Module-4_Unit-5_ NSNT Quantum Dots Introduction A quantum dot (QD) is an extremely small particle whose properties can be drastically changed merely by removing or adding an electron. In this parlance, an isolated atom can also be termed as a quantum dot. However, only a cluster of atoms or molecules demonstrates property variations. In biochemistry, QDs are often termed as redox groups. QDs are referred as quantum bits or qubits in nanotechnology. The dimensions of QDs are of the order of few nanometers. Nanotechnology is a multi-disciplinary field wherein the problem often includes studying biology, chemistry, computer science, electronics, etc. For example, consider a hypothetical biochip, which is grown analogous to a tree from a seed, and which might also include a computer inside it. In this analogy, both terms - a qubit or redox group - both are equally applicable. Besides, the classification of this chip as animate or inanimate, is difficult to accomplish. Each QD present in this chip may account for one or more data bits. In some cases, an electron within the QD may occupy several states, thus, a QD may also represent a byte of data, instead of a bit (in case of qubit). Alternately, one QD might be simultaneously involved in more than one computations. In addition to these applications, other sophisticated uses of a QD may include nano-mechanics, neural networks, high density data storage, etc. Applications of Quantum Dots (QDs) a. Solar Cells One of the most important features of silicon of use in electronic devices (e.g. transistors) is doping. Wherein small quantities of various elements can be added to silicon in order to generate either an excess (n-type) or deficiency (p-type) of electrons in it, thereby enhancing the material’s conductivity. Additionally, p-n junction can be created by joining n-type and p-type silicon, which are the building blocks for almost all the devices in electronic industry. Figure 1 TEM images of InAs n-type QDs doped with silver, 3.3nm in diameter. Doping the QDs is a relatively new area of research, and both n-type and p-type QDs have been produced. Figure 1 shows the indium arsenide QDs which have been doped with silver to create n-type QD. Low cost solar cells can be realised by using QDs, as QDs can be easily prepared via simple and economic chemical processes. Additionally, thin-film photovoltaics can be prepared from QDs which have efficiencies comparable to those of traditional silicon cells. The enhancements in efficiency of QDs can be attributed to the different materials that can be used in their synthesis. Since some semiconductors can emit multiple electrons on absorbing one photon. In addition to this, the manipulation in their size and shape can result in absorption of light having different colours. However, the synthesis of efficient QD based solar cells has not yet been possible. For preparing solar cell, an n-type, and a p-type nanocrystals are required. When light is incident on a solar cell, electrons and holes are generated as light photons gets absorbed in the material. These electrons and holes must be separated so as to avoid their spontaneous recombination. The separation of these charge carriers results in the flow of electrons out of the semiconductor to the external electrical circuit. However, some of the electrons and holes recombine, this does not cause and electron (and thereby current) flow in the external circuit. This recombination is much pronounced in QDs than the large silicon crystals. Doping of the semiconductor nanocrystals for making p-n junction can separate the electrons and holes more efficiently. Conventionally, silicon is doped with P or B atoms, however, this cannot be done with the QDs because of their nanometer size. For comparison, a 4 nm QD comprises ~1000 atoms. Addition of few dopant atoms can result in expelling these atoms from the nanocrystal. Solar Cell Basics In a typical solar cell, the absorption of light results in the generation of electron-hole (e-h) pairs. When the e-h pair is bound, it is termed as exciton. An internal electric filed (present in p-n junctions or Schottky diodes) can separate this pair, leading to the flow of electrons and holes, which in turn cause an electric current. This internal field can be generated by differential doping of one part of the semiconductor with electron donating atoms (n-type doping), and doping the other part with electron accepting atoms (p- type doping). This results in the formation of a p-n junction within the semiconductor. The e-h pair is generated only when the energy of the absorbing photon in greater than the bandgap of the material. Thus, (a) the photons of lower energy than the bandgap are not absorbed, and (b) photons with higher energy than the bandgap are quickly (within ~10−13 seconds) thermalised to the band edges, decreasing