, and exceptional resistance to ion migration. IPSCs incorporating CFC ETL yield an impressive efficiency of 26.55%, surpassing that of PCBM-based devices (24.82%). Notably, CFC-based devices maintain 96.0% and 95.1% of their initial efficiency after 1,000 hours under ISOS-L-1 and ISOS-D-2 protocols, respectively, showcasing their exceptional operational stability. Furthermore, CFC-based ETLs demonstrate high efficiency and long-term operational stability across large-area devices (1 cm2), 14.4 cm2 mini-modules, and wide-bandgap devices (1.68 eV and 1.77 eV). This work highlights FC as a scalable and commercially viable ETL, advancing the industrialization of high-performance perovskite photovoltaics.

Results
Kilogram-Scale Fullerene Composite Synthesis
The FC was synthesized via a Bingel reaction between bis((3-methyloxetan-3-yl)methyl) malonate (BOM) and C60 (Fig. 1a). By adjusting the reaction time, three different FCs with varying component ratios were obtained (FC1, FC, and FC3). Specifically, the reaction times for FC1, FC, and FC3 were 10 hours, 20 hours, and 30 hours, respectively. Using our custom-designed scaled-up reaction apparatus, kilogram-scale fullerene products can be readily obtained (Fig. 1b). This result highlights the excellent scalability of the reaction and underscores its strong potential for practical and industrial applications. The presence of C60, BCM, and BCB components was confirmed using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS) (Supplementary Fig. 1). The molar ratios of the components are as follows: for FC1, the proportions of C60, BCM, and BCB are 44%, 39%, and 17%, respectively; for FC, the proportions are 27%, 50%, and 23%; and in FC3, the proportions are 21%, 41%, and 38% (Fig. 1c). The reproducibility of the ternary composition was confirmed by HPLC (Supplementary Fig. 2). The yields of the FCs obtained at different reaction times are provided in Supplementary Table 1. The FC achieved a high yield of up to 96%, and a commercial-scale cost analysis indicates that the synthesis cost of FC is significantly lower than that of the widely used commercial PCBM (Supplementary Tables 2 and 3). We then fabricated IPSC (device structure: ITO/HTL/PVSK/ETL/BCP/Ag, Supplementary Fig. 3) using these various ETLs. As depicted in Fig. 1d and summarized in Supplementary Table 4, devices incorporating CFC ETL exhibited superior efficiencies compared to those using PCBM and binary mixtures of FC (Supplementary Fig. 4). Statistical data on device performance (Supplementary Fig. 5) indicate that CFC-based devices show significant improvements in open-circuit voltage (V OC), short-circuit current density (J SC), and fill factor (FF) relative to PCBM-based devices. Notably, the device using CFC achieved an efficiency of 26.55%, surpassing the performance of devices with PCBM (24.82%) and those with CFC1 and CFC3 (Fig. 1e). Furthermore, the forward and reverse scan data demonstrate that IPSCs based on CFC exhibit reduced hysteresis compared to those based on PCBM (Supplementary Fig. 6 and Supplementary Table 5). The device performance also demonstrates high robustness against variations in the CFC layer thickness (Supplementary Fig. 7).

We also evaluated the stability of the devices under maximum power point (MPP) tracking conditions with one sun illumination. As shown in Fig. 1f, the unencapsulated device based on CFC maintained 96.0% of its initial efficiency after 1,000 hours under the ISOS-L-1 protocol, whereas the PCBM-based device preserved only 47.9% of its initial efficiency. Additionally, we assessed the thermal stability of the devices. The CFC-based device retained 95.1% of its initial efficiency after 1,000 hours of aging under the ISOS-D-2 protocol at 85 ℃, while the PCBM-based device maintained only 66.7% of its initial efficiency (Fig. 1g). Meanwhile, the CFC-based devices demonstrated superior ambient storage stability compared to the PCBM-based devices. (Supplementary Fig. 8).

Film Characteristics of the Fullerene Composite
To understand the reasons behind the changes in device stability, we employed time-of-flight secondary ion mass spectrometry (ToF-SIMS) to track the distribution of Ag+, Ag2I+, C60+, Pb+, and FA+ ions in PCBM- and CFC-based devices before and after 1,000 hours of MPP tracking. As shown in Fig. 2a and Supplementary Fig. 9, the electrodes of the PCBM-based devices exhibited significant darkening, indicating severe corrosion. A large quantity of Ag+ ions was detected in the PCBM and PVSK layers, and FA+, Pb+, and Ag2I+ species were specifically identified in the silver electrode. This suggests that the PCBM ETL is unstable under long-term operational conditions, allowing ions such as FA+, Pb2+, and I- from the PVSK to penetrate the PCBM layer and reach the silver electrode, resulting in electrode corrosion and PVSK degradation. Cross-sectional scanning electron microscope (SEM) images of the device after 1,000 hours of MPP tracking (Supplementary Fig. 10) showed significant aggregation in the PCBM film and noticeable changes in the PVSK phase. X-ray diffraction (XRD) patterns (Supplementary Fig. 11) presented a significant increase in the PbI2 peak in the PVSK film with PCBM as the ETL, indicating substantial degradation of the PVSK. In contrast, in CFC-based devices, FA+ and Pb+ ions were mainly accumulated in the PVSK layer, with no apparent corrosion observed on the electrodes (Fig. 2b). This observation suggests that the CFC layer effectively blocks ion diffusion from the PVSK layer to the silver electrode, reducing electrode corrosion and PVSK degradation. Additionally, cross-sectional SEM images showed that the CFC film maintained its intact structure, and the PVSK layer preserved its morphology throughout the same MPP tracking period, with no significant changes observed in the XRD patterns of the PVSK phase. Based on these findings, we attribute the improved device stability to the compact and stable CFC-based ETL, which effectively eliminates molecular aggregation and reduces ion diffusion from the PVSK to the silver electrode.

To further elucidate the mechanism underlying the enhanced device stability, we prepared FC film from chlorobenzene solution and found that, after heating at 100 °C (Fig. 2c), it became insoluble in chlorobenzene, indicating that FC undergoes cross-linking at this temperature. The results from annealing-time-dependence and solvent-resistance tests on the CFC film (Supplementary Figs. 12 and 13) reveal that an 8-minute annealing step is sufficient to induce the formation of a stable crosslinked fullerene film. X-ray photoelectron spectroscopy (XPS) revealed that the C-O peak shifted from 533.9 eV before annealing to 534.1 eV after annealing (Supplementary Fig. 14), consistent with oxetane-ring opening and cross-link formation. Fourier-transform infrared spectroscopy (FTIR) further supported this conclusion, as the characteristic oxetane absorption peak at 983 cm-1 disappeared after annealing (Supplementary Fig. 15). In accelerated aging tests involving continuous annealing at 100 °C, optical microscopy images (Fig. 2d and 2e) revealed pronounced granular aggregation in PCBM films after 200 hours, indicating significant self-aggregation. In contrast, CFC films showed no noticeable changes before and after aging, confirming their superior thermal stability.

The compactness of the functional layer is crucial for the stability of PVSK devices. To evaluate this, we measured the densities of CFC and PCBM films using the Beer-Lambert law. First, we measured the absorbance of FC and PCBM films at different concentrations (Supplementary Fig. 16) and established the relationship between absorbance and concentration (Supplementary Fig. 17). Using the UV-visible (UV-Vis) spectra of dissolved films solution and the thickness variation from FC to CFC films (Supplementary Fig. 18), the densities of CFC and PCBM films were calculated to be 1.0670 g/cm3 and 0.8427 g/cm3, respectively (Fig. 2f). Additionally, we monitored the changes in water contact angle over time for different fullerene ETLs (Supplementary Fig. 19), revealing that CFC-based films exhibited superior hydrophobicity and stability. These findings suggest that dense and stable CFC films not only protect PVSK from water and oxygen erosion but also effectively prevent ion diffusion from the PVSK to the metal electrode, thereby enhancing device stability.

Building on the exceptional properties of CFC, we propose a schematic diagram to illustrate its functional mechanism (Fig. 2g). The cross-linkable fullerene components within CFC (BCM and BCB) undergo synergistic cross-linking to form a robust network that effectively encapsulates and stabilizes C60. This strategy enables the formation of a low-cost, high-density fullerene composite ETL with improved electron mobility and stability, thereby promoting the fabrication of highly efficient and stable IPSCs.

Enhanced Charge Transport in the Fullerene Composite
Proper energy level alignment with PVSKs and high electron mobility are also crucial for fullerene-based ETLs. We characterized the band structure of fullerene films using ultraviolet photoelectron spectroscopy (UPS) and UV-Vis absorption spectroscopy. As shown in Fig. 3a, the valence band maxima (E VBM) for CFC and PCBM films were determined to be -5.65 eV, and -5.66 eV, respectively. Combining these results with UV-Vis absorption spectra (Supplementary Fig. 20), the conduction band minima (E CBM) for CFC and PCBM films were calculated to be -3.90 eV and -3.93 eV, respectively (Fig. 3b and Supplementary Table 6). These results indicate favorable energy-level alignment between CFC and the perovskite absorber, as further confirmed by KPFM measurements (Supplementary Fig. 21). We also fabricated electron-only devices with an ITO/Al/Fullerene/Al structure and recorded the I-V curves under dark conditions. As shown in Fig. 3c, the electron mobility of the CFC film was calculated to be 4.02 × 10-4 cm2 V-1 s-1, which is higher than that of PCBM (1.76 × 10-4 cm2 V-1 s-1). This indicates that the CFC film demonstrates superior electron transport capability compared to PCBM films.

The interfacial interactions between the ETL and PVSK facilitate efficient carrier extraction and defect passivation.37,38 In the X-ray photoelectron spectroscopy (XPS) of PVSK/CFC films (Fig. 3d), the Pb 4f peak shifts towards lower binding energies, suggesting a coordination interaction between the C=O groups in CFC and Pb2+. Further confirmation of this interaction is provided by FTIR measurements, where the C=O stretching peak of CFC shifts from 1749.6 cm-1 to 1738.5 cm-1 upon mixing with PbI2 (Supplementary Fig. 22), in agreement with the XPS results (Supplementary Fig. 23). Theoretical calculations indicate that the PVSK/CFC interface exhibits stronger interactions and more effective defect passivation compared to PVSK/PCBM. (Supplementary Fig. 24). We therefore evaluated the trap density using the SCLC method. As shown in Fig. 3e, the trap-filled limit voltages (VTFL) of the devices based on PCBM and CFC were 0.191 V and 0.098 V, respectively, corresponding to trap densities of 9.90 × 1014 cm-3 and 5.08 × 1014 cm-3 (detailed calculations in Supplementary Note 2). The significant reduction in trap density indicates that CFC can effectively passivate surface defects in PVSK films.

