a question re solar cells

[RichD]

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?

 

[mike]

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?

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

 

[Martin Brown]

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?

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

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

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

http://dspace.mit.edu/bitstream/handle/1721.1/59937/676836192.pdf?...1

 

[benj]

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

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.

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

http://dspace.mit.edu/bitstream/handle/1721.1/59937/676836192.pdf?...1

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.

 

[RichD]

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.

How is that different than power conversion efficiency?

 

[RichD]

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.

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

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

 

[benj]

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

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

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

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.

 

[Jasen Betts]

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)

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.

 

[[email protected]]

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.

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

 

[[email protected]]

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

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.

 

[Peter Fairbrother]

How is that different than power conversion efficiency?

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