Annotated Bibliography
Benson, Scott W. "Solar Power for Outer Planets Study." 8 Nov. 2007. Microsoft Powerpoint
file.
This is a PowerPoint presentation given to the Outer Planets Assesment group in
2007 by Scott Benson of the Glenn Research Center at the National Aeronautics and
Space Administration. His slides go thru the specifications of the solar panels used in the
farthest missions. In 2007, these included the Dawn, Rosetta, and Juno satellites. Juno
travelled the farthest to study Jupiter. NASA would like to use solar panels when
studying planets farther than Jupiter who ever, the low intensity low temperature effect
hinders the efficiency of the solar panels past that threshold. While efficiency of the panel
increases as the temperature decreases, the low light intensity causes significant drops in
the overall efficiency. He presented a number of ways to counter act the LILT effect
including the use of quantum dots in the future. The presentation also touches on
different technologies to improve efficiency including, UltraFlex, SquareRigger, and
Stretched Lens Array SquareRigger. Solar panels may be fitted with a linear refractive
concentrator in an attempt to overcome the effect of the low light intensity in the deeper
reaches of space. He also includes that if solar panels are effectively included in the solar
missions, they would substantially decrease the payload needed for energy supply.
While just the slides of the presentation, there is a significant amount of
information included. There are many specification numbers and overviews of
technology that would be somewhat time consuming to find elsewhere. However, this
source would have been much more complete with a video of Benson’s presentation.
Boeing. "Solar Power Satellite." Boeing. Ed. Boeing. Boeing, n.d. Web. 19 Oct. 2013.
<http://www.boeing.com/boeing/history/boeing/solarsat.page>.
Boeing recounts its history of solar power satellite from the 1970s to 2008. Solar
power satellites were a novel concept in the years of the energy crisis designed to harness
energy from the sun and beam it back down to Earth collecting stations. This concept has
not yet been deployed due to lack of funding as well as the political dissent by many
countries because solar power satellites have the potential to be used as weapons of mass
destruction. However, the idea remains on the table and may serve as an alternative
energy source as the fossil fuel supply slowly runs out. Solar power satellites essential are
massive earth-orbiting aggregations of billions of silicon solar panels. The power
collecting from these solar panels are then converted to microwave energy that is then
sent back down to earth in through huge antennas. The DC power is then used to power
many home throughout the country. Scientists at Boeing argue that while the initial cost
of creating the satellites will be large, the impressive return on investment will make
solar power satellites worthwhile. Fossil fuels will eventually run out and are subject to
price fluctuations and politics. The sun is expected to shine for another 6 billion years
and is free!
This article provides some very interesting information about solar power
satellites. However, it does not explore the controversy of the satellites in enough detail.
This is expected considering Boeing won the grant from NASA to develop the satellite,
and was responsible in many of the key developments in the field. The information in the
article is best suited to readers who seek to gain a historical background of the
developments of the satellites and not a through understanding of science.
Bonacorso, F., et al. "Graphene Photonics and Optoelectronics." Nature (2010): 1-15. Web. 30
Aug. 2013. <http://arxiv.org/pdf/1006.4854.pdf>.
This is a review article from the Nature. Bonacorso and other prominent members
in nanotechnology review graphene as a potential material in photonics and
optoelectronics. They claim, that graphene allows electrons to travel through the lattice
behaving as dirac fermions. Dirac fermions travel with great speed and efficiency. The
reserachers claim that graphene acts as two-dimensional gas opposed to a solid substance.
Therefore, graphene is a prime candidate for use in fiberoptics and other disciplines
which require high speed transport. Graphene also has usage in optoelectronics because
of its uinque properties. It refects 0.1% of visible light. Its high absorbancy has potential
in solar cells because a variety of light sorouces can be converted into energy. Graphene
also has the higheset saturable effiencency of any known material. The researches also
looked at electron hole repair, an essential feature of conducting electricity. They found
that graphene had a high electron to phonon conversion but had this decreased as more
layers were added. The researchers also looked at graphene’s luminescence. Graphene,
after treated with oxygen plasma, was very luminescent. Organic compounds are needed
for cheaper electronics. Because graphene is organic and everywhere, it is very cheap.
Other researchers have been able to use photolumuse in luminscence graphene to look at
live cells. The remainder of the article explains how graphene can be produced for
optoelectronics.
This article was fairly helpful. Looked more at the physical properties that would
make graphene used in variety of disciplines opposed to to looking at the experimental
application of the material. It is availiable in full text from Arvix.
Brabec, Christoph J., et al. "Production Aspects of Organic Photovoltaics and Their Impact on
the Commercialization of Devices." MRS Bulletin 30 (2005): 50-52. Print.
This review article provides information and speculations on the efficiencies
organic solar cells must achieve before they are consider viable alternatives to traditional
silicon or other inorganic materials. The authors look at key facets in the manufacturing
and distributing of organic solar panels which must be addressed before the panels
become competitive on the market for power grids. In 2005, the authors speculated that
organic solar panels will be used for small electronic devices such as calculators and
watches because these devices have a three to five year life expectancy and need
relatively low power to operate. Life expectancy, cost per watt, and efficiency are all
major details that the organic solar cell industry must overcome if it enters in the solar
cell market. The authors suggest that organic solar cells must be less than one dollar per
watt to be used for a wide range of applications. Likewise, device efficiencies will have
to increase to around 10% and the life expectancy will have to increase to around 10
years. While these criteria are not impossible to achieve, it will take several years before
they can be achieved. New manufacturing methods are expected to bring down the cost
of manufacturing a solar cell as well as increases efficiency by improving the printing
quality of the polymers.
