Dear Sapna,

The reverse saturation current can be used to estimate the minority carrier diffusion length in the base layer of the solar cell. Assuming this layer is p-type as usual for commercial solar cells, then one can express the the reverse saturation current density J0= q n0 Dn/ Ln , where q is the electron charge, no is the electron minority carrier concentration in the p-base region, Dn is the diffusion constant of electrons and Ln is the minority carrier diffusion length. Here, it is assumed that the dark reverse current of the emitter layer is negligible.

The important point for the validity of the above equation is that the diffusion length must be smaller than the width of the p-base layer.

So, it is expected that the largest value for Ln will be equal to the base width in case that the base width is smaller than the diffusion length.

Expressed in an other way, if w is smaller than Ln, then the above equation will become:

j0= qDn n0/ w

I hope this will answer your question.

Best wishes

Thank you Prof. Abdelhalim Zekry for this valuable information.

As you mentioned that there is an assumption of negligible dark current in emitter region for calculating diffusion length of minority carrier in base material. In my device, dark current is not negligible for the emitter region as it is made up of amorphous silicon material. So, can I say that the larger diffusion length for this device is due to higher dark current flowing into amorphous layer? 

Dear Sapna,welcome again.

You can assume that the recombination current in the amorphous layer is much larger than the recombination current in the substrate as the minority carrier life time in the amorphous layer is much smaller than that of the substrate. However you have to bring an evidence for this assumption.

One of the methods to estimate the lifetime in the different regions of a pn junction is the open circuit voltage decay with time. You will find a fast decrease of the open curcuit voltage followed by a its slower decrease.

Small signal impedance as a function of frequency or the so called impedance spectroscopy can be used also to differentiate between the lifetime in the base and in the emitter.

Best wsihes

Thank you once again sir for your concern about my queries. 

The maximum diffusion length is equivalent to the base region thickness. this length depends on doping; defects; surface recombination.