N-type silicon solar cell with contact/protection structures

A solar cell is formed on an n-type semiconductor substrate having a p+ emitter layer by forming spaced-apart contact/protection structures on the emitter layer, depositing a blanket dielectric passivation layer over the substrate's upper surface, utilizing laser ablation to form contact openings through the dielectric layer that expose corresponding contact/protection structures, and then forming metal gridlines on the upper surface of the dielectric layer that are electrically connected to the contact structures by way of metal via structures extending through associated contact openings. The contact/protection structures serve both as protection against substrate damage during the contact opening formation process (i.e., to prevent damage of the p+ emitter layer caused by the required high energy laser pulses), and also serve as optional silicide sources that facilitate optimal contact between the metal gridlines and the p+ emitter layer.

FIELD OF THE INVENTION

This invention relates to solar cells, and more particularly to solar cells fabricated on n-type silicon wafers.

BACKGROUND OF THE INVENTION

Increasing solar cell efficiency is one of the most important goals for the solar cell research community, with the goal of enabling the lowest possible cost per watt for a given solar cell. A majority of current solar cells are fabricated on p-type Czochralski (Cz) silicon wafers with an n+ emitter layer. A problem with these conventional p-type silicon solar cells is that they suffer from light induced degradation and lower tolerance to metal impurities, which limits their efficiency to lower than 20%. As such, the particular goal of the solar cell research community is to develop high efficiency silicon solar cells having target efficiencies higher than 20%.

One approach currently being considered by the solar cell research community for achieving the >20% target efficiency is to use n-type silicon wafers with a p+ emitter layer in place of the currently used p-type wafers. N-type silicon wafers are known to avoid light induced degradation and have a higher tolerance to metal impurities than p-type silicon wafers, and therefore are believed to provide a solution for producing higher efficiency solar cells.

A problem currently faced by the solar cell research community in fabricating solar cells on n-type silicon wafers is finding a suitable dielectric material that can both passivate the p+ emitter layer, and can also be appropriately patterned to provide electrical connection to selected regions of the p+ emitter layer. The passivation (dielectric) layers on current solar cells fabricated on p-type wafers typically include silicon-nitride (SiNx) that is deposited using plasma-enhanced chemical vapor deposition (PECVD). Unfortunately, while PECVD deposited SiNxdielectric layer can effectively passivate the n+ emitter layer on p-type silicon wafers and has been used in mass production for many years, PECVD deposited SiNxcannot be used as a passivation layer on the p+ emitter layer of n-type silicon wafers because SiNx can only provide a positive fixed charge density, and thus can only passivate the surface where the minority carrier is holes (positive charger, that is n+ surface).

Recent study has proved that aluminum oxide (Al2O3) that is deposited using an atomic layer deposition (ALD) process is one of the most promising materials to passivate the p+ emitter of an n-type solar cell, and as such there has been a significant amount of research on ALD deposited Al2O3in the past a couple of years. That is, in order to compete with the very low labor costs available to Asian companies, Western solar cell manufacturers are forced to adopt high efficiency solar cell production processes, and the formation of passivation layers using ALD deposited Al2O3is believed to be one of the critical technologies for allowing high efficiency silicon solar cell production. Moreover, ALD deposited Al2O3dielectric film can also effectively passivate the n+ emitter layer as well, making it very promising for high efficiency interdigitated back contact (IBC) solar cells where both p+ and n+ emitter layers are on the same side (backside) and need to be passivated simultaneously.

The current problem facing the solar cell research community in utilizing ALD deposited Al2O3dielectric films is that, unlike PECVD deposited SiNxdielectric layers, silver paste can not fire through the ALD deposited Al2O3layer to make the metal contact with the underlying p+ emitter layer. Currently, most of the cells made in laboratory use photolithography method to make contact openings through the Al2O3layer, but this approach cannot be used in mass production due to the intrinsic high cost associated with the use of photolithography. Thus, how to achieve low-cost metallization through Al2O3passivation layer is one of the bottlenecks for the mass production of high efficiency solar cells passivated with Al2O3.

