Throughout history, gold has been one of the more high profile elements. Known for its brilliant color, gold has long been associated with status and nobility. It has been used, most popularly, for currency and jewelry. However, while we typically think of gold in terms of its more common macroscopic uses, it is also incredibly useful in the nanoscale realm. Nanoparticle are small particles, generally between 1 and 100 nanometers in diameter, of a material. They have a surprisingly high variety of uses in drug delivery, cancer detection, renewable energy, and even beer. They come in a variety of shapes, from cubes to wires to plates. Both size and shape can be tuned in order to best suit the particular desired application. Various forms of gold nanoparticles in particular have shown very promising results in improving the efficiency of solar cells, which take in light from the sun and convert it into energy. This energy can then be used to power our daily lives. Research and developments into solar energy are critical in the fight against climate change.
One of the most important features of a solar cell is its efficiency, or the percentage of the energy from sunlight that is converted to electrical energy and stored for later use. One of the hurdles in solar research is that this is typically very low, particularly in organic (also known as plastic) solar cells. Despite this, organic solar cells are very promising in terms of renewable energy due to their affordability, flexibility, and stability. Current work is being done to improve their functionality, in order to advance solar energy as a viable method to power modern society. This is where the gold nanoparticles come into play. By embedding the particles in the polymer layer of the cell, we can take advantage of localized surface plasmon resonance (LSPR). What this means is that the particles scatter sunlight, allowing a greater percentage to be converted into electrical energy, increasing efficiency. Surprisingly, these effects are not present in the bulk material, which means that solar cell entirely plated in gold would not be very practical. Gold nanoparticles are popular due to their stability and broad LSPR effects.
While the effects of pure gold nanoparticles are fairly well studied, two recent papers have looked at combining gold with other materials in order to maximize their benefits. In 2013, Korean researchers Baek et. al. investigated the effects of not only pure gold nanoparticles on their organic solar cells, but of silver shell-gold core nanocubes. Silver nanoparticles, by themselves, are not quite stable enough for the desired application. Gold nanoparticles, while great, do absorb a lot of light compared to the amount that they scatter. The light absorbed by the nanoparticles is not converted into usable energy, so minimizing their absorption is desirable. They found that individually, gold by 12% and 5% (in two different polymers). However, when they replicated these three samples with the nanocubes, they found the efficiency increased by 17% and 12%, respectively. The PBT7 nanocube-containing sample had an average efficiency of 8.74%, with a peak result of 9.19%. Essentially, the silver coated nanocubes are an improvement over the pure nanoparticles because they allow the the broad LSPR effects of the gold nanoparticles to be observed, but the silver coating helps to mitigate some of the absorption seen with pure gold.
Recently, a cheap, common polymer has been successfully used in this type of solar cell! Kim et. al., also based in Korea, examined the effects of coating their gold nanoparticles with polystyrene, which is an inexpensive and readily available polymer. The polystyrene enhances long term stability of the particles and thus the cell. This is important in the process of translating solar cells from the laboratory to a consumer market, while still allowing the cell to take advantage of the benefits seen in using pure gold nanoparticles. The polymer shell thickness is also easily adjusted. These polystyrene coated particles increased efficiency by about 9% when added to organic solar cells, up to 8.27%. While this is lower than the solar cells containing silver plated nanocubes discussed above, direct comparisons cannot be made as different polymers have slightly different responses. In addition, it is still higher than the pure polymers, showing that the polystyrene coated gold does improve the cell.
To conclude, gold nanomaterials are an exciting area of research, particularly relating to renewable energy. There a quite a few improvements that have been made recently, which opens up many doors to further research and hopefully to making organic solar cells more viable option for the consumer market. As this recent research shows, gold combined with other materials can lead to very significant improvements in solar conversion efficiency, sometimes with other benefits, such as stability. Longer term, gold nanoparticles may help us to combat climate change and reduce fossil fuel usage. There is still much work to be done in this exciting area of research, but these recent developments are incredibly promising.
1. Baek, S. et. al. Au@Ag Core–Shell Nanocubes for Efficient Plasmonic Light Scattering Effect in Low Bandgap Organic Solar Cells. ACS Nano. 2014, 8 (4), 3302-3312
2. Kim, T. et al, ; Au@Polymer Core–Shell Nanoparticles for Simultaneously Enhancing Efficiency and Ambient Stability of Organic Optoelectronic Devices. ACS Applied Materials and Interfaces. 2014, 6 (19), 16956-16965