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A G R E AT E R M E A S U R E O F C O N F I D E N C E present in all semiconductor materials.
When a photon with energy greater than the
junction bandgap enters the cell, an electron
is "pumped" from the valence band (below
the Fermi level) to the conduction band. This
causes the junction to create a forward bias
potential that moves the electron into the n-
type region. If an external circuit is connect-
ed to the junction (Figure 2), the "pumped"
electron moves around the circuit and re-
combines with a hole in the p-type region.
A major focus of solar cell researchers


Better Solar Cell
and users is improving cell efficiency and
maximizing energy extraction. This requires
I-V measurements to characterize perfor-


Testing: The Key to
mance and determine the load impedance
that best matches the cell's source imped-
ance. The best match is at Point A on the


Faster Development I-V curve in Figure 3. The cell's short circuit
current intersects the y-axis (Point B) and



and Production
its open circuit voltage intersects the x-axis
(Point C). As much as possible, systems
powered by solar cells should be designed
to transfer energy to the load at Point A. No
energy is delivered at Points B and C, and
progressively more is delivered as the oper-
James Niemann, Keithley Instruments, Inc. ating point approaches Point A.
Matching load impedance to source im-
Solar cell development and production requires extensive material pedance becomes even more important when cells are assembled
and device testing to improve efficiency and match individual cells into a solar panel array. If cells are sorted by matching their I-V char-
for panel construction. This makes fast testing highly desirable, but acteristics, those from a given bin can be assembled into a single ar-
requires an understanding of cell physics and their measurement im- ray. Each array can then be operated at its maximum power transfer
plications. point.
Most solar cell measurement problems are related to the high ca-
Solar Energy Collection and Conversion pacitance associated with their forward biased p-n junctions. Com-
In many ways, solar cells operate in a manner similar to the way pared to reverse biased junctions (e.g., photodetectors), forward bi-
plants use photosynthesis to convert solar energy. Both systems are
limited by quantum mechanics. Energy comes from the sun only in
Solar Irradiance
packets, expressed as E = hv, where E is energy, h is the Planck con-
stant, and v is the frequency of the photon. A photon packet must Photon Energy (eV)
have an appropriate quantum of energy or it is wasted. 4 3 2 1.5 1.0 0.8
2400
A good deal of inefficiency is due to wavelength sensitivity. In
photosynthesis, plants convert solar energy most efficiently at wave-
lengths in the range of 650nm to 680nm (near the peak wavelength 1800
AM0 Optical energy as
of solar radiation at the earth's surface