The influence of charge transfer and recombination dynamics at the PVSK/ETL interface was further investigated through time-resolved photoluminescence (TRPL) and steady-state photoluminescence (PL) measurements. Fig. 3f shows the TRPL spectra of PVSK films and PVSK films covered with different ETLs. The data, fitted with a biexponential function, reveal average PL lifetimes for PVSK, PVSK/PCBM, and PVSK/CFC films of 1003.11 ns, 30.06 ns, and 24.99 ns, respectively (Supplementary Table 7). The significant reduction in PL lifetime for PVSK films with an ETL suggests rapid electron extraction at the PVSK/ETL interface. Furthermore, the shorter PL lifetime for CFC compared to PCBM indicates a higher electron extraction efficiency. Additionally, the τ2 value for PVSK/CFC (34.12 ns) is greater than that of PVSK/PCBM (31.35 ns), which suggests a reduced density of defect states, in agreement with the trap density measurements. The PL results also show that PVSK films coated with CFC exhibit the lowest PL intensity, further confirming the effective electron extraction capability of CFC (Supplementary Fig. 25).

Moreover, transient photovoltage (TPV) and transient photocurrent (TPC) measurements were performed to study charge transfer and recombination mechanisms in the corresponding devices. As shown in Fig. 3g and Supplementary Table 8, the TPV decay lifetime for CFC-based device is 836.35 μs, which is longer than that of PCBM-based device (623.33 μs). Meanwhile, the TPC decay lifetimes for devices based on PCBM and CFC are 14.88 μs and 4.54 μs, respectively (Supplementary Fig. 26, Supplementary Table 9). The significantly shorter lifetime of the CFC-based device corresponds to faster charge extraction and transport, which accounts for its higher J SC. Fig. 3h presents the Mott-Schottky plots for devices based on PCBM and CFC. The built-in potential (V bi) for PCBM-based devices is 0.957 V, while that for CFC-based devices is 0.986 V. The higher V bi in CFC devices facilitates carrier extraction and transfer within the device, which helps explain the higher V OC observed in CFC-based devices. Additionally, the ideality factor calculated from V OC as a function of different incident light intensities decreases from 1.79 kT/q in PCBM-based devices to 1.48 kT/q in CFC-based devices (Supplementary Fig. 27), indicating effective suppression of defect-induced non-radiative recombination. Electrochemical impedance spectroscopy (EIS) results (Supplementary Fig. 28) also show a larger charge recombination resistance (Rrec) in CFC-based devices, further confirming the effective suppression of non-radiative recombination.

Wide Bandgap Inverted Perovskite Solar Cell Performance
To investigate the applicability of the CFC, we fabricated IPSCs with a 1.68 eV bandgap PVSK absorber, using an ITO/HTL/PVSK/CFC/BCP/Ag structure. As shown in Fig. 4a, the CFC-based 1.68 eV bandgap IPSC achieved a champion efficiency of 23.16%, featuring a VOC of 1.263 V, a JSC of 21.81 mA/cm2, and a fill factor (FF) of 84.08% (Supplementary Fig. 29, Supplementary Table 10). In comparison, the PCBM-based device exhibited a significantly lower efficiency of 21.21%. Additionally, the CFC-based device exhibits reduced hysteresis (Fig. 4b), achieving a steady-state efficiency of 23.14% (Fig. 4c). We further fabricated IPSCs using a 1.77 eV bandgap PVSK absorber (Fig. 4d). Compared with the PCBM-based device, which achieves a PCE of 18.77%, the CFC-based device demonstrated notable enhancements in V OC , J SC , and FF (Supplementary Fig. 30, Supplementary Table 11), resulting in a PCE of 20.39% with negligible hysteresis (Fig. 4e) and a steady-state PCE of 20.37% (Fig. 4f). The external quantum efficiency (EQE) spectra of the devices are shown in Supplementary Fig. 31, and the integrated J SC values are in good agreement with those derived from the J-V measurements. Their efficiencies are comparable to those of the high-performance 1.68 eV and 1.77 eV bandgap IPSCs currently reported (Supplementary Tables 12 and 13).

The stability of the device with a 1.68 eV bandgap PVSK absorber under the ISOS-L-1 protocol is depicted in Fig. 4g. The unencapsulated CFC-based device retains 95.1% of its initial efficiency after 1,000 hours. Similarly, the device with a 1.77 eV bandgap PVSK absorber maintains 95.4% of its initial efficiency under the same conditions (Fig. 4h). Notably, under the ISOS-L-1 protocol, the CFC-based devices outperform their PCBM-based counterparts, suggesting that CFC not only improves efficiency but also enhances the operational stability of IPSCs across different bandgaps. These results demonstrate the versatility of CFC across perovskite solar cells with diverse compositions and suggest its relevance for scalable device fabrication.

Scalable Photovoltaic Performance of Devices and Mini-Modules
To assess the performance of CFC across different scales, we fabricated both 1 cm2 single cells and 14.4 cm2 mini-modules. At the 1 cm2 scale, the champion CFC-based device achieved an efficiency of 25.36%, with a V OC of 1.191 V, a J SC of 26.02 mA cm-2, and an FF of 81.85%, outperforming the PCBM-based device (24.08%) (Fig. 5a). The advantage is further confirmed by the steady-state output efficiencies of 25.13% for CFC-based device versus 23.96% for PCBM-based device (Fig. 5b and Supplementary Table 14). Additionally, the CFC devices also demonstrated superior device-to-device reproducibility (Fig. 5c and Supplementary Fig. 32).

For the 14.4 cm2 mini-modules, CFC exhibited significant advantages. The champion CFC-based mini-module reached an efficiency of 23.81%, higher than the 22.17% of the PCBM-based counterpart, and showed a negligible hysteresis (Fig. 5d and Supplementary Table 15). This trend was further validated by the steady-state output efficiency measured near the MPP. The CFC-based mini-module stabilized at 23.19% over 120 seconds, compared to 22.18% for the PCBM-based device (Fig. 5e), which is consistent with the J-V scan results. Moreover, the CFC-based mini-modules displayed a narrower PCE distribution, indicating better performance uniformity after device scaling (Fig. 5f). The corresponding distributions of V OC, J SC, and FF are provided in Supplementary Fig. 33.

We further assessed the long-term operational stability of both 1 cm2 devices and 14.4 cm2 mini-modules through MPP tracking. The 1 cm2 CFC-based device retained 92.8% of its initial efficiency after 1,000 hours (Fig. 5g), while the PCBM-based device degraded to 77.9% within 400 hours. Similarly, the 14.4 cm2 CFC-based mini-module maintained 91.4% of its initial efficiency after 600 hours (Fig. 5h), whereas the PCBM-based device rapidly declined to 25.5% within just 200 hours. These results collectively demonstrate that the stability advantage of CFC is preserved under both area upscaling and prolonged operational conditions.

Methods
Materials. N, N-dimethylformamide (DMF, 99.8%), dimethyl sulfoxide (DMSO, 99.9%), 2-propanol (IPA, anhydrous, 99.5%), chlorobenzene (anhydrous, 99.8%), anisole (anhydrous, 99.7%), and cesium iodide (CsI, 99.9%) were purchased from Sigma Aldrich. Lead iodide (PbI2, 98%) and [4-(3,6-Dimethyl-9H-carbazol-9-yl)butyl]phosphonic acid (Me-4PACz, >99.0%) were purchased from TCI. Methylammonium iodide (MAI, 98.0%) and formamidinium iodide (FAI, 99.99%) were purchased from Greatcell Solar. Lead chloride (PbCl2, 99.9%) and methylammonium chloride (MACl, 95%) were purchased from Xi’an Polymer Light Technology Corp. C60 (99.5%) and (6,6)-phenyl-C61 butyric acid methyl ester (PCBM, 99.5%) were obtained from Liantan Technology Co., Ltd. Nickel oxide (NiOx, 99.999%) and Bathocuproine (BCP, 99%) was purchased from Advanced Electronic Technology.

Perovskite precursor solution. 1.53 eV bandgap perovskite precursor solution was prepared by mixing FAI, MAI, CsI, and PbI 2 in anhydrous DMF: DMSO=4: 1 (v: v) according to the stoichiometric formula of 1.6 M Cs 0.05 (FA 0.98 MA 0.02 ) 0.95 PbI 3 , with 20 mol% MACl. The 1.3 M FA 0.8 Cs 0.2 Pb(I 0.8 Br 0.2 ) 3 1.68 eV bandgap perovskite precursor was prepared by dissolving 178.88 mg of FAI, 419.51 mg of PbI 2 , 143.13 mg of PbBr 2 , and 67.6 mg of CsI in 1 mL mixed solvent of DMF and DMSO with a volume ratio of 3:1. For 1.77 eV bandgap perovskite precursor solution, FAI:CsI:PbBr 2 :PbI 2 (molar ratio=0.8:0.2:0.4:0.6) were dissolved in mixed DMF and DMSO solvents (volume ratio=1:1.4) at a concentration of 1.2 M.

Device fabrication. The pre-patterned ITO glass substrates underwent a thorough cleaning process including sonication with a cleaning agent, deionized water, IPA, and ethanol, followed by drying with high-pressure nitrogen and plasma treatment. Next, the NiO x HTL solution was applied to the substrate via spin-coating and annealed at 100 °C for 10 min. Following this, the Me-4PACz self-assembled molecular solution was applied to create a double-layered hole-transport layer. For the hole transport layer (HTL) solution, 10 mg/mL NiO x was dissolved in deionized water. After shaking for 20 minutes, the solution was filtered through a 0.22 μm polytetrafluoroethylene membrane before use. The self-assembled molecular layer solution was prepared by dissolving 0.5 mg/mL Me-4PACz in ethanol.

Subsequently, 80 μL of perovskite precursor solution was dropped onto substrates and spun at 2000 rpm for 25 seconds and 5000 rpm for 30 seconds. 10 seconds before the end of the program, 150 μL of anisole was poured onto the substrates. The substrates were then transferred to an annealing stage at 100 °C for 30 minutes. After cooling to room temperature, a PDAI 2 isopropanol solution (1 mg/mL) was spin-coated onto the perovskite films at 5000 rpm for 25 seconds, followed by annealing at 100 °C for 5 minutes. Following this, the FC chlorobenzene solution (20 mg/mL) was spin-coated at 2500 rpm for 30 seconds on the perovskite films and annealed at 100 °C for 10 minutes. Next, the BCP 2-propanol solution (0.5 mg/mL) was spin-coated at 5000 rpm for 30 seconds and annealed at 100 °C for 10 minutes. Finally, a 100 nm of Ag electrode was deposited through a mask by thermal evaporation under a pressure of 5.5×10-4 Pa.

Fabrication of mini-module. Firstly, 5 cm × 5 cm FTO glasses were etched using a picosecond laser to form P1 patterns. Prior to use, the etched glasses were cleaned and subjected to ultraviolet-ozone (UVO) treatment. Subsequently, the NiOx, Me-4PACz, perovskite, PCBM, and BCP layers were sequentially deposited on the patterned FTO glasses in accordance with the fabrication protocols for small-size solar cells. Following the layer deposition, the samples were etched again to form P2 patterns. After evaporating the Cu electrode, a third etching step was performed to create P3 patterns, thereby completing the series connection of 6 subcells. The scribing widths of P1, P2, and P3 were approximately 55 μm, 115 μm, and 35 μm, respectively. The total distance between P1 and P3 was about 210 μm, and the calculated geometric fill factor was approximately 96.5%. A 5 cm × 5 cm metal mask with an aperture area of 3.6 cm × 4 cm was used for the I-V test of these solar mini-modules.

