The key to using silicon in electronic devices such as transistors and solar cells lies in doping, or adding in small quantities of other elements, to create an excess of electrons (n-type) or positively charged holes (p-type) that change the material’s conductivity. N-type and p-type silicon are butted together to form p-n junctions, the basic building blocks of electronic devices such as solar cells, light-emitting diodes, and transistors.
For years, researchers have tried to do something similar with quantum dots, tiny semiconductor crystals a few nanometers in diameter. Now, a team of Israeli researchers has reported success. They have doped indium arsenide quantum dots to create n-type and p-type materials. The advance, published in the journal Science, could lead to new types of efficient, cheap, and printable thin-film solar cells.
Quantum dots hold promise for low-cost solar cells because they can be made using simple, inexpensive chemical reactions. Scientists have calculated that quantum dots could be used to make thin-film photovoltaics that are at least as efficient as conventional silicon cells, and possibly more efficient. The higher possible efficiency is because nanocrystals made of certain semiconductors can emit more than one electron for every photon absorbed. Plus, tweaking their size and shape changes the colors of light they absorb. “We could tune the nanocrystal absorption to match the solar spectrum,” says Uri Banin, a professor of chemistry at the Hebrew University of Jerusalem who led the new work.
Despite these advantages, no one has succeeded in making efficient quantum-dot solar cells. For that, you need n-type and p-type nanocrystals, says Eran Rabani, a chemistry professor at Tel Aviv University who was involved in the new work. In solar cells, the electrons and holes that are created when photons are absorbed have to be separated so that the electrons can travel out of the semiconductor to the external electric circuit. Some electrons and holes inevitably combine, but they combine much faster in quantum dots than in large silicon crystals. Doping semiconductor nanocrystals would provide a way for creating p-n junctions that separate electrons and holes efficiently, Rabani says.
Silicon is typically doped with phosphorus or boron atoms, but these materials do not work with quantum dots because the dots are so small. A 4-nanometer-wide nanocrystal contains about 1,000 atoms. Adding a few dopant atoms .can lead to their being expelled from the nanocrystals.
Some quantum-dot doping efforts have succeeded. Researchers have, for instance, doped them with magnetic manganese ions, but this technique does not introduce excess electrons or holes. Others have been able to make undoped nanocrystals n-type by injecting electrons into them. Still others have been able to dope thin films of nanocrystals.
The Israeli team, by contrast, is able to dope freestanding nanoparticles. “This is a major breakthrough here,” says Y. Charles Cao, a chemistry professor at the University of Florida in Gainesville. “The major advantage here is you [have] the building blocks for the bottom-up assembly of nanocrystal electronic devices.” Another plus, adds Cao, is that the method used to make the dots is easy and inexpensive and could be scaled up to make devices in large quantities.
Banin and his colleagues start with a solution of silver or copper compounds and gradually add it to a solution of indium arsenide nanocrystals. This results in silver-doped p-type dots or copper-doped n-type ones. Since the quantum dots are made in solution, they could be deposited on flexible plastic sheets using printers or roll-to-roll processes.