Plastic solar cells: energy of colours

Juan Bisquert

Juan Bisquert (1962, M. SC. Physics University of Ph. D. Physics Universitat de València, València 1985, 1992) is a Professor of applied physics at the University Jaume I of Castelló, where he directs the Group of photovoltaic devices and optoelectronic composed of 12 physicists and chemists. It has published over 190 articles in international journals and book chapters and has directed more than 20 research projects, including a Consolider project entitled 'Optoelectronic devices and photovoltaic hybrid renewable energy' (HOPE). It is 'referee' in more than 40 scientific journals, belongs to the editorial board of 'energy and environmental science', and acts as the evaluator of projects in over 10 countries in Europe, Asia and United States.

The main topics of current activity are the improvement of efficiency and stability of Grätzel cells, the application of quantum dots as sensitizers, the characterization of fully organic cells (mass heterojunction) and the study of organic injection and transport of cargo in LED (OLED). In the field of technology has created the company spin-off: Xop physics, S.L., which sells a humidity sensor for irrigation.

20Th century has been characterized by a huge increase in energy consumption per person, particularly in developed countries. The paradigm of the extension of the energy to the entire population was the motorized vehicle: cars, trucks, planes, facilitated the mobility of people and goods. The positive-ignition engine takes advantage of the remarkable concentration of energy available in the hydrocarbons which lead to the petrol or gas-oil (the concentration of energy both in volume and weight is essential, since the vehicle must carry their own fuel). But we are now aware that to use petroleum products, what we do is to eat in an irreversible manner reserves accumulated by the action of photosynthesis millions of years ago. Thus we released gas that had been set in the basement by the primordial plant, mainly CO₂. These gases interfere with the normal traffic of solar radiation and give rise to progressive warming of the surface of the Earth, with a probability very high result in a catastrophic global climate change. In immediate terms, the engine is noisy and exhaust gas polluting the atmosphere of the cities. So driving a car now begins to have a visible counterparts for a citizenship increasingly aware of these factors.

The present system of energy is not sustainable and will require important changes that should pay attention to the reality of the market forces, as the higher reality of the forces of nature. It is clear that scientific activity, technological and economic gives rise to profound changes in our environment, but it is no less true that once transformed the environment, we have forced to live in it. Cumulative greenhouse gases in the atmosphere over the past decades are already producing visible alterations in climate, and the risk of a global catastrophe is so serious that your solution may not differ to 'future generations', but that we will have to deal with us, who are here now. It is time to change attitudes and recourse to innovation. We will be more aware and accountable with regard to the impact of our activities on the environment we inhabit, and there will be a transformation of the remarkable system of energy that will profoundly affect our way of life. We will not only demand that the current cars travelling on increasingly more kilometres per litre of fuel, by increasing the efficiency of engines. Most likely we will see the extension of silent cars, powered by batteries powered by renewable energy, keep clean the air we breathe and will produce small quantities of CO₂. This change will be one of the significant vectors of general modification of the main sources of energy, which would lead to a multitude of new products, needs and customs.

The Sun is the largest source of energy that exists on Earth, and supplies a power of about 120,000 terawatt (TW) in the form of electromagnetic radiation consists of visible light and infrared. This power is about 10,000 times higher than the current consumption of all mankind. The growing urgency to find stable and carbon-free energy sources, has given rise to an extraordinary development of photovoltaic solar energy. Solar irradiance is produced with the same daytime cycle that governs all of our activities, so solar electricity is well adapted to the energy needs of the population. It is available everywhere and contributes to the autonomy of the energy supply, and finally, solar energy can be produced in distributed form, avoiding the losses of distribution in large transmission lines. In the future, probably not-too-distant incorporate solar cells that will produce energy benign and pollution-free, to our homes, in the ceiling on the facades and even on Windows, vehicles, and in the clothes, as in Figure 1.

Despite the notable advantages mentioned above, the use of solar electricity by photovoltaic converters is still limited, and the main reason is that the cost of the devices that convert the light of the sun into electricity, called solar cellsIt is quite high, so solar electricity, at a cost of about 4 euros per watt, cannot compete with other sources of generation (which often are not taken into account the costs associated with the emission of greenhouse gases)(, political instability, etc.). However, the growth of the global photovoltaic market is 40% in the last decade, and 100% in 2008. Solar energy has gone from being a curiosity to a few domestic kilowatts in the 1980s, to make an economy of large scale which is a serious source of supply already in order of gigawatts. When it will thin out the dust of the speculative and legislative battles which occurred recently in relation to large installations in Spain, we can surely see that the cost of solar cells and other components of the facilities is reduced progressively.

