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?

--
Rich

[mike]

On 4/29/2013 9:36 PM, RichD wrote:
> 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]

On 30/04/2013 05:36, RichD wrote:
> 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?
>
> --
> Rich

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


--
Regards,
Martin Brown

[benj]

On Tue, 30 Apr 2013 09:17:13 +0100, Martin Brown wrote:

> On 30/04/2013 05:36, RichD wrote:
>> 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?

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.

[Phil Hobbs]

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.

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510 USA
+1 845 480 2058

hobbs at electrooptical dot net
http://electrooptical.net

[Tim Wescott]

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...

--
My liberal friends think I'm a conservative kook.
My conservative friends think I'm a liberal kook.
Why am I not happy that they have found common ground?

Tim Wescott, Communications, Control, Circuits & Software
http://www.wescottdesign.com

[Phil Hobbs]

On 4/30/2013 1:13 PM, Tim Wescott wrote:
> On Mon, 29 Apr 2013 21:36:22 -0700, RichD wrote:
>
>> 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?
>
> 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...
>

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

[RichD]

On Apr 30, benj <b...@iwaynet.net> wrote:
> >>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?
>
> 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?

--
Rich

[RichD]

On Apr 30, Tim Wescott <t...@seemywebsite.com> wrote:
> >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.
>

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



--
Rich

[benj]

On Tue, 30 Apr 2013 14:02:30 -0700, RichD wrote:


> 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.

[Salmon Egg]

In article
<25fb2fe0-9d40-4154...@mq5g2000pbb.googlegroups.com>,

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.

--

Sam

Conservatives are against Darwinism but for natural selection.
Liberals are for Darwinism but totally against any selection.

[Jasen Betts]

On 2013-04-30, Tim Wescott <t...@seemywebsite.com> wrote:
> On Mon, 29 Apr 2013 21:36:22 -0700, RichD wrote:

> 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.

--
⚂⚃ 100% natural

--- news://freenews.netfront.net/ - complaints: ne...@netfront.net ---

[Phil Hobbs]

On 5/2/2013 7:04 AM, Jasen Betts wrote:
> On 2013-04-30, Tim Wescott <t...@seemywebsite.com> wrote:
>> On Mon, 29 Apr 2013 21:36:22 -0700, RichD wrote:
>
>> 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.
>

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

[dagmarg...@yahoo.com]

On May 2, 9:10 am, Phil Hobbs <pcdhSpamMeSensel...@electrooptical.net>
wrote:

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

> 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.

--
Cheers,
James Arthur

[dagmarg...@yahoo.com]

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.

--
Cheers,
James Arthur

[Peter Fairbrother]

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