Even though this article provided concise information, it had a lack of citations
that may decrease its validity. Additionally, it was written in 2005 and therefore is
somewhat antiquated. Therefore, it is best to find the most recent version of the article
with updated benchmarks.
Chandler, David L. "Graphene Electrodes for Organic Solar Cells: Researchers Identify
Technique That Could Make a New Kind of Solar Photovoltaic Panel Practical." MIT
News. Ed. Massachusetts Institute of Technology. Massachusetts Institute of Technology,
6 Jan. 2011. Web. 19 Oct. 2013. <http://web.mit.edu/newsoffice/2011/graphene-solar0106.html>.
Chandler writes in this news article about experiments conducted with graphene
to serve as an electrode in solar cells by Massachusetts Institute of Technology Professors
Jing Kong and Vladimir Bulovic. They found that graphene has the potential to replace
indium tin oxide in solar cells as a superior electrode. However, due to graphene’s
hydrophobic properties until recently graphene was not able to function to the higher
power conversion efficiency levels of indium tin oxide. This is unfortunate because
graphene is much more flexible, abundant and inexpensive. The team tried many
workarounds to overcome the hydrophobicity, including trying to remove water from the
manufacturing process, but ultimately found that the best results are produced when the
graphene is doped. This decreasing the hydrophobicity and also improves the power
conversion efficiency. Professor Kong suggested that graphene solar panels have more
versatile uses. Its relative transparency can used for windows, or on uneven surfaces
without the use of expensive mounting structures.
This article was fairly interesting however, it did not provide the necessary details
to fully understand the doping process. Therefore, readers would be most benefited if
they read the complete study which can be accessed through a link on this article.
Regardless, even for a news article, substantial information was lacking. It provided only
the briefest explanation of the teams research but relatively excessive background
information.
- - -. "Solar Power Heads in a New Direction: Thinner Atom-thick Photovoltaic Sheets Could
Pack Hundreds of Times More Power per Weight than Conventional Solar Cells." MIT
News. Ed. MIT. Massachusetts Institute of Technology, 26 June 2013. Web. 9 Oct. 2013.
<http://web.mit.edu/newsoffice/2013/thinner-solar-panels-0626.html>.
This article focuses on the use of two dimensional materials for solar panels,
including molybdenum disulfide, molybdenum diselenide, and graphene. Through
computer computational simulations, the researchers discovered that graphene has the
highest pound per pound efficiency of any material. This far exceeds standard silicon
solar cells in efficiency and in thickness. The lightness and thinness of graphene will help
decrease the overall cost of solar energy because it is much easier to mount the lighter
materials on roofs and cheaper to produce. Current solar panels require relatively large
amounts of purified silicon which is then covered with glass to prevent any damage from
occurring to them. In developing countries, the cost and the actual support systems
required to mount the solar panels make silicon panels a non-alternative. Likewise, in
satellites and other aerospace applications, weight and efficiency are major factors
because of the high cost per pound it takes to launch something into space. Professor
Jeffery Grossman asserts that two dimensional materials have large promise in energy
production and their efficiency can be greatly increased by layering the material on top of
one another or on beneficial substrates.
This article provided background and summation information on the study
conducted by Professor Grossman. There were very little statistics included and in-depth
exploration of the topic. However, there is a link to access the original journal article
from Nano Letters ® which will provide much more satisfactory information.
Chen, Yi, et al. "Optimizing the Light Absorption of Graphene-Based Organic Solar Cells by
Tailoring the Weak Microcavity with Dielectric/Graphene/Dielectric Multilayer."
Applied Physics Letters (2013): 1-6. Applied Physics Letters. Web. 16 Sept. 2013.
<http://scitation.aip.org/getpdf/servlet/GetPDFServlet?filetype=pdf&id=APPLAB000103
000006063301000001&idtype=cvips&doi=10.1063/1.4817801&prog=normal&bypassS
SO=1>.
This article discusses the practicality of replacing indium oxide with graphene in
solar cells. While many of graphene beneficial and unique properties occur because it is
2D, its thinness also has negative effects. Because graphene has is only .34 nm thick
opposed to indium oxide's 100 nm, the light that hits graphene is blueshifted. This
decreases the overall effiency of the graphene solar cell. In this experiment, researchers
tailor the weak microcavity in graphene to best absorb photons, thus increasing
efficiency. They researchers ultimately wanted to see how many photons were absorbed
into the active layer by using either indium oxide or graphene. They found that even
though graphene had a much higher absorption rate, this did not lead to a much higher
number of photons in the active layer compared to indium oxide. The biggest reason for
this decrease is graphene's weak microcavity. Therefore, the researchers inserted in layers
of tungsten trioxide to increase the thickness of the graphene layer and improve photon
absorption into the active layer. This led to an increase in absorption by 12.1% to the
maximum value of 21.1%. They also found that the absorption was best between 410636nm. Overall the addition of the tungsten trioxide layer led to a redshift in the light.
