BIPV Opportunities With Perovskite Materials

BIPV holds the promise to transform the PV markets to a genuine energy contender but there are many technical challenges that have arisen with technologies. Perovskite has been making the news for the promise of the material and the crystal structure benefits. With façades a focal point in BIPV, wafer-thin solar cells could transform glass fronts into power plants.

Above: Thin, light and supple: Heliatek vapour deposits a photoactive film in wafer-thin form onto a carrier film. The film can thus be used almost without any limits to produce electricity. Photo: Heliatek/Tim Duessen, Berlin.

Cities are eating up an increasing amount of heat and electricity. In order to reduce this consumption, buildings have to become increasingly efficient and integrate more renewable energies. New, printable photovoltaic semiconductors could help to boost this trend. They enable solar films and modules to be produced, which transform windows or façades into electric power generators. A new market is being created for the manufacturers of solar glass and modules.

The race for the best solar cells material has a new candidate: Perovskite. No other semi-conductor has enabled researchers to succeed in achieving such a dramatic development in efficiency levels.

"There is now an absolute hype surrounding Perovskite," says Thomas Unold, head of the Institute for Technologies (Institut fuer Technologien) at the Helmholtz-Centre Berlin.

The mineral promises to be efficient and at the same time inexpensive. Up to now it has not been possible to combine both these characteristics: currently the best silicon cells achieve an efficiency level of over 20%, but are expensive to produce.

Pigment and organic solar cells in turn can simply be printed on film, but often do not exceed an efficiency level just over ten percent.

Efficient improvement

With a Perovskite cell in contrast, the researchers at the University of California in Los Angeles (UCLA) recently achieved an efficiency level of 19.3 percent. Compared to the first Perovskite cells five years ago, the efficiency level has thus increased six-fold.

This is all the more remarkable as Perovskite can be easily and very economically processed. It consists of the universal commodities carbon, nitrogen, hydrogen, lead, chlorine and iodine, which can be vapor-deposited onto glass as a wafer-thin layer or printed on film and foils. The UCLA researchers produced a Perovskite layer of just around one millimetre thickness by vaporizing glass with organic molecules and lead crystals. Nevertheless, the cell generates almost as much electricity as a 180 micrometre thick silicon cell.

As a result, the high-performance light-weights could conquer the markets which were previously, to a great extent, taboo for photovoltaics. Building integrated photovoltaics for example, in short BIPV continues to be just a niche market, because the manufacture and installation of multi-functional BIPV modules is costly and expensive. Of the 3,300 Megawatt solar power output, which went online in 2013 in Germany, it is estimated that only around 100 Megawatt was integrated in building shells.

A market inhibitor: the BIPV elements are mostly project-orientated variations, which in terms of size, form, material, colour, varying transparency and design, are adapted to the respective building "“ individuality and the high planning expenditure have their price. Perovskite cells could help to reduce costs.

In addition, the technologies which come into consideration for BIPV have previously not been efficient enough. Often modules made of thin-layer silicon are available, but they rarely reach an efficiency level of 10% "“ too low to be able to compete with classic silicon cells on the roof, which convert almost twice the amount of light into electric energy. They themselves are only suitable to a certain extent for building integration: they are sawn straight out of blocks because they are simply too thick and inflexible for more complex BIPV applications.

BIPV breakthrough

Nevertheless experts are hoping for an imminent breakthrough in building-integrated photovoltaics, because it harbors immense climate protection potential. Although major cities only account for one percent of the Earth's total surface, they consume 75 percent of the primary energy used and cause 80 percent of greenhouse gas emissions.

"With a large part of their processes they have to be carbon dioxide-neutral, otherwise there is a threat of climatic collapse", warns scientist Christina Sager from the Fraunhofer Institute for Building Physics (Institut fuer Bauphysik - IBP) in Stuttgart.

