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2. Photovoltaic Principle, Cells Technologies, and Efficiencies 351

2. PHOTOVOLTAIC PRINCIPLE, CELLS TECHNOLOGIES, AND
EFFICIENCIES
2.1 PHOTOVOLTAIC PRINCIPLE

The photovoltaic cells use the photoelectric effect to produce direct current by ab-
sorption of solar radiation. This effect allows the cells to directly convert the light
energy of the photons into electricity by using a semiconductor material carrying
electrical charges [2]. The semiconductor material has two parts (Fig. 11.1), one
having an excess of electrons (type N) and the other a deficiency of electrons
(type P) [4].

2.2 PHOTOVOLTAIC CELL EFFICIENCY
An individual cell produces very little electrical power. To produce more power, the
cells are assembled to form a panel. Several cells connected in series increase the
voltage for the same current, while the connection in parallel increases the current
while maintaining the voltage. The output current and thus the power will be propor-
tional to the panel area. Efficiency in photovoltaic panels is the ability of a panel to
convert sunlight into energy. This value is important to choose the correct panels for
a specific photovoltaic system application. The photovoltaic efficiency is given
as [4]:

P out V pv $I pv
h pv ¼ ¼ (11.1)
P in S pv $G
where, P out is the output power; P in is the input power; V pv and I pv are, respectively,
PV voltage and current; S pv is the solar module surface; and G is the solar irradiance.
2
Under STCs (G ¼ 1000 W/m ,T a ¼ 25 C), the maximum efficiency will be cal-

culates as:
P pv max V pv max $I pv max V pv max $I pv max
h pv max ¼ ¼ ¼ (11.2)
P in S pv $G STC S pv $1000

Type N

PN junction
Type P

+
-
FIGURE 11.1
Photovoltaic cell technology.