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Definitions

• This disclosure relates in general to p-n junctions and in particular to a ceramic boron-containing doping paste and methods therefor.
• a solar cell converts solar energy directly to DC electric energy.
• a photodiode it permits light to penetrate into the vicinity of metal contacts such that a generated charge carrier (electrons or holes (a lack of electrons)) may be extracted as current.
• a generated charge carrier electrospray or holes (a lack of electrons)
• photodiodes are formed by combining p-type and n-type semiconductors to form a junction.
• Electrons on the p-type side of the junction within the electric field (or built-in potential) may then be attracted to the n-type region (usually doped with phosphorous) and repelled from the ⁇ -type region (usually doped with boron), whereas holes within the electric field on the n-type side of the junction may then be attracted to the p-type region and repelled from the n-type region.
• the n-type region and/or the p-type region can each respectively be comprised of varying levels of relative dopant concentration, often shown as n-, ⁇ +, ⁇ ++, p-, i-, ⁇ - ⁇ -, etc, .
• the built-in potential and thus magnitude of electric field generally depend on the level of doping between two adjacent layers.
• FIG. 1 a simplified diagram of a conventional solar cell is shown.
• a moderately doped diffused emitter region 108 is generally formed above a relatively light and counter-doped diffused region absorber region 110.
• the set of metal contacts comprising front-metal contact 102 and back surface field (BSF)/ back metal contact 116, are formed on and fired into silicon substrate 110.
• the emitter or field is formed by exposing the boron-doped substrate to POCI 3 (phosphorus oxychloride) ambient to form phosphosilicate glass (PSGj on the surface of the wafer.
• POCI 3 phosphorus oxychloride
• PSGj phosphosilicate glass
• the POCI 3 ambient typically includes nitrogen gas (N 2 gas) which is flowed through a bubbler filled with liquid POCS3. and a reactive oxygen gas (reactive 0 2 gas) configured to react with the vaporized POCI3 to form the deposition (processing) gas.
• N 2 gas nitrogen gas
• reactive oxygen gas reactive 0 2 gas
• the reduction of ⁇ 2 0 5 to free phosphorous is directly proportional to the availability of Si atoms.
• the substrates are loaded in either a back-to-back configurations with two substrates per slot, or in a single wafer per slot configuration, such tha all substrate surfaces exposed to the furnace ambient are doped with phosphorus.
• residual surface glass (PSG) formed on the substrate surface during the POCK deposition process may be remo ved by exposing the doped silicon substrate to an etchant, such as hydrofluoric acid (HF).
• etchant such as hydrofluoric acid (HF).
• the set of metal contacts, comprising front-metal contact 102 and BSF (back surface field)/back metal contact 116, are then sequentially formed on and
• the front metal contact 102 is commonly formed by depositing an Ag (silver) paste, comprising Ag powder (about 70 to about 80 wt% (weight percent)), lead borosilicate glass (frit) PbO-B 2 0 3 -Si0 2 (about 1 to about 10 wt%), and organic components (about 15 to about 30 wt%). After deposition the paste is dried at a low temperature to remove organic solvents and fired at high temperatures to form the conductive metal layer and to enable the silicon-metal contact.
• Ag silver
• BSF/ back metal contact 1 16 is generally formed from aluminum (in the case of a p-type substrate) and is configured to create a potential barrier that repels and thus minimizes the impact of minority carrier rear surface recombination.
• Ag pads [not shown] are generally applied onto BSF/ back metal contract 116 in order to facilitate soldering for interconnection into modules.
• the use of aluminum may also be problematic for multiple reasons. As a result of thermal expansion mismatch between the silicon wafer and the aluminum layer, an aluminum BSF tends to cause solar cell warping, which leads to difficulties in subsequent production processes and decreases the yield due to increased breakage. Aluminum is also a poor reflector for the red light that is not absorbed by the wafer, reducing the solar cell efficiency. In addition, aluminum generally provides sub-optimal passivation to the substrate rear surface.
