Multijunction and Energy Concentrating Solar Cells - Nanophysics of Solar and Renewable Energy - page 155

Figure 7.1 Efficiencies calculated for tandemcell combinations and the hypothetical intermediate
band cell. Note the common role of concentrated light intensity in improving the efficiency by
raising the cell open-circuit voltage ([87], p. 121, Figure 6.7)
7.1
Tandem Cells, Premium and Low Cost
Themost successful tandemcellshavebeenthose intheGaAssystem, similar totheone
mentioned as having achieved 41.6% ef ciency at a concentration of 364 suns. Such a
device would be used in a parabolic dish mirror concentrator, with an appearance
similar to that of Figure 5.5. Tracking of the sun and water-cooling are implied.
7.1.1
GaAs-based Tandem Single-Crystal Cells, a Near Text-Book Example
The concept of the tandem cell is clearly shown in the spectral response curves
published by the manufacturer Spectrolab, of the three-junction cell, shown in
Figure 7.2.
The complex multijunction GaAs-based cells are at a high state of development,
building on experience in the Soviet space program and in the wide application of
III - V semiconductor (GaAs) materials in commercial electronics. See Figure 6.5 ff.
in Chapter 6. An example close to the record 41.6% ef cient cell is shown in
Figure 7.3. The three main layers are evident from top to bottom, whose absorption
spectra were shown in Figure 7.2. Complexities in thismature technology include the
use of low-resistance PN junctions to series-connect adjacent cells, and provision for
back surface fields to inhibit recombination. The problem of maintaining the three
cells at optimum operating points consistent with identical current through each
junction has been apparently addressed successfully.