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  • Material Matters for Photovoltaics

Dec 2012
Marie Freebody, Contributing Editor,

Researchers are working to find ever-better materials for solar cell production.

It may be tricky times for much of the photovoltaics (PV) industry, but fervent research into solar cell materials has not ceased. From the innovative tailoring of organic molecules to efficient printable plastics, new research promises to boost efficiency and lower production costs.

There are hurdles aplenty when it comes to PV materials, and for each class of material, there are a host of obstacles to overcome. Some of the challenges include replacing electronics-grade silicon with a lower-cost alternative, upgraded metallurgical-grade (UMG) silicon, or UMG-Si; using copper-in-silicon cell technology rather than silver; searching for new materials that can be used as up- or down-converters; and finding new encapsulants to replace ethylene-vinyl acetate.

For thin-film manufacturers, there are questions about the availability of some constituent elements. For example, for copper indium gallium selenide (CIGS), there is competition with the flat-screen industry for the relatively rare and expensive element indium, and so a large global effort is under way to remove it and replace it with zinc or tin. For cadmium telluride (CdTe), there are concerns over the quantity of available tellurium and the toxicity of the precursor materials.

And for the thin-film, amorphous silicon, the problem is relatively low efficiencies compared with CIGS and CdTe.

An important step lies in tailoring the bandgap of III-V semiconductors such as GaInNAs as well as in refining UMG-Si and metamorphic III-V materials, said Dr. Andreas Bett, director of the Materials Solar Cells and Technology Div. and deputy director of the Fraunhofer Institute for Solar Energy Systems in Germany.

“The challenges are very specific for each material issue,” Bett said. He suggests that they may be overcome by adaptation of the purities/impurties in the PV materials so that the efficiency does not suffer; and for the modules, durability and reliability must be enhanced.

Although silicon remains the market workhorse for most of today’s solar cells, thin-film technologies such as CIGS are beginning to make their mark. But there is also an entirely different class of material capturing the imagination of some developers: organic PVs.

Research institutions and companies alike are putting their money into organic-based PVs, and with the wide range of possible materials and material classes that can be fabricated, it’s easy to see why.

The research laboratory at the Organic Photovoltaics Group at Karlsruhe Institute of Technology (KIT) in Germany. Courtesy of KIT.

“Organic materials allow for an easy modification of the molecules, as there are virtually infinite possibilities of designing organic molecules,” said Dr. Alexander Colsmann, head of the Organic Photovoltaics Group at Karlsruhe Institute of Technology (KIT) in Germany. “Due to the manifold of materials, organic PV probably has the highest potential for future innovations.”

Thanks to the flexibility of organic solar cells, they can be fabricated onto plastic foils, making them suitable for integration into almost any arbitrarily shaped surface. And from there the possibilities seem endless: Solar modules can be integrated into building facades and windows, and they open up new OEM applications in the automotive and consumer sectors.

Flexible organic solar cells can be fabricated onto plastic foils and integrated into almost any surface. Courtesy of Karlsruhe Institute of Technology (KIT) in Germany.

Organic solar cells are fabricated using low-cost printing and coating, such as gravure and screen printing, slot-die coating or spray coating in continuous roll-to-roll processes. Plastic carriers provide for the mechanical flexibility of the modules. And organic solar cells are characterized by low consumption of environmentally compatible resources as well as generally unproblematic disposal.

But it’s not all good news. Power conversion efficiencies of organic solar cells remain too low to compete with established inorganic solar cells. But this is where Colsmann believes his work on so-called tandem architectures could make a difference.

In this design, two solar cells with complementary absorption characteristics are stacked directly on top of each other to achieve better sunlight harvesting and more efficient energy conversion.

Cross-sectional transmission electron micrograph (TEM) of a step-graded Ga1-xInxAs buffer layer grown on germanium. The indium content is increased in seven steps from 1 percent to 17 percent (1-7), followed by another layer with 20 percent (8), which helps to fully relax the buffer. The buffer was successfully implemented in a metamorphic III-V solar cell, which achieved efficiency of 41.1 percent under concentrated sunlight. (TEM measured at Christian Albrechts University in Kiel, Germany). This image first appeared in Practical Handbook of Photovoltaics: Fundamentals and Applications, 2nd Ed. A. McEvoy et al, eds. Academic Press, Boston, pp. 417-448 (2012).

In a four-year project, which has received funding of €4.25 million from the Federal Ministry of Education and Research, the aim is to enhance the efficiency of organic solar cells to more than 10 percent. The KIT team is focusing its efforts on photoactive polymers that can be deposited onto flexible plastic foils and will be testing the solar cells in a real-life environment.

“With respect to future device fabrication by printing, one of the main challenges is to develop a device architecture that can be fabricated fully from solution; i.e., the deposition of several (typically seven to nine) functional layers on top of each other without dissolving any of the prior applied layers,” Colsmann said.

