Tiny wires promise intense energy: Energy generation materials
Given the limited reserves of fossil and nuclear fuels, it seems evident that our future energy demands will have to be met by renewable energy sources. Yet despite intense research the required technologies have not reached the point where they can directly compete with traditional energy sources.
Now the group of Ray LaPierre at the McMaster University, Canada, has reported one of the latest advances in the search for cost-efficient solar cells [Czaban et al., Nano Letters, 10.1021/nl802700u]. Their study centered on the development of photovoltaic cells based on GaAs core-shell nanowires.
“Conventional solar cells rely on single-crystal silicon cells for their operation, offering power conversion efficiencies in the range of only 15 to 20% for commercial systems, while being expensive to manufacture,” LaPierre tells Materials Today. “As a result, researchers have focused on reducing material costs by using thin absorber layers, inexpensive deposition technologies, and inexpensive substrates. However, these methods typically produce polycrystalline material, resulting in carrier recombination at bulk defects and low conversion efficiencies.”
Semiconductor nanowires present a promising way out of this problem. The tiny one-dimensional rods have lengths of a few microns, and thicknesses of just a few tens to 100 nm. Because they can be grown using self-assembly, the nanowires achieve excellent electronic and optical properties, comparable to those of traditional cells, while being much cheaper to manufacture. Their geometry also causes the incident light to be trapped inside the material, giving them a black appearance and resulting in very low reflection losses.
Each of the wires contains a single p-n junction, manufactured by adding dopants during the growth phase. Initially Be is added, resulting in a core of p-type material, which is then switched to Te, resulting in an outer layer of n-type material. Their study is the first to examine the effects of adding Te, as opposed to the more traditional Si, since as a group VII element Te is a n-type dopant both with respect to Ga (group III) and As (group V). They found that its addition affected both the geometry of the nanowires as well as the electronic properties.
The researchers also studied the energy conversion of the nanowires and found efficiencies up to 0.35%, marking an improvement from previous studies. While still limited, LaPierre is convinced that additional research will allow nanowire-based systems to surpass traditional solar cells.