WO2017111697A1

WO2017111697A1 - A method of fabricating a heterojunction all-back-contact solar cell

Info

Publication number: WO2017111697A1
Application number: PCT/SG2015/050506
Authority: WO
Inventors: Rolf Stangl; Johnson WONG; Thomas Mueller
Original assignee: Trina Solar Energy Development Pte Ltd.
Priority date: 2015-12-24
Filing date: 2015-12-24
Publication date: 2017-06-29

Abstract

A method of fabricating a heterojunction all-back-contact (ABC) solar cell, the method comprising the steps of: forming an emitter film on one side of a doped substrate; forming a conductive interlayer on the deposited emitter film; forming at least one channel, the channel cutting through at least the emitter film and the conductive interlayer; forming a continuous back surface field film on the deposited conductive interlayer and the channel formed; and forming a plurality of metallic grids on the deposited back surface field film, wherein at least a portion of at least one of the plurality of metallic grids is formed in the channel.

Description

A Method Of Fabricating A Heterojunction All-Back-Contact Solar Cell

The present invention relates broadly to a method of fabricating a heterojunction all-back contact solar cell.

P-type silicon wafers are typically used in a silicon wafer solar cell as the base substrate. Excess charge carrier separation can be achieved by forming homojunctions in the solar cell to improve the efficiency of a solar cell. For example, excess charge carrier separation can be achieved in a P-type silicon wafer solar cell by forming a full-area diffused p/n ⁺ homojunction (minority carrier collection) and a full-area diffused p/p ⁺ homojunction (majority carrier collection) by high temperature thermal diffusion process and high temperature contact firing respectively.

Further, solar cell efficiency can also be improved if an n-type silicon wafer is used as the base substrate. This is because the light-induced degradation observed in p-type silicon (due to metastable boron-oxygen complexes) may be avoided, thus achieving higher open-circuit voltages. In addition, electron-capture coefficient is usually higher than hole-capture coefficient in crystalline silicon. Thus, n-type silicon may cause lower minority carrier recombination rate.

Diffused homojunction point (or line) contacts or thin film deposited full area heterojunction contacts can also be formed in a solar cell to improve the efficiency of the solar cell. All-back-contact solar cells with all contacts formed at the rear side of the solar cell may avoid shading of the front side metallic grid and thus have an even higher efficiency at the expense of added complexity in patterning the rear surface of the wafer and the deposited layers.

Surface passivation is also important for enhancing the efficiency of silicon wafer solar cells. All sides of the wafer have to be efficiently passivated. If diffused homojunction point or line contacts are formed in solar cell (conventional homojunction approach), surface passivation is usually achieved by using electrically insulating passivation layers which may contain a large amount of interface charges (field effect passivation). The materials that are used as passivation layer include silicon nitride (SiN _x), which has large amount of positive interface charge, and aluminium oxide (AlO _x), which has large amount of negative interface charge. Small contact openings are formed within these electrically insulating passivation layers to form a contact to highly doped homojunction regions of the wafer. Conventional diffused homojunction contacts may form local contacts towards either a full-area diffused region or towards a local-area diffused region of the wafer. The latter approach increases the open-circuit voltage of the solar cell, as there are less recombination active regions within the wafer, even though the process of growing/depositing local-area contacts and patterning the diffusion mask may increase the cost and complexity of the fabrication process.

Further, if thin film deposited full-area heterojunction contacts are formed in a solar cell (conventional heterojunction approach), surface passivation is usually achieved by using an thin electrically-conducting intrinsic buffer layer. The thin intrinsic buffer layer is typically an ultrathin film (<10 nm) of intrinsic hydrogenated amorphous silicon (a-Si:H(i)) which is further covered by a thin (< 30 nm) p-doped or n-doped hydrogenated amorphous silicon (a-Si:H(p ⁺), a-Si:H(n ⁺)), in order to form the emitter and the back-surface-field (BSF) region of the solar cell. Alternatively, instead of using a-Si:H(i), its sub-oxides a-SiO _x:H(i), which provide better surface passivation, can also be used. Sometimes, the intrinsic buffer layer is omitted and a doped emitter layer or BSF layer is directly deposited on the base substrate, thereby accepting a slightly lower surface passivation but reducing the amount of layers in the solar cell. In order to form a full-area heterojunction contact, a transparent conductive oxide (TCO) layer is applied on top of the emitter or BSF layers. The TCO not only ensures lateral conductance, it also serves as an effective back reflector. A metallic grid is formed at the TCO to extract the current.

