Environmental Engineering Reference
In-Depth Information
further improvements. 1,200 mV is the minimum voltage required
to split water molecules using electrolysis.
The FF values measured for these devices are achieving the
maximum possible value for this parameter. FF values remain in
the mid-80% range, and the highest value recorded was 0.87. These
valuesare holdingfor0.5mmdiameter, 3
3mm
2
,5
5mm
2
,and
×
×
10mm
2
sizesolarcelldeviceswithoutanyreductionduringthe
gradual scaling-up process. These reproducible results indicate that
the losses due to series resistance and shunt resistance are at their
lowest for these device structures and hence the maximum possible
FF values have been achieved.
10
×
7.3.3
IPCE Measurements
The current density values measured for these devices are in the
range 10-12 mAcm
−
2
. These values are very low for devices based
on GaAs/AlGaAs systems when compared to the capability of these
materials and the new device structure. To explore the reasons
for these low current density values, incident photon to current
conversion e
ciency (IPCE) measurements have been carried out
onthesedevices.MeasurementsofIPCEofseveraldevicesshoweda
considerable loss in both blue and red regions. A typical example is
shown in Fig. 7.5.
One explanation for this severe loss is the remains of the 0.2
μ
m thick p-GaAs used as a capping layer. Tests have been carried
out to make sure the capping layer was completely removed during
chemical etching. Although the p-GaAs capping layer is completely
removed, the shape of the IPCE remained the same, indicating the
existenceofanotherreasoncausingtheselosses.Thisconclusionled
totheuseoftheelectronbeam-inducedcurrent(EBIC)techniqueto
further explore the possiblecauses.
7.3.4
EBIC Measurements
Cross-section EBIC experiments have been carried out on these de-
vices to investigate the PV-active region across the device thickness.
Figure7.6showsatypicalEBIC-linespectrumandanEBICimagefor
thisdevicestructure.Accordingtothesemeasurements,thewidthof
