Photovoltaic Power Generation - Renewable Energy: Technology, Economics and Environment - page 237

Due to the charge separation during irradiation, electrons accumulate within the
n-region, whereas holes accumulate in the p-region. Electrons and holes will ac-
cumulate until the repelling forces of the accumulated charges start to impede
additional accumulation; i.e. until the electrical potential created by the accumula-
tion of electrons and holes is balanced by the diffusion potential of the p-n-
junction. Then the open-circuit voltage of the solar cell is reached. The time to
achieve these conditions is almost immeasurably short.
If p- and n-sides are short-circuited by an external connection, the short-circuit
current is measured. In this operating mode the diffusion voltage at the p-n-
junction is restored. According to the operating principle of a solar cell, short-
circuit current increase is proportional and almost linear to solar irradiance (see
also e.g. /6-3/, /6-10/).
6.2 Technical description
The technical basics of photovoltaic power generation are outlined within the
following chapters. All explanations, including the indicated key figures, reflect
state-of-the art technology. High-end laboratory cells or modules might have a
better performance.
6.2.1 Photovoltaic cell and module
Structure. Fig. 6.6 shows the basic structure of a photovoltaic cell consisting of
p-conducting base material and an n-conducting layer on the topside. The entire
cell rear side is covered with a metallic contact while the irradiated side is
equipped with a finger-type contact system to minimise shading losses. Also full
cover, transparent conductive layers are used. To reduce reflection losses the cell
surface may additionally be provided with an anti-reflecting coating. A silicon
solar cell with such construction usually has a blue colour. By the incorporation of
inverse pyramids into the surface reflection losses are further reduced. The incli-
nation of the pyramid surfaces is such that photons are reflected onto another
pyramid surface, and thus considerably enhance the possibility of photon penetra-
tion into the crystal. Absorption of the solar light by these cells is almost com-
plete, the cells appear black.
Current-voltage characteristic and equivalent circuit. An illuminated solar cell
ideally can be considered as a current source provided with a parallel diode. The
photocurrent I Ph is assumed to be proportional to the photon flow incident on the
cell. The Shockley equation for ideal diodes (Equation (6.1) /6-3/) describes the
interdependence of current and voltage (current-voltage characteristic) of a solar
cell.