a question re solar cells

I recently attendesd a seminar on photovoltaic research. The speaker showed a graph, how the power efficiency drops as the recombination time of the minority carriers decreases. Can anyone expound on this?

And, the quantum efficiency drops, as well. What does that mean?

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Rich
Reply to
RichD
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The electrons have to get to the wire. That takes time. The any electron that recombines never gets out the wire.

Reply to
mike

If the electron recombines before it reaches the external connection that the energy that it had is just thermalised as heat.

I was a bit surprised to see how much they suffer when hot. eg

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Regards, 
Martin Brown
Reply to
Martin Brown

So what is "quantum efficiency"? It's simply the ratio of photons in to electrons out the wires. So if a photon comes in creates an electron-hole pair but they recombine before being collected for the output. Then that photon has no output. Hence QE is lower. Simple.

What do you mean. Only some of the technologies (especially the "compromise" ones) experience severe drop-off. The problem is efficiencies so low as to limit utility. They won't be saving the planet soon.

Reply to
benj

The available output voltage drops by a couple of millivolts per degree, like most other forward biased diodes. (*) If you're getting 0.55V at room temperature, you'll get only ~0.45 V at 70 C, which is almost a 20% drop.

Cheers

Phil Hobbs

(*) There are occasional exceptions, because I_S is a nonlinear function of current density--ordinary Si diodes' TC goes up to about -3 mV/K down in the picoamps.

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Dr Philip C D Hobbs 
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Reply to
Phil Hobbs

The quantum efficiency of a light-to-electric device refers to the proportion of photons that get converted to electrons.

I'm not sure why quantum efficiency would suffer as recombination time goes down unless (a) they're referring to the overall quantum efficiency (in which case power efficiency and quantum efficiency are just synonyms) or (b) low recombination time means a greater chance that an electron will never get knocked fully out of the valence band, and hence will never contribute to power generation.

But that's just guessing...

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Reply to
Tim Wescott

The maximum power point will move down in voltage as the temperature goes up, but not as fast as the open circuit voltage. Thus the amount of current lost as forward bias will increase with voltage, i.e. the operating quantum efficiency will drop.

The short-circuit QE shouldn't drop much, I shouldn't think.

Cheers

Phil Hobbs

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Dr Philip C D Hobbs 
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Reply to
Phil Hobbs

How is that different than power conversion efficiency?

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Rich
Reply to
RichD

So, it's possible to see high quantum efficency, but low power output efficiency?

That would be an argument for thin junctions, yes/no?

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Rich
Reply to
RichD

No. IF Quantum efficiency is the ratio of photons in to electrons out, then any inefficiency will reduce that ratio.

It's not that simple. Thin junctions reduce recombination but you lose efficiency if the light goes clear through the thin junction without being absorbed. So there are lots of compromises here. It's why this sort of thing has a lot of "art" to the designs.

Reply to
benj

The energy in a photon is proportional to its frequency. Ordinarily. only one electron-hole pair is produced per photon in a photodiode. The quantum efficiency is the number of such pairs divided by the number of photons. Thus the the theoretical spectral quantum efficiency cannot be greater than unity. For truly high energy photons, in soft x-rays and shorter wavelength, you start getting Compton scatter, bremsstrahlung, and other mechanisms that increase the quantum efficiency above unity.

If output is taken from the diode a a fixed voltage, the amount of energy will be proportional to the number of carriers leaving an electrode. For example, the electrons going from an anode into an external copper wire. On average these electrons are generated from various energy photons but all come out at the same terminal potential.

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Sam 

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Reply to
Salmon Egg

I can't see power efficiency being synonymous with quantum efficiency solars photons go in with an mean energy of around 2eV and you get electron current out (of a silicon photocell) at less than a third of that.

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Reply to
Jasen Betts

Solar cells are diodes working in forward bias. If you short-circuit the cell, you lose practically nothing to recombination (i.e. forward conduction), so you get all of the photocurrent, and therefore the maximum operating quantum efficiency. Unfortunately you get zero power, because P = VI.

If you open-circuit it, you get the maximum terminal voltage, i.e. the maximum energy per electron, but you waste all of the photocurrent forward biasing the diode, i.e. the operating quantum efficiency is zero.

In between, you get less than maximum voltage and less than maximum current, but since both are nonzero you also deliver power to the load.

The maximum power point is where d(VI)/dV = 0, i.e.

I + V dI/dV =0 so I/V = - dI/dV

If you increase the temperature, the forward voltage of the diode decreases just like any other diode, so if you keep the same operating voltage you start to lose current (i.e. the operating quantum efficiency goes down). To maintain maximum power, you have to reduce the operating voltage, which will increase the current some, but not all the way back to its lower-temperature value.

So if you want to maintain maximum power, you have to run at lower operating QE as the temperature increases, and the power you get is reduced.

There are other things going on too, primarily the increased ohmic resistance, which reduces the terminal voltage even more.

Cheers

Phil Hobbs

As I already pointed out, the maximum power point for a solar cell is where the amount of power you lose due to

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Dr Philip C D Hobbs 
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Reply to
Phil Hobbs

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You can get higher quantum efficiency by throwing a tarp over it, reducing the temperature. Makes 'em last longer too.

ed.

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Cheers, 
James Arthur
Reply to
dagmargoodboat

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I designed a really nifty solar space-heater a few years ago, only to realize the winter I designed it was running overcast/cloudy 8.5 days out of 10. (I measured "cloudy" intensity at 3-to-5% of clear, on typical days, so that's a lot of cold days.)

Fast-forwarding to today, there's a guy locally with a lot of solar panels selling for $1/W. This inspires an idea--air blown up under the panels could cool the panels, harvest the waste heat for heating, and I could be without heat or electricity both, all winter long.

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Cheers, 
James Arthur
Reply to
dagmargoodboat

Photoelectric efficiency is the power of the light falling on the cell divided by the energy output of the cell. It is seldom more than 20%.

There are several reasons for this - first, not all quanta are absorbed, some are reflected.

Second, some of the quanta which are absorbed do not create electron/hole pairs, and some electrons and holes recombine before they reach the cell's electrodes.

The proportion of quanta absorbed which produce electrons which reach the electrodes is often called the quantum efficiency, though technically the term is to mean the proportion of the quanta which fall on the cell which produce electrons which reach the electrodes. It is measured in electrons per photon.

Third, light falls on the cell in quanta with energies somewhere between

1.6eV (for red light) and 2.8 eV (for blue light). When a quantum is absorbed, some will have high initial energy and some lower initial energy, but the electrons they push out will all have the energy of the bandgap when they leave the electrodes, and some energy is always lost here.

Fourth, the cell has some electrical resistance.

fifth ...

-- Peter Fairbrother

Reply to
Peter Fairbrother

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