Environmental Engineering Reference
In-Depth Information
new materials with the right properties to absorb light and create
charge carriers effectively and (ii) fabricating suitable devices to
separateandtransportchargecarriersoutofthedeviceandthrough
an external circuit.
1.3 Solar Energy Materials
All materials around us can be classified into three main groups:
electrical conductors, semiconductors, and insulators. This classifi-
cationcanbedoneaccordingtotheelectricalconductivity(
σ
)orthe
energybandgap(
E
g
)ofthematerials.Table1.1summarisesthetwo
main properties of these different classes of materials, but it should
be noted that there is no clear demarcation among these classes.
For more details and specific properties of different materials, the
reader is requested to refer to already published topics in materials
[2-4], solid-state physics [5-7], and solid-state chemistry [8-9].
The materials used for the absorption of light and creation of
charge carriers come from semiconductor families. Over the past
70 years, scientists have identified many different semiconducting
materials with a wide variety of properties. Table 1.2 displays some
relevant elements from the Periodic Table, and Table 1.3 shows
many different semiconductors available today. The task for the
PV community is to identify the most suitable semiconductors to
effectively absorb light and create charge carriers for solar energy
conversion. High optical absorption, a direct instead of indirect
bandgap, and low density of recombination centres, together with
required mechanical strengths andstability,are some ofthedesired
properties for solar energy materials. In addition, these materials
should have the required qualities to make appropriate electronic
devices in order to separate and collect photo-generated charge
carriers.Whenthesematerialsarepreparedintheformofthinfilms
Table 1.1 A summary of two main properties of different classes of
electronic materials
Parameter
Electrical Conductors
Semiconductors
Electrical Insulators
cm)
−
1
10
8
-10
1
10
1
-10
−
8
10
−
8
-10
−
20
σ
(
≡
S
∼
∼
∼
∼
.
∼
.
∼
.
E
g
(eV)
0
3 or negativevalues
0
3-4.0
4
0-10.0