However, there are disadvantages for the approaches above. For example, conventional diffused homojunction silicon wafer solar cells suffer from a relatively low open-circuit (V _oc) potential. This is due to high recombination rate withinthe diffused regions and due to high contact recombination as the metallic grid is directly touching the solar cell absorber. Further, there are technological obstructions with regard to boron diffusion for forming p ⁺ contacts, which include a relatively low throughput, a very high thermal budget (> 1000 ºC), a large maintenance requirement for the tube (e.g. removal of boron powder) and an unstable diffusion process.

Heterojunction silicon wafer solar cells can attain relatively higher V _oc values, but the cost associated with the fabrication of heterojunction silicon wafer solar cells is also relatively higher. In particular, the formation of TCO layers, which is needed to ensure lateral conductance as well as rear side reflectance, requires an additional process and may thus add significant cost. In addition, due to parasitic absorptions in the TCO, and the need for specialized low temperature compatible screen printed pastes, it may be difficult to achieve high short-circuit current density (J _sc) and fill factors (FF) in heterojunction silicon wafer solar cells comprising front side contacts.

Recently, a high-efficiency contacting scheme is proposed for using heterojunction point-contacts in all-back-contact solar cells. In an all-back-contact heterojunction point-contact solar cell, diffused regions within the wafer are no longer needed to collect excess charge carriers in the solar cell absorber, as the huge amount of surface charge within the electrically insulating passivation layers can perform the same task (i.e. it accumulates electrons or holes near the surface of the wafer). Thus, charge carrier separation is no longer performed by a (homo or hetero) p ⁺/n or n ⁺/n junction but by alternating surface charges of two different electrically insulating passivation layers (i.e. AlO _x and SiN _x).

Excess charge carrier extraction can be performed by a local opening of the passivation layer and a subsequent deposition of heterojunction layers on top of the passivation layer. The heterojunction layers have an opposite doping as the surface charges of the passivation layers, i.e. the layers deposited on AlO _x (negative surface charge) should be effectively p-doped and the layers deposited on SiN _x (positive surface charge) should be effectively n-doped.

In contrast to full-area heterojunction contacts, it is not necessary to ensure perfect interface passivation in heterojunction point contacts as the fraction of the point-contacted area to the total area is well below 10%, thus a comparatively high interface recombination within these regions may be tolerated.

Heterojunction point-contact can also be formed using microcrystalline silicon (µc-Si:H), which has lower passivation quality but higher doping efficiency, instead of amorphous silicon (a-Si:H). Higher open-circuit voltage may be obtained in a heterojunction point-contact solar cell as compared to a homojunction point contact solar cell with the same geometrical dimensions. This is due to (1) a lower contact recombination rate as the band offsets of the heterojunction contacts block excess carrier of the solar cell absorber to reach the heterojunction material adjacent to the absorber and thus the metallic grid, and (2) there is no highly diffused region and thus there is no recombination active region within the solar cell absorber.

Conventionally, there are four types of high-efficient contacts for extracting excess electrons or holes from a solar cell absorber, including: (1) full-area diffused homojunction point-contacts, (2) locally-diffused homojunction point-contacts, (3) thin film heterojunction deposited full-area contacts, and (4) thin film heterojunction deposited point-contacts. With the exception of (4), all other contacts have already been successfully implemented in solar cells, thus proving their ability to reach high efficiencies (>20%) for silicon wafer solar cells. However, significant amount of local structuring is required on the wafer and/or the passivation layers for forming the contacts above. This is even more so for manufacturing an all-back-contact solar cells.

Forming full-area diffused homojunction point-contacts involves only one local opening process at the electrically insulating passivation layers (SiN _x or AlO _x). However, as the full-area diffused region within the wafer and the point-like metal-semiconductor interfaces are regions of high recombination, only relatively low open-circuit voltage can be obtained.

