EP4526930A1

EP4526930A1 - Localized passivated contacts for solar cells

Info

Publication number: EP4526930A1
Application number: EP23725464.4A
Authority: EP
Inventors: Paul Alejandro PROCEL MOYA; Olindo ISABELLA
Original assignee: Technische Universiteit Delft
Current assignee: Technische Universiteit Delft
Priority date: 2022-05-17
Filing date: 2023-05-15
Publication date: 2025-03-26
Also published as: WO2023224471A1; NL2031897B1

Abstract

The present invention is in the field of a semiconductor device sensitive to light, and specially adapted for the conversion of the energy of such radiation into electrical energy, in particular a silicon hetero-junction solar cell, or photovoltaic (PV) cell, a method for the manufacture thereof, and details thereof. Said solar cells comprise at least one hetero junction and typically two hetero junctions.

Description

Localized passivated contacts for Solar Cells

FIELD OF THE INVENTION

The present invention is in the field of a semiconductor device sensitive to light, and specially adapted for the conversion of the energy of such radiation into electrical energy, in particular a silicon hetero-j unction solar cell, or photovoltaic (PV) cell, a method for the manufacture thereof, and details thereof. Said solar cells comprise at least one heterojunction and typically two heterojunctions.

BACKGROUND OF THE INVENTION

A solar cell, or photovoltaic (PV) cell, is an electrical device that converts energy of light, typically sun light (hence “solar”), directly into electricity by the so-called photovoltaic effect. The solar cell may be considered a photoelectric cell, having electrical characteristics, such as current, voltage, resistance, and fill factor, which vary when exposed to light and which vary from type of cell to type.

Solar cells are described as being photovoltaic irrespective of whether the source is sunlight or artificial light. They may also be used as photo detector.

When a solar cell absorbs light it may generate either electron-hole pairs or excitons. In order to obtain an electrical current charge carriers of opposite types are separated. The separated charge carriers are “extracted” to an external circuit, typically providing a DC-current. For practical use, a DC-current may be transformed into an AC-current, e.g. by using a transformer. Typically solar cells are grouped into an array of elements. Various elements may form a panel, and various panels may form a system.

Wafer-based c-Si solar cells contribute to more than 90% of the total PV market. According to recent predictions, this trend will remain for the upcoming years and many years beyond. Due to their simplified process, conventional c-Si solar cells dominate a large part of the market. As an alternative to the industry to improve the power to cost ratio, the silicon heterojunction approach has become increasingly attractive for the PV industry, even though the relatively complicated process to deploy the proper front layers, such as a transparent conductive oxide (TCO) and an inherent low thermal budget of the cells limiting usage of existing production lines and thus result in a relatively small market share so far. A heterojunction is an interface that occurs between two layers or regions of dissimilar crystalline semiconductors. These semiconducting materials have unequal band gaps as opposed to a homojunction. A homojunction relates to a semiconductor interface formed by typically two layers of similar semiconductor material, wherein these semiconductor materials have equal band gaps and typically have a different doping (either in concentration, in type, or both). A common example is a homojunction at the interface between an n-type layer and a p-type layer, which is referred to as a p-n junction. In heterojunctions advanced techniques are used to precisely control a deposition thickness of layers involved and to create a lattice-matched abrupt interface. Three types of heterojunctions can be distinguished, a straddling gap, a staggered gap, and a broken gap.

A disadvantage of solar cells is that the conversion per se is not very efficient, typically, for Si-solar cells, limited to somewhat above 20%. Theoretically a single p-n junction crystalline silicon device has a maximum power efficiency of 32%. An infinite number of layers may reach a maximum power efficiency of 86%. The highest ratio achieved for a solar cell per se at present is about 44%. For commercial silicon solar cells, the record is about 26.7% (an interdigitated back-contacted silicon heterojunction solar cell). In view of efficiency, the front contacts may be moved to a rear or back side, eliminating shaded areas. In addition, thin silicon films were applied to the wafer. Solar cells also suffer from various imperfections, such as recombination losses, reflectance losses, heating during use, thermodynamic losses, shadow, internal resistance, such as shunt and series resistance, leakage, etc. A qualification of performance of a solar cell is the fill factor (FF). The fill factor may be defined as a ratio of an actual maximum obtainable power to the product of the open-circuit voltage and short-circuit current. It is considered to be a key parameter in evaluating performance. A typical advanced commercial solar cell has a fill factor > 0.75, whereas less advanced cells have a fill factor between 0.4 and 0.7. Cells with a high fill factor typically have a low equivalent series resistance and a high equivalent shunt resistance; in other words, less internal losses occur. Efficiency is nevertheless improving gradually, so every relatively small improvement is welcomed and of significant importance.

At present a solar cell having a full area front passivating contact is not attractive, such as due to highly absorptive materials used to build such a structure. That is the case of heavily doped poly-silicon and a-Si layers. In a poly-silicon case, the process requires a very thin polysilicon film for minimizing parasitic absorption loss, and in case of a-Si, the process requires e.g. an extra transparent conductive oxide (TCO) layer for supporting the carrier lateral transport.