What is needed is a low cost method for facilitating the mass production of high efficiency n-type silicon solar cells with the p+ emitter layers that addresses the problems set forth above. What is also needed are mass produced, high efficiency n-type silicon solar cells with p+ emitter layers that are manufactured using the method.

SUMMARY OF THE INVENTION

The present invention is directed to a method for facilitating the mass production of high efficiency, low cost n-type silicon solar cells with the p+ emitter layers that addresses the problems set forth above by forming contact/protection film structures on the p+ emitter layer using a print-type deposition and/or direct marking method (e.g., ink jet, screen printing, extrusion, etc.), depositing a blanket passivation layer (e.g., ALD deposited Al2O3), utilizing a non-photolithography patterning method (e.g., laser ablation, inkjet or screen print etching solution or paste) to form contact openings that expose the contact/protection structures, and then using a print-type deposition and/or direct marking method (e.g., ink jet, screen printing, extrusion, etc.) to form contact via structures in the contact openings and metal gridlines structures that are supported on the passivation layer. This method solves the passivation/metallization problem associated with n-type solar cells using of ALD deposited Al2O3because the contact/protection structures prevent damage to the wafer during formation of the contact openings by, e.g., laser ablation. That is, by placing an initial, compatible contact/protection layer on the p+ emitter material prior to passivation, a buffer is created that can eliminate damage to the emitter layer even when high-power laser ablation is utilized to form the contact openings through the passivation layer, thereby facilitating the use of high efficiency ALD deposited Al2O3. Metal gridlines can then be formed that contact the p+ emitter layer through the contact openings formed in the ALD deposited Al2O3, and then a firing step is performed to finish the metallization process. The same method may also be used to produce low-cost p-type solar cells with n+ emitter layers.

The present invention is also directed to a solar cell formed on a silicon wafer including spaced-apart contact structures disposed on an upper emitter layer, a dielectric passivation layer (e.g., ALD deposited Al2O3) disposed on the contact structures and other “exposed” portions of the emitter layer, metal gridlines disposed on the passivation layer, and metal via structures extending through associated contact openings defined in the passivation layer such that each metal via structure electrically connects an associated metal gridline to an associated contact structure, where the solar cell is characterized in that the contact structures have minimum lateral dimensions larger than the maximum lateral dimension of their corresponding contact openings (e.g., the X-axis width of each contact structure is greater than the X-axis width of each corresponding contact opening). Forming each of the contact structures to have minimum lateral dimensions that are greater than the corresponding maximum lateral dimensions of the contact opening facilitates reliable formation of the contact openings without damaging the underlying emitter layer, e.g., when the laser beam pulse is slightly off target. In one embodiment, the substrate includes a n-type body layer and a p+ emitter layer disposed between the upper substrate surface and the n-type body layer, and in an alternative embodiment the substrate includes a p-type body layer and a n+ emitter layer.

In accordance with an aspect of the present invention, the contact/protection structures serve both the purpose of protecting the underlying wafer from damage during formation of the contact openings, and also serve as contact structures for facilitating low resistance electrical connection between the metal gridlines and the emitter layer, and/or form a selective emitter structure. In one specific embodiment the contact/protection structures include a silicide forming metal (e.g., Ni, Co, Ti) that forms silicide structure at the metal/emitter junction. In another specific embodiment the contact/protection structures comprise aluminum disposed on the first portions of the upper surface, and a silicide-forming metal disposed on the aluminum, and the aluminum will diffuse into the p+ emitter layer to form the p++ selective emitter structure. According to another aspect, the contact/protection structures are optionally printed in a spaced-apart arrangement on the p+ emitter layer, e.g., in the form of dot-structures, continuous line structures, or dashed-line structures.

