CN106256025A - The self-aligned contacts part of back contact solar cell - Google Patents

Abstract

An aspect according to subject, it is provided that for the self-aligned contacts part of back contacts back junction solar battery.Described solaode includes that semiconductor layer, described semiconductor layer have on front side of light-receiving and the dorsal part relative with described front side and be attached to electric insulation backboard.The first metal layer with the base stage and emitter electrode that are self-aligned to base stage and emitter region is positioned on described semiconductor layer dorsal part.Battery interconnection is provided and is connected to patterning second metal level of described the first metal layer by via plug and is positioned on described backboard.

Description

Self-aligned contacts for back-contact solar cells

Cross References to Related Applications

This application claims the benefit of US Provisional Patent Application 61/954,116, filed February 26, 2014, which is hereby incorporated by reference in its entirety.

technical field

The present disclosure relates generally to the field of photovoltaic (PV) solar cells, and more particularly to self-aligned contacts for solar cells.

Background technique

With the adoption of photovoltaic solar cell technology as an energy generation solution on an increasingly widespread scale, fabrication and efficiency improvements related to solar cell efficiency, metallization, material consumption, and fabrication are required. Manufacturing cost and conversion efficiency factors are driving solar cell absorbers to thinner thicknesses and larger areas, thus increasing mechanical fragility, efficiency and complicating the handling and disposal of these thin absorber based solar cells - the fragility effect Especially increased relative to crystalline silicon absorbers.

In general, the solar cell contact structure comprises conductive metallization on the base and emitter diffusion regions - eg aluminum metallization connecting silicon in the base and emitter contact regions via relatively heavy phosphorous and boron regions respectively.

Description of drawings

The features, properties, and advantages of the disclosed subject matter may become more apparent from the following detailed description, set forth in conjunction with the accompanying drawings, in which like reference numerals indicate like features and in which:

1A to 1D are cross-sectional views of solar cells with self-aligned contact structures;

2A to 2E are cross-sectional views of a solar cell at various steps during fabrication of a butt-junction interdigitated back contact solar cell with self-aligned contacts;

3A to 3G are cross-sectional views of solar cells at various steps during the fabrication of non-butted junction interdigitated back contact solar cells with self-aligned contacts;

4A to 4E are cross-sectional views of solar cells at various steps during fabrication of non-butted junction interdigitated back contact solar cells with self-aligned contacts; and

5 is a cross-sectional view of a solar cell with a self-aligned contact structure consistent with FIG. 1A and with multi-level metallization.

detailed description

Therefore, there is a need for fabrication methods for back contact solar cells. In accordance with the disclosed subject matter, methods for the fabrication of back contact solar cells are provided. These innovations substantially reduce or eliminate the disadvantages and problems associated with previously developed back contact solar cell fabrication methods.

This patent relates to solar cells. In addition to those described herein, related patent applications, at least in part co-invention rights, that provide details of solar cell construction and fabrication include: U.S. Patent Application 14/179,526, filed February 2, 2014; US Patent Application 14/072,759 (published as US Publication 20140326295 on November 6, 2014); US Patent 13/869,928 filed on April 24, 2013 (published as US Publication 20130228221 on September 5, 2013) ; US Patent Application 14/493,341, filed September 22, 2014; and US Patent Application 14/493,335, filed September 22, 2014, all of which are hereby incorporated by reference in their entirety.

According to one aspect of the disclosed subject matter, self-aligned contacts for back-contacted back-junction solar cells are provided. The solar cell includes a semiconductor layer having a light receiving front side and a back side opposite the front side and attached to an electrically insulating backsheet. A first metal layer having base and emitter electrodes self-aligned to the base and emitter regions is positioned on the backside of the semiconductor layer. A patterned second metal layer providing battery interconnects and connected to the first metal layer by via plugs is positioned on the backplane.

These and other advantages, as well as additional novel features, of the disclosed subject matter will be apparent from the description provided herein. The purpose of this overview is not to describe the subject matter exhaustively, but to provide a brief, functional overview of the subject matter. Other systems, methods, features, and advantages provided herein will become apparent to those skilled in the art upon review of the drawings and detailed description. All such additional systems, methods, features and advantages included within this description are intended to be within the scope of the following claims.

The disclosed subject matter provides structures and methods for fabricating self-aligned contacts for back-contact back-junction solar cells. In particular, the disclosed subject matter and corresponding drawings provide low-damage, high-density silicon solar cells for forming thin silicon solar cells using self-aligned contacts for back-contact back-junction (e.g., interdigitated back-contact IBC) solar cells. Efficient and low-cost process flow. The described novel self-aligned contact structure can achieve high solar cell conversion efficiency. Additionally, solar cell fabrication methods with minimal or reduced process steps for forming solar cell structures with self-aligned contacts are described.

