Quantum Dot Sensitized Solar Cells

Cadmium-Selenide “Quantum Dots” for Sensitized TiO2 Solar Cells

 

Introduction

The success of next generation solar technology is heavily dependent on improving the power conversion efficiency factor of solar cells.  This is to say that the primary goal of current solar research is to increase the electrical power output (W) per global daily solar irradiation input (Wh/m2) of semiconductor materials.  Cadmium Selenide is of interest for producing nanocrystals that portray size-tunable optical absorption and emission spectra.  This material may someday be used to boost the efficiency of photo-electrochemical systems.  These nanocrystals commonly referred to as “quantum dots” have been researched for potential breakthroughs for over two decades.

Historical Notes

“Zero-dimensional semiconductor nanostructures” were first discovered by Russian solid-state physicist Alexey Ekimov [1] in 1981, grown within a porous glass substrate.  Brus provided proof of their existence through scanning electron micrograph.  It wasn’t until 1988 that the nanocrystals received their more palatable nickname: quantum dots.

In 1985 semiconductor nanocrystals were discovered in colloidal solutions by Bell Labs’ Louis E. Brus [2].  His discovery, revealed in two papers published in 1984 and 1985 [3] have been cited more than 3,500 times by the scientific community.  Brus’ experiments revealed that the values of minimum band gap for quantum dots were not fixed, but varied.  This had profound technological implications, especially in the field of optics.  Brus has been suggested for the Nobel Prize for his discovery.

Synthesis

There are two primary quantum dot types.  Cadmium-based, which is the industry standard, and a non-toxic alternative Zinc-based.  Cadmium is highly carcinogenic, and acts pre-synoptically on both cardiac and neuronal tissue.  Inhalation of cadmium dust causes pulmonary edema, renal failure and/or death.  These effects are rapid and non-reversible.  Great care must be taken in the handling of Cadmium powder.  Zinc may be substituted for Cadmium with similar results, but with a higher resulting band gap.

The focus of this paper is on Cadmium-based nanocrystals.  The synthesis of this material is as simple as heating Cadmium Oxide to 225° Celsius, then injecting a Trioctylphosphine Selenide.  Selenide binds to Cadmium as the second ingredient for the ionic structure to form.  Trioctylphosphine is used as a capping agent, to regulate crystalline size and stability.  As a main criteria, the energy band gap of the capping agent should be wider than that of the crystalline core [4]. The amount of time heated, in seconds, will determine the size of nanocrystals, and consequently band-gap value.  The red color emission is resultant from nanocrystals in size-magnitude of around 6 nm, whereas the blue spectra emission is from nanocrystals in the 2nm range.

ss

Crystal Structure

The crystalline structure of CdSe quantum dots are defined by their ionic bonds.  Electrostatic forces bond the electronegative Cadmium cations to complimentary Selenide anions.  The resultant ionic positioning is similar to that of ordinary sodium chloride.  The crystalline formation may be described as a 3d checkerboard pattern.

The size ratio of anion/cation reveals an octahedral crystal structure.   This next image depicts a unit cell visualization of CdSe.  As apparent from the image, Selenide ions are twice the size of Cadmium, causing the crystal lattice structure of CdSe to form a face centered cubic (FCC) structure also known as close cubic packing.

ll

Characterization

The electric properties are between bulk semiconductor and discrete molecules.  “However, unlike bulk semiconducting materials, quantum dots [are] too sparse to create the continuous valence and conduction bands typical of macroscopic conductors [5].”  Instead, quantum dots’ electronic structure mirrors the discrete electronic states found in single atoms.   Therefore, quantum dots owe their unique properties to the size regime in which they exist.  The overarching implications are, the larger the quantum dot, the smaller its associated band gap and the more it will behave like bulk semiconductor.

The optical properties may be attributed to quantum confinement.  As typical with semiconducting material, absorption of a photon with a sufficient energy state to meet material band gap energy, an electron-hole pair excitation will be created.  But, because the average excitation is smaller than the size of the quantum dot, the excitation is “squeezed” into the material generating confinement energy.  Therefore, the total flourescenseenergy of a fluorescing photon is the sum of band gap, confinement energy and excitation energy.  Because confinement effects dominate at this scale, energy levels operate on a digital spectrum where optical/electronic properties may be easily controlled.

