The present disclosure generally relates to photovoltaic devices, and more particularly to interconnection schemes for connecting photovoltaic cells.
Conventional photovoltaic cells, such as crystalline silicon solar cells, are generally inter-connected using a process referred to as “tabbing and stringing” whereby conducting contacts of adjacent photovoltaic cells are electrically connected (tabbed) to form a chain of devices connected in series (the string). A number of these strings are then packaged together to form a module that is installed on rooftops or other power generating locations. In a majority of conventional photovoltaic cells, one of the conducting contacts of each cell is positioned along the bottom surface of a silicon wafer in the form of a metallic layer, which is typically made up of aluminum metal or an aluminum alloy. The top contact of the photovoltaic cell is typically a screen-printed and baked conductive grid formed using a metallic paste, for example. The current collection portion of this grid and the part that is used for inter-connection is generally referred to as the bus-bar. As shown in
Not only do these interconnections (e.g., wires) require non-trivial additional space to be left between adjacent photovoltaic cells 102 but the distorted configuration (e.g., bends 110) can result in stresses and fatigue related failure during prolonged usage, particularly if subjected to significant thermal cycling. Additionally, this interconnection process (during module assembly of conventional silicon cells) is laborious and not readily automated. This has resulted in manufacturing inefficiencies and cost contributions.
The present disclosure is illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings and in which like reference numerals refer to similar elements and in which:
The present disclosure is now described in detail with reference to a few particular embodiments thereof as illustrated in the accompanying drawings. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present disclosure. It is apparent, however, to one skilled in the art, that particular embodiments of the present disclosure may be practiced without some or all of these specific details. In other instances, well known process steps and/or structures have not been described in detail in order to not unnecessarily obscure the present disclosure. In addition, while the disclosure is described in conjunction with the particular embodiments, it should be understood that this description is not intended to limit the disclosure to the described embodiments. To the contrary, the description is intended to cover alternatives, modifications, and equivalents as may be included within the spirit and scope of the disclosure as defined by the appended claims.
Particular embodiments relate to the formation, during nominal cell fabrication, of optimally sized and positioned Electrode Access Contacts (EACs) coupled to the top and bottom contacts of a, by way of example, conventionally shaped and sized thin film solar or photovoltaic (hereinafter photovoltaic) cell. In particular embodiments, the EACs are located and accessible on the top surface of each photovoltaic cell. Additionally, in particular embodiments, the EACs are distinctly formed such that they are readily identifiable by human or machine vision techniques, and thus can easily be distinguished for interconnection purposes. In various embodiments, the number, size, shape, and position of the EACs may vary according to whatever may be deemed optimal or most desirable for any particular photovoltaic cell.
According to particular embodiments, while depositing absorber layer 214 and the subsequent layers described below, one or more portions of a peripheral edge of the substrate is selectively masked such that a portion of the bottom contact layer 212 is left exposed. As described below, the exposed portion of the bottom contact layer 212 serves as the bottom EAC 236 for the photovoltaic cell 202. The masking can be accomplished in a number of ways including relatively more complex ones such as photo-lithography, which is customarily used for semiconductor processing. However one preferred embodiment would utilize specially designed sample holders 440, as illustrated in
In particular embodiments, sample holder 440 includes integrally formed (with sample holder 440) masking protrusions or tabs (hereinafter “tabs”) 442. Masking tabs 442 selectively mask desired portions of bottom contact layer 212 that will subsequently form the bottom EACs 236. Although in the described embodiment, masking tabs 442 integral with the sample holder are used to selectively mask the desired portions of bottom contact layer 212, it should be appreciated that any suitable means may be used to mask the desired portions of bottom contact layer 212 to form the bottom EACs 236. In various embodiments, bottom contact layer 212 may be selectively masked to produce one or more bottom EACs 236 having any desired shape or size (although it is typically desirable to maximize the area of the subsequently deposited absorber layer to maximize the light absorbed by the photovoltaic cell 202). By way of example, in the illustrated embodiment, two bottom EACs 236 will be formed. In an alternate embodiment, an entire peripheral edge of the bottom contact layer 212 may be masked by a masking tab 442. It should be appreciated that, in this way, the bottom EACs 236 may be formed integrally or concurrently with the conventional fabrication of the photovoltaic cell 202.