The term "self-aligned" describes the cell structure such that the heavily doped n+ and p+ regions under the base and emitter metal contacts are self-aligned with respect to the contact openings - shown for example in Figures 1A to 1D . Figure 1A is a cross-sectional view of a selective emitter solar cell with a self-aligned contact structure with a dopant diffusion region with a higher doping level just below the metal-to-silicon absorber contact (eg, greater than 1E18cm-3). By having heavily doped regions (n and p-type) in silicon for improved metal/silicon contact resistance and lower surface recombination rates at metal/silicon contacts, and by making heavily doped regions in solar cells With minimal doping (eg, greater than 1E 18 cm-2) and thus reducing the overall saturation current density, the self-aligned contact structure can provide higher solar cell efficiencies. Alternatively, the self-aligned contact structures disclosed herein can also be formed by using hetero/tunneling contacts through the barrier layer between metal and silicon - such as shown in FIG. 1B . 1B is a cross-sectional view of a solar cell with a self-aligned contact structure formed using hetero/tunneling contacts through a barrier layer between metal and silicon.

The advantage of the self-aligned structure is that the heavily doped regions are limited to under the contacts only where they are needed. If a contact opening needs to be aligned to a heavily doped with a non-self-aligned contact structure, then the heavy diffusion needs to be much wider than the contact opening to accommodate alignment tolerances. Compared to non-self-aligned contact structures, the provided self-aligned contact structures can be more efficient for two different reasons. First, heavy doping can be detrimental when used under passivating conditions, in other words, heavily doped and in some cases only useful when used under poor passivating conditions such as metals. Therefore, the self-aligned structure eliminates heavily doped regions under high-quality passivation conditions. Second, for non-self-aligned structures, two openings need to be made: the first one for doping and the second one for contact openings. If these openings are made using methods that are prone to damage in the silicon (eg, laser processing in some cases), then the self-aligned structure removes the outer nested openings and minimizes and in some cases eliminates damage from this step. Laser damage. Furthermore, in addition to the efficiency advantages, self-aligned structures may also require fewer process steps and thus reduce cell cost.

Table 1 below shows a front-end process flow for forming a selective emitter solar cell with self-aligned contacts and field emitters, such as that shown in FIG. 1A , using a dopant paste step.

1 Saw damage removal 2 APCVD boron doped Al2O3 + undoped SiO2 3 Laser ablation (ns UV and/or ps UV) 4 Dopant paste (phosphopaste printing - drying + boron paste printing drying) 5 diffusion annealing 6 Wet etching (e.g. dilute HF, SCl or dilute KOH based, etc.)

Table 1. Selective emitter solar cells with self-aligned contacts and field emitters with dopant paste.

Table 1 shows the process flow in which self-aligned contacts are used for fabricating high efficiency back contact back junction solar cells. As shown, step 1 is a saw damage removal step to remove damage from a wafer (e.g., a CZ wafer); however, the flow presented is equally applicable to epitaxially formed silicon processed while on a template The substrate, in which case step 1 sawing damage removal was replaced by porous silicon and epitaxial silicon deposition steps as described in detail herein. Thus, in epitaxial embodiments, the described front-end processing occurs on the exposed surface of the template-attached epitaxial substrate, after which the epitaxial liner can be released (e.g., mechanically or wet-etched) from the template in back-end processing. end. Importantly, the exemplary process flows provided are described in the context of fabricating high efficiency back-contact back-junction solar cells for descriptive purposes, and those skilled in the art may combine, add to, or remove from the overall process flow , alter, or move the various process steps disclosed. In other words, elements from each of the process flows described in the tables provided herein can be combined together or with other known methods of solar cell fabrication. As an example, refer to Table 1: The laser contact opening shown in Step 3 can be split into two steps (such as shown in Table 2) to separately form self-aligned contacts for base and emitter contacts only pieces; the dopant paste printing step shown in step 4 may have an additional third print of undoped paste on top of the already printed dopant paste (such as shown in FIG. 8 ). Furthermore, the wet etch step shown in step 6 of Table 1 and which removes the annealed dopant paste can be replaced by a dry HF vapor etch process, or can be skipped (i.e., removed) entirely for an all-dry front-end process. except step 6. Furthermore, the laser contact opening step shown in step 6 of Table 1 can be replaced by an etch paste process for a laser-less front-end process, which includes etch paste deposition, drying, and rinsing.

Table 2 below shows the front-end process flow for forming a selective emitter solar cell with self-aligned contacts using dopant paste and using a separate contact opening step.

1 Saw damage removal 2 APCVD boron doped Al2O3 + undoped SiO2 3 Laser ablation (ns UV and/or ps UV) 4 Dopant paste (phosphopaste printing + drying) 5 Laser ablation (ns UV and/or ps UV) 6 Dopant paste (boron paste printing + drying) 7 diffusion annealing 8 Wet etching based on HF/SKIP/HF vapor

Table 2. Selective emitter solar cells with self-aligned contacts with dopant paste and a separate contact opening step.

Table 3 below shows the front-end process flow for forming a selective emitter solar cell with self-aligned contacts with dopant paste and applying a diffusion barrier.

