What Are Quantum Dot Solar Cells?

In semiconductor materials, tiny crystals are often called quantum dots. This kind of quantum dot can lock the electrons in a very tiny three-dimensional space. When a beam of light shines on the electrons, the electrons will be excited to jump to higher energy levels. When these electrons return to the original lower energy level, they will emit a beam of a certain wavelength.

Quantum dots (QDs) are made of cadmium selenide (CdSe), cadmium telluride (CdTe), cadmium sulfide (CdS), zinc selenide (ZnSe), indium phosphide (InP) or indium arsenide (InAs), Or the CdSe core is covered with a layer of ZnS or CdS nanocrystals or semiconductor nanocrystals. When a semiconductor material absorbs a photon containing energy greater than its band gap, an electron is excited from the valence electron band to the conduction band, leaving a positively charged hole to form an electron-hole pair and one electron -The hole pair is called 1 exciton. Excitons are like artificial atoms with a radius of 1 to 10 nm, and their size depends on the nature of the semiconductor. Because the size of the semiconductor crystal and the size of the exciton are similar, the strong quantum confinement effect changes the characteristics of the exciton. As the crystal size decreases, the exciton operates more like a particle in a box. Its energy level is mainly determined by the size of the particles (boxes), not the nature of the entire semiconductor. Such semiconductor nanocrystals, which show a strong quantum confinement effect in three dimensions, are called quantum dots. The combination of electrons and holes generates light, and the length of the light wave is mainly determined by measuring the size of the quantum dots. The existing technology can prepare quantum dots with a particularly uniform size, and the width of the light band generated by the dots is 10-50 nm ^[1]

Quantum dots can be combined with specific antibodies or small molecules. Without changing their chemical characteristics, they can emit fluorescence with a specific wavelength after being excited by a light source to achieve target identification and detection. The combination of quantum dots and biological macromolecules such as nucleic acids, proteins, nutrient carriers, etc., usually have the following methods: electrostatic attraction method, conventional cross-linker connection method and biotin-avidin method, etc. ^[1]

Quantum dot markers can induce the generation of oxygen free radicals, so they are toxic to biologically active substances, but these toxicities can be reduced by coupling to protein molecules or by coating a layer of low-toxic substances. The combination of quantum dots and some proteins may cause the fluorescence of quantum dots to weaken or quench, such as copper / zinc-superoxide dismutase, which has a significant quenching effect on the fluorescence of CdSe quantum dots. If quantum dot marking is used in the preparation of test strips, an ultraviolet light source is needed for the observation of color changes during the use of the test strips. Compared with other types of test strips that can judge the detection results with naked eyes, the operation procedure is slightly more complicated ^[1]

Quantum dots are now widely used in biological laboratories to help researchers determine the structure or activity of biological cells. When quantum dots are irradiated with light pulses, they can produce a variety of colors, which can be observed by less advanced optical microscopes. If quantum dots are attached to the object to be studied, researchers can understand the movement of matter. Not only that, quantum dots can also be used to track the activity of drugs in the body, or to study the structure of cells and tissues in patients. Quantum dots can produce multiple colors of light, and the color of light depends on the size of the quantum dots. Researchers have created quantum dots that can produce more than 12 colors of fluorescence, and in theory can produce more colors. In this way, when a plurality of quantum dots are irradiated with laser light of a certain wavelength, multiple colors can be observed simultaneously and multiple measurements can be performed simultaneously. The quantum dots used in biological research need to be covered with a layer of material so that specific biological molecules can be tracked, which can be used in medical imaging technology. Foreign scientists have used quantum dots to label tumor cells for related research with live imaging systems.