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Keithley vFinal DR 2/9/09 11:16 Page 37
MANUFACTURINGAWARENESS
Converting awareness
Photovoltaic(PV) manufacturing is now firmly on a path of continuous improvement as it
gains momentum as a viable energy source. The semiconductor industry has been a
major template for PV manufacturing and a number of processes and tools are finding an
important role in PV manufacturing. One area is understanding the electrical flow and
output through characterisation. Mary Anne Tupta, Lead Applications Engineer at
Keithley Instruments, discusses the industry requirements for making I-V and C-V
measurements on solar cells and explains the capacity of a particular company system in
providing appropriate solutions.
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37
ecause of the increasing demand for Figure 1. Idealized
www
global energy and the limited supply of equivalent circuit of a
.solar
fossil fuels, the search for alternative sources of photovoltaic cell
power has become an imperative for industry and
-pv-management.com
society. Given that there is a vast amount of energy
available from the sun, devices that convert light
energy into electrical energy are becoming
increasingly important. Solar or photovoltaic (PV)
cells convert light energy into useful electrical model consists of current due to optical generation
power. These cells are produced from light (IL), a diode that generates a current [Is-(e
qV/kT
)], a
absorbing materials. When the cell is illuminated, series resistance (rs), and shunt resistance (rsh).
Issue IV 2009
optically generated carriers produce an electric The series resistance is due to the resistance of
current when the cell is connected to a load. the metal contacts, ohmic losses in the front
surface of the cell, impurity concentrations, and
A variety of measurements are made to determine junction depth and is an important parameter
the electrical characteristics of PV cells. because it reduces both the short circuit current
Characterising the cells often involves measuring and the maximum power output of the cell. Ideally,
the current and capacitance as a function of an the series resistance should be 0Ω (rs = 0). The
applied DC voltage. The measurements are usually shunt resistance represents the loss due to surface
done at different light intensities and temperature leakage along the edge of the cell or due to crystal
conditions. Important device parameters can be defects. Ideally, the shunt resistance should be
extracted from the current-voltage (I-V) and infinite (rsh =∞).
capacitance-voltage (C-V) measurements, such as
the conversion efficiency and the maximum power If a load resistor (RL) is connected to an illuminated
output. Electrical characterisation is also important PV cell, then the total current becomes:
to determine losses in the PV cell and is needed to
determine ways to make the cells as efficient as
possible with minimal losses.
Basic PV cell circuit and device parameters
A photovoltaic cell may be represented by the where: IS = current due to diode saturation
equivalent circuit model shown in Figure 1. This IL = current due to optical generation
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