Direct Energy, 2018a

6 PHOTOVOLTAICS 129
this reason, as temperature increases, the overall current produced by the
solar cell decreases roughly exponentially [76]. This eect on the current is
the main reason that solar cell eciency depends on temperature. Other
mechanisms are temperature dependent, but are less signicant [76].
6.6.2 Solar Cell Technologies
There are four major solar cell technologies being developed: crystalline,
thin lm, multijunction cells, and emerging photovoltaic technologies [77].
However, these categories are not distinct because some solar cells t into
multiple categories simultaneously. Figure 6.15, from [77], compares solar
cells of these technologies. More specically, it shows record eciencies
for each of these types of solar cells as well as the year the records were
achieved.
The rst category is crystalline, and these cells may be made from single
crystals or from polycrystalline material [78]. The rst generation of
solar cells was made with this technology. For a simple recipe for how to
produce a crystalline solar cell, see [69]. Most solar cells produced today,
around 80% of the market, are silicon cells in this category. Typical eciency
of a crystalline solar cell available today may be around 20% [78].
Polycrystalline solar cells are often cheaper and a bit less ecient than
single crystalline cells.
The second category is thin lm. To make these solar cells, thin lms
of semiconductors are deposited on a substrate such as glass or steel. The
substrate may be rigid or exible. The solar cell itself may be made of
layers of material only a few microns thick. Thin lm solar cells may be
cheaper than other types of solar cells [78]. Often they are less ecient
than crystalline cells, but they have other advantages [78]. One material
used to make thin lm solar cells is amorphous silicon. Another material
in use is CdTe, which has a energy gap 1.45 eV. Cadmium and tellurium
are both toxic, but they may be easier to deposit in thin lms than silicon.
The third category is multijunction, also called compound, solar cells.
These solar cells are made of a dozen or more layers of semiconductor
stacked on top of each other [78]. These layers form multiple pn junctions.
Larger gap semiconductors are on the upper layers, and smaller gap
semiconductors are closer to the substrate. These solar cells can be quite
ecient. Cells with eciency up to 46% have been demonstrated in labs
[77].
The last category is emerging technology solar cells. Multiple creative
strategies are being used to develop solar cells. Nanotechnology strategies
include using solar cells made from carbon nanotubes and from quantum