In particular, the electron transport layer and/or hole transport layer hinder the conversion of light into electrical power. In addition, to form such a transport layer, typically extra process steps in the manufacture of a solar cell are involved, such as a doping step, and the deposition of the transport layer, and possibly even (intermediate) cleaning steps, as well as extra or multi-chamber process tools, typically deposition tools. In view of cross-contamination between tools, it is typically better to use separate tools and/or multi-chamber tools. Extra tools or multi-chamber tools in addition add to the cost of production as more of the expensive clean-room surface is occupied thereby.

Solar cells deliver power electricity from sun light. It is noted that the efficiency of this PV-process usually is limited by material properties and the amount of light arriving to the absorber bulk. Normally, for low temperature processing of high efficiency solar cells, light management is limited by parasitic absorption of the light in front and rear layers. The reason for such parasitic absorption is that collecting layers are designed to cover bulk interfaces, to support the collection of charge. Indeed, such layers collect efficiently the charge inside the solar cell in spite of limiting the amount of absorbed light in the absorber bulk. Moreover, such collecting layers use materials that are not abundant in earth, such as indium.

Reference can be made to US2017162725A1 which recites a solar cell which includes a front side for light incidence, an opposite back side, a crystalline semiconductor substrate of a first or second conductivity type, a front side passivating region with a passivating layer and a conductive layer of the first type, a back side passivating region with a passivating layer and a conductive layer of the second type, a front side contact with one front side conductive material and front side electrical contacts on the front side conductive material, a front side light coupling layer on the front side, a back side contact opposite the front side contact and formed by back side conductive material and a back side electrical contact thereon. The front side has lower light absorption and better antireflective property. The front side conductive material is thinner in regions between and/or besides front side electrical contacts than in regions below front side electrical contacts. US2019057792A1 recites an electrically conductive composition for use in the preparation of an electrically conductive network, said composition comprising, based on the total weight of the composition: a) from 75 to 98 wt. % of a silver powder having a tap density of at least 4.0 g/cm³ and a specific surface area of less than 1.5 m2/g; b) from 1 to 10 wt. % of a binder resin; c) from 0 to 5 wt. % of a hardener; and, d) from 0 to 10 wt. % of solvent, wherein said composition is characterized in that, when heated to a temperature at which the silver powder starts to sinter, the binder resin is not yet fully cured or fully solidified. US2020168759A1 recites a method for manufacturing a substrate with a transparent conductive film, which includes: preparing a base body that has a top surface and a back surface and has an a-Si film coating at least one of the top surface and the back surface; and setting temperatures of the base body and the a- Si film to be in the range of 70 to 220° C. in a film formation space having a processing gas containing hydrogen, applying a sputtering voltage to a target, carrying out DC sputtering, and thereby forming the a-Si film on a transparent conductive film. CN10678404A recites a silicon-based heterojunction solar cell and a preparation method thereof. The silicon-based heterojunction solar cell comprises a crystalline silicon substrate, a first intrinsic amorphous layer, a second intrinsic amorphous layer, a first doping layer, a second doping layer, a first transparent conductive layer and a second transparent conductive layer, wherein the first transparent conductive layer and/or the second transparent conductive layer is a transparent conductive lamination formed by a metal film layer and a dielectric film layer; the metal film layer is a silver film layer, a gold film layer, an aluminum film layer, a copper film layer, a chromium film layer, a molybdenum film layer, a tungsten film layer, a niobium film layer or a film layer of other alloy material. The transparent conductive lamination herein has lower square resistance than a traditional transparent conductive oxide film layer, has higher reflectivity for medium to far infrared, and the performance of the silicon-based heterojunction solar cell can be improved. US2018138334A1 recites a method for manufacturing a solar cell. The method includes: forming a first tunneling layer on one surface of a semiconductor substrate; forming a first conductive region on the first tunneling layer so that the first conductive region includes a metal oxide layer having an amorphous structure; and forming a first electrode electrically connected to the first conductive region. US2017338356A1 recites a system for fabrication of a photovoltaic structure. The system can include a physical vapor deposition tool configured to sequentially deposit a transparent conductive oxide layer and a metallic layer on an emitter layer formed in a first surface of a Si substrate, without requiring the Si substrate to be removed from the physical vapor deposition tool after depositing the transparent conductive oxide layer. The system can further include an electroplating tool configured to plate a metallic grid on the metallic layer and a thermal annealing tool configured to anneal the transparent conductive oxide layer.

The present invention relates to an improved heterojunction solar cell and various aspects thereof and a simplified process for manufacturing the solar cell which overcomes one or more of the above disadvantages, without jeopardizing functionality and advantages.

