WUHAN, CHINA — A research team from the Wuhan National Laboratory for Optoelectronics (WNLO) and the School of Optical and Electronic Information at Huazhong University of Science and Technology (HUST) has reported a new advancement in all-perovskite tandem solar cells. By utilizing quantitative Silvaco TCAD simulations, the team has elucidated the fundamental physics of the tunnel junction, providing a definitive design rule to overcome efficiency bottlenecks in all-perovskite tandem solar cells.

The Bottleneck: Unbalanced Charge Tunneling
All-perovskite tandem solar cells are a high-potential technology with theoretical efficiencies reaching approximately 45%. However, their practical performance is often limited by the tunnel junction—the critical interlayer that connects the top and bottom sub-cells. In these devices, the junction typically consists of a SnO₂/metal/PEDOT:PSS structure.
The researchers found that the primary challenge lies in the intrinsic physical properties of the materials. In SnO₂, electrons have an effective mass m^* of approximately 0.2m₀, whereas holes in PEDOT:PSS have a much higher effective mass of roughly 4.8m₀. This discrepancy causes the hole tunneling probability to be four orders of magnitude lower than that of electrons, making hole transport the fundamental bottleneck within the tunnel junction.

The Breakthrough: The 5.1 eV Optimal Work Function
To address this imbalance, the team investigated how the work function (Φ_M) of the interlayer metal dictates the energy barriers at the semiconductor interfaces. By scanning Φ_M from 4.2 eV to 5.6 eV, the study identified a "sweet spot" at approximately 5.1 eV (representative of metals like Gold).
According to the findings:

1. Balanced Barriers: At Φ_M ≈ 5.1 eV the barrier for holes at the HTL/metal interface is minimized to approximately 0.2 eV, while a moderate barrier of ~0.5 eV is maintained for electrons at the ETL/metal interface.
2. Minimal Resistance: This balanced configuration enables efficient bidirectional tunneling, reducing the equivalent series resistance of the tunnel junction to a remarkably low ~10⁻² Ω·cm².

Impact on Future Solar Technology
The study establishes that -driven band alignment is the central design principle for engineering high-performance tunnel junctions. These results provide quantitative guidance for selecting materials and alloys to advance all-perovskite TSCs toward their theoretical efficiency limits.
(Left) Schematic illustration of the all-perovskite tandem solar cell structure, highlighting the ETL/Metal/HTL tunnel junction1. (Middle) Equivalent circuit representation of the tandem device. (Right) Simulated equivalent series resistance of the tunnel junction as a function of the interlayer metal work function, showing the optimal resistance minimum near 5.1 eV.

The work entitled “Tunnel junction simulation of all-perovskite tandem solar cells” was published in Frontiers of Optoelectronics (published on Jan. 7, 2026). (Front. Optoelectron., 2026, 19(1): 2 DOI:10.2738/foe.2026.0002)
DOI：10.2738/foe.2026.0002