When moving from our visible, everyday systems down to a system with nanometer dimensions, our way of describing the system changes from classical to quantum mechanical. At this miniature scale, which is the scale for a quantum dot (also called nanocrystal), the classical description simply does not represent the world properly. It is at this magnification that a solution to more efficient solar cells might be found.

The current maximum theoretical efficiency of the conventional silicon based solar cells is ~31%. These solar cells absorb high-energy photons from the sun when rays hit the uppermost layer of the cell, which is made of a crystalline semiconductor (e.g. silicon). Hot, or very energetic, electrons are created as a result of this photon bombardment. The electrons quickly cool by losing energy as phonons or heat to the band edges of the semiconductor band gap before the high-energy electrons can be extracted to produce an electrical current.

Fig. 1. The conduction band is the range of electron energies, higher than that of the valence band, sufficient to free an electron from binding with its individual atom and allow it to move freely within the atomic lattice of the material. Electrons within the conduction band are responsible for conduction of electric currents in conductors. The band gap is in between these two bands (Kotz, J. C., Treichel, P. & Townsend). Image designed by: Molly Hjorth Jensen

Consequently, this is a cooling process that causes irreversible energy loss.

However, on June 17, 2010, researchers at the University of Minnesota published the paper "Hot electron transfer from semiconductor nanocrystals" (DOI: 10.1126/science. 1185509) stating that if all the energy of these hot-electrons could be captured, the theoretical efficiency could reach ~66%. The discovery, spearheaded by graduate student William Tisdale and chemistry professor Xiaoyang Zhu (University of Texas, Austin), showed evidence of hot-electron transfer from lead selenide (PbSe) quantum dots (QDs) to the electron acceptor titanium dioxide (TiO2) wire.

Semiconductors do not conduct electricity easily, but can be encouraged to do so when energy is inputted. When energy has been supplied to the electron, here in the form of light, the relaxation of the electron depends on the dimensions of the material. These dimensions then determine whether the limits are classical or quantum mechanical.

In three dimension, hot carriers in these semiconductors can cool by sequential emission of heat. However, when a semiconductor is formed into a "zero-dimensional" QD, its properties change so that discrete electronic states arise through this boundary confinement. These discrete electronic states resemble discrete energy levels in an atom, which is a quantum mechanical feature. In small QDs, slow dissipation is predicted because the energy of the lattice vibrations is about 10 times less than the energy separation of the electronic states in the QD (DOI:10.1126/science.1159832).

Eray Aydil, a professor and researcher of the project, says that the evidence for hot-electron transfer "is a necessary but not sufficient step for building very high-efficiency solar cells." Press releases from the two host universities mentioned three critical steps towards an "ultimate solar cell" based on QDs.

"[Firstly], the theory says that quantum dots should slow the loss of energy as heat," said Tisdale. In 2008, a paper (DOI: 10.1126/science.1159832) published by Panday and Guyot-Sionnest (University of Chicago) confirmed that "slowing down the cooling of these electrons - in this case, by more than 30 times"- is possible. The technique is to force them into small volumes, which is the case for QDs.

Secondly, Zhu stated that "we need to be able to grab those hot electrons and use them quickly before they lose all of their energy." This is the scope of their new paper: examining the method of "how" to take out those electrons to do work with them.

Tisdale and his colleagues prepared samples consisting of one or two monolayers of PbSe nanocrystals, with diameters ranging from 3.3 to 6.7 nm. They then deposited these on TiO2 using the dip-coating method, so-called due to the submersion of TiO2 (substrate) into an oleic acid (OA)-PbSe solution to attach the QDs to the substrate/wire.

The OA-PbSe solution was synthesized by mixing lead oxide, oleic acid, and an alkene (1-octadecene). After applying pressure, the solution was purged with nitrogen and underwent a heat treatment. Then a selenide solution, a chemical that causes nanocrystal growth, was added. When the desired size for nanocrystals was obtained, toluene was added. The reaction vessel was cooled quickly to room temperature to stop the reaction.

Because OA forms a shell around the nanocrystal, it works as an electronic insulator, which means it is unable to transfer hot-electrons from the PbSe to the TiO2. Consequently, the OA shell must be removed. This was done by two different chemical treatments, one in which the OA-PbSe is treated with hydrazine (HYD) and the other with 1,2-ethanedithiol (EDT). Both gave similar interfacial valance band distances and energies.

To study the process of electron transfer, the researchers set up a laser system, in which a laser beam was split in two. When they recombined at the sample surface, conservation of momentum ensured that a second harmonic (SH) light is generated. This resultant light is collinear to both reflected beams. The change of this SH intensity is proportional to the energy separation that was generated when electron transfer occurred across the PbSe-TiO2 interface. By this method, hot-electron transfer was observed for all EDT-treated samples and those HYD-treated samples with small diameters.

Fig. 2. Dependence of the time-resolved second harmonic response after the two different chemical treatments. Smaller quantum dots show higher efficiencies. From William A. Tisdale et al. Science 18 June 2010: Vol. 328. no. 5985, pp. 1543 - 1547 DOI: 10.1126/science. 1185509 Reprinted with permission from AAAS.

"This is a very promising result," says Tisdale. "We've shown that you can pull hot electrons out very quickly – before they lose their energy. This is exciting fundamental science."

According to Zhu, the findings showed that highly efficient quantum-dot-based solar cells are "not just a theoretical concept, but an experimental possibility." However, this idea was met with skepticism. Professor Frederik Krebs, who is an expert on solar cells at Risø (the Danish National Laboratory for Sustainable Energy), questioned this theory. "It is at least conceivable that you can improve efficiencies by [this type of] clever means. I doubt however that you can imagine very efficient, very low-cost solar cells made in this way." He also added: "It will be scientific curiosities that are impossible to reduce to useful practice."

Even if it is experimentally possible, a third issue still needs to be solved: the loss of energy in the wire [TiO2] as heat. "Our next goal is to adjust the chemistry at the interface to the conducting wire so that we can minimize this additional energy loss," said Zhu.

He and his team are currently studying the application of QDs in making more-efficient solar cells.

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William A. Tisdale, K. J. W., Brooke A. Timp, David J. Norris, Eary S. Aydill, X.-Y. Zhu. Hot-Electron Transfer from Semiconductor Nanocrystals (incl. Supporting Online Material). Science 328, 1543, DOI:10.1126/science.1185509 (2010).

Author: Molly Hjorth Jensen

Reviewed by: Phuongmai Truong, Renee Gilberti, and Yangguang Ou

Published by Maria Huang