In the field of photovoltaics, perovskite is a particularly promising material that’s gunning for silicon’s crown. Not only is it great at absorbing energy from sunlight, but it’s thinner, lighter, more flexible, and easier and cheaper to manufacture.

Efficiency-wise, perovskite has shot up drastically in a little over a decade, from under 4% in 2009 to over 25% in 2021, now rivaling silicon. It works even better in so-called tandem cells, where multiple layers of materials are stacked on top of each other to harvest different wavelengths of light from the Sun. Perovskite-silicon tandem solar cells, for instance, recently passed the 30% efficiency milestone.

In the new study, a team of engineers has created and tested an all-perovskite tandem solar cell. How can a solar cell be all-perovskite but still tandem? The material’s thickness and chemical composition can be tweaked to allow it to tap into different parts of the solar spectrum, so two different versions of the material can be combined in one device.

“In our cell, the top perovskite layer has a wider band gap, which absorbs well in the ultraviolet part of the spectrum, as well as some visible light,” said Chongwen Li, co-lead author of the study. “The bottom layer has a narrow band gap, which is tuned more toward the infrared part of the spectrum. Between the two, we cover more of the spectrum than would be possible with silicon.”

Using this design, the team reported that a solar cell measuring 1 cm2 (0.15 in2) had a maximum efficiency of 27.4%, which would be a record for its type and impressive for any kind of solar cell. However, the team stops short at claiming the title, since an independent certification by NREL recorded an efficiency of 26.3% – just 0.1% short of the current official record-holder.

Where the cell did achieve a new record is in its voltage. The team measured an open-circuit voltage of 2.19 electron volts, the highest of any all-perovskite tandem solar cell.

Both of these impressive stats came thanks to tweaks made at the interface between the perovskite light-absorbing layer and the layer that carries electrons away. The team found that the electric field wasn’t consistent across the surface of the perovskite, meaning some electrons would be lost to the circuit. So the team added a thin coat of what’s called 1,3-propanediammonium (PDA), which evens out the charge of the surface.

The team says that future work will focus on boosting the solar cell’s efficiency by making the cell more stable, increasing the current, and expanding the size of the cell.

The research was published in the journal Nature.