Silicon-based solar cells Characteristics and production processes ...

Silicon-based solar cells – characteristics and production processes
recombination - τ Auger and trapping recombination - τ trap , according to the formula
[46]:
1 1 1 1
= + +
(13)
τ τ τ τ
rad
Auger
trap
For the impurity in Si which is lower than 10 17 atom/cm 3 the radiative
recombination is not significant. The trapping recombination is determined by the
impurity level resulting from the technological process or the silicon quality, and
generally, the lifetime of the carriers connected with its existence varies within the
range of 10 -3 -10 -6 s. The determining factor for the carrier lifetime, in the case when
the impurity concentration is above 10 18 atom/cm 3 , is the Auger recombination,
which causes a linear drop of τ Auger down to 10 -10 s for the impurity concentration
of 10 20 atom/cm 3 [16]. The dependence of the charge carrier lifetime, or the
proportional diffusion length from the impurity concentration to Si, is one of the
basic issues in the thick-layered technology of crystalline silicon solar cell production.
Silicon solar cells are produced on the basis of the p-type silicon with the acceptor
impurity concentration N A equaling 1,513 x 10 16 atom/cm 3 , which corresponds to the
base material resistivity of 1 Ωcm. For the N A value lower than the one given above,
one can obtain a lower cell voltage V oc , and for the N A value of the order of 10 17
atom/cm 3 and more, a drop of τ occurs, which is mainly caused by the Auger
recombination, and this implies a drop of both I sc and V oc . The donor doped area
of the n-type by the depth of about 0,4 µm is gradient in character, with the impurity
concentration in the surface area of ~ 10 22 atom/cm 3 . The surface resistance R ρ of this
area, for a typical cell, is at the level of 40 ÷ 50 Ω/□, which does not cause problems in
the creation of an ohm contact of a low resistivity ~ 3 mΩcm, between this area and
the electrode. For the decreasing value of R ρ the cell will have a lower I sc value, and
for R ρ at the level of 60 ÷ 80 Ω/□ technological problems occur, which are connected
with the proper metallization of the front electrode contact. This problem is often
solved through such a construction of the selective emitter of the solar cell which
contains an area whose R ρ has the value of e.g. 30 Ω/□ directly under the front
electrode paths, and an area with the R ρ value of ~ 100 Ω/□ between these paths.
Solar cells differ from other electron devices in a much larger surface, which
makes the latter’s effect on the cell parameters highly significant. Breaking the
crystal’s periodicity on the surface and the existence of unsaturated bonds causes the
appearance of additional acceptor energy levels localized in the silicon’s energy gap.
On the real Si surface, coated with a thin layer of natural oxide, one can find
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