design brief - Steven Keating's

nanocolumn InGaN photovoltaic design
Currently, the main obstacle to solar panel proliferation is cost. During the summer of 2009, I started work
under Dr. Joshua Pearce looking at ways to improve photovoltaic efficiency and reduce material expenses.
The topic of research was indium gallium nitride (InGaN), which is a new potential photovoltaic material.
Both the optical and structural properties of InGaN were of interest as InGaN has been shown to have a
tuneable band gap (from 0.4 eV to 3.4 eV depending on the indium content). This is an promising material,
as most semiconductors used for solar cells have fixed band gaps and the band gap of a material dictates
what frequencies of light the material can absorb. Most commercial solar cells are single-junction cells,
meaning that only one band gap is used to collect solar energy. Newer, multi-junction cells function similarly
to a stack of single-junction cells, and can collect a wider range of frequencies using different materials for
each junction. These higher efficiencies multi-junction cells currently exist, but are expensive and require
careful processing to get the different materials to have proper lattice matching. InGaN offers a unique
opportunity, as a multi-junction cell could be made with all of the junctions containing InGaN. The different
junctions would only have to have slightly varying compositions of indium to achieve the required varying
band gaps. This means lattice matching would be significantly easier, and the material costs are
inexpensive compared to the high-purity silicon currently used.
While the main driving factor for solar InGaN research is the tuneable band gap, another interesting avenue
is the microstructure. This past summer, my work revealed the nanocolumnar growth present in our method
of InGaN deposition. These nanocolumns offer an interesting method to easily creating a three-dimensional
solar cell. If the columnar structure could be maintained in a solar cell, the extra surface area and reflection
properties will improve absorption and efficiency when compared to the standard planar geometry. The
photos below detail the nanocolumnar structure observed in our deposited InGaN samples. My work
discovered these structures (seen below in scanning electron microscopy), linked their crystallinity to the
indium content, and modeled a multi-junction photovoltaic cell around this tuneable band gap
semiconductor.
For more information, please see our recently published paper:
S. Keating, M. G. Urquhart, D. V. P. McLaughlin, and J. M. Pearce. (2011)
Effects of Substrate Temperature on Indium Gallium Nitride Nanocolumn Crystal Growth. Crystal Growth
and Design. V.11, Pg. 565-568
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