• One solution may be to replace the blanket aluminum with a more reflective and better passivated layer in order to reduce charge carrier recombination and increase the absorption of long wavelength light. Additionally, the rear metal contact area may also be reduced to further optimize charge carrier recombination.
• a selective emitter solar cell architecture on the front of the wafer may be used to further optimize solar cell efficiency,
• a selective emitter uses a first lightly doped region optimized for low recombination, and a second heavily doped region (of the same dopant type) optimized for low resistance ohmic metal contact.
• FIG. 2 a simplified diagram is show r n of a solar cell with rear passivated and reduced rear area metal contact on a p- (boron doped) substrate 210 with an n+ (phosphorous doped) emitter region 220.
• a set of front metal contacts 222 connects to nr emitter region 220 through front surface SiN x layer 219 in order to form an Ohmic contact.
• SiN x layer 219 is generally configured to passivate the front surface as well as to minimize light reflection from the top surface of the sol ar cell.
• back metal contacts 216 connects with substrate 210 through back surface passivation layer 214 (such as SiN x ) in order to also make an Ohmic contact.
• back surface passivation layer 214 such as SiN x
• the solar cell conversion efficiency of this architecture may also be problematic, For example, the presence of a metal layer in direct contact with the weakly-doped base wafer will tend to result in high contact resistance (i.e., a non-Ohmic contact), in addition, direct contact between n+ layer 212 (a byproduct of the POCI 3 diffusion process) and the set of back metal contacts 216 will also tend to result in a shunted junction that further reduces device efficiency.
• One solution may be to use a doping paste to form a localized p+ (heavily doped) region between n+ layer 212 and the set of back metal contacts 216 in order to minimize detrimental shunting.
• a doping paste to form a localized p+ (heavily doped) region between n+ layer 212 and the set of back metal contacts 216 in order to minimize detrimental shunting.
• conventional dopant pastes is problematic since they are generally comprised of Si0 2 matrix with an addition of dopant containing compounds (see U.S. Pat. No. 4,104,091 and U.S. Pat. No. 6,695,903).
• the invention relates, in one embodiment, to a ceramic boron-containing dopant paste.
• the ceramic boron-containing dopant paste further comprises a set of solvents, a set of ceramic particl es dispersed in the set of solvents, a set of boron compound particles dispersed in the set of solvents, and a set of binder molecules dissolved in the set of solvents.
• the ceramic boron-containing dopant paste has a shear thinning power law index n between about 0,01 and about 1.
• FIG. 1 shows a simplified diagram of a traditional front-contact solar cell
• FIG. 2 shows a simplified diagram of a solar cell with rear passivated and reduced rear area metal contact on a p- (boron doped) substrate with an n+ (phosphorous doped) emitter region:
• FIGS. 3A-B show a set of diagrams of different solar cell configurations in which a ceramic boron-containing doping paste may be used to configure a beneficial (non-shunting) Ohmic contact between a rear metal electrode and substrate, in accordance with the invention
• FIG. 4 shows a simplified Ellingham Diagram, in accordance with the invention
• FIG. 5 shows the viscosity profiles for the two boron-containing doping pastes, in accordance with the invention
• FIG. 6 shows a simplified diagram showing a Spreading Resistance Profile plot of the majority carrier type and concentration in the diffusion region, in accordance with the invention
• FIG . 7 shows a simplified diagram of a boron dopant diffusion in an n-type substrate as generated with a boron-containing doping paste, in accordance with the invention
• FIG. 8 shows a simplified diagram of a boron dopant diffusion and a phosphorous dopant diffusion on an n-type substrate, in accordance with the invention.
• FIG. 9 shows a simplified process for the manufacture of boron-containing doping paste, in accordance with the invention.
• BSF may provide increased efficiency by allowing a low resistivity and low recombination contact to the bulk of the wafer.
• Such configurations are also problematic to manufacture since the presence of metal layer in direct contact with the weakly-doped base wafer will tend to result in a non-Ohmic contact, in addition, direct contact between a formed n+ layer (as a result of the POCI 3 diffusion process) and the set of back metal contacts will also tend to result in a shunted junction that further reduces device efficiency.
