Environmental Engineering Reference
In-Depth Information
Figure 6.2
A schematic diagram of a tandem solar cell fabricated by
connecting three p-i-n diodes in series through two tunnel junctions. See
also Colour Insert.
such a tandem solar cell is shown in Fig. 6.2, and the corresponding
energy band diagram is shown in Fig. 6.3. In this series connection,
then-typematerialofonedeviceisconnectedtothep-typematerial
oftheadjacentdevice(Fig.6.2).Inotherwords,theconductionband
ofonedeviceisplacedveryclosetothevalencebandoftheadjacent
device(Fig.6.3).Theideaistoconvertphotonswithdifferentenergy
ranges in the three different devices and collect the charge carriers
e
ciently to generate high power in the external circuit. Since the
solar cells are connected in series, the output voltages will add up
and the current through the system will be constant. Therefore,
for best results, the current produced by the three individual cells
should be equal and hence current matching is important in this
case.
6.2.2
Connection in Parallel
The other approach to fabricating a tandem solar cell is by
connecting a large number of solar cells in parallel. Figures 6.4
and 6.5 show a schematic diagram and an energy band diagram