SUMMARY OF THE INVENTION

The present invention relates in a first aspect to a solar cell, in particular a het- ero-junction silicon solar cell according, typically a front and rear contacted solar cell, and in a second aspect to a method for making such a solar cell. In this invention, a design of solar cells is provided using localized layers to collect efficiently the charge, and allowing the light to freely arrive to the light absorbing bulk. Furthermore, the present solar cells make use of collecting layers localized in a fraction of the bulk interface, with a reduction of the parasitic absorption. Additionally, it is an option to reduce the use of materials during the manufacture of the solar cells. In particular, the light collecting layers use materials that are not abundant in earth, such as indium. Inventors present a design of solar cells using localized layers to collect efficiently the charge and allowing the light to freely arrive to the absorber bulk. Furthermore, as the proposed design devise the use of collecting layers are localized to a small part of the bulk interface, it is a clear option to reduce and eventually avoid the consumption of materials during the processing. The present invention is in particular feasible with photovoltaic industry based in low temperature process for Silicon solar cells.

The invention involves the removal of a substantial part of the transparent conductive oxide (TCO) layer, in particular both the front and rear side TCO layer. More in particular the part of the TCO layer which is not underneath a contact or contact layer is removed. “Removal” is in view of typical prior art solar cells, which have a full TCO-layer. In the present invention the TCO needs only to be provided underneath the respective contact. The contact is considered to be an electrical contact, typically for flowing electrons externally away from the solar cell, such as to a converter, or to a power grid. The remainder of the front surface area or back surface area provides (direct) access for light to at least one light absorbing layer. In particular, the remainder of the front surface area or back surface area provides direct access to at least one light absorbing layer and is substantially free of other layers. In particular, the remainder of the front surface area or back surface area provides direct access to at least one light absorbing layer and is substantially free of other layers, apart from an optional antireflective coating or optional surface treated layer. The at least one contact, in particular a metal contact, more in particular a contact layer, and the TCO layer form a stack, optionally incorporating the doped silicon layer in the stack as well; the stack has a relatively small width, of all layers in the stack, such that only a small fraction of the respective surface is covered with the stack, the remainder of the surface being substantially free of other layers, part from the optional antireflective coating layer and/or optional surface treated layer. A stack is considered to relate to an orderly pile of layers, having a substantially similar width, in the present case, a relative narrow width. The present stack may be considered to be or form the electrical contact. A full TCO-layer can be provided, and then mostly removed, such as by etching. The removed TCO can be reused. Or, a mask or the like can be used to provide the TCO layer only partially on the surface available, and not on the surface occupied by the exemplary mask. Therewith, from the beginning less TCO-material is used. Also the performance of the obtained solar cell is comparable to prior art solar cells, albeit less material is used. In addition to the substantial absence of the TCO also the doped silicon layer may be likewise substantially absent. In addition, the TCO can be made even thinner. Therewith also less dopants are to be used. A proof-of-concept SHJ solar cell exhibits a VOC of >700 mV, a JSC of about 40 mA/cm², a FF of >80%, and an efficiency of >23%.

So due to the substantial removal of the TCO and optional doped silicon layers in the prior art structure of SHJ solar cells, the present invention clearly allows a simpler process, cost reductions, and material savings. Cost reductions come from both materials saving during the device manufacturing, and also simplification of fabrication tools, in particular the Plasma-Enhanced Chemical-Vapor-Deposition (PECVD). This invention is furthermore industrial relevant due to minimal use of scarcely available materials, such as In.

In the prior art, optimization of the design of low temperature processed solar cell are twofold: i) enhancing vertical transport of charge and ii) enhancing lateral transport of charge. It is well-known that lateral transport of charge is supported by collecting layers. However, the present solar cell makes use of such layers in a localized part of the interface. It enables more light to arrive inside the bulk. Therefore, the present inventive concept is different or even contrary to the common vision in the photovoltaic research. Typically the PV- research community is focusing efforts e.g. on achieving ultra-thin collecting layers to reduce the parasitic absorption in solar cells. This layers are typically still covering the complete absorbing bulk interface. Therefore, achieving such a level of minimized parasitic absorption is challenging from a processing point of view. On the contrary, the present inventors provide this layer localized only beneath metal contacts. Accordingly, it allows some flexibility regarding processing, for instance the thickness of collecting layers that has less impact in the illuminated area. The present invention is therefore better than available alternatives, as it uses less material and allows more flexibility in the manufacture of a solar cell. Indeed, thanks to the present invention, the use of ultra-thin layers is no longer a limitation for improving the performance of high efficiency solar cells. For implementing this invention the use of hard masks and its alignment is considered. Alignment may be relative to the present solar cell, or details thereof, such as alignment markers, may be relative to the hard mask, may be relative to an equipment used, in particualr a lithographic equipment, and so on, and combinations thereof. The impact of the present invention may be expressed in terms of less material consumption and the provision of more efficient solar cells, such as with a higher charge current density.