Transparent power plant: Transparent solar films can be laminated in between window panes. This produces tinted glazing, which at the same time provides shade and eco-electricity. Photo: Heliatek / Smack Communications, Berlin.

In her view more efficient buildings and renewable energies could bring about the desired trend turnaround. She stated that solar technology, in particular, could be effectively integrated in domestic houses. In cases where the modules could not be fixed directly to rooftops, they could serve as power-generating windows or act as a substitute for the overall concrete façade, Sager explains.

However, until the promising Perovskite cells can be used commercially, the researchers still have to master several challenges. "The development is just beginning", says Helmholtz researcher Unold.

The service life is regarded as the greatest hurdle. Perovskite is sensitive and degrades when it comes into contact with water. The cells must be designed in such a way, that even over a period of 20 years no moisture must be allowed to penetrate them. Leak-proof encapsulations, which were developed for organic light-emitting diodes are one possible solution.

In the meantime other promising technologies, which are currently ready for market introduction, have been able to drive forward the BIPV market. Dresden company Heliatek for example has developed an organic photovoltaic film, which can be produced both in transparent as well as tinted form. In non-transparent form it reaches an efficiency level of twelve percent, while the translucent variation has a reduced efficiency level down to around seven percent.

Compared to conventional silicon modules this is low, but in the area of organic photovoltaics it sets a new record. In addition, the flexible films can be embedded in curved formats such as glass car roofs or irregularly formed façades. As dimming films are, as a rule, also in demand in vehicles and offices, there is no additional maintenance expenditure, argues Heliatek boss Thibaut Le Séguillon. As a result, competitive prices are possible.

Flexible promise

Other companies are also banking on the concept of flexible and transparent cells in organic material. The Bavarian company Belectric as well as Crystalsol from Austria for example are working on printed polymer cells. Polymers are chemical combinations of long molecule chains, which can be enriched in a solution and then printed. Heliatek in contrast uses oligomers as light collectors, in other words shorter molecule chains.

In addition, it does not print, but vaporizes them in a vacuum onto a carrier film. Currently Heliatek is still operating pilot production. With solar films from this production line the company has just set up the first window façade in Dresden. The next step planned by the company is commercial production with an annual capacity of 100 Megawatt.

Algae house: In the façade of the "House with Bio Intelligent Quotient" in Hamburg algae use photosynthesis to produce heat for the apartments. Photo: IBA Hamburg GmbH / Johannes Arlt.

With BIPV a key, new area of operation could also be created for the glass industry. Among the module producers questions are being raised which they can only answer in cooperation with the glass sector: How can the solar films be integrated in the panes? How can the integration be effected as cost effectively as possible? Can work steps such as vapor-depositing on the photoactive materials be integrated in the glass finishing? "BIPV has not yet really asserted itself. But it is definitely essential for the glass and photovoltaics industry to come closer together", says Timo Feuerbach from the Glass Technology Forum (Forum Glastechnik) within the German Engineering Federation (VDMA).

The first cooperations are already in place. In this connection, Heliatek and Brussels-based flat glass manufacturer AGC Glass Europe last year concluded a development agreement for the integration of solar films in construction glass. AGC technology boss Marc Van Den Neste says that the glass/solar façade solution created by the two companies is opening up completely new possibilities for the architects and designers to combine creativity and energy efficiency with each other.

Field-tested showcase project: The roof of Berlin's main rail station clearly illustrates the advantages of BIPV: the modules generate electricity and at the same time transmit light. Photo: BSW-Solar / Paul Langrock.

It is not only due to the cooperation with Heliatek that AGC Europe is regarded as a trailblazer for the glass industry. Its factories are home to a fully-integrated production line which not only covers the production of glass but also its coating and further processing. Various functional coatings are available to photovoltaics manufacturers, for example electrical contact layers for thin-layer modules. A similar solar-orientated concept has otherwise previously been pursued solely by East German company F-Solar. They, too, have extended their production line at their own company to include coating systems.