The weak microcavity must be modified to allow the maximum number of photons to
enter the active layer or graphene solar cells will not outperform indium oxide.
While the information presented in the article is interesting and novel, it may be in
the very early stages of development or not a popular method of increasing efficiency.
The many researchers are focusing on plasmonic nanostructures, which can be made of
titanium dioxide, to increase photons converted into electrons.
Dume, Belle. "Graphene Could Make 'Perfect' Solar Cells." Physics World. Ed. Institute of
Physics. Institute of Physics Magazine, 11 Sept. 2011. Web. 5 Sept. 2013.
<http://physicsworld.com/cws/article/news/2011/sep/05/graphene-could-make-perfectsolar-cells>.
Dume reports on high efficiency graphene solar cells research conducted by a
team from University of Manchester and Cambridge University. The team was lead by
Andrea Grigorenko. The article begins with a brief overview of why graphene has
potential for solar cells, and optical communications. It then explains the benefits and
drawbacks of the team’s new cell. The researchers in this experiment were able to
increase the external quantum efficiency of the cells to 50%. External quantum efficiency
is how many photons hitting the surface are absorbed. Internal quantum efficiency is how
many of the photons that have been absorbed that create electron holes, which can
produced current. Graphene has high internal efficiency making it an ideal candidate for
photovoltaics, but its external efficiency is only 3%. The structures use an
electromagnetic field to pair the photons hitting the metal with electrons. By doing so,
more photons enter into the graphene portion of the cell and thereby increases the
external quantum efficiency. Graphene allows has a wide spectrum of light absorbancy
opposed to traditional cells. Scientists previously were working on creating a bandgap but
there is no need if the spectrum is so large. The researchers created titanium and gold
contacts to allow non-graphene devices to work with the graphene solar cell.
This article is concise, but informative. The review of the drawbacks and
advantages is especially helpful in determining the potential of this development. There is
also a link to the full journal article on the page
Echtermeyer, T. J., et al. "Strong Plasmonic Enhancement of Photovoltage in Graphene." Nature
(2011): 1-6. Arxiv. Web. 18 Sept. 2013. <http://arxiv.org/pdf/1107.4176.pdf>.
Researchers at the University of Cambridge and University of Manchester have
created plasmonic nanostructures that increased graphene efficiency in photovoltaics up
to 20 times its original value. Previous graphene photovoltaics suffered from three main
problems. The researchers report that graphene only stores = 2% of light, the electrons
that are produced from the structures are difficult to extract into current, and there is not
enough photocurrent for a complete flood illumination on both contacts of the device.
Plasmonic nanostructures enhance the electromagnetic field at the p-n junctions. In doing
so, the nanostructures are able to guide the stronger electromagnetic field energy into the
p-n junction increases energy effiency up to 20 times. Groups devices were created with
single layer graphene produced by micromechanical exfoliation and the contacts were
created with e-beam lithography. The design used for the experiment had 110nm gratings
and 300nm pitch. To determine the effectiveness of plasmonic nanostructures, the
researchers tested their device at a number of different wavelengths in the visible light
range. The highest photovoltage results when the laser is at the fingertips because this is
where the largest band bending occurs In between the fingers, there is an enhancement
but significantly smaller than at the fingertips. The largest enhancement is seen at 514
nm. The researchers also verified their results using theoretical methods. As predicted
514 nm wavelengths produced much higher voltage than at 633 nm.
This article is helpful but maybe slightly outdated. Published two years ago,
significant developments to plasmonic nanostructures have resulted since. Regardless,
this research contains the same basis for the later experiment. The full text of this article
can be obtained for arxiv.org.
Green, Martin A., et al. "Solar Efficiency Table (verision 39)." Progess in Photovoltaics:
Research and Applications 20.1 (2012): 12-20. Wiley Online Library. Web. 18 Oct. 2013.
<http://onlinelibrary.wiley.com/doi/10.1002/pip.2163/abstract;jsessionid=2C36B689B20
680C15C079798391E77DD.f02t03?deniedAccessCustomisedMessage=&userIsAuthenti
cated=false>.
The information presented by Green et al is a compilation of the records for solar
cell efficiencies. Solar efficiency tests were conducted at independent approved testing
sites after the cell passed met certain criteria. The researchers also include 10 notable
mentions in the article that did set the highest efficiency in their group but have potential
because the technology used in the cells is novel. The tables are broken down into the
type of solar cell (silicon, III-V, thin film chalcogenide, amorphous/nanocrystalline
silicon, photochemical, organic, and multijunction devices). Each group contain different
solar cells that fall under the overarching heading. Likewise, every solar cell in the table
has its reported efficiency, area, Voc , Jsc, FFd, test centre and date, and the
manufacturer. The highest efficiency single junction solar cell was produced by Alta
devices, and has an efficiency around 28.3%. This solar cell set the record in 2011 for the
highest efficiency solar cell to date. It is a III-V gallium arsenide thin film cell. The
highest efficiency multijunction solar cell is produced by AZUR and has an efficiency of
around 34.1%. It is a gallium indium phosphide/ gallium indium arsenide/germanium
monolithic solar cell. While it has exceptional efficiency, the majority of elements need
to fabricate it are very expensive and becoming increasingly rare.