1 Saw damage removal 2 APCVD boron doped Al2O3 + undoped SiO2 3 Laser ablation (ns UV and/or ps UV) 4 Dopant Paste (Phospho Paste Printing + Drying + Boron Paste Printing Drying) 5 APCVD USG deposition: 6 diffusion annealing 7 Wet etching based on HF/SKIP/HF vapor

Table 3. Selective emitter solar cells with application of self-aligned contacts with dopant paste and diffusion barrier.

Alternatively, the diffusion barrier deposition shown as APCVD USG deposition in step 5 of Table 3 can also be undoped paste printing.

The process flow embodiments of Tables 2 and 3 can be used to reduce autodoping from the dopant paste during diffusion annealing.

Table 4 below shows the front-end process flow for forming a non-butted junction solar cell with self-aligned contacts with diffusion barrier dopant paste printing, such as that shown in FIG. 1C . Figure 1C is a cross-sectional view of a non-butted junction solar cell with a self-aligned contact structure with a dopant diffusion region with a higher doping level just below the metal-to-silicon absorber contact ( For example, greater than 1E18cm-3).

1 Saw damage removal 2 Unadulterated paste printing + drying 3 APCVD boron doped Al2O3 + undoped SiO2 4 Laser ablation (ns UV and/or ps UV) 5 Dopant paste (phosphopaste printing - drying + boron paste printing drying) 6 diffusion annealing 7 Wet etching based on HF/SKIP/HF vapor

Table 4. Non-butted junction solar cells with self-aligned contacts with dopant paste printing and non-butted junctions with diffusion barrier paste printing.

Alternatively referring to the non-butted junction solar cell flow of Table 4, steps 2, 3 and 4 of Table 4 can be replaced by two steps of APCVD boron-doped silicon oxide (BSG1) deposition followed by picosecond (ps) CO2 laser - called Alternative implementations for self-aligned contacts printed with dopant paste and non-butted junctions with boron-doped silicon oxide by APCVD.

Table 5 below shows the fabrication process flow for a non-selective emitter solar cell with self-aligned contacts and using phosphorous dopant paste.

1 Saw damage removal 2 APCVD boron doped Al2O3 + undoped SiO2 3 Laser ablation (ns UV and/or ps UV) 4 Dopant paste (phosphopaste printing + drying) 5 diffusion annealing 6 Wet etching based on HF/SKIP/HF vapor 7 Laser ablation (ps UV)

Table 5. Non-selective emitter solar cells with self-aligned contacts with dopant paste.

Alternatively, Table 6 below shows the fabrication process flow for a non-selective emitter solar cell with self-aligned contacts and using phosphorus oxychloride POCl3 (POCl).

1 Saw damage removal 2 APCVD boron doped Al2O3 + undoped SiO2 3 Laser ablation (ns UV and/or ps UV) 4 diffusion annealing 5 POCl diffusion 6 Wet etching based on HF/SKIP/HF vapor 7 Laser ablation (ps UV)

Table 6. Non-selective emitter solar cells with self-aligned contacts with POCl-based diffusion.

Table 7 below shows the fabrication process flow for a non-selective emitter solar cell with self-aligned passivated base contact using dopant paste.

1 Saw damage removal 2 APCVD boron doped Al2O3 + undoped SiO2 3 Laser ablation (ns UV and/or ps UV) 4 Dopant paste (phosphopaste printing + drying) 5 diffusion annealing 6 Wet etching based on HF/SKIP/HF vapor 7 ALD deposition (Al2O3, TiO2, Al2O3+TiO2) 8 Laser ablation (ps UV)

Table 7. Non-selective emitter solar cells with self-aligned passivated base contacts with dopant paste.

Table 8 below shows the fabrication process flow for a solar cell with a self-aligned base tunneling/heterojunction contact such as that shown in FIG. 1B .

1 Saw damage removal 2 APCVD boron doped Al2O3 + undoped SiO2 3 Laser ablation (ns UV and/or ps UV) 4 diffusion annealing 5 Wet etching based on HF/SKIP/HF vapor 6 ALD deposition (Al2O3, TiO2, Al2O3+TiO2) 7 Laser ablation (ps UV)

Table 8. Solar cells with self-aligned tunneling/heterojunction contacts.

Table 9 below shows the fabrication process flow for a solar cell with self-aligned contacts without a heavy diffusion region below the base contact.

Table 9. Solar cells with self-aligned passivated base contacts.

Alternatively, the self-aligned contact structures and methods described herein can be applied to

Table 10 below shows the front-end process flow for forming a solar cell with self-aligned contacts with field bases, such as that shown in FIG. 1D . Figure 1D is a cross-sectional view of a solar cell with a field base and a self-aligned contact structure with a dopant diffusion region with a higher doping level just below the metal-to-silicon absorber contact (eg, greater than 1E18cm-3). Alternatively, for example, the HF vapor step 6 in Table 10 to remove the annealed dopant paste can be replaced by a wet etch step.

Table 10. Solar cells with field bases and self-aligned contacts with dopant paste.

