Researchers from King Abdullah University of Science and Technology (KAUST) in Saudi Arabia, in collaboration with fellow scientists from Taiwan and Japan, have laterally placed single layers of the semiconducting materials tungsten selenide and molybdenum sulfide to form atomically sharp monolayer junctions between semiconductors.
The new approach could help to mitigate contaminants and defects that can occur at the interfaces of vertically stacked layers in ultrathin electronic devices.
2D planar monolayer design
“We report solar cells based on an atomically sharp 2D monolayer WSe2-MoS2 p-n lateral heterojunction synthesized by two-step epitaxial growth,” says Jr-Hau He, associate professor of electrical engineering at KAUST. “The planar structure of the cells results in unprecedented omnidirectional light harvesting behavior, with merely 5% loss of PCE at high angles of incidence (AOIs) of 75°.” The team achieved power conversion efficiencies (PCE) up to 2.56% under AM 1.5 G illumination.
Most 2D solar cells reported recently are designed with two types of 2D materials stacked on top of one another. KAUST’s cell one the other hand features planar monolayer junctions. “This design is the first one using lateral heterojunction 2D monolayer materials for solar cell applications,” confirms the professor.
Ultrathin electronic devices that feature vertically stacked layers of semiconductors require complex manufacturing techniques, yet contaminants and defects can still sneak in between the different layers. “We overcome the problem of contaminants, quenching centers and alloy structures at the interface of the 2D heterojunction,” He notes, adding that they have also improved the omnidirectionality of the solar cell thanks to their new design.
Ultimately 5% conversion efficiency expected
“The efficiency of our device is 2.56%, which is also the highest efficiency we achieved so far in the lab,” He reports. “Based on further improvement of electrode and doping designs, a conversion efficiency above 5% can be expected.”
Key applications for this new type of atomically thin solar cell could be self-powered nanoelectronics, ultrathin electronic devices and transparent devices, such as flexible or transparent displays.
Designing solar cells of the future
The research advance reported by KAUST could have remarkable impact on the design of future generations of solar cells and on the way we use solar energy. “With high omnidirectionality, expensive solar tracking systems can be removed in the future,” agrees He. “By connecting our devices in parallel or stacked in 3D configuration, it is possible to convert comparable or even more power density per unit mass than conventional solar cells.”
He and his team are now trying to better understand the underlying kinetics and thermodynamics of 2D lateral heterojunctions in order to design more efficient cells.
This research work is reported in the article “Single atomically sharp lateral monolayer p-n heterojunction solar cells with extraordinarily high power conversion efficiency,” published in Advanced Materials.
Written by Sandra Henderson, Solar Novus Today