The data presented in this article is slightly outdated. Solar efficiency has
increased to 40% since the article was published. Therefore, readers are most benefited if
they can access the most recent version.
Henderson, Sandra. "First Evidence of Structural Degradation in Organic Solar Cells." Solar
Novus Today. Ed. Penny Pretty. Novus Media Today Group, 10 Jan. 2014. Web. 10 Jan.
2014. <http://www.solarnovus.com/first-evidence-of-structural-degradation-in-organicsolar-cells_N7316.html>.
Henderson is a research editor for Solar Novus, an international news website for solar
power for professionals. This article reports on the work of researchers at the Technical
Univeristy of Munich who utilized a novel way to examine the degradation of organic
photovoltaic cells in real time. Previously, researchers were aware that organic
photovoltaic cells lost efficiency has their lives continued, but were unable to provide
concrete real-time evidence indicating so. Using Grazing Incidence Small Angle X-Ray
Scattering, the researchers were able to witness the structural degradation of the solar
cells over a period of seven hours. They found that as time progressed, the active layers
began to grow larger but also separated. This increase in spacing leads to an overall
decrease in the productivity of the solar cells because it requires more energy for the
electrons to traverse the active layer and be transferred into current. While the researchers
used a basic model consisting of cylinders, they were able to understand the principals of
solar cell aging.
If readers wish to gain more information on the research conducted at the
Technical University of Munich, then they should read the full article. However, if
readers seek an overview of the information this article is perfect. However to understand
the technical details and results of the experiment, they must read the full study.
Hoppe, Harald, and Niyazi Serdar Saricifcti. "Organic Solar Cells: An Overivew." Journal of
Materials Research (2004): 1-22. Cambridge Journals. Web. 10 Jan. 2014.
<http://journals.cambridge.org/action/displayFulltext?type=1&fid=8108974&jid=JMR&
volumeId=19&issueId=07&aid=8108972&bodyId=&membershipNumber=&societyETO
CSession=>.
This article is an extensive overview describing the manufacturing, structure, and other
physical properties of organic solar cells. The authors discuss the various materials that
will be used in organic photovoltaic cells in the future as well as in current production.
They also describe the incorporation of these materials in to the different layer of the
solar cells extensively. Each layer of the solar cells is commented upon in regard to
function as well as incorporating carbon-based materials. Additionally, the authors
provide information on the different types of manufacturing techniques used today,
including evaporation and wet processing. They also provide analysis on the impacts on
morphology of each method, as the authors feel that poor morphology is a key factor in
decreased power conversion efficiency. There is also a section on the fundamental
physics that provides an explanation for the upper limits of the solar cell efficiency as
well as as the steps necessary to improve efficiency in the future.
While much of the information presented in this article is relevant to the
discussion of organic photovoltaics today, much of it is outdated. Readers should find the
most recent power conversion efficiency numbers to gain an accurate understanding of
organic photovoltaic cells potential. Additionally, many of the method suggested for
improving solar cells efficiency have been tested by scientist. Therefore, readers may
want to read those articles to gain a better understanding of the improvements offered by
the authors. Information on the fundamental physics of organic photovoltaic cells remains
the same.
"How Hubble Got Its Wings." European Space Agency. Ed. European Space Agency. European
Space Agency, 13 Dec. 2010. Web. 7 Nov. 2013.
<http://www.esa.int/Our_Activities/Space_Engineering/How_Hubble_got_its_wings/%2
8print%29>.
This article looks at the unique challenge faced by the European Space Agency to
design a solar array system for the Hubble Space Telescope. While, Hubble was launched
into orbit in 1990, the challenges faced by the team remain relevant today as engineers
look for the ways to incorporate solar power into their satellites. The solar array designed
for Hubble had to fit into the a circular space between the telescope and the rounded
cargo-bay doors of the space shuttle. The team eventually came up with a "rolled blinds"
solution in which the solar panels would be stored in furled until after launch. The solar
"blanket" would then be held in place using a complex system of booms. The engineers
were forced to design solar panels that did not crack when bent and could withstand
major temperature changes, every 96 minutes.. While the first design was found faulty
because of severe erosion caused by oxidation, the second design was much better.
Ultimately, new solar panels were made without exposing kapton or silver directly to the
oxygen in the upper atmosphere. It was coated or removed from the panels entirely. At
the time, solar panels operated with an efficiency of around 14%.
The ESA provides solid background on the designing of the solar arrays and
structural supports. However, to fully understand the statistics and design of the system,
readers would be most benefited if they found the full studies on the solar arrays. This
information is also not completely up-to-date. Solar panel efficiency has improved
significantly since time of production.
Jensen, Niels E., and Bruce Battrick, eds. Technology Programmes. Noorwijk: ESA Publication
Division, 2003. Print.
