Forget solar power. Researchers have developed environmentally friendly materials that can harvest energy from the great indoors.

An international team from Soochow University in China, Imperial College London, and the University of Cambridge found structures that convert the ambient light from a room's lightbulbs into energy that could someday be used to power wireless devices.

"By efficiently absorbing the light coming from lamps commonly found in homes and buildings, the materials can turn light into electricity with an efficiency already in the range of commercial technologies," said co-lead researcher Dr. Robert Hoye , a lecturer/assistant professor at Imperial College London. "We have also already identified several possible improvements, which would allow these materials to surpass the performance of current indoor photovoltaic technologies in the near future."

While devices have been able to capture indoor light (think: solar calculators), power-absorbing efficiency has remained low. The team therefore looked for gains in two "perovskite-inspired" materials.

Unlike the crystal structure known as perovskites, which have been used in fuel cells and sensors, "perovskite-inspired" materials are lead-free while still replicating the exceptional electronic properties of their original counterpart. The crystal arrangement of perovskite-inspired materials offer high-absorption coefficient and long-range charge transport — two valuable features for photovoltaic applications.

The two perovskite-inspired materials are bismuth oxyiodide and caesium antimony iodide-chloride.

While the perovskite-inspired materials are not as efficient at absorbing sunlight, the team discovered that the two compounds are much more effective at absorbing indoor light, with efficiencies that are promising for commercial applications.

The caesium antimony iodide-chloride has a derivative, perovskite-like crystal structure, except a diamond-shaped plane is removed to make the structure layered and not three-dimensional. Bismuth oxyiodide features a similar electronic structure to the lead-halide perovskites — that is, the electron clouds around the ions overlap in the crystal structure.

Both bismuth oxyiodide and caesium antimony iodide-chloride suit indoor photovoltaics because they absorb in the visible light range.

The perovskite-inspired materials could reach efficiencies in the 40-60% range in the future, according to the researchers, meaning that 40-60% of the power from indoor light could potentially be converted into electrical power. Such a figure would be substantially larger than what the commercial technology can deliver today, and would mean that smaller solar cell arrays could power larger electronics.

In the perovskite (left), the arrangement of atoms is regularly repeated in the crystal lattice. The perovskite-inspired materials (center and right) are structured differently and have absorption profiles that closely match with the indoor light spectrum.
In the perovskite (left), the arrangement of atoms is regularly repeated in the crystal lattice. The perovskite-inspired materials (center and right) are structured differently and have absorption profiles that closely match with the indoor light spectrum.
The researchers demonstrated that the power provided by these materials under indoor illumination is already sufficient to operate electronic circuits.

“Our discovery opens up a whole new direction in the search for green, easy-to-make materials to sustainably power our smart devices," said co-lead author and professor Vincenzo Pecunia  from Soochow University.

Pecunia and Hoye answered questions from Tech Briefs via email. Their responses are below.

Tech Briefs: What led to the idea to test out the perovskite-inspired material on indoor light?

So far, the groups working on perovskite-inspired materials have been razor-focused on using them to convert solar radiation to clean electricity. The problem is that most perovskite-inspired materials absorb visible light and not near-infrared light, which is a substantial part of the solar spectrum. As a result, perovskite-inspired materials have suffered from low efficiencies.

Indoor light, on the other hand, is spread mostly over the visible spectrum.

Considering the ability of lead-free perovskite-inspired materials to absorb visible light well, we thus thought that these environmentally-friendly materials could potentially make a difference in indoor photovoltaics.

Indeed, indoor photovoltaics is in rapidly growing demand to power smart devices, which are typically used indoors — for applications such as wellness and health monitoring, smart homes, smart cities, logistics, smart manufacturing. Such applications require the ubiquitous dissemination of smart devices in everyday objects and environments; therefore it is highly attractive to develop environmentally-friendly, efficient, and low-cost indoor photovoltaic technologies for that, which could power them perpetually by simply harvesting ambient light.

This is what led us to explore this area and to discover that perovskite-inspired material can convert indoor light into electricity with efficiencies already in the range exhibited by commercial indoor solar cells.

Tech Briefs: Why has the idea of harvesting indoor light been such a challenge?

Currently indoor solar cells are made from hydrogen-passivated amorphous silicon. One would find these in solar-powered calculators, for example. However, the efficiency of these devices is less than 10%. We emphasize that this is nevertheless sufficient to power small electronic devices, and the number of indoor solar cells being made is exponentially increasing.

But increasing the efficiency of indoor solar cells would, of course, allow larger electronics used indoors to be powered with smaller area photovoltaic arrays. The perovskite-inspired materials we explored have absorption profiles that are much closer to the indoor light spectrum than amorphous silicon. We calculated that in the future, fully optimized devices made from perovskite-inspired materials could display substantially higher efficiencies in the 40-60% range.

This was a discovery waiting to be made. As mentioned above, the community working on perovskite-inspired materials has been narrowly focussed on outdoor solar harvesting. We emphasize that the new materials being explored could also have significant impact in a wide range of other applications beyond solar cells.

Tech Briefs: Where is this capability especially valuable? Can you take us through a potential application?

We are in the midst of a revolution in which there is an exponentially growing number of small devices connected together via the cloud. This is called the Internet of Things ecosystem, which could enable smart homes and workplaces of the future. Many of these devices are autonomous and need a remote power source. The go-to option right now is to power these with batteries. But this will no longer become practical as the number of devices reaches the tens of billions in the near-future, due to the scarcity and toxicity of some elements used in batteries. Powering the autonomous devices with solar cells that can last over the lifetime of the device will be important for enabling the continued expansion in the number and complexity of the Internet of Things ecosystem.

What do you think? Will indoor light power our devices? Share your questions and comments below.