7 Crystalline Silicon Solar Cells: Heterojunction Cells
181
The capture cross section is 10
5 times larger for electrons than for holes regardless
of whether we have n-type material or p-type material. However in n-type material
the holes are the minority carriers and their density is less influenced by the capture
process than the density of majority carriers (electrons).
Thanks to the smaller capture cross-section of the holes, the latter are captured
less frequently by the impurities (here as an example, by the iron impurities) and have
longer lifetimes. At the same time, they are the minority carriers, which contribute
significantly to the open circuit voltage V oc . This is one reason why n-type silicon
leads to better cell properties than p-type silicon.
So, even in the hypothetical case where n-type material and p-type silicon would
have the same number (e.g. the same density) of impurities, and the same impurities
like iron (Fe), these impurities are less harmful in n-type silicon than in p-type silicon.
Thus, n-type silicon does not have to be gettered (see section e below), at all, in order
to obtain the material quality needed for solar cells.
(b) Carrier Lifetimes The lifetime of minority carriers is generally taken as a marker
for the quality of a solar cell material. The lifetime of minority carriers indicates how
long the minority charge carriers will, on an average, exist before they recombine
with carriers of opposite charge (e.g. with the majority carriers). If the lifetime is
too short, charge carriers cannot reach the junction from their point of origin in bulk
silicon and recombine before. For n-type material the lifetime of holes (minority
carriers) is up to approximately 10 ms, for p-type material the lifetime of electrons
(minority carriers) is only up to approximately 2 ms. The probability that the minority
charge carriers reach the junction and contribute to the solar cell current is, thus, two
times higher for n-type material.
20
(c) Carrier Mobilities In n-type material, the holes are the minority carriers and
their mobility is only 450 cm
2 /Vs. The mobility of the electrons is three times higher
at 1400 cm
2 /Vs. This then is the case in a cell with a back pn-junction: in the best
case, the charge carrier generation takes place in a cell near the back surface and the
hole diffuses to the back side by the shortest route. In the less favourable case, the
hole has to take a longer path through the bulk to reach the junction. But since the
mobility of the holes is relatively small, a high-quality bulk material is crucial, so
that even in unfavourable cases the charge carriers have a high probability to reach
the junction. n-type material satisfies these requirements, because the capture crosssection
21
σ p for holes in n-type silicon (minority carriers) is smaller than the capture
cross-section σ n for electrons in p-type silicon (minority carriers).
(d) Boron Oxygen Complex and Degradation Effects Furthermore, boron in silicon tends to form together with oxygen so-called boron-oxygen complexes,
22 which
20 Diffusion length L = (τ * D) 1/2 .
21 Explanation of capture cross-sections: Low capture cross-section means that activity radius around
impurities is small and recombination activity is low. High capture cross-section means that activity
radius around impurities is large, and recombination activity is high.
22 In addition to boron, iron and copper in combination with oxygen can also produce interference
effects.
181
The capture cross section is 10
5 times larger for electrons than for holes regardless
of whether we have n-type material or p-type material. However in n-type material
the holes are the minority carriers and their density is less influenced by the capture
process than the density of majority carriers (electrons).
Thanks to the smaller capture cross-section of the holes, the latter are captured
less frequently by the impurities (here as an example, by the iron impurities) and have
longer lifetimes. At the same time, they are the minority carriers, which contribute
significantly to the open circuit voltage V oc . This is one reason why n-type silicon
leads to better cell properties than p-type silicon.
So, even in the hypothetical case where n-type material and p-type silicon would
have the same number (e.g. the same density) of impurities, and the same impurities
like iron (Fe), these impurities are less harmful in n-type silicon than in p-type silicon.
Thus, n-type silicon does not have to be gettered (see section e below), at all, in order
to obtain the material quality needed for solar cells.
(b) Carrier Lifetimes The lifetime of minority carriers is generally taken as a marker
for the quality of a solar cell material. The lifetime of minority carriers indicates how
long the minority charge carriers will, on an average, exist before they recombine
with carriers of opposite charge (e.g. with the majority carriers). If the lifetime is
too short, charge carriers cannot reach the junction from their point of origin in bulk
silicon and recombine before. For n-type material the lifetime of holes (minority
carriers) is up to approximately 10 ms, for p-type material the lifetime of electrons
(minority carriers) is only up to approximately 2 ms. The probability that the minority
charge carriers reach the junction and contribute to the solar cell current is, thus, two
times higher for n-type material.
20
(c) Carrier Mobilities In n-type material, the holes are the minority carriers and
their mobility is only 450 cm
2 /Vs. The mobility of the electrons is three times higher
at 1400 cm
2 /Vs. This then is the case in a cell with a back pn-junction: in the best
case, the charge carrier generation takes place in a cell near the back surface and the
hole diffuses to the back side by the shortest route. In the less favourable case, the
hole has to take a longer path through the bulk to reach the junction. But since the
mobility of the holes is relatively small, a high-quality bulk material is crucial, so
that even in unfavourable cases the charge carriers have a high probability to reach
the junction. n-type material satisfies these requirements, because the capture crosssection
21
σ p for holes in n-type silicon (minority carriers) is smaller than the capture
cross-section σ n for electrons in p-type silicon (minority carriers).
(d) Boron Oxygen Complex and Degradation Effects Furthermore, boron in silicon tends to form together with oxygen so-called boron-oxygen complexes,
22 which
20 Diffusion length L = (τ * D) 1/2 .
21 Explanation of capture cross-sections: Low capture cross-section means that activity radius around
impurities is small and recombination activity is low. High capture cross-section means that activity
radius around impurities is large, and recombination activity is high.
22 In addition to boron, iron and copper in combination with oxygen can also produce interference
effects.