Buckyball Breakthrough: Loosening C60 Packing Pushes Perovskite Solar Cells to 27.2% Efficiency

Perovskite solar cells have spent the past decade shattering one efficiency record after another, yet a stubborn culprit has quietly siphoned away their potential: the very molecule engineers trusted to carry electrons out of the device. Now, a large international team led by researchers at the National University of Singapore, Shanghai Jiao Tong University and the University of Potsdam has identified the hidden mechanism behind this energy drain and devised an elegant fix. By infiltrating a vapor of simple trialkylamine molecules into the fullerene electron-transport layer, they loosened the tight packing of C60 buckyballs, suppressed a previously unrecognized recombination pathway, and pushed certified power conversion efficiency to 27.2 percent on a device with an active area of just over one square centimeter. The work, published in Nature Photonics, resolves a long-standing puzzle in perovskite photovoltaics and could reshape how electron contacts are designed across the field.

The heart of the problem lies at the interface between the perovskite light absorber and C60, a spherical carbon molecule resembling a microscopic soccer ball. In inverted, or p-i-n, perovskite solar cells, C60 sits directly on top of the perovskite film and serves as the electron-selective contact, whisking photogenerated electrons toward the external circuit while blocking holes. It is cheap, sublimable, chemically compatible with the delicate perovskite crystal, and forms smooth, uniform films. Yet devices built with C60 contacts consistently suffer from severe non-radiative recombination at that interface, a process in which photoexcited electrons and holes annihilate each other as heat instead of contributing to current or voltage. Previous research campaigns attacked this problem by passivating electronic trap states on the perovskite surface, assuming the perovskite itself was the defective party. Those efforts helped, but the recombination losses never fully disappeared, and the true origin of the loss remained maddeningly obscure.

The Singapore-led team approached the mystery from a different angle: instead of interrogating the perovskite, they scrutinized the electronic structure of the C60 film itself. Using density functional theory calculations, they showed that when C60 molecules pack closely together in a crystalline film, their lowest unoccupied molecular orbitals, the LUMOs, overlap strongly from molecule to molecule. This intermolecular coupling transforms what would be discrete, isolated molecular levels into extended, band-like electronic states that spread across the fullerene film and reach all the way down to the perovskite surface. In isolation, a single C60 molecule is a poor recombination center. But when its LUMO levels merge into a delocalized band that hybridizes with states at the perovskite surface, the interface gains an energetically accessible pathway for electrons and holes to meet and recombine non-radiatively.

To quantify how damaging these band-like states are, collaborators at the University of Potsdam computed carrier capture coefficients for the hybridized interface states, drawing on the multiphonon emission framework that describes how carriers shed energy into lattice vibrations. The calculations indicated that the C60-derived band-like states act as highly efficient recombination conduits, explaining why even well-passivated perovskite surfaces lose voltage the moment C60 is deposited on top. The team confirmed this experimentally by measuring quasi-Fermi-level splitting, a proxy for the maximum voltage a solar cell can deliver, before and after C60 deposition. Depositing pristine C60 onto an already passivated perovskite surface slashed the quasi-Fermi-level splitting by a striking 89 millielectronvolts, a loss large enough to shave roughly a full percentage point off the achievable efficiency.

The remedy the researchers devised is as chemically simple as it is conceptually elegant. Rather than modifying the perovskite, they exposed the C60 film to a vapor of trialkylamines, specifically a long-chain amine designated C18N. The amine molecules intercalate between the buckyballs, wedging them apart and disrupting the ordered molecular packing that enables LUMO overlap. X-ray diffraction measurements using grazing-incidence geometry at the Shanghai Synchrotron Radiation Facility documented the resulting loss of crystallinity in the fullerene film, while time-of-flight secondary ion mass spectrometry confirmed that the amine species distribute through the C60 layer. Transmission electron microscopy provided direct visual evidence of the structural changes. Crucially, the treatment preserves the electronic function of the contact: the decoupled C60 still conducts electrons efficiently, but its LUMO-derived band-like states are substantially weakened, closing the recombination channel at the perovskite interface.

