Large-Scale Fabrication, Learning Curves, and Economics Including Storage - Nanophysics of Solar and Renewable Energy - page 201

This article suggests that of the total estimated oil in the earth in 1900, the fraction
remaining is 0.58, or 1.4

10 12 barrels. It is agreed that no new oil is being generated
in the earths crust, that is, that oil is not renewable.
We have mentioned solar cells of various types, starting with single-crystal silicon
(Figure 6.5), multijunction cells using epitaxial layers of III - V semiconductors based
onGaAs (Figure 6.7), polycrystalline and thin lms of amorphous silicon (Figure 7.12),
CdTe (Figure 6.15), copper indium gallium selenide (CIGS; Figures 6.11 - 6.14), dye-
sensitized titania (Figures 6.16 - 6.18), and organic semiconductors (Figures 6.20 and
7.9). The concentrating cells (Figure 7.8) have to be provided, in addition, withmirrors
or lenses, tracking mechanisms, and cell cooling. In general, each type of cell has a
different set of fabrication methods, and it is likely that each method will have its own
learning curve as its total production accumulates. Onemight imagine amore detailed
version of Figure 10.1 with separate trajectories for separate technologies. The
trajectory for CdTe clearly will be below the average curve shown.
To rank the fabrication methods in order of decreasing cost, it is likely that the
liquid-phase epitaxy needed for the GaAs multijunction tandem cells is the most
expensive. The crystalline Si solar cell is built on a wafer of Si, which is typically
200 - 300 m thick, cut froma large crystal using a wire saw, which wastes also one wire-
width of the Si single-crystal boule per wafer. A recent initiative has been reported to
lower the cost of silicon cells by casting the wafers frommolten silicon. This potential
method avoids the crystal growing and sawing steps, saving both labor and raw
material, and is projected [127] to reduce the cost by 50%. Cells made in this way are
not yet on themarket. It is not clear what quality and quality control can be achieved in
such a process.
Polycrystalline silicon is conventionally deposited using chemical vapor deposi-
tion, and amorphous silicon using glow discharge. Large-size Si polycrystals,
sometimes called multicrystals, can be quite ef cient, because of the large
grain size, which reduces undesired recombination (see Figure 6.8 and caption of
Figure 6.4).

10.2
Learning Strategies for Module Cost

The cost per watt of product can be reduced by raising the efficiency of the cell. If one
doubles the efficiency, then one needs only half as many modules to deliver the same
power. Such savings apply both to the ingredients of the cells and, importantly, to the
glass plates, frames, wiring between cells and between modules, and to the land on
which the installation is placed. Improvements to cell ef ciency can be analyzed from
the physics of the cell, as discussed in Chapter 3, see Equations 3.69 and 3.70, and in
Chapter 6, see Equations 6.5 and 6.6.
To summarize factors affecting cell ef ciency, the power output is roughly the

product of the open-circuit voltage

V oc ¼ð k B T = e Þ ln ð J L = J rev þ 1 Þ ð 3 : 69 Þ