Alternatively, Table 11 below shows a front-end process flow for forming a solar cell with field base self-aligned contacts with etch paste and dopant paste printing, such as that shown in FIG. 1D .

1 Saw damage removal 2 APCVD undoped Al2O3 + undoped SiO2 3 Etching paste (printing, curing, washing) 4 Dopant paste (phosphopaste printing - drying + boron paste printing drying) 5 diffusion annealing 6 HF vapor

Table 11. Solar cells with field bases and self-aligned contacts printed with etch paste and dopant paste.

2A to 2E are process flow representations showing cross-sectional views of a solar cell at various steps during the fabrication of a butt junction interdigitated back contact solar cell with self-aligned contacts with dopant paste. Figure 2A shows an aluminum oxide (Al2O3) layer deposited (eg, by APCVD) on a silicon substrate/wafer. The aluminum oxide layer can also have an undoped silicate glass layer. Next, as shown in Figure 2B, a nanosecond (ns or ps) laser opens the base and emitter contacts. This step may also include a wet etch to remove any oxide residues (eg, aluminum silicon oxide residues). Next, as shown in Figure 2C, the dopant paste is paste printing in the emitter and base regions, followed by a diffusion anneal to drive in/diffuse the dopants and form the base and emitter regions. Next, as shown in Figure 2D, the dopant paste is exfoliated (eg, by wet etching). Metal is then printed and annealed on the base and emitter regions as shown in Figure 2E, resulting in minimal shunting risk.

3A to 3G are process flow representations showing cross-sectional views of a solar cell at various steps during the fabrication of a non-butted junction interdigitated back contact solar cell with self-aligned contacts with dopant paste. Figure 3A shows an aluminum oxide (Al2O3) layer deposited (eg, by APCVD) on a silicon substrate/wafer. The aluminum oxide layer can also have an undoped silicate glass layer. Next, as shown in Figure 3B, a nanosecond (ns) laser opens the base contact. This step may also include a wet etch to remove any oxide residues (eg, aluminum silicon oxide residues). Next, as shown in Figure 3C, a layer of undoped silicate glass is deposited (eg, by APCVD). The base and emitter contact openings are then ablated by a picosecond (ps) laser as shown in Figure 3D. Next, as shown in Figure 3E, the dopant paste is paste printing in the emitter and base regions, followed by a diffusion anneal to drive in/diffuse the dopants and form the base and emitter regions. Next, as shown in Figure 3F, the dopant paste is exfoliated (eg, by wet etching). Metal is then printed and annealed on the base and emitter regions as shown in Figure 3G, resulting in minimal shunting risk.

4A to 4E are cross-sectional views showing a solar cell at various steps during first fabricating a non-butted junction interdigitated back contact solar cell with self-aligned contacts with dopant paste using undoped paste process flow representation. Figure 4A shows an undoped silicon oxide (SiO2) paste printed on only the desired base regions of a silicon substrate/wafer. Next, as shown in FIG. 4B , deposit (for example, by APCVD) a doped layer (for example, a doped aluminum oxide layer Al2O3 or a doped borosilicate glass layer BSG1 ) and an undoped silicate glass (USG) layer. The undoped silicate glass layer may have a thickness three to four times thicker than the undoped layer. The base and emitter contact openings are then ablated by a picosecond (ps) laser as shown in Figure 4C. Next, as shown in Figure 4D, the dopant paste is paste printing in the emitter and base regions, followed by a diffusion anneal to drive in/diffuse the dopants and form the base and emitter regions. Next, the dopant paste is peeled off (eg, by wet etching). Metal is then printed and annealed on the base and emitter regions as shown in Figure 4E, resulting in minimal shunting risk.

Although the methods for fabricating self-aligned back-contact back-junction solar cells are described in the general context of CZ wafers, the methods are equally applicable in the case of epitaxially grown back-contact back-junction solar cells. Additionally, the method is applicable to thick crystalline silicon (e.g., with absorber thickness in the range of approximately 100um to 200um) as well as thin crystalline silicon back-contact back-junction solar cells (e.g., with absorber thickness in the range of approximately 5um to 100um). absorber thickness).

In general and specifically applicable to the process flow represented in the table below, the emitter or base are turned on sequentially (in either order) or simultaneously using various field dielectric removal techniques such as using a laser or wet etching or etching paste contacts. And then, a dopant source is deposited in the open contact, the dopant is driven into the silicon at high temperature, and the dopant source is selectively removed/etched while leaving the field dielectric unharmed by the etchant. This leaves the dopants driven into the silicon only in the area below where the contacts were opened, leaving a self-aligned structure.

