Environmental Engineering Reference
In-Depth Information
workfor(112)-and(220/204)-preferred-orientedCIGSlayers.This
work shows the dominance of the defect level at 0.97 eV (E
3
)for
(112)-oriented films and 1.08 eV (E
4
) for (220/204)-oriented films.
This clearly indicates that the Fermi-level pinning at E
4
will take
place for (220/204)-preferred-oriented layers, producing better
performance than (112)-oriented CIGS layers. This observation
needs further exploration and systematic research for e
ciency
to be maximised by the selection of more advantageous crystal
orientations.
5.8.4
Replacement of Mo Using TCO for Tandem and
Double-Faced Solar Cells
In the conventional device, the Mo back contact is an opaque metal
contact with a thickness of
∼
1
μ
m. However, the same device
structure can be developed on glass/transparent conduction oxide
(TCO) substrates instead. In this case, the solar cell can be used for
both tandem [52] and bi-facial solar cells. Therefore, this double-
sided illuminated solar cell will produce more electric power from
a given panel at least for some applications. Bi-facial solar cells will
be more attractive for applications with vertically fixed solar panels
inunshadedareasforpowergeneration.ThistypeofCIGSsolarcells
will also help in development of high voltage tandem solar cells for
use in solar hydrogen production by the electrolysis of water.
5.8.5
Further Improvements of the Device Structure
In both device structures, the i-ZnO layer plays several important
roles. However, this layer is hampering current collection due to its
presentpositionat(c).Boththesedevicestructurescanbeimproved
byaddinganebdblayeratthebackcontactandanhbdblayeratthe
frontmetalcontact.Inaddition,itisusefulatthecurrentpositionin
order to act as a pinhole plugging material. This improved device
structure for type-II device is shown in Fig. 5.8 and will produce
enhanced device parameters.
The back metal contact (Mo) used in the traditional CIGS solar
cell in fact, already has this advantage built into the device. The
research published in the literature shows the natural formation of
