Perovskite solar cells (PSCs), which utilize a hybrid organic–inorganic lead halide perovskite as the light-absorbing semiconductor, have emerged as a highly promising photovoltaic technology over the past decade^[1−5]. They have rapidly achieved power conversion efficiencies exceeding 25%, positioning them as strong competitors to traditional silicon solar cells in terms of performance^{[6, 7]}. However, stability remains a critical challenge in this field, as the performance of perovskite materials tends to degrade under prolonged exposure to heat, moisture, and operational stress. Fullerenes-molecular carbon cages exemplified by buckminsterfullerene (C₆₀) were first discovered in 1985 and earned a Nobel Prize in Chemistry for their unique structure and properties^[8]. Owing to their excellent electron affinity and mobility, these molecules have been widely adopted in organic electronics. In particular, fullerene derivatives like C₆₀ and its soluble variant [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) have played a key role in PSC design as electron-transport materials from the earliest generations of these cells^{[7, 9, 10]}. These fullerene-based layers facilitate electron extraction from the perovskite absorber and have contributed to record-breaking efficiencies, but they offer only limited interfacial stabilization^[11−15]. It is noteworthy that weak van der Waals interactions at the interface between the perovskite and conventional fullerene layers can lead to the formation of interfacial defects and mechanical degradation over time^{[14, 16−18]}. To overcome these limitations, recent research has focused on designing innovative fullerene-based materials that not only facilitate electron conduction but also actively enhance and protect the perovskite interface for long-term stability^{[19, 20]}.

Another innovative strategy centers on chemically modifying the fullerene itself to strengthen its interaction with the perovskite. In an international collaboration reported in Science (2025), researchers developed a C₆₀-derived ionic salt known as CPMAC to replace the standard C60 electron-transport layer^[20]. It is noteworthy that for over a decade C₆₀ has been a cornerstone of electron transport in PSCs, yet its unfunctionalized form interacts with the perovskite via only weak, non-covalent forces. The new CPMAC molecule, which stands for 4-(1’,5’-dihydro-1’-methyl-2’H-[5,6] fullereno-C₆₀-Ih-[1,9-c] pyrrol-2′-yl) phenylmethanaminium chloride, features a CH₂–NH₃⁺ methylammonium head group attached to the fullerene cage (Fig. 1). This structure facilitates the formation of ionic bonds at the perovskite interface. By creating a stronger electrostatic coupling between the electron transport layer and the perovskite, this ionic fullerene derivative effectively reduces interfacial defects and enhances the packing density of the interface, yielding a contact with approximately three times greater mechanical toughness compared to one made with pure C₆₀. Mechanistically, the improved interfacial bonding and packing mitigate non-radiative recombination losses and prevent mechanical delamination under prolonged stress. Solar cells incorporating CPMAC achieved power conversion efficiencies of up to ~26%, which is about 0.6% (absolute) higher than analogous devices employing pristine C₆₀–a significant improvement at these high efficiency levels. Furthermore, when subjected to prolonged thermal and humidity stress, the CPMAC-based cells exhibited only one-third of the performance drop observed in C₆₀-based cells over 2000 h. It is noteworthy that even in scaled-up perovskite mini-modules, the inclusion of CPMAC led to appreciable gains in both efficiency and stability, highlighting the practical relevance of this molecular innovation. By tailoring the molecular structure of a well-known electron acceptor, the researchers demonstrated that it is possible to retain the favorable electronic properties of C₆₀ while dramatically improving interfacial adhesion and durability.

In conclusion, interface engineering with advanced fullerene-based materials is proving to be a powerful route for elevating perovskite solar cells to the next level of performance and reliability. It is noteworthy that these innovations harness the unique chemistry of fullerenes-ranging from the incorporation of metal atoms within carbon cages to the attachment of ionic functional groups-to address the dual challenges of efficiency and stability in PSCs. The Nd@C₈₂-polymer coupling layer demonstrates how a hybrid material can provide both electronic advantages and environmental protection in a single interfacial layer. Meanwhile, the design of CPMAC underscores the impact of molecular-level modifications in reinforcing critical interfaces by forming chemical bonds where previously only weak forces existed. It is noteworthy that both approaches not only achieve impressive technical results but also open new avenues for further innovation. These advancements demonstrate the promising future of fullerene-based materials in perovskite solar cells, contributing to the next generation of high-performance, stable, and scalable solar technologies.