Device characterization. The J-V characteristics were tested using a Keithley 2400 source meter and a Xenon lamp solar simulator (Enlitech, AAA) within an N2-filled glove box. The tests were conducted under AM 1.5G irradiation intensity, calibrated with NREL calibrated Si solar cells equipped with infrared cut-off filters (KG-5). Measurements were taken in the range of 1.25 to -0.1V, with a voltage step of 0.01V and a delay time of 40 ms.

Device stability was evaluated following the ISOS-L-1 and ISOS-D-2 protocols.39 For the ISOS-L-1 test, unencapsulated devices were continuously exposed to a calibrated white LED light source (100 mW cm-2, AM 1.5G) under ambient conditions (50 ± 2 °C) to assess the effect of continuous illumination on device stability. For the ISOS-D-2 test, devices were stored in the dark at an elevated temperature of 85 °C in ambient air without humidity control to evaluate their thermal stability. No encapsulation was applied during either test, and the device performance was periodically measured using a source meter under standard testing conditions. For the device operating as an LED, the external quantum efficiency was determined by measuring the absolute radiation flux using a commercial system (Guangzhou Xi Pu Optoelectronics Technology Co., Ltd.) equipped with an integrating sphere (GPS-4P-SL, Labsphere) and a QE-Pro spectrometer (Ocean Optics).