• a beneficial (non-shunting) Ohmic contact may formed between rear metal electrode 216 and substrate 210 by a p+ (heavily doped) region between the metal layer and the base wafer with a ceramic boron-containing doping paste, in accordance with the invention.
• a ceramic boron-containing doping paste tends to be resilient to high temperature oxidizing processes (often associated with the dopant diffusion process), tends to mask ambient POCI 3 (the absence of which would counter- dope the local region to a detrimental n-type and thus shunt), and is compatible with HF-based acidic cleaning chemistries typically used after dopant deposition prior to the high temperature diffusion process (since silicon oxide is generally absent).
• Methods of depositing the ceramic boron-containing doping paste include, but are not limited to, screen printing, roll coating, slot die coating, gravure printing, flexogra hie drum printing, and inkjet printing methods, etc.
• FIGS. 3A-B a set of diagrams showing different solar cell configurations in which ceramic boron-containing doping paste may be used to configure a beneficial (non-shunting) Ohmic contact between a rear metal electrode and substrate, in accordance with the invention.
• FIG. 3 A shows a solar cell configuration in which a p+ blanket BSF, formed with a ceramic boron-containing doping paste, forms a non-shunting Ohmic contact with the set of rear metal contacts, in accordance with the invention.
• the presence of a p+ layer on the rear of the substrate will substantially reduce the detrimental impact of direct metal contact to the n+ and p- layers.
• a set of front metal contacts 333 connects to n+ emitter region 330 through front surface SiN x layer 319 in order to form an Ohmic contact.
• SiN x layer 319 is generally configured to passivate the front surface as well as to minimize light reflection from the top surface of the solar cell.
• SiN x layer 319 is replaced with dielectric passivation (such as SiO x or a SiO x /SiN x multilayer).
• set of back metal contacts 316 connects with substrate 310 through back surface passivation layer 314 (such as SiN x ) and blanket BSF 313 in order to make a non-shunting Ohmic contact.
• back surface passivation layer 314 such as SiN x
• blanket BSF 313 in order to make a non-shunting Ohmic contact.
• SiN x layer 314 is replaced with dielectric passivation (such as SiO x or a SiO x /SiN x multilayer).
• FIG. 3B shows a solar cell configuration in which a p+ localized BSF, formed with a ceramic boron-containing doping paste, forms a non-shunting Ohmic contact with the set of rear metal contacts, in accordance with the invention.
• a set of front metal contacts 322 connects to nr emitter region 320 through front surface SiN x layer 319 in order to form an Ohmic contact.
• SiN x layer 319 is generally configured to passivate the front surface as well as to minimize light reflection from the top surface of the sol ar cell.
• set of back metal contacts 316 connects with substrate 310 through back surface passivation layer 314 (such as Si x ) and localized BSF 323 in order to make a non-shunting Ohmic contact.
• back surface passivation layer 314 such as Si x
• localized BSF 323 in order to make a non-shunting Ohmic contact.
• a residual n+ floating junction created during the POCI3 diffusion process provided it does not provide a shunting path to n+ emitter region 320, helps to reduce charge carrier recombination.
• Non-Newtonian fluid refers to a fluid whose flow properties are not described by a single constant value of viscosity, or resistance to flow
• Shear thinning refers to a fluid whose viscosity decreases with increasing rate of shear. In general, shear thinning behavior is observed in colloidal suspensions, where the weak hydrostatic and electrostatic interaction between particles and their surface groups tends to increase viscosity in non-dynamic force regimes. The addition of a relatively small shear force overcomes the hydrostatic interaction and thus tends to reduce the viscosity of the fluid.
• the viscosity of the paste must be relatively low at high shear rates in order to pass through a screen pattern, but must be relatively high prior to and after deposition (at low or zero shear rates), in order not to ran through the screen or on the substrate surface respectively,
• shear thinning is the result of particle-to-particle interactions in the fluid. Functionalization of the particle surface with surface groups increa ses inter-particle interactions resulting in stronger shear thinning behavior for the same solid loading.
