Environmental Engineering Reference
In-Depth Information
5.2 Summary of Accumulated Knowledge on CIGS-Based
Materials
Worldwide research groups are working on CIGS-based materials
anddevicestructurestoproducealow-costandhigh-e
ciencysolar
panel in order to convert solar energy directly into electricity [3-7].
In fact, a few institutes have already started the production of CIGS-
based solar panels on an industrial scale. The following sections
summarise the accumulated knowledge on this material to date
before moving on to discuss the physics behindthese solar cells.
5.2.1
Different Growth Techniques
Various techniques have been used to grow CuInSe
2
(CIS) and CIGS
materials.Theconventionalsemiconductorgrowthtechnique,orthe
melt-growth method, has been used to grow bulk materials [8-9] in
order to study their structural, optical and electronic properties.
ThinfilmsofCIGShavealsobeengrownbytheflashevaporation
[10], vacuum evaporation [11, 12], electrodeposition [13-17], and
spray pyrolysis [18] techniques. The most commonly used method
is the deposition of Cu, In, and Ga layers and then the incorporation
ofSethroughtheselenisationprocess.Somegroupsalsoincorporate
Sintothelayerinordertoengineerthebandgapofthismaterialand
hence increase the performance e
ciency of CIGS solar cells [19].
5.2.2
Structural, Optical, and Electronic Properties
Most layers produced for large-area device production show a
chalcopyrite structure with polycrystalline nature. The grain sizes
varyinthesub-micronandmicronregions,dependingonthegrowth
conditions used. This material has a direct bandgap, and its value
can vary between
∼
1.00 eV (the bandgap of CIS) and
∼
1.68 eV
(the bandgap of CuGaSe
2
), depending on the composition of the
layer. Since this alloy contains four or five elements, the control of
composition and, hence, its bandgap is a challenging task. However,
many research groups have achieved excellent devices, in excess of
15% e
ciency, with materials having bandgap values in the range