Other characterizations. Steady-state photoluminescence (PL) was recorded using an FLS1000 instrument (Edinburgh Instruments Ltd). High-performance liquid chromatography (HPLC) measurements were performed using an LC-16P system (Shimadzu). Time-resolved photoluminescence (TRPL) was measured with a 375 nm pulsed laser (EPL-375) excitation using time-correlated single photon counting (TCSPC) equipment (FLS1000, Edinburgh Instruments Ltd). Space-charge-limited current (SCLC) curves were collected using a Keithley 2400 instrument in the dark, with the device's dark J-V curve scanned in the range of -0.5 to 5 V, at a voltage step of 0.01 V and a delay time of 40 ms. Top-view and cross-sectional SEM images were taken using a field-emission scanning electron microscope (JEOL JSM-7610F) operating at 5 kV. X-ray diffraction (XRD) patterns were recorded using a Smart Lab X-ray diffractometer (Rigaku Corporation) with a Cu Kα radiation source (1.54 Å), an operating voltage of 40 kV, and a current of 30 mA. Ultraviolet-visible (UV-Vis) absorption spectra were recorded using an Ocean Insight FLAME Miniature Spectrometer in an N2-filled glovebox. X-ray photoelectron spectra (XPS) were measured using a Thermo Fisher Scientific K-alpha+ instrument with an Al Kα source, calibrated to the C 1s peak (284.8 eV). Infrared spectra (FTIR) were collected in attenuated total reflection (ATR) mode using a NICOLET iS50 FTIR spectrometer (Thermo Scientific). Incident photon-to-electron conversion efficiency (IPCE) spectra were tested using a QE-R666 system (Enlitech) in DC mode without bias to confirm the current density. The stable output of the device was measured by applying a constant voltage near the maximum power point (MPP) extracted from the J-V curve. Electrochemical impedance spectra (EIS) were obtained using a CHI660E electrochemical workstation, biased at 0.9 V in the dark. Mott-Schottky plots were monitored by varying the voltage from 0 to 1.4 V at 10 kHz using a Zahner electrochemical workstation. Dark J-V curves of the device were recorded using a Keithley 2400 source meter with a scanning range of -1.5 to 1.5 V. Ultraviolet photoelectron spectroscopy (UPS) spectra were measured using a Thermo Fisher Scientific ESCALAB Xi+ with a He-Iα (21.22 eV) UV light source. Water-contact angle measurements were performed using a JC2000D1 contact angle measuring device from POWEREACH. Time of Flight Secondary Ion Mass Spectrometry (ToF-SIMS) measurements (ION-TOF M6) were performed with the pulsed primary ions from a Cs+ (2 keV) liquid-metal ion gun for the sputtering and a Bi+ pulsed primary ion beam for the analysis (30 keV). The optical microscope images were recorded on a DM 250 polarized optical microscope.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "Tian et al. reported a one-pot synthesis of a multi-component fullerene composite, containing C60, the mono-adduct and the\nbisadduct. Upon the thermal annealing, the fullerene composite undergoes the cross-linking process to form a robust\nencapsulation network, improving the film stability and electron mobility. As a result, IPSCs using cross-linked fullerene\ncomposite as the ETL present a decent PCE of 26.55% with improved stability. However, the novelty is insufficient, since the\nthermal cross-linking concept is widely used for fullerene based ETLs and the mono-adduct has also been reported by the\nauthors (Angew. Chem. Int. Ed. 2024, 64, e202416703.). In addition, it is necessary to fabricate large-area PSCs to fully\nevaluate the superiority of the multi-component fullerene composite, including PCE, stability, reproducibility, etc. compared\nwith the classical PCBM. Therefore, we do not recommend it to publish on Nature Communication.\nThe authors need to address some other issues below we concerned before publishing elsewhere.\n1. The purity and reproducibility of the multi-component fullerene composite prepared by one-pot synthesis must be\ncharacterized, such as liquid chromatogram, since it is important to the photovoltaic performance of PSCs.\n2. How long does it take to fulfil the cross linking for the FC? It should provide the optimization details.\n3. The authors stated “X-ray photoelectron spectroscopy (XPS) revealed that the C-O peak shifted from 533.9 eV before\nannealing to 534.1 eV after annealing, consistent with oxetane-ring opening and cross-link formation”. Please provide the\nreference or experimental data to support the declaration.\n4. The unreacted C60 is not dissolved in chlorobenzene in Fig. 2c. Why?\n5. Generally speaking, fullerene derivatives with isomers or impurity would suffer from low electron mobility. Why the\nfullerene composite has the higher electron mobility than PCBM?\n6. Why the PL lifetime for PVSK/CFC is shorter than that of the PL lifetime for PVSK/PCBM.\n7. The relationship between charge carrier dynamics and photovoltaic parameters of PSCs should be clearly clarified.\n8. How about the photovoltaic performance of PSCs, if use C60:BCM or C60:BCB as the ETL?\n9. The XPS of the PVSK/PCBM film for Pb 4f should be provided to compare with the interaction PVSK with CFC. The\ncorresponding theoretical calculation would help to support the experimental result.\n10. The multi-component composite usually brings about disordered molecular stacking or defects. Why the trap density of\nCFC based PSCs is lower?\n11. The TPC decay lifetimes for devices based on PCBM and CFC are 14.88 μs and 4.54 μs, respectively, indicating a\nfaster electron extraction in CFC-based devices, rather than a slower carrier recombination rate. Please revise the statement\nin the text.\n12. Why built-in potential (Vbi) for PCBM-based devices is lower than that for CFC-based devices? Please explain it.", "reviewer_2": "In this study, the authors report a one-pot synthesis method for the kilogram-scale production of fullerene-based electron\ntransport layer (ETL) materials, designed for use in high-performance inverted perovskite solar cells (PSCs). Inverted PSCs\nare widely regarded as one of the most commercially viable architectures, wherein the fullerene-derived ETL is crucial for\nachieving high device efficiency and operational stability. Through molecular design, the authors developed a ternary\nfullerene composite as the ETL material. This ternary system exhibits a synergistic effect, leading to the formation of a stable\nand compact ETL with optimal electron mobility, which contributes to a remarkable power conversion efficiency of 26.55%\nand excellent operational stability. Moreover, the ETL demonstrates broad applicability across PSCs with various bandgaps,\noffering a promising strategy for enhancing both efficiency and stability. Notably, the material can be produced via a low-cost\nand scalable preparation route. This is a well-designed work that provides valuable insights and innovative solutions to\naddress ongoing challenges associated with fullerene-based electron transport layers in perovskite photovoltaics. Overall, I\nthink this work is suitable for the publication in Nature Communications, after addressing the following issues:\n1. Is it possible for the reaction pathway to produce multi-adduct fullerene derivatives? Can such by-products be completely\nremoved during the purification process? If trace multi-adduct fullerenes remain in the final material, could they adversely\naffect the photovoltaic performance of the device?\n2. The cost analysis in the manuscript appears to be based on laboratory-scale reagent prices. If calculated based on bulk\nprocurement pricing, what would be the anticipated cost of this type of fullerene?\n3. Since C60 itself lacks crosslinkable functional groups, is it stable within the crosslinked network, and is there a potential\nfor aggregation?\n4. The authors emphasize the \"low-temperature crosslinking and multi-component synergy\" properties of the ternary\nfullerene. What are its advantages compared to other crosslinkable fullerenes previously reported in lead-based perovskite\nsolar cells?\n5. KPFM data are critical for evaluating interfacial energy level alignment and analyzing charge transport behavior in\nphotovoltaic devices. The authors are encouraged to include such data and provide further discussion in the manuscript.\n6. The authors are encouraged to provide data on the influence of CFC layer thickness variation on the device performance.\n7. It is suggested that the authors include a statistical analysis of device performance across multiple batches, including\nmaximum, minimum, average values, and standard deviations.\n8. Several minor errors were identified: the horizontal axis in Figures 2a and 2b (TOF-SIMS) should be labeled as\n\"Sputtering Time (s)\"; in Supplementary Figure 8, \"wavenumber\" should be capitalized; and in Supplementary Figure 14, a\nspace is missing between the physical quantity and the unit symbol.\n9. As this study focuses on high-performance inverted perovskite photovoltaic devices, it is recommended that the authors\nincorporate recent research advances in the introduction section.", "reviewer_3": "In this manuscript, Hou et al. report on a kilogram-scale, one-pot synthesis of a ternary fullerene composite (FC), comprising\nC60, a bis((3-methyloxetan-3-yl)methyl) malonate-C60 mono-adduct (BCM), and its bisadduct (BCB). They found BCM and\nBCB undergo cross-linking at a low temperature of 100 °C, forming an encapsulation network that incorporates C60. Using\nthis composite as electron transporting layer, they achieved an efficiency of 26.55% for perovskite cells, surpassing that of\nPCBM-based cells (24.82%). Moreover, composite-based devices maintain 96.0% and 95.1% of their initial efficiency after\n1000 hours under ISOS-L-1 and ISOS-D-2 protocols, respectively.\nThis study provides a practical synthetic approach as well as a material with both excellent performance and stability, which\ncould have significant impact on the research field and the commercialization prospects of perovskite devices. Overall, I find\nthe manuscript suitable for publication in Nature Communications after the authors address the following minor issues.\n1. The authors are encouraged to clarify the specific role of each fullerene component within the composite when applied in\nsolar cells, and to explain how these species collectively contribute to the observed synergistic effects in efficiency and\nstability.\n2. According to Figure 2c, the precursor undergoes cross-linking upon heating to form the composite layer. The authors are\nasked to comment on the solubility of the resulting composite and how this property may affect its processability and\npotential scalability.\n3. The authors report devices based on perovskites with bandgaps of 1.68 eV and 1.77 eV. It would be helpful to compare\nthe performance of these devices with the current state of the art, citing relevant literature to better contextualize the achieved\nefficiency. In addition, since the composite electron-transport layer appears compatible with wide-bandgap perovskites, the\nauthors are encouraged to discuss its potential advantages in tandem solar cells.\n4. Figure 2g, the composite layer protected the invasion of perovskite against water and oxygen. It would be valuable to\nspecify what the condition used for the stability testing, such as humidity and oxygen.\n5. The manuscript presents TOF-SIMS data for devices after 1000 hours of maximum power point tracking (MPPT).\nHowever, corresponding data prior to operation are not shown. Including such a comparison would be useful to more clearly\nillustrate the positive effect of the composite on long-term operational stability.\n6. In Figure 4h, the PCE of the PCBM-based devices drops sharply at around 600 hours. The authors are encouraged to\ncomment on the possible reasons for this rapid degradation, as understanding this behavior would provide useful context for\nevaluating the advantages of the composite layer.\n7. In Figure S8, the characteristic peak of oxetane at 983 cm⁻¹ does not disappear after annealing. The authors should clarify\nthis observation and discuss its implications for the proposed cross-linking mechanism."}, "author_response": "Response letter to referees\nReviewer #1: Tian et al. reported a one-pot synthesis of a multi-component fullerene\ncomposite, containing C , the mono-adduct and the bisadduct. Upon the thermal\n60\nannealing, the fullerene composite undergoes the cross-linking process to form a robust\nencapsulation network, improving the film stability and electron mobility. As a result,\nIPSCs using cross-linked fullerene composite as the ETL present a decent PCE of 26.55%\nwith improved stability. However, the novelty is insufficient, since the thermal cross-\nlinking concept is widely used for fullerene based ETLs and the mono-adduct has also\nbeen reported by the authors (Angew. Chem. Int. Ed. 2024, 64, e202416703.). In\naddition, it is necessary to fabricate large-area PSCs to fully evaluate the superiority of\nthe multi-component fullerene composite, including PCE, stability, reproducibility, etc.\ncompared with the classical PCBM. Therefore, we do not recommend it to publish on\nNature Communication. The authors need to address some other issues below we\nconcerned before publishing elsewhere.\nResponse: We appreciate your review of our manuscript and your positive assessment\nof the device performance. We have carefully addressed all points raised, especially\nconcerning the work's novelty and large-area device validation. Accordingly, we have\nconducted the necessary additional experiments and incorporated the corresponding\nrevisions into the manuscript. We hope these updates meet with your approval.