This pamphlet, published by the European Space Agency, provided interesting
information on satellite power systems. Specifically, it focuses on solar power systems
opposed to a radioisotope thermal power conversion system. Solar cells have been used
on satellites since 1958, and have slowly increased in quality. Current solar cells used for
satellites are made from silicon. An ubiquitous semiconductor, silicon is the material of
choice because it possess a bandgap and can be doped to control current passage through
the cell. Scientist are also looking for more efficient ways to handle the P-N junction to
increase power conversion efficiency. Solar panels used on the Hubble Space Telescope
had a power conversion efficiency of about 14%. The low power conversion efficiency
forces scientists to design satellites with huge arrays of solar cells to power the satellites.
The size, weight and durability of solar cells are all logistical concerns for engineers
design satellites. Solar cells are exposed to extremely cold and extremely hot
temperatures and high amounts of radiation. When building satellites, scientist must
consider the effect of the elements on the system to determine the end-of-life conversion
efficiency expectation. Scientist are hoping to improve upon efficiency by using a
gallium-arsenide/ arsenium based solar panel in the near future.
This pamphlet provides a basic but engaging explanation of satellite power
systems, including how energy is store after it is produced. To gain a deeper
understanding, the reader would be best benefited by contacting the authors of the study
as there are not any citations. Overall, an effective introduction to a complicated subject.
Kaltenbrunner, Martin, et al. "Ultrathin and Lightweight Organic Solar Cells with High
Flexibilty." Nature Communications (2012): 1-7. Nature Communication. Web. 12 Dec.
2013. <http://www.nature.com/naturecommunications>.
This article provides a review of both the current organic solar cells as well as
original information on the development of a new ultrathin solar cell. The authors posit
that organic solar cells have greater potential in the development of flexible and thin solar
cells in comparison to traditional silicon based cells. THey were able to develop a solar
cell that retained functionality after being wrinkled and stretched to 50% and 30% of its
surface area. In each scenario, the solar cell was able to retain functionality after the cell
was decompressed. Additionally, there was actually a slight improvement in the
performance of the solar cell when wrinkled. The authors claim that this was a result of
the electron traveling through the bulk hetero-junction layer multiple times. Essentially, if
the electrons were not absorbed through the first pass through the material, then there is a
much higher likelihood that the electron will be absorbed. However, the devices showed
a significant decrease in the power conversion efficiency after multiple stretching and destretching. They report that after 22 stretches and de-stretches the efficiency of the device
had decreased by 71% however, the wire connecting the solar cell to the capacitor broke
before the device could completely degrade.
While this solar cell is a stark increase of the traditional models, there are many
comebacks that must be addressed before the cell can be incorporated commercially.
Readers should look for later publications by this group to discern further information on
the efficiency of this solar cell.
Kochergin, Vladimir, et al. "Aluminum Plasmonic Nanostructures for Improved Absorption in
Organic Photovoltaic Devices." Applied Physics Letters (2011): 1-3. Applied Physics
Leters. Web. 14 Dec. 2013.
<http://scitation.aip.org/content/aip/journal/apl/98/13/10.1063/1.3574091>.
While gold and silver plasmonic nanostructures have been used with both organic
and inorganic photovoltaic cells, aluminum has not been a popular choice and therefore,
not extensively researched. This article looks aluminum’s potential in increasing power
conversion efficiency by using it as nanostructures in the active layer of a solar cell. The
authors of this study used P3HT:PCBM based organic solar cell, as it is the most
researched and treated aluminum to prevent oxidation. They found that the aluminum
nanoparticles were able to increase the absorption by 50% with the P3HT:PCBM cell.
However, when used with a PCPDTBT:C70-PCBM solar cell, the absorption was
increased by 60%. Partially due to the novelty of using aluminum nanoparticles, the
authors state that the information they were able to gather does not match theoretical
models and has considerably sources of uncertainty. These include the red-shifting due to
aluminum’s unique optical properties. Additionally, the incorporation of the
nanoparticles also added considerable roughness to the cell which may have led to redshifting but also the discrepancies between theoretical and experimental designs.
While the research presented in the article is interesting, it is too novel to
accepted. The authors are able to provide basic groundwork for a concept that must be
developed upon. There are too many uncertainties at this point that question the validity
of the information.
Li, Xuanhua, et al. "Dual Plasmonic Nanostructures for High Performance Inverted Organic
Solar Cells." Advanced Materials 24.22 (2012): 3046-52. Wiley Online Journal. Web. 14
Dec. 2013. <http://onlinelibrary.wiley.com/doi/10.1002/adma.201200120/pdf>.
This article describes a successful attempt to increase the power conversion
efficiency of organic solar cells using plasmonic nanostructures and metallic
nanoparticles. Using both improvements, the authors were able to create a solar cell with
8.79% power conversion efficiency. While not as efficient as traditional silicon solar
cells, an over 8% power conversion efficiency is reasonably high for organic solar cells.
Using a novel fabrication technique, the group was able to incorporate nanoparticles in
the active layer and also the back-reflector layer. This increased the total light absorbed
into the cell. Additionally, by manufacturing at room temperature in a vacuum assisted
environment, the overall texture of the cells was improved, minimizing losses of
efficiency due uneven planarization. The nanoparticles and plasmonic structures worked
together to increase efficiency by covering the gaps in the other devices absorption
spectrum. The gold nanostructures work best between 400-600 nm, while the silver
nanostructures work most efficiently below 400 nm and above 600 nm. An added benefit
of the new manufacturing technique was the silver grating nanostructure was in direct
contact with the active layer; this will prevent short-circuiting in later devices.
