Developing clean alternatives to nuclear and fossil energy is essential for the growth of sustainable economies. One of the most attractive alternatives is photovoltaic (PV) technology, using the almost limitless power of the sun to generate electricity.
Among the range of technologies already available for the direct conversion of solar light to electricity, organic photovoltaics offer a range of benefits such as compatibility with flexible substrates and low weight. The fabrication process is highly versatile, economical and compatible with mass production through printing processes.
Presently, the OPV market is still in the early stages of development, with initial commercial products emerging for specialized markets. The short lifetime and low efficiency of current OPV technology obstructs its competitiveness when compared to inorganic solar cells, but these barriers are likely to be overcome soon.
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OPV technology could greatly expand the range of applications for solar power, thanks to its ease of manufacturing, flexibility and transparency.
What is Organic Photovoltaics (OPV)?
Organic photovoltaic, or OPV, cells convert electromagnetic energy into electricity using the photovoltaic effect. The active materials in OPV are layered organic chemicals, often formulated as printable inks - unlike conventional PV, which uses solid state semiconductors like silicon.
OPV cells consist of five main layers:
- The substrate, such as PET foil
- The anode, which must be a transparent conductor like indium tin oxide (ITO)
- The interlayer, normally made from conducting polymer PDOT:PSS, which helps to smooth the ITO surface and increases its work function
- The active layer includes p and n type semiconductors, and is where photons get converted to electrical current
OPV cells convert electromagnetic energy in the following way:
Firstly, the photons are transmitted through the outer transparent layers until they hit the active layer. Low-energy photons may pass right through the active layer, but if the energy of the photon is equal to or more than the bandgap of the semiconducting active material, the photon is absorbed. The energy from the photon makes the electrons and holes in the active layer decouple slightly to form excitons.
These excitons then diffuse to the interface between the p-type and n-type semiconductors, where the holes and electrons completely separate from each other, resulting in a voltage. This voltage is used to generate a direct current (DC), to power electrical devices or charge a battery.
Advantages of OPV
Organic solar cells have many obvious benefits over silicon-based solar cells - the materials they are made from are often more economical, and they can be made on flexible substrates and made translucent, creating new opportunities in industrial and product design.
OPV is becoming a popular prospect for building-integrated photovoltaics (BIPV), as it can be built into walls, facades, or even windows, much more easily than conventional silicon or thin-film solar cells.
The fabrication process for OPV is versatile and low cost. The use of primarily liquid organic semiconductors make the technology compatible with existing large-scale printing processes.
The lack of bulky solid layers also allows OPV cells to be made incredibly thin - this simplifies transportation and enables applications unavailable to silicon solar cells, as well as offering materials savings.
The rise of OPV could be a key source of growth for the chemical industry as well. Large chemicals manufacturers such as Merck, Dow and BASF have realized that the large-scale manufacture of OPV modules could create a significant market for specialized semiconducting organic chemicals.
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In January 2013, Heliatek's OPV cells were certified at 12% efficiency - a new world record, and one step closer to creating a true competitor to silicon-based photovoltaics. Image Credit: Heliatek.
Disadvantages of OPV
The efficiency and lifetime achievable by OPV cells has been much lower than inorganic solar cells to date, although OPV technology is improving rapidly.
The relative inefficiency of OPV is due to a fundamental property of the semiconducting material. In conventional photovoltaic cells, the active layer is made up of doped regions of crystalline silicon, which has a very regular, ordered structure. This allows the electrons and holes to move through the material very quickly, which makes it easy for the cell to separate them and generate a voltage.
In OPV, however, the active material is a random mixture of two kinds of organic molecule, in an unordered arrangement - especially when the compounds are fomulated as liquid inks for printing.
This random structure makes it much more difficult for the electrons and holes to move efficiently, which in turn makes it likely that the excitons will be absorbed and lost as heat energy before they can be separated enough to be useful.
OPV cells also have a shorter operating lifetime, as the organic chemicals are much more likely to decay over time than the crystalline silicon in conventional solar cells.
These challenges are not insurmountable - careful design of the OPV cells and active compounds is required to minimize the effects as far as possible, and many companies and researchers are working towards this goal.
Recent Developments
In 2012, researchers from the Karlsruhe Institute of Technology (KIT) commenced a four-year research program with an objective to enhance the organic cell efficiency more than 10%.
Dr Alexander Colsmann, Head of Organic Photovoltaics Group at the Light Technology Institute, KIT, used tandem architectures. In order to attain better sunlight harvesting and more efficient energy conversion, two solar cells having complementary absorption features are stacked atop each other.
According to the KIT scientists, they used innovative materials, developed novel device architectures and tested the solar cells in a real life environment. Dr Colsmann expressed that his vision is to be able to print solar cells similar to printing newspapers some day.
In January 2013, Heliatek, based in Dresden, Germany, achieved a record efficiency for organic solar cells of 12% in a 1.1 cm2 tandem cell. Fraunhofer ISE CalLab certified the efficiency of the cell efficiency, which was manufactured with a low temperature deposition process.
According to Dr. Martin Pfeiffer, CTO and co-founder of Heliatek, the low temperature technique they used had already been explored in the OLED market, but is also highly applicable to the similar semiconducting organic chemicals used in OPV.
Heliatek synthesizes and develops these molecules in-house. The organic solar cells are now approaching the performance of silicon solar cells in terms of efficiency.
Conclusion
Organic photovoltaic devices use organic molecules or polymers for the conversion of light to electricity. They are growing considerably in popularity due to the product design opportunities offered by their flexibility and transparency, along with their potential for low costs.
Researchers are constantly working on improving the lifetime and efficiency of photovoltaic devices and also low-cost, high-volume production techniques. It is believed that with the interest of leading players in OPVs, this market will grow considerably in the coming years.
Sources and Further Reading