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Nanomaterials geared to advance efficiency of PV cells

Posted: 25 Nov 2013     Print Version  Bookmark and Share

Keywords:solar cell  quantum dot  nanosphere 

Researchers at the A*STAR Institute of Materials Research and Engineering, the National University of Singapore and Nanyang Technological University, Singapore have developed a method for using nanostructures to increase the fraction of incoming light that is absorbed by a light-harvesting material. The method is ideal for use with high-efficiency solar cells.

Specially designed nanostructured materials can increase the light-absorbing efficiency of solar cells

The Sun is our most promising source of clean and renewable energy. The energy that reaches the Earth from the Sun in an hour is almost equivalent to that consumed by humans over a year. Solar cells can tap this massive source of energy by converting light into an electrical current. However, these devices still require significant improvements in efficiency before they can compete with more traditional energy sources.

Solar cells absorb packets of optical energy called photons and then use the photons to generate electrons. The energy of some photons from the Sun, however, is too small to create electrons in this way and so is lost. Xiaogang Liu, Alfred Ling Yoong Tok and their co-workers circumvented this loss using an effect known as up-conversion. In this process, two low-energy photons are combined to produce a single high-energy photon. This energetic photon can then be absorbed by the active region of the solar cell.

The researchers' device comprised a titanium oxide frame filled with a regular arrangement of air pores roughly half a micrometre across—a structure called an inverse opal. Spheres of the up-conversion material, which were 30nm in diameter, sat on the surface of these pores. Tiny light-sensitive quantum dots made of crystals of cadmium selenide coated these nanospheres.

The quantum dots efficiently absorbed incoming light, either directly from an external source or from unconverted photons from the nanospheres, and converted it to electrons. This charge then flowed into the titanium oxide frame. "The titanium oxide inverse opal creates a continuous electron-conducting pathway and provides a large interfacial surface area to support the up-conversion nanoparticles and the quantum dots," explained Liu.

Liu, Tok and the team tested the device by firing laser light at it with a wavelength of 980nm, which is not normally absorbed by cadmium selenide quantum dots. As expected, they were able to measure a much higher electrical current than the same experiment performed with a device without the up-conversion nanospheres. "We believe that the enhanced energy transfer and light harvesting may afford a highly competitive advantage over conventional silicon solar cells," added Liu.

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