Forming locally-diffused homojunction point-contacts involves an additional local diffusion process within the wafer, which usually adds considerable complexity and cost to the solar cell fabrication process. However, as compared to full-area diffused homojunction point-contacts, locally-diffused homojunction can exhibit a higher open-circuit voltage, as there is smaller recombination active diffused area in the wafer. However, there are still highly recombination active point-like metal-semiconductor interfaces for locally-diffused homojunction point-contacts.

Thin-film deposited heterojunction full-area contacts are able to achieve higher open-circuit voltage than other contacts described above. This is due to (a) the inherent advantage of heterojunctions to reduce the contact recombination rate and (b) there are no recombination active regions within the wafer. To form a heterojunction full-area contact, no structuring is required. However, the amount of patterning step required to form heterojunction full-area contact in an all-back-contact solar cell significantly increases. For example, both the p+ and n+ a-Si:H regions, as well as an additional electrically insulating passivation layer (for example SiN _x) separating these two layers, need to be defined with mutual alignment.

Similar to the process of forming locally contacted full-area-diffused homojunction contacts, thin film deposited heterojunction point contacts requires only one structuring step (i.e. the local opening of the electrically insulating passivation layer). In principle, a thin film deposited heterojunction point contacts can exhibit a higher open-circuit potential than the thin film deposited heterojunction full-area contact as the highly recombination active thin-film heterojunction layers are decoupled from the solar cell absorber (with the exception of the point contact regions). For all-back-contact solar cells, neither the expensive TCO layer is needed (as SiN _x or AlO _x are able to form efficient back reflectors) nor an additional insulating layer separating the emitter layer from the BSF layer are needed. However, if such heterojunction point contacts were to be incorporated into all all-back-contact solar cell structures, the amount of patterning needed is at least as complex as using full-area heterojunction contacts in all-back-contact solar cells.

All-back-contact solar cells usually comprise diffused homojunction point contacts or thin film deposited full-area heterojunction contacts. Structuring and alignment steps have to be performed within the wafer (e.g. formation of highly p-doped or n-doped regions within the wafer) and this requires a significant amount of masking steps. For all heterojunction all-back-contact solar cells, a comparable amount of structuring may be needed in order to form the interdigitating sequence of n-doped and p-doped heterojunction layers at the rear side of the wafer.

A need therefore exists to provide a method of fabricating a heterojunction all-back-contact (ABC) solar cell to address at least some of the above problems.

According to a first aspect of the present invention, there is provided a method of fabricating a heterojunction all-back-contact (ABC) solar cell, the method comprising the steps of:
forming an emitter film on one side of a doped substrate;
forming a conductive interlayer on the deposited emitter film;
forming at least one channel, the channel cutting through at least the emitter film and the conductive interlayer;
forming a continuous back surface field film on the deposited conductive interlayer and the channel formed; and
forming a plurality of metallic grids on the deposited back surface field film, wherein at least a portion of at least one of the plurality of metallic grids is formed in the channel.

The conductive interlayer may comprise a transparent conductive oxide (TCO) layer.

Forming at least one channel may comprise the steps of:
depositing at least one shadow mask on the one side of the base substrate; and
removing the at least one shadow mask, wherein removing the shadow mask forms the channel.

Forming at least one channel may comprise using laser ablation.

Forming an emitter film may comprise the steps of:
forming an emitter passivation layer on the one side of a base substrate;
forming a set of emitter passivation layer openings, the set of emitter passivation layer openings cutting through the emitter passivation layer; and
forming a full-area emitter layer on the emitter passivation layer, the emitter layer covers the set of emitter openings to form at least one emitter point contact.

Forming a continuous back surface field film may comprise the steps of:
forming a back surface field passivation layer on the deposited conductive interlayer and the channel formed;
forming a set of back surface field passivation layer openings, the set of back surface field passivation layer openings cutting through the back surface field passivation layer; and
forming a full-area back surface field layer on the back surface field passivation layer, the back surface field layer covers the set of back surface field passivation layer openings, thereby forming at least one back surface field point contact to the silicon wafer and forming at least one point contact to the conductive interlayer, thereby contacting the emitter layer.

The emitter layer and/or the back surface field layer may comprise a doped amorphous silicon layer.

