Quantum dot (Quantum Dot, QD) refers to nanocrystalline grains with a size around tens of nanometers, which are bound in the three-dimensional barrier layer, and the electrons and holes in the quantum dot are also bound in the barrier layer. Therefore, its All parties are quantum; QDs are also called artificial atoms due to their similarity to the structure of atomic energy levels. Low-dimensional quantum structures have many excellent properties and broad application prospects, and are a hot spot in semiconductor physics research. The successful preparation of low-dimensional semiconductor materials such as semiconductor superlattices, quantum wells, quantum wires, and quantum dots has greatly promoted the development of basic research on semiconductor physics and applied research on semiconductor devices.
(1) Advantages of quantum dots applied to solar cells Because low-dimensional materials such as quantum wells and quantum dots can change the absorption wavelength of devices such as lasers and light-emitting diodes, this principle can be introduced into quantum dot solar cells. Quantum dots used in solar cells have distinct advantages.
①Adjustable light absorption range. Because in the high-density quantum dot group, small-sized quantum dots can absorb high-energy photons, and large-sized quantum dots can absorb low-energy photons.
②Increase the energy transition time, and the thermionic electrons can be used, which provides the possibility for the production of thermionic solar cells.
③ The collection of photogenerated carriers by the solar cell can be improved through the resonant tunneling effect, thereby effectively increasing the short-circuit current of the solar cell. With the introduction of new concept solar cells, people’s enthusiasm for quantum dot solar cells is also getting higher and higher.
The concept of quantum dot solar cells is based on improving the conversion efficiency of cells, that is, increasing the photovoltage and increasing the photocurrent. Semiconductor solar cells of conventional bulk materials severely limit the thermalization of photogenerated carriers in the lattice, and a large portion of the energy above the band gap is lost due to the thermalization of carriers, while the size-limiting effect of quantum dots makes them have The discrete energy values can alleviate thermal scattering of carriers, while also increasing the probability of collisional ionization, generating more electron-hole pairs. By adjusting the energy band of quantum dots, the range of light absorption can be expanded and the photogenerated current can be increased.
(2) The structure of quantum dot solar cells The size of quantum dots can be compared with the de Broglie wavelength of electrons or the mean free path of electrons, and they have quantized energy levels. Designing the quantum dot structure in the i region of the p-i-n structure cell is equivalent to introducing an extra energy level in the forbidden band of the material, so that the transition of the solar cell’s absorption of light energy is transformed into three energy transitions, and the solar cell is realized. Better utilization of the spectrum. Moreover, the mini-bands (microstrips) generated by the coupling of multiple quantum dots also make it possible to transport carriers through the microstrips.
In 2001, V. Aroutiounian et al. of Yerevan National University in Armenia proposed a theoretical model of quantum dot solar cells, and calculated that the self-organized InAs quantum dot multilayer structure introduced GaAs cells, which can significantly enhance the conversion efficiency of cells. to set up multiple quantum dot layers in the intrinsic region of the p-i-n cell, and generate additional photo-generated current through the more absorption of sunlight by the quantum dots. Due to the strong electronic coupling between quantum dots, an intermediate band or a superlattice microstrip can be formed, so that the photogenerated carriers in the quantum dots can reach the P region and N region of the cell through tunneling, thereby being effectively used. collected to form a photocurrent.