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Researchers have developed solar cells that harvest more electrons than one photon produces one electron.
Researchers have recently developed a new kind of solar cell that captures the extra energy that is usually lost in sunlight. To date, this new type of solar cell is still less efficient than commercial solar cells in converting sunlight into electricity. However, if this process is improved, it will pave the way for the development of a new generation of more efficient solar cells.
For most materials, the conversion of sunlight into electricity has been fully understood. Different colors of photons have different energies. In the visible region, red and orange have less energy, whereas blue, violet, and ultraviolet photons carry more energy. When high-energy photons come in contact with semiconductor materials in solar cells, they transfer this energy to the semiconductor electrons, thereby stimulating them from a standstill and forming a current. In many cases, the high-energy photons of violet and ultraviolet light carry more energy than the energy needed to form the current. But this extra energy is lost in the form of heat.
A few years ago, scientists from multiple research groups reported that high-energy photons in sunlight can actually excite more than one electron, provided that the semiconductor they encounter consists of nanoscale particles called quantum dots. This process, known as Multiple Excimer Generation (MEG), gives researchers hope to improve the efficiency of solar cells by collecting these extra charges. However, manufacturing MEG solar cells that can work is not an easy task.
Last year, a team led by Bruce Parkinson, a chemist at Wyoming State University in Laramie, USA, reported in the journal Science that they had developed a device that covered a semiconductor quantum lead sulfide quantum dot. It can excite more electrons than the number of photons it receives, resulting in greater currents, which is what characterizes the MEG. However, compared with a practical application of solar cells, this device is more proof of the concept, because its conversion efficiency is too low.
Now, a team led by chemist Arthur Nozik of the National Renewable Energy Laboratory in Colorado reported that they have developed the first MEG solar cell that can work. Nozik said that the key to creating such a device is to think of a chemical synthesis method, followed by processing the quantum dots. At the time of synthesis, these quantum dots—consisting of lead and selenium particles about 5 nm in diameter—combine with long organic molecules. However, previous studies have shown that these long organic chains are like plastic insulators wrapped around wires.
So Nozik's team used their two colorless liquids, hydrazine and 1,2-ethanedithiol, to process their quantum dots so they were surrounded by short-chain organics. This allows the charge to move more easily and eventually brings the total efficiency of the solar cell to convert light to electricity to 5%. The research team reported on this research result in the latest issue of the journal Science. Although this efficiency is still lower than conventional silicon solar cells - about 20%, it is important that this device collects 30% more charge than photons hitting quantum dots, making it a real meaning. On the MEG solar cell.
Parkinson said: "They turned it into a real device and proved that it can collect real energy... This brings hope for the next generation of solar cell design." (Zhao Lu)
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