A Molecular Cage Outperforms the Soccer-Ball Fullerene in Tandem Solar Cells

Perovskite-silicon tandem solar cells stack two light-absorbing materials to surpass the theoretical efficiency limit of a single solar cell, already hitting more than 35 percent in laboratory records. Behind that headline number sits a small but decisive detail: the electron-contact layer between the perovskite and the silicon. For years it has been a fullerene molecule called C60 — the same soccer-ball-shaped carbon cage that won the 1996 Nobel Prize. A new material built around a borane-carbon cage, called carborane, has now been shown to do the same job better, more cheaply, and with a cleaner interface.

The hidden bottleneck at the contact

In any solar cell, the active layer must hand its absorbed electrons to a contact material that carries them out as current. C60 has long been the default choice for perovskite contacts, but it comes with three built-in limitations. It causes non-radiative recombination at the interface — electrons and holes recombine and waste their energy as heat rather than current. It forms a mechanically weak boundary that can degrade the device. And it absorbs some of the light that should reach the perovskite, parasitically stealing photons that are already scarce.

What carborane brings

The new electron-transport material centers on a meta-carborane cage decorated with a phenylamino group. Carboranes are a class of molecules where boron and carbon atoms form a stable, three-dimensional cluster — chemically related to the fullerenes but built with a different architecture. This alternative cage sidesteps C60's flaws: it suppresses the non-radiative recombination at the perovskite contact, avoids the parasitic light absorption, and can be deposited at lower processing energy. The result is a fullerene-free contact layer that keeps more of the cell's harvested light as usable current.

Full lab record, already patented

The carborane-based contact has been demonstrated in both single perovskite cells and perovskite-silicon tandem cells, led by a team at Helmholtz-Zentrum Berlin in collaboration with researchers at Kaunas University of Technology and partners. The material has since been patented and is already commercially available, meaning the leap is no longer confined to a prototype. As the solar industry pushes tandem cells past 35 percent toward commercial modules, the choice of contact layer becomes a concrete determinant of how efficient, durable, and cheap those modules can be.

Knowledge takeaway: perovskite-silicon tandem cells already exceed 35% efficiency, but the electron-contact layer is a hidden bottleneck; C60 fullerene causes recombination, weak interfaces, and parasitic absorption; a carborane-based cage contact sidesteps all three and is already patented and commercially available.