Domestic researchers have identified the bulk photovoltaic effect that occurs when two-dimensional semiconductor materials such as tungsten disulfide are rolled up, raising expectations for the development of new semiconductors, such as new-concept solar cells and infrared sensors that operate without charging.

Professor Park No-jeong's team at UNIST and Professor Kim Jeong-woo of Incheon National University announced on the 27th that they have identified the cause of the increase in bulk photocurrent in multilayer nanotube-shaped tungsten disulfide.

In addition, based on this, a material structure capable of increasing the magnitude of the photocurrent flowing through this semiconductor nanotube by more than 2.5 times was also theoretically proposed.

According to the analysis results, the cause of the increase in photocurrent is charge transport resulting from the multilayer structure of the nanotubes. This multilayer nanotube consists of flat tungsten disulfide rolled up to form a structure similar to tree rings.

In addition, theoretical simulation results were presented showing that if one sulfur (S) atom of tungsten disulfide (WS2) is replaced with selenium (Se), charge transfer between the walls of the multilayer nanotube becomes more active, and this effect can be increased by 2.5 times.

The research team predicts that this discovery will also help in developing new concept solar cell materials to overcome the theoretical efficiency limits of solar cells.

In the field of solar cells, the Shockley-Queisser law—which states that the efficiency of converting sunlight into electricity can never exceed 33.7%—has been absolute. This is due to the limitations of the pn junction used to apply a potential difference. In pn junction solar cells, increasing the current reduces the voltage, and increasing the voltage reduces the current. On the other hand, the bulk photovoltaic effect makes it possible to increase both the current and the voltage simultaneously.

The research team explained, “This achievement is thanks to our ability to perform density functional calculations, which can accurately analyze the electronic structure inside semiconductor materials that change in real time as they absorb light,” adding, “It will be helpful in developing novel solar cells, infrared sensors that operate without charging, and next-generation semiconductor devices.”

This research involved the participation of researcher Bum-Seop Kim from the Department of Physics at UNIST. The research results were published on June 10 in Nature Communications, a prestigious multidisciplinary journal. The research was conducted with support from the National Research Foundation of Korea.