DETAILED DESCRIPTION OF THE DRAWINGS

The present invention relates to an improvement in n-type solar cells. The following description is presented to enable one of ordinary skill in the art to make and use the invention as provided in the context of a particular application and its requirements. As used herein, directional terms such as “upper”, “upwards”, “lower”, “downward”, “front”, “rear”, are intended to provide relative positions for purposes of description, and are not intended to designate an absolute frame of reference. Various modifications to the preferred embodiment will be apparent to those with skill in the art, and the general principles defined herein may be applied to other embodiments. Therefore, the present invention is not intended to be limited to the particular embodiments shown and described, but is to be accorded the widest scope consistent with the principles and novel features herein disclosed.

FIG. 1is a partial perspective cut-away top view showing a solar cell100that is produced in accordance with an embodiment of the present invention. Similar to conventional solar cells, solar cell100is formed on a silicon wafer (substrate)101having an upper surface102, and includes a dielectric passivation layer120disposed over upper surface102, metal gridlines130-1and130-2disposed on an upper surface122of passivation layer120, and metal via structures135that extend through associated openings125defined in passivation layer120such that each metal via structure (e.g.,135-11) electrically connects an associated metal gridline (e.g., gridline130-1) to an associated region (e.g., region102-11) of upper surface102.

According to an aspect of the present embodiment, solar cell100is formed on an n-type silicon wafer (substrate)101including an n-type body (lower) region105and a p+ emitter (upper) layer107. As indicated inFIG. 1, p+ emitter layer107is disposed between the n-type body region105and upper surface102, and extends up to and includes upper surface102. An advantage of using n-type silicon wafers are known to avoid light induced degradation and have a higher tolerance to metal impurities than p-type silicon wafers, and therefore are believed to produce higher efficiency solar cells. However, novel aspects of the present invention may also be applied to p-type silicon wafers (substrates), e.g., where body region105comprises a p-type semiconductor material and emitter layer107comprises an n+ doping concentration. Therefore, the appended claims are not intended to be restricted to n-type silicon wafers (substrates)101unless the phrase “n-type silicon” is specified.

According to another aspect of the present invention, solar cell includes spaced-apart contact structures110-11,110-12,110-13,110-21and110-22, which, in the present embodiment, are deposited in the form of dot (e.g., round or oval) structures over corresponding first portions102-11,102-12,102-13,102-21and102-22of the upper surface102, respectively. Note that spaced-apart contact structures110-11,110-12,110-13are aligned under gridline130-1, and spaced-apart contact structures110-21and110-22are aligned under gridline130-2, which is parallel to gridline130-1. Note also that adjacent spaced-apart contact structures are separated by (second) regions of upper surface102that are entirely covered by passivation layer120(e.g., contact structures110-11and110-12are separated by region102-31, contact structures110-12and110-13are separated by region102-32, and contact structures110-21and110-22are separated by region102-33).

According to another aspect of the invention, contact structures110-11to110-22are disposed prior to formation of blanket dielectric passivation layer120such that dielectric passivation layer120is disposed at least partially on contact structures110-1and110-2. That is, because blanket dielectric passivation layer120is formed after contact structures110-11to110-22, at least a portion of the upper surface of each contact structures110-1and110-2is contacted by and disposed under a corresponding portion of passivation layer120.