The described fabrication methods can be further categorized by the source of dopants under the contacts. These can come from dopant pastes (eg phosphorus for n-type and boron for p-type) or deposited films in which dopants are incorporated, eg APCVD deposited boron or phosphorus doped SiO2 films. Finally, a mixed source, where the N+ and p+ dopant sources come from APCVD for one type of dopant and dopant paste for the other type of dopant. A further subclassification is defined by the technique of etching away/removing the dopant source, which applies to both wafer and epitaxy based absorbers and dopant source classification (dopant paste, APCVC film and mixed dopant source). As an example, for oxide-based dopant sources such as doped SiO2, a wet process with HF can be used, or a dry process with HF vapor phase etching can be deployed. If the field region is also SiO2, then wet HF selectivity is obtained since heavily doped SiOx films can be etched much faster than undoped films. Alternatively, the field region stack may contain Al2O3 (eg also deposited using APCVD). This membrane can have a high selectivity to HF solutions once treated at elevated temperatures, eg greater than 900°C. Alternatively, HF vapor also etches dopant sources very selectively.

In general, the two dopant sources can be screen printed dopant pastes if the contacts are opened at the same time. If the contacts are opened sequentially, a deposited film or mixed source for both contacts can be utilized.

Table 12 shows a front-end self-aligned contact fabrication flow that creates a split junction and is implemented using a dopant paste (eg, screen-printed dopant paste). In a split junction, the emitter doping does not abut the base contact doping and is separated by the background bulk doping of the base. Step 2 shows the deposition of the emitter followed by a cap. And while the emitter source is shown as an APCVD deposited boron doped Al2O3, it could also be a boron doped SiO2 layer or another dopant source layer deposited using a different means. The first laser ablation (step 3) is to open the separation between the emitter and base doping so that after annealing there is a separation between the junctions. The described protocol suggests the use of lasers ns UV and ps UV. This base window can also be produced using picosecond green laser, femtosecond laser or etching paste or photolithography. If a picosecond laser is used, this can be followed by a small wet etch of the silicon to remove the laser damage in the silicon. Step 5 of Table 12 is also done using picosecond green laser or femtosecond laser. Step 5 is a contact opening in the base window for the base contact and a contact opening for the emitter. Both contacts are opened in the same step, so the method of printing the dopant source should be selective printing on top of these contacts, e.g. screen printing of dopant paste (with thin dopant source film compared to blanket deposition). After the anneal in step 7 to drive the dopants into the two contacts, the dopant sources are wet etched or selectively etched using HF vapor. In a separate embodiment, the etch step (step #8) may be skipped if the source of dopant is conductive, like a silicon-based dopant source.

Table 12. Self-aligned contact fabrication flow using dopant paste to create split junctions.

In another embodiment, the contacts can be opened sequentially if there is a risk of co-diffusion during drying or dopant drive. In this case, first the base or emitter contact is opened, and the corresponding paste is printed and dried. Next, the other contact is opened, and the corresponding paste is printed and dried. Finally, drive both pastes simultaneously. This alternative avoids cross-contamination in the contacts during drying and burning.

In a more extreme case, if cross-contamination is an issue during dopant drive, then contact opening, dopant paste printing, drying/burning, and annealing can be performed on one type of dopant. This sequence is followed by the same steps repeated for the second type of contact. This results in two different anneals, in which case the thermal budget should be optimized.

In the butt junction implementation of the process flow in Table 12, steps 3 and 4 can be skipped and the contacts can be opened directly to the base and emitter.

Finally, in another variation, the field region can be terminated by a thin film that is resistant to the dopant source etchant chemistry. In cases where the dopant source is based on SiOx and the etch chemistry is based on HF, the cap layer can be APCVD based Al2O3 (undoped or doped) or titanium oxide (TiO2) or amorphous silicon (a-Si ).

Table 13 shows the process flow of the front-separated junction self-aligned solar cell using only APCVD-deposited film as dopant source. This procedure follows the same steps as Table 12 (with all the changes above) until step 4. At step 5, only one type of contact is opened first. In this case, the contact is an emitter contact (for n-type back contact cells). This is followed by the APCVD BSG film, which is the dopant source for emitter contact doping (step 6). Next, the base contact is opened and PSG is deposited using APCVD. In one variation, the order in which the emitter and base contacts are opened can be reversed. The butted version of the split junction procedure described in Table 13 skipped/removed steps 3 and 4 to create a butted junction.

Table 13. Self-aligned contact fabrication flow using APCVD doped dielectric films to create split junctions.

Table 14 below shows the front-end split junction self-alignment process flow using a hybrid approach. In this method, one dopant source is deposited APCVD film and another type of dopant source is printed dopant paste.

Table 14. Self-aligned contact fabrication flow for creating split junctions using mixed APCVD doped dielectric films and phosphorous-based dopant pastes.

The procedure of Table 14 shares the first four steps (and variations thereof) with Table 13. In step 5 of Table 14, the emitter contact is opened. The BSG is deposited in step 6 and the base contact is opened with a laser in step 7 (note that while the described flow suggests the use of a ps laser, nanosecond or femtosecond lasers with different wavelengths are not excluded as long as they satisfy the contact opening upon request). Subsequently, in step 8, a phosphorous-based dopant paste is printed and dried. Step 9 is an annealing step to drive dopants from the BSG and from the phosphopaste to create doped regions under the contacts, while step 10 removes dopant sources based on wet or HF vapor techniques. The butted version of the split junction procedure described in Table 14 skipped/removed steps 3 and 4 to create a butted junction.

