Photovoltaic (PV) cells are expected to be a large part of the solution to wean developed countries from their dependence on fossil fuels. Once used primarily for power generation in space and other remote locations, they are now increasingly used to meet commercial, industrial, and household energy needs. While PV technology presently supplies a relatively small fraction of total energy production, it is a rapidly growing source of renewable and sustainable energy, as shown in Table 1.

Today, two fundamental technologies are utilized to produce most photovoltaic cells — crystalline silicon, which accounts for roughly 90 percent of all solar cells produced, and thin films, which include amorphous silicon (aSi or a-Si), cadmium telluride (CdTe), and copper indium gallium (di)selenide (CIGS). The commercial PV industry is rapidly evolving, however, and it is difficult to predict which technology may ultimately prevail. There are several new technologies in development, any of which may ultimately displace today’s conventional technologies. Perhaps one of the most promising is organic PV (OPV).

Enlarge this picture Table 1

There are three “major classes” of PV materials: (1) inorganic semiconductors; (2) organic semiconductors; and (3) hybrid solar cells, which are a combination of organic and inorganic systems. The inorganic PV cells are comprised of the previously mentioned crystalline silicon wafers and thin film (a-Si, CdTe and CIGS) materials, while the OPV devices encompass the two families of small molecule and polymer semiconductors. Since the organic semiconducting materials can be formulated as inks, a major advantage of OPV cells is that they can be manufactured via printing technologies.

Polymer PV cells have a structure similar to polymer organic light emitting displays (OLEDs), and use similar materials. The active polymer layer is sandwiched between two conducting electrodes. One of the electrodes is transparent to let the light in (for PV operations) or out (for display applications), depending on the required function. In the case of a PV cell, light absorbed in the polymer layers creates a pair of negative (electrons) and positive (holes) electric charges. These charges are collected by the electrodes, forming an electric current, which can be used to drive an electronic device.

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An example of an OPV cell construction is depicted in Figure 1. The hole transport layer (HTL) and the photoactive layer (PL) perform the same functions as the inorganic semiconducting materials do to transform sunlight into electrical energy.

The transparent substrate, which can be fabricated from glass or transparent polymeric materials, and the transparent anode permit sunlight to impact upon the HTL and PL semiconducting structures. The transparent anode — for example, indium-tin-oxide (as is used in conventional PV cells) — also lets sunlight through and collects holes generated in the photoactive layer. The cathode, which can be a metal (e.g. aluminum), is used to collect electrons generated in the photoactive layer.

There are three basic types of OPV cells: (1) standard organic cells, typically made from semiconducting small molecules or polymers (e.g. pentacene); (2) cells that harness nanostructures to achieve higher efficiencies than standard cells (as illustrated in Figure 2); and (3) dye-sensitized solar cells (e.g. Gratzel cells), which use dyes to enable absorption of a broader set of light wavelengths (much like photosynthesis in plants).

Cell efficiency

Enlarge this picture Table 2

OPV technology had a slow start because the first materials showed efficiencies below 0.1 percent. Organic compounds have a relatively narrow absorption spectra, which has been (to date) a significant limiting factor for OPV performance.

A more efficient cell can produce more power from a given area of active material and the efficiency of the cell is tied to the material from which it is manufactured. Today, silicon cells typically deliver the greatest efficiency, as shown in Table 2. Inorganic thin films still lag behind, the exception being multijunction cells. These cells use multiple junctions to generate electricity from different wavelengths of light. (This technology could also be used with organic materials to boost efficiency.)

Cost and flexibility

Efficiency, however, is not the whole story. More important is cost per watt, which improves with increasing efficiency and declining manufacturing costs (materials and processes). This is where OPVs offer a significant advantage. Despite the fact that the efficiency of OPV cells still lags behind silicon devices, their cost, flexibility, and weight make them attractive and worth pursuing. For example:

Enlarge this picture Fig 2 The AIST Organic Photovoltaic Cell (Source: Objective Analysis, 2008)

• They can be manufactured using conventional screen printing processes or even inkjet printers. These processes, used for high-volume printing applications, have been highly refined and are relatively low in cost.
• Organic materials can be printed onto flexible substrates. This allows the use of very inexpensive substrates, simplifies handling, and enables reel-to-reel processing. It also allows a flexible solar cell to be integrated into a device’s packaging or case.
• Organics can be used to make a lightweight power source for portable products. Mobile phones, laptop computers, and the information appliances of the future may all have solar cells to supplement their batteries.