\n1. Novelty of this work\nWe fully understand your perspective that the concept of thermal cross-linking and the\nuse of mono-adduct fullerenes have been reported previously. However, the true\nnovelty of our work lies not in introducing “cross-linkable fullerenes” as a new concept,\nbut in proposing and validating a fundamentally new materials-design strategy and a\ndistinct route to functional implementation, as detailed below.\n(1) In our previous work (Angew. Chem. Int. Ed. 2024, 64, e202416703) and other\nrelated literature, the dominant research paradigm has followed the concept of a \"single\nmultifunctional molecule.\" This approach involves the deliberate design and synthesis\nof a mono-adduct fullerene derivative intended to simultaneously deliver both cross-\nlinking capability and charge transport within one molecular structure. However, this\nstrategy typically leads to increased complexity in molecular design, higher costs, and\ninherent compromises among the multiple targeted functions.\n(2) In this work, we shift decisively to a new paradigm of a “multicomponent,\nfunctionally cooperative system”. Instead of pursuing a single “perfect molecule”, we\nemploy a straightforward one-pot synthesis to construct a ternary fullerene composite\nin situ, comprising C , mono-adduct, and bis-adduct fullerenes. These components\n60\nnaturally fulfill complementary roles. C provides the fundamental electron-transport\n60\nbackbone. The mono-adduct component contributes cross-linkable motifs, enabling the\nformation of a robust network and imparting high stability. The bis-adduct component\nfurther enhances film stability while helping to regulate film morphology and energy-\nlevel alignment. Moreover, the component ratio can be precisely tuned simply by\nadjusting the reaction time, allowing for the simultaneous optimization of both the\ncross-linked network and charge-transport properties. The core breakthrough of our\nwork lies in the fundamental transition from engineering a single multifunctional\nmolecule to designing a synergistic multicomponent system.\n(3) In addition, compared with previously reported physical blending strategies (e.g.,\nPCBM/ICBA/C : Adv. Mater. 2024, 36, 2308706 and EES Sol., 2025, 1, 567), our one-\n60\npot route produces the ternary system via chemically in situ preparation, resulting in\nuniform and controllable composition with minimal batch-to-batch variation (relative\nstandard deviation, RSD < 3%, as detailed below). The product requires no complicated\nseparation or purification, offering a straightforward and highly reproducible process.\nCost analysis further indicates that the synthesis cost is reduced by approximately 55%\nrelative to monoadduct derivative and by roughly 65% compared to PCBM,\nhighlighting strong potential for practical and scalable applications.\n2. Large-area devices and mini-modules\nWe fully agree with your view that large-area device validation is essential, as it is a\ncritical step for assessing practical application potential. Following your suggestion, we\nsystematically performed additional experiments and have incorporated the\ncorresponding data into the revised manuscript.\n(1) We fabricated large-area devices with an active area of 1 cm2 and evaluated their\nperformance (Fig. R1a and b, Table R1, also Fig. 5a and b, Supplementary Table 14).\nThe CFC-based device delivers a champion efficiency of 25.36%, significantly higher\nthan the PCBM-based device (23.34%). More importantly, after 1,000 h of maximum\npower point (MPP) tracking, the CFC device retains 92.8% of its initial efficiency,\nwhereas the PCBM device drops to 78% after only 300 h (Fig. R1g, also Fig. 5g). These\nresults clearly demonstrate the dual benefits of CFC in improving both efficiency and\noperational stability at the large-area device scale.\n(2) To further verify scalability, we fabricated mini-modules with an active area of 14.4\ncm2 (Fig. R1d and e, also Fig. 5d and e, Supplementary Table 15). The CFC-based\nmini-module achieved an efficiency of 23.8%, outperforming the PCBM-based mini-\nmodule (22.14%). After 600 h of MPP tracking, the CFC mini-module retains 91.4% of\nits initial efficiency, markedly exceeding the PCBM-based device (Fig. R1h, also Fig.\n5h).\n(3) We further compiled the efficiency distribution statistics for both the 1 cm2 devices\nand the 14.4 cm2 mini-modules (Fig. R1c and R1f, also Fig. 5c and f). Compared with\nPCBM, CFC exhibits a more concentrated efficiency distribution at both scales, directly\nevidencing improved process tolerance and batch reproducibility during scale-up.\nThese above systematic results confirm that the CFC delivers comprehensive\nperformance advantages from small-area devices to large-area devices and mini-\nmodules, underscoring its strong potential for practical deployment.\nTo sum up, we sincerely appreciate your constructive comments, which have\nsignificantly helped improve the quality and depth of our work. We hope the\nexplanations and new results presented above adequately address your concerns.\nFig. R1 (a) Forward and reverse scan J-V curves and (b) stabilized power output (SPO)\nefficiencies for 1 cm2 devices. (c) PCE distribution statistics for 1 cm2 devices. (d)\nForward and reverse scan J-V curves and (e) SPO efficiencies for 14.4 cm2\nmini‑modules. (f) PCE distribution statistics for 14.4 cm2 mini‑modules. MPP tracking\nstability for (g) 1 cm2 devices and for (h) 14.4 cm2 mini‑modules.\nTable R1. Key photovoltaic parameters of IPSCs employing different ETLs.\nArea Sample V (V) J (mA/cm2) FF (%) PCE (%)\nOC SC\nPCBM Reverse 1.179 25.55 79.96 24.08\nPCBM Forward 1.177 25.57 78.05 23.49\n1 cm2\nCFC Reverse 1.191 26.02 81.85 25.36\nCFC Forward 1.177 26.02 81.68 25.02\nPCBM Reverse 6.95 4.16 76.67 22.17\nPCBM Forward 6.94 4.15 76.52 22.07\n14.4 cm2\nCFC Reverse 7.12 4.16 80.40 23.81\nCFC Forward 7.12 4.16 79.42 23.52\n1. The purity and reproducibility of the multi-component fullerene composite prepared\nby one-pot synthesis must be characterized, such as liquid chromatogram, since it is\nimportant to the photovoltaic performance of PSCs.\nResponse: We thank the reviewer for this valuable suggestion. We fully agree that the\npurity and batch-to-batch reproducibility of multicomponent fullerene composites are\ncrucial for device performance. To further verify the compositional controllability and\npurity of our products, we conducted systematic high-performance liquid\nchromatography (HPLC) characterization on samples collected at three representative\nreaction times (FC1, FC, and FC3). As shown in Fig. R2 (also Supplementary Fig 2),\nall chromatograms exhibit well-resolved and unambiguously assignable peaks,\ncorresponding to C , the mono-adduct, and the bis-adduct species, respectively, with\n60\nno other evident impurity peaks or signals from side products. These results\ndemonstrate that our one-pot synthetic strategy yields the targeted multicomponent\nsystem with high purity.\nMore importantly, quantitative analysis of the relative peak areas of the main peaks\nshows excellent consistency across independently prepared batches, with relative\nstandard deviations (RSDs) below 3%. This confirms the outstanding compositional\ncontrollability and batch-to-batch reproducibility of our synthesis, providing a solid\nmaterials basis for stable device performance.\nThe corresponding HPLC data and analysis have been included in the Supporting\nInformation. We thank the reviewer for helping us strengthen this important aspect of\nthe characterization.\nFig. R2 HPLC analysis of samples (a) FC1, (b) FC, and (c) FC3. Analysis was\nperformed using a 5PBB column with dichloromethane as the eluent at a flow rate of\n2.0 mL min-1 (each sample was analyzed from three independent batches).\n2. How long does it take to fulfil the cross linking for the FC? It should provide the\noptimization details.\nResponse: We sincerely thank the reviewer for this suggestion. To ensure that the cross-\nlinking process is sufficiently complete and reproducible, we systematically optimized\nthe annealing time and performed corresponding characterization. CFC films were\nprepared using different annealing durations of 2, 5, 8, 10, 12, and 15 min, and the\noptimal cross-linking conditions were identified based on the following results.\n(1) Solvent-resistance test: As shown in Fig. R3a and b (also Supplementary Fig. 12a\nand b), when the annealing time reached 8 min, the film remained intact after soaking\nin chlorobenzene, and the absorption spectra before and after soaking were nearly\nidentical. This indicates that a continuous cross-linked network had formed, yielding\nexcellent solvent resistance.\n(2) Chemical-structure evolution: We tracked the intensity change of the oxetane\ncharacteristic band at 983 cm-1 by FTIR (Fig. R3c, also Supplementary Fig. 12c). The\npeak intensity decreased markedly after 8 min of annealing, and further increasing the\nannealing time led to no noticeable additional change. This suggests that most reactive\nfunctional groups participate in cross-linking within 8 min, and the reaction is\nessentially completed at this time point.\nIt should be noted that cross-linking of FC occurs in the solid-state film, where\nmolecular diffusion and local steric hindrance may leave a small fraction of oxetane\ngroups unreacted. Nevertheless, as evidenced by the solvent-resistance results above,\nthe formed cross-linked network is sufficiently continuous and stable, and this does not\ncompromise the overall film performance.\nIn summary, we identify 8 min as the optimized annealing time to achieve sufficient\ncross-linking in the CFC films, and the corresponding details have been added to the\nrevised manuscript.\nFigure R3 (a) Photographs of CFC films before and after soaking in chlorobenzene\nunder different annealing times. (b) Absorption spectra of the corresponding films\nbefore and after solvent treatment. (c) FTIR spectra of CFC films annealed for varying\ntime.\n3. The authors stated “X-ray photoelectron spectroscopy (XPS) revealed that the C-O\npeak shifted from 533.9 eV before annealing to 534.1 eV after annealing, consistent\nwith oxetane-ring opening and cross-link formation”. Please provide the reference or\nexperimental data to support the declaration.\nResponse: We thank the reviewer for this suggestion. We provide an explanation based\non established electronic structure principles, relevant literature, and additional\nexperimental evidence\n(1) In XPS, the O 1s binding energy directly reflects the local electron density around\nthe oxygen atom. Cyclic ethers (the oxetane group in this work) feature ring strain and\na compressed C-O bond angle, which leads to a more localized electron density on\noxygen and stronger core hole screening, giving a lower binding energy (533.9 eV).\nAfter ring opening to form a linear ether linkage, the ring strain is released and the C-\nO bond becomes more polarized. As a result, the electron density on oxygen decreases\nand screening becomes weaker, leading to a higher binding energy (534.1 eV). This\nchange has been well established in previous studies (Chem. Rev. 2016, 116, 12150-\n12233; JACS Au 2022, 2, 723-730).\n(2) Beyond XPS, we tracked the cross-linking process using FTIR spectroscopy (Fig.\nR4, also Supplementary Fig. 15). After annealing, the characteristic oxetane band at\n983 cm-1 decreases markedly, while the features associated with linear C-O-C stretching\nvibrations emerge and intensify. The consistent trends observed from vibrational\nspectroscopy and photoelectron spectroscopy mutually corroborate the conclusion of\noxetane ring opening and linear ether formation. This trend is in good agreement with\nprior literature on oxetane ring opening and the concomitant formation of linear ether\n(C-O-C) linkages (Angew. Chem. Int. Ed. 2021, 60, 10935-10941).\nThe corresponding discussion and references have been added to the revised manuscript.\nFig. R4. FC and CFC FTIR spectra.\n4. The unreacted C is not dissolved in chlorobenzene in Fig. 2c. Why?\nCFC film remains intact in chlorobenzene, and the unreacted C incorporated in the\n60\nfilm is not dissolved or washed out. This behavior primarily stems from the physical\nconfinement effect provided by the cross‑linked network.\n(1) During thermal annealing, the oxetane groups on the mono-adduct and bis-adduct\nspecies undergo ring opening and form covalent cross-links, building a three-\ndimensional network. The unreacted C molecules are physically trapped and confined\n60\nwithin this network. The cross-linking markedly suppresses molecular mobility and\nsolvent penetration, which greatly hinders the diffusion and migration of C into the\n60\nsolvent, resulting in excellent macroscopic solvent resistance.\n(2) We performed an experiment by immersing a CFC film and a PCBM film in\nchlorobenzene under identical conditions. As shown in Fig. R5 (also Supplementary\nFig. 13), the PCBM film dissolves rapidly, whereas the CFC film remains intact with\nno noticeable dissolution, directly evidencing the solvent stability imparted by the\ncross-linked network.\n(3) This observation is also consistent with phenomena reported in the literature.\nFor example, Adv. Funct. Mater. 2014, 24, 6172-6177 and Nano Energy 2018, 49, 324-332\nindicate that solidified or cross-linked polymer networks can effectively encapsulate\ncomponents such as C and suppress their washout in solvents through geometric\n60\nconfinement.\nTherefore, the physical confinement by the cross‑linked network explains why C\n60\ncannot dissolve out of the CFC film.\nFig. R5 (a) Photographs of PCBM- and CFC-based films. (b) Photographs of PCBM-\nand CFC-based films after soaking in chlorobenzene.