The emitter passivation layer and/or back surface field passivation layer may comprise one selected from a group consisting of aluminium oxide (AlO _x) and silicon nitride (SiN _x).

The method may further comprise the step of forming at least one groove, the groove cutting through the back surface field layer, the conductive interlayer and emitter layer.

According to a second aspect of the present invention, there is provided a solar cell manufactured using the method as defined in the first aspect.

Embodiments of the invention will be better understood and readily apparent to one of ordinary skill in the art from the following written description, by way of example only, and in conjunction with the drawings, in which:

Fig.1

shows a flow chart illustrating a method of fabricating a heterojunction all-back-contact (ABC) solar cell according to an example embodiment.

Fig.2

shows a sectional view of a heterojunction ABC solar cell that comprises a p-type silicon substrate with full-area emitter contact and full-area BSF contact, fabricated in accordance with an embodiment of the invention.

Fig.3

shows a sectional view of a heterojunction ABC solar cell that comprises an n-type silicon substrate 02 with full-area emitter contact and full-area BSF contact, fabricated in accordance with an embodiment of the invention.

Fig.4

shows a sectional view of a heterojunction ABC solar cell that comprises a p-type silicon substrate with full-area emitter contact and point-contacted BSF, fabricated in accordance with an embodiment of the invention.

Fig.5

shows a sectional view of a heterojunction ABC solar cell that comprises an n-type silicon substrate with full-area emitter contact and point-contacted BSF, fabricated in accordance with an embodiment of the invention.

Fig.6

shows a sectional view of a heterojunction ABC solar cell that comprises a p-type silicon substrate with point-contacted emitter and point-contacted BSF, fabricated in accordance with an embodiment of the invention.

Fig.7

shows a sectional view of a heterojunction ABC solar cell that comprises an n-type silicon substrate with point-contacted emitter and point-contacted BSF, fabricated in accordance with an embodiment of the invention

Fig.8

shows a sectional view of a heterojunction ABC solar cell that comprises a p-type silicon substrate with point-contacted emitter and full-area BSF contact, fabricated in accordance with an embodiment of the invention.

Fig.9

shows a sectional view of a heterojunction ABC solar cell that comprises an n-type silicon substrate with point-contacted emitter and full-area BSF contact, fabricated in accordance with an embodiment of the invention.

Fig.10

shows sectional views of another heterojunction ABC solar cell that comprises a p-type silicon substrate with full-area emitter contact and full-area BSF contact, fabricated in accordance with an embodiment of the invention.

Fig.11

shows sectional views of another heterojunction ABC solar cell that comprises an n-type silicon substrate with full-area emitter contact and full-area BSF contact, fabricated in accordance with an embodiment of the invention.

Figure 1 shows a flow chart 100 illustrating a method of fabricating a heterojunction all-back-contact (ABC) solar cell according to an example embodiment. At step 102, an emitter film is formed on one side of a doped substrate, e.g. a silicon wafer. At step 104, a conductive interlayer, e.g. a transparent conductive oxide (TCO) layer, is formed on the deposited emitter film. At step 106, at least one channel is formed. The channel cuts through at least the emitter film and the conductive interlayer. At step 108, a continuous back surface field (BSF) film is formed on the deposited conductive interlayer and the channel formed. At step 110, a plurality of metallic grids is formed on the deposited BSF film, wherein at least a portion of at least one of the plurality of metallic grids is formed in the channel.

Figure 2 shows a sectional view of a heterojunction ABC solar cell 200 that comprises a p-type silicon substrate 202 with full-area emitter contact and full-area BSF contact, fabricated in accordance with an embodiment of the invention. The ABC solar cell 200 comprises a crystalline silicon substrate, represented by p-type base substrate 202. A passivation layer 204, (i.e. in the form of Aluminium Oxide, AlO _x), is formed on a front side 206 of the substrate 202. An optional full-area intrinsic layer (i-layer) 208, e.g. in the form of intrinsic a-Si:H or a-SiOx:H, is formed on a rear side 210 of the substrate 202. The formation of i-layer 208 on the rear side 210 may improve the rear passivation of the ABC solar cell 200. Subsequent carrier extracting layers are then formed on the i-layer 208.

