Scientists take in solar panels

Scientists take in solar panels

Swedish and Chinese researchers have developed solar cells that efficiently convert the light in the house into electricity. Although the amount of power that these solar panels provide is relatively limited, the researchers expect that the solar panels will be perfectly capable of supplying energy to the millions of products that will be part of the Internet of Things (see box) shortly.

All devices in your house are expected to be connected shortly. And that is handy. For example, your coffee machine may automatically become active as soon as your alarm clock rings. Or that your electric car ensures that it charged for that long trip that mentioned in your online agenda. But the Internet of Things does not only consist of devices. Sensors also play an essential role. They monitor the environment, and if changes occur in that environment, they can (de) activate mechanisms to guarantee your comfort. For example, a steady drop in air quality may be a reason for the sensors to increase the air purification system.

Similarly, a steady rise in temperature may be a reason to crank up the air conditioning. All these devices and sensors naturally require energy. And that is why there is an increasing demand for small, cheap renewable energy sources that can make batteries superfluous.

The Swedish and Chinese researchers looked at organic solar cells for this. These solar cells distinguish themselves in different ways from the solar panels that we see resting on more and more roofs. Organic solar cells can produce flexibly and reasonably cheaply. Besides, the solar cells can be adapted relatively easily to convert other wavelengths of light into energy. “Functional organic solar cells usually require a mix of two materials,” explains researcher Feng Gao to Scientias.nl. “When the solar cells are active, one material – also called donor material – donates electrons, while the other material accepts these – for that reason also called acceptor material.” The mix of donor and acceptor material ultimately determines which type of light convert the solar cells into energy. “In our study, we present a donor/acceptor combination that is adjusted so that visible light is absorbed.”

Limited yield

The scientists developed two variants of these organic solar cells. The one option was 1 square centimetre in size. The other 4 square centimetres. The researchers saw that the small solar cell converted around 26.1 per cent of the light energy into electricity. The larger solar cell had an efficiency of 23 per cent. Although the solar cells are, therefore, quite efficient, the amount of power they generate is limited. For example, when exposed to ambient light with an intensity between 200 and 1000 lux, the small organic solar cell was able to deliver slightly more than 1 Volt. “That’s because the light in the house is weak, much weaker than sunlight outside,” said Gao. But that limited yield does not make the solar cells any less enjoyable. “The digitization of our society is getting faster and faster, and the Internet of Things and smart devices form a fast-growing market. Many of these devices – which often used indoors – consume small amounts of energy and they can power-efficient solar cells by converting ambient light into electricity.”

The organic solar cell that the researchers optimized for the conversion of light indoors. Image: Thor Balkhed.

But where should these solar cells be placed indoors? “Organic solar cells are flexible and can, in principle, be used anywhere. and we can also give them different shapes and colours so that they can serve as decoration and at the same time drive our Internet of Things.”

However, it is not yet that far; there is still work to do. For example, Gao expects to make solar cells much more efficient. “And by adjusting the mix of donor and acceptor materials even more precisely. The calculation in our paper indicates that we can achieve efficiency above 40% in this way.” However, higher efficiency is not the biggest challenge. “In particular, we will have to investigate the stability of our solar cells before we can apply them.”