“Optical rectennas” (or sometimes “nantennas”) are a technology that is sometimes advertised as a path towards converting solar energy to electricity with higher efficiency than normal solar cells. I looked into them extensively as a postdoc a decade ago, wound up concluding that they were extremely unpromising, and moved on to other things. Every year or two since then, I run into someone who is very enthusiastic about the potential of optical rectennas, and I try to talk them out of it. After this happened yet again yesterday, I figured I'd share my spiel publicly!

(For some relevant background context, check out my write-ups on the fundamental efficiency limit of single-junction solar cells, and on the thermodynamic efficiency limit of any solar energy conversion technology whatsoever.)

1. What is a rectenna?

Rectenna is short for “rectifying antenna”, i.e. a combination of an antenna (a thing that can transfer electromagnetic waves from free space into a wire or vice-versa) and a rectifier (a.k.a. diode).

Rectennas are an old and established technology for radio-frequency (RF) electromagnetic waves. For example, if you have a very-low-power gadget, you can power it with a rectenna that scavenges energy from nearby commercial radio stations.

Basically, the commercial radio station emits an electromagnetic wave in free space, and the antenna converts that into an RF signal in a wire (“waveguide”). However, this signal is “AC”—its voltage cycles between positive and negative, at megahertz frequencies, averaging to zero. You can’t recharge a battery with such a signal; it would slightly charge the battery one nanosecond, then slightly discharge it a few nanoseconds later, etc. Hence the diode, which converts (part of) that energy to DC, allowing it to usefully charge batteries or power any other electrical component.

2. If RF rectennas can turn RF electromagnetic waves into electrical energy, why can’t optical rectennas turn sunlight into electrical energy?

Well, they can! That is, they can if you’re OK with very low power conversion efficiency. Very, very, very low. Like, 0.00…1% power conversion efficiency. I don't even remember how many zeros there were.

Are higher efficiencies possible for an optical rectenna? Yes! That is, if you’re collecting energy from an intense focused high-power laser, rather than from sunlight.

Why do I say this? There are two problems.

3. The easy problem: antennas

The easy problem is scaling down the antenna until it is nano-scale, such that the antenna is sized to absorb and emit sunlight-appropriate electromagnetic waves (e.g. 500 nm wavelength), instead of RF waves (e.g. 500,000,000 nm wavelength).

Making this nano-scale device, and making it inexpensive to mass-produce such that it covers an inexpensive sheet, and getting the antennas to absorb lots of sunlight, constitute the easy problem. This is tractable. It's not trivial, but if this were the only problem, I would expect commercial optical rectennas in short order.

Absorbing lots of sunlight was never the problem! If you want a surface to absorb lots of sunlight, just paint it black!

The hard part is getting useful electrical energy out of that absorbed sunlight. Which brings us to…

4. The hard problem: diodes

The hard problem is finding a diode which will rectify that energy. I claim that there is no commercially-available diode, nor any prototype diode, nor any computer-simulation-of-a-diode, nor even a whiteboard sketch of a possible diode, that is on track to rectify these electromagnetic waves and turn them into useful energy.

There are actually two problems: speed and voltage.

The speed problem is that almost all diodes stop rectifying signals if the frequency of those signals is too high. If memory serves, one common problem is that the diode has too high a capacitance, and another is that electrons can only move so fast. Remember, the sun emits electromagnetic waves with a frequency of around 500 THz = 500,000,000 MHz. This rules out almost all types of diodes.

And that's actually the less thorny problem, compared to:

The voltage problem is that, for the small wavelength of sunlight, you need a small antenna, and a small antenna has a small absorption cross-section with which it can collect light. So you wind up with very very little sunlight energy getting absorbed by any given antenna, and thus very little voltage in the attached circuit—if memory serves, well under a millivolt.

Alas, diodes stop being diodes if the voltage of the signal is extremely small. Just look at the IV curves:

If the diode doesn’t actually rectify, then the power is absorbed instead of converted to usable energy.

Taking these together, it turns out that there are diodes which are fast enough for optical frequencies (metal-insulator-metal “MIM” diodes), but they do not turn on sharply at ultra-low voltage. There are diodes which turn on sharper than usual at low voltage (“backwards diodes”), but I don’t think they can support such high frequencies. And even if they could, even these diodes are not remotely close to being sharp enough for our (IIRC sub-millivolt) signal.

There is no diode, to my knowledge, that can work for this device. During this little postdoc project, I spent quite a while scouring the literature, and even trying to invent my own crazy new device concepts, but failed to find anything remotely close to meeting these specs.

5. But what if we combine the power collected by many antennas into a single waveguide, to increase the voltage?

Alas, the second law of thermodynamics mandates a fundamental tradeoff between the absorption cross-section and the collection angle, of any antenna (or antenna array) whatsoever. If you make a bigger antenna, it will collect more light, but only when the sun is in exactly the right spot in the sky.

6. But what if we track the sun?

Well, then you lose ~100% of the light on cloudy days, and you lose 15% of the light even on clear days (e.g. the light from the blue sky). Worse, you need very accurate 2-axis mechanical tracking as the sun moves across the sky, which is expensive. More importantly, if you’re willing to bear those costs (of precise two-axis mechanical tracking and losing the diffuse light), then you might as well just use a big lens and a tiny solar cell, and then the solar cell can be one of those super-expensive multi-junction cells, which incidentally is already getting pretty close to the theoretical efficiency limit on solar energy conversion.

Anyway, we shouldn’t compare with the theoretical efficiency limit, but rather with a rectenna, which I very much doubt would exceed 1% efficiency even at the theoretical limit of maximum possible absorption cross-section. (Why is there a limit, as opposed to being able to track ever-more-accurately? Because the sun is a disc, not a point. So there’s only so much that you can cut down the light collection angle.)

7. But what if we track the sun virtually, with a phased array?

That only solves one of the many problems above, and anyway phased arrays don’t work because sunlight is broadband.

8. But what if we use an impedance converter?

I glossed over this above, but to translate from “there is only so much electrical energy in the waveguide at any given time” to “there is only so much voltage across the diode”, you also need to know the relevant impedance. If the impedance is high enough, you can get a higher voltage for the same electrcial energy.

Alas…

Problem 1 is that high impedance makes the diode speed problem even worse, by effectively increasing the RC time constant.

Problem 2 is that there seems to be a tradeoff between how much you increase impedance, and how broadband your impedance converter is. And sunlight is rather broadband.

I say "seems to be a tradeoff", in that I am unaware of a law of physics demanding such a tradeoff. But it seems to be the case for all the impedance-conversion techniques that I know of, or at least for the techniques that work for these kinds of very high frequency waves (e.g. things like quarter-wave impedance transformers).

9. But what if … something else?

Hey, what do I know? Maybe there’s a solution. Maybe the numbers I threw out above are misremembered, or maybe I flubbed the math during my postdoc project.

I would be very happy to see those people working on or excited about optical rectennas grappling with these problems, proposing solutions, and doing back-of-the-envelope calculations.

Instead, what I usually see is people going on and on about the “easy problem” (i.e. the antenna), and how they’re making such great progress on it, without even mentioning the “hard problem” (i.e. the diode).