8 CdTe and CuInGaSe 2 Thin-Film Solar Cells
207
Close space sublimation (CSS) is probably the most popular technique; here the
material is sublimated
8 at high pressure, with high substrate temperatures: resulting,
thus, in fast deposition. This, on an industrial scale, is transformed into the process
called vapour transport deposition (VTD).
Substrate temperature influences grain growth: the higher the substrate temperatures are, the larger the grain sizes will be with a consequent reduction in the number
of grain boundaries.
Larger grain size with low substrate temperature can be obtained by the standard
recrystallization treatment: the so-called CdCl 2 annealing treatment. This consists of
a heat treatment at 380–420 °C under a chlorine atmosphere (that can be provided by
a chlorine containing gas or by a deposition of CdCl 2 or of MgCl 2 on the CdTe layer).
This can be done in argon or in air atmosphere. This process step re-crystallizes the
grains and removes structural defects in both CdTe and in the intermixed layer of
CdTe-CdS, which is generally formed at the junction. Devices without this postdeposition annealing obtain conversion efficiencies not larger than 10%.
In the past CdTe has been deposited by a large amount of techniques, however
the most successful ones are CSS [4], RF Sputtering [5], thermal evaporation [6] and
VTD [7].
We will describe here only the VTD process in more detail, since it is the method
that is mostly applied for large-scale production.
Vapour Transport Deposition (VTD)
Vapour transport deposition is a technique that combines the features of being
an atmospheric pressure deposition and having a high substrate temperature like in
the close space sublimation technology (but in VTD the material is delivered on the
substrate by carrier gases).
In this way, the features of a high pressure and a non-vacuum low-cost process are
obtained, but, at the same time, it is much easier to refill the material sources, since
the crucibles are far away from the deposition region. This makes VTD particularly
suitable for industrial processes, so there is no wonder that Solar Cell Inc. (now
First Solar), in collaboration with the National Renewable Energies Laboratory, has
introduced it for their fabrication line.
The process of CdTe deposition consists of three different stages:
(1) Heating of the material from a powder feeder into the vaporizer where CdTe is
introduced in gas phase, (2) transporting CdTe into the deposition chamber through
a distributor by means of inert carrier gases, (3) depositing the layer on the substrate
at high temperature (coalescence
9 and growth of the crystals).
Up to now, this is the process that leads to the highest efficiency for CdTe solar
cells, 22.1% [1] fabricated at the batch CdTe facility at First Solar.
8 Sublimation is the transition of a substance directly from the solid to the gas phase, without passing
through an intermediate liquid phase.
9 Coalescence is the process by which two or more small grains merge during growth to form a
single larger grain.
207
Close space sublimation (CSS) is probably the most popular technique; here the
material is sublimated
8 at high pressure, with high substrate temperatures: resulting,
thus, in fast deposition. This, on an industrial scale, is transformed into the process
called vapour transport deposition (VTD).
Substrate temperature influences grain growth: the higher the substrate temperatures are, the larger the grain sizes will be with a consequent reduction in the number
of grain boundaries.
Larger grain size with low substrate temperature can be obtained by the standard
recrystallization treatment: the so-called CdCl 2 annealing treatment. This consists of
a heat treatment at 380–420 °C under a chlorine atmosphere (that can be provided by
a chlorine containing gas or by a deposition of CdCl 2 or of MgCl 2 on the CdTe layer).
This can be done in argon or in air atmosphere. This process step re-crystallizes the
grains and removes structural defects in both CdTe and in the intermixed layer of
CdTe-CdS, which is generally formed at the junction. Devices without this postdeposition annealing obtain conversion efficiencies not larger than 10%.
In the past CdTe has been deposited by a large amount of techniques, however
the most successful ones are CSS [4], RF Sputtering [5], thermal evaporation [6] and
VTD [7].
We will describe here only the VTD process in more detail, since it is the method
that is mostly applied for large-scale production.
Vapour Transport Deposition (VTD)
Vapour transport deposition is a technique that combines the features of being
an atmospheric pressure deposition and having a high substrate temperature like in
the close space sublimation technology (but in VTD the material is delivered on the
substrate by carrier gases).
In this way, the features of a high pressure and a non-vacuum low-cost process are
obtained, but, at the same time, it is much easier to refill the material sources, since
the crucibles are far away from the deposition region. This makes VTD particularly
suitable for industrial processes, so there is no wonder that Solar Cell Inc. (now
First Solar), in collaboration with the National Renewable Energies Laboratory, has
introduced it for their fabrication line.
The process of CdTe deposition consists of three different stages:
(1) Heating of the material from a powder feeder into the vaporizer where CdTe is
introduced in gas phase, (2) transporting CdTe into the deposition chamber through
a distributor by means of inert carrier gases, (3) depositing the layer on the substrate
at high temperature (coalescence
9 and growth of the crystals).
Up to now, this is the process that leads to the highest efficiency for CdTe solar
cells, 22.1% [1] fabricated at the batch CdTe facility at First Solar.
8 Sublimation is the transition of a substance directly from the solid to the gas phase, without passing
through an intermediate liquid phase.
9 Coalescence is the process by which two or more small grains merge during growth to form a
single larger grain.