Environmental Engineering Reference
In-Depth Information
Such an approach, in which three or more cells are fabricated, leads
to cost increase but the e
ciency gains are comparatively small. In
this arrangement, the
V
oc
will increase by a factor of
n
but the
J
sc
will reduce by the same factor, where
n
is the number of individual
devices included in the structure. The power output will remain
basically the same.
6.3.2
Advantages of Parallel Connections
With reference to the energy band diagram for parallel connections
(Fig. 6.5), e-h pairs are generated by high-energy photons in the
frontlayersandbylow-energyphotonsinthebacklayersofthecell.
The photo-generated electrons are accelerated towards the front
contact, and holes are accelerated towards the back contact. In this
case, the recombination process has been minimised due to the
improved slope and the shape of the energy band diagram. The
presenceofahighinternalelectricfieldinthedeviceenablescharge
carriers to achieve very high kinetic energies due to continuous
acceleration across the device structure. Therefore, all of the photo-
generatedchargecarriershavethepossibilityofreachingtheloadin
the external circuit. Various other loss mechanisms, such as surface
reflection, are common to both systems and, therefore, have been
ignoredinthisdiscussion.Sincechargecarriersundergocontinuous
acceleration, they can even create multiple charge carriers due
to impact ionisation, increasing the electric current in the device
(Fig. 6.6). In series connections, charge carriers are slowed down
at each tunnel junction, increasing recombination and reducing the
current collection in the external circuit.
In addition, the impurity PV [1] or inter-band PV effect [2] is
alsobuiltintodevicesthatareconnectedinparallel.Thelow-energy
photons at the IR end travel towards the back of the solar cell.
Any semiconductor material contains a number of defect levels
in the bandgap, and these levels can be utilised in this device
structure to increase useful charge carriers. A combination of two
IR photons could create one e-h pair in any region, and these
charge carriers are well separated, accelerated, and collected in the
external circuit (Fig. 6.6). Various combinations of IR photons could
produce many e-h pairs using different impurity levels present in