Traditional solar cells took advantage of the knowledge and the semiconductors employed in electronic technology, and a large part of the world market is supplied with Silicon cells. High temperatures of preparation, that expensive base materials are required to form crystals of Silicon. In addition, these solar cells, although they give high yields of conversion of light to electricity (about 15%) are very fragile and little manageable.

When these limitations to the expansion of solar photovoltaics were recognized in the 1980s, he began a remarkable effort of research and development to find cost effective and cheaper, alternative materials which are now bearing real fruit. These efforts resulted in several families of very promising semiconductor materials, referred to generically as 'of thin layer' ('thin film'), and which can be considered the second generation of solar cells. Thin-film materials are so called because they are better absorbers of the crystalline silicon light, and they work well with much smaller thickness of the active layer (a few micrometers, to hundreds of micrometers needing crystalline silicon to completely absorb solar radiation). In addition, it is not necessary to cut semiconductor layers from a Crystal, they can deposit on a substrate of large, glass, for example, significantly reducing the costs of preparation.

As we have said, after several decades of research and development, he has been to resolve a host of technical barriers to the production of effective solar cells of large area, and at this time there is a massive influx of inorganic thin film cells on the market. For example we are seeing a large amount of the production of amorphous silicon cells, because several companies such as Applied Materials (USA) provide turnkey machinery to produce cells very consistent and effective on a large scale, through a continuous process of deposition of Silicon from silane gas atoms. Although efficiency is relatively low (around 7%), the price of panel per watt is a fraction of the cells of silicon, and in addition, this type of factories, such as the already installed in Spain (t-solar and Gadir Solar) may produce large panels, 5 x 6 square feet, which are very attractive in numerous applications. These factors should add real production capacity of energy of each technology, which is not determined only by the performance of conversion. Effectively, energy per kW installed production depends on very sharply from the use of the light of low intensity, the ability to capture the lateral radiation, and the resistance of the cell to maintain his conversion (efficiency) factor when its temperature increases. Amorphous silicon cells behave better crystalline silicon in all these respects, and that is why, with a performance (equal area) which is about half, amorphous silicon can produce more energy after the year and it is remarkably attractive. If we add the reduction of manufacturing costs, we find a very attractive product for the conversion of energy even on a large scale.

But the most notable development in inorganic thin film technology is the tremendous growth of the production of CdTe (cadmium telluride) cells. In 2008 the company First Solar Inc. USA won the reduction of the costs of production to $1.1 per watt, with a production capacity of cells for a total of 1 gigawatt annual. These results have improved in 2009 with a spectacular reduction of costs to $0.85 per watt in 2009. With these advances, thin-film technologies, which have achieved a share of 12% of the market in 2008, may occupy 20% in 2010.

While large area inorganic cells are consolidated in the market, innovative third-generation technologies based on plastics and nanotechnologies break with force as promising elements for photovoltaic conversion. While inorganic cells (the crystalline silicon, both those of thin layer) are based on a unique semiconductor that performs the conversion of light to electricity, hybrid and organic solar cells adopt a different model of photovoltaic conversion in terms of the properties of materialsIt uses a mixture of these. This strategy can reduce demands for purity and used preparation of low-temperature chemical routes. There is the possibility of very low-cost devices when they occur on a large scale. In addition, this approach is very versatile in terms of the variation of materials and components. Therefore, these technologies have great potential advantages and innovative features that will facilitate application of integration of solar cells in multiple areas. An example is the complement of mobile charger that we showed you in Figure 1. In addition to the low cost of production, organic and hybrid solar cells are made easily in light and flexible substrate, and recognize any color variation.

The most developed version of hybrid cell is the cell of titanium sensitized nanoestructurado with dye, discovered by the Swiss scientist Michael Grätzel and colleagues in 1991, for short called DSC ('dye solar cell') or 'Grätzel cell'. These cells take as a basis a porous matrix of a metal oxide resistant and cheaper, as Guy₂ or ZnO, see Figure 2. Using about 10 nanometres in size oxide particles, gets a huge internal area for photovoltaic conversion. This function corresponds to an organic molecule, a dye that is the absorber of light. It is anchored in the inner surface of the matrix and when illuminated injected electrons to semiconductor. The internal circuit is completed with a conductive liquid that regenerates the oxidized dye.

The Grätzel cell advantages are numerous. Source materials are cheap and abundant, the preparation process is quite simple, and the device is very versatile to make configurations to suit different needs. For example, Figure 3 shows prototypes of DSC with the same structure but using different dyes, which make it possible to make the color of the cell of the desired shape. We should highlight the white cell, using a dye which only absorbs the infrared radiation of the solar spectrum, and therefore reflects all colors. Dye said also allows our vision transparent solar cells. On the other hand, since Titanium is compatible with a plastic substrate, flexible DSC can also be performed, and an example of plastic panels is presented in Figure 4. All these configurations are mechanically robust and can be manipulated with ease.

