Patent Application
20080251121
1. Field of the Invention
The present invention relates generally to solar cells, and more particularly to solar cell structures and fabrication processes.
2. Description of the Background Art
Solar cells are devices for converting solar radiation to electrical energy. They may be fabricated on a semiconductor wafer using semiconductor processing technology. Generally speaking, a solar cell may be fabricated by forming P-type and N-type active diffusion regions in a silicon substrate. Solar radiation impinging on the solar cell creates electrons and holes that migrate to the active diffusion regions, thereby creating voltage differentials between the active diffusion regions. In a back side contact solar cell, both the active diffusion regions and the metal grids coupled to them are on the back side of the solar cell. The metal grids allow an external electrical circuit to be coupled to and be powered by the solar cell.
One problem or limitation with solar cells is that their performance tends to degrade over time. In other words, solar cells tend to get less reliable and less efficient over time. Applicant believes that the present disclosure provides a solution which overcomes, or at least partially overcomes, the performance degradation problem in solar cells.
One embodiment relates to a structure for a solar cell. The structure includes a silicon substrate with P-type and N-type active diffusion regions therein. An oxynitride passivation layer is included at least over the P-type and N-type active diffusion regions. The structure further includes contact openings through the oxynitride passivation layer to the P-type and N-type active diffusion regions, and metal grid lines which selectively contact the P-type and N-type active diffusion regions by way of the contact openings.
Another embodiment relates to a method of fabricating a solar cell. P-type and N-type active diffusion regions are formed in a silicon substrate, and an oxynitride passivation layer is formed at least over the P-type and N-type active diffusion regions. In addition, contact openings are formed through the oxynitride passivation layer to the P-type and N-type active diffusion regions, and metal grid lines are formed which selectively contact the P-type and N-type active diffusion regions by way of the contact openings.
Other embodiments, aspects and features are also disclosed.
These and other features of the present invention will be readily apparent to persons of ordinary skill in the art upon reading the entirety of this disclosure, which includes the accompanying drawings and claims.
Note that the use of the same reference label in different drawings indicates the same or like components. Drawings are not necessarily to scale unless otherwise noted.
In the present disclosure, numerous specific details are provided, such as examples of structures and fabrication steps, to provide a thorough understanding of embodiments of the invention. Persons of ordinary skill in the art will recognize, however, that the invention can be practiced without one or more of the specific details. In other instances, well-known details are not shown or described to avoid obscuring aspects of the invention.
As discussed above, solar cells tend to get less reliable and less efficient over time. Applicant believes that at least part of this degradation is caused by the exposure of the solar cells to damp heat over time. Applicant further believes that such damp heat causes moisture to diffuse through the passivation layer on the device side of the solar cell.
Applicant believes that the present disclosure provides a solar cell structure, and method of manufacturing same, which prevents or reduces the diffusion of moisture through the passivation layer on the device side of the solar cell. As such, applicant believes that solar cells fabricated in accordance with embodiments of the invention will have less performance degradation over time. Solar cells manufactured according to the present disclosure should be better at maintaining reliability and efficiency under damp heat conditions.
As described further below, applicant has come up with modified processes to fabricate a solar cell structure so as to incorporate an oxynitride passivation layer in order to better shield the devices from the effects of moisture diffusion. Applicant further believes that the oxynitride layer will improve device performance by reducing surface recombination.
It may be desirable in the fabrication process to texture the front side 103 and the back side 104 by a wet etch process, for example, using potassium hydroxide an isopropyl alcohol. Texturing the front side 103 may be advantageous in improving the solar radiation collection efficiency.
The first dopant source 202 may comprise an N-type dopant, such as phosphorus. The second dopant source 204 may comprise a P-type dopant, such as boron. In one implementation, the dopant concentration in each dopant source may be uniform or substantially uniform. In another implementation, the dopant concentration in each dopant source may vary according to a concentration profile. Such a concentration profile may be accomplished by dividing each doping source region into multiple sub-regions to be printed, each sub-region having a heavier (N+ or P+) or lighter (N− or P−) concentration of dopants.
The dopants are diffused from the dopant sources (202 and 204) into the silicon wafer 101 by placing the wafer 101 in a furnace.
In accordance with an embodiment of the invention, the next step or steps may be performed so as to provide an oxynitride passivation layer 402. As mentioned above, applicant believes that such an oxynitride passivation layer 402 slows or prevents the diffusion of moisture into the solar cell substrate and, hence, provides for less performance degradation over time for the solar cell. It is believed that the oxynitride passivation layer 402 is superior to preventing deleterious effects of moisture diffusion in comparison to the conventional silicon dioxide passivation layer. It is further believed that the oxynitride layer will improve device performance by reducing surface recombination.
While
The front and back sides may be processed by wet etching so as to texture the surfaces (block 804). Texturing the front side may be advantageous in improving the solar radiation collection efficiency. In other processes, the front side may be textured by wet etching in a later process step. In some processes, the back side may be masked from the wet etching or be polished after the wet etching.
Doping sources (N-type and P-type) may be deposited on the device side (for example, the back side) (block 806). For example, the deposition may be performed by industrial ink jet printing or screen printing. Thereafter, the wafer may be placed in a furnace at high temperature so as to enable the dopants to diffuse from the sources into corresponding regions of the wafer (block 808).
The oxynitride passivation layer may then be grown on front and back surfaces by introducing nitrogen gas into the furnace during growth of silicon dioxide (block 810). In other words, the oxynitride layer may be grown in a furnace by introducing nitrogen gas, in addition to the conventional oxygen gas.
Therafter, contact openings may be formed through the oxynitride passivation layer on the device side of the wafer (block 812). Subsequently, a metal layer (for example, aluminum) may be deposited on the device side (block 814). The metal layer may then be patterned, for example, by printing with inkjet or screen printing, followed by wet etching (block 816).
While certain pertinent steps are shown in the two example processes of
While specific embodiments of the present invention have been provided, it is to be understood that these embodiments are for illustration purposes and not limiting. Many additional embodiments will be apparent to persons of ordinary skill in the art reading this disclosure.