In the present solar cells either front and rear (also indicated as back) contacts, or both, may be present. The present solar cell comprises at least one heterojunction and typically two heterojunctions. The heterojunctions may be of staggered type. The present solar cells comprise a substrate (10), wherein the substrate typically comprises Si and dopants, directly on the substrate, at least one intrinsic layer (11), directly on the intrinsic layer a doped silicon layer, which may or may not occupy the full surface area of the intrinsic layer, and on the doped silicon layer a transparent conductive oxide (TCO)-layer (12), which may or may not occupy the full surface area of the doped silicon layer. The doped silicon layer may function as electron transport layer [ETL], or as hole transport layer [HTL], respectively. Also the transparent conductive oxide (TCO)-layer (12) provides electron transport or hole transport, respectively, and a metal contact (15) in electrical contact with the TCO-layer. The substrate is typically a crystalline Si-substrate, such as a n-doped or p-doped crystalline Si layer 10, typically of 50 pm-300 pm thickness. The layers of the stack are typically provided directly on one and another, that is, without an intermediate layer, or in any case an intermediate layer of significance to the performance of the heterojunction solar cell.

In a second aspect the present invention relates to a method of producing a front-side and back-side contacted silicon hetero-j unction solar cell (100) according to the invention, comprising providing a substrate, such as a crystalline Si-substrate, optionally texturing the substrate, such as double-side texturing the substrate, thereafter immersing the substrate into a strong oxidizing solution, thereafter etching the oxidized substrate by dipping the oxidized substrate into an acidic solution, directly thereafter loading the etched substrate into a layer deposition tool, and depositing an intrinsic Si layer on at least one side of the etched substrate, thereafter providing at least one doped silicon layer on the at least one intrinsic Si- layer, in particular a layer that covers less than 20% of the front surface area or back surface area, respectively, thereafter depositing a transparent conductive oxide(TCO) layer on the at least one doped Si-layer, in particular after a first alignment of the solar cell, and then depositing metal contacts on the TCO-layer, in particular after a second alignment of the solar cell, more in particular wherein the first and second alignment each individually is with an accuracy of better than 20 pm lateral, such as better than 1 pm lateral.

In summary, the present invention provides a simplified fabrication process wherein solar cell precursors can be finished within a couple of steps, and which is a low cost and high throughput process, using compatible industrial standard metallization steps, solar cells featuring a high Voc, solar cells featuring a high Jsc & Voc, solar cells featuring a relatively high fill factor (FF), and wherein the design is applicable to both a front/rear contacted conventional solar cell architecture, a bifacial solar cell architecture, for interdigitated back-contacted (IBC) solar cells, and for both n- type and p-type bulk material.

Thereby the present invention provides a solution to one or more of the above mentioned problems.

Advantages of the present description are detailed throughout the description. References to the figures are not limiting, and are only intended to guide the person skilled in the art through details of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates in a first aspect to a silicon solar cell, and in a second aspect to a process for making such a solar cell.

Where multiple layers or the like of a similar or same material are present, characteristics of said layers apply to each layer individually. Also “bottom” and “top”, or “front” and “back” are relative terms, which terms may be interchanged in so far as applicable.

In an exemplary embodiment of the present solar cell the substrate is covered with at least one intrinsic layer 11, such as one intrinsic layer at the rear side, and one intrinsic layer at the front side, in particular wherein the at least one intrinsic layer 11 each individually is selected from intrinsic Si, such as (i)a-Si:H and (i)nc-Si:H, from intrinsic Si-di electrics, such as (i)a-SiO_x:H, (i)a-SiC_x:H, and (i)a-SiNx:H, or dielectric metal oxide passivation layer, and combinations thereof.

In an exemplary embodiment of the present solar cell the thickness of the intrinsic layer each individually is from 0.1 nm-50 nm, in particular 1-20 nm, such as 2-15 nm.

In an exemplary embodiment of the present solar cell the intrinsic layer each individually is textured, in particular with a same texturing as the substrate.

In an exemplary embodiment the present solar cell comprises at least one doped silicon layer 13,14, such as a p-doped silicon layer 13, and an n-doped silicon layer 14, in particular comprising a 5*10¹⁴-0.5*10¹⁹ dopants/cm³ n- or p-type doped crystalline Si layer 13,14, and/or wherein a doping concentration is preferably spatially constant.

In an exemplary embodiment of the present solar cell n-type dopants are selected from P, As, Bi, Sb and Li, and wherein p-type dopants are selected from B, Ga, and In.

In an exemplary embodiment of the present solar cell the doped silicon layer is provided in between the at least one contact and substrate.

In an exemplary embodiment of the present solar cell the doped silicon layer substantially covers the same surface area and the same amount of surface area as the at least one contact.

In an exemplary embodiment of the present solar cell the at least one contact is provided on a TCO layer 12, in particular wherein the material of the transparent conductive layer 12 is selected from Indium Tin Oxide (ITO), IOH, ZnO, or doped ZnO, such as Aluminium doped ZnO, doped Tin oxide, such as fluorine doped tin oxide, doped indium oxide, such as Indium Fluor Oxide (IFO:H), and Indium Tungsten Oxide (IWO).

In an exemplary embodiment of the present solar cell a thickness of the transparent conductive layer 12 is 10-200 nm, in particular 20-170 nm, more in particular 30-50 nm.

In an exemplary embodiment of the present solar cell the refractive index of the transparent conductive layer 12 is <2.2.

