Researchers at University of Tsukuba demonstrate precise mechanism by which sunlight generates electrical charge within cutting-edge solar cell film, yielding potential benefits for solar power industry

Tsukuba, Japan—The details of how charge carriers form within a solar cell and then generate an electric current have remained unclear. A breakthrough by a trio of researchers at the University of Tsukuba (Ibaraki, Japan), as reported in Applied Physics Express, helps clarify this. The group's study has provided detailed insight into how energy in the form of sunlight is absorbed by a layer of state-of-the-art material used in solar cells, and then converted into a form usable for generating electricity.

The study builds on earlier work on a specific type of solar cell. These solar cells leverage the different properties of materials arranged in adjacent layers for generating an electrical charge. When the packets of energy or photons (of which sunlight consists) strike these layers in solar cells, they free up or energize electrons from electron-donating segments, which can then move to electron-accepting segments. This produces a flow of electrons that forms the basis of a current. Enhancing efficiency in converting sunlight into electricity requires clarifying exactly what happens at an extremely small scale within the materials in solar cells. To date, this had proven to be a difficult task.

"The key to understanding our breakthrough is the concept of an exciton, which refers to an electron activated by the energy from sunlight and the 'gap' left behind by this activation," lead author Kohei Yonezawa says. "Using spectroscopy, we were able to clarify the carrier formation process in time domain. We were also able to determine temperature's lack of effect on this."
The latest findings should add to the established advantages of low-cost production and relatively high power-conversion efficiency of bulk heterojunctions within organic solar cells. Specifically, they could lead to further improvements in solar cells' energy efficiency, making this technology even more attractive to industry and the general public.

"We can now precisely analyze carrier formation process within solar cell films, which shows us the amount of time that these charge carriers take to form, and then to decay. This reveals the importance of locations near the interface of donor and acceptor regions for the charge carrier formation process," says coauthor Yutaka Moritomo, who hopes the insights obtained through the new approach will lead to further improvements in solar cell efficiency. "Now we know that excitons that have moved a long way to the interface between donor and acceptor regions do not contribute to charge generation, we should be able to develop new materials with even better charge generation."

The article "Temperature-Independent Carrier Formation Dynamics in Bulk Heterojunction" was published online at Applied Physics Express, at http://dx.doi.org/10.7567/APEX.8.112301

Original Paper

Kouhei Yonezawa, Takeshi Yasuda, and Yutaka Moritomo, Temperature-independent carrier formation dynamics in bulk heterojunction, Applied Physics Express, 8(11), DOI: 10.7567/APEX.8.112301(2015)

Laser pulses excite a blend of acceptor and donor materials. Changes in the infrared absorbance of the layer are measured to give insight into the behavior of excited donor (D*) and acceptor (A*) states at different temperatures.