The properties of nanowires — useful in a whole host of optoelectronic applications — are a consequence of the wire’s small diameter. At just a few tens of nanometers across, nanowires restrict the motion of an electron to just one dimension. This is practical for solar cells, allowing the electrons generated by light to be collected and transported to a metal electrode efficiently.

Zinc oxide (ZnO) in particular has shown itself to be an excellent material for this purpose. However, ZnO nanowires only absorb ultraviolet light, which limits their usefulness for harvesting the energy in sunlight. Kijung Yong and co-workers from the Pohang University of Science and Technology in Korea1 have now found a simple method for combing ZnO-based nanowires with quantum dots to broaden their absorption spectrum to include visible light.

Quantum dots are tiny semiconducting crystals that exhibit modified absorption properties related to their small size. This means that quantum dots can be synthesized to absorb light in a chosen part of the spectrum.

Yong and his team grew 100 nm-diameter ZnO nanowires on a glass substrate and applied a shell of cadmium sulfide (CdS) around each wire to ensure efficient charge transport between the quantum dot and the nanowire, and to prevent the generated charge from recombining. Cadmium selenide (CdSe) quantum dots just 6.5 nm across were then affixed to the nanowires from a chemical bath. Every step in the process was solution-based, making their approach much simpler compared to other methods and more suitable for large-scale production.

Fig. 1: Schematic illustration of the quantum dot–nanowire solar cell. Light absorption in the CdSe quantum dots generates electrons, and the nanowires transport the photogenerated electrons efficiently to the electrode, generating a current.© 2010 K. Yong

A solar cell based on just the ZnO–CdS nanowires without quantum dots absorbed light at wavelengths of up to about 550 nm (green) with a maximum quantum efficiency of 45% at 500 nm. The quantum-dot-functionalized nanowires, however, responded to wavelengths of up to 750 nm (infrared) and achieved an efficiency of 70% at 550 nm (Fig. 1).

Yong hopes to improve this efficiency even further in the future. “We need to optimize the nanowire structure, length and diameter, for example, and the density of the nanowire array,” he explains. “We will also develop more efficient electrolyte and electrodes for our quantum-dot-sensitized solar cells.”