A research team from the Wuhan National Laboratory for Optoelectronics and the School of Optical and Electronic Information of Huazhong University of Science and Technology has reported a new advance in all-perovskite tandem solar cells, solving a major bottleneck in tunnel crossing design.
All perovskite tandem solar cells are considered a high-potential photovoltaic technology, with a theoretical energy conversion efficiency approaching 45 percent. In practice, however, device performance has fallen short of these limits, partly due to losses and resistance at the tunnel junction connecting the upper high bandgap cell to the lower low bandgap cell.
In the work reported by the team, the tunnel junction is formed by a SnO2/metal/PEDOT:PSS stack connecting the electron transport layer of one subcell to the hole transport layer of the other. Using quantitative Silvaco TCAD simulations, the researchers investigated the fundamental physics of tunneling electrons and holes through this intersection and how these processes depend on material parameters and metalworking function.
The simulations show that the intrinsic properties of the charge transport layers create an inherent imbalance during tunneling. In SnO2, electrons have an effective mass of about 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 probability of hole tunneling about four orders of magnitude lower than the probability of electron tunneling, making hole transport the dominant bottleneck in the intersection.
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 metalworking function from 4.2 electron volts to 5.6 electron volts, they identified an optimal value near 5.1 electron volts, representative of precious metals such as gold, which balances the interface barriers for electrons and holes.
At this optimal work function, the energy barrier for holes at the metal interface of the hole transport layer is reduced to about 0.2 electron volts, while a moderate barrier of about 0.5 electron volts remains for electrons at the metal interface of the electron transport layer. This configuration allows efficient bidirectional tunneling rather than favoring one carrier type, which in turn reduces 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, as it allows the charges generated in each subcell to flow across the intersection with minimal loss. Their results highlight that work function-driven band alignment is the central design principle for designing high-performance tunnel connections in all perovskite tandem solar cells.
In addition to identifying a target work function, the study provides quantitative guidelines for selecting metals or metal alloys to realize such intersections in practice. It also links the microscopic tunneling behavior at the interfaces to macroscopic device-level merits such as series resistance and overall power conversion efficiency, providing manufacturers and device engineers with a clearer path to approaching the theoretical performance limits of all perovskite tandem designs.
The publication includes a schematic of the tandem device structure that highlights the role of the tunnel junction of the electron transport layer, the metal hole transport layer, an equivalent circuit representation of the two-terminal tandem device, and a plot of the simulated tunnel junction resistance as a function of the metalworking function. Together, these results map how small changes in the electronic structure between layers can yield big gains in operational efficiency.
The work is reported in the article “Tunnel junction simulation of all-perovskite tandem solar cells,” published in the journal Frontiers of Optoelectronics on January 7, 2026.
Research report:Tunnel crossing simulation of all-perovskite tandem solar cells
