30
Figure 3.10 I−V characteristics of Si pn junction solar cell under dark conditions and under
illumination with light of increasing intensity
Solar cell materials and e
ffi
ciency:
Conventional solar cells are made of Si single crystal and have an e
ffi
ciency of
around 22-24%, while polycrystalline Si cells have an e
ffi
ciency of 18%. A
schematic representation of such a cell is shown in figure 3.7. The e
ffi
ciency of
the solar cell depends on the band gap of the material and this is shown in
figure3.12. Polycrystalline solar cells are cheaper
to manufacture but have a
lower e
ffi
ciency since the microstructure introduces defects in the material that
can trap carriers. Amorphous solar cells have an even lower e
ffi
ciency but can
be grown directly on glass substrates by techniques like sputtering so that the
overall cost of manufacturing is lowered. There are also design improvements in
the solar cell that can enhance the e
ffi
ciency. PERL (passivated emitter rear
locally di
ff
used) cells, shown in figure 3.13, have an e
ffi
ciency of 24% due to the
inverted pyramid structure etched on the surface that enhances absorption.
32
Typical solar cells are made of the same material so
that the pn junction is a
homojunction. Some solar cell materials and their e
ffi
ciencies are summarized in
table 3.2. A comprehensive state of current research in di
ff
erent solar cell
technologies and their e
ffi
ciency is available in figure 3.14. Heterojunction solar
cells are also possible and they have the advantage of minimizing absorption in
regions other than the depletion region, but overall cost increases because of the
use of di
ff
erent materials and the tight processing conditions needed to produce
defect free interfaces. A schematic of such a cell based on GaAs/AlGaAs is
shown in figure 3.15. The shorter wavelengths are absorbed by the AlGaAs
layers
while the longer wavelengths, with higher penetration depths, are
absorbed by the GaAs layer. This leads to an overall e
ffi
ciency of around 25%,
see table 3.2. It is also possible to have a homojunction solar cell but with a
passivating layer of another material at the surface to reduce defects. This is
shown in figure 3.16. The surface passivating layer removes the
dangling bonds
and minimizes carrier trapping. The passivation layer is a thin layer of a higher
band gap material to minimize absorption. Similarly, amorphous semiconductor
materials like Si and Ge also have a passivating layer of H, a-Si:H or a-Ge:H, to
reduce dangling bonds. Another way of improving solar cell e
ffi
ciency is to have
more than one cell in tandem. These are called tandem solar cells and a schematic
is shown in figure 3.16. These consist of two pn junction solar cells, with the first
one having a higher band gap than the second. Thus, the shorter wavelengths can
be absorbed in cell 1, see figure 16, while the longer wavelengths are absorbed
in cell 2. The advantage is that a larger portion of the solar radiation is used so
that tandem cells have high e
ffi
ciency, see table1, but it also adds a layer of
complexity in growth and increases cost. Tandem cells can also be made using
amorphous Si:H and Ge:H. These are cheaper to make and more e
ffi
cient than
individual amorphous solar cell devices.