\n5. Generally speaking, fullerene derivatives with isomers or impurity would suffer from\nlow electron mobility. Why the fullerene composite has the higher electron mobility\nthan PCBM?\nResponse: We thank the reviewer for raising this important question. We agree that the\ngeneral statement “isomers or impurities typically reduce electron mobility” holds in\nmany cases, this applies mainly when bulky functional groups or unfavorable packing\nincrease the intermolecular spacing. In our multicomponent fullerene composite, the\nhigher electron mobility arises from deliberate component design and synergistic\neffects, which collectively improve molecular packing and the charge transport\npathways. Our rationale is as follows.\n(1) In fullerene films, electron transport is primarily governed by intermolecular\nhopping, and the intermolecular distance between fullerene cages is a key determinant\nof mobility (J. Phys. Chem. C 2014, 118, 21785-21797; Nano Lett. 2009, 9, 1085-1090).\nIf a fullerene derivative contains bulky substituents or forms disordered packing, the\nincreased molecular spacing can indeed deteriorate electron mobility.\n(2) The conclusion that “multicomponent mixtures inevitably lower mobility” is not\nuniversal. Recent studies (Adv. Mater. 2024, 36, 2308706; EES Sol., 2025, 1, 567) have\nshown that fullerene mixtures (C /PCBM/ICBA) can also achieve favorable\n60\ncharge‑transport performance.\n(3) In our ternary composite (CFC), each component plays a distinct and\ncomplementary role. The cross-linkable mono-adduct and bis-adduct species form a\nrobust covalent network skeleton upon thermal annealing, imparting excellent\nmechanical integrity and solvent resistance. Meanwhile, C , which exhibits high\n60\nintrinsic electron mobility, is physically confined within the interstitial voids of this\ncross‑linked network. There, it acts as a charge‑transfer relay, enhancing close\nintermolecular contact and connectivity within the film, thereby improving the overall\nelectron transporting performance.\nIn summary, CFC is not simply a random blend of different fullerenes but a designed\nfunctional composite: a covalent network ensures stability, while the confined C\n60\noptimizes packing and transport, ultimately leading to a high electron mobility. This\nconclusion is also supported by the corresponding mobility measurements (Fig. R6,\nalso Fig. 3c).\nFig. R6 Electron mobility measurement of PCBM- and CFC-based films.\n6. Why the PL lifetime for PVSK/CFC is shorter than that of the PL lifetime for\nPVSK/PCBM.\nResponse: We thank the reviewer for raising this question. In PVSK/ETL\nheterostructures, the PL lifetime is largely governed by the interfacial charge transfer\nrate. A shorter PL lifetime typically indicates more efficient charge extraction (Adv.\nEnergy Mater. 2020, 10, 1904134; Nat. Commun. 2017, 8, 15330).\nIn this study, the PVSK/CFC sample exhibits a shorter PL lifetime than PVSK/PCBM,\nwhich can be mainly attributed to the following synergistic factors.\n(1) The CFC film possesses a higher electron mobility than PCBM (Fig. R7a, also Fig.\n3c), providing a faster transport pathway for the extracted electrons and thus reducing\ncharge accumulation and recombination probability at the interface.\n(2) XPS measurements of Pb 4f (Fig. R7b, also Supplementary Fig. 23) and\ntheoretical calculations (Figure R7c, also Supplementary Fig. 24) suggest a stronger\nchemical interaction at the PVSK/CFC interface. This interfacial coupling is expected\nto lower the charge transfer barrier and thereby accelerate electron injection and\nextraction from the perovskite into the CFC layer.\nTherefore, the shortened PL lifetime can be ascribed to the stronger chemical\ninteraction at the PVSK/CFC interface together with the higher electron mobility of\nCFC. We have expanded the related discussion in the revised manuscript.\nFig. R7 (a) Electron mobility measurement of PCBM- and CFC-based films. (b) XPS\nPb 4f spectra of PVSK, PCBM/PVSK, and CFC/PVSK.\n7. The relationship between charge carrier dynamics and photovoltaic parameters of\nPSCs should be clearly clarified.\nResponse: We thank the reviewer for this valuable suggestion. To clearly elucidate the\nintrinsic links between carrier dynamics and the photovoltaic parameters, we have\nsupplemented relevant analysis and discussion in the revised manuscript. The detailed\nanalysis is outlined below.\n(1) J is governed jointly by the generation of photocarriers and the charge collection\nSC\nefficiency. Transient photocurrent (TPC) measurements (Fig. R8a, also\nSupplementary Fig. 26) show that the decay time constant of the CFC-based device\n(4.54 μs) is markedly shorter than that of the PCBM-based device (14.88 μs). This\nindicates that CFC enables faster charge extraction and transport, which is consistent\nwith the higher J achieved in CFC device.\nSC\n(2) Non-radiative recombination losses within the device, particularly at the interfaces,\nare a key factor limiting the open-circuit voltage. Transient photovoltage (TPV)\nmeasurements (Fig. R8b, also Fig. 3g) show that the CFC device exhibits a longer\nrecombination lifetime (836.35 μs versus 623.33 μs). This indicates that non-radiative\nrecombination at the perovskite/ETL interface is effectively suppressed, and the\nreduction in interfacial recombination loss is an important reason for the higher open-\ncircuit voltage achieved with the CFC device.\n(3) FF is influenced by several factors, mainly including series resistance, and\nrecombination resistance. Dark electrochemical impedance spectroscopy (EIS) analysis\n(Fig. R8c, also Supplementary Fig. 28) shows that the CFC device exhibits a higher\nrecombination resistance. This suggests that interfacial recombination is suppressed.\nThe suppressed recombination together with enhanced extraction and transport favors\na higher FF.\nIn summary, TPC, TPV, and EIS measurements collectively establish a complete logical\nchain connecting microscopic carrier dynamics to macroscopic photovoltaic parameters.\nOwing to faster charge extraction, stronger suppression of recombination, and reduced\ntransport losses, CFC contributes to the overall improvement in power conversion\nefficiency. The corresponding discussion has been added to the revised manuscript.\nFig. R8 (a) TPV decay of different devices. (b) TPC decay of different devices. (c)\nNyquist plots of different devices.\n8. How about the photovoltaic performance of PSCs, if use C :BCM or C :BCB as\n60 60\nthe ETL?\nResponse: We thank the reviewer for this suggestion. As suggested, we fabricated\ndevices using C :BCM and C :BCB (employing the same component ratios as in the\n60 60\nternary system) as the ETLs. As shown in Figure R9 (also Supplementary Fig. 4),\nboth C :BCM- and C :BCB-based devices demonstrate relatively inferior\n60 60\nperformance compared to the CFC-based device, which can be attributed to insufficient\nfilm uniformity and continuity within these binary ETLs.\nOverall, neither binary system surpasses the ternary CFC system proposed in this work\nin terms of device performance or cost‑effectiveness. The relevant data and\naccompanying discussion have been incorporated into the revised manuscript.\nFigure R9 J-V curves of devices based on C :BCM, C :BCB, and CFC ETLs.\n60 60\n9. The XPS of the PVSK/PCBM film for Pb 4f should be provided to compare with the\ninteraction PVSK with CFC. The corresponding theoretical calculation would help to\nsupport the experimental result.\nResponse: We thank the reviewer for this constructive suggestion. In response, we have\nnow included the Pb 4f XPS spectra for the PVSK/PCBM film in the revised manuscript\nto enable a direct comparison with the PVSK/CFC interface. As shown in Fig. R10a\n(also Supplementary Fig. 23), the Pb 4f peak for the PVSK/CFC sample shows a more\npronounced shift relative to that of PVSK/PCBM, indicating a stronger chemical\ninteraction at the CFC/perovskite interface. To further support the experimental\nfindings, we have also carried out corresponding density functional theory (DFT)\ncalculations. The calculated adsorption energies (Eads) follow the trend PCBM\n(−1.12 eV) < BCM (−1.34 eV) < BCB (−2.30 eV), as displayed in Fig. R10b (also\nSupplementary Fig. 24). This trend confirms that the CFC system exhibits a stronger\nchemical interaction with the perovskite surface compared to PCBM. The relevant data\nand discussion have been incorporated into the revised manuscript.\nFig. R10 (a) Pb 4f XPS spectra collected from PVSK, PCBM/PVSK, and CFC/PVSK\nfilms. (b) DFT‑calculated adsorption energies of PCBM, BCM, and BCB on the FAPbI\n3\nsurface.\n10. The multi-component composite usually brings about disordered molecular\nstacking or defects. Why the trap density of CFC based PSCs is lower?\nResponse: We thank the reviewer for raising this point. We agree that conventional\nmulti-component blends often suffer from disordered packing and increased defects due\nto poor compatibility between different molecules. However, the CFC system in this\nwork is specifically designed to overcome these challenges through synergistic\ncross‑linking and component confinement, which collectively suppress defect\nformation. The key reasons are outlined below.\n(1) Unlike physically blended mixtures, the cross‑linkable mono‑adduct and bis‑adduct\nspecies in CFC form a covalent three‑dimensional network during thermal annealing.\nThis network acts as a structured scaffold that templates the molecular packing.\nThe unreacted C molecules are physically confined and densely packed within the\n60\ninterstitial voids of this network. This “welding” effect minimizes random molecular\narrangements, reduces intermolecular spacing, and creates continuous, low‑resistance\npathways for electron transport. As a result, the charge‑transport channels become more\nuniform and ordered, lowering the probability of trap‑state formation.\n(2) Both XPS and DFT calculations confirm that CFC exhibits stronger chemical\ninteraction with the perovskite surface compared to PCBM. This interaction not only\naccelerates charge extraction but also effectively passivates under‑coordinated Pb²⁺ and\nhalide vacancies at the perovskite/ETL interface. The suppressed interfacial\nnon‑radiative recombination is directly evidenced by the longer recombination lifetime\n(TPV) and higher open‑circuit voltage in CFC‑based devices.\nIn summary, the CFC system is a functionally integrated composite where cross‑linking\nprovides structural order, confined C ensures dense packing and fast transport, and\n60\nenhanced interfacial interaction passivates defects. These synergistic effects\ncollectively explain why the trap density in CFC‑based PSCs is lower than that in\ncontrol devices.\n11. The TPC decay lifetimes for devices based on PCBM and CFC are 14.88 μs and\n4.54 μs, respectively, indicating a faster electron extraction in CFC-based devices,\nrather than a slower carrier recombination rate. Please revise the statement in the text.\nResponse: We appreciate your valuable suggestion. We have revised the relevant text\naccordingly in the manuscript. “The TPC decay lifetimes for devices based on PCBM\nand CFC are 14.88 μs and 4.54 μs, respectively (Supplementary Fig. 26,\nSupplementary Table 9), indicating a faster electron extraction process in the CFC-\nbased devices.”\n12. Why built-in potential (V ) for PCBM-based devices is lower than that for CFC-\nbi\nbased devices? Please explain it.\nResponse: Thank you for raising this insightful question. The built‑in potential (V ) is\nbi\nclosely related to the energy level alignment and interfacial electronic coupling between\nthe perovskite and the ETL. The higher V observed in CFC‑based devices can be\nbi\nattributed to the following factors, supported by both experimental and theoretical\nevidence:\n(1) XPS measurements (Fig. R11a, also Supplementary Fig. 23) reveal a more\npronounced shift in the Pb 4f peak for the CFC/perovskite interface compared to the\nPCBM/perovskite interface, indicating stronger chemical interaction between CFC and\nthe perovskite surface. This interfacial coupling facilitates better electronic contact and\nreduces the energy mismatch between the perovskite and the ETL, thereby enhancing\nthe effective band bending and increasing V (Nat Commun 2019, 10, 4593).\nbi\n(2) The stronger interaction also helps passivate interfacial defect states (e.g.,\nunder‑coordinated Pb2+ and halide vacancies). Fewer defects at the interface reduce\nFermi‑level pinning, allowing the perovskite Fermi level to shift closer to the\nconduction band. This results in a larger effective work‑function difference between the\ncontacts and, consequently, a higher V in CFC‑based devices. This is consistent with\nbi\nprevious report (Joule 2021, 5, 467-480).\n(3) We measured the work functions of PCBM and CFC films by ultraviolet\nphotoelectron spectroscopy (UPS), and thereby determined their EF positions. As\nshown in Fig. R11b (also Fig. 3a), the Fermi level of the CFC film is shallower than\nthat of PCBM. According to device physics, a larger EF difference corresponds to a\nstronger built in field and thus a higher V , which is fully consistent with the V values\nbi bi\ndirectly obtained from capacitance voltage (C-V) measurements.\nIn summary, the enhanced V in CFC‑based devices stems from stronger chemical\nbi\ncoupling, better energy‑level alignment, and reduced interfacial defect density\ncompared to the PCBM control.\nFig. R11 (a) Pb 4f XPS spectra collected from PVSK, PCBM/PVSK, and CFC/PVSK\nfilms. (b)UPS spectra of CFC and PCBM films.