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Lab Authors' Paper in Nano Letters Draws Strong Interest

October 15, 2007

A research paper showing how zinc oxide can be manipulated to become a good material for photovoltaic devices was among the most-accessed paper published by Nano Letters in the third quarter.

The paper, written by Joshua Schrier, Denis Demchenko and Lin-Wang Wang in CRD’s Scientific Computing Group, laid out the calculations that narrowed the band gap – the energy difference between the top of the valence band and the bottom of the conduction band – of zinc oxide. To effectively absorb light, materials must have band gaps which match the distribution of photons that pass through the earth’s atmosphere, from the sun.

Nano Letters, published by the American Chemical Society, recently unveiled the 20 most downloaded or viewed papers from July through September. Schrier’s paper, “Optical Properties of ZnO/ZnS and ZnO/ZnTe Heterostructures for Photovoltaic Applications,” ranked No. 10 and was also co-authored by Paul Alivisatos, head of the Material Science Division at Berkeley Lab.

The high cost of the conventional photovoltaic material, silicon, and the scant alternatives for building solar energy equipment for the mass market have prompted scientists to explore nanostructure devices. Previous experimental work at Berkeley Lab by Alivisatos and co-workers has demonstrated how semiconductor nanocrystals can be used to make inexpensive devices. However, these nanocrystals have typically been made from materials such as cadmium selenide (CdSe) and cadmium telluride (CdTe), which contain expensive and toxic elements.

Schrier and his collaborators turned to zinc oxide (ZnO), an abundant, non-toxic and stable compound. ZnO alone isn’t an ideal material because it has a large band gap of 3.4 electron volt (eV). But stacked with another material, it could create a composite structure with a much smaller band gap, the researchers wrote, and they tested their theory with zinc sulfide (ZnS) and zinc telluride (ZnTe).

Using resources at the National Energy Research Scientific Computing (NERSC) center, the scientists calculated the optical properties of ZnO/ZnS and ZnO/ZnTe using plane-wave norm-conserving pseudopotential density function theory (DFT) with PETot code. They include a band-corrected pseudopotential scheme because DFT is inadequate for calculating the value of band gap.

They studied the composite materials in two forms: planar superlattices and core/shell quantum wires. The researchers considered a superlattice model composed of ZnO/ZnS and another composed of ZnO/ZnTe. They also examined whether applying strain to either superlattice models could further narrow its band gap. With the quantum wires, the scientists only considered the structure composed of ZnO/ZnS.

Results of their work demonstrated significant reductions of the band gap for using both composite materials and structures, compared with using bulk ZnO alone. Strained superlattices showed a further reduction.

Moreover, the ZnO/ZnS core/shell nanowires proved to be a better structure than superlattices for potential use in photovoltaic devices. “The ZnO/ZnS core/shell nanowire improves upon both the band gap and oscillator strength of its superlattice counterpart,” the researchers noted in the paper.

Using the Shockley-Quiesser model for calculating the idea solar cell efficiency, Schrier and his fellow researchers obtained an efficiency limit of 19 percent for the ZnO/ZnS superlattice, 30 percent for the ZnO/ZnTe superlattice and 23 percent for the ZnO/ZnS core/shell quantum wires. The bulk ZnO alone, on the other hand, could only yield a limit of 7 percent.