Researchers from China's Northwestern Polytechnical University and Shaanxi Joint Laboratory of Graphene (NPU) and Southern University of Science and Technology have developed a new interfacial strategy that addresses a stubborn weak point in inverted perovskite solar cells (PSCs): the instability of the fullerene (C60) electron transport layer.
C60 and its derivatives are popular choices for electron transport layers in inverted PSCs thanks to their high electron mobility and favorable energy-level alignment with the perovskite, but suffer from intrinsic drawbacks. Its electronic disorder promotes trap-assisted recombination, which narrows the quasi-Fermi level splitting and drags down the open-circuit voltage. Just as importantly, C60's high molecular symmetry and weak intermolecular bonding make it prone to thermodynamically driven aggregation as the film forms - which worsens under light and heat. As the fullerene agglomerates, contact across the perovskite/C60 interface deteriorates, interface resistance climbs, charge transport suffers, and the device degrades faster. Functionalized C60 derivatives can improve compatibility and morphology, but the added groups often leave the material even more vulnerable to stress-induced breakdown. To break this cycle, the team designed an acceptor-donor-type metallopolymer, "polycarbolong," and used it to build what they call a corrugated polycarbolong interlocking (CPI) layer at the interface.
The polymer embeds transition-metal d orbitals within a π-conjugated backbone, creating a dπ-pπ-conjugated system that is unusually robust against external stress, while intramolecular charge transfer boosts its electron-transport capability. Its acceptor-π-acceptor-donor framework carries triphenylphosphine-based "claws" that grip C60 firmly, and terminal substituents that passivate undercoordinated lead (Pb²⁺) sites in the perovskite. This bidirectional coordination - binding both the perovskite's dangling lead ions and the electron-rich C60 - encourages uniform C60 deposition while simultaneously passivating defects and improving charge transport. The polymer's corrugated shape adds a mechanical benefit, acting as a stress buffer that redistributes strain and reinforces adhesion across the interface. A practical bonus comes from the phosphonium substituents, whose polycationic character improves the polymer's solubility in isopropanol, easing processing.
The team reported substantial performance gains. Optimized devices reached a power conversion efficiency of 27.43%, among the highest reported for polymer-modified PSCs, along with a certified steady-state efficiency of 26.93%. A larger 1-cm² device delivered 25.27%. The cyano-substituted version proved especially durable under the standardized ISOS testing protocols. Unencapsulated cells retained 94.8% of their initial efficiency after 2,200 hours of continuous 85°C heating (ISOS-D-2) and 95.6% after 2,200 hours under one-sun illumination (ISOS-L-1), and held onto 98.9% of their initial efficiency across 12 cycles of alternating 12-hour light and 12-hour dark periods at 85°C (ISOS-LC-2). Encapsulated devices retained 92.1% of their original efficiency after 1,000 hours of damp-heat testing (ISOS-D-3).
The work highlights how interlocking polymer engineering at the perovskite/C60 interface can suppress fullerene aggregation and, in doing so, push inverted perovskite solar cells toward both higher efficiency and the long-term operational stability that commercial deployment demands.