In a variation of the procedure in Table 14, the order of BSG2 (step 6) and phosphorus dopant paste (step 8) is reversed. The base contact is opened first, followed by the phosphopaste. This is followed by emitter contact and BSG2 deposition, and the rest of the flow is similar.

In another variation, the mixed dopant source is based on APCVD PSG and dopant paste boron such that doped SiO2 is APCVD deposited to make the base contact while boron based dopant paste is used to make the emitter contact. This variation has a further variation in which the sequence of contact opening and its accompanying dopant source has two possibilities.

Table 15 below is a front-end process flow showing a variation of the hybrid approach of Table 14, where both dopant paste and doped dielectric film are used as dopant sources for doping below the base and emitter contacts.

Table 15. Self-aligned contact fabrication flow for creating split junctions using a hybrid APCVD-doped dielectric film and phosphorous-based dopant paste, where the split contacts are opened by diffusion annealing, eliminating the risk of dopant co-diffusion

In a variation of Table 15 compared to Table 14, the emitter and base contacts were separated by APCVD-PSG and diffusion annealing. This is to reduce the risk of dopant co-diffusion during the diffusion anneal. Co-diffusion is when the dopant source from the base or emitter contact diffusion region (phosphorous or boron) of the dopant paste (phosphorus or boron) moves through the gas phase into the other polarity (base or emitter) process. This process can be avoided, for example, by placing a solid phase dopant source (APCVD-PSG) on top of the PSG and adding an anneal before the next contact-emitter contact opening step, as shown in Table 15. In some cases the paste is phosphorous and the base is turned on first (for n-type back contact cells), and in one variation the paste is boron and the emitter is turned on first.

A variation of the process flow of Table 15 forms a butt junction by skipping steps 3 and 4, as shown in Table 16 below. Throughout this disclosure, the variations described in Bonding Table 15 apply equally to the butt bonding process.

Table 16. Self-aligned contact fabrication flow for butt junctions using hybrid APCVD-doped dielectric films and phosphorous-based dopant pastes, where separation contacts are opened by diffusion annealing, eliminating the risk of dopant co-diffusion.

In the variations of Tables 15 and 16, the risk of co-diffusion can be avoided by eliminating APCVD-PSG or eliminating diffusion annealing.

It should be noted that all self-aligned process flows and variations thereof described thus far are equally valid for epitaxially grown thin film solar cells. A representative process flow corresponding to the method outlined in Table 12 (split junction with dopant paste) is shown in Table 17 for epitaxial thin film solar cells. The epitaxial process can use the HF vapor method to keep the process mostly dry while the epitaxial absorber is still on the template. All other embodiments regarding docked and split junctions with mixed dopant sources or all APCVD dopant sources (shown for CZ wafers) are equally valid for epitaxial solar cells by modifying the procedure based on Table 16. This application provides a more detailed flow around other aspects of epitaxial formation. The self-aligned properties and methods of fabrication thereof can be combined with any of the previously discussed variations of epitaxy and CZ wafer based process flows.

Table 17. Self-aligned contact fabrication flow for creating split junctions using dopant pastes on epitaxially formed substrates.

After the completion of the base and emitter regions on the backside of the solar cell, the solar cell structures described herein can utilize a multi-level metallization structure, such as a two-level metallization structure, including base and emitter metallization on the cell first level metallization (M1), and a second level metal (M2) that collects power (voltage and current) from the first level metal (thus completing the solar cell metallization) and can also form cell-to-cell interconnects. The second level metal (M2) may include an interdigitated pattern of base and emitter current collecting fingers, and optionally include solar cell base and emitter bus bars (e.g., from the base and emitter bus bars, respectively strip extended M2 base and emitter fingers). First level metal (M1) may include interdigitated back contact metallization with relatively fine pitch interdigitated fingers (much finer pitch than second level metal pitch) orthogonal/perpendicular or (in some cases Middle) Arranged parallel to the interdigitated fingers of M2. The relatively thin electrically insulating backsheet formed between M1 and M2 and attached to the solar cells provides solar cell structural support, M1 electrical isolation, and allows solar cell fabrication (especially M2 fabrication and solar cell frontside processing) process improvements. The backplane sheet may be a continuous flexible material closely CTE-matched to the solar cell semiconductor substrate material (e.g., crystalline silicon for silicon solar cells), which is laminated or otherwise Attached to e.g. back contact/back junction solar cells.

