Researchers: Two types of solar-cell materials are better than one

Jan. 17, 2008 – Combining two methods for making solar cell materials appears to yield better results than either one alone, according to researchers from the U. of California/Santa Cruz, China, and Mexico who say their nanocomposite thin film doped with nitrogen and sensitized with quantum dots performs “better than predicted.”

The work combines two methods used to engineer solar cell materials: doping thin films of metal oxide nanoparticles (e.g. titanium dioxide, with other elements e.g. nitrogen); and quantum dots, which inject electrons into a metal oxide film to increase its solar energy conversion. Both doping and quantum dot sensitization extend visible light absorption of the metal oxide materials.

The group, led by U.CA/SC prof. Jin Zhang, prepared films with thicknesses of 150-1110nm, with titanium dioxide particles (average size 100nm), doped the lattice with nitrogen atoms, and chemically linked CdSe quantum dots for sensitization. The resulting hybrid material offered nitrogen doping to absorb a broad range of light energy (including energy from the visible region of the electromagnetic spectrum), while the quantum dots enhanced visible light absorption and boosted the material’s photocurrent and power conversion.

“We initially thought that the best we might do is get results as good as the sum of the two, and maybe if we didn’t make this right, we’d get something worse,” said Zhang, in a statement. “But surprisingly, these materials were much better.” In fact, the nanocomposite achieved an “incident photon to current conversion efficiency” (IPCE) as much as 3x greater than the sum of the IPCE for materials developed separately with either method (doped with nitrogen or embedded with CdSe quantum dots). Zhang explained that it may be easier for the charge to “hop around” in the nanocomposite material, with the quantum dot sensitizing and the nitrogen doping at the same time.

Next up in the work is optimizing parameters with three materials “that we can play with to make the energy levels just right,” Zhang stated. “What we’re doing is essentially ‘band-gap engineering.’ We’re manipulating the energy levels of the nanocomposite material so the electrons can work more efficiently for electricity generation,” he said. “If our model is correct, we’re making a good case for this kind of strategy.”

The work is being funded by the US Department of Energy, the National Science Foundation of China, and the U. of California Institute for Mexico and the United States (UC-MEXUS).


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