A channel 212 may be formed by depositing a shadow mask (not shown) on the i-layer 208, covering a predetermined area. A full-area emitter film, represented as emitter layer 214, is subsequently formed on the i-layer 208. In the example embodiment, the emitter layer 214 is an n-type amorphous silicon, a-Si:H(n ⁺). This is followed by a formation of a full-area TCO layer 216 on the emitter layer 214. The shadow mask is then removed for forming a channel 212 that cuts through the emitter layer 214 and the TCO layer 216.

In another embodiment, the channel 212 may be formed without using a shadow mask. The full-area emitter layer 214 is formed on the i-layer 208. This is followed by the formation of the full-area TCO layer 216 on the emitter layer 214. The channel is subsequently formed by laser ablation process to remove a portion of at least the emitter layer 214 and TCO layer 216. It will be appreciated by a person skilled in the art that different methods may be used to form the channel 212.

After forming the channel 212, a full-area BSF film, represented as BSF layer 218, is formed on the deposited TCO layer 216 and the channel 212. In the example embodiment, the BSF layer 218 is a p-type amorphous silicon, a-Si:H(p ⁺). Next, a plurality of metallic grid, represented as emitter grid 220a and BSF grid 220b, are formed on the deposited BSF layer 218. As shown in Figure 2, the emitter grid 220a is formed outside a region of the channel 212. The BSF grid 220b is formed within the region of the channel 212. In an embodiment, the

metallic grid

220a, 220b may be formed by low temperature screen printing of interdigitating metal fingers. The process of forming the

metallic grids

220a, 220b using this method may require a rough degree of alignment.

Figure 3 shows a sectional view of a heterojunction ABC solar cell 300 that comprises an n-type silicon substrate 302 with full-area emitter contact and full-area BSF contact, fabricated in accordance with an embodiment of the invention. Here, the ABC solar cell 300 comprises an n-type base substrate 302. A passivation layer 304, (i.e. in the form of Silicon Nitride SiN _x), is formed on a front side 306 of the substrate 302. A full-area i-layer 308 is formed on a rear side 310 of the substrate 302. Similar with the solar cell as described above with respect to Figure 2, a channel 312 may be formed by depositing and removing a shadow mask (not shown) on the i-layer 308 or carrying out laser ablation process later in the fabrication process. A full-area emitter layer 314 is subsequently formed on the i-layer 308, In the example embodiment, the emitter layer 314 is a p-type amorphous silicon a-Si:H(p ⁺). This is followed by the deposition of the full-area TCO layer 316. A full-area BSF layer 318 is then formed on the TCO layer 316 and the channel 312. In the example embodiment, the BSF layer 318 is an n-type amorphous silicon, a-Si:H(n ⁺). The metallic grids, represented as emitter grid 320a and BSF grid 320b, are then formed on the deposited BSF layer 318.

Figure 4 shows a sectional view of a heterojunction ABC solar cell 400 that comprises a p-type silicon substrate 402 with full-area emitter contact and point-contacted BSF, fabricated in accordance with an embodiment of the invention. The ABC solar cell 400 comprises a p-type base substrate 402. In an embodiment, an i-layer (not shown) may be formed on a rear side 404 of the substrate 402. An ultrathin intrinsic buffer layer may be used as the i-layer for rear passivation of the solar cell 400.

A full-area emitter layer 406 and TCO layer 408 are subsequently formed on either the rear side 404 of the substrate 402 or the i-layer, if the i-layer has been formed on the rear side 404 of the substrate 402. The emitter layer 406 is an n-type amorphous silicon a-Si:H(n ⁺). A channel 410 may be formed by laser ablation process. As shown in Figure 4, the channel 410 cuts through a portion of the TCO layer 408, the emitter layer 406 and also the substrate 402.