• n is a Power Law Index (or Rate index).
• Equation 4 can be rewritten by taking a natural logarithm of both sides
• a refractory ceramic matri selected for thermal stability in contact with the silicon substrate may be combined with a boron doping source to form the ceramic boron- containing doping paste, During the high temperature diffusion process, boron is allowed to diffuse into the substrate, while ambient phosphorous is blocked by the ceramic material.
• an Ellingham diagram can show the change in Gibbs free energy ( ⁇ 6) with respect to temperature for various reactions including oxidation of different metals.
• Gibbs free energy is generally the capacity of a system to do non-mechanical work and G measures the non- mechanical work done on it.
• Equation 6 shows the reduction reaction that may take place when a metal oxide is placed in contact with a silicon substrate, This reaction will result in injection on metallic impurities into the wafer resulting in poor device performance:
• Equation 6 The reaction shown in equation 6, can be split into a sum of two half reactions shown in Equations 7A and 7B. Equation 7 A can be rewritten as Equation 7C to match the typical format of oxidation reactions:
• Equation 7C [0055] The Gibbs free energy of the overall reaction shown in Equation 6 will then be
• FIG. 4 a simplified Ellingham Diagram is shown, in accordance with the invention. Change in the Gibbs free energy (— ⁇ ) in kj/mol is shown along vertical axis 304 for multiple oxidation reactions, while the reaction temperature in °K is shown along horizontal axis 302.
• oxides which result in a greater reduction in free energy than the oxidation of silicon are thermodynamically stable in contact with silicon at an elevated temperature as they result in a positive AG as described in Equation 6.
• Suitable cerami c materials include (Ti0 2 ) 416, aluminum oxide (A1 2 0 3 ) 418, magnesium oxide (MgO) 420, and calcium oxide (CaO) 422, and combinations thereof.
• Additional ceramic materials include, but are not limited to: metal carbides, MCx, metal nitrides, Nx, metal silicates, MSiOx, Si0 2 , and C.
• a suitable solid dopant source shall be configured to deliver sufficient dopant while minimizing silicon substrate contamination.
• suitable dopants include boron nitride (BN ), boron oxide (B 2 0 3 ), boron carbide (B 4 C), any of the phases of boron silicide (3 ⁇ 4Si), where x ::: 2,3,4,6, and other borides of metals that form silicon compatible binary oxides, such as TiB x , MgB x , HfB x , GdB x , LaB x , ZrB x , TaB x , CeB Xj C and mixtures thereof.
• Additional boron dopants include elemental boron, boron powder, boron- doped Si powder, B-C-Si compositions, MOx ⁇ B 2 0 3 , and mixtures thereof.
• B-C-Si compositions include, but are not limited to:
• the ceramic material and the boron dopant source are dispersed in a set of solvents, such as alcohols, aldehydes, ketones, carboxyiie acids, esters, amines, organosiloxanes, halogenated hydrocarbons, and other hydrocarbon solvents.
• solvents such as alcohols, aldehydes, ketones, carboxyiie acids, esters, amines, organosiloxanes, halogenated hydrocarbons, and other hydrocarbon solvents.
• the set of solvents may be mixed in order to optimize physical characteristics such as viscosity, density, polarity, etc.
• binder In addition, in order to optimize viscoelastic behavior of the paste for screen printing, a set of high molecular weight (HMW) polymer molecules, called binder, is added.
• the binder is one of polyacrylates, polyaeetals and their derivatives, polyvinyls, a cellulose
• the particle surface of the ceramic material may be treated with a ligand or capping agent in order to disperse in a set of sol vents and optimize shear thinning behavior.
• a capping agent or ligand is a set of atoms or groups of atoms bound to a "central atom" in a polyatomic molecular entity. The capping agent is selected for some property or function not possessed by the underlying surface to which it may be attached.
• Both sets of boron-containing doping pastes were produced by dispersing a mixture of boron-containing particles and metal oxide particles in a solution of ethyl cel lulose binder 1.5% (wt) and 75.5% (wt) terpineol solvent.