A comparative analysis of these approaches with traditional fullerene materials (PCBM or C₆₀) highlights the advantages of targeted molecular engineering. Conventional fullerene-based layers like PCBM and C₆₀ are effective electron acceptors and have been indispensable in achieving high initial performance in PSCs; however, their inert surface chemistry offers little opportunity for chemical bonding with perovskite layers. In contrast, the Nd@C₈₂-polymer interface and the CPMAC ionic fullerene both introduce stronger interactions at the interface-one through a composite material that physically and electronically couples to the perovskite, and the other through ionic chemical bonds that intimately link with the perovskite lattice. It is noteworthy that both of the new designs maintain or even improve charge-transport characteristics compared to traditional fullerenes: The endohedral Nd@C₈₂ ensures that the polymer-based layer remains electrically conductive, while the polar functional group in CPMAC actually improves the electronic alignment and defect passivation at the interface. By contrast, a purely polymeric interlayer (without a conductive filler) would typically hinder electron extraction, and an unmodified C₆₀ layer would leave interfacial traps that facilitate recombination. Thus, these fullerene innovations elegantly circumvent the limitations of their predecessors. It is noteworthy that both strategies also serve as built-in "stabilizers" for the device: The Nd@C₈₂-PMMA layer acts as a nano-scale encapsulant, and the CPMAC layer’s ionic bonding confers superior mechanical integrity to the interface. Traditional PCBM or C60 layers, lacking these features, often require additional barrier coatings or careful device encapsulation to attain comparable lifetimes.

One cutting-edge approach involves a magnetic endohedral metallofullerene integrated into a polymer matrix to form a robust interface layer. In recent study published in Nature (2025), Lin et al. reported an Nd@C₈₂–polymer coupling layer that simultaneously enhances electron extraction and provides in-situ encapsulation of the perovskite active layer^[19]. The Nd@C₈₂ molecule, consisting of a neodymium (Nd) atom encapsulated within a C₈₂ carbon cage, was incorporated into polymethyl methacrylate (PMMA) to form a composite interlayer. This engineered interface facilitates ultrafast electron transport while physically protecting the underlying perovskite from environmental factors and ion migration. It is noteworthy that the Nd@C₈₂ component acts as an electromagnetic coupling medium at the molecular interface, inducing interface polarization that promotes more efficient charge separation and extraction. At the same time, the polymer matrix serves to block moisture ingress and suppress the interdiffusion of ionic species (such as migrating halide ions) from the perovskite, factors that are known to cause long-term degradation. This dual functionality effectively overcomes the typical trade-off encountered with purely polymer interlayers, which often improve stability at the expense of charge transport due to their insulating nature. As a result, perovskite devices incorporating the Nd@C₈₂-PMMA layer achieved a remarkable power conversion efficiency (PCE) of 26.78% (with a certified 26.29% PCE) on small-area cells, and 23.08% on a 16 cm² module. Equally important, the unencapsulated cells retained approximately 82% of their initial efficiency after 2500 h of continuous operation at 65 °C. Under more rigorous damp-heat conditions (an ISOS-D-3 stability test), devices with this interface showed negligible degradation, maintaining over 99% of their performance after 1000 h. It is noteworthy that such stability far exceeds the performance of comparable PSCs utilizing traditional C₆₀ or PCBM layers, underscoring the broader significance of this molecular interface engineering strategy. By combining a conductive metallofullerene with a protective polymer, the researchers effectively demonstrated a pathway to synergistically optimize both efficiency and durability in perovskite photovoltaics.