Besides the usual PV concerns of efficiency, cost and stability, the transfer of lab processes to a real production environment is also an important task and must be achieved while preserving the efficiencies achieved in the lab.

The problem is that most scale-up processes come at a cost of lower efficiencies resulting from larger electrical pathways and a higher probability of shunts. In particular, the presence of wiring accounts for shading and, hence, a reduction in efficiency.

“In general, the efficiency and stability of organic molecules need further improvement,” Colsmann said. “Moreover, the production and material costs have to come down in order to reduce the euro-per-watt peak energy costs. All this can be accomplished with new material classes.”

There seems to be no common material when it comes to organic PVs; every supplier develops its own organic materials, and these depend on the intended use; e.g., for facade integration, the color of the solar cell plays an important role. For window integration (window shadowing), the materials must appear color-neutral.

Highly efficient organic PV solar cells are produced in the R&D lab. Heliatek recently set a world record cell efficiency of 10.7 percent. Courtesy of photographer Tom Baerwald, Berlin.

Although photoactive polymers are under investigation at KIT, at a nearby solar cell company in Dresden, Germany, small molecules are being tested instead.

Heliatek GmbH and its partners, which include the global chemical company BASF of Ludwigshafen and the University of Ulm, both in Germany, are using a new and proprietary class of oligomers. The oligomer, a molecule that consists of a few monomer units, comprises an acceptor-donor-acceptor structure with an extended donor-type block in the middle where charge carriers are well delocalized.

The photoactive layers of Heliatek’s organic photovoltaic cells are formed by a nanocrystalline blend of such oligomers and fullerene C60, where the task of the oligomer is to absorb the sunlight and transport the positive charges (holes); the fullerene transports the negative charges (electrons).

“They combine ultrastrong and broad absorption with good self-organization properties, good charge transport and the possibility to get high photovoltages by energetic fine-tuning,” said Martin Pfeiffer, chief technology officer at Heliatek. “Here, the synthesis is less complex, the materials can be purified by vacuum gradient sublimation, and the processing does not require solubility. Furthermore, vacuum deposition is an established and well-controllable process for mass manufacturing, and the vacuum deposited layers are very closely packed and morphologically very stable.”

In the past six years, Heliatek has managed to increase the efficiency of its oligomer-based PV cells from below 5 percent to more than 10 percent as part of the European Union-funded project X10D.

The aim at completion is to achieve 12 percent efficiency on cell level (1 cm2) and 9 percent on module level (above 100 cm2). The targets concerning extrapolated lifetime are 20 years for modules on glass and 10 years for modules on flexible polymer foil.

“Heliatek’s world-record cell with 10.7 percent efficiency is based on an absorber that mainly harvests the spectral range between green and red,” Pfeiffer said. “Accordingly, there is an obvious potential for further improvement by designing two modified absorbers, one with an absorption spectrum shifted to shorter wavelengths to harvest more blue light, and the other one with the absorption spectrum shifted to longer wavelengths to make use of near-infrared light.”

As with Colsmann’s polymer-based PVs, a tandem concept can once again be applied here in which both absorbers can be stacked on top of one another to create a series interconnection where voltages add up.

In another project, the European competitiveness of existing PV technologies is being targeted using advanced thin-film technologies. Scalenano is a €10 million initiative comprising 14 partners, including the Electrodeposition Group at Luxembourg’s Laboratory of Photovoltaics, which is headed by Dr. Phillip Dale.

A 2-µm-thick Cu2ZnSnSe4 p-type semiconductor absorber layer is produced from a reaction of metal stacks with vapor phase selenium within 1 min at 500 °C inside a rapid thermal processing unit. Courtesy of the University of Luxembourg.

The goal is to replace manufacturing processes that use large amounts of energy with lower-energy-cost ones. For example, metals, which often are vacuum sputtered requiring high vacuum, could be replaced with electrodeposition, which is done under ambient conditions.

“The University of Luxembourg is developing the p-type semiconductor Cu2ZnSn(S,Se)4 for use in thin-film solar cells by an electrodeposition and annealing route,” Dale said. “Cu2ZnSn(S,Se)4 is seen as the successor material to Cu(In,Ga)(S,Se)2. Concretely, our role in the project is to electrodeposit thin layers of Cu, Sn and Zn onto a molybdenum substrate to form a precursor. The precursor is then subsequently annealed in a chalcogen (sulfur or selenium) and tin chalcogenide environment.”

By the end of the project, the Scalenano team hopes to achieve 10 percent efficient devices on a 0.5 cm2 substrate and to bring down the cost of efficient solar cells, which Dale hopes will eventually allow solar technology to reach the masses.

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