Horizons Tec solar and hand crank powered NOAA weather radio, LED flashlight, and cell phone charger. Pixy.org

We use a lot of smart devices these days. Most of them use lithium-ion batteries, which discharge quickly, contain toxic chemicals, and can even catch fire. But a few are solar-powered The next generation of solar cells will be based on a mineral called perovskite.

Perovskite solar cells cost less to make than the familiar silicon-based ones, but they also require toxic lead. So scientists are investigating perovskite-inspired materials with safer metals.

I wrote about perovskites a few years ago. With all the progress since then, we still won’t have perovskite solar cells on our rooftops anytime soon. Nor will utilities adopt them. But, as it turns out, solar cells from perovskite-inspired materials already work very well with indoor light.

Solar? Indoors?

A few years ago, I went to Walmart to look for a digital clock that showed seconds. I had to ask a clerk where to find the clocks. A solar clock caught her eyes, and she wondered why anyone would want to buy a clock that only worked outside.

Well, technically, “solar” has to do with the sun. The proper word for converting light to electricity is “photovoltaic.” But who wants to use such a long word all the time? So we use solar instead. By the same token, I might as well use “perovskite solar cell” instead of “perovskite-inspired materials solar cell.”

I bought that clock. It worked well for a couple of years then quit. My solar NOAA radio didn’t last very long, either. Someday, it appears, perovskite-inspired materials will work better than anything available now.

What are perovskites and perovskite-inspired materials?

Example of a smart meter that has the ability to reduce load, disconnect-reconnect remotely, and interface to gas and water meters. Wikimedia Commons

Perovskite is a mineral with a particular crystalline structure, such as calcium titanium oxide. It is easily synthesized. Its useful properties include superconductivity and magnetoresistance. Many consider perovskites the future of solar technology, well suited for low-cost and very efficient thin-film photovoltaics.

Perovskite solar cells react to a wider spectrum of light wavelengths than silicon, which makes them more efficient. They are also relatively easy and inexpensive to make, among other advantages. Therefore, they promise to lower the cost of solar energy. Their flexibility makes them suitable for more applications than silicon cells.

They do have some important drawbacks, however. While the perovskite layer costs relatively little, the completed cells have mostly gold electrodes, which are expensive. Perovskite solar cells deteriorate quickly when the air is moist. The decay products damage the electrodes.

But what’s worse, the most commonly used perovskite solar cell uses a lead or tin halide to harvest the light. There is no safe level of exposure to lead. Promising substitutes include antimony, tin, and germanium

Tin perovskites cells are less toxic than lead, but also less efficient. The lead halide cells have achieved 20% efficiency under laboratory conditions. Tin so far only manages 6%. Research continues into how to overcome these problems.

Perovskite-inspired materials, on the other hand, seek to replicate the electronic structure of perovskites without regard to the crystal structure. In other words, they do not resemble calcium titanium oxide.  They just work in a similar way. Suitable lead substitutes include indium, tin, antimony, and bismuth. It is these materials that show such promise for the next generation of perovskite solar cells.

The promise of perovskite-inspired materials

Smart watch (Apple). Wikimedia Commons

Perovskite-inspired materials may someday be suitable for generating electricity outdoors, but not yet. They do not absorb sunlight efficiently enough, but they do absorb visible light very well.

Big Data and the Internet of Things promise to intensify the demand for various kinds of sensors and other wireless devices. Soon enough, battery-powered devices will no longer be practical for some applications.

Hydrogenated amorphous silicon, the industry standard for indoor solar applications, has an efficiency rating from 4.4-9.2%. Lead-halide perovskites have demonstrated 36% efficiency indoors, but the lead makes them too toxic for practical use there.

Antimony and bismuth have similar electronic structures. These make up only two of several perovskite-inspired materials that can potentially work well in indoor devices.

They are also much less toxic and abundant enough for large-scale commercial use. And collectors made with compounds of these metals can also be made in aesthetically appealing colors. Their indoor efficiency (4-5%) greatly exceeds their outdoor efficiency (1%). The industry-standard silicon compound for indoor use doesn’t perform much better than that.

So the perovskite-inspired materials already perform well enough to power thin-film transistor circuits. Those are the circuits needed for wearable electronics, smart sensors, and the Internet of Things. What’s more, they work while the devices move from one place to another, with the variation of illumination such movement entails.

In other words, we can now envision wearable devices and other smart technology powered by perovskite-inspired materials instead of a battery.

As always, however, it can take a long time between demonstrating something works in a lab and actual products on the market. When we can actually buy something with indoor perovskite solar cells, they’ll probably be even more efficient than they are now.

Sources:

Green materials can power smart devices at home, office / IANS, Energy World, November 16, 2020
Indoor solar cells based on lead-free perovskite-inspired materials / Emiliano Bellini, PV Magazine. November 24, 2020
Lead‐free perovskite‐inspired absorbers for indoor photovoltaics / Youheng Peng et al., Advanced Energy Materials. November 3, 2020
Perovskite-inspired photovoltaics: Best practices in materials characterization and calculations / Robert L.Z. Hoye et al. US Department of Energy Office of Science. [2015]
Perovskite solar / Perovskite-Info. December 1, 2020