Environmental Engineering Reference
In-Depth Information
Table 1.4 A summary of main properties of p-type and n-type
semiconductors
p-Type Semiconductors
n-Type Semiconductors
•
•
The majority chargecarriers areholes.
The majority chargecarriers are electrons.
•
•
The minority chargecarriersare electrons.
The minority chargecarriersare holes.
•
•
These semiconductors are doped
These semiconductors are doped with
with externally added acceptors, native
externally added donors, nativedefects,
defects, or variation in composition.
or variation in composition.
•
•
The Fermi level is positioned close to the
The Fermilevel is positioned close to the
valenceband.
conduction band.
forcing them in opposite directions, towards the two electrical
contacts on both sides of the junction. If the built-in electric
field is weak or non-existent, photo-generated charge carriers
will recombine again and the photo-current will be low or zero.
Therefore, the key to improving the PV activity is to fabricate a
healthy depletion region, with a strong built-in electric field, using
a semiconductor with high optical absorption properties and low
recombination centres.
Theactionofasolarcellbasedonap-njunctionisdemonstrated
by animation on Dharme's blog at: www.apsl.org.uk to help
beginners in this field visualise the PVprocess.
1.4.2
p-i-n Junctions
The production of a healthy depletion region depends on the
doping concentration of the two semiconductor regions used to
fabricate the p-n junction. Low doping concentrations produce a
wide depletion region, and heavy doping concentrations create
a thin depletion layer, which are not helpful for PV conversion.
Optimising the doping concentration to create a depletion region
width comparable to the thickness of a thin film solar cell, typically
in the order of 2
μ
m, to achieve the best performance is a
real challenge faced by material scientists and device physicists
during the research and development phase. The exact value of the
depletion region width depends on the device design planned for a
particular purpose.