output. Part (a) causes a reduction in current, and thermalisation decreases the voltage. Consequently, a trade-off exists between voltage and current in a semiconductor cell. This can be partially avoided by employing multiple junctions. Complete balance evaluations envisage a maximum efficiency of 31% for a monolithic solar cell. A little bit of mathematics can show that the maximum efficiency (i.e., 31%) corresponds to a bandgap of 1.3-1.4 eV, or the near infrared (NIR) spectrum. Silicon’s bandap (1.1 eV) is closest to this value, and this is one of the key factor in the predominance of silicon in solar cell applications. Notwithstanding this advantage, the maximum efficiency achievable with silicon is ~29%. This value can be enhanced via ‘tandem’ or, multi-junction approach, wherein multiple single-junction cells of different bandgaps are vertically stacked. As an example, a 2-layer cell should have one layer of 1.64 eV and second with 0.94 eV, resulting in maximum efficiency of 44%. Similarly, a 3-layered (1.83, 1.16, 0.71 eV) gives the value of 48%. Theoretical calculations have predicted the maximum efficiency of ~86% for an infinite layered cell, where the thermodynamic losses limit the efficiency to this value. What About Quantum Dots QDs are semiconductor particles whose dimensions are less than the exciton Bohr radius, resulting in quantum confinement effect being dominant in deciding their properties. Due to the confinement effects, the electron energy levels become finite. The situation is similar to an atom, and that is why, QDs are often termed as ‘artificial atoms’. By adjusting the size and shape of the QDs, these energy levels can be tuned, which then defines the bandgap. QDs can be made with different sizes, thereby changing their bandgap without any modification in the underlying material or the synthesis processes. Ideally, the size of the dot can be changed by simply controlling the synthesis time and temperature. Owing to the tunable bandgap, QDs are interesting for solar cell applications. Single junction cells of PbS (lead sulfide) colloidal QDs demonstrate bandgaps in the range of far infrared, which is difficult to obtain in traditional cells. Since the maximum portion of the solar energy reaching the Earth is in infrared and near infrared regions, a QD, owing to its bandgap in the right spectrum can make use of this enormous energy. Furthermore, colloidal QDs can be easily synthesised. Being suspended in a colloidal liquid, they are easily handled throughout production process. Colloidal QDs can be produced in large amounts and the dots can be spread on desired substrate via spin coating. Dye Sensitized Solar Cell (DSSC) These are the most recent cell design, wherein a sponge like layer of TiO2 is used as semiconductor value and also provides mechanical support. During construction, an organic dye (e.g., ruthenium-polypyridine) is filled in the sponge which injects electrons into TiO2 upon photoexcitation. Since ruthenium is a rare metal, the cost of the dye is high. Since the discovery of DSSC, the use of QDs as a substitute to the organic dyes has been considered. Owing to the possibility of tuning the bandgap, other design parameters (such as material sued for other parts of the cell) can also be changed. For instance, the researchers have developed a cell wherein the rear electrode is in contact with a film of QDs, thereby eliminating electrolyte, and resulting in a depleted heterojunction. The cell had an efficiency of 7.0%, which is superior to solid-state DSSC devices, but less than devices using liquid electrolytes. Multi-junction In cells absorbing multiple frequencies, cadmium telluride (CdTe) is usually used. A colloidal suspension of CdTe crystals in spin coated over a substrate (e.g., glass slide). Such cells do not employ QDs, since low scale production of QDs is expensive. Hot-Carrier Capture The efficiency of the cell can also be improved by capturing the surplus energy of the electron when emitted from a single bandgap material. In materials like silicon, the emission site is at a fair distance from the electrodes (where electrons are collected); electrons interact with he crystal lattice, dissipating their surplus energy as heat. So the electrons do not have extra energy. For increasing these interactions, amorphous silicon was studied, however, the large amount of defects inherent to amorphous materials suppressed their advantages. Modern thin-film cells have less efficiency than conventional silicon cells. To alleviate these problems due to defects, thin uniform films can be prepared from nanostructured donating species. However, these structures suffer from other issues inherent to QDs, e.g., high resistivity, heat retention. Multiple Excitons Researchers in 2004 reported that in QDs, multiple excitons can be created by absorption of one photon.