Cadmium-Selenide “Quantum Dots” for Sensitized TiO2 Solar Cells
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.
“Zero-dimensional semiconductor nanostructures” were first discovered by Russian solid-state physicist Alexey Ekimov  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 . His discovery, revealed in two papers published in 1984 and 1985  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.
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 . 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.
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.
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 .” 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 energy 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.
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 devices. Specifically, improving the absorption of Titanium Nan-oxide in dye sensitized solar cells.
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 . The control dye-sensitized solar cell operating under the same light intensity achieved an efficiency of 0.05%.
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%.
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 National Nanotechnology Initiative. (2014). Nanotechnology 101. http://www.nano.gov/timeline
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 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
 Melville, Jonathan. (2015). Optical Properties of Quantum Dots. UC Berkley College of Chemistry. https://www.ocf.berkeley.edu/~jmlvll/lab-reports/quantumDots/quantumDots.pdf
 International Journal of Photoenergy. (2012). CdSe Quantum Dots for Solar Cell Devices. http://www.hindawi.com/journals/ijp/2012/952610/[ Readmore. ]