Environmental Engineering Reference
In-Depth Information
and BP Solar have been successful in manufacturing
∼
1m
2
solar
panels based on CdTe. Both Antek and First Solar grow CdTe
using close-space sublimation, but BP Solar grew CdTe using
electrodeposition.Rampingupofmanufacturingcapabilitiestoover
1 GW per annum in 2009, First Solar made a huge stride in the
thin-filmsolarpanelproduction.AlthoughBPSolarterminatedCdTe
solar panel manufacturing programme together with a few other
solar technologies in 2001-2002, this production line successfully
demonstrated the manufacturability of CdTe solar panels using
electrodeposition [4].
Since manufacturing capabilities are well established in the
industry, what is now required is to improve the understanding
of material issues and physics behind these devices in order to
increase the conversion e
ciency. This process will rapidly reduce
the $/W figure well below $1/W. This chapter briefly presents the
conventional device model used to describe the PV activity of CdTe
solar cells and introduces an alternative concept to describe the
devicestructureusingadvancesthattookplaceintheresearchfield.
Basedonthisnewscience,waysoffurtherimprovementoftheCdTe
solar cells are proposed. The chapter also briefly presents the latest
experimental observations proving these suggestions.
4.2 The Conventional Model for a Glass/Conducting
Glass/CdS/CdTe/Metal Solar Cell
The main subject of this chapter is the CdTe-based thin-film solar
cell,whichconsistsoftwosemiconductinglayers.Thewide-bandgap
n-type CdS (
E
g
=
2.42 eV) layer serves as the window material,
and the narrow-bandgap CdTe (
E
g
=
1.45 eV) layer is used as
the absorber material. If the CdTe layer is p-type, then the active
junction is a simple p-n-type hetero-junction and the required
internal electric field within the device is provided by this interface.
To date, various methods have been used to grow CdS and CdTe
and fabricate this solar cell device. A genuine p-n junction device
could have a typical structure of metal-1/n-CdS/p-CdTe/metal-2,
providing two ohmic contacts for the collection of photo-generated
current, as shown in Fig. 4.1. Metal-1 serves as the ohmic contact to