In an exemplary embodiment of the present solar cell the work function of the TCO layer 12 is from 2 eV to 8 eV, in particular 3.4 eV to 6.4 eV.

In an exemplary embodiment of the present solar cell the work function of the TCO layer 12 is 3.4 eV to 4.7 eV in case of the TCO-layer mainly transporting electrons.

In an exemplary embodiment of the present solar cell the work function of the TCO layer 12 is 4.7 eV to 6.4 eV in case of the TCO-layer mainly collecting holes.

In an exemplary embodiment of the present solar cell the TCO layer each individually is textured, in particular with a same texturing as the substrate.

In an exemplary embodiment of the present solar cell the contact, in particular a metal contact, more in particular a contact layer, the TCO layer, and optionally the doped silicon layer form a stack, in particular a stack of substantially the same shaped layers, more in particular wherein a width of the doped silicon layer (HTL or ETL) > a width of the TCO layer, which width of the TCO layer > width of the contact layer. The width of an underlying layer may be slightly larger than the width of the layer directly on top thereof, such as 1-10% wider, such as 2-5% wider. In a preferred embodiment of the present solar cell, the contact, in particular a metal contact, more in particular a contact layer, the TCO layer, and the doped silicon layer form a stack, in particular a stack of substantially the same shaped layers. The respective widths in the stack, or in general of layers on top of one and another, may therefore have substantially the same width. A width of the layer lines each individually may be from 1-300 pm, such as 20-200 pm.

In an exemplary embodiment of the present solar cell a remainder of the front surface area or back surface area provides access to at least one light absorbing layer and is substantially free of other layers, apart from an optional antireflective coating and/or optional surface treated layer

In an exemplary embodiment of the present solar cell the at least one contact is provided as a strip, in particular at least one of a circular shaped strip, a multigonal shaped strip, a rectangular strip, and a spiral shaped strip, more in particular as a strip with a width of 0.01-200 pm, in particular a width of 0.05-50 pm, such as 1-5 pm.

In an exemplary embodiment of the present solar cell the at least one light absorbing layer is surface treated, in particular wherein the treatment is selected from treatment with a gas, and treatment with a plasma, more particular wherein in the treatment hydrogen is used, or wherein CO2 is used, or wherein in the treatment oxygen is used, even more in particular wherein substantially only hydrogen is used, or wherein substantially only oxygen is used. In an example a pressure of 1.1 to 4.4 mbar, a power of 55-90 mW/cm², and a temperature of 160-200 °C may be used, each individually. Preferred values are a pressure of 2.0-2.2 mbar, a power of 80-90 mW/cm², and a temperature of 180-185 °C. The surface treatment is aimed at changing the surface of the layer, in particular of the doped silicon layer or the intrinsic silicon layer respectively. It provides a better condition/passivation of the absorber bulk. And also it provides a better interface for the application of ARC layer (s)

In an exemplary embodiment of the present solar cell the substrate 10 is a single sided or double sided flat substrate 10 surface.

In an exemplary embodiment of the present solar cell the substrate 10 is a single sided or double sided textured substrate 10 surface (ISO 4287: 1997), in particular textured with a surface roughness Ra of 1-20 pm, such as 2-10 pm.

In an exemplary embodiment of the present solar cell the textured surface has an aspect ratio (height depth of a textured structure) of 2-10.

In an exemplary embodiment of the present solar cell the substrate has a thickness of 1-500 pm.

In an exemplary embodiment of the present solar cell comprising 10¹⁴-10¹⁷ do- pants/cm³ n- or p-type doped substrate 10.

In an exemplary embodiment of the present solar cell the substrate 10 comprises 1 * 10¹²-l .0* 10²¹ n- or p-type dopants/cm³, in particular 2*10¹⁴-10¹⁸ dopants/cm³, more in particular 5*10¹⁴-10¹⁶ dopants/cm³, such as 8*10¹⁴-3*10¹⁵ dopants/cm³.

In an exemplary embodiment of the present solar cell the substrate 10 has a resistivity of 0.1-1000 ohm*cm at 300K, more in particular 1-100 ohm*cm, such as 5-10 ohm* cm.

In an exemplary embodiment of the present solar cell the at least one contact covers less than 5% of the front surface area or back surface area, in particular wherein the at least one contact covers less than 1% of the front surface area or back surface area, more in particular less than 0.5%.

In an exemplary embodiment of the present solar cell a width of the doped silicon layer and a width of the TCO layer and a width of the contact layer each individually are from 1 pm-300 pm, in particular from 20 pm-200 pm, such as from 80 pm-150 pm.

In an exemplary embodiment of the present solar cell the contact comprises a metal, wherein the metal of the contacts independently comprises at least one of Cu, Al, W, Ti, Ni, Cr, Ag.

In an exemplary embodiment of the present solar cell a thickness of said metal contacts 13 is 200 nm-50 pm, in particular 1-25 pm.

In an exemplary embodiment of the present solar cell the metal contact 13 is selected from a metal layer, a metal grid, a metal line, or a combination thereof.

