In June 2019 Elektronik Tidningen published a reportage with Jacinto Sá about Peafowl’s Invisible Solar Cells

Link for the Swedish version of the article

Invisible solar cells: Soon you can build handsome sensor nodes

We will soon be flooded by IoT sensors. Their design becomes more important than many have yet realized. The Uppsala-company Peafowl contributes with solar cells that blend in.

Silicon solar cells will always be more effective and cheaper than the plasmonic cells of Peafowl. However, Peafowl does not compete in efficiency but in beauty, and hopes to find markets where aesthetic considerations are crucial.

You don’t have to feel like a failure if you were startled and did not recognize “plasmonic solar cells”. The research area is so new that even colleagues struggle to keep up when Peafowl’s CEO and head of research is trying to explain.

– There is still a lot that the research does not even know about these materials.

His name is Jacinto Sá and he is one of the researchers who presented the foundational results. Moreover, he is still alone in trying to industrialize them.

– We are the first in the world to use plasmonic nanoparticles as light absorbers. Others have tried to use plasmonics to enhance light absorption by spreading the light. But no one has tried to use it directly as absorption material.

Peafowl’s solar cells do not attempt to compete with silicon solar cells for general energy production. They will also never be offered for purchase separately. When you encounter them, they will be integrated into other products. They will supply energy-efficient electronics that work without cables and batteries.

– Generally, we develop power supply for smart gadgets.

The concept of developing solar cells for integration into products is unusual. The goal is almost always to supply energy to the grid.

Transparency is an illusion, because it is not possible to generate electricity without absorbing photons. However, a human eye looks straight through the 500 nm thin cell.

It can be toned in different colors. Today the completely colorless cell delivers between one-tenth and one-fifth of watts per square meter. Peafowl believe they can reach one watt – one hundredth of a silicon solar cell. If you accept a color tone, you can obtain up to five watts, maybe ten.

 The numbers are valid with the formula Peafowl currently uses. There is an exciting development potential that we will return to.

One of the customer development projects is Chromogenics, an older spin-off from Uppsala University. The company produces a film whose transparency can be controlled electronically from transparent to completely opaque. The film changes transparency when one of its layers is charged and discharged.

The application is dynamic window glass with adjustable toning. In one installation, the technology has managed to save 40 percent of the energy costs for heating and cooling.

Chromogenics has until now mainly been constrained to new constructions where power cables could be drawn to the windows. With Peafowl’s cells it is sufficient to just replace the window glass. This allows for existing houses to be easily retrofitted. Competing photovoltaic technology has not been transparent enough.

Peafowl’s film is added as an extra layer over the glass. It does not change the optical properties. Because the surface is large, the energy required for the electronics is generated without problems.

It even produces surplus effect.

– We have been thinking about what we can use the extra power generated for – maybe embedding an extra sensor in the glass?

A sensor for the outside temperature, for example – toning might for example not be desired when the sun shines on a cold Swedish winter morning.

Peafowl is looking for more products to integrate its solar cells into. They should have profiles that match the power and robustness that Peafowl’s current technology delivers.

Sometimes the solar cell can completely eliminate the use of a battery. Other times it extends the operating time. When the IoT nodes are scaled up to volumes of billions, the battery changes will be a logistical nightmare, with each change many times more expensive than the battery itself. Energy harvesting that extends operating time or replaces batteries will become increasingly important.

In another customer development project, Peafowl is looking at low-power screens, for example for smart tags in department stores. A third application is wireless sensor nodes on low power standards such as Lora.

But how can Peafowl compete against silicon solar cells and their significantly higher efficiency?

– I always say that if it can be done with silicon it will be done with silicon – it is abundant, it is cheap and has a good reputation. But fortunately there are many situations where silicon is not good enough. For example, when aesthetics are important.

He talks about the Danish company that demonstrated an in-house solar cell room sensor. The customer’s CEO dismissed it directly.

– Not because it did not work, but for the aesthetics. Danish ceilings are white. A black thing in the corner became too distracting.

Jacinto Sá quotes the customer’s CEO:

-It looks like an insect and I just want to go there and smash it!

-I believe many sensors will be placed where they are very visible, and I think the appearance will often be a crucial parameter, not just an extra spice.

Jacinto Sá thinks that more people than the Danish CEO will reject tacky IoT nodes.

-It will be difficult to accept sensors everywhere if they are ugly and intrusive. And they must be everywhere if, for example, they are to manage a building automation system well.

Today’s energy and environmental problems need to be solved and with his aesthetic contribution to the development, Jacinto Sá wants to make sure that good solutions are not slowed down by the disdainful detail that they are intrusive and unattractive.

Another prerequisite Jacinto Sá had to go for industrialization was that the technology itself is environmentally friendly. Unfortunately, this does not apply to all types of solar cells.

Peafowl hopes that one of the customer projects will result in a finished product in 1–1.5 years.

Furthermore, Peafowl is developing the core technology. The next formula may be ready in 3-5 years. At that point Peafowl hopes to be able to turn up the power and supply sensor nodes that may both transmit and receive data, do their own computations and manage control. An example would be an IoT node that turns on the light when people are in the room.

Another further phase is on the very outermost research frontier. It is probably theoretically possible to significantly increase efficiency – perhaps up to 50 watts per square meter. And still retain colorless transparency. Then we’re dealing with effects that could, for example, power a laptop.

Jacinto Sá has patented ideas that he thinks may work. For example, such a solar cell must also absorb IR light.

Peafowl was founded in 2018. Almi invested SEK 4 million in May 2019. Private investors contributed another million. 

The Uppsala Innovation Center helped to assess early on whether there was anything worth commercializing here. Others who have supported Peafowl are Vinnova and EIT Innoenergy.

Jacinto Sá is a university lecturer at the Department of Chemistry-Ångström and a chemist at origin.

– But many people think that I am a physicist, because I have developed a lot of the technology to study these processes with laser and x-ray. Academically, I am known for my work on plasmonics and spectroscopy.

The company Peafowl will soon be made up by six people. Jacinto Sá is not only a scientific leader but also the CEO, but in that role he will soon be replaced.

His wife Cristina Paun is a co-founder. Her last name means peacock – a male Peafowl.

It’s no coincidence. The feathers of the peacock are really brown. The fantastic colors are created by interferences in the nanostructures of the feathers. Peafowl’s color tones arise through the same effect – by modifying the shape of the nanostructures.

The physics are so new they are almost un-known

The 500 nm thin solar cells of Peafowl will be inkjet printed at Acreo in Norrköping. They consist of three layers – one collecting positive charges, one collecting negative charges, and the active layer of Peafowl in between.

All metallic nanomaterials have a so-called plasmonic behaviour. Jacinto Sá was the one who proved, seven years ago, that light could eject electrons and holes in plasmonic materials and generate electricity. Until then plasmonics had been all about generating high temperatures.

The knowledge about the physics is so new that all his results have not been published yet.

Plasmonic materials are excellent at absorbing light. Therefore, they can be built virtually transparent.

Nobody can give a definite answer yet about the theoretical efficiency. It is complicated. One photon can for example generate multiple electrons and thus an “efficiency” above 100 percent.