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Photovoltaic cell efficiency question. - Page 3

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Posted by frigitar on March 24, 2009, 1:34 am
 

Haha, that question had me in a bit of knot for a while!

Well I think this wafer would surely absorb more green wavelengths.
Any "reflection" only occurs due to the actual color of the cell
itself.
So as long as our panels are not green in colour I guess we are
absorbing all the green that the Silicon can. Unfortunately, Silicon
is not so good at absorbing green in the first place :(

Posted by BobG on March 24, 2009, 2:00 am
 

==========================
=====================
What determines the peak frequency of the silicon freq resp? The
dimensions of the silicon molecules? In other words,  how do you dope
up the molecule to move the resonance from 800nm to 500nm? Smaller
wavelenth, faster frequency... you'd need to make it lighter somehow.
Dope it with something lighter like Boron? The PV wafer already has a
P layer and an N layer. How about turn it over, so the other layer is
exposed to the sun?

Posted by daestrom on March 24, 2009, 11:45 pm
 

==============================================
What determines the peak frequency of the silicon freq resp? The
dimensions of the silicon molecules? In other words,  how do you dope
up the molecule to move the resonance from 800nm to 500nm? Smaller
wavelenth, faster frequency... you'd need to make it lighter somehow.
Dope it with something lighter like Boron? The PV wafer already has a
P layer and an N layer. How about turn it over, so the other layer is
exposed to the sun?



I don't believe its the atomic weight but the energy needed to 'lift' an
electron out of its orbit.  Once freed from an atom, the electric field of
the depletion zone at the junction moves it across to the other side so it
can't recombine directly back to the atom it came from.  So it has to flow
through the external circuit to get back to the atom it came from.  (well,
actually it just bumps another electron that bumps another, and another...
until finally one bumps back into the original atom of silicon).

The energy to 'lift' an electron has to come in one 'packet' of light, it
can't work it's way out gradually (quantum mechanics).  So a single photon
has to have that much energy.  (Photons with shorter wavelength (towards the
blue end of the spectrum) have more energy.)  This is known as the
'photo-electric effect'.  If the photon is only slightly more energy than
needed, the electron is 'lifted' but with a bit extra 'speed' when it's
free.  The extra energy is quickly dissipated as heat as the electron
'collides' with other atoms.

If a photon has a lot more energy than needed, then it may 'lift' the
electron out of the atom's orbit and re-emit a low energy photon that
carries off the left-over energy.  This is 'Compton scattering'.

So the issue is to find a way to make it take less energy to free an
electron from silicon.  Kind of against 'natural law' of silicon.  Of course
different doping can change the electric field in the depletion zone and
that can help a bit.  Or try something besides silicon.  But the next larger
electron 'shell' gets pretty vague and messy.

daestrom


Posted by BobG on March 25, 2009, 1:45 am
 Thanks for the explanation.... Somehow I knew it wasnt something
simple like mass and resonance. I always thought the 'serendipity' of
the color of the solar radiation and the color of leaves and
chlorophyll was a divine symbiosis. I was trying to imagine an analogy
that a pv cell that was sensitive to green radiation would be more
efficient somehow.

Posted by Eeyore on March 25, 2009, 12:37 pm
 

daestrom wrote:


http://en.wikipedia.org/wiki/Planck 's_constant

Graham


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