It is obvious that with this ease of varying colors, as well as substrates, the DSC can be adapted to a large number of new applications. An important feature of the DSC is its ease of integration with architectural elements, vehicles, etc. Dyesol, a company based in Australia which is dedicated to the supply of materials and machinery for the production of DSC, has promoted several projects in Europe which already seek to make DSC for integration in architecture. The project Chose, in Rome, developed cells dye translucent in substrate of glass for facades of buildings, while Chorus, in Cardiff, Great Britain, explore the DSC on substrate of steel that can be integrated into industrial roofs.

The presence of organic compounds in the hybrid cells, allows to include a myriad of materials with desirable properties, but on the other hand, these materials are less resistant to degradation than inorganic materials. For a long time, there were doubts, in the field of research, on the viability of the cells of Grätzel to withstand years of operation in high temperatures that occur when the cell is heated in the Sun. However, at the present time these reservations have been overcome. Now many laboratories, both scientific and industrial, are carrying out the process of scaling to large cells, as well as the Assembly of pilot lines. Cells are produced in large quantities, and you can check its consistency and durability. There has also been a remarkable effort for viscous liquids, gels, sealants and dyes to allow prolonged operation. Several laboratories are conducting tests similar to amorphous silicon cells, to guarantee the quality of their products. On the other hand, the efficiencies of conversion of the record (less than 1 square centimeter area) cells are increased. Professor Peng Wang, of the Institute of chemistry applied in Chengchun, China, is an expert in the development of dyes advanced that it has been reported in 2009 several times efficiencies higher than 11.1% to equivalent to a Sun lighting. And in the Materials Research Society, held in Boston in December 2009 Congress, Michael Grätzel reported the first cell of dye that exceeds the efficiency of converting light to electricity of 12%.

Everything suggests that the DSC technology is taking off the flight at this time to join the market as an option, perhaps at the small beginning compared with technologies already established, but with potential to become a very important option for photovoltaic conversion. The DSC have a remarkable ability to attract diffuse or low-intensity radiation, and are excellent for products of interior cells, such as the prototype of lamp produced by Sony Corp., Figure 5. As we have explained, by its transparency and color custom properties can integrated into Windows, facades, roofs of vehicles, etc. And they also constitute a remarkable choice for the conversion of energy on a large scale. It is necessary to increase efficiencies and stability, but this process can occur quickly, as it depends largely on capital investments to develop the technology with the incentive to get a good point of departure in a future market. In addition, the low cost of manufacturing and machinery to produce DSC, in comparison with the reference of amorphous silicon cells, reduces the investment required to install a production plant. Under the demands of capital can lead to a rapid expansion of the hybrid photovoltaic technologies, the barrier to the boot is much smaller. Certainly, the investment and industrialization projects are already a reality. For example 3GSolar is a company based on capital investment in Tel-Aviv DSC technology has developed over the past years. In cooperation with Professor Arie breach of the University of Bar - Ilan, have developed an innovative system to extend the active area of cells by internal protection of contacts. Thus 3GSolar has shown stable panels, and its immediate objective is to make cells of 15 x 15 cm with efficiency of 7.5%, which will allow an efficiency of 7% at the panel level, to achieve a production of 8 annual megawatts at a cost of manufacturing of $1.40 per watt.

Another technology that is currently experiencing a spectacular development are the cells based on mixtures of organic compounds. The activity in this area was also beginning around 1990, when they began to use richly conductive plastics. In 1991, Professor Richard Friend and his collaborators demonstrated at the University of Cambridge, United Kingdom, the LED (Electroluminescent device) organic, quickly gave rise to commercial applications, and is today normally used in mobile phone screens. Organic LEDs are also beginning to address large television market. On the basis of these efficient polymers emitters of light, Nobel Prize in chemistry, Professor Alan j. Heeger of the University of California at Santa Barbara, and numerous partners in Europe, made solar cells consisting of a conjugated polymer blendsabsorber of light, and another aceptador polymer of electrons, the latter usually based on the famous molecule in the form of football known as Fullerene.

Plastic cells have a number of features that make them a promising device with a myriad of applications. For example, printing technologies can be used to produce equal solar cells that become periodic. In addition, adapt naturally to portable devices for electronics, and identification or credit cards. To develop the field of organic electronics, functional cells appear as the ideal complement to provide the electricity they need new devices with autonomy.

