The present invention relates to solar cells. More particularly, the present invention relates to improved solar cell metalized contacts, and methods of their manufacture.
In typical solar cells, solar radiation illuminates at least one surface of the solar cell (typically referred to as the front side). In order to achieve a high energy conversion efficiency of incident photons into electric energy, an efficient absorption of photons within a silicon wafer substrate is important. In certain cell structures (described further below) this is achieved by a low (parasitic) optical absorption of photons within all layers except the wafer itself. For the sake of simplicity the impact of the wafer's geometrical shape (a surface texture such as pyramids is usually formed on crystalline wafer surfaces or other modifications of a flat surface are applied) is not specifically addressed herein, because it is understood that the surfaces may be textured in any shape beneficial for improved solar cell efficiency.
The choice of layers and their composition plays an important role in solar cell fabrication. Typically the number of layers, and each layer's associated processing steps (pre-clean, semiconductor film deposition, patterning-etch, pre-clean, metal deposition, and metal pattern-etch; etc.) contribute to cell complexity and corresponding manufacturing costs. Metallization is a particularly important feature of solar cells, and the demanding economics of solar cell manufacturing and deployment dictate stringent controls in manufacturing costs, and optimization wherever possible.
The present invention provides a solar cell structure and a method of manufacture which provide the benefits of low shadowing of the solar cell, commonly caused by excessive surface coverage from the metal electrodes, a high conductivity of the metal grid, and minimized carrier recombination underneath the metal contacts on, e.g., the front illuminated side of the cell, or any other side of the cell. The techniques disclosed enable use of multifunctional layers which also include integral electrical contacts, and manufacturing techniques which decrease the number of materials and processing steps needed, thereby reducing solar cell manufacturing costs.
The present invention addresses the requirement for reduced complexity and corresponding manufacturing costs and processing steps by selectively converting the electrical conductivity state of a single, e.g., deposited dielectric insulating film, using direct laser energy impingement on the film, to form solar cell electrical contacts and interconnects without multiple deposition and patterning steps.
In that regard, the present invention, in one aspect, is a solar cell including an upper layer that provides at least one function to the solar cell (e.g., transparent dielectric film, antireflective film, passivation, etc.); wherein the upper layer includes a material that can be converted into an electrically conductive contact using selective laser irradiation impingement. The resulting electrical contact provides, e.g., an electrically conductive path to at least one region below the upper layer of the solar cell through the dielectric insulator. Metal plating may be subsequently formed over the selectively formed electrically conductive contact.
In one example, the material comprises a metal-nitride composite material, and the impinging laser irradiation selectively oxidizes the nitride resulting in the conversion of the material from a dielectric insulator into an electrically conductive contact, in, e.g., an oxidizing environment containing gaseous oxygen.
In another example, the material comprises a metal-carbide composite material, and the impinging laser irradiation selectively modifies the oxidization state of the metal-carbide composite, resulting in the conversion of the material from a dielectric insulator into an electrically conductive contact, in, e.g., an oxidizing environment containing gaseous oxygen.
In another example, the material comprises metal ions, and the laser irradiation reduces metal resulting in the formation of the electrical contact, in, e.g., a reducing environment containing gaseous hydrogen or forming gas or methanol or ethanol.
The upper layer may be formed over an underlying doped region including a doped semiconductor material, wherein dopants in the upper layer are of the same dopant type as the doped semiconductor material. The laser irradiation causes diffusion of the upper dopants into the underlying doped region, wherein the transformed region of the thin film dielectric layer forms an electrical contact with the underlying doped region. As an example, aluminum forms a P-type dopant when diffused into a silicon substrate.
The disclosed structures, methods, and products formed by these methods, and all related techniques form part of the invention.
Further, additional features and advantages are realized through the techniques of the present invention. Other embodiments and aspects of the invention are described in detail herein and are considered a part of the claimed invention.
