Page 168 - A Comprehensive Guide to Solar Energy Systems

Chapter 8 • Photovoltaics: The Basics 169

FIGURE 8.18 Spectral distribution of solar radiation power [3].

8.4.1 PV Cell Efficiency Limit

For PV cells, the efficiency of conversion of irradiative energy to electrical energy is very
important. As discussed in section 8.2.1, the number of carrier generated electrons de-
pends strongly on the type of semiconductor (bandgap, band structure). only photons
with energy hc/λ > W g can generate electron–hole pairs. due to the thermalization pro-
cess, the part hc/λ − W g of the absorbed energy is transformed into heat. If all the irra-
∞ hc
λ d entering the cell body is absorbed, the irradiation power
diation power ∫ Φ () λ ∫λmin∞hcλΦ 0 (λ)dλ
0
/
hc W g λ min λ
∫ W g Φ () λ ∫λminhc/WgWgΦ 0 (λ)dλ
λ d is transferred into electron–hole pair generation. Assuming an ideal PV
0
λ min
cell with only radiative recombination and ideal junction characteristic without parasitic
resistances, it is possible to state that the ultimate limit for the maximum conversion ef-
ficiency η u is dependent on the absorbing material bandgap and the spectral distribution
of the incident radiation power (Quasier–shockley criteria) as
/
hC W R
∫ W g Φ () λ
λ d
0
λ min = η . (8.27)
∞ hC u
∫ λ Φ () λ
λ d
0
λ min ∫λminhc/WgWgΦ 0 (λ)dλ∫λmin∞h
cλΦ 0 (λ)dλ=ηu.
The dependence of the maximum efficiency of conversion on the bandgap energy for
solar radiation spectra AM 1.5 [3,6,7] is shown in Fig. 8.19. In this case, the ultimate effi-
ciency limit for one absorbing material is 33% for material with a bandgap of W g = 1.36 eV.
This value can change if another type of radiation is used (the maximum efficiency can
be obtained using monochromatic light with photon energy only a little higher than W g ).

8.4.1.1 Tandem Structures
one possible way of overcoming the Quasier–shockley limit for PV cell efficiency is by us-
ing so-called tandem structures, that is, stacks of cell structures of materials with different