One source1 estimates that, with a power conversion efficiency of only 10 percent, the cost per watt for OPVs could beat both thin film and crystalline silicon. Since organic PV technology is still very new, there is reason to hope that efficiencies will continue to increase and costs to drop.

Materials

One of the attractive aspects of OPVs is that they do not compete with the semiconductor market for materials. To date, the vast majority of photovoltaic devices employ some sort of silicon-based technology, putting the market on a collision course with the semiconductor industry since both markets demand large quantities of silicon. However, market forces can change this situation, as recent increases in silicon production and the depressed business climate have resulted in a significant drop in silicon wafer prices for PV applications.

Organic materials are abundant and provide a broad range of materials for potential OPV solutions. There is always the possibility that some new material will be developed that will significantly improve performance and lower processing costs. In the meantime, there are a number of options being pursued to increase OPV efficiency. These include nanomaterials to improve light collection, more transparent thin films, improved light-trapping schemes, and multijunction topologies.

The use of nanostructured material cells has led to more efficient charge separation and efficiencies are currently in the 3 to 5 percent range. Work in this area is still primarily a research-based focus for universities and institutes, and for some companies, such as Cambridge Display Technology (CDT).

A wide variety of materials and structures is being studied. For example, Konarka and Sustainable Technologies International (STI) are working on Gratzel cells, which use a dye-sensitized nanostructured titanium oxide. In Japan, the National Institute of Advanced Industrial Science and Technology (AIST), Mitsubishi Corp., and Tokki Corp. have jointly developed an organic thin-film solar cell, shown in Figure 2, based on a plastic substrate. The active elements consist of three layers — phthalocyanine, fullerenes (a nanomaterial), and lithium fluoride — between two electrodes.

In addition to efficiency and manufacturing process optimization, there are other issues to address in order to develop a sustainable OPV industry. A key issue is the establishment of viable supply chains to supply the raw materials used to manufacture OPVs with the purity required by such systems.

Presently, OPV devices have shorter operational lifetimes than do conventional PV devices. The majority of near-term PV applications lies outside of consumer applications, with the greatest opportunities being the generation of electricity for domestic and industrial purposes (grid electricity), and these applications have a required lifetime of 20 to 30 years. Organic materials degrade more rapidly over time than inorganic materials. The high levels of exposure to sun that solar cells must endure not only cause degradation but — in the case of dye-sensitized cells — fading. There are also concerns about the effects of sunlight and heat on lightweight flexible substrates, especially plastics.

Applications

Military and remote services are expected to be initial key drivers for the development and deployment of OPV devices. Lightweight, roll-up power sources would be ideal for people requiring electricity in remote locations, such as field researchers, mountaineers, and military personnel. For example, tents made from large-area, flexible solar panels could be used by aid agencies to power vaccine refrigerators or other vital medical equipment.

The technology’s promise of low cost also makes organic PV cells well-suited as power sources in a range of toys, novelties, greeting cards, and small portable products such as electronic calculators. This market is already established and can use a product that has a relatively short lifetime, especially if the cells are printable, inexpensive, lightweight, and flexible. A key high-volume application is as a supplemental power source for point-of-sale displays, both as packaging and at a shelf level. On the other hand, the economics of such a system rely heavily on the efficiency of the OPV cells.

Conclusion

Organic PV has many technical hurdles to overcome. Improving cell efficiency and lifetime are the keys to expanding the range of potential products that can use the technology. Most leading developers are concentrating on these issues. Applications are currently aligned with opportunities where the use of a flexible cell offsets its lack of efficiency. The first wave of applications is for consumer-type products such as battery chargers and integration of PV cells into textiles and clothing. In time, technology improvements will open up new applications.

References

1) Organic Photovoltaic Solar Cells: Recent Advancements in Efficiency. Christopher J. Musto. Literature Seminar, November 15, 2007.
www.chemistry.illinois.edu/research/materials/seminar_abstracts/2007-2008/Musto.pdf