The subject matter which is regarded as the invention is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other objects, features, and advantages of the invention are apparent from the following detailed description taken in combination with the accompanying drawings in which:
a depicts a partial cross-section of a solar cell on which selective laser irradiation is used on, e.g., an insulating dielectric upper layer material comprising, e.g., metal containing compounds, in accordance with an aspect of the present invention;
b depicts laser-exposed areas in selected areas are converted by laser irradiation, forming conductive metal contacts from the dielectric insulating material, and wherein the contacts directly contact a lower layer;
c depicts contacts which may penetrate into or even through the upper layer into a lower layer, if the metal containing compounds are of the same type of dopants as those in the lower layer;
d depicts the created contacts used as a seed layer for a thickening plating step;
a depicts a partial cross-section of a second type of solar cell on which selective laser irradiation is used on an upper layer comprising, e.g., metal containing compounds, in accordance with an aspect of the present invention;
b depicts laser-exposed areas in which conductive metal contacts are created;
c depicts the created contacts used as a seed layer for a subsequent thickening plating step;
a depicts a partial cross-section of a solar cell on which selective laser irradiation is used on an upper layer comprising, e.g., metal containing compounds, in accordance with an aspect of the present invention;
b depicts laser-exposed areas in which metal seed layer contacts are created in the upper surface of the material, forming isolated or buried conductors;
c depicts the created contacts used as a seed layer for a subsequent thickening plating step;
a-b depict using varying intensities of laser energy irradiation used to create varying depths of electrical contact areas and/or interconnect lines in accordance with an aspect of the present invention, wherein some of the converted material penetrates fully through the material forming contacts to the substrate, while some of the material is only converted near the surface, forming interconnects which are isolated from the substrate, but may be electrically integrated with the contacts to the substrate; and
The present invention is directed to effecting a local change of a solar cell's layer composition by laser irradiation, during which a metal contact to the underlying layer(s) or across the front surface is established through or embedded into, e.g., an insulating dielectric. In one embodiment, the metal contacts can be interconnected to form a continuous contact grid of, e.g., fingers and/or bus-bars.
This local change in chemical composition is achieved for films which comprise metal containing compounds, for example, aluminum nitride, titanium oxide, aluminum oxide, boron nitride, silicon carbide or silver containing transparent layers. Some of these materials can be transparent binary ceramics. Another exemplary class of materials includes transparent conductive oxides (TCOs) such as aluminum doped zinc oxide or fluorine doped tin oxide or indium tin oxide or zinc tin oxide, etc.
Many of these metallic compounds have ideal optical properties for solar cells, namely they have a wide band-gap (in the range of 6 eV), providing high optical transparency; and appropriate refractive index (in the range of 1.8-2.4), providing effective anti-reflective coatings for many types of solar cells in typical applications.
Moreover, these metal containing compound films can provide very effective surface passivation of the solar cell substrate and/or upper layers, thereby reducing surface interface states and resulting in low surface carrier recombination losses.
Therefore, this invention presents a very effective structure and method of formation of multi-functional films in solar cells.
In one embodiment, local change of the chemical film composition can convert the film from an insulator to a conductor through a thermally activated oxidation of, e.g., a metal-nitride compound or metal carbide compound, resulting in removal or change in relative concentration of the nitride, metal or other oxides in the resulting converted material, in which case an oxidizing environment such as in air or in pure oxygen may be required. Alternatively, the change in chemical film composition can involve a reduction of the metal containing compound to metal, and in those cases a reducing material may be required such as gaseous hydrogen or forming gas or liquids like ethanol or methanol.
In a certain embodiments of the invention, films containing metals that act as a p-type dopant in the adjacent semiconductor material are used on top of p-type semiconductor layers. For silicon as the semiconductor material, examples are aluminum, gallium or indium. This way an out diffusion of e.g., aluminum into the underlying region can be provoked by the laser treatment of the film and a localized p-type doping underneath the contacts is achieved. This doping reduces contact recombination. Accordingly, films containing metals that act as an n-type dopant in the adjacent semiconductor material are used on top of n-type semiconductor layers. For silicon as the semiconductor material, some examples are arsenic, antimony or bismuth. This way an out diffusion of e.g. bismuth into the adjacent region can be provoked by the laser treatment of the film and a localized n-type doping underneath the contacts is achieved.
More generally, the thin upper layer may be deposited over a thin film layer which is a doped semiconductor material, wherein the metal containing compounds in the thin upper layer are of the same dopant type as the thin film doped semiconductor material.
Alternatively, the thin upper layer may be deposited over a semiconductor substrate which contains a heavily doped surface region, wherein the metal containing compounds in the thin upper layer are of the same dopant type as the heavily doped surface region of the semiconductor substrate.
In either case, the laser irradiation may cause diffusion of metal into the underlying doped region of the substrate or into the underlying doped semiconductor thin layer. The solar cell may be heat treated after laser irradiation to cause diffusion of metal into the underlying doped region of the substrate or into the underlying doped semiconductor thin film layer.
The invention can be applied to many solar cell structures, including any of those listed in the above-incorporated patent applications. The following are merely examples, but the invention is not limited to these examples.
