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The challenge of the Sun

A new generation of flexible cells tries to overcome limitations and increase the use of photovoltaic energy worldwide

In March of this year, the United Arab Emirates inaugurated a large solar power generation plant with a capacity of 100 megawatts in the Madinat Zayed desert, enough to supply 20,000 homes. The project, located in one of the sunniest, hottest regions in the world, is the largest of its kind to employ this source of energy, which is considered renewable, abundant and non-polluting. The production of solar or photovoltaic energy is growing worldwide at a rapid pace, nearly 50% per year, but it still represents a very small percentage of global energy production, about 1%. In Brazil, it is only 0.01% of the total. According to the International Energy Agency, photovoltaic energy generated by all power plants in the world totaled 67 gigawatts (GW) in 2011, the equivalent of five Itaipu hydroelectric dams. One major obstacle to the wider use of this energy source is the high cost of solar panels and the other equipment that make up the system.

To overcome this problem, universities, research institutes and companies in several countries, including Brazil, are working to develop a new line of solar cells that cost less to produce than the silicon wafers currently used in conventional modules. The solution is what is known as third generation solar cells—silicon wafers were the first generation, and inorganic thin films were the second—and they  mainly consist of two types: organic photovoltaic (OPV) or dye-sensitized solar cell (DSSC). OPV cells are called this because they use carbon-based semiconductor materials to convert the energy in light into electricity. DSSCs, however, are based on an oxidation-reduction chemical reaction. They are also called hybrid, since they are made of both inorganic and organic materials. They are built between two layers of glass and contain a liquid electrolyte, usually a solution containing an iodine salt. Dye-activated cells absorb solar radiation, resulting in separation of charges (positive and negative), which produces energy. Neither the hybrid nor the organic cells are sold on a large scale yet. It is estimated that it will take at least three more years for this to happen.

Several new solar cell technologies have been studied in recent years, with the goal of finding a more efficient alternative to cells based on crystalline silicon. “In general, the third generation cells, which also include those made with quantum dots [miniscule semiconductor crystals], multi-junction and hot carriers [with a highly energetic charge], make better use of the photons that reach them,” says researcher Fernando Ely, of the Organic Electronics Group at the Renato Archer Center for Information Technology (CTI), in the city of Campinas. The Renato Archer CTI has produced advanced research on the development of third generation solar cells. Its researchers have already developed several flexible cell prototypes with an area of 60 mm x 40 mm using quantum dots and are currently working to improve them. “In addition to addressing the main limitations that hinder the sale of these devices, our group seeks to generate intellectual property in order to transfer this knowledge to the productive sector at a later date. Our activities also include studies to increase the conversion efficiency of the new solar cells by using functional additives, developing new manufacturing techniques for continuous processing—a technique called roll-to-roll—and producing transparent electrodes based on carbon nanotubes.” Activities at the Renato Archer CTI are funded by FAPESP, the National Council for Scientific and Technological Development (CNPq) and the Brazilian Innovation Agency (Finep).

The companies Dye-Sol, in Australia, and G24 Innovations, in Great Britain, are leaders in the development of dye-sensitized cells (DSSC). On the other hand, OPV research is headed by Heliatek and the Fraunhofer Institute for Applied Polymer Research (IAP), both in Germany. In Brazil, two companies are working on the development of third-generation cells. FlexSolar, headquartered in Joinville, Santa Catarina,  signed an agreement with the IAP in 2012 to develop flexible organic solar cells. The project, worth €4.8 million—about R$12.5 million—stipulates that initial production will be principally in Europe, but that after two years, the devices will also be manufactured in Joinville. According to a statement posted on the Fraunhofer Institute website, the idea for the project arose during the visit of FlexSolar president Bernard Schmidt to an international trade fair for organic electronics in Munich in June of last year. Four months later, the parties signed an agreement. The company is a subsidiary of Cromotransfer, also in Joinville, which has been developing printing technologies for the textile and packaging industries for 15 years. FlexSolar was created to transfer this know-how to the area of photovoltaics, since the manufacturing of organic solar cells uses printing methods similar to those used in the printing industry.

The other Brazilian company, called Tezca Solar Cells, is located in the Campinas High Technology Park (Ciatec). Established in late 2008, the start-up has already developed several DSSC prototypes in the laboratory, called TezcaFlex, and hopes to build a pilot factory this year. “Right now, we are performing durability tests on our cells. We aim to bring new investors into the business to start manufacturing on a commercial scale by 2016,” says Agnaldo Gonçalves, one of Tezca’s founding partners.  His objective is to build low-power, flexible solar modules with the thickness of a sheet of paper for use in mobile electronic equipment such as mobile phone batteries. In developing the technology, Tezca has been supported by FAPESP, which finances an Innovative Research in Small Businesses Program (Pipe) project, and CNPq.

