Environmental Engineering Reference
In-Depth Information
Sunlight
Grid type electrical contact
n-ZnO:Al
i-ZnO
n-CdS
CIGS
Mo
Glass
Figure 5.1
A schematic diagram of a thin-film CIGS solar cell showing the
main components of the device structure (not toscale).
5.3.2
Frequently Used Energy Band Diagram
In order to describe the PV action, a rectifying interface available
within the device structure should be identified. The frequently
used energy band diagram for this purpose is shown in Fig. 5.2,
and the research directions are usually guided according to this
device concept. The analysis of experimentally observed results,
scientific thinking, and the future development programmes are
heavily dependent on this device concept. In the scientific research
process, it is essential and a good practice to examine the existing
wisdomfromtimetotimetochecktheiraccuracy.Otherwise,errors
could propagate through the scientific literature, hindering positive
progress.Carefulandcloseobservationofthisenergybanddiagram
raises two main concerns.
(i) This energy band diagram has been constructed using Ander-
son's electron a
nity rule and the electron a
nity values of
semiconducting materials used. By taking the vacuum level as
the reference and the electron a
nity of ZnO, the conduction
band edges of both i-ZnO and n-ZnO:Al have been drawn
accurately. However, this contradicts the most important solid-
statephysicsprincipleofthepositionoftheFermilevelofi-ZnO.
