206
A. Romeo
substrate, transported near the deposition zone. Nowadays, FTO is made in an “inline CVD” system where the tin oxide is deposited on the “just manufactured” float
glass.
7
8.3.2.2 Buffer Layer
In the typical configuration, the buffer layer or window layer (n-part of the junction)
is the CdS layer. CdS has a bandgap of 2.4 eV, which does not allow having a complete transparency for wavelengths below 400 nm. The “parasitic” optical absorption
in the CdS layer is the predominant loss in photocurrent, making it necessary to minimize the thickness of CdS. High efficiency CdTe/CdS solar cells have been achieved
using CdS films with a thickness below 100 nm. A reduction from 120 to 60 nm
would result in a gain in the 400 nm wavelength region, with a subsequent increase
of about 10% in short-circuit current density.
Recently also magnesium tin oxide Mg x Zn 1−x O (MZO) has been introduced as
a buffer layer. MZO results in higher transparency compared to CdS. Moreover,
the band-gap of MZO can be tuned by changing the substrate temperature during the
deposition, and by doing so it is possible to control the Mg content. This results in
an increase in efficiency, in particular in terms of short-circuit current density and of
fill factor.
This new buffer layer works very well when a thin layer of CdSe is introduced in
the CdTe structure near the junction. Comparison of spectral response measurements
of solar cells made with a CdSe buffer, between CdS and CdTe layers suggests that
the improved current density can be attributed to a specific design of the junction.
The additional insertion of CdSe improves spectral response both in the blue
region (between 300 and 500 nm) and in the long wavelength region (825–930 nm):
this results in an increase in short-circuit current density. In fact CdSe has a lower
band gap than CdTe, allowing for the absorption of photons with lower energy.
In summary the insertion of an intermediate buffer layer of CdSe and substitution
of CdS with MgZnO, allows avoiding the “parasitic” absorption due to CdS, improves
the junction quality and drastically increases efficiency.
8.3.2.3 CdTe
In the standard configuration, the absorber consists of a CdTe layer, with a thickness
ranging from 1 to 8 µm. Its crystal structure and quality affect both the carrier
concentration and the carrier lifetime, having an effect on both the open-circuit
voltage and on the short-circuit current density.
7 Float glass is a sheet of glass made by floating molten glass. Generally float glass has a melting
temperature around 500 °C and contains alkali elements.
A. Romeo
substrate, transported near the deposition zone. Nowadays, FTO is made in an “inline CVD” system where the tin oxide is deposited on the “just manufactured” float
glass.
7
8.3.2.2 Buffer Layer
In the typical configuration, the buffer layer or window layer (n-part of the junction)
is the CdS layer. CdS has a bandgap of 2.4 eV, which does not allow having a complete transparency for wavelengths below 400 nm. The “parasitic” optical absorption
in the CdS layer is the predominant loss in photocurrent, making it necessary to minimize the thickness of CdS. High efficiency CdTe/CdS solar cells have been achieved
using CdS films with a thickness below 100 nm. A reduction from 120 to 60 nm
would result in a gain in the 400 nm wavelength region, with a subsequent increase
of about 10% in short-circuit current density.
Recently also magnesium tin oxide Mg x Zn 1−x O (MZO) has been introduced as
a buffer layer. MZO results in higher transparency compared to CdS. Moreover,
the band-gap of MZO can be tuned by changing the substrate temperature during the
deposition, and by doing so it is possible to control the Mg content. This results in
an increase in efficiency, in particular in terms of short-circuit current density and of
fill factor.
This new buffer layer works very well when a thin layer of CdSe is introduced in
the CdTe structure near the junction. Comparison of spectral response measurements
of solar cells made with a CdSe buffer, between CdS and CdTe layers suggests that
the improved current density can be attributed to a specific design of the junction.
The additional insertion of CdSe improves spectral response both in the blue
region (between 300 and 500 nm) and in the long wavelength region (825–930 nm):
this results in an increase in short-circuit current density. In fact CdSe has a lower
band gap than CdTe, allowing for the absorption of photons with lower energy.
In summary the insertion of an intermediate buffer layer of CdSe and substitution
of CdS with MgZnO, allows avoiding the “parasitic” absorption due to CdS, improves
the junction quality and drastically increases efficiency.
8.3.2.3 CdTe
In the standard configuration, the absorber consists of a CdTe layer, with a thickness
ranging from 1 to 8 µm. Its crystal structure and quality affect both the carrier
concentration and the carrier lifetime, having an effect on both the open-circuit
voltage and on the short-circuit current density.
7 Float glass is a sheet of glass made by floating molten glass. Generally float glass has a melting
temperature around 500 °C and contains alkali elements.