I recently attended the 2013 Materials Research Society’s (MRS) Spring Meeting from April 1st to 5th in San Francisco, California. The MRS brings together members of industry, academia, and government to discuss the latest in materials research across a wide variety of disciplines. There were 56 parallel technical sessions, an exhibit, and a wide variety of tutorial sessions taught by leading scientists and engineers. I presented a poster entitled, “Flux engineering for height dependent morphological control of branched nanowires” in a section focused on nanostructured semiconductors and nanotechnology. I attended talks primarily focused on nanowire growth and applications. Numerous talks focused on the use of nanowires in photovoltaic devices that I believe are of interest to the Canadian Photovoltaic Innovation Network. Here I will briefly discuss a couple of highlights.
Results from a paper recently published in Science detailing high performance solar cells consisting of nanowire arrays were presented by a member of The Nanostructure Consortium at Lund University in Sweden.1 P-i-n junction indium phosphide nanowire arrays were employed in the devices, resulting in a maximal efficiency of 13.8% at one sun. InP nanowires have extremely low surface recombination velocities, removing the need for surface passivation as required by nanowire composed of alternative materials (such as Si). Interestingly, the devices exhibited short circuit current densities at 83% of the highest performance planar InP cells, while only covering 12% of the surface (as compared to 100% surface coverage in planar devices). The authors concluded that ray optics is not suitable to model the interaction of light with subwavelength nanostructures due to resonant light trapping. As a result, the authors suggested that nanowire PV devices could potentially reduce the amount of material required to fabricate cells by producing photocurrents comparable to planar devices.
An interesting talk entitled, “Band-gap and structural engineering of semiconductor metal oxides for solar energy conversion,” described the use of 1-D nanostructures (nanowires) to serve as direct pathways for charge extraction in dye-sensitized solar cells (DSSCs).2 In this work, zinc oxide (ZnO) nanowires were used due to their high electron mobility. In a typical nanoparticle film, electrons undergo “zig-zag” transport, increasing transport time and the probability for recombination or trapping. As a result, much of the generated charge carriers are not collected, leading to low performance. Direct “straight-line” conductive pathways are provided for electrons by implanting ZnO nanowires into the nanoparticle film. As a result, charge collection efficiency is significantly improved. The implementation of ZnO nanowires improved efficiency in DSSC devices by 26.9% in the best performing device.
1. Wallentin, J. et al. Science 339, 1057, (2013).
2. Bai, Y. et al. Advanced Materials 24, 5850, (2012).
Ph.D Candidate, Year 2
Electrical and Computer Engineering Department, University of Alberta