Supplementary information is available for this article from Wiley Online Journal.
It is recommended that the reader download these files for reference as they contain
relevant charts and tables used in the study. More recent versions of this article should
also be accessed to determine if the researchers executed an experiment to determine the
electrical effects of the nanostructures, opposed to stimulation.
Miller, Kevin. Personal interview. 22 Nov. 2013.
This interview was conducted with high school physics teacher, Kevin Miller.
While his background is in electrical engineering, he was able to provide some basic
information on the physics behind solar energy. He discussed the hurdles that organic
photovoltaic cells must overcome before they can be seen as a viable source of energy in
the future. These include lower prices, better reliability and life expectancy. Miller
explained that while organic photovoltaic cells have potential because carbon is an
inexpensive element and has many unique characteristics such as flexibility and
recyclablity, though consumer will not purchase it until prices have dropped.
Additionally, Miller discussed the physics behind using Groups III-VI in the active
layers. He explained that these elements have external valence electrons that are easier to
knock loose than other elements. If the electrons are knocked lose from the element, they
then can be captured and harnessed as a current. Groups III-VI have easily detaching
electrons and therefore solar cells produced with these elements have greater power
conversion efficiency.
Miller provided a solid foundation for understanding the science behind solar
cells. However, this interview did not provide any in-depth information nor did it discuss
in detail improving the efficiency of organic photovoltaic cells. Therefore, the majority of
researchers would be most benefited if they read a more in-depth source.
Ostman, Sarah. "A Step toward Better Electronics: Researchers Develop a New Way to Oxidize
Promising Graphene." Northwestern University. Ed. Northwestern University.
Northwestern University, 20 Feb. 2012. Web. 19 Oct. 2013.
<http://www.northwestern.edu/newscenter/stories/2012/02/graphene-electronicsmccormick.html>.
This news article describes efforts made by Northwestern University Professor
Mark C. Hersam to induce a band gap in graphene. Pristine graphene lacks a bandgap
which prevents the material from ever stopping a current. Therefore, while graphene is an
excellent conductor, it cannot be used in traditional electrical applications because it
lacks the “off-switch.” Until an effective bandgap is induced, graphene will not be able to
replace silicon as the material of choice in electronics. However, researchers are currently
trying to overcome this barrier by using Hummer’s method to artificially create the
bandgap. This method creates the gap, but in the process also introduced many impurities
and destroys the morphology of the graphene thus severely decreasing the value of
graphene’s unique and exceptional values. Professor Hersam uses an ulta-vacuum
chamber to introduce oxygen homogeneously into the graphene. By doing so, the oxygen
is homogeneously integrated into the lattice and remains relatively undamaged allowing
for effective testing. By heating up a tungsten rod to 1500 degrees celius, the team was
able to split 02 into two molecules which then uniformly bonded into the graphene
lattice.
Ostman provides a brief overview of Professor Hersam’s studies. However,
readers would be most benefited if they read his complete study to understand the details
and process used more completely.
Park, Hyesung, et al. "Interface Engineering of Graphene for Universal Applications as Both
Anode and Cathode in Organic Photovoltaics." Scientific Reports (2013): n. pag. Nature.
Web. 19 Oct. 2013.
<http://www.nature.com/srep/2013/130402/srep01581/pdf/srep01581.pdf>.
This articles looks at a number of problems associated with indium tin oxide
(ITO) in solar cells as wells as the obstacles in utilizing graphene in solar cells. The
researchers note that while ITO is generally the material of choice in solar cells, it often
corrodes the device due to its high acidity thus decreasing the cell’s lifetime. Graphene, a
much more stable and flexible material, has potential for replacement but generally lacks
the power conversion efficiency of ITO cells because graphene is not able to correctly
bond with polymer, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)
(PEDOT:PSS), the hole injection layer. To overcome the hydrophobic properties of
graphene, the researchers created a double hole injection layer made of poly(3,4ethylenedioxythiophene)-block-poly(ethylene glycol) (PEDOT:PEG) which is doped
with perchlorate. Adding the second layer, allows for PEDOT:PSS to serve as the hole
injection layer efficiently and PEDOT:PEG(PC). The double HIL also prevents
PEDOT:PSS from corroding the lower layers of the solar cell. The researchers found that
the device created using graphene electrodes has a power conversion efficiency within
10-15% of the indium tin oxide devices. They further posit that the graphene devices will
have greater efficiency as more layers of graphene are included or the graphene is
chemically treated.
Park et al. focus on graphene’s potential use as an anode or cathode in solar cells.
However, they also include information on a variety of other relevant topics including
reversed geometry and logistical concerns such as scalability and cost. Overall, the article
provides very helpful information.
-
- -. "Doped Graphene Electrodes for Organic Solar Cells." Nanotechnology (2010): n.
pag. IOP Science. Web. 19 Oct. 2013. <http://iopscience.iop.org/09574484/21/50/505204/pdf/0957-4484_21_50_505204.pdf>.