According to yet another aspect of the invention, contact structures110-11to110-22are disposed prior to formation of openings125and via structures135such that each via structure135(e.g., via structure135-11) electrically connects an associated metal gridline (e.g., gridline130-1) and an associated contact structure (e.g., contact structure110-11). In the present embodiment, contact structures110-11to110-22function both to enhance an electrical connection between via structures135and p+ emitter layer107, and to prevent damage to substrate101(e.g., p+ emitter layer107) during formation of openings125. To facilitate the contact function, each contact structure110-11to110-22comprises a conductive metal that is deposited directly on upper surface portions102-11and102-12. The conductive contact metal can be either the same as or the different from material subsequently used to form gridlines130-1and130-2. In accordance with a specific embodiment, each contact structure110-12to110-22comprises a silicide-forming metal (e.g., Nickel, Cobalt, or Titanium) such that optional silicide structures (e.g.,113-1and113-2are formed along the interface between each contact structure110-1and110-2and first portions102-11and102-12of the upper surface102, respectively. In yet another specific embodiment, each spaced-apart contact structure110-1and110-2comprises an aluminum layer disposed on upper surface portions102-11and102-12, and a silicide-forming metal (e.g., Ni, Co, Ti) layer disposed on the aluminum layer, such that optional p++ emitter structures (not shown) are formed along the interface between each contact structure110-1and110-2and the first portions of102-11and102-12of the upper surface102, respectively. To facilitate the protection function, each contact structure has a minimum lateral dimension (i.e., measured in the same direction parallel to the plane defined by upper surface102) that is larger than the maximum lateral dimension of the contact openings in the corresponding direction (e.g., as shown inFIG. 1, the minimum diameter D1measured in the X-axis direction of dot-type contact structure110-11is larger than the maximum diameter D2of corresponding contact opening125-11, also measured in the X-axis direction). As described below, by forming contact structures110-11to110-22with minimum lateral dimensions that are substantially larger than the maximum lateral dimensions of their associated contact openings125-11to125-22, the contact structures reliably serve to prevent damage to substrate101when the contact openings are formed, e.g., by laser ablation.

According to a presently preferred embodiment of the present invention, passivation layer120is formed by ALD deposited Al2O3. As mentioned above, a benefit of providing contact structures110-11to110-22between metal gridlines130-1/2and p+ emitter layer107is that the contact structures serve to protect substrate101during formation of openings through passivation layer120, thereby facilitating the use of high energy laser pulses to ablate selected portions of passivation layer120in order to form contact openings125without damaging p+ emitter layer107. The present inventors have determined that laser ablation process is well-suited for generating contact openings through ALD deposited Al2O3, but may cause significant damage to upper surface102that could prevent suitable connection between the subsequently formed metal gridlines130-1and130-2and p+ emitter layer107. By providing contact structures110-11to110-22on upper surface102as a protective structure, laser energy passing entirely through passivation layer120is prevented from reaching upper surface102by these contact structures, thereby greatly reducing the chance of damage to p+ emitter layer107during the laser ablation process, and thereby facilitating the formation of passivation layer120using ALD deposited Al2O3, which in turn facilitates the low cost production of high efficiency solar cells on n-type silicon wafers.

FIG. 2is a flow chart showing a generalized method for producing solar cells (or other electrical circuits) on a silicon substrate having an emitter layer according to an embodiment of the present invention. Although the present invention is described herein with particular reference to the formation of solar cells on n-type silicon wafers having p+ emitter layers, the method shown inFIG. 2may also be utilized to form solar cells on p-type silicon wafers having n+ emitter layers. Further, the method shown inFIG. 2may also be used to form other electrical circuits that would benefit from the use of a low cost blanket passivation layer such as ALD deposited Al2O3.

Referring to the upper portion ofFIG. 2, the generalized method begins by depositing spaced-apart contact/protection structures on the upper surface of the silicon wafer (block210). In one embodiment, to facilitate the contact function described above, the spaced-apart contact/protection structures may be formed using a contact metal such as aluminum, a silicide-forming metal, a combination of contact and silicide forming metals. In an alternative embodiment, the spaced-apart contact/protection structures may be formed using a sacrificial material such as an organic material that serves the protection function described above, and is then removed (e.g., fully removed by the laser ablation or using a suitable etchant) after forming the contact openings and prior to forming the metal gridlines/vias. In accordance with an aspect of the present invention, the spaced-apart contact/protection structures are formed using a printing method (e.g., inkjet printing or screen printing) or another non-lithographic method in order to avoid the high production costs associated with photolithography.

After forming the spaced-apart contact/protection structures, a blanket dielectric passivation layer is deposited over the contact structures and over the remaining exposed portions of the upper surface (block220, seeFIG. 2). In a presently preferred embodiment, the process of forming the blanket passivation layer comprises depositing a thin layer Al2O3, using an ALD process, which is known in the art. In an alternative embodiment, the blanket passivation layer is formed using another dielectric material such as silicon oxide, which can be formed by using sputtering, CVD or thermal growth.

