Environmental Engineering Reference
In-Depth Information
Figure 7.12
Anenergybanddiagramofasolarcellunderilluminationand
open circuit condition. Note the reduced R&G process by separating photo-
generated charge carriers by the
∼
3
μ
m solar cell device thickness.
The expected improvements have been experimentally demon-
strated by producing the highest
V
oc
=
1,141-1,171 mV values
=
withthelargestpossibleFF
0.85-0.87values.Thecurrentdensity
values were low due to undesirable doping concentrations in the
frontaswellasbackendsofthisdevice.Then-typeSidoping
concentration has been reduced using MOVPE, demonstrating a
substantialimprovementin
J
sc
,increasingfrom
∼
12to24mAcm
−
2
,
and the improved IPCE spectrum is shown in Fig. 7.10. But the p-
type doping concentration cannot be adjusted using this growth
technique.TheMBEgrowthtechniqueistherightmethodtoachieve
low C-doping levels in the front of the device, to observe the next
drastic improvement.
It is anticipated that high current density values are obtained
afterthereductionofp-typedopingto
∼
5
×
10
14
to
∼
5
×
10
15
cm
−
3
levels in the front layers. There are many other possible im-
provements that could be introduced to this device structure for
further development. These include matching of semiconductor
layer thicknesses to the solar spectrum in order to improve the
effectivelightabsorptionandthemodificationofelectricalcontacts,
including the electron back diffusion barrier (ebdb) and hole back
diffusion barrier (hbdb) layers at both ends, as reported in papers
published in 2009 [16, 17].