A New Molecular Layer Could Make Tandem Solar Cells More Efficient and Durable

A small layer tackles several big solar-cell problems

A thin molecular layer that most people will never see could help determine whether next-generation solar cells become practical. An international research team has developed a carborane-based material called mCB-FMN to replace C60, a fullerene commonly used to carry electrons out of high-performance perovskite solar cells.

In laboratory devices, the replacement improved both electrical performance and the physical bond between layers. Single-junction perovskite cells gained 1.5 percentage points in absolute efficiency compared with reference cells using C60. Perovskite-silicon tandem devices gained 2.4 percentage points.

Why C60 became a bottleneck

Perovskite cells absorb sunlight efficiently and can be stacked on silicon to capture a broader part of the solar spectrum. Many leading designs rely on a very thin C60 layer to collect electrons. It works well and can be deposited by vacuum processing, but it also introduces several weaknesses.

Some electrical charge is lost at the boundary between C60 and the perovskite. The material can absorb light that should reach the active layers, and the interface may be mechanically weak enough to delaminate over time. Exposure to oxygen can create additional degradation at that boundary. These problems matter even more in tandem cells, where every layer must transmit or collect light with minimal loss.

What the new molecule changes

The researchers built mCB-FMN around a cage-like cluster containing boron and carbon. It can be deposited as a uniform film at a lower temperature than C60, reducing energy use and thermal stress during fabrication.

Measurements showed more efficient electron extraction and fewer non-radiative recombination losses, in which captured solar energy disappears as heat instead of becoming electricity. The new layer also passivated defects, allowed more light to reach the active materials and improved the growth of the protective tin-oxide layer deposited above it.

Mechanical testing found stronger adhesion across the perovskite, transport layer and tin-oxide stack. That is important because a highly efficient cell is of little value if temperature changes, vibration or manufacturing stresses cause its layers to separate.

Commercially available does not mean commercially proven

The material has been patented and brought to market by a specialist supplier, which could make it easier for other laboratories and manufacturers to test. Its compatibility with vacuum deposition is also valuable because the method is already relevant to scalable tandem-cell manufacturing.

However, the study demonstrates a material and laboratory devices, not years of outdoor operation in full-size solar modules. Researchers still need to establish long-term stability under heat, humidity, ultraviolet radiation and repeated thermal cycling. The cost of synthesizing and depositing the material at industrial scale must also compete with an established C60 supply chain.

Why the result matters

Solar progress is often presented as a race for one headline efficiency record. This work addresses a less visible combination of challenges: electrical loss, unwanted light absorption, oxygen sensitivity and weak adhesion. Solving several of them with the same layer could be more valuable to manufacturers than a narrow efficiency gain alone.

The result does not mean perovskite-silicon panels are ready to replace conventional silicon everywhere. It does, however, identify a plausible new family of electron-transport materials that could help tandem cells move from impressive laboratory results toward durable commercial hardware.