8 CdTe and CuInGaSe 2 Thin-Film Solar Cells
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Moreover, alkali treatment (among them Rb and Cs) increases the bandgap at
the surface of CIGS improving the junction quality. This has allowed one to reach
efficiencies above 22%.
8.2.2.2 Buffer Layer
CIGS solar cells were originally made with CdS as buffer layer deposited by evaporation; but this limits the current density since with its bandgap of 2.4 eV, CdS
absorbs visible light from 400 to 500 nm.
A significant improvement has been obtained with the strong reduction of CdS
thickness, obtained by replacing thermal evaporation with chemical bath deposition
(CBD) that assures a higher conformal coverage; and by adding a highly resistive and
transparent thin ZnO layer. On the other hand, attempts to replace CdS completely
by ZnO did not give satisfying results. Therefore CdS by CBD is mainly used to
protect CIGS against sputter damage; it etches the CIGS surface and passivates the
grain boundaries.
One of the most important improvements for the buffer layer is the application of
ZnS as buffer layer, which was obtained by substituting Cd with Zn in the chemical
bath. Increase of the bandgap from 2.4 eV for CdS to 3.6 eV for ZnS has been shown.
This also reduces the lattice mismatch between CIGS and (Zn,Cd)S.
ZnS and ZnO are II–VI semiconductors with wide bandgaps: 3.8 and 3.3 eV,
respectively. The most successful buffer layers from this group of materials contain
oxygen and sulphur in combination with hydrogen in the form of hydroxides.
The bandgap of ZnO 1−x ,S x exhibits a large bowing (modification of bandgap by
intermixing of two materials, in this case ZnO and ZnS) with a minimum bandgap
of 2.6 eV for x = 0.5. At the moment this solution is applied in large-scale module
production.
8.2.2.3 Front Contact
Today, CIGS solar cells are mainly fabricated with a double layer of ZnO:Al/ZnO,
generally deposited by RF-sputtering. Doping of the conducting ZnO layer is
achieved by group III elements (B, Al, Ga, In, Tl), particularly with aluminium. The
presence of an intrinsic ZnO layer increases the open-circuit voltage and prevents
the solar cell from shunts.
203
Moreover, alkali treatment (among them Rb and Cs) increases the bandgap at
the surface of CIGS improving the junction quality. This has allowed one to reach
efficiencies above 22%.
8.2.2.2 Buffer Layer
CIGS solar cells were originally made with CdS as buffer layer deposited by evaporation; but this limits the current density since with its bandgap of 2.4 eV, CdS
absorbs visible light from 400 to 500 nm.
A significant improvement has been obtained with the strong reduction of CdS
thickness, obtained by replacing thermal evaporation with chemical bath deposition
(CBD) that assures a higher conformal coverage; and by adding a highly resistive and
transparent thin ZnO layer. On the other hand, attempts to replace CdS completely
by ZnO did not give satisfying results. Therefore CdS by CBD is mainly used to
protect CIGS against sputter damage; it etches the CIGS surface and passivates the
grain boundaries.
One of the most important improvements for the buffer layer is the application of
ZnS as buffer layer, which was obtained by substituting Cd with Zn in the chemical
bath. Increase of the bandgap from 2.4 eV for CdS to 3.6 eV for ZnS has been shown.
This also reduces the lattice mismatch between CIGS and (Zn,Cd)S.
ZnS and ZnO are II–VI semiconductors with wide bandgaps: 3.8 and 3.3 eV,
respectively. The most successful buffer layers from this group of materials contain
oxygen and sulphur in combination with hydrogen in the form of hydroxides.
The bandgap of ZnO 1−x ,S x exhibits a large bowing (modification of bandgap by
intermixing of two materials, in this case ZnO and ZnS) with a minimum bandgap
of 2.6 eV for x = 0.5. At the moment this solution is applied in large-scale module
production.
8.2.2.3 Front Contact
Today, CIGS solar cells are mainly fabricated with a double layer of ZnO:Al/ZnO,
generally deposited by RF-sputtering. Doping of the conducting ZnO layer is
achieved by group III elements (B, Al, Ga, In, Tl), particularly with aluminium. The
presence of an intrinsic ZnO layer increases the open-circuit voltage and prevents
the solar cell from shunts.