After formation of the dielectric layer, contact openings are formed through the dielectric passivation layer such that each contact opening exposes a portion of a corresponding contact structure (block230,FIG. 2). In accordance with an aspect of the present invention, the contact openings are formed using a laser ablation method or another non-lithographic method (e.g., inkjet or screen printing etching solution or etching paste) in order to avoid the high production costs associated with photolithography. Laser ablation is currently preferred for forming the contact openings because it is believed the laser can be suitably targeted and controlled to ablation dielectric material disposed over the contact structures, and it is a dry, fast, clean and non-contact method to form the contact openings. In accordance with a specific embodiment, the laser is directed onto portions of the passivation disposed over each contact structure (e.g., see contact structure110-11inFIG. 1), and the laser beam is shaped such that the maximum lateral dimension of the contact opening (e.g., opening125-11having lateral dimension D2inFIG. 1) formed by the ablation process is smaller than the minimum lateral dimension of the underlying contact structure (e.g., lateral dimension D1of contact structure110-11; shown inFIG. 1).

After forming the contact openings, metal gridlines are formed on the upper surface of the dielectric layer such that each metal gridline is electrically connected to the substrate's upper surface by way of at least one via structure that extends through an associated contact opening (block240,FIG. 2). In accordance with an aspect of the present invention, the metal gridlines are formed using a printing method (e.g., inkjet printing or screen printing) or another non-lithographic method in order to avoid the high production costs associated with photolithography. In an alternative embodiment, gridlines are formed using the co-extrusion process described, for example, in co-owned and co-pending U.S. patent application Ser. No. 12/266,974, entitled Micro-Extrusion Printhead Nozzle With Tapered Cross-Section, which is incorporated herein by reference in its entirety. Note that when the contact structures are retained after forming the contact openings, a lower end of each via structures contacts the upper surface of an associated contact structure, and electrical connection between the metal gridlines and the substrate is made by way of the contact structures. Alternatively, when the contact structures are formed using sacrificial material that is removed after the contact openings are formed, the lower end of each via structures contacts the substrate's upper surface, or contacts another material disposed on the upper substrate surface (e.g., a material deposited into the contact openings or after the sacrificial material is removed).

FIGS. 3(A) to 3(G)are simplified cross-sectional side views illustrating a method for producing a solar cell according to an exemplary embodiment of the present invention.

FIG. 3(A)shows an exemplary silicon wafer/substrate101A at the start of the production method. Silicon wafer/substrate101A includes an upper surface102A and has an n-type bulk body105A with a thin p+ emitter107A layer formed between upper surface102A and bulk body105A using methods well-known in the art such as boron diffusion.

FIG. 3(B)is a simplified diagram illustrating the formation of spaced-apart contact structures110-1A and110-2A on p+ emitter layer107A using an inkjet head150that is positioned over upper surface102A and caused to move relative to substrate101A and to eject a metal-bearing ink, in the form such as a liquid ink or a paste, onto upper surface regions102-11A and102-12A. In the disclosed embodiment, at a first time (T1) inkjet head150is positioned over surface region102-11A and actuated to eject a first ink quantity155-1such that contact structure110-1A is formed on substrate101A. Inkjet head150is then moved such that, at a second time (T2), inkjet head150is positioned over surface region102-12A, and is then actuated to eject a second ink quantity155-2that produces contact structure110-2A. Note that the remaining portions102-21A,102-22A and102-23A of upper surface102A remain exposed, with contact structures110-1A and110-2A being separated by exposed region102-22A. In alternative embodiments substrate101A maybe moved relative to inkjet head150. In one specific embodiment contact structures110-1A and110-2A are printed such that they have a thickness of about 100 nm to about 5 μm. In another specific embodiment, contact structures110-1A and110-2A are formed using the same metal (e.g., silver) that is subsequently used to form the metal gridlines. In a presently preferred embodiment, contact structures110-1A and110-2A are formed using a metal that can form low contact resistance with silicon, such as the silicide forming metals Ni, Co, and Ti. In yet another embodiment, an acceptor metal such as Al is printed onto surface regions102-11A and102-12A, and then a silicide-forming metal such as Ni is printed onto the acceptor metal. The advantage using a two-layer metal structure is that, during firing the acceptor metal (e.g., Al) diffuses into the p+ emitter layer to form a p++ selective emitter structure, and the silicide forming metal also forms a very low contact resistance interface to the emitter structure, which further increases the solar cell's efficiency. Alternatively, the contact structure can also be formed by using the ink that only contains an acceptor metal such as Al.

