All perovskite tandem solar cells are regarded as a high potential photovoltaic technology, with theoretical power conversion efficiencies approaching about 45 percent. In practice, however, device performance has lagged behind these limits, in part because of losses and resistance at the tunnel junction that connects the wide bandgap top cell to the low bandgap bottom cell.
In the work reported by the team, the tunnel junction is formed by a SnO2/metal/PEDOT:PSS stack that links the electron transport layer of one sub cell to the hole transport layer of the other. Using quantitative Silvaco TCAD simulations, the researchers examined the fundamental physics of electron and hole tunneling through this junction and how those processes depend on material parameters and metal work function.
The simulations show that the intrinsic properties of the charge transport layers create an inherent imbalance in tunneling. In SnO2, electrons have an effective mass of roughly 0.2 times the free electron mass, while holes in PEDOT:PSS have an effective mass of about 4.8 times the free electron mass. This large disparity makes the hole tunneling probability about four orders of magnitude lower than the electron tunneling probability, turning hole transport into the dominant bottleneck in the junction.
To address this, the team focused on how the work function of the interlayer metal controls the energy barriers at both semiconductor interfaces. By scanning the metal work function from 4.2 electron volts to 5.6 electron volts, they identified an optimal value near 5.1 electron volts, representative of noble metals such as gold, that balances the interface barriers for electrons and holes.
At this optimal work function, the energy barrier for holes at the hole transport layer metal interface is reduced to about 0.2 electron volts, while a moderate barrier of around 0.5 electron volts remains for electrons at the electron transport layer metal interface. This configuration enables efficient bidirectional tunneling rather than favoring one carrier type, which in turn lowers the equivalent series resistance of the tunnel junction to the order of 10^-2 ohm square centimeters.
The authors describe this optimal configuration as a golden bridge within the tandem architecture, because it allows charges generated in each sub cell to flow across the junction with minimal loss. Their results highlight that work function driven band alignment is the central design principle for engineering high performance tunnel junctions in all perovskite tandem solar cells.
Beyond identifying a target work function, the study provides quantitative guidance on selecting metals or metal alloys to realize such junctions in practice. It also links the microscopic tunneling behavior at the interfaces to macroscopic device level figures of merit, such as series resistance and overall power conversion efficiency, giving manufacturers and device engineers a clearer path to approach the theoretical performance limits of all perovskite tandem designs.
The publication includes a schematic of the tandem device structure that emphasizes the role of the electron transport layer metal hole transport layer tunnel junction, an equivalent circuit representation of the two terminal tandem device, and a plot of simulated tunnel junction resistance as a function of metal work function. Together, these results map out how small changes in interlayer electronic structure can produce large gains in operational efficiency.
The work is reported in the paper "Tunnel junction simulation of all-perovskite tandem solar cells," published in the journal Frontiers of Optoelectronics on January 7, 2026.
Research Report:Tunnel junction simulation of all-perovskite tandem solar cells
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