A sun made up of coloured solar cells provided the electricity for the 2015 world exhibition in Milan. Transparent dye-cells adorn the  façade of the new conference centre at the EPFL, and the departure hall of Geneva airport. This revolutionary technology was invented by Michael Grätzel, who has been a Professor at the EPFL since 1977.

Transforming light energy into electrical energy: red and orange dye-sensitised solar cells.

Michael Grätzel celebrated his 71st birthday in May at the Universal Exhibition Expo 2015 in Milan, and was delighted at the enormous birthday present: 80 dye solar modules arranged in the form of a sun at the highest point in the exhibition, evidence visible from afar that the so-called Grätzel cells have now made the breakthrough from the laboratory to practical applications. “These panels are not only beautiful to look at, they are very much in demand”, says the EPFL Professor of Physical Chemistry.

It was over 25 years ago that Michael Grätzel had the idea of obtaining solar energy by using a dye, and he patented his invention in 1992. He had been inspired by plants, which absorb light using their leaf pigment chlorophyll, thus making photosynthesis possible. In conventional solar cells a semi-conductor, usually silicon, converts the sunlight into electricity. Grätzel cells consist of several layers. An organic dye captures the light, releasing electrons, which are conducted via a porous layer of minute titanium dioxide particles to the negative electrode. Another layer serves as the positive electrode, a conducting glass layer on which a thin coating of graphite or platinum serves as a catalyst. Using this method one can even obtain electrical current to run a small fan using raspberry juice, in a laboratory demonstration.

 “There are many types of red dye we can use in our solar cells”, explains Michael Grätzel. The development of green and blue substances for industrial applications on a large scale is proving rather more difficult. “We are currently working on this.” Although the chemist has passed the standard age of retirement, he continues to carry out research at the EPFL with the contractual consent of the university administration, but without drawing a salary. “When you are no longer competitive, it is better to step down and enjoy the rest of your life in retirement. But for me that time has not yet arrived”, the Professor says.

In 2015 the King Faisal International Science Award was added to his many distinctions, along with the associated generous research funding for a new project. “If I were to retire now it would also be a big disappointment for my co-workers and our sponsors, as well as for our industry partners”, adds Michael Grätzel, whose articles have been cited more than 164,000 times. This puts him in the top ten in international rankings for the most-cited chemists.

Electricity even in diffuse light

The advantage of his invention: Grätzel cells are inexpensive to manufacture; they can convert indirect and diffuse light, and can be deposited on flexible thin sheets. On the downside, there is the low efficiency and lack of long-term stability which are still causing problems. However, in the course of time improvements will be made here, and the applications will become cost-effective. One example is the “sun” put together using dye-cells that provide some 16 kilowatts of power, which was put up to adorn the Austrian pavilion at the Expo Milano 2015. These panels were made by the Swiss firm “Glass 2 Energy” (G2E), a startup that was founded in 2011. They specialise in the encapsulation of the modules in glass, which ensures a longer useful life. Glass 2 Energy also installed solar cells similar to those at the Expo, on a guardrail in Geneva airport in 2013. “I am filled with a feeling of pride when I see these panels, as I go through to the security check before leaving on a flight”, says Michael Grätzel, who is often travelling. In autumn 2015 he visited South Korea, where yet another company has started producing transparent dye-panels. In fact one of the first licences for using this solar technology was acquired by the Swiss firm Solaronix, which was founded as a spin-off of the EPFL in 1993. Solaronix made quite a stir when it installed 65 red, green and orange coloured pillars of solar cells on the west façade of the new EPFL SwissTech Convention Center.

The west façade of the SwissTech Convention Center, consisting of vertical panels of red, green and orange dye-sensitised solar cells of their inventor, Prof. Michael Grätzel.

The installation that rises to a height of 15 metres, and extends to a width of more than 36 metres, casts a warm light into the enormous entrance hall, while at the same time offering protection against too intense sunlight. “This technology leads the field when one wishes to integrate solar cells vertically in a building, allowing a part of the light to filter through”, explains Michael Grätzel. Compared to conventional silicon cells, the energy produced is about 50 % more, because the transparent panels also convert the light well even when they are not optimally aligned towards the sun and the light is only diffuse. The conference centre expects an electricity production of 8,000 kilowatt hours a year from the dye-cells.

Another use for which the Grätzel cells are particularly well-suited is wearable applications, for example as a sheet on a backpack to charge the mobile phone. The “Gratzel Solar Backpack”, which displays a Swiss cross, was invented and brought to market by a Welsh company.” Now the computer accessories manufacturer Logitech is also selling solar-powered keyboards that are charged up using the ambient light. “The market looks particularly promising for wearable electronics”, the inventor says. “But for façades there is the problem of how to set up the production capacity.” The automation that is needed calls for an investment of millions of Swiss francs, which the scientist asserts will certainly pay off considering the worldwide growth of solar energy, even in a niche market.

Science meets classical art

However much Michael Grätzel may be pleased about the applications, his main passion is still the science behind them: “The thirst for knowledge and new discoveries is what motivates every researcher”, he says. One has to stay curious but also observe well and be able to evaluate the test results correctly. “One should not overlook the unexpected”, this is how he describes one of his recipes for success – otherwise someone else will make the discovery. His coming to Lausanne more than 35 years ago was by chance – and indeed a happy chance. Born in Saxony, he studied in Berlin, where he also planned to stay, following a visit to the US, when a professorship became available at the EPFL which offered him the best conditions for his work. The young researcher happily took the chance, learned French within a few months, and has remained loyal to the university to this day even though he was later offered a position at the university of his former chosen home country.

Another passion of his is music, which Michael Grätzel would have liked to make a career of. Even as a youth he was an excellent piano player. “Once I even gave a concert”, he remembers. “I kept the placard with my name on it.” But he has no regrets about missing out on a career as a pianist: “My brother Matthias is an opera singer”, the scientist says. So he knows how difficult life as a musician can be and how much competition there is. It would be even more difficult for a pianist than for someone with a good voice. “That is a God-given gift, which one can make good use of. But practically anyone who has some talent for music can learn to play the piano.”

Even today he sometimes accompanies a violinist on the piano, but generally he is occupied with his invention, even while at home. He is installing the same panels that provided electricity for electric vehicles at the Expo in Milan, in his fruit garden. The EPFL Professor plans to charge up his electric car using the energy gained from the sun. Unfortunately the project has met with some delays. Point being, he wanted the Grätzel cells in his garden to be made using a different colour than the usual red dye that is easy to produce: “I wanted to have green panels, and there was not enough green dye available.” Plenty to do, then, for the researcher.