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| Fig. 1: Compiled values of the highest confirmed photo-conversion efficiencies for research solar cells to date, for a wide variety of photovoltaic technologies currently investigated. [4] (Courtesy of the U.S. Department of Energy) |
All living beings need energy to survive-- energy to keep our streets and homes lit at night; energy to produce food to survive; energy to keep us warm in the dark winters. To meet our energy needs, humans have largely depended on fossil fuel resources such as coal, oil, and natural gas. In recent years, the earth's population has grown exponentially, due to advances in healthcare and sanitation. [1] However, the fossil fuel resources needed to support this growing population are limited and dwindling. To make matters worse, we can no longer ignore climate change phenomena across the globe that is the result of years of unregulated use of fossil fuels for energy. In order to create a sustainable future, we must turn to and develop alternative energy resources.
The alternative energy resources we have today include solar, wind, nuclear, and hydroelectric energy. However, massive and expensive undertakings are needed to build the wind farms, nuclear plants or dams necessary to harness wind, nuclear and hydroelectric energy respectively. On the other hand, installing just a few solar panels on top of your home is all it takes to reduce one's energy dependence on fossil fuels in a non-negligible way. These solar panels then absorb sunlight to produce usable electricity which can in turn be used to power our homes. Although solar farms that capture the sun's energy on a much larger scale do exist, there is clearly a much smaller barrier to entry for solar energy. As a result, amongst the other alternative sources of energy, solar energy has the added benefit of giving individuals the freedom to reduce their fossil fuel dependence, regardless of the prevailing government policies regarding energy.
In fact, solar cells allow for the harvesting of the sun's energy, which is arguably "free". Scientists have already calculated that the earth provides more than enough energy than we need to power our world with commercially available solar panels. [2] In the United States of America, government policies such as the "Sunshot Initiative" in the Department of Energy help to make solar energy more accessible, and cheaper than ever. [3] These factors all come together to make solar energy a potentially cost-effective and economically viable alternative to fossil fuels.
Commercially available solar cells today are overwhelmingly silicon-based, made from the same materials found in the chips and processors that power your cellphones and computers. Silicon-based solar cells have demonstrated high single cell efficiencies of up to 25% but are limited by their rigidity, and require the use of expensive fabrication facilities. [4]
Organic photovoltaics (OPVs) are a relatively new class of solar cell with great potential. OPVs are so named, since the active material (ie. the part of the solar cell that interacts with the light to form free electrons and holes that can then be separated to form an electric current), is made of large conductive molecules predominantly made up of Carbon, Hydrogen and Oxygen. This means that the main ingredients that make up OPVs are abundantly found on this earth, and have the potential to be cheaply obtained using well-established methods of chemical synthesis. At the same time, the active material in OPVs can be solution-processed and fabricated using well-established processes such as roll-to-roll printing on flexible substrates. Roll-to-roll printing is a highly efficient, high-throughput process that enables cost- effective scaling up of the production process.
When OPVs were first discovered, they were fabricated by layering a donor material on top of an acceptor material analogous to the p-n junction in silicon devices. However, these first OPVs performed dismally due to short recombination lifetimes. In other words, when OPVs were exposed to sunlight, free carriers were formed, but unable to escape the device. To overcome this problem, an alternative bulk-heterojunction junction (BHJ) structure was independently proposed by Hiramoto, Yoshino, and Heeger, which allowed for significant improvements in device efficiency. [5] The BHJ active layer is created by co-mixing the acceptor and donor material in the same solvent, before depositing the material to form a thin film. This mixing in solution creates additional interfaces between the donor-acceptor material, allowing for more efficient separation of electrons and holes at the interface, such that more current could be extracted from the device. Next, to complete the device, different metals with appropriate workfunctions to accept either the newly freed electron or hole were placed on each side of the active layer. To reduce the probability that a free electron or hole would recombine before it reached the metal electrodes, the active layer is often thin, on the order of a 100 nm. Due to the high absorption coefficient of these active materials, despite being thin, they are able to absorb significant amounts of light. [6] This design requirement had the added benefit of making OPVs thinner, lighter, and more flexible than its silicon counterparts. Beyond the goal of maximizing efficiency for energy consumption, these features could become increasingly important as our electronic devices become smaller and more portable.
While OPVs clearly demonstrate several benefits over traditional silicon- based solar cells, they still have a long way to go in terms of efficiency and reliability before they can truly take over the solar market. For example, most of the active materials used in OPVs are not air stable, causing their electrical properties to degrade over time. With the use of good encapsulation however, such as those used in commercially available organic light-emitting diode televisions, this problem can be alleviated in the foreseeable future. On the other hand, as can be seen from Fig. 1, state of the art OPVs only have an efficiency of 12%, nearly half that of silicon-based solar cells. Other emerging types of solar cells, such as perovskite cells, have demonstrated significant efficiencies, and garnered much interest within the scientific community as well. In order to continue to stay competitive and relevant amongst the different solar cells available today, greater understanding of the device physics and mechanisms that govern the efficiency of these solar cells is needed.
© Annabel Chew. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] D. Carrington, "World Population to Hit 11Bn in 2100 - With 70% Chance of Continuous Rise," The Guardian, 18 Sep 14.
[2] N. Rolston, "Reliability of Organic Photovoltaics," PH240, Stanford University, Fall 2014.
[3] A. Mileva et al., "SunShot Solar Power Reduces Costs and Uncertainty in Future Low-Carbon Electricity Systems," Environ. Sci. Technol. 47, 16 (2013).
[4] M. A. Green et al., "Solar Cell Efficiency Tables (Version 46)," Prog. Photovoltaics 23, 7 (2015).
[5] C. Sekine et al., "Recent Progress of High Performance Polymer OLED and OPV Materials For Organic Printed Electronics," Sci. Technol. Adv. Mater. 15, 034203 (2014).
[6] M. B. Askari, "Introduction to Organic Solar Cells," Sustain. Energy 2, 85 (2014).