 

Applications

Quantum dots may be considered revolutionary Nano electronic devices.  The applications range from OLED display, lasers, single-electron transistors, biological imaging, and more.  The focus of this report, however, is the possible impact on the efficiency of photo-electrochemical devicesSolarHigh. Specifically, improving the absorption of Titanium Nan-oxide in dye sensitized solar cells.

 

Homemade Dye-Sensitized Solar Cells.  Dyed with: Black Cherries (top), Blackberries (left), Raspberries (right).

Dye-sensitized solar cells are a next generation solar technology, and they are much easier to fabricate than traditional silicon-based cells!  Two glass plates with a special Transparent Conductive Oxide (TCO) coating act as electrodes.  Sintered to opposing glass electrodes are a titanium Nano-oxide paste and a platinum catalyst.

Titanium Nano-Oxide does not readily absorb sunlight.  Generally, experimentalists dye these cells red to capture low-energy photons.  This allows dye-sensitized cells operate with a high capacity factor (CF) in diffuse-light conditions, like on cloudy days.

Dye-Sensitized Solar Cell Design

Rather than dying the titanium anode with, for example, blackberries, dying them with quantum dots would enable the control of the cell’s optical properties.  Theoretically, we could calibrate the solar cell’s performance.  And, there have been instances where this idea has been investigated.  An experiment performed by the Advanced Technology and New Materials Research Institute in Egypt resulted in quantum dot-sensitized solar cell performance of 0.08% under a light intensity of 100 mW/cm2 [6].  The control dye-sensitized solar cell operating under the same light intensity achieved an efficiency of 0.05%.

Conclusion

Quantum dots are a fascinating Nano-crystal that is small enough to provide access to the quantized world.  With the proper resources to mitigate hazards associated with Cadmium, CdSe microcrystals appear relatively simple to experiment with.  And, initial findings of the efficiency of quantum dot-sensitized solar cells from the scientific community are encouraging.  This is certainly a step in the right direction for this developing technology.  But, there is a long way to go to overcome silicon mono-crystalline current efficiency rating of 22%.

 

 

 

 

 

 

References

[1] Ekimov, Alex. (1981). http://www.jetpletters.ac.ru/ps/1030/article_15644.pdf

[2] National Nanotechnology Initiative. (2014). Nanotechnology 101. http://www.nano.gov/timeline

[3] Journal of Chemical Physics, 79[11]: 5566-71; and 80[9], 4403-9. (1985).

[4] Guyot-Sionnest. Hines, M.A. (1996).  Synthesis and Characterization of strongly luminescing ZnS-Capped CdSe Nanocrystals.  The journal of Physical Chemistry. https://www.researchgate.net/profile/Guyot-sionnest_Philippe/publication/231656202_Synthesis_and_Characterization_of_Strongly_Luminescing_ZnS-Capped_CdSe_Nanocrystals/links/5489bb8d0cf225bf669c71bf.pdf

[5] Melville, Jonathan. (2015). Optical Properties of Quantum Dots. UC Berkley College of Chemistry.  https://www.ocf.berkeley.edu/~jmlvll/lab-reports/quantumDots/quantumDots.pdf

[6] International Journal of Photoenergy.  (2012). CdSe Quantum Dots for Solar Cell Devices.  http://www.hindawi.com/journals/ijp/2012/952610/

[ Readmore. ]

2010 BP oil Spill

2010 BP Oil Spill

 

The US depends upon a constant oil supply in order to remain an economically competitive nation, this much is certain.  It remains questionable whether the push for offshore drilling will grant the security of energy independence.  But in the midst of the largest man-made disaster of our time, visible from space, the dark-side of oil extraction drifts into public view.  I am talking about, of course, the oil spill in the Gulf of Mexico on April 16th, 2010.  Eleven workers of the oil rig Deepwater Horizon were killed, and oil gushes from its broken pipes 5,000ft below the ocean’s surface.  British Petroleum (BP) having ownership of the sunken Deepwater Horizon has been provided sole responsibility of stopping the leak, and cleaning-up the mess.

Over a month after the initial explosion and with the leak still not stopped, this disaster continues to be a large environmental and economic concern for Louisiana and its surrounding areas.  Despite the best efforts of the coast guard, volunteers, and BP engineers, severe damage is being done:

gross

6 million gallons have shown up in Louisiana coast.