In a multi-level metallization design, for example, a first level metallization M1 (eg, fine pitch interdigitated metallization comprising aluminum or another suitable metal) and a second level metal M2 (eg, comprising aluminum, copper, etc.) are included. or coarse-pitch interdigitated metallization of suitable conductive metal), M1 may include interdigitated base and emitter lines (e.g., base-emitter finger spacing less than 2mm and in some cases ) and M2 (in some cases the interdigitated fingers are substantially orthogonal/perpendicular to the M1 fingers and the base-emitter spacing is much thicker than M1) serves as the M1 base and Electrical connectors between emitter lines (ie M1 pattern without busbars, while optional battery busbars can be placed on M2 pattern). The metal layers in the disclosed multi-level metal design are separated by a dielectric or electrically insulating layer, such as a resin/fiber based prepreg material or a suitable plastic or polymer based material, forming a layer for placement on a continuous backplane A continuous backsheet for each of the plurality of solar cells in the solar array. Importantly, the backplate should preferably be relatively closely CTE (coefficient of thermal expansion) matched to the CTE of the semiconductor absorber (e.g., crystalline silicon) in order to minimize CTE mismatch stress or warpage effects during thermal processing, e.g. , a specially formulated aramid fiber resin prepreg that provides a close CTE match to silicon while providing flexibility, electrical insulation, thermal and chemical stability, and other desirable handling and reliability characteristics, such as effective crack-free laminated. The M1/M2 interconnect structure includes conductive material-filled vias through an insulating layer (e.g., an insulating dielectric layer such as a pre-preg backplane) positioned between M1 and M2, the insulating layer being in the patterned M2 layer. Laminated or attached to the backside of the solar cell after formation.

In particular, the provided solar cells can utilize a two-level metallization scheme comprising: a first-level contact metallization (M1) preferably without bus bars (although optional bus bars can be used) using a relatively thin Patterned metal (for example, thin aluminum formed by screen printing of aluminum paste or inkjet printing of aluminum ink or plasma sputtering from an aluminum target followed by laser ablation or wet etch patterning) on the backplane layer formed directly on the backside of the solar cell prior to pressing; and a second level of thin patterned metal M2 (e.g., comprising approximately 3 to 5 microns thick Al or approximately one to several microns copper, which in either case is optionally capped with a solderable coating such as tin), which is formed after backplane lamination. The patterned M2 layer can also be formed by electroplating or lamination and patterning using highly conductive metal foils including copper or aluminum. The M1 and M2 layers are separated by a backplate and interconnected at designated areas by conductive via plugs (conductive via plugs may be formed during M2 formation). M1 has a fine pitch pattern, and M2 is preferably orthogonal (or substantially perpendicular) to M1 and has a coarse pitch pattern (thus, fewer base and emitter fingers compared to M1 ). Patterned M2 completes the cell-level electrical metallization and can also provide cell-to-cell electrical interconnection for multiple solar cells laminated to a continuous backsheet, thus eliminating the need for individual cell-to-cell straps/busses in some cases / soldering as needed. In addition, when the array/module electrical interconnection design requires, M2 can form an array/module level bus or interconnection.

In some cases, voltage and current scaling (eg, higher voltage and lower current solar cells) can relax and reduce M2 conductivity requirements and constraints. For example, using thinner M2 metal (e.g., approximately 2 to 5 micron thick via PVD) compared to thicker M2 metallization (e.g., approximately 50 to 80 micron thick electroplated copper) Evaporated aluminum or approximately 1 to several microns of copper formed by plasma sputtering or evaporation). Importantly, the thickness of the M1 and M2 metallization layers can also be adjusted based on the number, size and shape of the interdigitated fingers on the M1 and M2 layers. It may be advantageous for M1 to be patterned with finer interdigitated fingers than the interdigitated fingers of M2. However, the presented cell structure and fabrication embodiments are applicable to various dual-level metallization schemes utilizing the backplane and M2 metallization layers.

Directly on the backside of the cell prior to backsheet lamination, e.g. Form the solar cell base and emitter contact metallization patterns. This first metallization layer (hereinafter referred to as M1 ) defines a solar cell contact metallization pattern, such as fine pitch interdigitated back contact (IBC) conductor fingers, which define the base and emitter regions of the IBC cell. The M1 layer extracts the solar cell current and voltage (thus, the solar cell power) and delivers the solar cell electrical power through conductive via plugs formed in the backsheet to the first layer of highly conductive solar cell metallization formed after M1. The second level/layer (herein referred to as M2). The conductive via plugs may be formed simultaneously during the formation of the patterned M2 layer, for example after laser drilling of the vias in the backplane layer.

The backsheet material attached to the backside of the solar cell and placed between the patterned M1 and M2 layers can be a thin (e.g., between approximately 25 microns and 1 mm and in some cases between approximately 25 microns and 1 mm) of polymeric material. 250 micron) sheet, the polymeric material has a sufficiently low coefficient of thermal expansion (CTE) closely matched to the semiconductor absorber layer so as to avoid excessive thermally induced stress and warpage to the solar cell array. Furthermore, the backsheet material should meet the process integration requirements of the back-end cell fabrication process, especially chemical resistance during wet texturing of the cell front side and PECVD deposition of passivation and anti-reflective coating (ARC) layers on the front side period of thermal stability. Moreover, the electrically insulating backplane material should also meet module-level lamination process and long-term reliability requirements. While a variety of suitable polymeric materials (e.g. plastics, fluoropolymers, prepregs, etc.) and suitable non-polymeric materials (e.g. glass, ceramics, etc.) can be used as backsheet materials, the choice of backsheet material depends on a number of considerations, Including (but not limited to) material cost, ease of process integration, reliability, flexibility, mass density, etc.

