Researchers at the University of California (UC) Riverside have discovered that the combination of two inorganic two-dimensional (2D) materials produces a quantum mechanical process that could significantly increase the efficiency of photodetectors, leading to revolutionary new ways of collecting solar energy.
In research described in the journal Nature Nanotechnology, the UC Riverside researchers have used the transition metal dichalcogenides tungsten diselenide and molybdenum diselenide to achieve the effect known as electron-hole multiplication. Electron multiplication involves making multiple electron-hole pairs for each incoming photon. This can dramatically increase the efficiency of a photovoltaic cell in converting light into electricity.
The 2D materials tungsten diselenide and molybdenum diselenide can help achieve this effect because both have energy levels that determine how electrons behave. In general, an electron (or any object) with high potential energy will move to a place of lower energy, and gain kinetic energy in the process. Like a ball rolling off the edge of a table or down a staircase, electrons will gain kinetic energy when there is a sudden lowering of potential energy.
“The energy levels for each material chosen in our study varies, and so forms an offset at the interface of the two materials,” explained Nathaniel Gabor, an assistant professor at UC Riverside, who led the research team, in an e-mail interview with IEEE Spectrum. “By combining selected materials, this energy offset forms a ‘staircase’ for an electron to step down, and subsequently gain kinetic energy. This offset also means that if you can get an electron in one material into a high-energy state (for instance, by using a particle of light, or photon) then it may gain kinetic energy.”
This highly energetic electron then drops down into lower energy levels of the second material, releasing its excess energy. In the analogy of a ball rolling off the table—when it hits the floor, it will give off some of its energy as sound. “In our system, this excess energy can instead generate another electron, and now there are two (or more) electrons that are free to conduct,” adds Gabor.
The actual devices that the UC Riverside researchers are fabricating with this combination of materials are photocells that are the single pixels of a larger photodetector array, such as a typical pixel array in a megapixel-scale SLR camera.
“Each of our devices is very small, but is the working element of a larger scale photodetector or photovoltaic array,” says Gabor. “For instance, in a digital camera, each pixel is a few microns (micrometers) on an edge, but many hundreds of microns thick. Our devices are also a few microns on an edge, but are only about as thick as 9 atoms stacked one on top of the other.”
While the researchers have fabricated devices, their work thus far has been aimed at demonstrating a new process. As a result, the team has not yet integrated these devices into a photovoltaic array.
Nonetheless, Gabor and his colleagues believe that the underlying mechanism is quite exciting since it came about by the combination of two separate materials. “In the future, engineers will likely develop photovoltaic cells that combine such materials, each of which can be specifically chosen to make the photovoltaic array more efficient,” he adds.
The results are promising with the devices exhibiting extremely low voltage operation in a photo-multiplying mode. When a voltage of about 1.2 V (the typical voltage in a household battery) is applied, the researchers observed a four-fold multiplication of the device current resulting from charge multiplication.
“In standard silicon, this process requires 10-100 Volts,” says Gabor. Since almost all commercial semiconductor products must operate at the 1 V level, our device proves to be a significant advance for such low power, high efficiency operation.”
In order for this technology to move to real-world applications, Gabor says they will need to develop a better understanding of how to synthesize the individual layers of these devices at a large scale using chemical vapor deposition methods. It will also require an optimized method to transfer one layer onto the other, and eventually cover them with a transparent, conducting, and flexible material such as graphene.
“Like the first transistor in the 20th century, the materials and processes going into our device prototype are new, and so each individual photocell is fabricated through a time-intensive process,” says Gabor. “A huge effort in the engineering community is currently underway to overcome some of these challenging problems, but lab-to-commercial realization may be accelerated given the potential for high efficiency device operation.”
For Gabor and his team, this initial research opens up the question of whether energy generation technologies can be made more efficient using devices that behave quantum mechanically.
Gabor adds: “We intend to explore more on the quantum optoelectronic properties of these devices, and look into ways in which understanding the quantum mechanical behavior of electrons allows us to solve very big problems.”
Dexter Johnson is a contributing editor at IEEE Spectrum, with a focus on nanotechnology.