
Black silicon photodetector breaks the 100% efficiency limit - gmays
https://phys.org/news/2020-08-black-silicon-photodetector-efficiency-limit.html
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floatingatoll
The most relevant single sentence from the paper seems to be:

> _Special emphasis is put on the UV range (λ=200-350nm) where we show that
> extremely high response, more than 130%, can be achieved at zero bias._

The material they constructed turns out to react well to UV light, so well
that it proves their technique is worth further investment.

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sreekotay
Can someone decode/ELI5 please? (or did i just miss the joke...)

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compumike
Normal p-n junctions (including solar panels and ordinary semiconductors) are
light sensitive because one photon of light can strike and transfer its energy
into creating one electron-hole pair (promoting one electron from the valence
band to conduction band). This is called "optical generation", in contrast to
"thermal generation". These electron-hole pairs are generated in the depletion
region of the junction where they are quickly swept across by strong electric
fields that arise from the junction itself. But in general, one photon = one
electron-hole pair -- at most.

Avalanche photodiodes already exist and convert a single incident photon into
multiple charge carriers. They do this using an externally-applied electric
field. This externally-applied electric field enables "impact ionization"
which means that one mobile charge carrier seems to be able to knock another
one loose.

(As a thought experiment: imagine a bed of pebbles. Toss in another pebble,
and it's likely to kick one of the existing pebbles free. Now, pretend the bed
of pebbles is at a 45-degree angle to gravity. Toss a pebble in and it's more
likely that the ejected pebble will itself kick another pebble loose when it
lands. Make the angle steep enough and you get an "avalanche", hence the
name.)

As far as I can tell, the novelty here is that the high electric field is
created by the geometry and materials at the surface of the nanoscale needle
structure, so some version of the avalanche effect happens without needing to
apply an external electric field.

In the headline, efficiency refers to quantum efficiency (electrons per
photon); conservation of energy still applies.

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fizzled
how does the avalanche photodiode provide more electrons that required to
generate the external field?

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dan_hawkins
It doesn't. The role of this kind of diode is not to generate energy but to
detect faint amounts of light, to achieve that you spend additional energy on
the external field.

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fizzled
Thanks, that makes way more sense.

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peter_d_sherman
"Aalto University researchers have developed a black silicon photodetector
that has reached above 130% efficiency. Thus, for the first time, a
photovoltaic device has exceeded the 100% limit..."

Well, looks like the apparently correct physics which, up until now, defined
the limit on photovoltaic efficiency -- _was wrong_... <g>

 _Physics is such a fickle subject..._ <g>

"You cannot change the laws of physics!"

-Star Trek's Scotty (James Doohan)

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an4rchy
This is super cool.

Excited to see the way this breakthrough will be absorbed into different
industries.

Can anyone comment on the impact of this with regards to LIDAR tech?

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osamagirl69
It is probably not very useful for lidar, because the effect only works for UV
wavelengths. At the normal eye-safe infrared wavelengths used for lidar the
quantum efficiency of this type of detector is basically 0.

It does work well for general low light imaging and nightvision applications,
you can get a black silicon sensor quite high QE (~80%-ish depending on what
wavelength ranges you are interested in) commercially from
[https://www.sionyx.com/](https://www.sionyx.com/)

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crazygringo
So if "100%" efficiency is defined as one photon generating one electron...

...then what would _total_ energy conversion be according to this metric?
Where the energy of the electric current _equals_ the energy of the photons?
No energy lost? If this is above 130%, what's the ceiling? (Even if no
materials exist that could ever achieve it.)

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Armisael16
Total energy efficiency would entail extracting electron-volt from the photon.
There isn’t any real cap here except how much energy you can stuff into a
vaguely stable photon and how much energy your PV cell can withstand.

PV cells usually excite at one one energy level; any photon with more energy
just gives the electron extra speed, which is wasted. This paper is claiming
to be able to extract the excess kinetic energy from the electron through
collision by colliding with another electron and knocking it loose.