FIG. 3(C)is a simplified diagram illustrating the deposition of a blanket Al2O3passivation layer120A over the contact structures110-1A and110-2A and remaining exposed portions102-21A,102-22A and102-23A of upper surface102A using an atomic layer deposition (ALD) method according to known techniques. An optional anneal is performed after forming blanket Al2O3passivation layer120A. Alternatively, a separate passivation annealing process is not used, and Al2O3dielectric layer120A is annealed together with the gridline metal firing process (discussed below).

FIG. 3(D)is a simplified diagram illustrating the formation of contact openings125-1A and125-2A through Al2O3passivation layer120A such that each contact opening125-1A and125-2A respectively exposes an upper surface portion112-1A and112-2A of corresponding contact structures110-1A and110-2A. In the disclosed embodiment, at a third time (T3) laser head160is positioned over contact structure110-1A and is actuated to generate a laser pulse LP1that removes a portion of Al2O3passivation layer120A, thereby forming contact opening125-1A. Note that laser head160is controlled such that opening125-1A has a maximum diameter (lateral dimension) D2that is smaller than the minimum diameter (lateral dimension) D1of contact structure110-1A. At subsequent time T4, laser head160is then moved such that laser head160is positioned over contact structure110-2A, and is then actuated to generate a pulse LP2that produces contact opening125-2A, thereby exposing a portion of upper surface112-2A. In alternative embodiments substrate101A maybe moved relative to laser head160, or a suitable scanning mechanism (e.g., a rotating mirror, Galva scanning or polygon scanning) may be used to direct laser pulses LP1and LP2to their target regions.

FIG. 3(E)is a simplified diagram illustrating the subsequent formation of metal gridlines130-1A and130-2A on upper surface122A of Al2O3passivation layer120A such that each metal gridline (e.g., gridline130-1A) is electrically connected to a corresponding contact structure (e.g., contact structure110-1A) by way of at least one via structure (e.g., via structure135-1A) that extends through an associated contact opening (e.g., opening125-1A). In the disclosed embodiment, at a time T5an inkjet head170is positioned over opening125-1A and actuated to eject an ink quantity175-1to form metal via135-1A in opening125-1A such that it contacts structure110-1A, and to form gridline130A-1on Al2O3passivation layer120A such that it contacts metal via135-1A. Inkjet head170is then moved such that, at a time T6, inkjet head170is positioned over opening125-2A and is then actuated to eject an ink quantity175-2such that metal via135-2A is formed in opening125-2A such that it contacts structure110-1A, and then gridline130A-2is formed on Al2O3passivation layer120A such that it contacts metal via135-2A. Alternatively, metal gridlines130-1A and130-2A are deposited using a non-lithographic printing method such as screen printing or co-extrusion.

FIG. 3(F)is a simplified diagram illustrating a final firing (annealing) process that is performed after gridline formation. In the embodiment that silicide foaming metal is used for the contact structure, it is expected during the firing process metal silicide structures113-1A and113-2A are respectively formed between each contact structure110-1A and110-2A and the underlying silicon p+ emitter layer107A. In the embodiment that aluminum or another acceptor metal (e.g., B or Ga) is deposited first during the contact structure deposition, it is expected during the firing process aluminum diffuses into the p+ layer to form a selective emitter structure, that is, the regions of the emitter layer underlying the contact structure, regions107-11A,107-12A becomes a p++ region and have sheet resistance smaller than the regions not covered by the contact structures, such as regions107-21A and107-22A.

