Plasmonic solar cells in nanotechnology by ricksitterly

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									Running Head: PLASMONIC SOLAR CELLS IN NANOTECHNOLOGY            1




                    Plasmonic Solar Cells in Nanotechnology
                   Nanoreport 3: Nanotechnology in Solar Power
                               Richard A. Sitterley




                                Excelsior College
                         Introduction to Nanotechnology
Running Head: PLASMONIC SOLAR CELLS IN NANOTECHNOLOGY                                                  2




                                                Abstract


This report discusses the role of nanotechnology in solar power generation. Developments in thin-

film solar cells are analytically described, with specific details on the functionality, advantages, and

challenges associated with recent developments in solar cells. Also discussed are specific

enhancements to thin-film solar cells, with special focus the role of metallic nanoparticles in

plasmonic solar cells. Two different types of plasmonic solar cells are covered in this report, surface

layer plasmonic cells and sandwiched layer plasmonic cells, as well as the ideal type of metallic

nanoparticles for use in these devices. Interactions between nanoparticles and light are described,

in reference to the specific role of metallic nanoparticles as applied to plasmonic solar cells.

Problematic issues such as material cost, weight, and degradation are covered.
Running Head: PLASMONIC SOLAR CELLS IN NANOTECHNOLOGY                                                3




                                             Introduction




Nanotechnology has had a significant impact on new developments in solar technology. By

converting sunlight into electricity, solar power is beneficial to our environment, and may potentially

bring reliable electricity to remote locations throughout the world. However, solar power has faced

considerable challenges as an emerging technology, and has not replaced fossil fuels as our primary

source of energy. Two major challenges faced by the solar power industry are the high cost and low

efficiency of solar cells. Developmental efforts are currently searching for new ways to overcome

these obstacles in order to




                                        Thin-film Solar Cells




Nanotechnology has enabled the production of thin-film solar cells, which are considerably lighter

and cheaper to manufacture. This report will later focus on the plasmonic effect of metallic
Running Head: PLASMONIC SOLAR CELLS IN NANOTECHNOLOGY                                                   4

nanoparticles upon a thin-film solar cell. However, when discussing plasmonic solar cells, a basic

understanding of thin-film solar cells is useful. A thin-film solar cell is made by depositing a thin

layer of photovoltaic semiconductor material upon a substrate, usually made of inexpensive metal or

glass. Through the photovoltaic effect, light introduces photons to the photovoltaic material, which

dislodge electrons from its molecular structure, creating a flow of current within a solar cell

(Poortmans & Arkhipov, 2006). This photon induced current is used as an electrical power source,

and is the basic mechanism that allows solar panels to supply electricity.




It is important to point out that not all of the light that contacts a solar panel is converted to

electricity, as some of the light is reflected back into space. Also, a certain amount of light

absorbed by a solar panel is converted to thermal energy, and does not contribute to the electrical

energy generated by a solar cell. Currently, modern thin-film solar cells are considerably less

efficient at converting light into electricity, with even the highest quality cells operating at about 14

percent efficiency, as recently reported by the solar technology company MiaSole (Rhodes, 2010).

This means that about 86 percent of the incident light escapes the solar cell and is bounced back

into the environment. These high performance solar cells are made from copper, indium, gallium,

and selenium (Rhodes, 2010). They are commonly referred to as CIGS. Other companies also

produce CIGS solar cells, with varying levels of efficiency. Subtle differences in the production

process and level of precision can account for differences in CIGS efficiency ratings. The standard

efficiency of thin-film solar cells throughout the industry is about six to eleven percent (Heckeroth,

2010). Traditional solar panels use a much thicker layer of high-grade silicon semiconductor,
Running Head: PLASMONIC SOLAR CELLS IN NANOTECHNOLOGY                                                   5

housed under sheets of glass and framed in aluminum. These traditional panels currently have a

peak efficiency around 18 percent (Heckeroth, 2010). The problem with these traditional solar

panels is their weight and, more importantly, the greater necessary quantity of high-grade silicon

per square inch, which drives up production cost considerably. Regardless of production

techniques, thin film solar cells are inherently limited in their efficiency due to the material

properties and the extremely thin semiconductor itself which, although lighter and cheaper, limits its

ability to absorb light (Poortmans & Arkhipov, 2006).




