"Black Silicon" Advances Imaging, Solar Energy 114
waderoush writes "Forcing sulfur atoms into silicon using femtosecond laser pulses creates a material called 'black silicon' that is 100 to 500 times more sensitive to light than conventional silicon, in both the visible and infrared spectrums, according to SiOnyx, a venture-funded Massachusetts start-up that just emerged from stealth mode. Today's New York Times has a piece about the serendipitous discovery of black silicon inside the laboratory of Harvard physicist Eric Mazur. Meanwhile, a report in Xconomy explains how black silicon works and how SiOnyx and manufacturing partners hope to use it to build far more efficient photovoltaic cells and more sensitive detectors for medical imaging devices, surveillance satellites, and consumer digital cameras."
Re:Current PV cells are already up to 40% efficien (Score:5, Informative)
Not all photons have the same energy (wavelength), and this is for precision imaging not power generation. Note it's more "sensitive" not more efficient.
Re:Current PV cells are already up to 40% efficien (Score:3, Informative)
Re:Current PV cells are already up to 40% efficien (Score:5, Informative)
Read carefully: they said 500x more sensitive than silicon, not 500x more sensitive than PV cells.
It's a bit like if they said that by reacting hydrogen with oxygen, they created a compound 700 times denser than oxygen. That doesn't mean it's 700 times denser than the densest material known.
Re:Bad science writers annoy me... (Score:5, Informative)
A pure silicon crystal ingot and a doped silicon wafer are entirely different. You want a pure crystal to grow the ingot as large as possible. To make silicon useful you take the wafer sliced form the ingot, ant it has to be doped (ie add impurities) amongst many other steps.
Some impurities are introduced while growing the crystal, but most are added after the fact.
It just depends on what you're using the silicon for.
Re:Current PV cells are already up to 40% efficien (Score:4, Informative)
Just to be a bit more explicit, sensitivity probably refers to one of two things.
The first would be an increase in quantum efficiency; that would be an increase in the ratio of photons detected to those impacting. In a photovoltaic cell this would lead to improved efficiency. Current scientific detectors, that I've looked into anyway for a research project I'm involved in, max out at maybe 70%, with most reasonably priced ones being 25%-35%. (The 70% ones tend to be things like photomultiplier tubes which require power input to achieve a high reverse voltage, so they're certainly not useful for PV cells.)
The second aspect would be to decrease the noise or dark count so that its capable of detecting dimmer and dimmer light sources, and in order to get the > 100% improvements this is definitely a large aspect of what the new method has done. Unfortunately I know more about the applications and figures of merit than the semiconductor stuff, so I can't say much about this other than I hope this opens up some new application possibilities.
Will it help in imagers? (Score:3, Informative)
The problem with most uncooled imagers isn't insufficient sensitivity any more. It's thermal noise. Unless this improves the S/N ratio, it won't help for uncooled imagers. That's why digital cameras which increase sensitivity in darkness show more and more noise as less light is received.
Cooled imagers, though, as in astronomy and fancier night vision equipment, might benefit. Cooling is done to reduce the random photons from heat within the imager. So cooled imagers do run into the sensitivity limitations of silicon, and might benefit.
But that's an exotic application. Cooled imagers are found mostly in military, space, and astronomy. Some require liquid nitrogen. It's not a mainstream technology.
Re:So what's the catch? (Score:4, Informative)
Gentlemen... (Score:3, Informative)
They already spilled the beans - femtosecond laser pulses against silicon wafer in sulfur hexafluoride gas.
The problems probably are:
1. femtosecond laser pulses aren't exactly easy to make
2. the power density of the beam (if they increase the spot size, the power density goes down, meaning it's more costly and difficult to expose larger portions of the wafer at once, hence increasing time and cost)
3. sulfur hexafluoride - ummm hexafluoride anything is probably not the safest thing to deal with, hence - increasing cost
4. effects of oxide formation post-processing probably increases problems
5. thermal noise ... probably not much of an issue, plus I don't think they're talking about far IR photons, just IR that would normally be picked up in a GaAs detector
6. there is no mention of what wavelength of laser light they're using, so if it's something in the UV range, they'd need more expensive optics, increasing costs yet again.