An advantageous material choice for the backsheet material is prepreg, and more particularly prepreg based on aramid fiber resin. In some cases, nonwoven aramid fibers are especially advantageous. In general, prepregs are reinforcement materials that are pre-impregnated with resin and are easy to use to create composite parts (prepregs can be used to create composites faster and easier than wet lay-up systems). Prepregs are manufactured by combining reinforcing fibers or fabrics with specially formulated pre-catalyzed resins using equipment designed to ensure consistency. Inexpensive prepreg materials are commonly used in printed circuit boards.

A vacuum laminator can be used to attach a backsheet (eg, a prepreg sheet) to the solar cell backside. After applying a combination of heat and pressure, a thin backsheet (eg, a prepreg sheet) is permanently laminated or attached to the backside of the partially processed (or even fully processed) solar cell. In the case of partially processed solar cells, subsequent post-lamination fabrication process steps may include: (i) completion of the texturing and passivation process on the sun-facing side (front side) of the solar cell, (ii) completion of the solar cell Completion of highly conductive metallization (M2) on the backside of the solar cell (which may include part of the solar cell backsheet). A highly conductive metallization M2 layer comprising emitter and base electrodes (e.g. comprising aluminum, copper or silver, where aluminum and/or copper are preferred over silver due to much lower material cost) is formed on the attached to the laminated backsheet on the backside of the solar cell.

In the formation of the backplane (on or in and around the M1 layer), a higher conductivity M2 layer is formed on the backplane. Vias are drilled (for example by laser drilling, etching, or a combination of partial laser drilling followed by etching) in the backplane (in some cases as many as hundreds or thousands of vias per solar cell), and the vias The pores may have a diameter (in some cases tapered) in the range of approximately 50 up to 500 microns, especially in the diameter range of about 100 to 300 microns. These vias land on pre-designated landing pad areas of M1 for electrical connection between the patterned M2 and M1 layers through conductive plugs formed in the vias. In some cases, the via may be covered or at least partially filled with conductive metallization, and M2 may be deposited in a separate step, and in other cases in the same M2 deposition or formation step, M2 is deposited at least in part completely cover or partially fill the via. Subsequently or in combination with via filling and conductive plug formation, a patterned highly conductive metallization layer M2 is formed (e.g., by plasma sputtering, electroplating, evaporation, or combinations thereof, using materials including, for example, aluminum, Al/NIV, Al/NiV /Sn or copper or solder-coated copper M2 material). For IBC solar cells with fine pitch interdigitated back contact (IBC) fingers (e.g., hundreds of fingers) on M1, the patterned M2 layer can be designed to be orthogonal to M1, in other words, Rectangular or tapered M2 fingers substantially perpendicular to the M1 fingers. Due to this orthogonal deformation, the patterned interdigitated M2 layer can have much fewer and wider IBC fingers than the M1 layer (eg, about 10 to 50 times fewer relative to the M1 fingers). Therefore, the M2 layer can be formed with a much coarser pattern with wider IBC fingers than the M1 layer. Optional solar cell bus bars can be positioned on the M2 layer instead of the M1 layer (in other words, M1 without bus bars) to eliminate the electrical shading losses associated with the bus bars on the cells. Since both base and emitter interconnects and bus bars can be positioned on the M2 layer on the backsheet of the solar cell, it is possible to provide two the electric access of the person.

5 is a cross-sectional view of a solar cell with a self-aligned contact structure consistent with FIG. 1A and with multi-level metallization. Specifically, Figure 5 shows a first-level metal M1 to second-level metal M2 emitter connection through a backplane via, where the interdigitated fingers of the second-level metal M2 are orthogonal to the interdigitated fingers of M1 Finger-shaped and patterned.

The previous description of the exemplary embodiments is provided to enable any person skilled in the art to make or use the claimed subject matter. Various modifications to these implementations will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other implementations without departing from the use of innovative capabilities. Thus, claimed subject matter is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

All such additional systems, methods, features and advantages included within this description are intended to be within the scope of the following claims.

Claims (1)

1. A back-contact back-junction thin solar cell, comprising: A deposited semiconductor layer comprising: light-harvesting front-side surface, which has a passivation layer, doped base region, and a doped backside emitter region having an opposite polarity to the doped base region; a backside passivation dielectric layer and a patterned reflective layer on the backside emitter region; Self-aligning backside emitter contacts and backside base contacts, the contacts being connected to metal interconnects, forming a first-level interdigitated pattern on the backside of the back-contact back-junction thin solar cell metallization patterns; and at least one permanent support reinforcement positioned on the back side of the back contact back junction thin solar cell; and a second metal layer separated from the first layer by the permanent backside support stiffener structure, the second layer being locally separated by an interdigitated pattern of holes in the permanent backside support stiffener structure contacts the first level metallization pattern.