In accordance with the present invention, and with reference to the solar cell under process 10 of
With reference to
In a subsequent step (
The present invention can use Gaussian or top hat laser profiles. The formation of precise, e.g., top-hat laser profiles (e.g., known to be a controlled flat top profile rather than Gaussian) can be effected using very high power (>300 W) lasers to enable direct writing of repetitive features, with the machined features being defined by e.g., masks, translation stages, and/or scanners. Laser sources used may be high power multimode sources. The laser source wavelength, pulse width, repetition rate, and pulse energy are chosen to best suit the process requirements. Examples of such laser sources include diode pumped solid state Nd:YAG and Excimer lasers. Other examples include pulsed (Q-Switched) lasers or continuous wave lasers. The laser may be operated at a wavelength and pulse width at which laser energy effects the requisite material conversion into contacts. Used together, the laser power, beam profile, wavelength, pulse frequency are all parameters which can be used to adjust the laser absorption or coupling to a given metal containing compound film, and thereby adjust the depth profile of the converted material to form either full-depth contacts or isolated/buried interconnect lines, or other required structures.
In accordance with another aspect of the present invention, and with reference to the solar cell under process 20 of
The laser irradiation in one embodiment converts the metal containing compound material to a more metallic, electrically conductive contact material, and contacts 21 to the polysilicon layer 23 are formed. (As discussed above, not shown here, the metal may penetrate into or even through the upper layer 22 into lower layers 23.)
In a subsequent step (
In accordance with another aspect of the present invention, and with reference to the solar cell under process 30 of
As a result, no external alignment is necessary during subsequent metal plating (i.e., plating becomes self-aligned to the seed layer). Since the seed structure for the electrodes is embedded in the film, mechanical adhesion problems of the electrode are resolved. In-situ heat treatment of the metal contacts formed by laser irradiation may also be employed to reduce contact resistance by alloying the metallic compound or by forming intermetallic compounds with the plated metal.
The solar cell structure and formation techniques of the present invention have the benefit over the prior art that localized contacts can be created by the laser with much smaller feature sizes than standard printing or deposition techniques. The present invention also enables the formation of metal lines from a film (12, 22, 32) that is a functional film of the solar cell already, e.g. an antireflection coating, transparent film, surface passivation, etc., negating the need for other upper layers to be deposited on the cell upper surface. Therefore, the non-treated areas of the film (12, 22, 32) do not need to be patterned, removed or replaced, saving cost and manufacturing time.
In accordance with another aspect of the present invention, and with reference to the solar cell under process 50 of
In accordance with another aspect of the present invention, and with reference to
The term “contact” is used broadly herein to connote any type of conductive structure.
The term “metal containing compound” is used broadly herein to connote a material which can be converted into an electrically conductive contact according to the techniques of the present invention.
The present invention is applicable to contact formation on any side of a solar cell (e.g., front side, back side, etc.), or between junctions, buried within a multi junction solar cell.
One or more of the process control aspects of the present invention can be included in an article of manufacture (e.g., one or more computer program products) having, for instance, computer usable media. The media has embodied therein, for instance, computer readable program code means for providing and facilitating the capabilities of the present invention. The article of manufacture can be included as a part of a computer system or sold separately.
Additionally, at least one program storage device readable by a machine embodying at least one program of instructions executable by the machine to perform the capabilities of the present invention can be provided.
The flow diagrams and steps disclosed herein are just examples. There may be many variations to these diagrams or the steps (or operations) described therein without departing from the spirit of the invention. For instance, the steps may be performed in a differing order, or steps may be added, deleted or modified. All of these variations are considered a part of the claimed invention.
Although preferred embodiments have been depicted and described in detail herein, it will be apparent to those skilled in the relevant art that various modifications, additions, substitutions and the like can be made without departing from the spirit of the invention and these are therefore considered to be within the scope of the invention as defined in the following claims.
This application claims the benefit of previously filed U.S. Provisional Application entitled “Localized Metal Contacts By Localized Laser Assisted Reduction Of Metal-Ions In Functional Films, And Solar Cell Applications Thereof,” filed 22 Apr. 2009 and assigned application No. 61/171,491; and is related to the commonly-assigned, previously filed U.S. Provisional Application entitled “High-Efficiency Solar Cell Structures and Methods of Manufacture,” filed 21 Apr. 2009 and assigned application No. 61/171,194; and to commonly-assigned, co-filed International Patent Application entitled “High-Efficiency Solar Cell Structures and Methods of Manufacture” filed as Attorney Docket No. 3304.001AWO and assigned application number ______. Each of these applications is hereby incorporated by reference herein in its entirety. All aspects of the present invention may be used in combination with any of the disclosures of the above-noted applications.
Filing Document | Filing Date | Country | Kind | 371c Date |
---|---|---|---|---|
PCT/US10/31881 | 4/21/2010 | WO | 00 | 11/15/2011 |
Number | Date | Country | |
---|---|---|---|
61171491 | Apr 2009 | US |