CIGS thin-film solar cells currently have an advantage over those made from amorphous silicon, as

CIGS cells are much cheaper to produce (Rhodes, 2010). Regardless of the type of thin-film

semiconductor material used, thin-film solar cells have a tremendous cost advantage over traditional

solar panels because they are lighter, easier to transport and install, and require a great deal less

high-grade silicon material to construct (Poortmans & Arkhipov, 2006).




Considering the advantages of thin film solar cells, such as reduced production cost and weight, the

biggest challenge facing this technology is its efficiency. Currently, fossil fuels such as coal provide

the most energy per dollar spent. This is partially why society has not completely switched to green

technology, such as solar and wind energy, as a primary power source. Despite being better for the

environment, it is difficult for consumers to turn to alternative energy sources when these

alternatives will end up costing more money. Developers need to shoot for a “win-win” situation,

where the consumer can save money while also reducing their dependence on fossil fuel. This is
Running Head: PLASMONIC SOLAR CELLS IN NANOTECHNOLOGY                                                   6

why researchers are looking into new ways to boost the efficiency of thin-film solar cells. If solar

technology can reach higher efficiency at a lower cost, it has a strong chance of becoming our

primary energy source in the future. Solar energy will almost certainly play a bigger role in our

future energy needs, but it is difficult to determine exactly how much.




                                         Plasmonic Solar Cells




Plasmonic solar cells use metallic nanoparticles to increase the efficiency of thin-film solar cells.

This is a new technique made possible by advances in nanotechnology. How these new solar cells

work is by introducing nanoparticles to a solar cell to increase light absorption. The purpose of

these nanoparticles is “trap” light in the solar cell by reducing the amount of sunlight that is

reflected from the solar cell and back into the air (Catchpole & Polman, 2008). Light that is

reflected from a solar cell cannot be used to generate electricity, thus reducing reflected light is

critical to increasing a solar cell's efficiency. These metallic nanoparticles do this by scattering the

escaping light, changing its path in a way that promotes directing it back to the solar cell

(Catchpole & Polman, 2008). There are two different types of plasmonic solar cells that will be

covered in this paper, both of which operate by this basic principle. In both cases, it is important

to understand the plasmonic effect in its relevance to solar cells.




                                         The Plasmonic Effect
Running Head: PLASMONIC SOLAR CELLS IN NANOTECHNOLOGY                                                    7

When light hits the surface of a metal, waves may be induced along the surface of the metal as its

surface electrons are excited. This electrical disturbance on the surface of a material is known as a

surface plasmon (Catchpole & Polman, 2008). If a metal particle is small enough, as with a metallic

nanoparticle, the particle itself will vibrate from this light induced wave activity. The resonant

vibration of the particle serves to scatter the incident light that contacts these particles from

various angles. This vibration is an important to the function of a plasmonic solar cell, as it can

redirect light rays traveling in various directions, toward the solar cell (Catchpole & Polman, 2008).




                        Surface layer Nanoparticles for Plasmonic Solar Cells




One method of constructing a plasmonic solar cell involves depositing metallic nanoparticles directly

on the surface of a solar cell (Catchpole & Polman, 2008). This is advantageous because it requires

minimal modification to existing production methods used to produce the solar cell itself (Catchpole

& Polman). Surface applied nanoparticles can be used to increase light absorption in virtually any

type of solar cell, which means this is a useful way to improve the performance of CIGS and

amorphous silicon cells. Steps are added to existing processes to effectively adhere the

nanoparticles to the surface of the cell, at a desired dispersion ratio. The dispersion of the particles

is important because too many particles per square inch will eventually thicken the topical layer of

nanoscopic particles to a point that they are blocking an unacceptable amount of light from

contacting the photovoltaic layer of the solar cell. There is an ideal level of dispersion, represented

by a curve, at which there there is a peak level of light scattering with minimal light obstruction to
Running Head: PLASMONIC SOLAR CELLS IN NANOTECHNOLOGY                                                   8

