Dear Djaber,

Welcome,

The basic difference between the front and back illumination comes from the collection process of the generated electron holes resulting from the incident solar radiation. In addition to the nature of the absorption of the incident radiation. It is so that the shorter wavelength radiation will be absorbed nearer to the incident surface while the longer wave lengths penetrate  to a longer distance. If the minority carrier lifetime of the mobile charge carriers is equal throughout the thicness of the cell and the surface recombination is same and they have to diffuse the same diffusion length to the field collecting region, then the conversion efficiency may be the same.

But it is so that the collecting field region lies between the pn junction in the top of the cell. Also, the carrier lifetime is varying across the thicness of the solar cell with the recombination velocity increases near the top and bottom surfaces.

Because of the nearer field region the generated electron hole pairs  generated nearer to them have more change to be collected rather than to recombine and get lost.

So, and since the shortwave length radiation will be absorbed near the surface and the field region, most of them will get collected. 

While the back illumination will lead to more generation of electron hole pairs near the bottom and far from the collecting junction laying on the top, and consequently the probability of their recombination will be smaller than their collection.

Consequently the photcurent of the front illumination will be basically larger than that of the back illumination.

wish you success 

Trying to summarize Prof Zekry's detailed explanation . . . . basically light from the rear results in fewer photons being turned into useful electrons. So less power and less efficiency.

Actually, there is three basic phenomena in the junction based solar cells, (1) Light absorption and charge generation, (2) Charge separation, and (3) Charge collection. After light absorption, the charge carriers are generated. The absorption of light is wavelength dependent, the light up to 900 nm wavelength absorb with in 30 micron. Even low wavelength light (up 500 nm absorbs with in 1 micron very close to junction). In case of front illumination, most of charge carriers are generated near the junction and are separated efficiently and hence collection efficiency is high. Therefore, performance is high. However, in back illumination, the most of charge carriers generate away from junction hence collection efficiency is very low. Therefore performance is also low. Please see http://www.sciencedirect.com/science/article/pii/S0038092X13003629

In some cases, the metalized contact grid on the back contact may be more dense to reduce the serial resistance, but at the same time it also shades larger area and allows fewer photons to enter the cell.

Hi, everyone!

Thank you a lot for your answers. I have a question regarding the efficiency concept of bifacial solar cells. How can you define the efficiency of bifacial solar cells, knowing that there are two possible illumination sides, unlike mono-facial solar cell?