Page 155 - A Comprehensive Guide to Solar Energy Systems

156 A CoMPrehensIVe GuIde To soLAr enerGy sysTeMs

∞ ∞
∫
λ x d
x
λ x d
G () = G(; ) λ = ∫ αλ βλ Φ)(; ) λ. (8.6)
)(
(
tot
Gtot(x)=∫0∞G(λ;x)dλ=∫0∞α(λ)β(λ 0 0
)Φ(λ;x)dλ.
The number of carriers generated in solar energy conversion depends strongly on the
type of semiconductor (bandgap, band structure).
Photons with too low energy are not absorbed and their energy cannot be transferred in
excess carrier generation. surplus energy of photons with energy higher than the bandgap
energy is mostly transformed into heat. Therefore, only a part of incident solar energy can
be converted in free charge generation and, consequently, into electric power. The part
of the solar spectrum converted in carrier generation in the case of crystalline silicon is
shown in Fig. 8.4.
In some materials, after photon absorption an exciton can be generated, that is, an excited
electron/hole pair that is still in a bound state due to the Coulomb forces between the par-
ticles. such exciton can diffuse in material and it can dissociate in an electron–hole pair after
obtaining an additional energy higher than its bounding energy, or recombine. exciton gen-
eration is an important phenomenon in organic materials and at organic–inorganic material
junctions. Further details will be discussed in Chapters 11 and 12.

8.2.2 Carrier Recombination

As discussed in the previous section, excess carrier pairs are created by photogeneration
and the carrier concentration is higher than it is at thermodynamic equilibrium. The sys-
tem tends to reach equilibrium and free electrons and holes recombine (electron takes

FIGURE 8.4 Energy converted in carrier generation in the case of crystalline silicon.