Monolithic perovskite/Cu(In,Ga)Se₂ tandem solar cells have reached a new performance milestone, with researchers reporting a certified power conversion efficiency of 30.57% for a small-area device. The work, published in Nature Energy, addresses one of the most persistent obstacles facing this type of photovoltaic technology: losses at the interfaces where the two semiconductor subcells are electrically connected. By redesigning both the intermediate recombination layer and the perovskite electron-extraction interface, the researchers achieved higher efficiency while also improving operational and thermal stability.
Tandem solar cells generate electricity by stacking materials that absorb different portions of sunlight. In the reported architecture, a wide-bandgap perovskite cell is placed on top of a Cu(In,Ga)Se₂, or CIGS, bottom cell. The perovskite captures much of the visible spectrum, while CIGS absorbs lower-energy near-infrared photons that pass through the top layer. This division of the solar spectrum can produce more electrical power than a conventional single-junction cell, whose efficiency is constrained by the trade-off between light absorption and voltage generation.
Although the concept is highly promising, the two-terminal monolithic design is difficult to manufacture. The subcells must be physically integrated, optically aligned and electrically connected through an intermediate recombination layer. Any roughness on the textured CIGS surface, incomplete film coverage or defect-rich boundary can cause carriers to recombine before they are collected. These interface losses reduce the tandem’s voltage and fill factor, while also creating pathways for chemical degradation. The new study focuses on controlling these losses at both critical junctions rather than optimizing only one side of the device.
The first part of the strategy uses a nanoparticle-assisted nickel oxide, or NiOₓ, intermediate recombination layer. NiOₓ serves as a hole-selective material and helps connect the perovskite top cell with the CIGS bottom cell. However, depositing a uniform ultrathin layer over a textured semiconductor surface can be challenging. The researchers used nanoparticles to improve the layer’s ability to follow the underlying topography, producing more conformal coverage and reducing exposed regions where unwanted recombination or electrical leakage could occur.
The intermediate layer must do more than simply connect the two subcells. It needs to support efficient transport of holes from one absorber while allowing electrons from the other absorber to recombine with them in a controlled way. This process, known as interfacial recombination, electrically links the subcells in a monolithic tandem. According to the study, the engineered NiOₓ layer helped optimize the energy-level alignment between adjacent materials, lowering transport barriers and improving the extraction of photogenerated carriers. Better alignment can increase the voltage retained by the tandem and reduce resistive losses during operation.
The second part of the approach targets the boundary between the perovskite absorber and C₆₀, a fullerene-based electron-transport material commonly used in perovskite solar cells. Defects at this interface can act as traps, capturing electrons and holes and allowing them to recombine without contributing to the external current. The researchers introduced a bimolecular co-passivation treatment designed to neutralize multiple types of interfacial defects at the same time. By chemically stabilizing the contact and modifying its electronic properties, the treatment reduced trap-assisted recombination and promoted more efficient electron extraction into C₆₀.
The combined effect of the two interface treatments produced a champion small-area tandem with a reported efficiency of 31.09%. Independent certification measured the device at 30.57%, while its steady-state efficiency reached 30.32% under continuous operation. The active area of this device was 0.0539 square centimetres. Importantly, the strategy also translated to a larger device measuring 1.0298 square centimetres, which achieved a reported efficiency of 29.44% and a certified value of 28.85%. Maintaining performance as device area increases is a major challenge because larger cells are more vulnerable to coating defects, current non-uniformity and resistance losses.
The results are particularly notable because efficiency was not achieved at the expense of durability. The optimized devices retained approximately 94% of their initial efficiency after more than 3,500 hours of storage. Under continuous operation, they preserved about 91% of their starting performance after more than 750 hours. When heated to 70 degrees Celsius, the devices retained roughly 90% of their efficiency after 960 hours. These tests suggest that suppressing interfacial defects can improve both carrier transport and resistance to degradation, although longer testing under standardized outdoor conditions will be needed to determine how the cells perform over years of real-world use.
The study demonstrates that interface engineering may be one of the most effective routes toward practical perovskite/CIGS tandem photovoltaics. Rather than treating the boundaries between layers as passive connections, the researchers designed them as active components that control energy alignment, defect chemistry, carrier recombination and mechanical coverage. The combination of nanoparticle-assisted NiOₓ and bimolecular co-passivation addresses two different bottlenecks in the same device, helping bridge the gap between laboratory-scale efficiency records and scalable tandem manufacturing. If the approach can be adapted to larger modules and industrial deposition processes, it could accelerate the development of high-efficiency solar panels capable of converting a broader fraction of sunlight into electricity.
Subject of Research: Interface engineering for efficient and stable monolithic perovskite/Cu(In,Ga)Se₂ tandem solar cells
Article Title: Improving interface-mediated carrier transport for efficient perovskite/Cu(In,Ga)Se₂ tandem solar cells
Article References: Zeng, L., Wang, W., Tang, L. et al. Improving interface-mediated carrier transport for efficient perovskite/Cu(In,Ga)Se₂ tandem solar cells. Nature Energy (2026). https://doi.org/10.1038/s41560-026-02125-3
Image Credits: AI Generated
DOI: https://doi.org/10.1038/s41560-026-02125-3
Keywords: Perovskite solar cells, Cu(In,Ga)Se₂, tandem photovoltaics, interface engineering, NiOₓ, C₆₀, carrier transport, defect passivation, solar-cell stability, photovoltaic efficiency