Park et al. investigates graphene’s potential as an electrode in organic
photovoltaic cells. While theoretically, graphene should serve as an excellent electrode,
the researchers found that it did not work because the poly(3,4ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) could not effectively
bind with graphene. This is due to graphene’s hydrophobicity. Therefore to overcome
this, the team positively doped the graphene with gold trichloride: doing so decreased the
hydrophobicity and improved the overall power conversion efficiency of the device. Prior
to doping, the graphene/PEDOT:PSS device had a power conversion efficiency of around
0.57%, 0.75% and 1.37%. Indium tin oxide devices had an efficiency of around 0.63%,
1.33%, and 1.77%. After doping, performance of the graphene devices increased to
around 1.51% and 1.63%. The researchers had three generations each with different
amounts of organic polymers, thus three different power conversion efficiencies.
However, the team noted that the devices made with doped graphene had a much higher
failure rate than the devices fabricated with indium tin oxide. They also argue that the
doped graphene devices had a slightly lower power conversion efficiency because
graphene has higher sheet resistance.
This study was very interesting. However, it is slightly outdated. Recent studies
have found better ways to decreases the hydrophobicity of graphene and have thus
created devices with much higher power conversion efficiencies. Therefore the reader
should read their latest studies.
Penn Electric Racing. "How Solar Cells Work." Penn Electric Racing: The Quest for Fossil Fuel
Free Performance. Ed. Penn Electric Racing. U of Pennsylvania, 2013. Web. 19 Oct.
2013. <https://fling.seas.upenn.edu/~electric/dynamic/?page_id=244>.
This website is maintained by the Penn Electric Racing Team to provide
information on their cars, tournaments and projects. They also provide some general
information that reader must understand about electricity and electric cars including the
differences between batteries and capacitors, alternative energy types and how solar cells
work. The brief, but apt, overview on solar cells explains that solar panels are made of
two different charged sections of silicon. Generally, silicon has space for four additional
electrons in its third electron shell. However, when combining boron with silicon, there
material only has three electrons in the third shell thus becoming positive doped.
Likewise, when phosphorus is combined with silicon, the material has five electrons in
the third shell thus becoming negatively doped. When the sunlight strikes the cell, a
disturbance is created in the electric field of the P-N junction. In order to restore
neutrality, and a full 3rd shell, the electrons try to go to the opposite charge of the cell.
However, the solar cells are created with alternating positive and negative sections of
silicon which then forces the electrons to create an electric current as they travel around
the cells searching for equilibrium. Solar cells have an efficiency of around 18% but this
power conversion efficiency can be improved by using various modifications.
Solar panels are a complicated topic and this website effectively simplifies some
of its complexities. It would be improved if the website touched upon the other parts of
the cell.
Piliego, Claudia, et al. "Synthetic Control of Structural Order in N-Alkylthieno[3,4-c]pyrrole4,6-dione-Based Polymers for Efficient Solar Cells." Journal of American Chemical
Society (2010): 7595-97. American Chemical Society. Web. 19 Oct. 2013.
<http://pubs.acs.org/doi/abs/10.1021/ja103275u>.
Piliego et al. focus on the different side chains attached to N-alkylthieno[3,4c]pyrrole-4,6-dione and their effect on the power conversion efficiency of organic
photovoltaic cells. Here, the group uses modified cells prepared by Lecerec et al. with a
power conversion efficiency of around 5.5%. They then changed the alkyl groups on the
polymer to: for P1 to ethylhexyl, and ethyl for P2 and P3 chains. Using these subgroups,
the group was able to increase the effiecency to between 4.0% and 6.8%. Additionally, in
order to reduce the effects of an uneven surface, the cells were coated with 1,8diiodooctane. P1 and P2 had the most improvement from the addition of 1,8diiodooctane. P3 showed minimal improvement. This article provides some very
technical information about a certain bulk heterojunction polymer used in solar cells.
Overall, the article is geared only towards those who fully understand chemistry and the
structure of solar cells. Otherwise, the reader is at a loss to understand the research
presented.
Shwartz, Mark. "Stanford Scientists Build the First All-Carbon Solar Cell." Stanford News. Ed.
Stanford News. Stanford University, 31 Oct. 2012. Web. 19 Oct. 2013.
<http://news.stanford.edu/news/2012/october/carbon-solar-cell-103112.html>.
Shwartz reports on the Bao group’s creation of an all-carbon solar cell. This is the
first solar cell that was completely made with carbon, including the electrodes. While this
device did not meet the power conversion efficiency of silicon solar cells, it still contains
great promise in science. Professor Zhenan Bao claims that carbon’s unique properties
will allow for the creation of solution based solar panels which can be coated on
buildings and cars. However, she admits that there are substantial efficiency hurdles that
must be overcome first for everyday use. The carbon based solar cell had an efficiency of
less than 1%. The researchers claim that the spectrum that the cell currently absorbs is
infrared and not visible light. They also note that their current manufacturing process led
to many defects in the cell layers that also decreased efficiency. Future cells will be built
to have smoother surfaces. However, the current cells may be able to function better in
extremely high or low temperatures. The active layer was made of buckminsterfullerene,
and other components were made mostly with carbon nanotubes and graphene.
