In most industrialized countries, solar panels account for only a quarter to a third of the overall cost of building a solar farm. All the other expenses—additional hardware, financing, installation, permitting, etc—make up the bulk of the cost. To make the most of all these other costs, it makes sense to pay a bit more to install efficient panels that convert more of the incoming light into electricity.

Unfortunately, the cutting edge of silicon panels is already at about 25 percent efficiency, and there’s no way to push the material past 29 percent. And there’s an immense jump in price between those and the sorts of specialized, hyper-efficient photovoltaic hardware we use in space.

Those pricey panels have three layers of photovoltaic materials, each tuned to a different wavelength of light. So to hit something in between on the cost/efficiency scale, it makes sense to develop a two-layer device. This week saw some progress in that regard, with two separate reports of two-layer perovskite/silicon solar cells with efficiencies of well above 30 percent. Right now, they don’t last long enough to be useful, but they may point the way toward developing better materials.

Wearing layers

The idea behind two-layer—called tandem—photovoltaic devices is very simple. The top layer should absorb high-energy photons and convert them to electricity while remaining transparent to other wavelengths. Then, the layer underneath it should absorb lower energy photons. Silicon, which tends to have peak absorption toward the red end of the spectrum, is a great candidate for the lower layer. That leaves the question of what might make sense to put on top of it.

Perovskites make an appealing candidate. They’re an entire class of materials that are defined by the structure of the crystals they can form; they can be made from a huge variety of unrelated chemicals. That has some considerable advantages since it means you can potentially identify some very inexpensive source materials that can combine into a perovskite crystal. Many perovskites will also readily form from a solution of the raw materials, potentially allowing us to put a photovoltaic perovskite coating on a huge range of hardware.

The big problem has been that a lot of these crystals aren’t especially stable and will break down into raw materials over time. And that time can be as little as weeks to months for some of the more promising materials. There has been some progress in extending their lifespan, but we’re still not at the point where it makes sense to manufacture perovskite panels.

The other good thing about perovskites is that, by choosing the raw materials carefully, you can tune the peak wavelength absorbed by the resulting crystal. So you can pick a wavelength that pairs well with silicon. And there have been a few demonstrations that tandem perovskite/silicon cells work, but the efficiencies haven’t been much above what silicon can achieve on its own.

https://arstechnica.com/?p=1952188