Environmental Engineering Reference
In-Depth Information
elsewhere [1-4]. The four most important steps for an e
cient
solarcell are satisfied forthis structure (Fig. 7.1) according to solid-
state physics principles, namely (i) the absorption of a major part
of the solar spectrum, (ii) the creation of e-h pairs in the presence
of an internal electric field, (iii) the effective separation of charge
carriers towards two electrical contacts and (iv) transport through
an external electric circuit. Because the device starts from a large
bandgap p-type semiconductor and is completed with a narrow
bandgap n-type semiconductor, the probability of impact ionisation
and the impurity PV effect has been increased. The detailed
working of this device is illustrated with an animation placed on
the website Dharme's blog at www.apsl.org.uk (Tunnel Junction
File).
The first growth of this device structure was made using metal
organicvapourphaseepitaxy(MOVPE)-GaAs/AlGaAsmaterials.The
devices exhibited
V
oc
=
1,150-1,175 mV,
J
sc
=
10.3-12.3 mAcm
−
2
,
fill factor (FF)
=
0.85-0.87, and conversion e
ciency (
η
)
=
11-
12%. It is worth noticing the achievement of the highest-reported
V
oc
for a single device [5, 6] together with the highest achievable FF
values.Theseparameterswereindependentlyverifiedbymeasuring
in five different laboratories in Europe and United States, including
National Renewable Energy Laboratory (NREL).
Attempts made to improve the current density were presented
in chapter 7. During this second growth, the
V
oc
=
900 mV,
J
sc
=
24 mAcm
−
2
,FF
=
0.86, and
η
=
18.6% values were observed
with additional information. The variation of
V
oc
is noteworthy,
and the new information emanating for GaAs-based solar cells is
summarised in chapter 11. The overall conversion e
ciency for the
graded bandgap multi-layer solar cell has improved from
∼
12% to
18.6% during the second attempt.
8.3 Search for Experimental Evidence of the Impurity
PV Effect
ThedevicestructureshowninFig.1.16wasdesignedtoincorporate
impurity PV effect using native defects available closer to the GaAs
substrateatthebackofthedevice.Theexperimentswere,therefore,