\nReviewer #2 (Remarks to the Author):\nIn this study, the authors report a one-pot synthesis method for the kilogram-scale\nproduction of fullerene-based electron transport layer (ETL) materials, designed for use\nin high-performance inverted perovskite solar cells (PSCs). Inverted PSCs are widely\nregarded as one of the most commercially viable architectures, wherein the fullerene-\nderived ETL is crucial for achieving high device efficiency and operational stability.\nThrough molecular design, the authors developed a ternary fullerene composite as the\nETL material. This ternary system exhibits a synergistic effect, leading to the formation\nof a stable and compact ETL with optimal electron mobility, which contributes to a\nremarkable power conversion efficiency of 26.55% and excellent operational stability.\nMoreover, the ETL demonstrates broad applicability across PSCs with various\nbandgaps, offering a promising strategy for enhancing both efficiency and stability.\nNotably, the material can be produced via a low-cost and scalable preparation route.\nThis is a well-designed work that provides valuable insights and innovative solutions\nto address ongoing challenges associated with fullerene-based electron transport layers\nin perovskite photovoltaics. Overall, I think this work is suitable for the publication in\nNature Communications, after addressing the following issues:\nResponse: We sincerely thank the reviewer for the very positive recommendation, and\nwe provide our responses to each comment below.\n1. Is it possible for the reaction pathway to produce multi-adduct fullerene derivatives?\nCan such by-products be completely removed during the purification process? If trace\nmulti-adduct fullerenes remain in the final material, could they adversely affect the\nphotovoltaic performance of the device?\nResponse: We sincerely thank the reviewer for raising these questions, which are\ncrucial for a comprehensive evaluation of synthetic controllability, purity, and the\nresulting device performance.\n(1) We agree that, alongside the target products, a small fraction of multi‑adduct species\ncan be generated during synthesis. By precisely controlling the one‑pot reaction\nconditions, we successfully suppress the formation of such by‑products to a low level.\n(2) Owing to the higher polarity of multi‑adduct species compared with the target\nmono‑ and bis‑adducts, we employed a straightforward ethanol‑washing step to\neffectively remove them. High‑performance liquid chromatography (HPLC) analysis\nof the purified product (Fig. R12a, also Supplementary Fig. 2b) shows no discernible\npeaks attributable to multi‑adduct species, confirming that their content falls below the\ndetection limit. This result ensures high compositional consistency and batch‑to‑batch\nreproducibility.\n(3) To explicitly assess the potential influence of residual multi‑adduct species, we\nperformed control experiments in which defined amounts of multi‑adduct fullerene\n(approximately 1 mol% and 3 mol%) were intentionally blended into the ternary CFC\ncomposite. As summarized in Fig. R12b and Table R2, the addition of 1 mol%\nmulti‑adduct species does not cause any noticeable degradation in key photovoltaic\nparameters. However, when the content reaches 3 mol%, a clear reduction in the\nshort‑circuit current density (J ) is observed, leading to an overall drop in efficiency.\nSC\nThis decline is likely due to the disruption of the ordered molecular packing of the\nfullerene matrix, which impedes efficient electron transport.\nIn summary, our one pot synthesis and purification protocol can effectively remove\npotential multi-adduct byproducts, and the impact of trace levels of multi adduct\nfullerene species on device performance is negligible. The relevant data and discussion\nhave been added to the revised manuscript.\nFigure R12 (a) HPLC chromatogram of the purified CFC product. (b) J-V curves of\nthe devices fabricated with different ETLs.\nTable R2. Comparison of photovoltaic performance for the devices fabricated with\ndifferent ETLs.\nSample V (V) J (mA/cm2) FF (%) PCE (%)\nOC SC\nCFC 1.193 26.28 84.62 26.55\nCFC-Multi-adduct-1% 1.189 26.20 84.26 26.25\nCFC-Multi-adduct-3% 1.189 25.77 83.94 25.71\n2. The cost analysis in the manuscript appears to be based on laboratory-scale reagent\nprices. If calculated based on bulk procurement pricing, what would be the anticipated\ncost of this type of fullerene?\nResponse: We thank the reviewer for this question. In response, we have recalculated\nthe synthesis cost of the CFC material based on kilogram‑scale bulk pricing of the key\nstarting materials.\nAs summarized in Table R3 (also Supplementary Table 2), the raw‑material unit costs\ndecrease substantially under scaled‑up procurement compared to laboratory‑scale\npurchasing. Our updated analysis indicates that the projected production cost of FC can\nbe lowered to approximately 229.7 CNY per gram. This cost is not only significantly\nlower than that of high‑purity PCBM, whose typical cost exceeds 653 CNY per gram\n(Table R4, also Supplementary Table 3), but also underscores the strong\ncost‑competitiveness of our one‑pot synthetic strategy for potential large‑scale\nmanufacturing.\nWe thank the reviewer for this suggestion, which allowed us to strengthen the value\nproposition of this work from an industrial perspective. The detailed cost analysis table\nhas been included in the Supporting Information.\nTable R3. Estimated cost of FC.\nChemical Dosage Price Cost\nC 0.0964 kg 250000.0 ¥/kg 24100.0 ¥\no-DCB (anhydrous) 4.3 L 800.0 ¥/L 3440.0 ¥\nBOM 0.0510 kg 11725.0 ¥/kg 598.0 ¥\nDBU 0.0285 kg 809.0 ¥/kg 23.1 ¥\nEthanol (AR) 5.0 L 14.0 ¥/L 70.0 ¥\nFC1 product 0.1130 kg 256982.3 ¥/kg 29039.0 ¥\nFC product 0.1264 kg 229738.9 ¥/kg 29039.0 ¥\nFC3 product 0.1143 kg 254059.5 ¥/kg 29039.0 ¥\nTable R4. Estimated cost of PCBM.\nChemical Dosage Price Cost\nC 0.1000 kg 250000.0 ¥/kg 25000.0 ¥\nMethanol 15.0 L 32.0 ¥/L 480.0 ¥\nSulfuric acid 0.5 L 41.0 ¥/L 20.5 ¥\n4-methylbenzenesulfonyl\n0.0750 kg 300.0 ¥/kg 22.5 ¥\nhydrazide\nSodium methoxide 0.0175 kg 350.0 ¥/kg 6.1 ¥\nPyridine 1.4 L 700.0 ¥/L 980.0 ¥\no-DCB 4.3 L 800.0 ¥/L 3440.0 ¥\nCS 2.5 L 50.0 ¥/L 125.0 ¥\n2\nn-Hexane 8.5 L 97.0 ¥/L 824.5 ¥\nToluene 5.0 L 56.0 ¥/L 280.0 ¥\nSilica gel 25.0000 kg 37.0 ¥/kg 925.0 ¥\nPCBM product 0.0500 kg 653784.0 ¥/kg 32689.2 ¥\n3. Since C itself lacks crosslinkable functional groups, is it stable within the\n60\ncrosslinked network, and is there a potential for aggregation?\nResponse: We thank the reviewer for raising this important question regarding the\nstability of C within the cross‑linked network and its potential for aggregation.\n60\n(1) In our system, mono‑ and bis‑adduct fullerene derivatives bearing oxetane groups\nare introduced. Under mild thermal annealing, these components form a\nthree‑dimensional covalent network. This network significantly restricts molecular\nmotion, thereby effectively suppressing the long‑range diffusion and migration of C\n60\nwithin the solid film. As a result, the risk of thermally‑ or solvent‑induced phase\nseparation or aggregation of C is fundamentally minimized. This physical\n60\nconfinement mechanism is well supported by literature precedents (Adv. Funct. Mater.\n2014, 24, 6172‑6177).\n(2) To directly assess the stability of C in the network, we performed thermal aging\n60\ntests on CFC films at 100 °C for 200 h. As shown in Fig. R13 (also Fig. 2d and e), the\naged film remains uniform and dense with no visible signs of aggregation, providing\nclear evidence of the stabilizing effect imparted by the cross‑linked matrix.\nIn summary, although C itself does not participate in the cross-linking reaction, it is\n60\nfirmly confined within the cross-linked network, and this physical confinement is\nsufficient to ensure its long-term stability under device operating conditions. The\ncorresponding data have been added to the revised manuscript.\nFig. R13 Polarized optical microscopy images of PCBM and CFC films on glass\nbefore and after thermal aging at 100 °C for 200 h.\n4. The authors emphasize the \"low-temperature crosslinking and multi-component\nsynergy\" properties of the ternary fullerene. What are its advantages compared to other\ncrosslinkable fullerenes previously reported in lead-based perovskite solar cells?\nResponse: We sincerely thank the reviewer for raising this critical question. Compared\nwith many previously reported cross-linkable fullerenes, the low temperature\ncrosslinked ternary fullerene system (CFC) developed in this work offers distinct\nadvantages in the following three aspects.\n(1) Most reported cross-linkable fullerenes require temperatures above 150 °C to\ncomplete cross-linking (Table R5). In contrast, our system achieves effective solid-\nstate cross-linking under a mild condition of 100 °C. This feature markedly reduces\nthermal stress on the underlying perovskite absorber and the overall device stack, and\nhelps avoid risks associated with elevated temperatures, such as component\nvolatilization, phase transitions, or interfacial degradation. As a result, it provides key\nprocess compatibility for realizing devices with high efficiency and robust stability.\n(2) Unlike conventional cross-linkable fullerenes that are typically based on a single\nmolecular design, our work leverages functional design and synergy among three\ncomponents, thereby breaking the common trade off among solution processability,\ncross-linking enabled robustness, and charge transport. Through this cooperative design,\nthe final crosslinked film simultaneously exhibits excellent solvent resistance and high\nelectron mobility, enabling comprehensive performance improvements.\n(3) The CFC material is synthesized via a simple one‑pot route that eliminates the need\nfor complex multistep separation or purification. The process is concise, high‑yielding,\nand highly reproducible, with minimal batch‑to‑batch variation (RSD < 3%). This\napproach not only reduces material costs but also underscores strong potential for\nscalable manufacturing.\nTable R5. Cross-linking conditions of the reported cross-linkable ETLs.\nETLs Cross-linking conditions Ref.\nC &PCBCB 200 ℃/10 min ACS Energy Lett. 2016, 1, 648-653\n60\nc-C -SAM 250 ℃/1 h Nat. Commun 2016, 7, 12806.\nCL-PCBM 200 ℃/30 min Adv. Mater. 2023, 35, 2207656.\nC-PCBSD 160 ℃/30 min Adv. Mater. 2024, 36, 2406246.\nFC 100 ℃/10 min This work\nIn summary, this work not only introduces a new cross‑linkable ETL material but, more\nimportantly, presents and validates a low‑temperature, synergistic, and scalable\nmaterials‑design strategy. It offers a fresh pathway to address long‑standing challenges\nin perovskite solar cell ETLs, such as high‑temperature incompatibility, limited\nmultifunctionality, and process complexity.\n5. KPFM data are critical for evaluating interfacial energy level alignment and\nanalyzing charge transport behavior in photovoltaic devices. The authors are\nencouraged to include such data and provide further discussion in the manuscript.\nResponse: We thank the reviewer for this suggestion. Following your recommendation,\nwe performed KPFM characterization on both PVSK/CFC and PVSK/PCBM samples.\nThe corresponding results are shown in Fig. R14, also Supplementary Fig. 21 and\nhave been included in the revised manuscript.\nThe KPFM data reveal that PVSK/CFC exhibits a lower surface potential compared to\nPVSK/PCBM. This suggests that CFC can extract electrons from the perovskite more\neffectively, which may be attributed to the stronger interfacial interaction and favorable\nenergy level alignment between CFC and the perovskite (Nat. Commun. 2024, 15,\n4922). This finding aligns with the trends observed in UPS measurements. Moreover,\nthe CFC sample shows a notably more uniform surface potential distribution compared\nto the PCBM reference. This enhanced uniformity is expected to mitigate local charge\naccumulation and recombination, thereby improving the overall charge-collection\nefficiency.\nFig.R14 (a) Surface potential maps and (b) corresponding line‑scan profiles of\nPVSK/PCBM and PVSK/CFC obtained by KPFM.\n6. The authors are encouraged to provide data on the influence of CFC layer thickness\nvariation on the device performance.\nResponse: We thank the reviewer for this important suggestion. Following your\nrecommendation, we prepared ETLs with different thicknesses by varying the\nconcentration of the CFC precursor solution (15, 20, and 30 mg mL⁻¹), and\nsystematically evaluated the effect of ETL thickness on photovoltaic performance.\nAs shown in Fig. R15 (also Supplementary Fig. 7), AFM measurements confirm the\ncorresponding variation in ETL thickness. Within the achievable thickness range\n(approximately 30-60 nm), the average efficiency exhibits only minor fluctuations. This\nindicates that the CFC ETL enables efficient charge extraction and transport across a\npractical thickness window.\nSuch high thickness tolerance represents a significant advantage for scalable fabrication,\nas it implies a wider processing window and improved process robustness. The\ncorresponding statistical data and thickness‑dependent performance trends have been\nincorporated into the revised manuscript.\nFigure R15 (a-c) Thickness of CFC films measured by AFM. (d-g) Statistical\ndistribution of the corresponding photovoltaic parameters: V , J , FF, and PCE.\nOC SC\n7. It is suggested that the authors include a statistical analysis of device performance\nacross multiple batches, including maximum, minimum, average values, and standard\ndeviations.\nResponse: We thank the reviewer for this valuable suggestion. In the revised\nmanuscript, we have added statistical analysis of the four key photovoltaic parameters,\nV , J , FF, and PCE, based on several independent device batches. The results are\nOC SC\npresented in Fig. R16 and detailed in Table R6 (also Fig.1d, Supplementary Fig. 5\nand Supplementary Table 4), which include the maximum, minimum, mean, and\nstandard deviation for each parameter. The corresponding figure and table have been\nincorporated into the revised manuscript.