This article was very interesting and including a fair amount of information
through direct interviews with the authors of the stories. There was also an accompanying
video interview with Professor Bao that provided insight on the solar cell and what its
future will look like. However, to fully understand the construction of the cell, readers
should look to the full study. The link is provided.
Song, Justin C. W., et al. "Hot Carrier Transport and Photocurrent Response in Graphene." Nano
Letters (2013): 1-4. Arxiv. Web. 21 Sept. 2013. <http://arxiv.org/pdf/1105.1142v2.pdf>.
This article looks at the effect of carrier multiplication on graphene for potential
use in optoelectronics. While this idea has been theoretically analyzed, it had not been
experimental tested until this study. The researchers found that there was poor lattice
cooling with high carrier multiplication. This allows for better photodectors because the
hot carrier electrons leave a very distinct pattern that allow for a specific experimental
fingerprint. Hot carrier electrons were used in this experiment because the thermocurrent
generated by them leads to much higher efficiencies in photoresponsiveness. The
efficiency can futher be increased when the hot carrier electrons are combined with a
slower cooling lattices. If the lattice is does not cool quickly, the carrier electrons will
reach the contact still hot leading to better photoresponsiveness. The carrier
multiplication occurs from one phonon exciting many electrons, which in turn are hot,
and allow for better overall efficiency in power and photoresponsivenss. Eventually, the
team would like to use graphene with boron nitride to further exploit the effect, and the
sign changes in the p-n junctions.
While this article is based off of an experiment, it does provide substantial
background on the theoretical possibilities of hot carrier electron transport and carrier
multiplication. Therefore, there are many equations used to describe both the theoretical
background and the experiment. Overall, this is a fairly recent study, yet to be compared
to too many others on the same topic. The article can be found on arxiv.org.
Wang, Yu, et al. "Large Area, Continuous, Few-layered Graphene as Anodes in Organic
Photovoltaic Devices." Applied Physics Letters (2009): n. pag. Applied Physics Letters.
Web. 17 Oct. 2013.
<http://scitation.aip.org/docserver/fulltext/aip/journal/apl/95/6/1.3204698.pdf?expires=13
82047149&id=id&accname=guest&checksum=C28E4B015F5770BBC9F0A2EF712CA
435>.
Wang et. Al report upon their research using graphene as an anode in photovoltaic
devices opposed to indium tin oxide. Their research is driven by a need to find a cheap
alternative to ITO because it is becoming an increasingly rare, and expensive material,
and because the material is very fragile. Graphene while theoretically inexpensive
because it is created from a hydrocarbon gas has not had too much success in as an
anode. The researchers claim that the defects in graphene, including the overlapping of
small flakes and structural defects created by using graphene chemically reduced from
graphene oxide. Pristine, unmodified, chemically produced graphene has a power
conversion efficiency of around 0.21% as an anode. This is a much lower value than
indium tin oxide at 3.10% power conversion efficiency. To increase the power
conversion efficiency, the researchers used a treated the graphene with ultraviolet
radiation for ten minutes and exposed another sample to pyrene buanoic acid
succidymidyl ester (PBASE). The device modified with PBASE performed the best in
trial because it allowed the graphene to more efficiently line-up with the orbitals of the
PEDOT:PSS layer, made it easier for graphene to function as an open circuit and
facilitated hole injection. However, this device had a power conversion efficiency of
1.71% and the UV treated graphene had an efficiency of 0.74%.Graphene efficiency can
further be improved by modifying its surface to be more hydrophillic.
This article was fairly interesting. Different researchers are using graphene as
different components of solar cells. Graphene does display promise as a components of
the solar cell but it seems unlikely that there will be an all carbon organic solar cell.
Readers are most benefited if they examine the different components of solar cells before
reading this article.
Zhou, Yinhau, et al. "Recyclable Organic Solar Cells on Cellulose Nanocrystal Substrates."
Nature: Scientific Reports (2013): n. pag. Print.
This article looks at the potential use of cellulose nanomaterials and other organic
materials to develop easily recyclable photovoltaic cells. Most organic photocells have
not been a viable alternative to the traditional silicon cells. They have only been able to
achieve a maximum of 10.6% power conversion efficiency, and shorter lifetimes.
Therefore, the researchers postulate that, in order to make organic photovoltaic cells an
alternative, the inexpensiveness factor must be exploited and they should be marketed as
cheap, throw-away cells. Because silicon solar cells rely upon expensive petroleum based
and rare earth metals. The researchers have been able to develop solar cells on cellulose
nanocrystals that have a power converision efficiency of 2.7%. These cells are a major
improvemetnt from previous cells that had a power conversion efficiency of 0.4% The
low power conversion efficiency was most likely due to the unequal distribution of
nanocrystal. The researchers were able to create a smoother surface and thus improve the
efficiency. These photovoltaic cells still need to reach better efficiency to be a viable
alternative to silicon solar cells. The researchers estimate an efficiency of 5% must be
achieved before organic solar cells can be popularly used.
This article was very interesting. Building organic solar cells is a popular topic,
however, it is was interesting to see the idea learn about the idea of recyclable solar cells.
However, scientific reports is not an official part of the nature journal and therefore, this
information should be triangulated to determine how accurate the information is.