Hi,
I have been trying to measure EQE of silicon solar cells manufactured in our lab. Supposedly, the EQE should be "zero" after the bandgap wavelength (~1100 nm). Apparently, this is not the case and at about 1100 nm the EQE is about 20%. I am wondering if anyone can explain this phenomenon and bring reasons why this happens?
P.S.1: I have looked at many references and this is the case for them as well.
M.A.Green et. al., Prog. Photovolt: Res. Appl. 24, 3 (2016)
P.S.2: Attached is EQE graph of standard Si photodiode which I measured. It also shows about 20% of EQE above bandgap wavelength.
The bandgap of silicon is somewhat ill-defined. The density of states below the bandgap just tails off, and it is a question of definition where you place the bandgap. Si still has a considerable density of states below 1100nm, therefore you can still get reasonable EQE, in particular for thick silicon wafers. See e.g. here: http://pveducation.org/pvcdrom/materials/optical-properties-of-silicon
The bandgap of silicon is somewhat ill-defined. The density of states below the bandgap just tails off, and it is a question of definition where you place the bandgap. Si still has a considerable density of states below 1100nm, therefore you can still get reasonable EQE, in particular for thick silicon wafers. See e.g. here: http://pveducation.org/pvcdrom/materials/optical-properties-of-silicon
It may be due to high base diffusion length, low rear surface recombination velocity or up conversion of incident photons.
the one that we have is a mono-crystalline Si cell (15.5 cm * 15.5 cm). It is a single pn junction with positive contact on front and negative contact on the rear surface. 4-wire I-V measurement is done to capture the characteristics.
I agree on the up-conversion of the incident photons. Other reasons I found include: not a fully monochromatic wavelength (usually with a 5nm FWHM), uncertainty in beam signal (~2%), and not an exact bandgap (lower or higher than 1.14eV that results in wavelengths higher or lower than 1100nm).
I fully agree with Mr. Ehler, you can measure band to band absorption for wavelengths above 1100 nm as you can read in:
http://scitation.aip.org/content/aip/journal/adva/5/6/10.1063/1.4923379
In my experience the EQE for Si-solar cells typically does fall below 1% around 1200 nm. Therefore your measurement of 20% at 1100 nm seems perfectly reasonable.
I'm more curious that the Max is at 65%, you must have a lot of reflection or parasitic absorption at the front.
I would like to add to what is said by the respected colleagues that an important point to consider is that silicon is indirect band gap semiconductor. It can absorb photons with the assistance of phonons. That is it can absorb photons with energy lower than the energy gap by the the phonon energy which is on the average amounts to the thermal energy of about 25 meV at room temperature.
The other effect is the relatively high doping which adds allowed states directly under the conduction band edge. There are also the emitter region and the back surface field region which are heavily doped creating impurity bands.
More important is the band-gap narrowing due to heavy doping. This effect leads to what is called red shift of the edge of the bands.
Temperature rise would decrease the band gap energy and also affect a red shift.
The effect of the back materialization will enhance the relatively long wave absorption as it effectively increases the optical path length in the material.
Best wishes
Thanks for all the responses...all the responses totally make sense and are really interesting....
Just as noted above: the reason maximum EQE is about 65% is due to high reflection at the surface...
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