Environmental Engineering Reference
In-Depth Information
Figs. 6.4 and 6.5. In addition, the use of electrodeposition satisfies
a number of key criteria necessary for PV development, namely, its
low cost, scalability, and manufacturability, ultimately contributing
to increased device e
ciencies and low production costs. These
advantages of electrodeposition have been described in chapter 3.
6.4 Conclusions
The reconsideration of the solid-state device principles behind
the fabrication of tandem solar cells allows us to draw several
conclusions:
Thefabricationoftandemsolarcellsusingaseriesconfiguration,
through tunnel junctions, is ine
cient in charge carrier separation
and collection. At each tunnel junction, photo-generated electrons
and holes approach one another, are decelerated, and are allowed
to recombine. The addition of more tunnel junctions is actually
detrimental to charge carrier separation and collection. Any
improvement observed in the device performance to date is likely
due to improvements in the quality of the material achieved due
to growth of a number of layers on top of one another using the
most expensive and well-established metalorganic chemical vapour
deposition (MOCVD) or molecular beam epitaxy (MBE) techniques.
It is a remarkable achievement of nearly 40% e
cient solar cells
using complex device structures containing tunnel junctions — a
resultofthreedecadesofresearch.Theremovalofthisdisadvantage
fromthedevicewillleadtotheformationofdeviceswithe
ciencies
well above 40%.
Ontheotherhand,thefabricationofmulti-layergradedbandgap
solar cells using a parallel configuration has several advantages.
Variation of the conduction type from n
+
to p
+
combined with
bandgap variation from high to low from the front to the back
of the solar cell provides many positive results. Photo-generated
charge carriers at any depth of the device are effectively separated,
accelerated, and collected in the external circuit.
Continuousaccelerationacrossthewholedevicethicknesscould
produce high-speed carriers and, therefore, results in multi-charge
carrier production through impact ionisation. In addition, the
