22.4 Solar Cells
625
(a)
(b)
Fig. 22.55 Global sunshine distribution in a January and b July. The sunshine fraction is the actual number of bright
sunshine hours over the potential number, and is thus expressed as a percentage figure. The color scale reaches from 0
to 100%. The sunshine data are in a 0.5 degree grid based on data from [1872]
illumination (Fig. 22.56) is given by
I = I s
exp (βV ) − 1
− I L ,
(22.60)
with I L being the current due to generation of excess carriers by the absorption of the sunlight. Assuming
a simple n
+ p-diode solar cell model, the current consists of two components: the depletion layer
current j DL from carriers absorbed in the depletion layer (field region) and the diffusion current j D
from absorption in the neutral region ( j = I /A).
For the drift current out of the depletion layer of width w it can be assumed that it is collected fast
and recombination plays no role. Thus (cmp. (22.31))
j DL (λ) = e n ph (λ) [1 − R(λ)] [1 − exp(−α(λ) w)] ,
(22.61)
where λ is the wavelength of the incident radiation, R the reflectance of the surface, α the absorption
coefficient and n ph (λ) the photon flux (number of photons per area and time) at the given wavelength.
For a solar spectrum, an integral needs to be performed over the spectral distribution:
j DL =
j DL (λ) dλ ,
(22.62)
the total photon flux being n ph =
n ph (λ) dλ.
625
(a)
(b)
Fig. 22.55 Global sunshine distribution in a January and b July. The sunshine fraction is the actual number of bright
sunshine hours over the potential number, and is thus expressed as a percentage figure. The color scale reaches from 0
to 100%. The sunshine data are in a 0.5 degree grid based on data from [1872]
illumination (Fig. 22.56) is given by
I = I s
exp (βV ) − 1
− I L ,
(22.60)
with I L being the current due to generation of excess carriers by the absorption of the sunlight. Assuming
a simple n
+ p-diode solar cell model, the current consists of two components: the depletion layer
current j DL from carriers absorbed in the depletion layer (field region) and the diffusion current j D
from absorption in the neutral region ( j = I /A).
For the drift current out of the depletion layer of width w it can be assumed that it is collected fast
and recombination plays no role. Thus (cmp. (22.31))
j DL (λ) = e n ph (λ) [1 − R(λ)] [1 − exp(−α(λ) w)] ,
(22.61)
where λ is the wavelength of the incident radiation, R the reflectance of the surface, α the absorption
coefficient and n ph (λ) the photon flux (number of photons per area and time) at the given wavelength.
For a solar spectrum, an integral needs to be performed over the spectral distribution:
j DL =
j DL (λ) dλ ,
(22.62)
the total photon flux being n ph =
n ph (λ) dλ.