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"Quantum coaxial cables" offer solar solution

Combining GaP and GaN in intricately designed nanowires could increase solar power efficiency and give tailored bandgaps, according to calculations performed at the US National Renewable Energy Laboratory.

Materials scientists in Colorado are developing nanowires to improve the efficiency of solar energy harvesting by emulating coaxial cables, like those you may find in your hi-fi.

Two different semiconductor materials, such as GaP and GaN, can be arranged in the same wire to provide distinct transport channels for electrons and holes, preventing the unwanted recombination of the charge carriers, which contributes to the low efficiency of solar panels.

In the work carried out at the National Renewable Energy Laboratory (NREL) and Lawrence Berkeley National Laboratory, the nanowires are arranged with one material acting as a central cable (the core) around which the other material is layered (the shell), like the separate copper layers in conventional coaxial leads.

Calculations showed that in a conducting composite wire, electrons resided in the GaN layer and holes in the GaP layer, irrespective of which formed the core and shell.

However, Yong Zhang from the NREL team indicates that although these effects are theoretically possible, practical demonstration has yet to be achieved.

“GaN-GaP and GaP-GaN core-shell nanowires have been synthesized using thermal CVD by others,” he explained, “although the structures are imperfect and the sizes are somewhat larger than what is needed to observe the quantum effects discussed in the paper.”

“We are working on proof-of-principle devices.”

A further advantage of the combination nanowires is that a new effective bandgap is formed, potentially absorbing energy at wavelengths better suited to photovoltaic energy generation.

“The new bandgap arises from alignment between the conduction band of GaN and the valence band of GaP, offering a tunable range from IR to UV,” Zhang explained.

“The actual bandgap includes a number of other important effects: strain, quantum confinement and the polarization effect at the core-shell interface.”

ZnO nanowires have already been successfully demonstrated as replacements for nanoparticles in dye-sensitized solar cells (DSCs), a rapidly developing photovoltaic technology.

Zhang's group envision that their “quantum coaxial cables” could match the increased rate of electron transport attained by the nanowires, while the ability to better match the solar spectrum could offer still greater efficiency improvement and device performance.

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