In an exemplary embodiment of the present solar cell at least one of the front surface and the back surface is provided with an anti -reflective coating 16, in particular an anti- reflective coating 16 on the surface area not covered by the contact.

In an exemplary embodiment of the present solar cell the layer underneath the anti- reflective coating is surface treated 17, such as surface treated with H2, with O2, or a combination thereof. Optionally further chemical species may be present, either reactive species, or inactive species, such as nitrogen, or He.

In an exemplary embodiment of the present solar cell the Voc is >700 mV, in particular > 725 mV, such as > 730 mV.

In an exemplary embodiment of the present solar cell a J_sc is > 30 mA/cm², in particular > 38 mA/cm², such as > 39 mA/cm².

In an exemplary embodiment of the present solar cell a fill factor (FF) of >75%, in particular > 80%, such as > 82.5%.

In an exemplary embodiment the present solar cell has an efficiency of > 23%, in particular >23.8%, such as > 23.9%.

In an exemplary embodiment of the present solar cell the solar cell is a back-contacted solar cell, such as an interdigitated back-contacted solar cell, or wherein the solar cell is a back and front contacted solar cell.

In an exemplary embodiment of the present method deposition of the TCO-layer and/or the metal contacts and optionally provision of the doped silicon layer a hard mask is used. For forming a plurality of solar cells these are typically provided in a tray or pod. Such a tray may house tens of wafers. These wafers are then subsequently treated through various process steps, such as deposition of a layer. For PECVD deposition a metal sheet could be used, considered to be a hard mask. The hard mask is carefully aligned with the wafer, or likewise, the solar cell being formed in the relevant process step. For alignment, at least one alignment marker may be used, and typically four markers are used. Using optical means the mask and the wafer can be aligned precisely, typically with an accuracy of 20 um or better, and using advance optical techniques an accuracy of 1 um or even 100 nm can be obtained relatively easy. It is noted that such an accuracy is (more) than sufficient. The hard mask may be provided with ad-hoc engineered, laser-cut openings, which openings provided deposition of the relevant layer on the relevant part of the surface area, and prevent deposition elsewhere, such as with the present contact layer, with the present TCO layer, and optionally with the present doped layer. The mask can be cleaned regularly such that the deposited material on the mask can be recovered, and reused. As such these localized layers (doped lines, TCO lines, and metal lines) could be deposited, among other techniques, also via hard mask.

In an exemplary embodiment of the present method contacts and/or contact layers are provided by metal deposition and lift off of non-contact areas, screen printing, and electrical plating.

The present invention therefore relates to a silicon hetero-j unction solar cell (100) as claimed and/or obtained by the method as claimed, comprising at least two elements as mentioned in the claims, and/or comprising at least one further element as mentioned in the description.

The invention is further detailed by the accompanying figures and examples, which are exemplary and explanatory of nature and are not limiting the scope of the invention. To the person skilled in the art it may be clear that many variants, being obvious or not, may be conceivable falling within the scope of protection, defined by the present claims.

SUMMARY OF FIGURES

Figure la,b shows an example of a prior-art solar cell.

Figures 2a,b-3a,b show a schematic representation of an example of the present solar cell.

Fig. 4 shows experimental results.

DETAILED DESCRIPTION OF FIGURES

In the figures :

100 hetero-junctions Si-solar cell

10 Si-substrate, typically crystalline,

11 intrinsic silicon layer

12 transparent conductive oxide layer

13 p-doped silicon layer

14 n-doped silicon layer

15 contacts or contact layer

16 anti -reflective coating layer

17 surface treated layer

Various exemplary embodiments of the present solar cell are detailed below.

The solar cell of fig. 1 relates to a prior art solar cell. Fig. la shows a typical cross-section comprising a crystalline silicon substrate 10, on both sides thereof an intrinsic a-Si:H layer 11, a p-doped 13 or n-doped layer 14 on the intrinsic layer, respectively, a TCO layer 12 covering the doped layer, and metal contacts 15 on the TCO-layer. Fig. lb shows a perspective view of the top layers. The bottom layers are formed likewise.

The solar cells of figs. 2-3 relate to the present inventive solar cell. Fig. 2a shows a typical cross-section comprising a crystalline silicon substrate 10, on both sides thereof an intrinsic a-Si:H layer 11, a p-doped 13 or n-doped layer 14 on the intrinsic layer, respectively, a TCO layer 12 only partially covering the doped layer, and metal contacts 15 on the TCO-layer. Fig. 2b shows a perspective view of the top layers. The bottom layers are formed likewise. Optionally an anti -reflective coating 16 may be present. Before providing the ARC-layer the underlying layer, in this case the intrinsic layer, may be surface treated 17.