An electrically insulating passivation layer, represented as BSF passivation layer 412, is subsequently formed on the TCO layer 408 and the channel 410. In the example embodiment, the BSF passivation layer 412 is in the form of aluminium oxide, AlO _x. Aluminium oxide, AlO _x exhibits a large amount of negative surface charges and is of opposite polarity as the base doping of the p-type substrate 402. Local openings, represented as BSF layer openings are formed, e.g. by laser ablation, thereby creating

openings

414a, 414b, 414c towards the conductive interlayer 408 contacting the emitter layer 406, and openings 414d inside the channel 410 towards the silicon substrate 402. The

openings

414a, 414b, 414c, 414d cut through the BSF passivation layer 412. A full-area of the BSF layer 416 is then formed on the BSF passivation layer 412. The BSF layer 416 is a p-type amorphous silicon a-Si:H(p ⁺). In other embodiments, the BSF layer 416 can also be a p-type microcrystalline silicon, μc-Si:H(p ⁺), as will be understood by persons skilled in the art. As can be seen in Figure 4, the BSF layer 416 covers both the

openings

414a, 414b, 414c towards the conductive interlayer, and the opening 414d inside the channel towards the silicon substrate, to form contacts to the full-area heterojunction emitter 406 (via the conductive interlayer 408) and to form BSF heterojunction point contacts respectively. Metallic grids, represented as emitter grid 418a and BSF grid 418b are then formed on the BSF layer 416.

In an embodiment, a laser scribing process may be carried out to separate the electron-extracting contacts and the hole-extracting contacts at the BSF layer 416, i.e. by removing a small area of the BSF passivation layer 416 in the area between the emitter grid 418a and the BSF grid 418b. However, the BSF layer 416 typically has low conductivity and low thickness (typically less than 30 nm). In addition, the distance between the

metallic grids

418a, 418b is relatively large (typically more than 50 μm) and therefore the BSF passivation layer 412 may also be simultaneously used to electrically insulate the electron-extracting contacts from the hole-extracting contacts of the solar cells, thus laser scribing may not be necessary. With the electrical insulation provided by the formation of BSF passivation layer 412, local internal shunting that is caused by contact of the BSF layer 416 and emitter layer 406 or TCO layer 408 may be avoided. As a result, metallic grids with equal width can be used for both the emitter grid 418a and BSF grid 418b.

Figure 5 shows a sectional view of a heterojunction ABC solar cell 500 that comprises an n-type silicon substrate 502 with full-area emitter contact and point-contacted BSF, fabricated in accordance with an embodiment of the invention. The solar cell 500 can be fabricated using the same process as the solar cell 400 described above with respect to Figure 4. However, the ABC solar cell 500 comprises an n-type base substrate 502. The emitter layer 506 in this embodiment is a p-type amorphous silicon a-Si:H(p ⁺) and the BSF layer 516 is an n-type amorphous silicon a-Si:H(n ⁺). The BSF passivation layer 512 is in the form of Silicon Nitride, SiN _x. Silicon Nitride, SiN _x exhibits a large amount of positive surface charges and is, in this embodiment, of opposite polarity as the base doping of the n-type substrate 502.

Figure 6 shows a sectional view of a heterojunction ABC solar cell 600 that comprises a p-type silicon substrate 602 with point-contacted emitter and point-contacted BSF, fabricated in accordance with an embodiment of the invention. Here, an electrically insulating passivation layer, represented as emitter passivation layer 604, is formed on a rear side 606 of the substrate 602. The emitter passivation layer 604 is in the form of Silicon Nitride, SiN _x. Local openings, represented as emitter

passivation layer openings

608a, 608b, 608c are formed, e.g. by laser ablation. The emitter

passivation layer openings

608a, 608b, 608c can be seen in Figure 6 cutting through the emitter passivation layer 604.

A full-area emitter layer 610 is subsequently formed on the emitter passivation layer 604, covering the emitter

passivation layer openings

608a, 608b, 608c to form emitter heterojunction point contacts. In the example embodiment, the emitter layer is an n-type amorphous silicon a-Si:H(n ⁺) or microcrystalline silicon, μc-Si:H(n ⁺). A full-area TCO layer 612 is then formed on the emitter layer 610, followed by formation of a channel 614 by laser ablation process.

An electrically insulating passivation layer, represented as BSF passivation layer 616, is subsequently formed on the TCO layer 612 and the channel 614. The BSF passivation layer 616 is in the form of aluminium oxide, AlO _x. Local openings, represented as BSF

passivation layer openings

618a, 618b, 618c, 618d, are formed by laser ablation.