The electrical consequences were immediate and dramatic. The quasi-Fermi-level splitting loss after C60 deposition dropped from 89 millielectronvolts to just 41 millielectronvolts with the amine treatment alone, and fell further to 17 millielectronvolts when combined with conventional perovskite surface passivation. In other words, the team recovered nearly all of the voltage that the fullerene contact had been silently destroying. Photoluminescence quantum yield measurements and impedance spectroscopy corroborated the reduction in non-radiative losses, and femtosecond transient absorption experiments traced the extended carrier lifetimes that result from shutting down the interfacial recombination pathway.

Translated into full devices, the interfacial engineering delivered inverted perovskite solar cells with a certified power conversion efficiency of 27.2 percent and a stabilized output of 26.6 percent on an active area of 1.02 square centimeters. That certification matters: laboratory efficiency records are sometimes reported on tiny cells measured under non-standard conditions, whereas this result was independently verified on a device large enough to be technologically relevant. The achievement places these cells among the most efficient single-junction perovskite devices ever reported and demonstrates that the remaining headroom to the theoretical Shockley-Queisser limit is increasingly limited by losses that engineers can now name, locate and suppress.

Efficiency alone does not make a solar technology viable; stability under real-world conditions does, and this is where perovskites have historically stumbled. The amine-treated devices proved remarkably resilient, retaining more than 98 percent of their initial efficiency after 1,000 hours of continuous illumination under ambient air, without protective encapsulation tricks beyond standard device architecture. This durability suggests that loosening the fullerene packing does not introduce new degradation pathways, and may even contribute to interface robustness by reducing the recombination-driven heating and ionic stress that accelerate failure. For a technology often criticized for degrading within months, thousand-hour endurance at near-record efficiency is a persuasive counterargument.

Beyond the headline numbers, the study carries a broader conceptual lesson for the field. For years, passivation strategies have focused almost exclusively on the perovskite side of every interface, treating the transport layers as chemically inert bystanders. This work demonstrates that the transport layer’s own electronic structure, down to the way its molecules pack and couple, can be the dominant source of recombination. The implication extends well beyond C60: any molecular contact material whose frontier orbitals delocalize into band-like states could be quietly sabotaging device performance, and the same intermolecular decoupling logic could be applied to organic semiconductors, small-molecule contacts and even the charge-selective layers in tandem and multi-junction architectures.

The research also showcases the power of combining atomistic simulation with synchrotron characterization and device engineering under one coordinated program, spanning institutions in Singapore, China and Germany. With perovskite-silicon tandems now approaching and exceeding 29 percent efficiency, every millielectronvolt of interfacial loss recovered translates directly into commercial advantage, and the trialkylamine vapor treatment is a low-cost, scalable post-deposition step compatible with existing fabrication flows. One of the corresponding authors, Yi Hou, is the founder of a perovskite commercialization company, underscoring how quickly laboratory insights of this kind are flowing toward manufacturing. If the buckyball’s destructive band-like states can be tamed with a puff of amine vapor, the path from record-setting cells to durable, mass-produced perovskite modules looks considerably shorter than it did before this interface was finally understood.

Subject of Research: Suppressing interfacial recombination at the C60 electron-selective contact in inverted perovskite solar cells through vapor-phase trialkylamine passivation

Article Title: Intermolecular decoupling of C60 suppresses interfacial recombination in perovskite solar cells

Article References: Intermolecular decoupling of C60 suppresses interfacial recombination in perovskite solar cells. (n.d.). https://doi.org/10.1038/s41566-026-02030-x

Image Credits: AI Generated

DOI: 10.1038/s41566-026-02030-x

Keywords: perovskite solar cells, C60 fullerene, interfacial recombination, trialkylamine passivation, electron transport layer, power conversion efficiency, non-radiative recombination, quasi-Fermi-level splitting, vapor-phase treatment, device stability, Nature Photonics, photovoltaics