the panel. This dispersion may vary depending on the exact particle size and type, but for the

purpose of this report it will suffice to say that there is a peak dispersion without going deep into

the calculation method of finding this dispersion. It is unavoidable that these nanoparticles will

obstruct and reflect light away from the cell, to a certain extent, but their small size and plasmonic

vibration minimize their obstruction to a negligible level, where they effectively direct more light to

the solar cell than away from it. The challenge here is simply finding the ideal number of particles

per square inch to maximize the efficiency of the cell. This is important because it costs additional

additional time and money to apply the nanoparticles to the surface. This application of surface

layer nanoparticles currently enhances the efficiency of thin-film solar cells by 20 percent

(Catchpole & Polman, 2008).




                                   Sandwiched Layer Nanoparticles




Another method of using nanoparticles to enhance solar power efficiency is to sandwich them in

between two transparent light absorbing subcells in a tandem polymer solar cell (Marcus, 2011).

Researchers at the University of California, Los Angeles (UCLA) are taking this approach to

improving solar cells. This particular project used gold nanoparticles to produce the desired

plasmonic effects in the solar cells. The interconnecting layer of gold nanoparticles serves to better

utilize the reflective properties of the nanoparticles, as it takes advantage of the light scattering

properties of the particles as they scatter light both toward and away from the interconnecting

layer. By comparison, the surface layer nanoparticle approach to plasmonic cells scatters light into
Running Head: PLASMONIC SOLAR CELLS IN NANOTECHNOLOGY                                                    9

the solar cell to some extent, but surface nanoparticles also deflect some light away from the solar

cell. Sandwiched layer naoparticles can effectively scatter light above and below the

interconnecting layer, which enhances light aborption to both subcells. Although this type of

plasmonic cell is a more efficient in thoery, this application is still in development.




The interconnecting layer of gold nanoparticles currently only enhances the device's performance by

about twenty percent, which is the same reported improvement that surface layer nanoparticles

provide to a solar cell (Marcus, 2011). As this method is more complex and early in development, it

is expected to improve over time and hopefully surpass other other types of plasmonic cells. This

approach to plasmonic solar cells is made possible by highly transparent solar cells that UCLA

researchers have developed (Marcus). The function of these transparent cells is beyond the scope

of this report, but they are expected to provide many new capabilities in the solar industry.




                                 Ideal Types of Metallic Nanoparticles




In both types of plasmonic solar cells, the main focus is to use nanoparticles that will reflect light

while minmizing the amount of light that is absorbed by the nanoparticle itself. So far, silver

nanoparticles are the ideal type of metallic nanoparticle, as they offer desirable light scattering

properties at a lower price than gold nanoparticles. In plasmonic applications, the nanoparticle must

also have a desirable electron configuration in its molecules in order to vibrate appropriately from

light induced waves. This is why metallic nanoparticles are used for plasmonic solar cells. Metallic
Running Head: PLASMONIC SOLAR CELLS IN NANOTECHNOLOGY
10
nanoparticles with this capability include copper, tin, gold, and silver.




                                         Identified Challenges




An obvious challenge that plasmonic cells strive to overcome has to do with poor light absorption by

thin layers of semiconductive material used in thin-film solar cells. Thin layers of material generally

absorb light less effectively than thick layers of the same material. This makes sense, as we can

demonstrate this property using common materials around the home. Light passes through a single

sheet of paper very easily compared to a thick stack of papers. Solar cells are no exception to this

principle. It follows that the advantage of reduced weight and production cost is accompanied by

reduced light absorption efficiency. Thus the advantage of a thin-film solar cell is also directly

related to its weakness. We could thicken our semiconductive layer to increase light absorption,

but this would require more costly semiconductor and also increase the weight of each cell.

Plasmonic cells offer a unique solution in that the size and weight of the nanoparticles is negligible,

and the cost, depending on the type of metallic particle used, is relatively low.




                                      Internet Research Results




Through internet research, I have discovered some new processes that are in development to

overcome issues with oxidation of the metallic particles deposited on the surface of solar cells.

