(Source: Jarl Fr. Erichsen/Scanpix,)
Black metal -- nanostructured metal PV layers -- is an extension of "black silicon concept

Researchers at the Lawrence Livermore National Laboratory (LLNL), the national lab associated with the Univ. of California, have cranked up the volume on black metal materials research, coming up with a way to more reliably produce the finnicky, but promising nanostructured class of metals.

I. Multiple Exciton Generation, Plasmons: Routes to Higher Theoretical Efficiency

Black metal is inspired by the black silicon concept, which employs laser lithography and reactive ion etching to sculpt the surface at a nanoscale level and create nanopillars/nanopits/nanocavities that trap light.  This in turn allows the material to more fully harvest solar energy, generating surface plasmons (SPs) (oscillating electrons) which can be used to create a harvested charge.  The low reflectivity lends the resulting materials a black appearance to the human eye.

Solar cells create an electron hole/pair (known as an "exciton").  The exciton pair travels through its source material, reaching another semiconducting material.  But only the electron component of the exciton can travel across this p-n junction (diode) (the hole cannot).  This leads prevents the resulting electrons from recombining with their holes, leading to a net charge in the accepting layer and current.

Black metal
Wafers are seen coated in an aluminum black metal thin film. [Image Source: LLNL]

In traditional semiconductor cells, such as polycrystalline silicon solar cells a single photon can only excite a single exciton.  Typically photons from our sun have more than twice the amount of energy needed to excite one electron, so in these materials after excitation this extra energy is lost.  Thus the maximum theoretical efficiency of traditional thin film cells is around 45 percent.  Real world efficiency is even lower, as the electrons and holes sometimes recombine on the way to the p-n junction leading to other losses.  The upside is that typically absorbance in these cells is high as the materials are transparent and thick.

Newer solar cells tend to use nanotechnology to produce thin films with special quantum effects.

One major research push is to work on the exciton generation, in so-called "excitonic solar cells" such as quantum-dot based dye-sentized solar cells (DSSCs) and other new/exotic material cells.  The goal here is  to find materials that create multiple excitons from a single photon, a phenomena known as MEG (multiple-exciton generation).  Why must a nanofilm be used?  Typically these materials' excitons have a greater tendency to recombine, so they require a smaller nanoscopic p-n junction to try to isolate the electrons as quickly as possible (by making the distance from excitation location to junction as short as possible).

Quantum dots
Quantum dots don't just look pretty, they have some handy physics quirks too!
[Image Source: Elec-Intro]

One strategy is to simply layer multiple layers of quantum dots of different sizes to maximize MEG and absorption.  Another advanced approach is to use metal nanostructures to first "soak up" (so to speak) the photon's energy (instead of having the exciton-generating layer directly absorb it).  In this approach, the energy is temporarily stored in electron oscillations (surface plasmons (SPs)).  These plasmonic charge effects are then used to produce excitons in an associated organic nanolayer.  

One reason this approach is attractive is that metal nanostructures tend to be very good at trapping light via scattering it along a surface, thus there's less reflection.  Another reason this approach is attractive ist that plasmons can also be created from quantum thermoelectric effects, leading to a device that can both recapture waste heat and capture solar energy.

Black Metal Nanowires
The nanowire "black metal" thin film used in the LLNL work is shown. [Image Source: AIP/LLNL]

The black metal nanofilms created in this study could be applied to both thermoelectric and photoelectric sensing or power-generating devices, when combined with a second layer that produces excitons.

II. Back in Black

The LLNL work improves on past black metal devoting itself to the art of making a layer of ordered nanopillars about 50 nanometers in width and between 250 and 1400 nanometers (1.4 µm) in height.  Nanowires thin films of gold, aluminum, and silver were tested.

The study indicates that taller nanowires perform the best.  And surprisingly the aluminum -- while more reflective (88% avg. reflection) than gold (74%) in unpatterned flat films -- performs the best in black metal films, covering the broadest array of spectrum including the entire visible range.  Aluminum has an average reflectivity of only around 30% at a nanowire height of ~1.3 µm.  

Nanowire reflectivity
Longer nanowires reflected less. [Image Source: LLNL/AIP]

Silver may also be a viable choice for solar thin films, as it has the "lowest losses in the visible and it is most efficient for transferring energy from the plasmon modes into the absorptive dielectric material." Gold, meanwhile has significant potential for biosensing, according to the team.

The team found that 3 µm was the upper limit to the benefits of multiple resonances, as the plasmon quantum effects fade, leading to less efficient capture.  Thus the optimal nanowire height for efficient capture appears to be around 1-3 µm.

LLNL engineer Tiziana Bond comments, "[This study] represents cutting-edge work in the area of plasmonics, the broadband operation obtained with a clear design and its implication for the photovoltaic yield."

Senior author Tiziana Bond (center, bottom) with (left to right) with Elaine Behymer (LLNL Engineering Directorate), Allan Chang, and Mihail Bora (first author) [Image Source: LLNL]

Ms. Bond and LLNL physicist Mihail Bora led a team of nine collaborators to make and test the black metal.  They published their results in the peer-review journal Applied Physics Letters, where it was distinguished by being named the cover article for its issue.  Ms. Bond is the senior author of the work, while Mr. Bora is the first author.

Sources: Applied Physics Letters [abstract], LLNL

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