EP4609437A1

EP4609437A1 - Simplified processing of interdigitated-back-contacted silicon heterojunction solar cells

Info

Publication number: EP4609437A1
Application number: EP23799023.9A
Authority: EP
Inventors: Paul Alejandro PROCEL MOYA; Yifeng ZHAO; Liqi CAO; Katarina KOVACEVIC; Olindo ISABELLA
Original assignee: Technische Universiteit Delft
Current assignee: Technische Universiteit Delft
Priority date: 2022-10-27
Filing date: 2023-10-25
Publication date: 2025-09-03
Also published as: WO2024091116A1; NL2033406B1

Abstract

The present invention is in the field of a simplified process for making solar cells, or photovoltaic (PV) cell, with transparent contacts and a silicon bulk material. Said solar cells comprise at least one hetero junction and typically two hetero junctions. The invention provides solar cells with good operating characteristics, such as in terms of conversion efficiency, fill factor, and current gain.

Description

SIMPLIFIED PROCESSING OF INTERDIGIT ATED-B ACK-CONTACTED SILICON HET¬

EROJUNCTION SOLAR CELLS

FIELD OF THE INVENTION

The present invention is in the field of a simplified process for making solar cells, or photovoltaic (PV) cell, with transparent contacts and a silicon bulk material. Said solar cells comprise at least one heterojunction and typically two heterojunctions. The invention provides solar cells with good operating characteristics, such as in terms of conversion efficiency, fill factor, and current gain.

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 an 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 towards 2020 and many years beyond. Due to their simplified process, conventional c-Si solar cells dominate a large part of the market. As alternative to the industry to improve the power to cost ratio, the silicon heterojunction approach has become increasingly attractive for 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 negligible market share so far. A heterojunction is the 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 some 20%. Theoretically a single p-n junction crystalline silicon device has a maximum power efficiency of about 30%. 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 25.6%. 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.

In aspect of multi-layer structures relates to the so-called work-function. In physics the work function relates to a minimum thermodynamic work (i.e., energy) needed to remove an electron from a solid to a point in the vacuum outside the solid surface. Here "outside" means that the final electron position is far from the surface on the atomic scale, but still too close to the solid to be influenced by ambient electric fields in the vacuum. The work function is considered not to be a characteristic of a bulk material, but rather a property of the surface of the material and hence depending on crystal face and possible contamination, surface charge, etc. The workfunction may be expressed in eV. Typically at an interface between two different material there is a mismatch, such as in terms of the work-function. A “loss” in work-function may occur at the interface.

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 an alternative thereto solar cell cells with back contact are provided, such as with interdigitated back contacts. US2016/0020342 Al is an exemplary document reciting such interdigitated back contacts (IBC). Silicon hetero-j unction (SHJ)-IBC solar cells can achieve a photoconversion efficiency of 26.7% for a record for single-junction c-Si cell. As the collection of photogenerated electrons holes are on the rear side of the solar cells, IBC-SHJ solar cells benefit from shading- free front surface and largely reduced parasitic absorption as compared to standard front/back- contacted (FBC) solar cells. However, a typical prior art fabrication process of IBC-SHJ solar cells comes with high complexity and manufacturing costs, which limit their implementation in industry. This is partly because the state of the art IBC-SHJ solar cells involve separate patterning steps for electron contact stack and hole contact stack. Moreover, doped Si-based layers are typically used for forming the electron and hole contact stacks of the state of the art IBC-SHJ solar cells. Those doped Si-based layers are deposited by capital-intensive PECVD, and the deposition process also involves the use of flammable and toxic gases, such as SiHt, B2H5, PH3, etc.

Transition metal oxides (TMOs) may be considered for c-Si based heterojunction (SHJ) solar cells in view of their ability to induce efficient carrier selectivity and mitigate parasitic absorption losses resulting in clear current gain. Among TMOs, molybdenum oxide (MoO_x) is promising for applications as hole transport layer (HTL). MoO_x layer, in combination with a thin intrinsic passivation a-Si:H layer and a transparent conductive oxide (TCO) has in fact demonstrated conversion efficiency of 23.5%. However, (i)a-Si:H/MoO_x exhibits a weak thermal stability in air/moisture hindering the carrier selectivity in contrast to conventional SHJ cells. Consequently, devices with TMOs usually suffer from lower fill factor (FF) and possibly S-shaped J-V characteristics as compared to solar cells with doped silicon carrier selective HTLs.