I just want these SiOnyx people to do this with Uranium Hexafluoride. I want them to do it NOAW!
That should have a beautifully large cross-section, gobbling up lots of photons, and would give the nuclear industry something to do besides fission. This is just a guess, maybe there's some fucked up reason UF6 just wouldn't work for this purpose. I just like the idea of increasing the absorption band of photovoltaics.
meh ... back to reading the Modern Physics textbook.
The company name is a good choice, it sounds like Psionics, which implies of Psi, the Greek letter used to represent wavefunctions, on top of the onyx for black.
Re:Efficiency isn't important - $/Watt IS (Score:3, Informative)
We are getting there. There are several companies that are currently making a large profit on Solar Cells. The basic science has all been performed. We know what material systems work the best (Silicon, CIGS, CdTe). There have been several improvements on production method of the last several years, as well. I personally believe ribbon silicon has the greatest promise. However, if researches can get solution deposited, nano-particle devices up to decent efficiencies, they could rule the market.
The business market is starting to catch on, as well. First Solar stock tripled in price in six months last year. Honestly, raw material is our biggest limit. As these PV manufacturers ramp up production, Silicon, Indium, Tellurium, and perhaps even Cadmium prices are going to rise. However, with proper recycling, solar cells can easily fill the impending energy deficit.
photodetectors-yes; solar cells-NO (Score:3, Informative)
Re:Current PV cells are already up to 40% efficien (Score:4, Informative)
The sensitivity they are referring to is the amount of electrons released by the incident light - Amps of current per Watt of sunlight. Sunlight has a broad spectrum, and this technique allows more of the infrared portion of the spectrum (which is a lot) to cause electrons to flow.
However, and this is important, they achieved this by lowering the bandgap energy of the silicon. Why is that important? Remember that power, when it comes to electronics, is current times voltage. The voltage of a solar cell (open circuit voltage) is more or less the bandgap energy (divided by one electron charge). So, yeah, they get more electrons to flow for the same amount of incident sunlight, but the cell's voltage has also been lowered. Do you end up with more or less power as a result? Does the greater current overcome the lowered voltage? Since they haven't actually published data on a solar cell made from this technique, there isn't really a way to tell for certain.
My guess is that they won't be able to get vast power gains - possibly lower ones. The reason for this is that, right now, one photon with energy greater than the bandgap energy has a chance to create one electron-hole pair. If the photon has more energy than the bandgap energy, it doesn't make a correspondingly more energetic electron-hole pair. Even if the photon had twice the bandgap energy, it can't make two electron-hole pairs. So, a blue photon creates as much useful electrical energy as a red photon, despite the fact that the blue photon has more energy in it. One can play around with the bandgap energy of the PV cell to make better use of the high energy photons, but at the cost of excluding lower energy photons like infrared and red. More info here [google.com]. This is why the solar cells with greatest efficiency are actually multi-junction cells [wikipedia.org] - several solar cells with different bandgap energies stacked on top of each other, each tuned to a different portion of the solar spectrum.
The article mentions how these guys should be able to use their black silicon to create multiple electron-hole pairs from a single photon. In order to do that, however, they have to provide a bias voltage. In that case, the solar cell is sucking power, not producing it. That's fine if what you want is a very sensitive photo sensor - it's basically a solid-state photomultiplier tube. It's not a way to generate electrical power.
Re:So what might this mean for cameras? (Score:3, Informative)
The sensors in cameras are already many many times more sensitive to light then the rods in your eyes.
Sensors exist that can detect single photons (if properly cooled). However the sensors are not as flexible as the human eye. They tend to have a linear response to light intensity rather then the eyes Log response (rods and cones don't actually respond to the intensity of light but the signal generated by a change seems to be log) although some sensors exist that produce log outputs.
Sensors are hitting thier sensitivity limits (think low light photography) but that is in terms of sensor noise. The human eye has far more noise to but it has a massive chunk of grey matter behind it that really helps filter the noise out.
Another difference is that the cones in you eye do not respond to the intensity of light they respond to the change in the intensity of light. Sensors respond directly to the intensity of light.
So improving the sensitivity of a sensor is unlikely to have any much impact on normal photography, perhaps it may reduce the dark noise.