Following deposition of the absorber layer 214, the substrate 210, bottom contact layer 212, and absorber layer 214 may be annealed at 308 and subsequently cooled. In particular embodiments, a buffer (window) layer 216 is then grown or otherwise deposited over absorber layer 214 at 310. Again, buffer layer 216 and the subsequently deposited layers described below are masked by masking tabs 442 thereby leaving portions of the bottom contact layer 212 exposed to form the bottom EACs 236 of the photovoltaic cell 202. By way of example and not by way of limitation, buffer layer 216 may be an n-type semiconducting layer formed from, by way of example, CdS or In2S3, among other suitable materials, and have a thickness in the range of approximately 30 to 70 nm.
In particular embodiments, an i-type layer 218 is grown or otherwise deposited over buffer layer 216 at 312. By way of example and not by way of limitation, i-type layer 218 may be formed from ZnO and have a thickness in the range of approximately 70 to 100 nm. At 314, a top contact layer 220 may then be deposited over the i-type layer 218. In particular embodiment, top contact layer 220 may be formed from a conducting material such as, by way of example and not by way of limitation, AZO (Al2O3 doped ZnO) or IZO (Indium Zinc Oxide, e.g., 90 wt % In2O3/10 wt % ZnO), and have a thickness in the range of approximately 0.5 to 1.5 μm.
In particular embodiments, an optional conducting grid 222 including bus bars 224 (which may be integrally formed with grid 222) is also deposited at 316 over the top contact layer 220. Any of the aforementioned layers may be deposited by any suitable means such as, by way of example, physical vapor deposition (PVD), including sputtering or evaporation, chemical vapor deposition (CVD), electroplating, plasma spraying, printing, solution coating, etc, while being held by sample holder 440 and selectively masked by masking tabs 442. Conventional processes such as edge isolation, deposition of an anti-reflective coating, and a light soaking, among others, may then follow prior to pre-testing, sorting, packaging, and shipping.
Those of skill in the art will appreciate that
As illustrated in
As illustrated in
Furthermore, in some embodiments, an entire peripheral edge of the bottom contact layer 212 may be masked by a masking tab 442 such that the bottom EAC 236 extends along most or all of one or more sides of the photovoltaic cell 202. In such an embodiment, a single tab may be used to electrically an entire side of the bottom contact layer 212 of one cell with an entire side (e.g. bus bar 224) of the adjacent cell. Not only would this interconnection arrangement be even less susceptible to stresses, but it may also provide a physical barrier that seals the space between the adjacent cells. In one embodiment, this sealed space may then be injected or otherwise filled with a filler material.
The interconnects 234 may be applied with any suitable means including soldering, bonding, ultrasonic bonding/welding, etc. One advantage of using the EACs described is that it would be amenable to novel interconnection schemes in which the interconnections 234 are embedded in a top cover material, for example, in some designated pattern. By way of example, the interconnections 234 may be laid out in a pattern that corresponds to the desired layout of the chain of photovoltaic cells 202. The pattern of interconnects 234 may then be positioned simultaneously over the pattern of photovoltaic cells, or vice versa. In this case all of the photovoltaic cells 202 of a given module may be interconnected in a single-step process through laser or ultrasonic welding or some similar process.
In conclusion, a major advantage of this interconnection scheme would be its ease of automation and the fact that the interconnections themselves would be co-planar and relatively stress-free. The EACs and interconnections would also permit very high packing densities to be achieved due to the absence of connections running over and under adjacent cells.
In particular embodiments, photovoltaic cells 202 are fabricated on relatively smaller-sized substrates such that they will have the general appearance and dimensions of conventional silicon solar cells (for example, square or pseudo-square 157 mm2 or 210 mm2 cells), although other arrangements may be suitable. In particular embodiments, this facilitates their use as drop-in replacements for equivalent sized and shaped silicon-based cells and as such will be compatible with the large global installed base of solar module manufacturers.
The present disclosure encompasses all changes, substitutions, variations, alterations, and modifications to the example embodiments described herein that a person having ordinary skill in the art would comprehend. Similarly, where appropriate, the appended claims encompass all changes, substitutions, variations, alterations, and modifications to the example embodiments described herein that a person having ordinary skill in the art would comprehend.
This application claims the benefit, under 35 U.S.C. §119(e), of U.S. Provisional Patent Application No. 61/230,241, entitled INTERCONNECTION SCHEMES FOR PHOTOVOLTAIC CELLS, filed 31 Jul. 2009, and hereby incorporated by reference herein.
Number | Date | Country | |
---|---|---|---|
61230241 | Jul 2009 | US |