Organic photovoltaics: Generating power and creating shade

The German Pavilion at Expo Milano 2015 models the future of solar energy with its “solar trees,” artificial plants that use organic photovoltaics rather than silicon, maximizing its efficiency.


Façades and roofs offer plenty of space for the climate-friendly generation of electricity from sunlight. And today’s solar cells are also entirely compatible with modern architecture. One example of that is the “solar trees” in the German Pavilion at the Universal Exposition 2015, in Milan. This is the first time that organic photovoltaics have been utilized on a large scale. Merck KGaA, Darmstadt, Germany is providing important components for this system.

The solar tree is an artificial 12-meter-high plant at the German Pavilion of the Universal Exposition 2015 in Milan. The solar tree is an artificial 12-meter-high plant at the German Pavilion of the Universal Exposition 2015 in Milan.

The solar tree is an artificial 12-meter-high plant at the German Pavilion of the Universal Exposition 2015 in Milan.

It’s an extraordinary object – a 12-meter-high plant that seems to be growing through the roof of the pavilion. Once it’s out in the open air, the thin stem of the plant quickly widens and ends in a calyx that spreads out like a wide sail to provide shade for the observers standing beneath it.

The artificial plant is the architectural and technical masterpiece of the German Pavilion (see below). It consists mainly of bent steel and a milky-looking textile. Inside it are solar cells that close the top of the calyx like a lid and shimmer with a blue-green flow in the sunlight. This play of colors continues down one side of the conical stem, where a thin line of organic photovoltaics stretches down to the ground. 

Generating power and creating shade

Façades and roofs offer plenty of space for the climate-friendly generation of electricity from sunlight.

Harvesting energy from the solar tree

The designer of this object, the architects at Schmidhuber in Munich, Germany, have named it the “solar tree.” Five of these solar trees can be seen for the first time in the German Pavilion at the Expo Milano 2015, which will run from May to October under the slogan, “Feeding the Planet, Energy for Life.” The German Pavilion “Fields of Ideas” offers visitors to the Expo insight into innovative and at times surprising approaches to human nutrition in the future and invites visitors to take action themselves. The solar trees are a distinctive architectural element of the pavilion, which will address themes such as innovative technologies, the economical use of resources and space, and modern architecture in a natural setting with an integrated format.    

The solar trees “grow” out of the interior of the pavilion. The solar trees “grow” out of the interior of the pavilion.

The solar trees “grow” out of the interior of the pavilion.

The solar trees will play a special role by providing power for the pavilion. They will offer shade while simultaneously using organic photovoltaics to generate electricity. “This is the first time that we are presenting this technology in connection to an architectural project,” says Richard Harding, Head of Business Development for photovoltaic products at Merck KGaA, Darmstadt, Germany. “Our aim is to demonstrate that the solar trees are suitable for large-scale use on roofs and facades.”

Richard Harding

Head of Business Development for photovoltaic products

Printed electronics

Organic photovoltaic films offer a number of advantages over conventional silicon cells. For example, the production process can be carried out at room temperature and are printed in extremely thin layers with “inks” on transparent plastic film, similarly to the way a newspaper is printed on paper. The special inks used in printing consist of formulated blends of materials which, after coating, create electricity when exposed to light.

Another advantage is that these cells are not rigid and heavy, but lightweight and flexible. They can be glued on curved surfaces as though they were a second skin. Initial applications for this technology already exist – for example, on cell phone charging bags and at bus stops. The aim is now to equip roofs and façades with solar cells that use organic photovoltaics. 

Solar cells on roofs and façades can develop their other unique characteristics. First, they generate electricity no matter the angle sunlight falls on their surface. The solar trees thus supply energy not only in the horizontally-oriented calyx cover that is directly hit by sunlight, but also in the vertically-oriented stem, which receives diffuse light. If the stem were covered by conventional silicon cells, the electricity yield in this area would be reduced to almost zero. Secondly, the different parts of the solar cell shimmer in different colors, depending on the composition of the ink used to produce them. This is a property that offers entirely new possibilities for building design.    

Longer service life in daily use

The cells for the solar trees were jointly developed by Belectric and Merck KGaA, Darmstadt, Germany. Belectric is part of ARGE OPV (see below) and manufactures the cells and processes them into modules, and Merck KGaA, Darmstadt, Germany provides the ink. So far, this technology has been developed in two stages. First, the modules’ efficiency was increased from less than one percent to more than four percent of the incident solar energy. In other words, this still-young technology already offers about one-fifth of the yield of traditional silicon cells. In the second stage, the service life of the modules was increased from a starting level of just a few hours to more than 1,000 hours, even under aggressive laboratory conditions. “That corresponds to several years of daily use,” says Harding.

Testing the technology

At Expo 2015 in Milan, the solar cells, whose diameter varies between 35 centimeters and one meter, will be used on a total area of more than 300 square meters, which is bigger than a tennis court. At maximum insolation, the five solar trees deliver a total power output of up to five kilowatts. This amount of electricity can power about 50 desktop PCs operated at full load, or five modern refrigerators.

After the Universal Exposition, the plan is to market this exemplary technology all over the world. In Germany alone, around three billion square meters of façades and roofs could be equipped with solar panels. This would be bigger than the surface area of the German federal state of Saarland.

Users will not have to worry about the future performance of the energy source. That’s because the amount of energy received by the earth from the sun in a single hour can cover the energy needs of the earth’s population for a whole year. 


German Pavilion:

On behalf of the German Federal Ministry of Economics and Energy, Messe Frankfurt has been entrusted with the organization and running of the German Pavilion at Expo 2015 in Milan. The design, planning, and realization of the German Pavilion have been taken on by ARGE, a consortium made up of Schmidhuber (pavilion’s spatial concept, architecture, and general planning), Milla & Partner (content concept, exhibition, and media design), and Nüssli Deutschland (project management and construction).



The Organic Photovoltaic Technology Consortium (ARGE OPV) is the partner network for the development and installation of the organic photovoltaic technology in the German Pavilion. The ARGE OPV is made up of the companies BELECTRIC OPV, Carl Stahl GmbH, Hager SE, U.I. Lapp GmbH, Merck KGaA, Darmstadt, Germany, and Schmidhuber. The project is being supported by the German Federal Ministry of Education and Research. 

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