• Plot 506 is comprised of a mixture of 5% (wt.) boron silicide and 12%) (wt) aluminum oxide powders in a solution of ethyl cellulose binder 1.5% (wt) and 81.5% (wt) terpineol solvent. Fitting the shape of the viscosity curve with Equation 5, a slope of -0.687 corresponding to an n of 0.313. As previously described, the slope is equivalent to n-1.
• Plot 508 is comprised of a mixture of 5% (wt.) boron carbide and 1 8% (wt) titanium dioxide powders in a solution of ethyl cellulose binder 1.5% (wt) and 75.5% (wt) terpineol solvent. Fitting the shape of the viscosity curve with Equation 5, a slope of -0.6561 corresponding to an n of 0.3439.
• an n between about 0.01 and about 1.0 is preferable, an n between about 0.2 and about 0.8 is more preferable, and an n between about 0.325 is most preferable.
• FIG. 6 a simplified diagram showing a Spreading Resistance
• Profile plot of the majority carrier type and concentration in the diffusion region in accordance with the invention.
• On vertical axis 604 is the carrier concentration in cm "3 and on horizontal axis 602 is the depth of the measurement from the surface of an n-type (phosphorous doped) substrate.
• a boron-containing doping paste comprising of a mixture of 5% (wt.) boron carbide and 18% (wt) titanium dioxide powders in a solution of ethyl cellulose binder 1.5% (wt) and 75.5% (wt) terpineol solvent was deposited on the n-type substrate that was previously cleaned in an HF solution.
• the wafers were dried in a box oven at 70°C for 30 minutes to remove the solvent in an ambient containing nitrogen.
• the n-type substrate is exposed to about a 6: 1 mixture HF/HC1 at about room temperature and for about 2 minutes to reduce surface contamination.
• the n-type substrate is placed in a diffusion furnace and heated in an N2 ambient at about 900°C for about 60 minutes in order to diffuse the p-type dopant into the n-type substrate, which is subsequently beveled for generation of the Spreading Resistance Profile plot.
• a bias of about 5mV is applied across two tungsten carbide probe tips placed about 20 urn apart onto the doped n-type substrate. Between each measurement along the beveled surface, the probes are raised and indexed a pre-determined distance down the bevel.
• boron dopant has diffused from the printed ceramic boron-containing doping paste into the n-type silicon wafer, resulting in a p-n junction depth of approximately 0.3 microns, an acceptable depth for the formation of a proper contact to the silicon solar cell.
• Peak concentration of electrically active boron atoms at the substrate surface is approximately 2* 10 1 ( l/cnr ), matching the solid solubility of boron in sil icon at the temperature of diffusion.
• FIG. 7 a simplified diagram is shown of boron dopant diffusions in an n-type substrate as generated with two ceramic boron-containing doping pastes, in accordance with the invention.
• Vertical axis 702 shows the measured sheet resistivity in Ohm/square as measured for substrate areas underneath the deposited ceramic boron-containing doping paste and for field areas (i.e., areas without the printed boron-containing doping paste).
• a first ceramic boron-containing doping paste 706 (corresponding to plot 506 in
• FIG. 5 deposited on n-type substrate 714, was comprised of a mixture of 5% (wt.) boron silicide and 12% (wt) aluminum oxide powders in a solution of ethyl cellulose binder 1.5% (wt) and 81.5% (wt) terpineol solvent.
• a second ceramic boron-containing doping paste 710 (corresponding to plot 508 in FIG. 5) as deposited on n-type substrate 716, was comprised of a mixture of 5% (wt.) boron carbide and 1 8%> (wt) titanium dioxide powders in a solution of ethyl cellulose binder 1 .5% (wt) and 75.5%> (wt) terpineol solvent.
• Each ceramic boron-containing doping paste was deposited onto an n-type silicon substrate that was previously cleaned in an HF solution. After deposition, the substrate was dried in a box oven at 70°C for 30 minutes to remove the solvent. The substrate was then immersed in a dilute aqueous HF:HC1 mixture to reduce surface contamination. After a DI water rinse and drying, the substrate was heated in a hot wall diffusion tube in an inert ambient at 900°C for one hour.
