Self-charging mobile phones

Photovoltaic solar cells enable sunlight to be converted into electrical energy. We can see examples of photovoltaic solar panels on the roofs of houses, shopping centres, parking meters, lighting systems, etc. In all these cases, the photovoltaic cells are designed to work outdoors using sunlight, and most use silicon, which is a material that absorbs high amounts of the infrared radiation present in the solar spectrum and also in incandescent light bulbs.

The main sources of illumination for the interiors of buildings (schools, factories, homes, etc.) are fluorescent tubes and LED lamps, systems that are much more energy-efficient, as their spectrum is designed to maximise visible radiation whilst minimising the infrared region. However, this lack of infrared radiation from indoor building lighting makes it difficult for silicon solar cells to generate electricity indoors. For example, there are commercially available kits with solar cell panels and batteries that can generate the 15 W needed to charge a mobile phone in the sun, but they cannot be used inside buildings.

Another example of the difficulty of trying to generate electricity from indoor light is provided by the company Samsung, which in 2021 tried, with only limited success, to replace the 1.5V alkaline batteries used in television remote controls with solar cells due to the environmental impact caused by batteries (they calculated that globally they could save up to 99 million batteries in seven years).

Because of this intrinsic limitation of silicon, researchers are investigating new materials that combine the high absorption of visible (i.e. non-infrared) light with the ability to generate an electrical charge. Such materials could, it is hoped, be used to produce solar cells that are efficient inside buildings.

Recently, solar cells based on perovskites, polymers and organic molecules have been manufactured in the laboratory, with very promising results. Interestingly, this new family of solar cells uses layers that are much thinner than the usual silicon ones, with typical thicknesses of approximately 0.1 mm (they are about 10 times thinner than the thickness of a human hair).

This property has enabled researchers to manufacture solar cells on flexible substrates, which could pave the way for a whole range of design applications that are currently completely impossible with silicon cells. Furthermore, this flexibility allows for industrial roll-to-roll manufacturing and the creation of large surfaces.

These developments therefore make it possible to imagine smart homes illuminated by energy-efficient LEDs, whose residual energy could then be harnessed using solar cells located in nearly transparent flexible layers on pieces of furniture, thus providing enough energy to charge a mobile phone. In this way, all sensors, actuators and communication networks could operate without needing to be connected to the mains electricity supply because they would have their own autonomous power system that is both wireless and free of any additional cost.

This research is part of the project 'Use of organic solar cells as an energy source for wireless sensor networks in smart homes - 2022PGR-DIPTA-URV4', funded by Tarragona Provincial Council under the agreement it has signed with the URV.