\nFig. R16 Distributions of (a) V , (b) J , (c) FF, and (d) PCE for devices incorporating\nOC SC\ndifferent ETLs.\nTable R6. Performance of 1.53 eV PSCs based on different fullerene ETLs\nV (V) J (mA/cm2) FF (%) PCE (%)\nOC SC\nAverage 1.175±0.013 25.51±0.39 80.39±1.55 24.09±0.60\nPCBM\nChampion 1.173 25.65 82.53 24.82\nAverage 1.178±0.005 25.93±0.20 82.89±0.76 25.31±0.26\nCFC1\nChampion 1.181 26.05 83.80 25.79\nAverage 1.188±0.004 26.25±0.17 83.88±0.50 26.17±0.15\nCFC\nChampion 1.193 26.27 84.62 26.55\nAverage 1.170±0.006 26.02±0.17 82.96±0.26 25.23±0.14\nCFC3\nChampion 1.179 26.06 82.96 25.48\n8. Several minor errors were identified: the horizontal axis in Figures 2a and 2b (TOF-\nSIMS) should be labeled as \"Sputtering Time (s)\"; in Supplementary Figure 8,\n\"wavenumber\" should be capitalized; and in Supplementary Figure 14, a space is\nmissing between the physical quantity and the unit symbol.\nResponse: We thank the reviewer for their helpful suggestions. We have made the\nfollowing corrections: the x‑axis labels in Fig. 2a and 2b have been updated to\n“Sputtering Time (s)”, the term “Wavenumber” in Supplementary Fig. 15 has been\ncapitalized, and a space has been inserted between the numerical value and its unit in\nSupplementary Fig. 22, as shown in Fig. R17. We sincerely apologize for these\noversights and have thoroughly reviewed the manuscript for similar formatting issues.\nFigure R17 (a, b) TOF‑SIMS depth profiles of PCBM‑ and CFC‑based devices after\n1000 h of MPP tracking. (c) FTIR spectra of FC and CFC. (d) FTIR spectra of CFC/PbI .\n2\n9. As this study focuses on high-performance inverted perovskite photovoltaic devices,\nit is recommended that the authors incorporate recent research advances in the\nintroduction section.\nResponse: We sincerely thank the reviewer for this suggestion. We have updated the\nIntroduction to include the latest progress in inverted perovskite solar cells (Science\n2025, 390, 638-642; Nat. Mater. 2025, 24, 1265-1272).\nReviewer #3 (Remarks to the Author):\nIn this manuscript, Hou et al. report on a kilogram-scale, one-pot synthesis of a ternary\nfullerene composite (FC), comprising C , a bis((3-methyloxetan-3-yl)methyl)\n60\nmalonate-C mono-adduct (BCM), and its bisadduct (BCB). They found BCM and\nnetork that incorporates C . Using this composite as electron transporting layer, they\n60\nachieved an efficiency of 26.55% for perovskite cells, surpassing that of PCBM-based\ncells (24.82%). Moreover, composite-based devices maintain 96.0% and 95.1% of their\ninitial efficiency after 1000 hours under ISOS-L-1 and ISOS-D-2 protocols,\nrespectively. This study provides a practical synthetic approach as well as a material\nwith both excellent performance and stability, which could have significant impact on\nthe research field and the commercialization prospects of perovskite devices. Overall,\nI find the manuscript suitable for publication in Nature Communications after the\nauthors address the following minor issues.\nResponse: Thank you very much for the reviewer’s positive evaluation of our work.\nYour encouragement and constructive suggestions are highly valuable for improving\nand strengthening the quality of the manuscript.\n1. The authors are encouraged to clarify the specific role of each fullerene component\nwithin the composite when applied in solar cells, and to explain how these species\ncollectively contribute to the observed synergistic effects in efficiency and stability.\nResponse: We thank the reviewer for this critical question. C , mono‑adduct, and\n60\nbis‑adduct fullerene fulfill distinct yet closely coordinated roles, working in synergy to\nenhance device performance.\n(1) Mono‑adduct fullerene maintains relatively high electron mobility while providing\na fraction of cross‑linkable sites. Bis‑adduct fullerene, with its higher number of\ncross‑linkable functional groups, acts as a cross‑linking hub, efficiently forming a\nthree‑dimensional covalent network that offers mechanical strength and excellent\nsolvent resistance to the film. Concurrently, C , owing to its high intrinsic electron\n60\nmobility, functions as high‑mobility transport nodes within the cross‑linked network,\nthereby significantly improving the overall electron mobility of the film.\n(2) The covalent network formed by the bis‑ and mono‑adduct fullerenes physically\nconfines C within its structure, effectively suppressing the migration and aggregation\n60\nof C during device fabrication and operation. This confinement helps maintain the\n60\nintegrity of charge‑transport pathways, thereby contributing to enhanced long‑term\noperational stability. Through this synergistic interaction, the cross‑linked film\nultimately develops into a dense and highly efficient electron‑transport layer.\nIn summary, the three components work in a complementary manner, enabling the\ndevices to achieve both high power‑conversion efficiency and excellent operational\nstability. The related discussion has been updated in the revised manuscript.\n2. According to Figure 2c, the precursor undergoes cross-linking upon heating to form\nthe composite layer. The authors are asked to comment on the solubility of the resulting\ncomposite and how this property may affect its processability and potential scalability.\nResponse: We thank the reviewer for raising this important question concerning the\nsolubility and processability of the FC composite.\n(1) As shown Fig. R18, after thermal annealing and cross‑linking of the FC precursor,\nthe resulting CFC film exhibits excellent stability in common organic solvents used for\nperovskite device fabrication, such as chlorobenzene and toluene. This confirms the\nsuccessful construction of a robust three‑dimensional covalent network structure.\n(2) Prior to cross‑linking, FC shows good solubility in common organic solvents,\nenabling the preparation of high‑quality, uniform films via solution‑based methods such\nas spin‑coating. Subsequent thermal annealing induces solid‑state cross‑linking,\npermanently converting the soluble coating into an insoluble network. This\ncharacteristic not only suits laboratory‑scale spin‑coating but also ensures compatibility\nwith scalable deposition techniques, including slot‑die coating.\nIn summary, CFC material not only significantly reduces material cost but also realizes\na fullerene‑based ETL that combines solution processability, high stability, and high\nelectron mobility. This offers a promising material design strategy for the fabrication of\nefficient, stable, and scalable perovskite solar cells.\nFig. R18 Photographs of CFC films after soaking in chlorobenzene (CB), toluene (Tol),\nand isopropanol (IPA).\n3. The authors report devices based on perovskites with bandgaps of 1.68 eV and 1.77\neV. It would be helpful to compare the performance of these devices with the current\nstate of the art, citing relevant literature to better contextualize the achieved efficiency.\nIn addition, since the composite electron-transport layer appears compatible with wide-\nbandgap perovskites, the authors are encouraged to discuss its potential advantages in\ntandem solar cells.\nResponse: We thank the reviewer for this valuable suggestion. In accordance with your\ncomment, we have systematically compared the photovoltaic parameters of our\nwide‑bandgap perovskite devices (with bandgaps of 1.68 eV and 1.77 eV) against\nrecently reported high‑performance studies. These comparisons are summarized in\nSupplementary Tables R7 and R8 (also Supplementary Table 12 and 13). The results\nshow that our devices achieve efficiencies and stabilities that are competitive with the\ncurrent state of the art, further confirming the excellent performance of CFC as an ETL\nin wide‑bandgap perovskite systems. We have also added a discussion on the\napplicability of CFC in tandem solar cells. Given its good compatibility with\nwide‑bandgap perovskites, along with its favorable electron‑transport properties and\nstability, CFC shows strong promise as an electron‑transport layer for future\nhigh‑efficiency tandem devices. The corresponding content has been incorporated into\nthe revised manuscript.\nTable R7. Performance of 1.68 eV PSCs based on different fullerene ETLs\nYear ETL PCE Ref.\n60\n60\n60\n60\n60\nTable R8. Performance of 1.77 eV PSCs based on different fullerene ETLs\nYear ETL PCE Ref.\n60\n60\n60\n60\n60\n60\n4. Figure 2g, the composite layer protected the invasion of perovskite against water and\noxygen. It would be valuable to specify what the condition used for the stability testing,\nsuch as humidity and oxygen.\nResponse: We thank the reviewer for this valuable suggestion. In accordance with your\nsuggestion, we have added the relevant experimental details in the revised manuscript.\nAs shown in Fig. R19 (also Supplementary Fig. 8), the long‑term ambient stability\ntest of the unencapsulated devices was performed under ambient air conditions, with a\ntemperature maintained at approximately 25 °C and a relative humidity of about 40 %.\nFig. R19 Long‑term ambient air stability of unencapsulated PCBM‑ and CFC‑based\ndevices monitored over 1000 h.\n5. The manuscript presents TOF-SIMS data for devices after 1000 hours of maximum\npower point tracking (MPPT). However, corresponding data prior to operation are not\nshown. Including such a comparison would be useful to more clearly illustrate the\npositive effect of the composite on long-term operational stability.\nResponse: We thank the reviewer for this suggestion. Following your recommendation,\nwe have supplemented the time‑of‑flight secondary ion mass spectrometry (TOF‑SIMS)\ndepth‑profiling data of the fresh (unaged) devices (Fig. R20, also Supplementary\nFigure 9). A direct comparison between the initial state and the data after 1000 h of\nMPP tracking clearly shows that the CFC composite layer effectively suppresses ion\nmigration and interfacial side reactions during prolonged operation. This comparative\nanalysis highlights the crucial role of CFC in ensuring the long‑term operational\nstability of the devices. The related discussion has been updated in the revised\nmanuscript.\nFig. R20 TOF‑SIMS depth profiles of (a, b) PCBM‑ and (c, d) CFC‑based devices\nbefore and after 1000 h of MPP tracking.\n6. In Figure 4h, the PCE of the PCBM-based devices drops sharply at around 600 hours.\nThe authors are encouraged to comment on the possible reasons for this rapid\ndegradation, as understanding this behavior would provide useful context for evaluating\nthe advantages of the composite layer.\nResponse: We thank the reviewer for their helpful suggestion. The sharp decline in the\nefficiency of PCBM-based devices after approximately 600 hours of operation can be\nprimarily attributed to the self-aggregation tendency of PCBM molecules under\ncombined light and thermal stress, as shown in Fig. R21 (also Fig.2d and 2e).\nThis aggregation transforms the ETL from an initially dense and continuous film into a\nrough and locally discontinuous morphology, degrading interfacial contact and providing\nadditional pathways for ion migration. Once aggregation reaches a critical point, device\nparameters may exhibit a rapid \"cliff-like\" drop.\nIn contrast, the cross-linked CFC composite forms a robust network structure,\neffectively suppressing degradation caused by both molecular aggregation and ion\nmigration. This leads to a significant enhancement in long-term operational stability.\nFig. R21 Comparison for PCBM and CFC films before and after thermal aging at\n100 °C for 200 h.\n7. In Figure S8, the characteristic peak of oxetane at 983 cm⁻¹ does not disappear after\nannealing. The authors should clarify this observation and discuss its implications for\nthe proposed cross-linking mechanism.\nResponse: We thank the reviewer for raising the question regarding the residual\noxetane characteristic peak at 983 cm-1 after annealing. The significant decrease of this\npeak without its complete disappearance is mainly due to the spatially confined\nenvironment of the reaction in a solid‑state film, where restricted molecular diffusion\nhinders some functional groups from reaching reactive sites. This phenomenon is also\ndocumented in the literature. For instance, studies on thermally cured oxetane systems\n(Polymer 2008, 40, 310-316) clearly indicate that an insoluble cross‑linked network can\nstill be formed even when the characteristic vibrational band persists. The persistence\nof the 983 cm-1 peak does not indicate film instability. We confirm the formation of an\ninsoluble cross-linked network by a solvent-immersion test in chlorobenzene, where\nthe CFC film remains intact while the PCBM film dissolves rapidly as show in Fig.\nR22 (also Supplementary Fig. 13).\nFig. R22 Photographs of PCBM- and CFC-based films (a) before and (b) after\nsoaking in chlorobenzene."}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors have well addressed the issues in the revised manuscript. Therefore, we recommend it to be published on\nNature Communications without any change.", "reviewer_2": "The authors have made detailed response to the reviewers' comments. The revision is greatly improved in the current form.\nSo I think this work is suitable for publication in Nature Communications now.", "reviewer_3": "The authors addressed all my previous concerns, it is recommended for publishing in Nature Communications in its current\nform."}, "author_response": ""}]

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