180
S. Leu and D. Sontag
This equation is discussed in detail in Chap. 3.
For BSF and PERC cells approximate values for the density of minority carriers
are:
Standard BSF cell 10
14 cm
−3
PERC-cell
10
15 cm
−3 .
The reason for these relatively low densities of minority carriers is the poor
passivation of BSF and PERC cells and the high value for J 0 .
This is quite different in the case of the HJT cell: Here the very good passivation
leads to a higher density of minority carriers
HJT cells 10
16 cm
−3 ,
which according to (7.3) leads to higher values of V oc because J 0 is lower.
7.3 n- and p-Type Wafers
The entire development of standard solar cells is based on the use of p-type, e.g.
boron-doped crystalline silicon. This development is historically due to the fact that
in the early days of photovoltaics, wafers were manufactured from p-doped waste
products of the semiconductor Industry. Over the years, the amount of waste was no
longer sufficient and much effort was made to produce high-quality p-type silicon
specifically for photovoltaics. In the past p-type material was cheaper to produce
than n-type material. Today, both kinds of materials have more or less the same
production costs. In principle, the question naturally arises why one generally uses
n-type crystalline silicon for heterojunction (HJT) solar cells and not p-type silicon.
There are several reasons which are listed in the next sections:
(a) Capture Cross-sections In the comparison of n-type and p-type silicon, the
key quantity “capture cross-section” plays a decisive role. The capture cross-section
indicates the size of the capture radius for electrons or holes, for capture by foreign
atoms (impurities).
19 In the following example, we assume that iron is the dominant
impurity in silicon. We will investigate the cross section capture of iron for electrons
and holes more closely.
As an example we consider capture cross section of iron [13] for electrons (e) and
holes (h):
σ e = 3.5 × 10
−11 cm
−2
σ h = 4.5 × 10
−16 cm
−2
19 Actually the term «capture cross-section» is used not only in the case of impurities, but in the
case of all recombination centres or defects—as an example it is also used in the case of “dangling
bonds”, which appear in amorphous silicon layers (see Chap. 6).
S. Leu and D. Sontag
This equation is discussed in detail in Chap. 3.
For BSF and PERC cells approximate values for the density of minority carriers
are:
Standard BSF cell 10
14 cm
−3
PERC-cell
10
15 cm
−3 .
The reason for these relatively low densities of minority carriers is the poor
passivation of BSF and PERC cells and the high value for J 0 .
This is quite different in the case of the HJT cell: Here the very good passivation
leads to a higher density of minority carriers
HJT cells 10
16 cm
−3 ,
which according to (7.3) leads to higher values of V oc because J 0 is lower.
7.3 n- and p-Type Wafers
The entire development of standard solar cells is based on the use of p-type, e.g.
boron-doped crystalline silicon. This development is historically due to the fact that
in the early days of photovoltaics, wafers were manufactured from p-doped waste
products of the semiconductor Industry. Over the years, the amount of waste was no
longer sufficient and much effort was made to produce high-quality p-type silicon
specifically for photovoltaics. In the past p-type material was cheaper to produce
than n-type material. Today, both kinds of materials have more or less the same
production costs. In principle, the question naturally arises why one generally uses
n-type crystalline silicon for heterojunction (HJT) solar cells and not p-type silicon.
There are several reasons which are listed in the next sections:
(a) Capture Cross-sections In the comparison of n-type and p-type silicon, the
key quantity “capture cross-section” plays a decisive role. The capture cross-section
indicates the size of the capture radius for electrons or holes, for capture by foreign
atoms (impurities).
19 In the following example, we assume that iron is the dominant
impurity in silicon. We will investigate the cross section capture of iron for electrons
and holes more closely.
As an example we consider capture cross section of iron [13] for electrons (e) and
holes (h):
σ e = 3.5 × 10
−11 cm
−2
σ h = 4.5 × 10
−16 cm
−2
19 Actually the term «capture cross-section» is used not only in the case of impurities, but in the
case of all recombination centres or defects—as an example it is also used in the case of “dangling
bonds”, which appear in amorphous silicon layers (see Chap. 6).