Solar Cell Efficiency Smackdown: Eye of Moth Vs. Petal of Rose

Moth eyes were last year’s big thing in solar cell biomimicry, and  now it looks like a new challenger has stepped into the ring. A research team from Germany’s Karlsruher Institut für Technologie has zeroed in on the nanoscale structure of rose petals — specifically, the El Toro variety — to improve thin-film solar cell efficiency by up to 13%.

The El Toro Rose

If the name El Toro doesn’t ring a bell, our friends over at the UC–Davis Foundation Plant Services can provide you with a rundown of aliases, including Gladiator and Red Mountain Peak as well as the somewhat less aggressive Uncle Joe and just plain old Toro.

Whatever you call it, El Toro is no shrinking violet. A large single-stem variety hybridized at the legendary Kern nursery in Ohio, it reeks of strength with deep red petals, dark leathery leaves, and probably a lot of thorns:

That looks rather intimidating, right?

Rose Petals & Solar Cell Efficiency

The researchers note that plants have received quite a bit of attention for their anti-reflective and water-repelling powers, which serve as an inspiration for self-cleaning solar panels and other materials.

Applying their unique surface structure to solar cell efficiency is another story. You can find the new study under the title “Flower Power: Exploiting Plants’ Epidermal Structures for Enhanced Light Harvesting in Thin-Film Solar Cells” in the journal Advanced Optical Materials.

Do take a look if only to enjoy the rose-worthy presentation that precedes the nuts and bolts (and to find out how the team hit up El Toro as a research subject), but for those of you on the go, here’s the takeaway:

…the hierarchical structures decorating the petals of Rosa ‘El Toro’ are replicated into a transparent resist layer to form highly efficient light harvesting elements that can be applied to any photovoltaic technology.

KIT elaborates in a press release for the new solar cell efficiency study:

…every single replicated epidermal cell works as a microlense. The microlense effect extends the optical path within the solar cell, enhances the light-matter-interaction, and increases the probability that the photons will be absorbed.

With improved efficiency, thin-film solar cells can be made even more thin and flexible, widening the span of their potential application.

By the way, you can probably DIY this yourself. With a good pair of tweezers you can actually peel a transparent layer from the surface of rose petals and other plants. Drop us a note in the comment thread if you have any luck.

Eye Of Moth Vs. Petal Of Rose

The study also reveals a pathway to understanding how and why the rose petal structure could top moth eyes as a solar cell efficiency enhancer.

The anti-reflective property of the moth eye is attributed to neatly arranged arrays of microscopic structures. According to the research team, when deployed on a thin-film solar cell, these structures improve light collection but do not change the direction of the photons.

In contrast, the surface of rose petals consists of disorganized structures with additional “ribs” adding to the disarray. This has the effect of redirecting photons in addition to reducing losses from reflection, as explained in the abstract:

…since those surfaces evolved as efficient sunlight harvesters, they possess broadband and omnidirectional optical properties, as targeted in PV applications. Those attributes make them particularly well-suited to tackle the substantial optical losses often occurring in thin-film solar cells by using a single plant-inspired structure for both light collection and light trapping purposes.

With that new knowledge in hand, next steps for the team include a further examination of how disorganization affects solar cell efficiency.

If you’re new to the whole thin-film solar cell topic, check out this short film on our sister site Planetsave, produced by Joel Jean and Sarah Luppino of the US Energy Department’s Energy Frontier Research Center at MIT.

What Now, Nukes?

Nuclear investor Bill Gates used the occasion of last year’s historic COP21 Paris climate talks to talk up nuclear energy with his new “Breakthrough Coalition,” but it seems that thin-film solar and other renewables are skipping ahead.

Regardless of advances in nuclear R&D, high costs and public antipathy are working against the commercial use of nuclear energy.

Here in the US, for example, the energy-sucking state of California will close its last nuclear plant by 2025 and make up the difference with renewable energy, energy storage, and energy efficiency.

According to one estimate, consumer savings from the transition will hit as much as $1 billion.

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Images: top via KIT, bottom via FPS/UC-Davis.