Louisiana fishing/beaches have been closed down.

Estimated 20 million gallons spilled so far

Estimated 4million gallons a day

20 times worse than Exxon Valdez

Giant underwater plum of oil/dispersants

Clean-up cost is $450,000,000 and increasing at $10,000,000 a day

Warm water loop current could take it to Florida

(MSNBC, 2010)

 

A Matter of National Security, or not…

 

If an American company can sell its petroleum product for $3 per gallon in any other part of the world, there is no incentive to sell for anything less in America.  So while the business of energy remains private, no amount of domestic drilling can save us from the law of supply and demand.  The multinational corporate mindset is to follow the path of highest profitability.    The Deepwater Horizon for example, had been flying the flag of a small group of islands off the coast of Australia named the Marshall Islands.  The oil rig Deepwater Horizon had no official ties to the Marshall Islands, but by claiming a nationality of choice allows corporations to choose which countries regulations their offshore drilling machinery must abide.

The Organization of Petroleum Exporting Countries (OPEC) has had more influence on the global petroleum market than any single nation. OPEC is a consortium of twelve oil producing countries: Algeria, Angola, Ecuador, Iran, Iraq, Kuwait, Libya, Nigeria, Qatar, Saudi Arabia, the United Arab Emirates, and Venezuela.  In 1973 OPEC attempted an embargo against the US that lasted less than two months.  “While its members were giving up oil revenues, its oil was still reaching the United States because of diverted shipments from Europe.” Today America spends more money on crude oil imports from Canada and Mexico than we purchase from all of the Persian Gulf.

One can also make the argument that corporations drilling on domestic soil may not necessarily be American, and inversely American oil producers often operate outside of the country.  Prior to the spill in the Gulf of Mexico, BP had been selling their image in America as “Beyond Petroleum,” presumably to blend-in as a local company and hide an already tarnished reputation.

“British Petroleum was linked to bribery in Azerbaijan and Kazakhstan in 2001, and 2002, and the Financial Times commented that ‘while the days of government ownership have long passed, BP’s ties with the British government are still so close that rivals call it [Blair Petroleum].”

With support from the US military, American petroleum multinationals have gained access to many of the world’s largest oil deposits.  The Iraq Oil Ministry was the first major building to be occupied by American soldiers in Iraq (Phillips, 2006). This ministry was home to thousands of seismic portraits of oil fields, which provided no strategic military value.  If there had been any question, this spoke loudly to the reason why America was invading Iraq.

For the greater extent of my 26 yr. life, oilmen have dominated the American presidency.  The last decade of which, both the president (George W. Bush) and vice president (Dick Cheney) have commonly referred to themselves as “Texas oilmen.”  George W. used his presidency, in-part, to tie the hands of the American regulatory system. “President Bush has installed more than 100 top officials who were once lobbyists, attorneys or spokespeople for the industries they oversee.”  Minerals Management Service, in charge of overseeing the petroleum industry is certainly no exception.  An infamous “revolving door” was created at MMS when former BP executive Sylvia Baca was appointed head of division of Minerals Management Service.

            Minerals Management Service boasts the second highest income for the American government, receiving $10 billion in revenue for 2009.  Taxes are undoubtedly the biggest income.  During a presidential news conference, 27MAY10, President Obama describes the Minerals Management Service as “plagued with corruption for years and has given the oil industry leverage to regulate themselves.”  As evidence, the president offers that under current law the Interior Department only has 30 days to review an exploration plan submitted by an oil company, which does not give sufficient time to do the review defaulting in a PASS.

            A somewhat less direct way that the oil industry influence political decisions is through lobbying.  British Petroleum reportedly spent $20 million in Washington lobbying in 2009 alone.  This does point to an interesting question.  Should foreign business men be given such access to influencing American political policy?

 

Effects of Petroleum (Hydrocarbons) on Life

 shinny

            Petroleum weathers from initial spill-site to shore, going from fluid rainbow sheen to sticky and tar-like.  Near the spill-site, where the oil is still thin, hydrocarbons are volatile, reactive, toxic, and highly flammable.  The stickier weathered form of oil contains cancer-causing Polycyclic Aromatic Hydrocarbons (PAH).  Plants and small animals along the shore are smothered to death, turtles perish from food-contamination, and birds suffer hypothermia as the oil strips their feathers of weatherproofing.

