Semiconductor device manufacturing: process – Making device or circuit responsive to nonelectrical signal – Responsive to electromagnetic radiation
Utility Patent
1998-02-25
2001-01-02
Smith, Matthew (Department: 2825)
Semiconductor device manufacturing: process
Making device or circuit responsive to nonelectrical signal
Responsive to electromagnetic radiation
C438S583000, C438S584000, C438S587000, C438S591000, C136S244000, C136S255000, C257S458000
Utility Patent
active
06168968
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of fabricating integrated thin film solar cells. More particularly, the present invention relates to a process of patterning a rear reflection electrode layer in an integrated thin film solar cell.
2. Description of the Related Arts
The thin film solar cell has an integrated structure in which a plurality of photoelectric power generation elements (unit cells) typically including a transparent electrode layer, an amorphous semiconductor photoelectric conversion layer and a rear reflection electrode layer of metal, these layers are laminated in this order on a light-transmitting insulating substrate of glass or the like. The unit cells are electrically connected in series for achieving a redetermined photoelectric power generation in the thin film solar cell. Accordingly, each layer is subjected to a patterning process after the formation of the layer.
Here, a current loss due to a sheet resistance of the transparent electrode layer is accumulated as a series resistance component loss (series loss) of the thin film solar cell and lowers a curve factor value (F.F.) of an I-V characteristics of the solar cell. Therefore, the patterning process needs to be performed so that the unit cells are electrically connected in series without increasing the series resistance component loss. For performing the patterning process, a laser scribing method is established which is advantageous in terms of the number of steps and the costs as compared with the mask evaporation or photoetching method.
The patterning process employing the laser scribing method makes use of the laser beam characteristics having a high energy density and being excellent in directivity and is suitable for fine patterning process. The laser to be used for the patterning process may be, for example, a YAG laser, an excimer laser or the like, and its wavelength and intensity may vary depending on the intended use thereof. Also, it is possible to perform selective patterning process by utilizing the difference in light absorptivity of an object to be processed. Especially, a YAG laser is often used because of its low running costs, simple apparatus construction and elimination of high pressure corrosive gas from the process.
A conventional process for fabricating an integrated thin film solar cell is as follows. First, a transparent electrode layer formed on a light-transmitting insulating substrate is segmented into a plurality of elongate strips for isolation (a first scribing step). Then, an amorphous photoelectric conversion layer is deposited on the resulting substrate, and a patterning process is performed for segmenting only the amorphous photoelectric conversion layer at a position shifted from a first scribing line without damaging the transparent electrode layer (a second scribing step). Subsequently, a rear reflection electrode layer is deposited on the resulting substrate and a patterning process is performed for segmenting the rear reflection electrode layer at a position shifted from a second scribe line to a side opposite to the first scribe line (a third scribing step).
Generally, a laser scribing method is employed in the patterning process for segmenting the transparent electrode film layer (the first scribing step). This is due to the fact that it is possible to form trenches easily through scanning a surface of the object to be processed with a laser beam by moving the object or the laser beam, so that the manufacturing process is simplified and also it is possible to prevent decrease in the effective power generation area because the trenches are formed to have a narrow width of 100 &mgr;m or less.
The patterning process for segmenting only the amorphous photoelectric conversion layer (the second scribing step) makes use of the fact that the light absorption of the transparent electrode layer and the amorphous photoelectric conversion layer varies greatly depending on a wavelength of light. For example, a beam of a secondary harmonic generation (SHG having a wavelength of 0.532 &mgr;m) of the Nd-YAG laser is applied, whereby the beam is transmitted through the transparent electrode layer without damaging it and is absorbed by the amorphous photoelectric conversion layer to remove a portion of an amorphous photoelectric conversion layer by ablation so as to form the trenches.
The patterning process for segmenting the rear reflection electrode layer (the third scribing step) is performed by one of the following methods.
First method employs applying the beam of the second harmonic generation (SHG) of the Nd-YAG laser from the light-transmitting insulating substrate side to allow the beam to be absorbed by the amorphous photoelectric conversion layer to remove a portion of the rear reflection electrode layer together with a portion of the amorphous photoelectric conversion layer by ablation so as to form the trenches.
Second method employs applying a laser beam directly onto the rear reflection electrode layer to remove a portion of the layer.
Third method employs printing a resist on the rear reflection electrode layer by screen printing method to remove a portion of the rear reflection electrode layer by chemical etching.
Among the above-described three patterning steps (the first, second and third scribing steps), the most problematic one is the patterning process for segmenting the rear reflection electrode layer (i.e., the third scribing step). This is due to the following reason. The patterning process for segmenting the rear reflection electrode layer is performed as the final step after each layer of the integrated thin film solar cell is formed by deposition. Therefore, by employing the third method, a pressure is exerted on the substrate by contact with a mask or a squeegee at the time of screen printing, causing a very small short circuit (shunt path leak) in the surface of the thin film solar cell and lowering the curve factor value (F.F.) of the I-V characteristics of the solar cell.
By employing the first method, a short circuit occurs by contact of adjacent rear reflection electrodes, which are separated by the patterning, with each other due to burrs (flashes) of Al or Ag constituting the rear reflection electrode layer. A short circuit occurs also because a rear transparent conductive layer (for example, ZnO or ITO) formed between the rear reflection electrode layer and the amorphous photoelectric conversion layer for improvement of sensitivity in the longer wavelength adheres to the trenches by becoming a conductive sublimate when a portion of the rear reflection electrode layer is blown away by ablation of the amorphous photoelectric conversion layer.
The rear reflection electrode layer is made of Al or Ag and is highly reflective as well as being highly conductive. Therefore, the second method has a drawback that the laser beam is reflected by the rear reflection electrode layer so that the energy of the laser beam is not utilized effectively in the patterning process.
FIG. 3
shows a relationship between the wavelength of the laser beam and the reflectivity. Al and Ag show a high reflectivity of 80% or more with respect to the laser beam having a wavelength of 0.4 &mgr;m or more. Therefore, when a beam of a fundamental harmonic generation (wavelength of 1.064 &mgr;m) of the Nd-YAG laser is applied for patterning, almost all of the beam is reflected by the rear reflection electrode layer. If the patterning process is conducted with high laser power in order to compensate for the energy loss caused by the reflection, the beam is absorbed indiscriminately by the underlying amorphous photoelectric conversion layer and the transparent electrode layer because the wavelength lies in a near infrared region. Accordingly, it is impossible to conduct a selective film patterning process, and may cause problems such as severance of all the layers and short circuit due to fusion of the rear reflection electrode layer by local heating.
Even if the beam of the second harmonic generation (0.532 &mgr;m) of
Kidoguchi Susumu
Umemoto Akimasa
Lee Granvill
Nixon & Vanderhye P.C.
Sharp Kabushiki Kaisha
Smith Matthew
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