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Organic solar cells - innovative materials for next generation PV


Every hour and a half the Sun irradiates the Earth with a quantity of energy equal to that consumed by the world’s entire population in a year. Eni believes in the development of this energy source, with the aim of increasing production from renewables.
The Research Centre for Non-Conventional Energy - Eni Donegani Institute is committed to the development of innovative technologies to build next-generation PV devices, based on organic semiconductors (smart materials), which have significant advantages in terms of cost, markedly lower than traditional silicon-based PV applications, for integration in buildings and for portable electronics and other applications requiring electricity but far from power grids.

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Solar energy is an inexhaustible and widely available source. The Sun sends to the Earth an amount of energy 6000 times greater than the entire energy consumption of the planet. The effective capture of just a fraction of this energy could be enough to meet the global energy demand for both now and the next future.
Traditional PV technologies are mainly based on silicon, a semiconductor¹ that generates an electric current when illuminated, thanks to the photovoltaic effect². For more specific applications, other inorganic semiconductors may be used, such as the highly toxic cadmium telluride, or gallium arsEnide, which is mostly used in aerospace applications due to prohibitive costs.  
The research for next generation PV aims at replacing silicon with organic semiconductors, which are special polymers (therefore carbon based). The new solar cells basically consist of two electrodes and a layer of organic semiconductor in between, absorbing the incident solar light and generating electrons; these are then collected by the electrodes, producing an electric current. The new technologies can produce lightweight and flexible devices for consumer electronics, that can be easily fitted in buildings and vehicles, also exploiting curved or complex surfaces.
The manufacturing processes of organic cells do not require extreme environmental conditions and are already used for conventional printing (for example ink-jet printing and gravure); consequently production costs, including energy consumption, are significantly lower than those of traditional silicon cells. The final result is a better environmental compatibility.

Currently most of the commercial photovoltaic (PV) devices are made of a single material, silicon, that, despite being one of the most common elements on the earth's surface, requires lengthy and costly manufacturing to be used in PV.
This technology, even if consolidated, has also some drawbacks:

  • high energy consumption to obtain a silicon purity sufficient for cell production;
  • the electricity produced is still relatively expansive when compared to that from fossil fuels;
  • silicon modules are difficult to fit inside buildings, transport and portable electronic devices, because of the material rigidity and fragility.

 

Replacing silicon with organic semiconductors, Eni aims to overcome these problems and develop next generation PV technologies which, having lower production costs, lower energy consumption and consequently shortest payback periods, will produce flexible and lightweight devices, that can be easily fitted also on curved surfaces and in portable electronic devices.


The Research Centre for Non-Conventional Energy - Eni Donegani Institute in Novara produces innovative materials for PV generation, the so-called smart materials, consisting of a highly coloured polymer chain (called “donor‘) and a fullerene derivative (called “acceptor‘), an organic compound discovered in recent decades during studies of interstellar chemistry. The donor effectively absorbs the light and exploits the energy transfering its electrons to the acceptor, while the latter conveys the electron up to the electrode (cathode). At the same time the donor carries the positive charge to the counter-electrode (anode).
The development of an efficient material for manufacturing polymer solar cells is a complex and lengthy process. The design of a new smart material, its synthesis and its following characterisation may involve several months of work. The synthesis takes place through complex chemical reactions, during which the “building blocks‘, relatively simple molecules, are combined to obtain the desired product. For the property characterisation, different chemical-physical analysis techniques (e.g. UV-visible spectroscopy, fluorescence, cyclic voltammetry, EPR, Raman, NMR, GPC and Plasma) are required. Of the many systems produced, only the most promising are tested on real devices in order to verify the characteristics and photovoltaic properties, steps that may require additional months of work.
Thanks to molecular modelling, it is possible to reduce experimental work and focus research on the most attractive materials. With quantum-mechanical calculation techniques it is possible to predict a priori the photovoltaic characteristics of a smart material from its structure, without materially proceeding with its preparation and characterisation.


Since 2007, Eni has been working on the entire production chain of the next generation PV technologies based on organic semiconductors.
The Research Centre for Non Conventional Energy - Eni Donegani Institute in Novara was the first R&D centre in Europe, and is still one of the few, to be equipped, with the entire development chain on the same site: molecular modelling of quantum photoactive molecules, synthesis and characterization of polymers and fullerene derivatives, formulation of inks and their deposition, response of the PV modules to the various light frequencies, assessment of thelectrical efficiency, construction of solar cell prototypes, their morphological characterisation with SEM, TEM and AFM microscopy.
The final devices are produced using deposition techniques similar to traditional printing, for example ink jet printing or gravure printing. Here is a brief description of the process: the smart materials are dissolved in a solvent to form a photoactive ink, which is deposited on continuous rotating cylinders in a machinery similar to a classical printer. The cylinders transfer the ink to a flexible and transparent plastic substrate (such as polyethylene teraphthalate - PET), that has previously been covered with a conductive layer of indium and tin mixed oxide, acting as a transparent electrode. Between one deposition and another, the film is dried.


Organic solar cells Eni has already identified and patented a number of promising benzotriazole-based materials (Patent N° WO2010046114), which gave good results in the laboratory. Construction of a pilot plant for the continuous production of PV modules is expected to begin in 2013.

The main advantages of this technology are:

  • the possibility of exploiting solar energy, available across the entire planet and completely free;
  • using areas and surfaces not accessible to traditional PV devices, thanks to architectural and structural integration;
  • technological integration in devices that cannot use traditional solar cells, such as portable electronics, transportation and applications requiring energy but far from power grids;
  • manifacturing processes that are widely available, relatively inexpensive and not requiring extreme production environments (e.g. high vacuum or high temperatures).

 

Eni research in this field has already received important awards:

  • Eni Award 2009 - recognition for innovation in Eni: for the identification and synthesis of novel conjugated polymers usable for the construction of photovoltaic cells of new generation
  • National Award for Innovation 2009 (Prize of Prizes): the prestigious award, established by the President of the Italian Republic to enhance and support the best in innovation and creativity in companies, universities, government departments, organisations or individual inventors and to foster a culture of innovation in Italy, presented by President Giorgio Napolitano to the group of researchers from the Centre for Research Eni Donegani that produced the patent for new polymers, which can create innovative photovoltaic cells which can be converted into electricity, also solar radiation, at higher wavelengths.

 

Among the numerous scientific articles written about the technology, the most popular are:

  • one of the 100 most downloaded articles from a scientific journal in 2012: Polymer- and carbon-based electrodes for polymer solar cells: toward low-cost, continuous fabrication over large area - Po' R., Carbonera C., Bernardi A., Tinti F., Camaioni N. – published in “Solar Energy Materials and Solar Cells" 100, 97-114 (2012)
  • Key scientific article 2012 on Reginnovations.com, quoted by Technology Review (the magazine of MIT), Applied Physics Letters and the specialised site DailyE : Comparison between theoretical and experimental electronic properties of some popular donor polymers for bulk-heterojunction solar cells - Longo L., Carbonera C., Pellegrino A., Perin N.,Schimperna G., Tacca A., Po R. – published in "Solar Energy Materials and Solar Cells" 97, 139-149 (2012)
  • one of the 100 most downloaded articles from a scientific journal in 2010: The role of buffer layers in polymer solar cells - Po R., Carbonera C., Bernardia A., Camaioni N. – published in "Energy & Environmental Science" 4, 285-310 (2011)
  • among the  1% of the most cited articles from a scientific journal: Polymer Solar Cells: Recent Approaches and Achievements - Po R. e al. – published in "Journal of Physical Chemistry C" 114, 695-706 (2010)




Last updated on 11/01/13