Researchers at China’s Nanjing University have fabricated a perovskite-organic tandem solar cell featuring a bottom organic subcell that uses a low-bandgap (LBG) acceptor to enhance near-infrared (NIR) absorption while reducing voltage losses.
“A key factor limiting the efficiency of perovskite-organic tandem solar cells is the relatively low photocurrent density generated by the LBG organic subcells compared with their wide-bandgap (WBG) perovskite counterparts,” corresponding author Qingdong Zheng told pv magazine. “This photocurrent imbalance largely originates from the substantial overlap between the external quantum efficiency (EQE) spectra of the two subcells near the absorption edge of the perovskite layer. Therefore, the development of highly efficient near-infrared (NIR) acceptors is essential for minimizing spectral overlap, improving photocurrent matching, and ultimately enhancing tandem device performance.”
The researchers said previous studies have used various nonfullerene acceptors (NFAs), developed through molecular engineering strategies, to enhance NIR absorption in LBG organic solar cells. These acceptors have enabled perovskite-organic tandem devices to achieve certified power conversion efficiencies ranging from 22.0% to 26.4%.
To push efficiencies higher, the scientists designed and synthesized a novel NFA, dubbed Zh-F, with an optical bandgap of 1.23 eV. The material features a strongly electron-donating heteroheptacene core designed to narrow the optical bandgap and extend light absorption further into the NIR region.
To further reduce voltage losses and improve the long-wavelength photoresponse, the researchers combined Zh-F with BTP-eC9, a high-performance NFA from the Y-series family of organic semiconductors. The two acceptors were paired with the polymer donor PM6 to form a ternary heterojunction, providing complementary light absorption and improved spectral utilization.
The resulting PM6:BTP-eC9:Zh-F ternary organic solar cell achieved a power conversion efficiency of 19.84%, an open-circuit voltage of 0.853 V, a short-circuit current density of 29.00 mA cm⁻², and a low voltage loss of 0.510 V.
“Overall, incorporating Zh-F into the PM6 system simultaneously enhances NIR photon harvesting, charge transport, exciton dissociation, and recombination dynamics while limiting voltage loss, making it highly suitable for perovskite-organic tandem solar cells,” Zheng said.
The organic cell was combined in a tandem architecture with a perovskite top cell that achieved a power conversion efficiency of 19.45%, an open-circuit voltage of 1.32 V, a short-circuit current density of 17.63 mA cm⁻², and a fill factor of 83.65%.
The researchers built the monolithic perovskite-organic tandem cell on a glass and indium tin oxide (ITO) substrate. The perovskite top cell consists of a nickel oxide (NiOₓ) hole-transport layer (HTL), a Ph-4PACz self-assembled monolayer (SAM), a wide-bandgap perovskite absorber, and a buckminsterfullerene (C₆₀) electron-transport layer (ETL).
An interconnecting recombination layer electrically couples the perovskite front subcell with the low-bandgap organic rear subcell. The organic cell comprises a Ph-2PACz hole-selective layer, the organic photoactive absorber, an ETL based on the perylene-diimide interfacial material PDIP, and a silver (Ag) metal contact.
The resulting architecture forms a monolithic two-terminal perovskite-organic tandem device, with the perovskite front cell harvesting higher-energy photons and the organic rear cell extending light harvesting into the near-infrared region.
Under standard illumination conditions, the tandem cell achieved a power conversion efficiency of 27.35%, an open-circuit voltage of 2.16 V, a short-circuit current density of 15.35 mA cm⁻², and a fill factor of 82.39%. A reference tandem device based on a conventional acceptor, by contrast, reached an efficiency of 25.69%.
“The tandem device achieved a certified efficiency of 26.88%, marking the highest verified efficiency reported for a perovskite-organic tandem solar cell,” Zheng stated. “Beyond its impressive efficiency, the unencapsulated device also showed good stability, retaining 80% of its initial performance after around 744 hours of continuous 1-sun illumination.”
The tandem cell was described in “Achieving 27.35% efficiency in perovskite-organic tandem solar cells by improving near-infrared absorption with a low-band-gap acceptor,” published in Joule.
In June 2025, researchers from the Solar Energy Research Institute of Singapore (SERIS) claim to have achieved a power conversion efficiency of 26.4% for a tandem cell with the same configuration.
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