Fig. 3a shows a typical cross-section comprising a crystalline silicon substrate 10, on both sides thereof an intrinsic a-Si:H layer 11, a p-doped 13 or n-doped layer 14 only partially provided on the intrinsic layer, respectively, a TCO layer 12 only partially covering the doped layer, and metal contacts 15 on the TCO-layer. Fig. 2b shows a perspective view of the top layers. The bottom layers are formed likewise. Optionally an anti -reflective coating 16 may be present. Before providing the ARC- layer the underlying layer, in this case the intrinsic layer, may be surface treated 17.

The above types of solar cell are indicative of the versatile design of the present invention.

J_sc (mA/cm²) Voc (V) FF(-) r](%)

Baseline (Fig. 1) 39.54 0.732 0.851 24.64

W/O TCO (Fig. 2) 39.59 0.732 0.828 23.98

W/O TCO&doped layer(Fig. 3) 39.94 0.733 0.836 24.45

So for the J_sc a slightly higher J_sc for solar cells W/O front layers is obtained. The difference can be more apparent when working with an ARC. For the V_oc no changes are found with respect to a baseline solar cell. For the fill factor (FF) the baseline device has a higher FF, which is considered due to more surface for the current of collecting carriers. However, the present solar cell structures are more sensitive to interface transport. For the efficiency p almost the same p is found. It is noted that there is a trade-off of FF and Jsc for new devices.

The invention although described in detailed explanatory context may be best understood in conjunction with the accompanying figures.

It should be appreciated that for commercial application it may be preferable to use one or more variations of the present system, which would similar be to the ones disclosed in the present application and are within the spirit of the invention.

Claims

1. A solar cell (100), in particular a heterojunction solar cell, comprising a substrate (10), in particular wherein the substrate comprises silicon, more in particular crystalline Si, at least one P-N junction, in particular a heterojunction, and at least one contact (15) selected from a front contact and a back contact, wherein the at least one contact (15) is provided on a TCO layer (12) characterized in that the at least one contact, in particular a metal contact, more in particular a contact layer, and the TCO layer form a stack, and the at least one contact covers less than 20% of the front surface area or back surface area, respectively, and wherein a remainder of the front surface area or back surface area provides access of light to at least one light absorbing layer and is substantially free of other layers.

2. The solar cell according to claim 1, wherein the substrate is covered with at least one intrinsic layer (11), such as one intrinsic layer at the rear side, and one intrinsic layer at the front side, in particular wherein the at least one intrinsic layer (11) each individually is selected from intrinsic Si, such as (i)a-Si:H and (i)nc-Si:H, from intrinsic Si-dielectrics, such as (i)a- SiO_x:H, (i)a-SiC_x:H, and (i)a-SiNx:H, or dielectric metal oxide passivation layer, and combinations thereof, and/or wherein the thickness of the intrinsic layer each individually is from 0.1 nm-50 nm, in particular 1-20 nm, such as 2-15 nm, and/or wherein the intrinsic layer each individually is textured, in particular with a same texturing as the substrate.

3. The solar cell according to any of claims 1-2, comprising at least one doped silicon layer (13,14), such as a p-doped silicon layer (13), and an n-doped silicon layer (14), in particular comprising a 5*10¹⁴-0.5*10²¹ dopants/cm³ n- or p-type doped crystalline Si layer (13,14), and/or wherein a doping concentration is preferably spatially constant, and/or wherein n-type dopants are selected from P, As, Bi, Sb and Li, and wherein p-type dopants are selected from B, Ga, and In.

4. The solar cell according to claim 3, wherein the doped silicon layer is provided in between the at least one contact and substrate, and/or wherein the doped silicon layer substantially covers the same surface area and the same amount of surface area as the at least one contact.

5. The solar cell according to any of claims 1-4, wherein the material of the transparent conductive layer (12) is selected from Indium Tin Oxide (ITO), IOH, ZnO, or doped ZnO, such as Aluminium doped ZnO, doped Tin oxide, such as fluorine doped tin oxide, doped indium oxide, such as Indium Fluor Oxide (IFO:H), and Indium Tungsten Oxide (IWO), and/or wherein a thickness of the transparent conductive layer (12) is 10-200 nm, in particular 20- 170 nm, more in particular 30-50 nm, and/or wherein the refractive index of the transparent conductive layer (12) is <2.2, and/or wherein the work function of the TCO layer (12) is from 2 eV to 8 eV, in particular 3.4 eV to 6.4 eV, and/or wherein the work function of the TCO layer (12) is 3.4 eV to 4.7 eV in case of the TCO- layer mainly transporting electrons, and/or wherein the work function of the TCO layer (12) is 4.7 eV to 6.4 eV in case of the TCO- layer mainly collecting holes, and/or wherein the TCO layer each individually is textured, in particular with a same texturing as the substrate.

6. The solar cell according to claim 5, wherein the stack is formed by the contact, in particular a metal contact, more in particular a contact layer, the TCO layer, and optionally a doped silicon layer, in particular a stack of substantially the same shaped layers, more in particular wherein a width of the doped silicon layer > a width of the TCO layer, which width of the TCO layer > width of the contact layer and/or wherein a remainder of the front surface area or back surface area provides access to at least one light absorbing layer and is substantially free of other layers, apart from an optional antireflective coating and/or optional surface treated layer.

