Quantum dots amplify solar cell output

Quantum dots are already being used commercially to boost the output and expand the colour range of ultra-high-definition televisions. Quantum dots, however, can also be used to absorb light to boost the output of photovoltaics, photocatalysts, light sensors, and other optoelectronic devices according to Brookhaven National Laboratory (Upton, N.Y.).

"Our particular 2-D material system (SnS2) [tin disulfide] is similar to Si [silicon], in that it has an indirect band gap, not providing sufficient electroluminescence for light-emitting diodes, but there are other 2-D materials with high luminescence such as MoS2 [molybdenum sulfide] WS2 [tungsten disulfide] which can be used in view of the Qdot-2D hybrid architectures for LEDs," Mircea Cotlet, the physical chemist who led this work at Brookhaven Lab's Center for Functional Nanomaterials (CFN ), a Department of Energy (DoE) Office of Science User Facility, told EE Times in an exclusive interview.

Figure 1: Research team showing off their quantum-dot doped nanomaterial (back to front: Chang-Yong Nam and Mircea Cotlet of Brookhaven Lab's Center for Functional Nanomaterials with Stony Brook University graduate students Prahlad Routh and Jia-Shiang Chen). (Source: Brookhaven)

Instead of emitters, Brookhaven concentrated on receivers of optical energy in layered metal dichalcogenide semiconductors—dual chalcogen anions (ions with more electrons than protons) plus one or more electropositive elements usually sulfides, selenides, and tellurides, rather than oxides—in Brookhaven's case sulfide). Brookhaven did the work with Stony Brook University (including doctoral candidate Prahlad Routh) and the University of Nebraska.

The researchers call their material a hybrid because they dope the electrical conductivity of layered tin disulfide semiconductor with the light harvesting of different spectrums of light from various sized quantum dots. In more detail, the quantum dots absorb the frequency of light corresponding to its diameter, then transfer their energy to the tin disulfide semiconductor. As a result, they prove to be a good candidate for solar light conversion and other similar light sensors.

Figure 2: Single nanocrystal spectroscopy identifies the interaction between zero-dimensional CdSe/ZnS nano crystals (quantum dots) and two-dimensional layered tin disulfide, whose strength increases with increasing number of tin disulfide layers. (Source: Brookhaven)

Two-dimensional tin disulfide also has a high surface-to-volume aspect ratio, unfortunately its light absorption needed to be boosted by the quantum dots to make commercially feasible light harvesters even feasible. But by adding the quantum dots, and observing the boost of 500 per cent in light conversion to electricity as a result, convinced the researchers they were onto something good.

"This energy transfer is the process that dramatically increased in the light absorption of our 2-D tin sulfide material," Cotlet told EE Times.

In the lab, the researchers found that increasing rather number of tin disulfide layers likewise improved the performance of the material, and proved the concept by successfully building a photo-FET (field effect transistor).

Next the researchers are going to attempt to optimize the cheap grown of their material using chemical vapour deposition, before attempting to commercialize the process.

The research is being funded by the DoE's Office of Science.

- R. Colin Johnson
  EE Times