Olson & Brackman
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 bandgap 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 QDsand, hence, for a potential efficiency of QDs that exceeds the theoretical limit ofnormal dyes.2 Maintaining quantum confinement requires the radii of the QDs to be 2-10nm, and the distance between the dots will need to be greater than twice the radii (4-20nm), 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
Scheme 1: CdSe QDs interspersed among TiO2 nanoparticles in a QDSSC, and the electron flow through the cell.
Experimental Section: CdSe QD Characteristics
CdSe quantum dots work well as photovoltaic dye sensitizers because they are semiconductors with optimal band gaps (~1.7eV) and high absorption coefficients.[4] The UV-Vis and the X-ray diffraction (XRD) spectra were recorded usingCdSenanofiber 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 1100nm. The absorption spectrum shows that the QDs have a high absorption at shorter wavelengths in the visible spectrum. The CdSeQDs show better absorption at higher annealing temperatures: The lines in the XRDspectrum 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 CdSenanocrystals 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 fromCd(NO3)24H2O substrates. SEM analysis shows the crystal clusters for each, while the TEM shows individual particles for further analysis.
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.
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~500nm. C) Hollow spheres with d~600-700nm.
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)24H2O and the classic solvent trioctylphosphine oxide (TOPO). Each cadmium precursor gives different shapes. It has been found that with Cd(NO3)24H2O 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 CdSenanocrystals. TOPO is used traditionally because it gives nanoparticles with quantum dot sizes. While other solvents, such as fatty acids, give crystals witha rather broad range of diameters (25nm or more) and broad size distributions, the solvent TOPO affordsCdSe crystals with a maximum diameter of about 12nm.
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.
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