Aliexpress solar cells as photodiodes--not

So 10 uF per square metre. That sounds like a lot, until I worked it out for the Osram SFH2700FA I'm using for a new design. 0.59 x 0.59 mm and

4.6 pF, which works out to 13 uF per square metre. It's a PIN device, which is supposed to reduce the capacitance. Or am I missing something?

I like the Osram parts so far, but it's not a very demanding application bandwidth wise. It _is_ very space constrained, hence the small area. It's the best of a similar bunch I tested for sensitivity.

The capacitance of my single poly-Si solar cell 102mm*102mm is 4.5 μF, shunt resistance = 200 Ohm. It is possible to neutralize the capacitance at a certain frequency using inductance.

I don't suppose reverse biasing will help much ? How about bootstrapping ?

Brian

I misspoke--mine are 30x50 _millimetres_. So it's more like 1 millifarad per square metre, or 100 nf/cm**2. Really good PIN photodiodes run about 40-100 pf/cm**2 when fully depleted, about 5-7x that at zero bias.

I'm out of the lab today, but I'll try resonating the capacitance and see what kind of Q I get. I need to work around 220 Hz, which is far enough from harmonics of both 50 and 60 Hz for my purposes--at 1.5 uF, that needs a 300-mH inductor.

There's such a thing as a parametric gyrator, so it might even be possible to use a Y5V cap and some magic to make a sufficiently-quiet simulated inductor.

(I'll get a nanoamp of photocurrent if I'm lucky, so the Q has to be high or I'm better off with a smaller detector.)

I don't really think that the nasty $3 AliExpress gizmo is the right answer for the actual measurement, but it's worth checking out the parameter space.

That sounds like a lot, until I worked it out

Cheers

Phil Hobbs

Lots of big ICs have on-chip bypass caps. I measured one Xilinx FPGA that had hundreds of nF caps on various supply rails. So I don't worry about bypassing much.

I guess you can make a lot of c when the dielectric is nanometers thick.

I'm trying to explore that as well as some other possibilities that don't involve big honkin' chunks of iron.

The transimpedance amp forces its input noise e_Namp across the detector capacitance, resulting in a 1-Hz current noise of

i_N = 2 pi f * C_d * e_Namp

In a lowpass system, the total noise from this source goes up quadratically with bandwidth.

For a given photocurrent, we can figure out where we get into the shot noise limit by finding where the two contributions are equal. Comparing the Johnson noise of the load resistor to the shot noise, we find that we're shot noise limited when

4 k T

----- < 2 e I_photo. e R_L

Rearranging this a bit, a photodiode plus load resistor system is shot noise limited when

I_photo R_L > 2 k T / e,

which is 51 mV at room temperature. This is a super-useful rule of thumb.

For a 1-nA photocurrent, I'd need a 50 Mohm load resistance to reach the shot noise, assuming the e_N*C noise doesn't dominate. Avoiding that sinister fate at a frequency f means that

2 pi f C_d * e_N < sqrt(2 e I_photo) ,

or

sqrt(e I_photo/2) e_N < ----------------- . pi f C_d

Note that e_N has to include the Johnson noise of the series resistance of the detector, and that any significant contribution from the shunt resistance has to be taken into account as well.

With center frequency of 220 Hz, 1.5 uF capacitance, and a 1-nA photocurrent, to stay in the shot noise we'd need

e_N < 9 pV/sqrt(Hz) including the series resistance contribution, which is, um, challenging. ;)

Cheers

Phil Hobbs

Thanks.

At very low photocurrents, reverse bias is not always a win even with PIN photodiodes. You reduce the e_N*C noise by a factor of 5 or thereabouts, but if the leakage current is 100x the photocurrent, that winds up being worse.

For amorphous thin-film solar cells, you'd expect high doping levels to reduce series resistance, which means that reverse bias won't reduce the capacitance much. (The 'I' in 'PIN' refers to a thick layer of very low-doped (intrinsic) silicon that can be fully depleted at a reasonable voltage.)

Bootstraps and TIAs both exhibit the e_N * C mechanism, but a good JFET (or pHEMT, at least above 1 MHz) is a lot quieter and has lower input current and capacitance than an op amp.

Cheers

Phil Hobbs

And has a huge epsilon as well.

Cheers

Phil "bobbing for photons" Hobbs

At best, the shunt resistance of the solar cell is of the order of 1000 Ohm(( Maybe VTS3080 is a better option?

cubically> with bandwidth.

Turns out that based on resonating the cell with a 330-uh inductor, its series resistance is only a couple of ohms and its capacitance is 0.82 uF at 10 kHz.

It might be pretty interesting to resonate it with half a henry or so. I expect it should be possible to get that much inductance with an ungapped permalloy core, as used in current transformers. Getting the right value might be more challenging. One of those Y5V varactors of Joerg's might be the ticket. ;)

Cheers

Phil Hobbs

A regular gapped ferrite might do it.

The B65887E1000J041 gaped core pair offers an Al of 1000nH (+/-5%) per root turn, and an RM14 core has about 100mm^2 winding area, so you could put on a thousand turns of fine wire - up to 0.3mm OD, which isn't all that fine.

Ferroxcube does seem to do an RM14 core with a centre hole for a screw-in inductance adjuster. You might have to go for a core with a bigger gap to get one where the adjustor could be guaranteed to cover the whole range, but with a thousand turns to start with adding a or subtracting single turns should let you get pretty close.

Interwinding capacitance won't be trivial, but it won't be anything like even a nanofarada either.

TDR it! Sort of. With a function generator and a scope.

That shows capacitance, esr, and esl all at once.

A Y5V-varactor parametric gyrator would be really fun to build.

I haven't gone into it very deeply yet, but if it's possible at low frequency, it could be very interesting. You don't need much voltage headroom for a 1-nA photocurrent at any reasonable impedance level.

Cheers

Phil Hobbs

A 50 mV photocurrent signal level across the 1000 Ohm shunt junction corresponds to a 50,000 nA signal. How to avoid the noise generated by this parasitic internal shunt resistance of the solar cell?

1nA * 1000 Ohm = 1µV

Nice paper.

Aren't the nonlinear dielectrics kinda leaky too?

There must be something useful that can be done with horrible nonlinear caps.

There is one shock line that has a slab of ceramic sandwiched between two metal plates. A pulse gets faster as it travels down the line, becoming a pretty fast kilovolt edge at the end.

As Dmitriy suggested, the shunt resistance isn't great on these devices, so we'll have to dig a bit deeper.

Cheers

Phil Hobbs

Why did you pick amorphous cells rather than monocrystalline silicon?

Because they're super cheap. I just tested a 25x8 mm amorphous cell with a glass substrate (harvested from one of those solar-powered hula dancer figurines) and it measured 5 meg, so it ain't necessarily impossible.

Cheers

Phil Hobbs