This illustration shows solar power conversion using a lateral WSe2-MoS2 heterojunction. Image: © 2017 WILEY VCH.
This illustration shows solar power conversion using a lateral WSe2-MoS2 heterojunction. Image: © 2017 WILEY VCH.

Super-thin photovoltaic devices underpin solar technology, and so efficient ways to produce them are keenly sought. Researchers at King Abdullah University of Science & Technology (KAUST) in Saudi Arabia have now combined and rearranged different semiconductors to create so-called lateral p-n heterojunctions. They hope this simple process, reported in a paper in Advanced Materials, will transform the fabrication of solar cells and self-powered nanoelectronics, as well as ultrathin, transparent, flexible devices.

Two-dimensional (2D) semiconductor monolayers, such as graphene and transition-metal dichalcogenides like tungsten diselenide (WSe2) and molybdenum disulphide (MoS2), have unique electrical and optical properties that make them potential alternatives to conventional silicon-based materials. Recent advances in material growth and transfer techniques have allowed scientists to manipulate these monolayers. Specifically, vertical stacking of the monolayers has led to ultrathin photovoltaic devices, but requires multiple complex transfer steps. These steps are hampered by various issues, such as the formation of contaminants and defects at the monolayer interface, which limit device quality.

"Devices obtained using these transfer techniques are usually unstable and vary from sample to sample," says Meng-Lin Tsai, lead researcher and a former visiting student of KAUST’s Jr-Hau He. Tsai adds that transfer-related contaminants significantly affect device reliability, while electronic properties have also proven difficult to control by vertical stacking.

To fully harness the exceptional properties of these 2D materials, Tsai's team, under the mentorship of He, joined them together horizontally rather than vertically to create monolayers featuring lateral WSe2-MoS2 heterojunctions, which they incorporated into solar cells. Under simulated sunlight, the cells achieved a greater power conversion efficiency than their vertically stacked equivalents.

The researchers synthesized the lateral heterojunctions by consecutively depositing WSe2 and MoS2 on a sapphire substrate. Next, they transferred the materials onto a silicon-based surface for photovoltaic device fabrication.

High-resolution microscopy revealed that the lateral junction displayed a clear separation between the semiconductors at the interface. Also, the researchers detected no discernable height difference between the two semiconductor regions, consistent with an atomically thin interface.

These interfacial characteristics signaled success. "Our structures are cleaner and more ideal than vertically stacked assemblies because we didn't need the multi-step transfer procedure," explains Tsai.

Furthermore, the lateral heterojunctions mostly retained their efficiency despite changes to the orientation of the incident light. Being able to accept light coming from any direction means expensive solar tracking systems will not be needed.

According to Tsai, the implementation of lateral heterojunctions in more complex circuits and interconnects may result in higher performance than possible with conventional solar cells and so the team is working on the next steps. "We are trying to understand the underlying kinetics and thermodynamics of these heterojunctions to design more efficient cells," he adds.

This story is adapted from material from KAUST, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.