Zinc oxide nanowires

Morphology is an important factor affecting device efficiency in nanostructured heterojunction solar cells. To achieve optimized power conversion efficiency, each component must be positioned within the device architecture with a high level of control. Better controlling randomly organized nanostructured electrodes attracts research interest, especially for those cells incorporating quantum dots and absorbing dyes. Reporting in Nanotechnology, researchers employ position control of hydrothermally grown zinc oxide nanowires to improve charge transfer and efficiency of ZnO nanowire/quantum dot solar cells.

The researchers find that electron-beam lithography, a top-down approach, is a suitable prototyping method for templating the growth of ZnO nanowires. It allows the understanding and control of nanowire array geometry. The ZnO material system is selected because highly crystalline nanowires can be grown by hydrothermal synthesis – a low temperature, substrate independent technique with large area reproducibility.

Sputtering for alignment

Following high-resolution templating, the nanowire branching and alignment is measured as a function of process variables such as template hole size, substrate, seed layer annealing, and deposition method. It is found that sputtered seed layers yield good nanowire alignment and minimal branching, with each template hole yielding only one nanowire. It is possible to fabricate high quality arrays of ZnO nanowires at photovoltaic-appropriate geometries, on a number of substrates.

Device cross section (left) and an array of ZnO nanowires (right).
Device cross section (left) and an array of ZnO nanowires (right).

Maximizing efficiency

Applying these newly fabricated and well-controlled nanowire arrays to solar cells and other devices is an important next step. The control over the array geometry achieved in this work can be used to maximize the efficiency of nanowire/quantum dot solar cells. Lessons-learned from the research will be used to test the ideal nanowire pitch and impact of nanowire branching and alignment.

Furthermore, the geometric control and analysis provided by this research can be used in many other areas of nanotechnology. For example, in mechanical energy harvesters, light collecting or emitting devices, transistors, other crystalline nanowire growth and much more.

More information can be found in the journal .

About the author

Samuel M Nicaise is a PhD student in Electrical Engineering at MIT with a research focus on high-resolution lithography for templated self-assembly. Jayce J Cheng is a PhD student in Materials Science and Engineering at MIT with a research focus on controlled solution-based growth of nanowires for understanding optoelectronic devices.

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