It is hard to say what effect the Exxon/Valdez oil spill had on marine life, as the area was not well documented before the spill.  Killer whales on the other hand, had been well tracked in Alaska.  A group of 22, known as the Prince Albert Transients, have their own dialect, eat mammals instead of fish, and generally do not intermingle with other killer whale populations. Nine whales disappeared in the first winter after the spill, including two females and two children. This group is expected to die out, as they have no remaining females of reproductive age.  This does not bode well for our cetacean friends in the Gulf of Mexico.

 

Cetaceans of the Gulf of Mexico

 

Balaenidae

1NorthRight

Northern right whale Balaena glacialis

 

Balaenopteridae

2BluWhale

Blue whale Balaenoptera musculus

Fin whale Balaenoptera physalus

Sei whale Balaenoptera borealis

Bryde’s whale (S)a Balaenoptera edeni

Minke whale Balaenoptera acutorostrata

Humpback whale Megaptera novaeangliae

 

Physeteridae

3sperm

Sperm whale (S) Physeter macrocephalus

 

Kogiidae

4dwarfsperm

Pygmy sperm whale (S) Kogia breviceps

Dwarf sperm whale (S) Kogia sima

 

Ziphiidae

ziphius-female-2008may05_d7_rwb_0078-resize

Cuvier’s beaked whale (S) Ziphius cavirostris

Blainville’s beaked whale Mesoplodon densirostris

Sowerby’s beaked whale Mesoplodon bidens

Gervais’ beaked whale Mesoplodon europaeus

 

Delphinidae

6spotted

Melon-headed whale (S) Peponocephala electra

Pygmy killer whale (S) Feresa attenuata

False killer whale (S) Pseudorca crassidens

Killer whale (S) Orcinus orca

Short-finned pilot whale (S) Globicephala macrorhynchus

Rough-toothed dolphin (S) Steno bredanensis

Fraser’s dolphin (S) Lagenodelphis hosei

Bottlenose dolphin (S) Tursiops truncatus

Risso’s dolphin (S) Grampus griseus

Atlantic spotted dolphin (S) Stenella frontalis

Pantropical spotted dolphin (S) Stenella attenuata

Striped dolphin (S) Stenella coeruleoalba

Spinner dolphin (S) Stenella longirostris

Clymene dolphin (S) Stenella clymene

 

BP-oil-spill

            We are already seeing a health-decline of volunteers in the Gulf.    A few have been hospitalized with claims of headaches, nausea, dizziness, coughing, sore throats, and other flu-like symptoms.  These symptoms are signs of chemical poisoning, requiring special health-care.  Only after volunteers started being hospitalized does BP acknowledge demands for protective equipment provisions of any kind, claiming that nobody yet knows the health risks.  Yet in 2007, a study done by the Korea Centers for Disease Control and Prevention found locals near the Hebei Spirit oil tanker spill suffered from “headache, nausea, dizziness, fatigue, tingling of limbs, sore throat, cough, runny nose, shortness of breath, itchy skin, rash and sore eyes.”  Hitting a bit closer to home, 6,722 workers of the 1989 Valdez oil spill reported suffering from upper-respiratory illness.

Related

            An impressively well hidden oil spill, 1.5 times bigger than the Valdez oil spill, is centered under a residential area in Brooklyn, New York.  Teresa Toro, who lives just two blocks away from the so far uncontaminated Newton Creek, says “when the wind is just right, I can smell it blowing off the creek. Sometimes we can’t open our windows.”  Vapor tests performed in the area in 2005 relayed dangerous levels of Methane (natural gas) and Benzene.  “It’s up to 35 or 36 people that I know that have had cancer just on this block,” says Tom Stagg, another block resident.  Local residents speculate the source of the leak to be a large explosion that happened in the city’s sewer system in 1950.  The explosion is rumored to have sent manhole covers (avg. 100lbs) “raining down on the populace.”  Chemical analysis shows ExxonMobil to be the producer of the polluting oil in question, though they deny any involvement. No efforts are currently being made to clean up the spill; it is 55-acres and presumably growing larger.

[ Readmore. ]