Environmental Engineering Reference
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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].
 
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