Incidentally EP 3664157 Al recites a solar cell (10) including a semiconductor substrate (12), a conductive region (32, 34) disposed in the semiconductor substrate or over the semiconductor substrate, and an electrode (42, 44) electrically connected to the conductive region. The electrode includes a first electrode part (42a, 44a) and a second electrode part (42b, 44b) disposed over the first electrode part. The second electrode part includes a particle connection layer (426) formed by connecting a plurality of particles including a first metal and a cover layer (428) including a second metal different from the first metal and covering at least the outside surface of the particle connection layer. US 11251325 B2 recites a photovoltaic device comprising a silicon-based substrate (2) having a p-type or n-type doping, with an intrinsic buffer layer (4) situated on said substrate. A first silicon layer (6) of a first doping type is situated on predetermined regions (4a) of the intrinsic buffer layer. The first layer has interstices (5) between said predetermined regions (4a). The first silicon layer comprises at least partially a microcrystalline layer at its side away from the substrate. A microcrystalline silicon layer (8) of a second doping type is situated on said first silicon layer (6). A third silicon layer (10) of the second doping type is situated on said intrinsic buffer layer at the interstices, the third silicon layer being amorphous at its side facing said silicon-based substrate and comprising an at least partially microcrystalline layer portion to the side away from the intrinsic buffer layer.

The present invention relates to an increased efficiency Si-based 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 Si-based solar cell according to claim 1, and in a second aspect to a method for making such a solar cell. The invention involves a simplified processing silicon heterojunction (SHJ) solar cells, in particular for of inter- digitated-back-contacted (IBC) solar cells. For conventional IBC-SHJ solar cells, the rear side of the solar cells requires patterning for both electron contact stacks and hole collection contact stacks, which are typically doped Si-based layers deposited by a capital-intensive Plasma-Enhanced Chemical Vapor Deposition (PECVD) tool. These compulsory patterning steps, such as photolithographic patterning, result in high complexity and high manufacturing costs for fabricating conventional IBC-SHJ solar cells. The present inventors now have found to use carefully selected transition metal oxides (TMOs) (e.g. with different work functions) on top of a pre-patterned contact stack to efficiently collect electrons and holes without the prior art additional patterning steps. Specifically, after patterning the first electron (or hole) contact stack, the selected TMO layer can now be deposited full area on the rear side of the crystalline silicon (c-Si) wafer without patterning. The collection of electrons (or holes) can happen through where the first electron (or hole) contact stack are stacked with the TMO layer. The collection of holes (or electrons) is realized by the TMO layer, that is not stacked on top of the first electron (or hole) contact stacks. In this way, at least one patterning step less and thus cheaper process can be achieved in particular for manufacturing IBC-SHJ solar cells as compared to the conventional process. For instance, as seen in the figures, Molybdenum oxide (MoO_x) is selected to be deposited full area on top of a patterned phosphorus-doped hydrogenated nanocrystalline silicon layer ((n)nc-Si:H). The electron collection happens through the (n)nc-Si:H/MoO_x contact stack while the MoO_x alone acts as hole collectors. Inventors have now demonstrated the effectiveness of electron collections through the proposed (n)nc-Si:H/MoO_x via both the advanced Technology Computer- Aided Design (TCAD) simulations and fabricated solar cells. The hole collection by MoO_x is also proved to be efficient based on both the simulations and fabricated solar cells. Therefore, with the present approach, the fabricating process of high-efficiency IBC-SHJ solar cells is simplified with lower costs. Besides, there is no needed for a second doped Si-based layer as electron or hole transport layer, thus the PECVD equipment can be simplified with at least one doping chamber less, thus reducing the capital investment for the PECVD tool and reducing the use of toxic and flammable gases used for PECVD depositions. In an initial experiment, inventors have successfully demonstrated the production of the claimed solar cells, such as a IBC-SHJ solar cell, featuring this novel MoO_x-based selective passivating contact with an acceptable efficiency. The efficiency of IBC-SHJ solar cell is expected to be improved to above 24% after those optimizations.

So, in the present invention, after patterning the first electron (or hole) contact stack, the selected TMO layer can be deposited over a part or full area on the rear side of the crystalline silicon (c-Si) wafer without patterning. Therefore, at least one patterning step less is required, and thus a cheaper and high throughput process is provided, which may be used for manufacturing of IBC-SHJ solar cells. Moreover, a TMO layer, e.g. MoO_x, may be deposited by thermal evaporation, unlike doped Si-based layers which are deposited by PECVD. Therefore, by replacing at least one doped Si-based electron or hole transport layer by a TMO-layer, e.g. MoO_x, the use of flammable and toxic gases of the PECVD process is reduced/absent. The present invention relates to a carefully selected combinations of the pre-patterned electron or hole contact stack, and the subsequently deposited TMO layer, and a good control of the material properties by optimized processing. As there is no need for a second doped Si-based layer, the simplification of the multi-chamber PECVD tool for solar cells manufacture is possible, and this results in lower production costs.

The present invention relates to a single or heterojunction Si-based solar cell 100 comprising a silicon bulk material 10, a contact stack of layers 14,15,16 provided in contact with the silicon bulk material, characterized in that the contact stack of layers 14, 15, 16 is patterned into at least one hole collection region 21 and into at least one electron collection region 22, wherein the at least one electron collection region 22 comprises a negatively doped Si-layer 15, such as an n nc-Si:H layer, and wherein the contact stack of layers comprises at least one vertical conducting layer comprising at least one transition metal oxide 14, wherein the at least one vertical conducting layer is configured to transport electrons in the at least one electron collection region and wherein the at least one vertical conducting layer is configured to transport holes in the at least one hole collection region, in particular wherein the at least one electron collection region 22 and the at least one hole collection region 21 are provided on the same side of the solar cell. Figs. 1-3 show schematic representations of exemplary embodiments. The present invention further relates to a method pf producing said solar cell.

