13.3.3
bottom cell because its nc-Si:H film is the thickest layer in the device and has to absorb
most of the red and infrared, whereas nc-Si:H is not that absorptive. Secondly,
intermediate reflector layers are used as a tool to redistribute the light between the
junctions above and below them. More specifically, the top and bottom junctions are
separated with a low reflective index material, such as nc-SiO x :H. Because of the
refractive index difference of the top absorber and the nc-SiO x :H, more light is reflected
back into the top cell. Thus, the a-Si:H top cell can be made thinner, which makes it less
sensitive to light-induced degradation.
Making thin-film silicon solar cells
To get a better understanding of the fabrication of thin-film silicon solar cells, we take a
look at the production process used at the Else Kooi Laboratory in Delft, the Netherlands.
Before the deposition can start, the sample has to be cleaned in an ultrasonic cleaning
bath. During cleaning, dirt and dust particles are removed, which could lead to a shunt
between the front and back contacts in the final solar cell. Now, the TCO layer can be
deposited. In Delft, we can deposit ZnO:Al or ITO with sputtering processes. After
sputtering, ZnO:Al can be etched with acids in order to achieve a crater-like structure for
light scattering. Alternatively, the sample may already be covered with a TCO layer, for
example SnO 2 :F from the Japanese Asahi company, which already has a pyramid-like
structure for light scattering because of its deposition process. Also on SnO 2 :F a 5–10 nm
thick ZnO:Al layer is deposited that protects the SnO 2 :F from being reduced by hydrogenrich plasma present during the deposition of the thin-film silicon layers. During sputtering,
the zinc oxide target is bombarded using an ionized noble gas like argon. The generated
aluminium zinc oxide species are sputtered into the chamber and deposited onto the
substrate.
Before the thin-film silicon layers are deposited, a thin Al strip is deposited on the
side of the sample, that will act as an electric front contact when the cells are measured.
Processing of the different thin-film silicon layers often happens in multi-chamber
setups that allow each layer to be processed in a separate chamber and therefore can
prevent crosscontamination, e.g. from p and n dopants, which would reduce the quality of
the layers. After the sample is mounted on a suitable substrate holder, it enters the setup
via a load lock, in which the substrate is brought under low pressure before it’s moved into
the processing chambers. This avoids the processing chambers becoming contaminated
with various unwelcome atoms and molecules present in ambient air. Then the sample is
brought into a central chamber, from where all process chambers can be accessed.
bottom cell because its nc-Si:H film is the thickest layer in the device and has to absorb
most of the red and infrared, whereas nc-Si:H is not that absorptive. Secondly,
intermediate reflector layers are used as a tool to redistribute the light between the
junctions above and below them. More specifically, the top and bottom junctions are
separated with a low reflective index material, such as nc-SiO x :H. Because of the
refractive index difference of the top absorber and the nc-SiO x :H, more light is reflected
back into the top cell. Thus, the a-Si:H top cell can be made thinner, which makes it less
sensitive to light-induced degradation.
Making thin-film silicon solar cells
To get a better understanding of the fabrication of thin-film silicon solar cells, we take a
look at the production process used at the Else Kooi Laboratory in Delft, the Netherlands.
Before the deposition can start, the sample has to be cleaned in an ultrasonic cleaning
bath. During cleaning, dirt and dust particles are removed, which could lead to a shunt
between the front and back contacts in the final solar cell. Now, the TCO layer can be
deposited. In Delft, we can deposit ZnO:Al or ITO with sputtering processes. After
sputtering, ZnO:Al can be etched with acids in order to achieve a crater-like structure for
light scattering. Alternatively, the sample may already be covered with a TCO layer, for
example SnO 2 :F from the Japanese Asahi company, which already has a pyramid-like
structure for light scattering because of its deposition process. Also on SnO 2 :F a 5–10 nm
thick ZnO:Al layer is deposited that protects the SnO 2 :F from being reduced by hydrogenrich plasma present during the deposition of the thin-film silicon layers. During sputtering,
the zinc oxide target is bombarded using an ionized noble gas like argon. The generated
aluminium zinc oxide species are sputtered into the chamber and deposited onto the
substrate.
Before the thin-film silicon layers are deposited, a thin Al strip is deposited on the
side of the sample, that will act as an electric front contact when the cells are measured.
Processing of the different thin-film silicon layers often happens in multi-chamber
setups that allow each layer to be processed in a separate chamber and therefore can
prevent crosscontamination, e.g. from p and n dopants, which would reduce the quality of
the layers. After the sample is mounted on a suitable substrate holder, it enters the setup
via a load lock, in which the substrate is brought under low pressure before it’s moved into
the processing chambers. This avoids the processing chambers becoming contaminated
with various unwelcome atoms and molecules present in ambient air. Then the sample is
brought into a central chamber, from where all process chambers can be accessed.