The present invention described above provides several advantages over conventional methods and solar cells. First, the method does not require any photolithography or wet chemical etching, making the method fast, clean, and low cost. Second, the use of contact structures greatly simplifies the production process. That is, while using laser ablation to make contact openings through an SiNx layer directly deposited on n+ emitter layer has been widely studied for p-type silicon solar cells and it has been approved that selective removal of the SiNx without damaging the underlying n+ emitter layer, the present inventors believe using the same laser ablation process to ablate an Al2O3passivation layer will be very difficult because Al2O3is a very stable material and has very large band gap (Crystalline Al2O3has a band gap of ˜8.8 eV and ALD amorphous Al2O3has a band gap of ˜6.4 eV). Because of this large band gap, much higher laser energy is needed to remove or ablate Al2O3, making it very difficult to only selectively remove Al2O3without damaging or removing the underlying p+ emitter layer. By forming the contact structures between the Al2O3passivation layer and the p+ emitter layer with a suitable thickness (i.e., the contact structures can be made significantly thicker than the p+ emitter layer, e.g., a few microns versus the typical 0.3 to 0.4 μm thickness of p+ emitter layers), the contact structures can be used to absorb any damage that would otherwise be caused by the high energy laser, thereby reliably preventing damage to the p+ emitter layer. The contact structures thus greatly improve the solar cell's tolerance for laser ablation, and thus improve production yields, as well as reduce the cost of the required laser system. Moreover, the present invention provides for very low contact resistances through the use of silicide-forming metals in the contact structures. Previous work has demonstrated that using nickel to form nickel silicide reduces the specific contact resistance by almost two orders of magnitude lower than conventional firing through silver paste. Finally, the present invention facilitates selective emitter structure formation by inkjet printing an acceptor metal such as Al, which will diffuse into p+ emitter layer to form a p++ region. This process will allow the formation of a selective emitter structure. It has been well known that a selective emitter structure can improve the absolute efficiency by about 1%.

Although the present invention has been described with respect to certain specific embodiments, it will be clear to those skilled in the art that the inventive features of the present invention are applicable to other embodiments as well. For example, in addition to depositing the spaced-apart contact structures in the form of spaced-apart dot structures (e.g., contact structures110-1and110-2; seeFIG. 1), the metal ink can be deposited in way that forms other structural patterns as well, all of which are intended to fall within the scope of the present invention. For example,FIG. 4shows a solar cell100B in which spaced-apart continuous line contact structures110-1B and110-2B are deposited on substrate101using the various methods described above, where via structures135are formed in associated contact openings125and extend through passivation layer120to provide electrical connections between gridlines130-1and130-2and substrate101by way of continuous line contact structures110-1B and110-2B. As indicated by solar cell100C inFIG. 5, when contact structures110-1C and110-2C are formed as continuous lines, the contact openings125-1C and125-2C may be formed as continuous line openings, thereby facilitating the formation of continuous line via structures135-1C and135-2C under gridlines130-1and130-2. Note that the widths of continuous line via structures135-1C and135-2C (i.e., measured in the X-axis direction) is smaller than the width of continuous line contact structures110-1C and110-2C for protection purposes (as described above). Alternatively,FIG. 6shows a solar cell100D in which spaced-apart contact structures110-11D,110-12D,110-13D form a first dashed-line structure, and110-21D and110-22D form a second dashed-line structure. These dashed-line structures are deposited on substrate101using the various methods described above, and via structures135are formed in associated contact openings125and extend through passivation layer120to provide electrical connections between gridlines130-1and130-2and each dashed-line contact structure segment. Further, although the present invention is described with specific reference to the formation of ALD deposited Al2O3, the current invention can also be used in other dielectric material passivated solar cells and interdigitated back contact solar cells.