Environmental Engineering Reference
In-Depth Information
makes use of the naturally occurring defect levels to create more
chargecarriersusingtheimpurityPVeffect,reducingthecompeting
recombination and generation (R&G) process. It is important to
note that in this structure, the photo-generated charge carriers are
separated by the whole thickness (about 3
μ
m) of the active solar
cell structure and, hence, the probability of R&G process has been
further reduced.
7.1.2
Incorporation of Impact Ionisation
Thereisanothermechanismofchargecarriercreationincorporated
inthisdevicestructure.Theelectronsexcitedintoimpuritylevelsby
IR photons can be promoted to the conduction band by either using
another IR photon or by impact ionisation. The electrons created
by UV and visible photons at the front of the device [1] accelerate
through the structure, gaining adequate momentum to knock off
electrons trapped in defect levels at the rear, into the conduction
band — process (a), Fig. 7.1. The transmission losses are, therefore,
reduced through the combination of IR excitation of electrons to
defect levels and promotion of these electrons to the conduction
bandbyimpactionisation.Photo-generatedelectrons createdatthe
frontofthedevicemayalsoacquireenoughenergytobreakanother
bond—band-to-bandtransition,asshownbyprocess(b),Fig.7.1—
close to the back contact of the device due to a smaller bandgap
present in this region of the solar cell. Therefore, impact ionisation
could help in two different ways to create more charge carriers in
this device.
The composition grading adds
∼
0.70 V to the voltage difference
across the device due to the grading over 3
μ
m thickness.
This corresponds to an enhancement of internal electric field by
∼
2.30 kVcm
−
1
.Thisadditional electric fieldwillenhancethecharge
separation and the impact ionisationprocess in these devices.
7.2 Summary of Growth and Process Details of the
Device Structure
AdevicestructurewithanenergybanddiagramasshowninFig.7.1
was first fabricated using the GaAs/AlGaAs system grown by the
