Researchers at Nanjing University and the Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, have developed a new near-infrared-absorbing acceptor molecule that pushes two-terminal perovskite-organic tandem solar cells to a power conversion efficiency of 27.35%, certified at 26.88%, alongside improved operational stability.
Perovskite-organic tandem solar cells have drawn growing interest because both the perovskite and organic layers can be processed from solution at room temperature and remain flexible, opening the door to vehicle-integrated and building-integrated photovoltaics, while the wide-band-gap perovskite layer's filtering of ultraviolet light is expected to improve operational stability relative to other tandem architectures. Even so, certified efficiencies for this tandem type have lagged behind other perovskite-based tandems such as perovskite-silicon and all-perovskite designs, largely because the low-band-gap organic subcell generates less photocurrent than the wide-band-gap perovskite subcell above it. That mismatch stems from heavy overlap between the two subcells' external quantum efficiency (EQE) spectra near the perovskite's band-gap edge, which suppresses the organic subcell's usable photocurrent once it's stacked underneath a perovskite layer.
To address this, the team designed a new low-band-gap acceptor molecule, Zh-F, built around an electron-rich heteroheptacene core without sp3-hybridized bridging atoms, with partially fluorinated side chains added to tune molecular crystallinity. The design lowers Zh-F's optical band gap to 1.23 eV and extends its absorption onset beyond 1,000 nm, while keeping its energy levels compatible with the polymer donor PM6. Rather than using Zh-F alone, the researchers incorporated it as a third component into the established PM6:BTP-eC9 organic blend, creating a ternary low-band-gap organic subcell whose EQE spectrum redshifts relative to a standard binary blend: it gives up some photoresponse right at the wavelength range where the perovskite subcell's absorption already dominates, in exchange for a substantially enhanced photoresponse further into the near-infrared, beyond that overlap region.
The resulting ternary organic solar cell achieved a high open-circuit voltage of 0.853 V with a low voltage loss of 0.510 V and a short-circuit current density of 29.00 mA/cm², addressing the voltage penalty that has typically accompanied other low-band-gap near-infrared acceptors with deep LUMO energy levels. Paired with a 1.82 eV wide-band-gap perovskite subcell in a two-terminal tandem architecture, the combined device reached a power conversion efficiency of 27.35%, with a certified value of 26.88%, while also showing enhanced operational stability relative to earlier perovskite-organic tandem designs.
The authors frame the result as evidence that deliberately engineering an organic subcell's EQE profile, rather than simply extending an acceptor's absorption range, is an effective route to narrowing the efficiency gap between perovskite-organic tandems and other perovskite-based tandem architectures, offering a design strategy that other groups working on near-infrared acceptors could apply to further improve this device class.
