Photodetectors are ubiquitous in life, such as cameras, mobile phones, remote control devices, solar panels, and even the spacecraft's panels. With only micrometers of thickness, these small devices can convert light into electrical energy and then generate electrical signals. Since the invention of the photodetector, increasing its photoelectric conversion efficiency has become one of the primary goals for making photodetectors. In recent days, physicists at the University of California, Rivers and Bays have developed a new type of photodetector that can subvert the existing collection of solar energy by mixing two different inorganic materials and then creating a quantum-mechanical process. The research results were published in Nature Nanotechnology. The team's researchers stacked two atomic layers of tungsten selenide (WSe2) on the monoatomic layer of molybdenum diselenide (MoSe2). The result of this stacking is very different from the existing layer structure characteristics, allowing researchers to customize and operate electronic engineering at the thinnest scale. In an atom, the state of the electrons determines their energy level. When electrons move from one state to another, it will absorb or release energy. When the electrons are above a certain energy level, the electrons can move freely. When electrons move to a low energy level, the emitted energy is enough to loosen another electron. The researchers observed that when a photon strikes the WSe2 layer, it will loosen an electron and let it move freely in the WSe2 layer. At the junction of WSe2 and MoSe2, this free electron will fall to the MoSe2 layer. The energy released by the electrons during the drop will "kick" the other electron from the WSe2 layer to the MoSe2 layer, eventually obtaining two free electrons and generating electricity. Figure | Energy level diagram of the WSe2-MoSe2 device. A photon (1) incident on the WSe2 layer will kick out an electron (2), making it free to move on WSe2 (3). At the junction of the two materials, the electrons fall to MoSe2 (4). The energy released by the electrons during the drop will "kick" the other electron from the WSe2 layer (5) to the MoSe2 layer (6), eventually obtaining two free electrons and generating electricity. "What we observe is the emergence of a new phenomenon," said Nathaniel M. Gabor, research team leader and Assistant Professor of Physics. "Usually, when an electron leaps between energy levels, it loses energy. However, in our experiment, the lost energy leads to the production of another free electron, which doubles the original efficiency. This principle combines more than The improved design of the theoretical efficiency limit will have an extensive impact on the design of new ultra-high precision optoelectronic devices in the future." "The electrons originally excited by photons on the WSe2 layer have energy that is lower in the WSe2 layer." Gotemeh Barati, a graduate student in the Gabor Quantum Materials and Optoelectronics Laboratory and one of the paper's co-first authors, said. “By applying a small electric field, this electron will be transferred to MoSe2, at this time the energy of the electron is high on this new layer, which means that it can release part of the energy. This part of the released energy is consumed in the form of momentum. Disperse and simultaneously 'kick' the other electron in WSe2." In the existing solar panel model, one photon can generate at most one electron. However, in a prototype device developed by these researchers, a photon can generate two or more electrons through a so-called electron multiplication technique. The researchers explained that in the smallest materials, the volatility of the electrons is reflected. Although this is macroscopically difficult to understand, it is entirely possible for one photon to produce two electrons at its tiny scale. When a material, such as WSe2 or MoSe2, whose scale is close to the wavelength of electron fluctuations, its behavior begins to become unexplained, unpredictable, and mysterious. "It's like the wave is confined to the wall," Gabor said. “Quantum mechanically, this changes all the limitations. The combination of two different but ultra-small materials leads to a new doubling process, like 2+2=5!†"Ideally, on solar cells, we hope that light words can be transformed into multiple electrons," said Max Grossnick, another graduate student in the team and one of the paper's co-first authors. "Our paper shows that this is possible." Barati noted that by increasing the temperature of the device, more electrons will be produced. "We observed electron doubling when the device was at 340 Kelvin (150 degrees Fahrenheit, 67 degrees Celsius), above room temperature," she said. “And almost no material shows this phenomenon at room temperature. As we continue to increase the temperature, we can see more electrons.†In conventional optoelectronic devices, electron multiplication often requires the application of a high voltage of 10-100V. However, in this new type of device, the observation electron doubling only needs to apply 1.2V voltage, which is equivalent to a AA battery. “The low power consumption brought about by this low-voltage operation indicates the arrival of revolutionary changes in the design of photoelectric detectors and solar cell materials.†He explained that the efficiency of photovoltaic devices is determined by a simple competition. Light energy is converted into useless heat energy or useful electrical energy. "Ultra-thin materials can balance this competition and increase power while limiting heat production," he said. Gabor goes on to explain that the quantum mechanics phenomenon that his team discovered is similar to the phenomenon that cosmic rays pass through the atmosphere. When a high-energy cosmic ray comes in contact with Earth's atmosphere, a series of new particles will be produced. At the same time, he also believes that the team's findings will be applied in unknown areas. "Those materials that are only one atom thick are almost transparent," he said. “It is envisaged that we will see them attached to paintings in the future, or installed as solar cells on windows. Because these materials are very flexible, they can also be expected to be integrated with fabrics and applied to wearables. Photovoltaic devices may not appear to produce energy-generating clothing in the future, making the energy harvesting technology invisible." 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