• the region under the ceramic boron-containing doping paste was substantially p-type, with a resistivity between about 60 Ohm/sq and about 80 Ohm/sq, with an average of about 70 Ohm/sq.
• the field region 708 w r as substantially n-type, with a much higher resistivity between about 100 Ohm/sq and about 275 Ohm/sq, with an average of about 180 Qhm/sq, corresponding to the bulk of the n-type wafer.
• the region under the ceramic boron-containing doping paste was substantially p-type, with a resistivity between about 70 Ohm/sq and about 90 Ohm/sq, with an average of about 80 Ohm/sq.
• the region under field 708 was substantially n-type, with a much higher resistivity between about 125 Ohm/sq and about 375 Ohm/sq, with an average of about 225 Ohm/sq, corresponding to the bulk of the n-type wafer,
• the ceramic boron-containing paste is counter- doping the n-type substrate with boron (p-type) dopant
• FIG. 8 a simplified diagram is shown of simultaneous boron dopant diffusion (as generated with boron-containing doping paste) and phosphorous dopant diffusion (as generated with a POCl 3 process) on an n-type substrate, in accordance with the invention.
• Vertical axis 802 shows the measured sheet resistivity in Ohm/square as measured for substrate areas underneath the deposited boron-containing doping paste and for field areas (i.e., areas without the deposited boron-containing doping paste),
• a first boron-containing paste 806 (corresponding to plot 506 in FIG. 5), deposited on n-type substrate 814, was comprised of a mixture of 5% (wt.) boron silicide and 12% (wt) aluminum oxide powders in a solution of ethyl cellulose binder 1 .5% (wt) and 81 .5% (wt) terpineol solvent.
• a second boron-containing paste 810 (corresponding to plot 508 in FIG. 5) as deposited on n-type substrate 816, was comprised of a mixture of 5% (wt.) boron carbide and 18% (wt) titanium dioxide powders in a solution of ethyl cellulose binder 1 ,5% (wt) and
• Each boron-containing paste was screen printed onto an n-type silicon substrate that was previously cleaned in an HF solution. After deposition, the substrate was dried in a box oven at 70°C for 30 minutes to remove the solvent, The substrate was then immersed in a dilute aqueous HF:FIC1 mixture to reduce surface contamination.
• the substrate was heated in a hot wall diffusion tube in an inert ambient at 900°C for one hour followed by exposure to a phosphorous (n-type) dopant source in a diffusion furnace with an atmosphere of POCI3, N 2 , and 0 2 , at a temperature of about 850°C for about 60 minutes.
• a phosphorous (n-type) dopant source in a diffusion furnace with an atmosphere of POCI3, N 2 , and 0 2 , at a temperature of about 850°C for about 60 minutes.
• the residual PSG glass layer on the substrate surface was subsequently removed by a BOB cleaning step for 5 minutes.
• the boron-containing paste is both counter-doping the n-type substrate, and blocks ambient phosphorous generated during the POCI3 diffusion process.
• FIG. 9 a simplified sample process for the manufacture of a boron-containing doping paste is shown, in accordance with the invention.
• the boron-containing particles are combined with an optional dispersant and a first set of solvents into a first mixture.
• ceramic particles are combined with a second set of solvents into a second mixture.
• a binder is combined with a third set of solvents in a third mixture.
• the first, second, and third mixtures are combined and then mixed and milled.
• the terms "dopant or doped” and “counter-dopant or counter-doped” refer to a set of dopants of opposite types. That is, if the dopan t is p-type, then the counter-dopant is n-type. Furthermore, unless otherwise dopant-types may be switched.
• the silicon substrate may be either mono-crystalline or multi-crystalline.
• Advantages of the invention include a doping paste that is resilient to high temperature oxidizing processes (such as the POCI 3 diffusion process), is able to mask ambient POCI 3 , and is compatible with HF-based acidic cleaning chemistries.

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