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Chemical Identity & Function of CdSe Quantum Dots

Improving the efficiency of solar cells in converting photons to electrical current is a

continuing goal of photovoltaics. With the advent of dye-sensitized solar cells (DSSCs), new

methods of achieving this goal have become available, and one of the new methods consists in

the use of quantum dots (QDs) as the sensitizer.1,2 A quantum dot is a semiconducting crystal

nanoparticle (in this case, CdSe) which serves as a spatially confined potential well for

electrons.1,3 Thus it is not just the chemical properties of the CdSe material, but the CdSe

quantum dots function as sensitizers because of material properties which are the result of the

shape and size of the quantum dots (Scheme 1).

These QDs have found application in lasers, in light emitting diode (LED) uses, and in

optoelectronic devices, in addition to their uses in solar cells and photovoltaics.1,3 The QDs offer

more control over the band gap energy and they are highly tunable by control of their sizes, and

this feature allows to increase the range of photon wavelengths that can be absorbed.1,3 Impact

ionization allows for quantum yields greater than unity in QDs and, hence, for a potential

efficiency of QDs that exceeds the theoretical limit of normal dyes.2 Maintaining quantum

confinement requires the radii of the QDs to be 2-10 nm, and the distance between the dots will

need to be greater than twice the radii (≈ 4-20 nm), which is normal for solar cell materials. This

also will grant a strong absorption coefficient for the solar cell.1 However, the toxicity of

cadmium, a heavy metal, remains a concern for CdSe QD applications. There also remain

technological challenges to achieving an appropriate shape and orientation for the QDs for

optimal performance and with regard to engineering QD devices with optimal excitation

energies.2

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Scheme 1: CdSe QDs interspersed among TiO2 nanoparticles in a QDSSC, and the electron flow through the cell.

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Experimental Section: CdSe QD Characteristics

CdSe quantum dots work well as photovoltaic dye sensitizers because they are

semiconductors with optimal band gaps (~1.7 eV) and high absorption coefficients.4 The UV-

Vis and the X-ray diffraction (XRD) spectra were recorded using CdSe nanofiber crystals (Figs.

1 and 2). These nanofibers were electrodeposited on indium tin oxide coated glass substrate with

a pH of approximately 2. The UV-Vis spectrum was recorded between 250 and 1100 nm. The

absorption spectrum shows that the QDs have a high absorption at shorter wavelengths in the

visible spectrum. The CdSe QDs show better absorption at higher annealing temperatures: The

lines in the XRD spectrum are defined better at 350 and 410 °C indicating that the cubic

structure has changed to a hexagonal structure. The sizes and shapes of the CdSe nanocrystals

were analyzed by scanning electron microscopy (SEM) and transmission electron microscopy

(TEM) (Figs. 3 and 4). Differing crystals structures can be formed depending on the cadmium

source used in the synthesis.5 Multi-foots rods were formed from CdCl2, flower-like structures

were formed using CdBr2, and hallow spheres were formed from Cd(NO3)2•4H2O substrates.

SEM analysis shows the crystal clusters for each, while the TEM shows individual particles for

further analysis.

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Figure 1: A) Electrodeposited and annealed optical absorption spectrum of CdSe. B) At 250

°C. C) At 350 °C. D) At 410 °C.

Figure 2: A) Electrodeposited X-ray diffraction spectrum of CdSe. B) Annealed at 250 °C. C)

At 350 °C. D) At 410 °C.

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Figure 3: SEM spectra of: A) Multi-foots rods from CdCl2. B) Flower-like structures from

CdBr2. C) Hollow spheres from Cd(NO3)2 hydrate.

Figure 4: TEM spectra of: A) Multi-foots rods. B) Flower-like structures with d ~ 500 nm. C)

Hollow spheres with d ~ 600-700 nm.

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Synthesis of CdSe Nanoparticles

The creation of CdSe quantum dots depends largely on the cadmium precursor as well as

the solvent system and associated ligands; the Se is dissolved in the solvent to combine with the

resulting Cd2+ ions. We focus on the precursor of Cd(NO3)2•4H2O and the classic solvent

trioctylphosphine oxide (TOPO). Each cadmium precursor gives different shapes. It has been

found that with Cd(NO3)2•4H2O the CdSe products tend to take on a hollow spherical shape

(Scheme 2).6 The solvent system incorporated into the synthesis usually determines the size of

the CdSe nanocrystals. TOPO is used traditionally because it gives nanoparticles with quantum

dot sizes. While other solvents, such as fatty acids, give crystals with a rather broad range of

diameters (25 nm or more) and broad size distributions, the solvent TOPO affords CdSe crystals

with a maximum diameter of about 12 nm.

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Scheme 2: Initial cadmium precursor with product shapes. Also show the size variations of

CdSe crystals. The smaller crystals are what will be needed for the quantum dots.

                                                                                                                         

1.  Rafaelle, R. P.; Castro, S. L.; Hepp, A. F.; Bailey, S. G. Progress in Photovoltaics: Research and Applications

2002, 10, 433 – 439.

2. Nozik, A. J. Physica E 2002, 14, 115 – 120.

3. Kumar, S.; Sagar, L. K. Chem. Commun. 2011, 47, 12182 – 12184.

4. Kois, J.; Bereznev, S.; Gurevits, J.; Volobujeva, O. Materials Letters 2013, 95, 110 – 113.

5. Li, J.; Tang, X.; Lu, Z.; Qian, Y. Journal of Alloys and Compounds 2010, 497, 390 – 395.

6. Qu, L.; Peng, Z. A.; Peng, X. Nano Lett. 2001, 1, 333 – 337.