Environmental Engineering Reference
In-Depth Information
7.5 Summary
A solar cell design based on p-type window materials has been
used to minimise loss mechanisms and achieve high performance
parameters in solar cells. This design has been experimentally
tested using the well-researched GaAs/AlGaAs system, grown by
the MOVPE method. Device parameters were assessed in different
laboratories using I-V and incident photon to current conversion
e
ciency (IPCE) measurements. Independently verified device
parameters of
V
oc
=
1,141-1,171 mV,
J
sc
=
10-12 mAcm
−
2
,and
FF
=
0.85-0.87 have been achieved for initial devices. The overall
e
ciency values measured for devices of different sizes — 0.5 mm
diametercontactsand3
×
3,5
×
5,and10
×
10mm
2
—wereinthe
range of 11-12%. The
V
oc
achieved in this work exceeds reported
values in the literature, and FF values have reached their maximum
possiblevaluesinthemid-80%range.EBICandSIMSmeasurements
revealed the reasons for the low
J
sc
values. It has been found that
the doping concentrations at both front and back ends of the device
were too high, producing partially depleted devices for the first
growth run. The reduction of doping concentration at the back end
of the device improved the short circuit current density from
∼
12
24 mAcm
−
2
, improving the e
ciency to 18.6%. Considerable
improvementsin
J
sc
and,hence,theoveralle
ciencyareanticipated
byoptimisingp-dopingconcentrationsatthefrontendofthedevice
structure. This chapter invites an MBE grower to reduce the p-type
dopingconcentrationto
∼
10
15
cm
−
3
inthefrontofthedevice,which
cannot be achieved by the MOVPE method. The work also opens
doors for InGaN alloy-based graded bandgap devices to achieve
very high e
ciencies. This optimisation could yield a considerable
improvement ofthe overall e
ciency ofthis device.
∼
to
References
1. C.Kittel(1996)
Introduction to Solid State Physics
,7thedn,JohnWiley&
Sons.
2. I. M. Dharmadasa (2005)
Sol. Energy Mater. Sol. Cells
,
85
, 293.
3. I. M. Dharmadasa, J. S. Roberts, and G. Hill (2005)
Sol. Energy Mater. Sol.
Cells
,
88
, 413.
