Batteries: thermoelectric and photoelectric – Photoelectric – Cells
Reexamination Certificate
1999-03-31
2001-01-09
Robinson, Ellis (Department: 1772)
Batteries: thermoelectric and photoelectric
Photoelectric
Cells
C136S255000, C438S029000, C438S042000, C438S071000, C204S192260
Reexamination Certificate
active
06172297
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a solar cell, comprising of a semiconducting material in which the incident radiation energy can generate charge carriers. The charge carriers are separable by an electric field and then conductible by first and second electrically conductive contacts. Whereas at least on one surface of the semiconductor groove-like depressions are formed, with at least one flank-like area and of which at least one of said flank-like areas carries directly or indirectly electrical conductive material, used for the formation of said first electric contact. Furthermore the invention relates to the method for the fabrication of a solar cell comprising a semiconductor material in which due to the incident radiation energy charge carriers can be generated, which are separatable by an electric field and carried then conductible by first and second conductive contacts, where on one side of the semiconductor surface groove-like depressions are formed, with flank-like areas to which electrical conductive material for the formation of the electrical conductive contacts is applied.
For industrial manufacturing of the metal contacts of conventional solar cells screen printing is the most common method. This method works by pressing a metal consisting paste through a stencil like screen onto the desired location of the solar cell surface. Upon that process the paste will be sintered by elevated temperatures between 700° C. and 800° C. This removes the solvents from the paste and gains a good electrical conductive and mechanical stable contact to the silicon surface. The main advantage of this method is the simplicity and the therefore resulting low production costs. But this method has a number of disadvantages. Limited by the manufacturing method, the width of screen printed contact lines is typically not smaller than 100 &mgr;m. From this width a relatively high percentage (12%) of light shadowing results. This high shadowing requires a large contact line distance of at least 3 mm. This requires again a highly diffused emitter, to keep the resistance losses at an acceptable level. Another difficulty of the screen printed solar cells is the relatively high contact resistance of the metal-semiconductor-contact. It is necessary to produce a high surface concentration of doping atoms to gain a low contact resistance between the semiconductor and the metal grid. This requirement of a highly diffused emitter with a high surface doping concentration results in a bad sensitivity in the ultraviolet spectral region of sun light. This can be explained by the fact that ultraviolet light is absorbed very close to the surface of the solar cell, and so electrons and holes are generated close to the surface. Because of the high doping level in this region of the cell recombination takes place very fast and therefore ultraviolet light does not contribute to the current produced in the solar cell.
One way to reduce the above mentioned problems of screen printed solar cells is the Buried Contact Solar Cell (BCSC) (U.S. Pat. No. 4,726,650). This type of cell has narrow and deep grooves (width:depth 1:2-1:7) at the surface which dictate the positions and the shape of the metallization. The metallization is generated by the method of electroless deposition. During this deposition the metal fills the grooves totally or nearly totally. By generating very narrow grooves (20-50 &mgr;m) the shadowing could be reduced compared to screen printed solar cells, but this method also has a number of disadvantages. To employ the seemingly simple method of electroless deposition, a number of very high energy consuming process steps are necessary. In addition to the elevated temperature process step of the emitter diffusion at around 850° C., at least two other high temperature processes are needed (every step requires several hours at around 1000° C.) (C. B. Honsberg et al.; Conf. Rec. 24
th
IEEE Photovoltaic Specialists Conference, Hawaii, 1994, P. 1473-1476). The first of these two additional high temperature steps will create a diffusion barrier. This barrier consists of a thick thermal oxide for the subsequent deep diffusion in the grooves. The second step consists of the deep diffusion in the grooves itself. This mainly serves the reduction of corrosion between the deposited metal fingers normally consisting of nickel, copper and silver. The corrosion would produce current shunts in the solar cell. The metallization step requires also a large number of different galvanic bathes and produces environmentally critical waste. In addition this type of metallization produces a large contact surface between the metal and the semiconductor due to the shape of the grooves. This relatively large contact area reduces the open circuit voltage of the cell and therefore the efficiency.
It is commonly known that vacuum evaporated contacts have the best contact properties compared to the above mentioned methods. The method of shallow angle evaporation (Borden et al Proc. 16
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IEEE Photovoltaic Specialists Conf., San Diego, 1982, P. 574 ff, and Hezel Proc. 13
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European Photovoltaic Solar Energy Conf., Nice, 1995, S. 115-118).) is a promising possibility for the manufacturing of metal contacts. For this method closely running parallel grooves have to be produced on the solar cell surface, which normally have a half circle or v-shaped cross section. These narrow elevations are employed as shadow edges for the subsequent evaporation of the metal contact material under a shallow angle to the substrate surface. The common disadvantages of the perpendicular shadow mask evaporation are eliminated with the help of the shallow angle evaporation. Due to the self aligning elevations the use of shadow masks for definition of the front contacts can be avoided and the machine capacity can be enormously increased compared to conventional evaporation method. This is due to the possibility of spacing of the wafers very closely in the evaporation chamber, because of the very shallow angle of the wafers to the evaporation source (Hezel, Proc. 13
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European Photovoltaic Solar Energy Conf.; Nice, 1995, P. 115). In addition, the use of the evaporated material is much higher using this method compared to the shadow mask evaporation. Using the conventional method, the evaporation material is deposited mainly onto the shadow masks and afterwards has to be removed from the masks employing work intensive etching steps. The introduced method allows for a variation of metallization finger width simply by changing the evaporation angle, since the groove width is normally dictated by the used fabrication tool (e.g. diamond dicing blade, wire and slurry). The distance of the contact fingers is also given by the fabrication tool. Cells of this type are achieving currently efficiencies of 18.6% (Verbeek et al., Proc. 25tz IEEE Photovoltaik Specialits Conf., Washington D.C., 1996, P. 521). A sharp shadow of the elevations can only be achieved up to a distance of 500 &mgr;m. This is due to divergence of the material beam caused by the scattering of the material at the shadow edges, the dimension of the evaporation source and the pressure in the evaporation chamber. The maximum finger distance of this cell type is therefore limited to less than 500 &mgr;m. Besides which using v-shaped grooves the light shadowing is relatively high since the fingers are at a angle of 35° to the substrate basis. Another point is the relatively high amount of silicon which has to be removed from the cell, since the grooves are closely spaced. The last point increases the wear of the tooling and the manufacturing costs.
In EP 0 561 615 A2 a circuit of separate IR detectors in an advantageous closely spaced manner and compact arrangement is described. Single elements can be connected by vertical running conductors to the surface of the electrical elements. This is a method which is commonly used for connecting single solar cells to solar cell modules. It is not required that the necessary electrical connecting element runs over the whole length of
Hezel Rudolf
Metz Axel
Dennison, Scheiner Schultz & Wakeman
Institut fur Solarenergieforschung GmbH
Miggins Michael C.
Robinson Ellis
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