Direct Energy, 2018a

128 6.6 Solar Cells
to day and location to location. In a bright sunny area, a solar cell may
receive around 0.1 cm W [73, p. 7].
2
Even if energy from the sunlight reaches a solar cell, the energy is not
converted to electricity with perfect eciency. There are multiple reasons
for this ineciency, and some of these reasons relate to the fact that not
all light that hits a solar cell is absorbed. Light may heat up the solar
cell instead of exciting electrons to create electron-hole pairs [74]. Alternatively,
light may be reected o the solar cell surface [74]. Many solar
cells have an antireection coating to reduce reections, but they are not
eliminated. The surface of other solar cells are manufactured to be rough
instead of smooth to reduce reections. Furthermore, if a photon hits an
electron that is already excited, the photon will not be absorbed. Additionally,
solar cells have wires throughout the surface to capture the produced
electricity. These wires are often thin and in a nger-like conguration.
Light that hits these wires does not reach the semiconductor portion of
the solar cell and is not eciently converted to electricity. To reduce this
issue, wires of some solar cells are made from materials that are partially
transparent conductors, such as indium tin oxide or tin oxide SnO 2 [74]. Indium
tin oxide is a transparent conductor with a moderately high electrical
conductivity of σ =10 6 Ω·m 1 [75].
Other reasons that solar cells are not perfectly ecient have to do with
what happens after a photon excites an electron. An electron may be
excited, but it may decay before it gets swept from the junction [74]. A
photon may excite an electron to a level above the conduction band, but the
electron may quickly decay to the top of the conduction band losing some
energy to heat. Internal resistance in the bulk n-type or p-type regions
may convert electricity to heat. There may also be internal resistance of
wiring in the system. Also unmatched loads make solar cells less ecient
than matched loads [74].
The voltage across and the current produced by an illuminated solar
cell are both functions of temperature. Reference [76] demonstrates, both
theoretically and experimentally, that eciency of a solar cell decreases
as temperature increases. A number of mechanisms occurring in a solar
cell are dependent on temperature. First, as the temperature increases,
the allowed energy levels broaden. For this reason, the energy gap E g ,
which is proportional to the voltage produced by the solar cell, is smaller
at higher temperatures. As temperature increases, this voltage produced
by the solar cell decreases roughly linearly [76]. Second, the current due
to recombination of electron-hole pairs at the junction is a function of
temperature. At higher temperatures, more electron-hole pairs recombine
at the junction, so the overall current produced by the solar cell is less. For