7. The solar cell according to any of claims 1-6, wherein the at least one contact is provided as a strip, more in particular as a strip with a width of 0.01-200 pm, in particular a width of 0.05-50 pm.

8. The solar cell according to claim 7, wherein the at least one light absorbing layer is surface treated, in particular wherein the treatment is selected from treatment with a gas, and treatment with a plasma, more particular wherein in the treatment hydrogen is used, or wherein in the treatment oxygen is used, even more in particular wherein substantially only hydrogen is used, or wherein substantially only oxygen is used.

9. The solar cell according to any of claims 1-8, wherein the substrate (10) is a single sided or double sided flat substrate (10) surface, and/or wherein the substrate (10) is a single sided or double sided textured substrate (10) surface ISO 4287: 1997, in particular textured with a surface roughness Ra of 1-20 pm, such as 2-10 pm, and/or wherein the textured surface has an aspect ratio (height: depth of a textured structure) of 2-

10.

10. The solar cell according to any of claims 1-9, wherein the substrate has a thickness of 1- 500 pm, and/or comprising 10¹⁴-l 0²¹ dopants/cm³ n- or p-type doped substrate (10), and/or wherein the substrate (10) comprises l*10¹²-0.5*10¹⁹ n- or p-type dopants/cm³, in particular 2*IO¹⁴-1O¹⁷ dopants/cm³, more in particular 5*10¹⁴-10¹⁶ dopants/cm³, such as 8*10¹⁴-3*10¹⁵ dopants/cm³, and/or wherein the substrate (10) has a resistivity of 0.1-1000 ohm*cm at 300K, more in particular 1-100 ohm*cm, such as 5-10 ohm*cm.

11. The solar cell according to any of claims 1-10, wherein the at least one contact covers less than 5% of the front surface area or back surface area, in particular wherein the at least one contact covers less than 1% of the front surface area or back surface area, more in particular less than 0.5%, and/or wherein a width of the optional doped silicon layer and a width of the TCO layer and a width of the contact layer each individually are from 1 pm-300 pm, in particular from 20 pm-200 pm, such as from 80 pm-150 pm, and/or wherein the contact comprises a metal, wherein the metal of the contacts independently comprises at least one of Cu, Al, W, Ti, Ni, Cr, Ag, and/or wherein a thickness of said metal contacts (13) is 200 nm-50 pm, in particular 1-25 pm, and/or wherein the metal contact (13) is selected from a metal layer, a metal grid, a metal line, or a combination thereof.

12. The solar cell according to any of claims 1-11, wherein at least one of the front surface and the back surface is provided with an anti-reflective coating (16), in particular an anti-re- flective coating (16) on the surface area not covered by the contact.

13. The solar cell according to claim 12, wherein the layer underneath the anti -reflective coating is surface treated (17), such as surface treated with H2, with O2, or a combination thereof.

14. The solar cell according to any of claims 1-13, wherein the VOC is >700 mV, in particular > 725 mV, such as > 730 mV, and/or wherein a J_sc is > 30 mA/cm², in particular > 38 mA/cm², such as > 39 mA/cm², and/or a fill factor (FF) of >75%, in particular > 80%, such as > 82.5%, and/or having an efficiency of > 23%, in particular >23.8%, such as > 23.9%.

15. The silicon hetero-j unction solar cell (100) according to any of claims 1-14, wherein the solar cell is a back-contacted solar cell, such as an interdigitated back-contacted solar cell, or wherein the solar cell is a back and front contacted solar cell.

16. Method of producing a front-side and back-side contacted silicon hetero-j unction solar cell (100) according to any of claims 1-15, comprising providing a substrate, such as a crystalline Si-substrate, optionally texturing the substrate, such as double-side texturing the substrate, thereafter immersing the substrate into a strong oxidizing solution, thereafter etching the oxidized substrate by dipping the oxidized substrate into an acidic solution, directly thereafter loading the etched substrate into a layer deposition tool, and depositing an intrinsic Si layer on at least one side of the etched substrate, thereafter providing at least one doped silicon layer on the at least one intrinsic Si-layer, in particular a layer that covers less than 20% of the front surface area or back surface area, respectively, thereafter depositing a transparent conductive oxide (TCO) layer on the at least one doped Si-layer, in particular after a first alignment of the solar cell, and then depositing metal contacts on the TCO-layer, in particular after a second alignment of the solar cell, more in particular wherein the first and second alignment each individually is with an accu- racy of better than 20 pm lateral, such as better than 1 pm lateral.

17. Method according to claim 16, wherein deposition of the TCO-layer and/or the metal contacts and optionally provision of the doped silicon layer a hard mask is used, and/or wherein contacts and/or contact layers are provided by metal deposition and lift off of noncontact areas, screen printing, and electrical plating.

18. Silicon hetero-j unction solar cell (100) according to any of claims 1-15 and/or obtained by the method according to claim 16 or 17, comprising at least two elements as mentioned in the claims, and/or comprising at least one further element as mentioned in the description.