Openings

618a, 618b, 618c, form at least one point contact to the conductive interlayer 612, thereby contacting the emitter layer 610, and opening 618d forms at least one back surface field point contact to the silicon wafer. A full-area of the BSF layer 620 is formed on the BSF passivation layer 616. The BSF layer 620 is a p-type amorphous silicon a-Si:H(p ⁺) or microcrystalline silicon, μc-Si:H(p ⁺) in the example embodiment.

Metallic grids

622a, 622b are then formed on the BSF layer 620.

Figure 7 shows a sectional view of a heterojunction ABC solar cell 700 that comprises an n-type silicon substrate 702 with point-contacted emitter and point-contacted BSF, fabricated in accordance with an embodiment of the invention. In contrast to the ABC solar cell 600 described above with respect to Figure 6, the ABC solar cell 700 comprises an n-type base substrate 702. In the example embodiment, the emitter passivation layer 704 is in the form of aluminium oxide, AlO _x and the emitter layer 710 is a p-type amorphous silicon, a-Si:H(p ⁺) or microcrystalline silicon, μc-Si:H(p ⁺). Further, the BSF passivation layer 716 is in the form of Silicon Nitride, SiN _x and the BSF layer 720 is an n-type amorphous silicon a-Si:H(n ⁺) or microcrystalline silicon, μc-Si:H(n ⁺).

Figure 8 shows a sectional view of a heterojunction ABC solar cell 800 that comprises a p-type silicon substrate 802 with point-contacted emitter and full-area BSF contact, fabricated in accordance with an embodiment of the invention. An emitter passivation layer 804, in the form of Silicon Nitride, SiN _x, is formed on a rear side 806 of the substrate 802. Emitter

passivation layer openings

808a, 808b, 808c are formed by laser ablation. A full-area emitter layer 810 is subsequently formed on the emitter passivation layer 804, covering the emitter

passivation layer openings

808a, 808b, 806c to form emitter heterojunction point contacts. The emitter layer 810 is an n-type amorphous silicon a-Si:H(n ⁺) or microcrystalline silicon, μc-Si:H(n ⁺). A full-area TCO layer 812 is then formed on the emitter layer 810, followed by formation of a channel 814 by laser ablation process. A full-area BSF layer 816 is then formed on the deposited TCO layer 812 and the channel 814. In the example embodiment, the BSF layer 816 is a p-type amorphous silicon, a-Si:H(p ⁺) or microcrystalline silicon, μc-Si:H(p ⁺).

Metallic grids

818a, 818b are then formed on the BSF layer 816.

As can be seen in Figure 8, the emitter passivation layer 804 for forming the point-contacted emitter does not insulate the emitter layer 810 from the BSF layer 816. In order to avoid local internal shunting caused by contact of BSF layer 816 and emitter layer 810 or TCO layer 812, laser scribing is carried out to form at least one groove 820 cutting the BSF layer 816, TCO layer 812 and emitter layer 810, separating the electron-extracting contacts 822 from the hole-extracting contacts 824 of the solar cells.

Figure 9 shows a sectional view of a heterojunction ABC solar cell 900 that comprises an n-type silicon substrate 902 with point-contacted emitter and full-area BSF contact, fabricated in accordance with an embodiment of the invention. In the example embodiment, the ABC solar cell 900 comprises an n-type base substrate 902. The emitter passivation layer 904 is in the form of aluminium oxide, AlO _x and the emitter layer 906 is a p-type amorphous silicon, a-Si:H(p ⁺) or microcrystalline silicon, μc-Si:H(p ⁺). Further, the BSF layer 916 is an n-type amorphous silicon a-Si:H(n ⁺) or microcrystalline silicon, μc-Si:H(n ⁺).

Figures 10 and 11 show sectional views of another heterojunction ABC solar cells 1000 with full-area emitter contact and full-area BSF contact, fabricated in accordance with an embodiment of the invention. In the solar cells as described with respect to Figures 1 and 2, there may be local internal shunting at the point of contact between the BSF layer and TCO layer or emitter layer. The voltage may be tied to the emitter voltage since the TCO layer is much more conductive laterally as compared to the BSF layer. There may be a voltage drop between the BSF layer and the substrate which may cause some current leakage, thereby affecting the current-voltage characteristics of the solar cell in the same way as a parasitic shunt element.

