Method for manufacture of a solar cell

Details Method for manufacture of a solar cell Method for manufacture of a solar cell

2002-10-18

2004-06-08

Talbot, Brian K. (Department: 1762)

Coating processes

Electrical product produced

Photoelectric

C427S255270, C427S255290, C427S255370, C427S255393, C427S255394

Reexamination Certificate

active

06746709

ABSTRACT:

BACKGROUND OF THE INVENTION
The invention relates to a method for manufacture of a solar cell by the formation of a hydrogenous layer containing silicon in the form of a passivation and/or anti-reflexion layer on a substrate comprising or containing silicon such as a wafer or film.
More than 80% of all solar cells are currently made from crystalline silicon wafers manufactured either using the Czochralski method or by means of block casting. A silicon mass in the form of a round column or large block is here crystallized and then sawn into individual wafers. This high proportion will presumably increase markedly in the coming years on account of new production capacities, since many manufacturers prefer the production techniques tried and tested for many years and based on crystalline silicon wafers to the new technologies.
Thin-film solar cells are currently being discussed as future alternatives to solar cells of silicon wafers (typical thickness around 300 &mgr;m); in comparison with solar cells made of crystalline silicon wafers, they make do with considerably less semiconductor material (thickness approx 1-10 &mgr;m). These cells can be deposited using a variety of methods directly onto large glass surfaces and therefore hold out the promise of considerable potential cost reductions. Amorphous silicon thin-film solar cells with efficiencies in the 6 to 8% range are already commercially available. Higher efficiencies can be achieved with composite semiconductors such as CdTe or CulnS
2
. Solar cells made from these materials arc currently being tested in pilot production lines (A. Abken et al., Proc. 16 FPVSEC, 2000; D. Cunningham et al., Proc. 16th EPVSEC, 2000). Whether these materials will make headway in the long term is not at present clear, as some of them are toxic or are available only in small quantities. There are high hopes for material-saving crystalline Si thin-film solar cells, since silicon is environmentally compatible and has unlimited availability. These cells are however still at a very early stage (R. Brendel et al., Proc. 14th EPVSEC, p. 1354, 1991; K. Feldrapp et al., Proc. 16th EPVSEC, 2000).
A second alternative to conventional manufacture of solar cells from crystalline silicon wafers is the use of silicon films. Here silicon is directly crystallized as a film in the thickness necessary for solar cells. This avoids the considerable cutting losses entailed by the classic block casting or Czochralski methods. The Edge-defined Film-fed Growth (EFG) method is already in industrial use, and permitting the manufacture of very high-quality silicon films. The latest developments are aimed at the reduction of the film thickness to approx. 100 &mgr;m. Unlike the block casting or Czochralski methods, the film method permits a marked reduction in the manufacturing costs, since the ratio of cutting losses to wafer volume does not increase here as the wafers become thinner. For that reason, silicon films could dominate the market during the long-term transition from the present wafer technology to thin-film technology.
A central factor in all silicon solar cells is the effective life of the charge carriers generated by light in the crystal volume. This must be sufficient to permit all charge carriers to diffuse to the metal contacts if possible and hence reach the connected circuit. This applies to the currently dominant block-cast and Czochralski-type wafers, to the silicon films which will presumably find greater application in the medium term, and to the crystalline silicon thin-film solar cells which might be possible in the future.
The effective charge carrier life of crystalline silicon is limited by crystal defects (offsets or flaws), by crystal impurities (including metal atoms), and by the quality of the crystal surface (e.g. dangling bonds). A sufficient avoidance of crystal defects and impurities and the manufacture of an ideal surface even during the crystal and wafer manufacture is not possible due to technological obstacles or for economic reasons. Attempts are therefore being made in the downstream solar cell manufacturing processes to improve the originally often short charge carrier life of the silicon wafers.
This is possible by a subsequent reduction of the impurities (Gettern) (I. J. Caballero et al., Proc. 16th EPVSEC 2000), by electronic “alleviation” of crystal defects by adding atomic hydrogen into the crystal (hydrogen volume passivation) (B. L. Sopori et al., Solar En. Mat. & Solar Cells 41/42, p. 159, 1996), and by depositing surface coatings to prevent charge carrier combination on the surface (electronic surface passivation) (A. Aberle, R. Hezel, Progr. in PV 5, p. 29, 1997). Processes in this respect can be of crucial importance for good solar cell efficiencies and are therefore already in use by industry in various designs.
For hydrogen volume passivation of silicon solar cells, known methods include the hydrogen plasma, tempering in forming gas and diffusion of hydrogen from a hydrogenous silicon nitride surface layer (SiN). For electronic surface passivation the known methods include oxidation of the silicon surface (S. Wenham et al., Solar En. Mat. & Solar Cells 65, p. 377, 2001) and application of a hydrogenous silicon nitride surface layer (A. Aberle, R. Hezel, Progr. in PV 5, p. 29, 1997). Of all the known methods, the application of a hydrogenous SiN surface layer is the only one that can achieve both processes at the same time. For this reason, more and more solar cell manufacturers are using SiN layers in their production. A further advantage of SiN surface coatings is that they have in addition to their passivation properties excellent optical parameters, allowing them to be used as effective anti-reflexion coatings.
Hydrogen volume passivation with the aid of a SiN surface coating takes place in two process steps, first the hydrogenous SiN layer is applied to the surface of the silicon wafer. In so doing, a small proportion of atomic hydrogen can already penetrate into a surface-near area of the silicon wafer. This is followed by a high-temperature treatment at temperatures in excess of 700° C. At these high temperatures, a relatively large amount of atomic hydrogen in the surface layer is freed and diffuses deep into the silicon crystal (B. L. Sopori et al., Solar En. Mat. & Solar Cells 41/42, p. 159, 1996; J. Jeong et al., J. Appl. Phys. 87 (10), p. 7551, 2000). The electronic surface passivation with the aid of a SiN surface coating is achieved by two effects. Firstly, the hydrogen contained in the layer collects at the silicon surface and passivates dangling silicon bonds, such that these become electronically ineffective. Secondly, fixed insulator charges are created in the layer, which by their influence generate in the silicon an electrical field which leads to a strengthening of the electronic passivation effect (charge carriers are kept away from the surface and hence cannot be lost) (A. G. Aberle et al., Solar En. Mat. & Solar Cells 29, p. 175, 1993). The manufacturing methods of SiN surface coatings blown for solar cell applications are:
a) Parallel-plate Plasma:
In this method, process gases containing silicon and nitrogen, preferably silane and ammonia, are excited in a low pressure system by a plasma discharge and brought to a reaction. The plasma discharge is generated between two parallel plates by applying an A.C. voltage. This is typically in the kHz or MHz frequency range, with a voltage of 100 to 1000 V (R. Reif in: handbook of Plasma Processing Technology, Noyes, N.J., 1990, p. 269 ff.).
b) Remote Microwave Plasma:
Ammonia and nitrogen is excited in a low pressure plasma outside or in a separate area of the coating chamber and then passed to the substrate. On the way there, a siliceous process gas (as a rule silane) is admixed. The excited nitrogenous gas reacts with the siliceous gas, leading to a layer deposition on the substrate.
c) LPCVD
Nitrogenous and siliceous process gases are brought thermally to a reaction in a low pressure system at temperatures in excess of 700° C. In view of the high te

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