Advantages and challenges
Low power consumption manufacturing and low manufacturing costs are the main advantages offered by third generation cells. A term widely used in power generation is the energy or financial payback—which is how long it takes for the investment to pay for itself, or how much time is needed to produce the same amount of energy that was expended in manufacturing. “In the case of crystalline silicon photovoltaic panels, the energy payback is around four years, while in OPV systems, payback should be less than one year,” says Ely. Another distinguishing feature of these new cells is that they can be used in the manufacture of large flexible panels, made of plastic or fabric, using simple printing methods from the graphics industry, enabling the production of lightweight solar modules of various sizes. Moreover, organic and dye-sensitized cells have high photoconversion rates using artificial light, which enables their use in indoor environments such as offices, factories and homes.

Because they are lightweight, flexible, and semi-transparent, the range of applications of OPV cells and DSSCs is broader than that of previous generations. They can be used to recharge the batteries of low-power consumer electronics such as mobile phones, cameras and tablets. They can also be integrated into building façades, windows or skylights—an application known as building integrated photovoltaics (BIPV)—or even in special clothing, jackets and backpacks, allowing the user to collect energy while moving. “The U.S. Army is planning to use these panels in  soldiers’ uniforms and tents to provide power for electronic equipment or provide lighting,” says Ely. Another idea is to use OPVs in street fixtures, such as bus stops, as a source of energy for advertising displays and signage.

For all this to become a reality, however, two major challenges must be overcome: their low efficiency and the reduced lifetime of the new devices. The rate of conversion of light energy into electrical energy—the ratio of the number of photons reaching the cell to the amount of electrical energy produced—is still very low in third generation cells. The maximum, uncertified efficiency index obtained so far is 12.1% for OPV cells and for 11.4% for DSSCs. In crystalline silicon cells, the maximum efficiency is 24.7%, twice as high. These values refer to small cells with an area of approximately 1 cm2—in panels with a large area, conversion efficiency drops sharply. The low performance of organic cells is explained by the failure to absorb infrared light, which has a wavelength greater than 900 nanometers, and energy losses due to recombination of electrical charges. “The best way to tackle the problem is to develop new organic semiconductors or composite systems with nanomaterials,” says Ely.

The short life of these cells, however, is the result of oxygen or moisture inside them. When light strikes them, especially ultraviolet (UV) light, the presence of moisture and oxygen gives rise to undesired elements that react with the organic semiconductors, changing their chemical structure and functionality. The solution in this case is to produce OPV cells in an inert atmosphere and then wrap them with impermeable films. For DSSCs, the problems are related to reliability, durability and the manufacturing engineering process. One way to overcome these problems is to replace the liquid electrolyte to prevent leaks, and replace some expensive materials used in manufacturing, such as the platinum catalyst and ruthenium, one of the chemical elements in the dye.

For OPV panels to become commercially viable, Ely thinks a conversion efficiency of 10% and a useful life of 10 years will be necessary. With these numbers, the cost per watt would be around $0.10. There is no reliable data of the cost of photovoltaic energy in Brazil, but in Germany, one of the most advanced countries in the use of this type of energy, the value per watt, assuming a crystalline silicon panel with an efficiency of 12% to 14%, is $1.50. “Brazil has great solar energy potential. Therefore it is important that we master this technology. Since third-generation solar cells are not yet being marketed, I see this as a great opportunity for Brazil to strengthen intellectual property rights, manufacture and market these devices,” says Fernando Ely.

1. Organic semiconductor architectures for electronic devices (nº 2006/57399-9); Modality Young Investigator Grant; Coord. Fernando Ely/Renato Archer CTI; Investment R$ 299,265.87 (FAPESP).
2. INCT Namitec – National Institute of Science and Technology for Micro and Nano-Electronic Systems (FAPESP Grant nº 2008/57862-6 and CNPq Grant nº 573738/2008-4). Modality Thematic Project (FAPESP) and National Institutes of Science and Technology (CNPq); Coord. Jacobus W. Swart / Renato Archer CTI; Investment R$4,251,055.34 (FAPESP) and R$5,693,114.45 (CNPq).