knjiga solarna energija dobra

SOLAR CELL FUNDAMENTALS 97
0.85
0.80
FF
4.0
0.75
I SC
3.5
0.70
V OC
[V], FF
0.65
0.60
0.55
V OC
L n < W p
L n > W p
3.0
2.5
I SC
[A]
0.50
2.0
10 −6 10 −5 10 −4 10 −3
Base lifetime tn
[s]
Figure 3.17 Effect of base lifetime on solar cell performance for the solar cell parameters in
Table 3.2. The minority-carrier diffusion length (L n = √ D n τ n ) is equal to the base thickness (W P )
when τ n = 25.7 µs
Front surface recombination for solar cells with contact grids on the front of the
device is really an average over the front surface area of the relatively low surface
recombination velocity between the grid lines and the very high surface recombination
velocity of the ohmic contact. An expression for the effective front surface recombination
velocity is given by [16]
⎡
(
(1 − s)S F G N τ p cosh W )
N
− 1 + p o (e qV/AokT − 1) ⎢
L p
⎣ s D cosh W ⎤
N
p L p
L p
sinh W + S F
⎥
N
⎦
L p
S F,eff =
(
(1 − s)
[p o (e qV/A okT
− 1) + G N τ p cosh W )]
N
− 1
L p
(3.144)
where S F is the surface recombination velocity between the grid lines and G is the
average generation rate in the emitter region. It is obvious that S F,eff is dependent upon
the solar cell operation point. This is better seen in Table 3.3 where some special cases
are illustrated (assuming L p ≫ W N ).