Photoelectric devices that convert light energy into electrical energy play a vital role in clean energy technology. They usually need to be coupled with batteries that store and capture energy, but researchers have now built a device that combines photoelectric charge generation with charge storage. The excited electrons can be retained for at least one week until they are discharged as current. The team said that the device may be used for power generation, photodetectors, or light-based memory.
A good optoelectronic device contributes a charge carrier to the current almost every time it absorbs a photon; in other words, it has a high "external quantum efficiency" (EQE). The problem is that negatively charged electrons and positively charged holes produced by photons often recombine soon after they are produced. One way to increase the EQE of the device is to temporarily trap charge carriers before recombination occurs—for example, at crystal defects.
Yucheng Jiang of the University of Science and Technology of Suzhou in China and his colleagues began to use this strategy in a device called a van der Waals heterojunction, in which two materials maintain contact through a relatively weak van der Waals interaction. They use tungsten diselenide (a semiconductor material) and transparent conductor strontium titanium oxide (STO). On the surface of the STO, the team produced a nearly two-dimensional "electron gas" (a state in which electrons move freely and independently) through surface treatment. The use of electron gas as a component of this heterojunction is new and has led to new characteristics of the device.
The interface area between the two materials forms a so-called pn junction, which is a common structure in solar cells. Generally, photons generate electron-hole pairs that can be separated by voltage, although some inevitably recombine. Jiang and his colleagues hope that in their structure, recombination may occur more slowly than other pn photovoltaic devices. But to their surprise, they found that photogenerated charge carriers can last a long time. After irradiating their device with a blue laser and then storing it in the dark at 30 K for up to 7 days, when they connected it to the circuit, they could draw a large current (2.9 mA). The light-excited charge is stored without significant degradation, just like in a battery that can be charged and discharged at will. They named this new phenomenon a rechargeable photoconductor.
Researchers believe that trapping occurs in a part of the tungsten diselenide film, called the space charge zone, adjacent to the interface of the STO crystal. Here, photo-induced holes can accumulate and remain until a large enough applied voltage draws them into the circuit. Then the device is ready to be charged by light again. The EQE of the device is 93.8%, and many photovoltaic cells are considered "high-performance" if their EQE is greater than 50%. However, Jiang stated that the device needs to be cooled to around 30 K to keep the stored charge stable, which may limit it to certain applications unless improvements with other materials can significantly increase the operating temperature.
Heterojunction devices can also act as optical storage. Information will be input and stored by light pulses until it is read out as current pulses. "We don't know what the limit of this stability is, but we think the storage life is almost unlimited," Jiang said.
Other layered materials have previously been used to fabricate heterojunction devices with photo-storage capabilities [2, 3]. But Jiang said that in these cases, the trapping process is not easy to turn on and off because it involves "stronger" traps (defect locations in the crystal).
Sun Zhimei, a materials engineer at Beijing University of Aeronautics and Astronautics in China, said the observed effects may be important, but he needs to see more data to be sure that researchers fully understand the mechanism.
The paper "proposed a new device structure for photoelectric conversion and storage," said Mark Hersam, an expert on low-dimensional nanoelectronic materials at Northwestern University in Illinois. "As the field of van der Waals materials increasingly uses opportunities to form heterogeneous structures that include many other types of materials, this type of work may be further elaborated in the future," he said.
Philip Ball is a freelance science writer in London. His latest book is "Modern Mythology" (University of Chicago Press, 2021).
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