Environmental Engineering Reference
In-Depth Information
6.3.1
Disadvantages of Series Connections
Atandemsolarcellconnectedthroughtunneljunctionshasasevere
disadvantage, as shown in the energy band diagram (Fig. 6.3). The
photo-generated electrons of one cell are brought closer to the
photo-generatedholesoftheadjacentcell.Thesecarriersareslowed
downattheinterfaceduetotheexistenceofaninternalelectricfield
acting in the opposite direction. This increases the recombination
of carriers at this interface through the tunnel junction. Figure 6.3
shows the equilibrium situation under illuminated conditions and
the short circuit mode showing the direction of flow of photo-
generated charge carriers. Under illumination, the device structure
is equivalent to a forward-bias situation by a voltage
V
oc
. This
is a supporting condition for increased recombination, and the
separationofphoto-generatedchargecarriersisseverelyhampered
bythetunneljunction.Thee
ciencyisreducedsinceconditionsare
created within the solar cell that promote recombination of photo-
generated charge carriers without allowing them to flow through
the external circuit. An animation of the working of this device
can be found in Dharme's blog at www.apsl.org.uk. In this case,
photons create charge carriers within the device, but the carriers
killed within the device before they are separated and transported
through the external circuit. Although light harvesting is effective,
charge carrier separation andcollections are severely hampered.
Ideally, the electrons created should be injected into the con-
duction band of the adjacent cell. Instead, the conduction electrons
of one cell and holes of the adjacent cell are brought towards the
tunnel junction (Fig. 6.3) and slowed down at that interface due to
theoppositeinternalelectricfieldspresentnearthetunneljunction.
In fact, if all three solar cells absorb an equal number of photons,
the external circuit will experience only the holes produced by the
front cell and the electrons produced by the back cell (Fig. 6.3). In
such a situation, there is no real advantage in having more cells.
Any improvements observed in the past are most likely due to
improvements in the quality of the material made when numerous
layers are grown on top of one another, starting from the back
contact. In fact, more tunnel junctions introduced in the structure
increase the number of interfaces where e-h pairs can recombine.
