Some Self-Assembly Required

Investigating Solid-State Arrays of Semiconductor Quantum Dots

In the search for more efficient methods for generating solar power, one promising field of inquiry involves solid-state arrays of semiconductor quantum dots (QDs). Quantum dots have properties — such as strong light-absorbing capability and the tendency to self-assemble into ordered arrays — that make them attractive for applications in solar energy conversion.

Dr. Emily Weiss was awarded a Postdoctoral Energy Fellowship by the ACS Petroleum Research Fund for her investigation into QDs and the design and study of new architectures for solid-state QD arrays that would more efficiently convert absorbed light energy into electrical energy. Quantum dots’ potential in this area has not yet been realized, because researchers have not been able to optimize either their solid-state conductivity, or the contacts they form with electrodes within solar cells.

The challenge of Weiss’ research was to use a special property of QDs: namely, the fact that their electronic structure is dictated by their size. Weiss’ team therefore designed and constructed multi-size arrays that incorporated cadmium selenide QDs and contained stacked layers of QDs, each layer with its own size (see figures). The resulting structure formed both a redox and energy gradient, and allowed the team to vary the size of the QDs next to the electrode, while keeping the sizes of the QDs in the bulk of the array constant in order to probe the properties of the QD-electrode interface.

Weiss and company have obtained two main results from their research so far. The first is that, in the dark (with no light excitation of the QDs), the rate-limiting step for conduction of electrons through the QD junction is the transport between the QDs and the (polymer-covered) electrode. The amount of energy (here, applied voltage) necessary to induce transport across this interface was very sensitive to the size of the QDs at the interface (and not sensitive to the sizes of the QDs in the bulk). This result is important for QD-based solar cells, because it implies that the QD-electrode interface can be optimized by varying the size (and, probably, the material) of the QDs.

The second result was that, by studying photo-induced conduction in a set of multi-size QD arrays with various sizes, it was possible to pinpoint the location within the junction at which charge-separation occurs. In this case, it was at the interface between the QDs and the polymer-covered electrode. However, the same technique (which Weiss and her team call size-selective excitation) can be used to map charge transfer processes in more complicated QD-based solar cells as well.

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