However, there are some impediments to perform this kind of applications. In contrast to the DSC, functional cells lack a rigid matrix of semiconductor. The internal morphology of the cell, i.e. the mixture of organic materials, may experience changes during the operation to a Sun lighting. In addition, conjugated polymers are relatively reagents in the atmosphere, which degrades the properties of the cell. For these reasons, and also by the very low efficiencies that are managed at the beginning, for many years these cells were not considered to be a convincing candidate for lead to industrial products. DIN embargo, in the past three years, approximately, the situation has changed radically. In 2007 the Heeger group reported for the first time efficiencies above 5%, and since then there is a sustained increase, which currently reaches 7%. Important chemical industries have launched development projects to obtain more effective molecules in these cells. For this reason, the current development of the materials is wrapped in secrecy characteristic of competition to obtain the intellectual property of the best materials. Through the improvement of fotoactivos materials, and with robust barriers of capping, Konarka, a company with capital of capital investment based on California, and under the technical direction of Heeger, has shown small panels that maintain the parameters without degradation for a year in the open. Also in Europe are carried out important demonstration projects which tested the technical ede manufacturing to produce large amounts of organic cells. Figure 6 shows panels of the national laboratory for sustainable energy in Risø, on Denmark. In this project, led by Professor Frederik Krebs, there have been organic honeycombs of 1 x 1.7 metres, each consisting of 24 modules of 20 x 25 cm. Each panel produces approximately 11 Watts.

During the Decade of 1990-2000, the research activity in the tumor cells in Spain was very low. He was the Professor Pedro Salvador, researcher at the CSIC, who first performed work on DSC, in collaboration with researchers from Berlin. At the beginning of the 2000s, the Group of Juan Bisquert, Universitat Jaume I of Castelló, began to get outstanding results which have subsequently had a great impact in terms of the operating principles and methods for characterization of DSC.

Recently, the scientific activities in these areas of research have experienced a remarkable extension. As the international scientific community progressed in this type of study, many Spanish researchers were interested in the topic. In 2007 received a major boost to the investigation with a so-called Consolider Project HOPE (organic for renewable energy and hybrid devices), directed by Juan Bisquert. The project involves an allocation of EUR 4,000,000 to a team of about 100 researchers for 5 years. This project is to coordinate and promote research on organic and hybrid devices with a very applied approach, in line with the ongoing development of this field of research. Subsequently, the Government of Spain has provided another impetus to these issues to subsidize the project Fotomol, devoted to activities pre-industrial, and directed by Emilio Palomares, ICREA researcher at the Institut D'investigacions in chemistry of Catalonia, Tarragona. Therefore, groups Bisquert and Dovecotes made today DSC devices, and group of Hernán Míguez, researcher at the CSIC of the Institute of materials in Seville, investigates the coupling of crystals Photonic dye cells to improve their optical properties. We must also highlight the results of Professor Tomás Torres, of the Universidad Autónoma de Madrid, which has synthesized a set of colours based on ftalocianinas with a large capacity of absorption of photons in the red portion in infrared of the solar spectrum. On the other hand, Dr. Henk Bolink performs organic cells at the Molecular Science Institute of Valencia, and Roberto Pacios, researcher of the Mondragón Ikerlan Research Centre, has made small modules of organic cells for integration with other instruments.Thanks to these contributions, and the other groups involved in the project Consolider HOPE (http://www.consoliderhope.uji.es/), we can say that currently the Spanish researchers constitute a notable force in the field of organic and hybrid solar cells, and are able to offer new ideas and materials.

However, in the field of industrialization of hybrid and organic solar cells, the situation is less encouraging. On the one hand we have the competitive research began with many years of delay with regard to countries of the North of Europe and Asia. On the other hand, although at this time the basis of expertise and scientific knowledge is available in several universities and technological centres, the launch of industrialization projects requires risky investments. Here we have the disadvantage of the lack of culture of investment in scientific and technological innovations. However, in the recent past there have been many developments which demonstrate that seemingly 'safe' and quickly 'cost-effective' investment not always are so. It is possible to begin a change of mentality to make flow investments to high-risk bets, but with enormous potential benefits. As we have explained in this article, at the present time, there is the coalescence of scientific innovations on organic and hybrid solar cells made in recent years, competitive products that will come to the market in the coming years. It is therefore the right time to participate in the competition to develop more attractive products.

Finally, we must indicate the necessity that the policies of public support, which they encourage photovoltaic installations, have a greater degree of planning and serenity to avoid ups and downs that have occurred recently, and they can thus give rise to a stable and sustainable PV industry development. Above all, such incentives should support the innovative elements which give an advantage in an extremely competitive market. It is also very important to provide that consumers can incorporate the production of photovoltaic energy in their homes with small installations, eliminating the lentísimas administrative barriers that exist today.

Author's note

I am very grateful to Tsukasa Yoshida, Tsutomu Miyasaka, Reiko Ogura, Torben Damgaard Nielsen and Frederik Krebs, for providing the images to illustrate this article. Our work in hybrid and organic solar cells is mainly funded by MCIN Consolider HOPE CSD2007-00007 and MAT2007-62982 projects, and the Prometheus 09I179 project of the Generalitat Valenciana.