14.3
Figure 14.2: (a) Physical vapour deposition (PVD) with resistive evaporation;(b) PVD with electron beam evaporation.
With kind permission from G. Papakonstantinou [89].
For resistive evaporation, the metallic source is loaded in an open boat (crucible),
which is heated resistively by applying a high current. For electron beam evaporation, the
metallic source is loaded in a water-cooled crucible and is irradiated by an intense electron
beam, which heats the source. The electron beam is emitted by a tungsten filament and
directed by strong magnetic fields. In both methods, the metallic source evaporates and its
atoms follow straight trajectories targeting the substrate.
Evaporation is performed under a high vacuum, in which the pressure ranges from
10
−3 to 10
−6 Pa. This pressure range is sufficient to control the thickness and the oxidation
level of the deposited layer [91]. Usually, the metallic sources are located at the bottom of
the chamber, such that the metallic vapour flows upwards. If the substrate is mounted on a
rotating holder, the homogeneity of the deposited layer can be improved.
Screen printing technology
Screen printing (SP) is another method to fabricate metal front and back contacts. Unlike
evaporation, SP is a method in which the metallic source is liquid in the form of a viscous
paste. SP is the most extensively used technique in industry [92]. It is a fast,
wellestablished, reliable and cost-efficient method that has dominated the mass production
of solar cells. In principle, all shapes and patterns can be realized with the SP technology
[93].
As illustrated in Figure 14.3, screen printers consist of three basic structures: the
frame, the screen and the squeegee. These structures are responsible for transferring the
metallic paste onto the surface of the substrate in the shape of the grid pattern. The rigid
frame serves as a supporting structure for the screen. The screen consists of an interwoven
thinwire mesh, which is fixed to the frame under high tension. It has openings in the shape
of the grid pattern. Prior to the printing process, the substrate and the screen are not in
intimate contact. During the printing process, the squeegee moves across the surface of the
screen and sweeps the metallic paste. While the squeegee moves, it applies a force onto
the screen and coerces the metallic paste to pass through its openings. As a result, the
squeegee causes the screen to deflect and come locally into contact with the substrate at
the position of the squeegee. At the same time, the screen peels off from the substrate
behind the travelling squeegee, whereas the paste remains on the substrate. In this manner,
the desired metallization pattern is reproduced on the surface of the substrate.
Figure 14.2: (a) Physical vapour deposition (PVD) with resistive evaporation;(b) PVD with electron beam evaporation.
With kind permission from G. Papakonstantinou [89].
For resistive evaporation, the metallic source is loaded in an open boat (crucible),
which is heated resistively by applying a high current. For electron beam evaporation, the
metallic source is loaded in a water-cooled crucible and is irradiated by an intense electron
beam, which heats the source. The electron beam is emitted by a tungsten filament and
directed by strong magnetic fields. In both methods, the metallic source evaporates and its
atoms follow straight trajectories targeting the substrate.
Evaporation is performed under a high vacuum, in which the pressure ranges from
10
−3 to 10
−6 Pa. This pressure range is sufficient to control the thickness and the oxidation
level of the deposited layer [91]. Usually, the metallic sources are located at the bottom of
the chamber, such that the metallic vapour flows upwards. If the substrate is mounted on a
rotating holder, the homogeneity of the deposited layer can be improved.
Screen printing technology
Screen printing (SP) is another method to fabricate metal front and back contacts. Unlike
evaporation, SP is a method in which the metallic source is liquid in the form of a viscous
paste. SP is the most extensively used technique in industry [92]. It is a fast,
wellestablished, reliable and cost-efficient method that has dominated the mass production
of solar cells. In principle, all shapes and patterns can be realized with the SP technology
[93].
As illustrated in Figure 14.3, screen printers consist of three basic structures: the
frame, the screen and the squeegee. These structures are responsible for transferring the
metallic paste onto the surface of the substrate in the shape of the grid pattern. The rigid
frame serves as a supporting structure for the screen. The screen consists of an interwoven
thinwire mesh, which is fixed to the frame under high tension. It has openings in the shape
of the grid pattern. Prior to the printing process, the substrate and the screen are not in
intimate contact. During the printing process, the squeegee moves across the surface of the
screen and sweeps the metallic paste. While the squeegee moves, it applies a force onto
the screen and coerces the metallic paste to pass through its openings. As a result, the
squeegee causes the screen to deflect and come locally into contact with the substrate at
the position of the squeegee. At the same time, the screen peels off from the substrate
behind the travelling squeegee, whereas the paste remains on the substrate. In this manner,
the desired metallization pattern is reproduced on the surface of the substrate.