Due to the extremely short travel distances of photo-generated carries, the electric field needs to be present at the origin of photo-generation, in order to assist carrier travel and immediately separate electrons and holes and thus to avoid their recombination. The field assisted travel distance is given by drift length. For optimum utilization of the incident irradiance, the electric field must extend throughout the entire cell thickness. this is accomplished by inserting an i- layer between the p-and n-layers, p-i-n diode is formed.
With the relatively low penetration depths and, with low values of i-layer thickness, the electric field strength E(x), determined by built in voltage and the i-layer thickness, reaches a sufficient magnitude for efficient carrier separation.
For collection efficiency: collection is governed by the drift length L(drift) of both electrons and holes with in the intrinsic layer...the drift length L(drift) has a value that is about 10 times larger than the minority carrier diffusion length L(diff)
for more information read THIN FILM SILICON SOLAR CELLS by Aravind Shah first edition.
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