To understand the secret of perovskite solar cells’ high performance, we have to talk about the microscopic mechanism of their light absorption and energy conversion. The curious process goes something like this: photons from the sun hit the absorption layer of the battery and are immediately absorbed. The energy of the photons stimulates electrons that were previously bound around the nucleus to form free electrons (e-). Since matter as a whole must remain electrically neutral, electrons excited simultaneously produce an additional positively charged counterpart, known in physics as a hole (h+).
Such pairs of electron-hole pairs are often referred to as excitons in the scientific literature. Because electrons have a negative charge on one hole and a positive charge on the other, they have electrostatic attraction to each other. In some materials, such as organic solar cells, excitons have a high binding energy of about 400 millielectron volts, which requires a strong built-in electric field to separate excitons to form independent electrons and holes. Perovskite cells have exciton binding energies of only 50 to 70 millielectron volts, which means that at room temperature the electron-hole pairs can be easily separated within the material.
Excitons are separated into electrons and holes and transported to the cathode and anode respectively. The negatively charged free electrons are transferred in the titanium dioxide (TiO2) layer and reach the transparent electrode, and then reach the metal electrode through the external circuit, as shown in figure 2. The positively charged hole diffuses to the hole transport layer and eventually reaches the metal electrode. At this point, the holes are combined with electrons, and the current forms a loop to complete the transportation of electric energy. Perovskite solar cells separate the light absorption process from the current transport process, and only one kind of charge is transported in one medium, which avoids the disadvantages of high photoelectric carrier compound rate and short carrier life in silicon-based and thin-film solar cells. Therefore, perovskite solar cells have high photoelectric conversion efficiency.
As a direct band-gap material, hybrid perovskite not only greatly reduces the light absorption layer, but also has a strong light absorption ability. Specific for photovoltaic devices needed for the absorption layer thickness, the first generation of solar cells and the second generation of solar cells, respectively, need about 300 microns and 2 microns thickness, and represented by perovskite light absorption layer in less than 0.4 microns (about the thickness of the A4 paper 220) membrane layer, can get more than 20% of the photoelectric conversion efficiency. Moreover, its light absorption coefficient is very large, and its light absorption ability is an order of magnitude higher than that of traditional dyes. It has a good absorption ability for almost all photons from ultraviolet to near-infrared.
We have also noticed that perovskite solar cell is a ternary component material. There are three elements to choose from at each position of ABX. Unlike crystal silicon, only p-type silicon or n-type silicon can be mixed with impurity atoms.
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