The TCO layer 1016 may be formed in such a way that the edge of the TCO layer 1016 is at a distance from the intersecting point of the emitter layer 1014 and BSF layer 1018, as shown in Figures 10 and 11. As a result, the BSF layer 1018 may have the same voltage as the base substrate 1002 at all points of contact. This may significantly reduce current leakage. In order to form the TCO layer 1016 in the solar cell 1000, after the formation of the emitter layer 1014, an additional shadow mask may be used to form a wider channel for the TCO layer 1016.

Embodiments of the present invention provide a method of fabricating a heterojunction all-back-contact (ABC) solar cell. The process of patterning the layers can be completed by shadow mask deposition or laser-assisted ablation which does not require stringent alignment tolerances. Further, a conductive interlayer is formed between the emitter and BSF film. As a result, various steps in fabricating ABC solar cell that involve aligned patterning and sacrificial mask formation/removal may be avoided. In other words, all of the layers, including emitter/BSF layers and passivation layer, can be full-area deposited and none of those have to be masked and subsequently removed in the fabrication process of the solar cell.

The solar cell fabricated using the method disclosed in the embodiments comprises only full-area or point-contacted heterojunction contacts at the junctions between the crystalline silicon substrate and the emitter/BSF layer. There is also no diffused area underneath the contacts. Thus, the solar cell may reach higher open-circuit voltage as compared to the conventional homojunction point contact due to reduced contact and bulk recombination.

It will be appreciated by a person skilled in the art that numerous variations and/or modifications may be made to the present invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects to be illustrative and not restrictive.

Claims

A method of fabricating a heterojunction all-back-contact (ABC) solar cell, the method comprising the steps of:
forming an emitter film on one side of a doped substrate;
forming a conductive interlayer on the deposited emitter film;
forming at least one channel, the channel cutting through at least the emitter film and the conductive interlayer;
forming a continuous back surface field film on the deposited conductive interlayer and the channel formed; and
forming a plurality of metallic grids on the deposited back surface field film, wherein at least a portion of at least one of the plurality of metallic grids is formed in the channel.
The method as claimed in claim 1, wherein the conductive interlayer comprises a transparent conductive oxide (TCO) layer.
The method as claimed in claim 1 or 2, wherein forming at least one channel comprises the steps of:
depositing at least one shadow mask on the one side of the base substrate; and
removing the at least one shadow mask, wherein removing the shadow mask forms the channel.
The method as claimed in claim 1 or 2, wherein forming at least one channel comprises using laser ablation.
The method as claimed in any one of the preceding claims, wherein forming an emitter film comprises the steps of:
forming an emitter passivation layer on the one side of a base substrate;
forming a set of emitter passivation layer openings, the set of emitter passivation layer openings cutting through the emitter passivation layer; and
forming a full-area emitter layer on the emitter passivation layer, the emitter layer covers the set of emitter openings to form at least one emitter point contact.
The method as claimed in any one of the preceding claims, wherein forming a continuous back surface field film comprises the steps of:
forming a back surface field passivation layer on the deposited conductive interlayer and the channel formed;
forming a set of back surface field passivation layer openings, the set of back surface field passivation layer openings cutting through the back surface field passivation layer; and
forming a full-area back surface field layer on the back surface field passivation layer, the back surface field layer covers the set of back surface field passivation layer openings, thereby forming at least one back surface field point contact to the silicon wafer and forming at least one point contact to the conductive interlayer, thereby contacting the emitter layer.
The method as claimed in any one of claim 5 or 6, wherein the emitter layer and/or the back surface field layer comprises a doped amorphous silicon layer.
The method as claimed in any one of claims 5 to 7, wherein the emitter passivation layer and/or back surface field passivation layer comprises one selected from a group consisting of aluminium oxide (AlO _x) and silicon nitride (SiN _x).
The method as claimed in any one of claims 5 to 8, further comprising the step of forming at least one groove, the groove cutting through the back surface field layer, the conductive interlayer and emitter layer.
A solar cell manufactured using the method as claimed in any one of the preceding claims.