PORTLAND, Ore. A new shape for semiconductor nanocrystalstetrapods, rather than simple spheres, rods and diskscould double the efficiency of "plastic" solar cells, according to the inventor of tetrapods.
Paul Alivisatos, the inventor of semiconductor tetrapods and a professor at the University of California at Berkeley, said they promise to convert twice as much incident light into electricity. Tetrapods also promise to improve chemical sensors, biomedicine and optoelectronic devices, as well as serving as strengthening additives to plastic composites.
"We have been studying these materials [II-VI semiconductors like cadmium tellurium, CdTe] because we already take rodsshapes of it and put them inside plastic solar cellsthe rods are the light-absorbing materials inside these solar cells," said Alivisatos, Chancellor's Professor of Chemistry and Materials Science at Berkeley and director of the Materials Sciences Division at the Lawrence Berkeley National Laboratory.
"It turns out that tetrapods could be better for [plastic solar cell] application, because the rods need to always point in the same direction, and tetrapods will always do that. You just scatter them on a surface and they all point up."
The researchers said tetrapods could double the efficiency of plastic solar cells.
Tetrapods are shaped like schoolyard "jacks" since they come to rest on their three downward pointing legs. On closer inspection, the nucleus of a tetrapod is shaped like an upside-down pyramid with legs protruding from its four faces.
Nanocrystals are manufactured in only three simple shapesspheres, rods and disks. Many different types of semiconductor materials can be fabricated in these shapes. Alivisatos said he hopes his technique for growing crystalline tetrapods will advance the industry's
understanding, leading to broader use of the tetrapod shape for
chemical sensor applications, biomedical diagnostics and
Alivisatos said careful selection of the type of semiconductor used is critical since his tetrapod growing technique
depends on "polytypism," in which the crystal must have two different ways of packing atoms that are close to each other in energy. He used III-V, IV and II-VI semiconductors since all
have the advantage of nearby energy levels enabling any of
them to switch between atom-packing methods using the technique.
When energy levels are very close together, as in the CdTe semiconductor material, Ailivisatos found a chemically induced manner of switching between which structure is
more stable part-way through the nanocrystal's formation. For CdTe, the first packing method forms what chemists call a zinc-blender core (akind of nanoscale pyramid), later switching to the wurtzite-rod packing method, which grows the legs of the tetrapod.
"We form an initial packing that makes a pyramid, and then when the crystal gets bigger it switches to the other kind of packing, making the arms that come out from the faces of the pyramid to form the tetrapod," the researcher said.
The "trick" Alivisatos found to induce the nanocrystal to switch packing methods, was by binding a specific organic molecule (phosphonic acid) to the surface of the pyramid. In the presence of the phosphonic molecule, CdTe will switch from zinc-blender (pyramid building during "nucleation") to wurtzite-rod (leg formation) during its growth phase.
Alivisatos grew the legs to lengths of 50 nm or more and found that they formed according to the controllable kinetic
mechanisms previously observed while making nanorods,
namely that higher Cd/Te ratios resulted in longer arms with more light-gathering capability.
He also discovered that higher concentrations of phosphonic acid yielded larger arm diameter, which determines the bandgap of the material.
Alivisatos and his staff are attempting to embed tetrapods into hybrid nanocrystal--polymer plastic solar cells instead of rods so that the tetrapod's improved efficiency can be measured.
Alivisatos is meanwhile looking for new shapes to make such as "branching" tetrapods. "The next step for us to try to find a way to get the legs to branch again, forming a branching tree-like structure," he said.