Environmental Engineering Reference
In-Depth Information
7
Multijunction and Energy Concentrating Solar Cells
The single-junction solar cell wastes a bulk of the energy in the incoming light, as we
have seen in detail in Chapter 6. Photons of energy greater than the bandgap lose the
excess energy to heat, and photons of energy less than the bandgap go through the cell
without absorption.
An intuitive solution to the problemis the opposite limit of a series of cells with graded
bandgaps, so that there is in the stack somewhere a semiconductor that will optimally
convert the energy of each photon. By extension of the Shockley
-
Quiesser analysis,
this picture can be veri
ed. In the limit of an in
nitely subdivided tandem cell, that is
a series connection of an in
nite array of single-junction cells, the ef
ciency
approaches, but does not reach, that of the Carnot cycle
g¼
1
T
c
/T
s
¼
1
300/
¼
6000
0.95. A concentrated in
nite tandem cell is predicted in Figure 7.1 [87] to
reach 86.5%, close to the 85.4% ef
ciency, called the Landsberg limit in connection
with thermal solar power installations in Chapter 5. The curves in Figure 7.1 are
numerically based on an AM
1.5 spectrum as shown in Figure 5.2. In the case of
tandem cells, the calculation assumes that the power from each cell is extracted
independently, while in practice the series-connected stack of cells is a two-terminal
device and a constraint of equal current through each cell limits the power extracted.
A small error may come from this difference, but in fact series-connected three-gap
tandem cells have been demonstrated to operate at ef
ciency as high as 41.6% [88].
The record setting cell is a three-gap GaInP/GaInAs/Ge lattice-matched cell
produced by Spectrolab, a subsidiary of Boeing Corporation, operated at 364-sun
concentration. This is a structure grown epitaxially on a single crystal of Ge. We will
discuss such cells later in this chapter. The junction technology is called liquid-phase
organometallic epitaxy, and is an offspring of the general method of molecular beam
epitaxy (MBE), used with success for compounds based on GaAs. It is credible that
the ef
ciency in concentrating tandem cells with up to
five different bandgaps may
reach 50%. What is open to question is the market price that would apply to large-
scale, complex systems of this type. It may also be mentioned that apart from the
practical tandem arrangement of multijunctions, light-splitting optics can be used to
illuminate a parallel array of junction devices. An ef
ciency record has been
established by such an arrangement, although the cost associated is likely to be high.
