9.1.5
recombination is represented by the recombination of the minority carriers in the
quasineutral regions, the n is equal to 1. However, when other recombination mechanisms
occur, the n can have a value of 2. A high n value not only lowers the FF, but since it
signals a high recombination, it leads to a low V oc . Eq. (9.3) describes a maximum
achievable FF. In practice the FF is often lower due to the presence of parasitic resistive
losses.
Conversion efficiency
The conversion efficiency is calculated as the ratio between the maximal generated power
and the incident power. As mentioned above, solar cells are measured under the STC,
where the incident light is described by the AM1.5 spectrum and has an irradiance of I in =
1000 W/m
2
,
Typical external parameters of a crystalline silicon solar cell as shown are; J sc ≈ 35
mA/cm
2
, V oc up to 0.65 V and FF in the range 0.75 to 0.80. The conversion efficiency lies
in the range of 17 to 18%.
Example
A crystalline silicon solar cell generates a photo current density of J ph = 35 mA/cm 2 . The wafer is doped with
10 17 acceptor atoms per cubic centimetre and the emitter layer is formed with a uniform concentration of 10 19
donors per cubic centimetre. The minority-carrier diffusion length in the ptype region and n-type region is 500 ×
10 −6 m and 10 × 10 −6 m, respectively. Further, the intrinsic carrier concentration in silicon at 300 K is 1.5 ×
10 10 cm −3 , the mobility of electrons in the p-type region is μ n = 1000 cm 2 V −1 s −1 and holes in the n-type region is
μ p = 100 cm 2 V −1 s −1 . Assume that the solar cell behaves as an ideal diode. Calculate the built-in voltage, the
open circuit voltage and the conversion efficiency of the cell.
J ph
=
350Am −2 .
N A
=
10 1 7 cm −3 = 10 23 m −3 .
N D
=
10 19 cm −3 = 10 25 m −3 .
L N
=
500 × 10 −6 m.
L P
=
10 × 10 −6 m.
D N
=
(k B T/q)μ n = 0.0258V × 1000 × 10 −4 cm2V −1 s −1 = 2.58 × 10 −3 m2s −1 .
D P
=
(k B T/q)μ p = 0.0258V × 100 × 10 −4 cm2V −1 s −1 = 2.58 × 10 −4 m2s −1 .
Using Eq. (8.16) we calculate the built-in voltage of the cell,
recombination is represented by the recombination of the minority carriers in the
quasineutral regions, the n is equal to 1. However, when other recombination mechanisms
occur, the n can have a value of 2. A high n value not only lowers the FF, but since it
signals a high recombination, it leads to a low V oc . Eq. (9.3) describes a maximum
achievable FF. In practice the FF is often lower due to the presence of parasitic resistive
losses.
Conversion efficiency
The conversion efficiency is calculated as the ratio between the maximal generated power
and the incident power. As mentioned above, solar cells are measured under the STC,
where the incident light is described by the AM1.5 spectrum and has an irradiance of I in =
1000 W/m
2
,
Typical external parameters of a crystalline silicon solar cell as shown are; J sc ≈ 35
mA/cm
2
, V oc up to 0.65 V and FF in the range 0.75 to 0.80. The conversion efficiency lies
in the range of 17 to 18%.
Example
A crystalline silicon solar cell generates a photo current density of J ph = 35 mA/cm 2 . The wafer is doped with
10 17 acceptor atoms per cubic centimetre and the emitter layer is formed with a uniform concentration of 10 19
donors per cubic centimetre. The minority-carrier diffusion length in the ptype region and n-type region is 500 ×
10 −6 m and 10 × 10 −6 m, respectively. Further, the intrinsic carrier concentration in silicon at 300 K is 1.5 ×
10 10 cm −3 , the mobility of electrons in the p-type region is μ n = 1000 cm 2 V −1 s −1 and holes in the n-type region is
μ p = 100 cm 2 V −1 s −1 . Assume that the solar cell behaves as an ideal diode. Calculate the built-in voltage, the
open circuit voltage and the conversion efficiency of the cell.
J ph
=
350Am −2 .
N A
=
10 1 7 cm −3 = 10 23 m −3 .
N D
=
10 19 cm −3 = 10 25 m −3 .
L N
=
500 × 10 −6 m.
L P
=
10 × 10 −6 m.
D N
=
(k B T/q)μ n = 0.0258V × 1000 × 10 −4 cm2V −1 s −1 = 2.58 × 10 −3 m2s −1 .
D P
=
(k B T/q)μ p = 0.0258V × 100 × 10 −4 cm2V −1 s −1 = 2.58 × 10 −4 m2s −1 .
Using Eq. (8.16) we calculate the built-in voltage of the cell,