Researchers are looking for innovative methods of depositing metallic nanoparticles in a way that
Running Head: PLASMONIC SOLAR CELLS IN NANOTECHNOLOGY
11
prevents their oxidation. This is especially relevant to silver and copper particles, which oxidize

easily. The Department of Chemistry at Northwestern University developed some useful solutions

to this problem. To protect silver nanoparticles on plasmonic solar cells, one effective technique is

to deposit a layer of transparent conductive oxide over the surface of the solar cell, after dispersing

the silver nanoparticles on its surface. (Standridge

& Schatz, 2009). One way of doing so is to use atomic layer deposition to deposit 7.7nm of

titanium dioxide over the cell, which creates a pin-hole free barrier that is a suitable barrier to

prevent oxidation of the silver particles (Standridge & Schatz). Another option, which provides a

thinner barrier layer, is to first deposit an 0.2nm adhesion layer of aluminum trioxide prior to

depositing the titanium dioxide barrier. The inclusion of the adhesion layer allows fully protective

barrier of titanium dioxide to be achieved at a 5.8nm deposited thickness (Standridge & Schatz).

This thinner barrier can be achieved through fewer ALD cycles of TiO2, which cuts down on

production time. It should be noted that protective barriers thicker than 10nm showed undesirable

“red shifting” of incident light and an overall reduced light collecting capacity of the cell, which is

why both methods had a total barrier thickness of less than 10nm.




                                               Conclusion




In conclusion, it is evident that researchers are working hard to bring solar technology to consumers

by improving the efficiency and affordability of solar devices. As nanotechnology continues to

develop and make its way into the industry, we can expect to see more applications for solar power.
Running Head: PLASMONIC SOLAR CELLS IN NANOTECHNOLOGY
12
Additionally, as nanotechnology provides advancements that permit devices to operate on less

power, solar power may play a bigger role in powering portable electronics. One thing that is clear

from the information gathered in this report is that solar technology is improving at steady pace,

and may eventually help reduce our dependence on fossil fuels as an electrical power source.




                                            Bibliography


Poortmans, J. & Arkhipov, V. (2006). Thin Film Solar Cells. Hoboken, NJ: Wiley


Catchpole, K. & Polman, A. (2008). Plasmonic Solar Cells. Optics Express, Vol. 16, Issue 6,
December 22, 2008. Retrieved from
http://www.sciencedaily.com/releases/2008/12/081223172705.htm




Marcus, J. (2011). Tiny Gold Particles Boost Organic Solar Cell Efficiency: Plasmonic Technique
Helps Enhance Power Conversion by Up to 20 Percent. ScienceDaily, August 16, 2011. Retrieved
from
http://www.sciencedaily.com/releases/2011/08/110816162311.htm
Running Head: PLASMONIC SOLAR CELLS IN NANOTECHNOLOGY
13


Heckeroth, S. (2010). The Promise of Thin-Film Solar. Mother Earth News, March, 2010.
Retrieved from
http://www.motherearthnews.com/Renewable-Energy/Thin-Film-Solar-Utility-Scale-PV-
Power.aspx




Rhodes, C. (2010). 14% Efficiency for Thin-Film Solar Cells, but Where Will the Indium Come
From? Forbes, July 7, 2010. Retrieved from
http://www.forbes.com/sites/energysource/2010/07/07/14-efficiency-for-thin-film-solar-cells-
but-where-will-the-indium-come-from/

Standridge
, S. & Schatz
, G. (2009). Toward Plasmonic Solar Cells: Protection of Silver
 Nanoparticles via Atomic Layer Deposition of TiO
. Langmuir, Volume 25. Retrieved from
 https://docs.google.com/viewer?a=v&q=cache:bD6yB2sQsOQJ:chemgroups.northwestern.edu/hup
p/Publications/260.pdf+silver+plasmonic+solar+cells&hl=en&gl=us&pid=bl&srcid=ADGEESjrNWt4FF
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        qned13fLHpyJq2uIlXwemWKD63Dg&sig=AHIEtbQAzFtC_B9iKL8I2egsDAjGkKEWqw

								
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