Silicon has long been the go-to material for solar cell technology, and for good reason: It's inexpensive, it's stable and it's efficient. Unfortunately in that last regard silicon is fast approaching its theoretical limit, but pairing it up with other materials could help break through that ceiling. Now, researchers at EPFL and CSEM have developed a new technique for combining silicon and perovskite solar cells, and reported an efficiency of 25.2 percent – a record for that combination.

Straight silicon solar cells currently on the market max out around 20 to 22 percent efficiency, which isn't bad but doesn't give the technology much more room to grow. Perovskite has reared its head in recent years as a decent alternative, shooting up from a 3.8 percent efficiency in 2009 to over 20 percent for a multilayered perovskite cell in 2016. Still, it's more expensive than plain old silicon and has its own efficiency ceiling to contend with.

Pairing perovskite and silicon in one solar cell could help play to the strengths of both materials. Perovskite is better at converting green and blue light to electricity, while silicon specializes in red and infrared, so together they can capture a wider range of the spectrum.

"By combining the two materials, we can maximize the use of the solar spectrum and increase the amount of power generated," says Florent Sahli and Jérémie Werner, authors of the study. "The calculations and work we have done show that a 30 percent efficiency should soon be possible."

The team's new silicon-perovskite solar cell has achieved an efficiency of 25.2 percent. That tops previous tandem cells that managed 13.7 percent in March 2015, and an indium-doped solar cell that reached 24.5 percent in November 2016.

The main hurdles of these tandem cells are in the manufacturing process. Normally, perovskite would be deposited as a liquid onto the surface, but silicon's texture makes that difficult. It's made up of a sea of "pyramids" about five microns high, which lets it trap and absorb light better. But that means liquid perovskite pools in the valleys and leaves the higher "peaks" uncoated.

A microscope image of silicon pyramids covered with perovskite EPFL

"Until now, the standard approach for making a perovskite/silicon tandem cell was to level off the pyramids of the silicon cell, which decreased its optical properties and therefore its performance, before depositing the perovskite cell on top of it," says Sahli. "It also added steps to the manufacturing process."

To cover the peaks and valleys of silicon in equal measure, the researchers first use evaporation to create an inorganic base layer, which covers the pyramids. Then, a liquid organic solution is added by way of spin-coating, which seeps into the pores of the base layer. Finally, the team heats the substrate to 150° C (302° F), which lets a layer of perovskite crystallize over the top, forming a thin film that covers the entire silicon surface.

Although it might sound like a lot of extra work, the researchers say that the process is relatively simple, and could be incorporated into existing production lines with only a few extra steps. That would help the new tandem cells be produced without inflating the cost too much.

"We are proposing to use equipment that is already in use, just adding a few specific stages," says Christophe Ballif, co-author of the study. "Manufacturers won't be adopting a whole new solar technology, but simply updating the production lines they are already using for silicon-based cells."

The research was published in the journal Nature Materials.

Source: EPFL