Razor-thin and flexible: This display, made of organic electronics, was developed at Arizona State University. | Image: U.S. Army RDECOM/Wikimedia Commons

The possibilities seem quite fantastic. Transparent solar cells could meet some 40 percent of the USA’s energy needs, wrote the American materials scientist Richard Lunt in a high-profile paper back in 2017. He reckoned on being able to install this innovative product on up to seven billion square metres of window surface across the USA. The organic solar cells that Lunt has developed allow visible light through, and absorb only ultraviolet and near-infrared light. But at present, they can only reach an efficiency quotient of ten percent, which is considerably less than the 20 percent achieved by classical solar cells under laboratory conditions. All the same, the gap is narrowing, and the commercial breakthrough for organic solar cells is coming ever closer.

Today, most electronic components are based on silicon. But researchers are coming up against the limits of this inorganic semi-metal in matters of construction and efficiency. By contrast, electronics made of organic polymers offer immense potential because they can be made extremely thin, flexible and transparent. These so-called conductive polymers have a special electron distribution that enables them to absorb and emit light, and to conduct electricity. “What’s exciting about them is how they can change their characteristics on a molecular level”, says Frank Nüesch, a physicist at EMPA in Dübendorf. They can be mixed with dyes, nanoparticles or salts, and thus offer researchers a vast number of new application possibilities.

In principle, you can use organic polymers to make solar cells, light-emitting diodes (LEDs), transistors, sensors, antennas and electronic circuitry. “Imagine a window that generates energy during the day, and at night transforms into a lamp – or a sliding roof on a car that powers an internal battery”, says Renana Gershoni-Poranne, a chemist working at ETH Zurich. She is investigating conductive polymers with the goal of developing new compounds with innovative characteristics. Already today, for example, there are materials that can be applied swiftly and cheaply to any smooth surface by means of thermal evaporation, and that can even be printed as ink – to make pictures of flowers or leaves, for example. “We could make large-scale lighting elements that offer a more pleasant source of light, or sensors for the digital society and the Internet of things”, says Nüesch.

Silicon is still more efficient

For a long time, one of the biggest problems was the degree of efficiency of these organic components. But researchers have meanwhile developed efficient light diodes and organic solar cells that under laboratory conditions have achieved an efficiency quotient of more than 17 percent. There are also improvements in their durability. When they are unprotected, these polymers quickly fade in the light and lose their functionality. One solution would be to enclose the components in glass. Already today, 60 percent of all smartphone screens have economical, organic light-emitting diodes (OLEDs).

It is more difficult to manufacture foldable displays for mobile devices. The Chinese company Huawei brought one such display onto the market in 2019, but it had to be withdrawn because it was not durable enough. Folding is a challenge because the conductive, transparent layer in today’s mobile phones is too brittle. Researchers at the EU project Treasores have recently developed a possible solution, says Nüesch. It comprises electrodes made of carbon nanotubes.

All the same, developing market-ready products remains a big challenge. Photovoltaics researchers, for example, are trying to develop a competitor solar cell made of perovskite. “At present, a huge amount of research funding is being invested into the field, at the cost of organic photovoltaics”, says Nüesch. But in buildings – where aesthetics play an important role – organic solar cells are more likely to become established.