114
S. Leu and D. Sontag
We have learned that recombination (see Chap. 4) can occur on the surface of the
wafer because at that point the crystal structure is broken abruptly; creating so-called
“dangling bonds” (see also Chap. 6). These recombination centres are also saturated
by the hydrogen in the silicon nitride and can then no longer form recombination
centres. Silicon nitride has, thus, two functions: (1) it acts as antireflection layer
(leading to an increase in the short-circuit current I SC ) and (2) also as passivation
19
layer (leading to an increase in the open-circuit voltage V OC ). At the back of the
solar cell we also have to ensure a passivation of the surface. There, we form a “Back
Surface Field”.
(d) Back Surface Field
Electron-hole pairs generated by long-wavelength light in the lower half of the wafer,
where there is only weak diffusion and a negligible electrical field, should be collected, before they can recombine. Without any special measures they would be
“sucked up” by the metal contact on the backside, where they would all recombine. To avoid this, one coats the backside with aluminium, which creates, thus, a
small backside electric field. At higher process temperatures, the aluminium forms
an electrically active alloy with the silicon, thereby establishing a heavily doped
p-type region, generating a p
+ /p junction. This reduces the effective surface recombination velocity of electrons at the back surface. As discussed in Chap. 4, surface
recombination is one of the major loss mechanisms in solar cells. The so-called Aluminium Back Surface Field (Al-BSF) ensures that electrons, which are close to the
back side, do not recombine there, but are directed back into the bulk of the silicon
wafer. In addition to the passivation property, the aluminium layer simultaneously
takes over the task of conducting the electric current on the back side—and, like the
phosphorous in the case of phosphorus diffusion, also ensures that the silicon wafer
is cleaned during the gettering process.
The backside metallization consists of an aluminium layer, which contains about
80% aluminium particles with a size of a few μm, and which is coated with a thin
oxide layer. Another important component of the aluminium paste is 5% boron or
borosilicate glass frits. The remainder consists of organic solvents to guarantee the
rheology
20 and viscosity,
21 so that the paste can be printed using a Screen Printing
process. At around 800 °C, silicon dissolves in the aluminium during the firing step
and forms an alloy [8]. The entire high-temperature process takes about 40 s; thereby
a stable eutectic
22 between silicon and aluminium is formed with about (by weight)
19 Silicon nitride is also positively charged. The positive charge in the silicon nitride improves the
accumulation of electrons at the front surface, creating an accumulation region in the n-type doped
emitter and thereby reducing the effective surface recombination velocity.
20 Rheology (from ancient Greek ·ε‹ν “rhein”) is the science that deals with the deformation and
flow behaviour of materials.
21 Viscosity is a property of liquids (fluids) and gases. The higher the viscosity, the thicker the fluid;
the lower the viscosity, the thinner (more flowing) the fluid is.
22 Eutectic alloys have a clearly definable melting point, the so-called eutectic point. Because in
eutectics all components solidify simultaneously and this occurs at a much lower temperature than
would be the case with the pure components, a fine and uniform microstructure is formed.
S. Leu and D. Sontag
We have learned that recombination (see Chap. 4) can occur on the surface of the
wafer because at that point the crystal structure is broken abruptly; creating so-called
“dangling bonds” (see also Chap. 6). These recombination centres are also saturated
by the hydrogen in the silicon nitride and can then no longer form recombination
centres. Silicon nitride has, thus, two functions: (1) it acts as antireflection layer
(leading to an increase in the short-circuit current I SC ) and (2) also as passivation
19
layer (leading to an increase in the open-circuit voltage V OC ). At the back of the
solar cell we also have to ensure a passivation of the surface. There, we form a “Back
Surface Field”.
(d) Back Surface Field
Electron-hole pairs generated by long-wavelength light in the lower half of the wafer,
where there is only weak diffusion and a negligible electrical field, should be collected, before they can recombine. Without any special measures they would be
“sucked up” by the metal contact on the backside, where they would all recombine. To avoid this, one coats the backside with aluminium, which creates, thus, a
small backside electric field. At higher process temperatures, the aluminium forms
an electrically active alloy with the silicon, thereby establishing a heavily doped
p-type region, generating a p
+ /p junction. This reduces the effective surface recombination velocity of electrons at the back surface. As discussed in Chap. 4, surface
recombination is one of the major loss mechanisms in solar cells. The so-called Aluminium Back Surface Field (Al-BSF) ensures that electrons, which are close to the
back side, do not recombine there, but are directed back into the bulk of the silicon
wafer. In addition to the passivation property, the aluminium layer simultaneously
takes over the task of conducting the electric current on the back side—and, like the
phosphorous in the case of phosphorus diffusion, also ensures that the silicon wafer
is cleaned during the gettering process.
The backside metallization consists of an aluminium layer, which contains about
80% aluminium particles with a size of a few μm, and which is coated with a thin
oxide layer. Another important component of the aluminium paste is 5% boron or
borosilicate glass frits. The remainder consists of organic solvents to guarantee the
rheology
20 and viscosity,
21 so that the paste can be printed using a Screen Printing
process. At around 800 °C, silicon dissolves in the aluminium during the firing step
and forms an alloy [8]. The entire high-temperature process takes about 40 s; thereby
a stable eutectic
22 between silicon and aluminium is formed with about (by weight)
19 Silicon nitride is also positively charged. The positive charge in the silicon nitride improves the
accumulation of electrons at the front surface, creating an accumulation region in the n-type doped
emitter and thereby reducing the effective surface recombination velocity.
20 Rheology (from ancient Greek ·ε‹ν “rhein”) is the science that deals with the deformation and
flow behaviour of materials.
21 Viscosity is a property of liquids (fluids) and gases. The higher the viscosity, the thicker the fluid;
the lower the viscosity, the thinner (more flowing) the fluid is.
22 Eutectic alloys have a clearly definable melting point, the so-called eutectic point. Because in
eutectics all components solidify simultaneously and this occurs at a much lower temperature than
would be the case with the pure components, a fine and uniform microstructure is formed.