April 14, 2011 — A new type of solar photovoltaic cell using an inorganic core/shell nanowire structure has been fabricated and tested by scientists at Xiamen University and the University of North Carolina. The "quantum coaxial cable" nanostructure efficiently harvests visible-wavelength light using stable, high-bandgap semiconductors.
Arrays of core/shell nanowires had previously been theorized as a potential structure that could absorb the broad range of wavelengths present in sunlight. High-bandgap semiconductors are generally considered not effective at absorbing most of the available wavelengths in solar radiation by themselves. For instance, high-bandgap zinc oxide (ZnO) is absorptive in the UV but transparent in the visible.
The team of researchers created ZnO nanowires with a zinc selenide (ZnSe) coating to form a type-II heterojunction that has a significantly lower bandgap than either of the original materials. Arrays of the structured nanowires were able to absorb light from the visible and near-IR wavelengths.
"High-bandgap materials tend to be chemically more stable than the lower-bandgap semiconductors that we currently have," noted team member Yong Zhang from the University of North Carolina. "And these nanowire structures can be made using a very low-cost technology, using a chemical vapor deposition (CVD) technique to grow the array. In comparison, solar cells using silicon and gallium arsenide (GaAs) require more expensive production techniques."
Past attempts to use high-bandgap materials did not use the semiconductors to absorb light but instead involved coating them with organic dyes that accomplished the photoabsorption and simply transmitted electrons to the semiconductor material. In contrast, the team’s heterojunction nanowires absorb the light directly and efficiently conduct a current through nano-sized coaxial wires, which separate charges by putting the excited electrons in the wires’ ZnO cores and the positively charged holes in the ZnS shells.
The nanowires were created by first growing an array of six-sided ZnO crystal wires from a thin film of the same material using CVD. The technique created a forest of smooth-sided needle-like ZnO crystals with uniform diameters (40 to 80 nm) along their length (approximately 1.4µm). A somewhat rougher ZnS shell was then deposited to coat all the wires. Finally, an indium tin oxide (ITO) film was bonded to the ZnS coating and an indium probe connected to the ZnO film, creating contacts for current generated by the cell. The photoresponse threshold of the cell was measured to be 1.6 eV, making it responsive to the UV to near-IR range.
"The expanded use of type II nanoscale heterostructures also extends their use for other applications as well, such as photodetectors — IR detectors in particular," noted Zhang.
The findings were publised in the Journal of Materials Chemistry: Zhiming Wu et al., Journal of Materials Chemistry, issue 16, p. 6020, 2011; DOI: 10.1039/C0JM03971C. Access the article here.