It is found that with the present invention it is possible to deposit hole transport layers, such as MoO_x layers in a wider operational range (such as by PECVD, thermal evaporation, atomic layer deposition, PVD, and sputtering) by introducing e.g. a prior PECVD plasma pretreatment.. That is considered important, because they do not have a doped emitter underneath the MoOx transport layer (see fig. 7). This pre-treatment (i) mitigates the interaction of the hole transport layer with (i)a-Si:H and (ii) strongly supports the charge transport ascribing this to the reduction of the dipole strength and hinders the reactivity of the interface with oxygen. The optimized plasma treatment gave a S-shape-free J-V curve with FF comparable to a reference device. Further thickness optimization demonstrated that the MoO_x layer can be further reduced down to 3.5 nm with no electrical losses by the presence of the plasma treatment (PT). This result is in agreement with the simulated optimal hole transport layer thickness for FF maximization. Finally, both electrical simulations and experimental JV showed that the proposed approach consisting of plasma treatment (PT) + the hole transport layer (treatment time of 130-360 seconds, such as 300 seconds) results in very limited losses as compared to the cell with only the hole transport layer, such as MoO_x.

The present invention provides a simplified process, as described. The present solar cells obtained by the present process have as advantages e.g. a good work-function and/or a limited loss of work-function over the interface of the hole transport layer/pre-treated layer, a mitigated dipole on the interface, a good conversion efficiency, a good transparency, a low parasitic absorption, good carrier collection, a not very complex structure, a high Voc, a high Jsc, and a high fill factor. The present invention makes use of various techniques in order to solve one or more of the prior art problems and provides further advantages; these advantages relate to measurable characteristics (see above effects) of the obtained devices and hence constitute noticeable physical differences over e.g. the prior art. No annealing step is required. A high efficiency solar cell (> 22% efficiency) is provided. With some simple optimization steps an efficiency of 25-26% is feasible. In the present solar cells front and rear (also indicated as back) contacts are present. The present solar cell typically comprises at least one heterojunction and typically two heterojunctions. The present solar cell 100 comprises a hole transport layer 12, which sometimes is also referred to as contact layer, or collector layer, characterized in that the hole transport layer (12) comprises at least one transition metal oxide, wherein the hole transport layer (12) has a thickness of 1.5-9 nm, wherein the hole transport layer (12) is provided on a plasma pre-treated surface layer (12a), wherein the plasma pre-treated surface is a surface passivation layer, wherein the surface passivation layer is an a-Si:H pre-treated layer (12a). Therewith in particular a loss in work-function is minimized, and the dipole at the plasma treated surface is limited as well.

The above and other improvements lead to a short circuit current of > 39 mA/cm², and/or an FF of > 70%, preferably FF>75%, preferably >77%, such as > 80%, and/or a Voc of 700-730 mV, and/or a conversion efficiency of > 21%, and/or an absorption coefficient <20 xlO⁴ cm'¹ in the range 3-4eV. Such relates to an improvement of 1-3% over comparable prior art devices, which is a relative improvement of 5-15%. For return of investment such a difference is considered huge.

The present method is considered to be relatively simple and reduces process time and use of equipment. Mostly fabrication tools thereof are already part of standard production lines. Therefore the present invention may be considered commercially available from the start since it does not require development of additional process tools.

In summary the present invention provides a simplified fabrication process wherein solar cells 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 due to the full passivated contacts, solar cells featuring a high Jsc & Voc due to the high transparency of the passivating contacts, solar cells featuring a relatively high fill factor (FF) due to the lowly doped c-Si regions near the interfaces, and wherein the design is applicable to both a front/rear contacted conventional solar cell architecture, a bifacial solar cell architecture 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 single or heterojunction Si-based solar cell according to claim 1, and in a second aspect to a process for making such a solar cell.

In an exemplary embodiment of the present solar cell the at least one hole collection region 21 covers 10-90% of a surface area of a side of the solar cell, and wherein the at least one electron collection region 22 covers 10-90% of a surface area of the same side of the solar cell, in particular of the back side of the solar cell.

In an exemplary embodiment of the present solar cell the work-function loss of the at least one electron collection region is < 1.0 eV, preferably < 0.6 eV, more preferably < 0.5 eV, such as < 0.35 eV,.

In an exemplary embodiment of the present solar cell the work-function loss of the at least one hole collection region is <1.0 eV, preferably < 0.6 eV, more preferably < 0.5 eV, such as < 0.35 eV,.

In an exemplary embodiment of the present solar cell the work-function loss of the at least one hole collection region differs from the of the at least one hole collection region by at least 10%, in particular by at least 50%.

In an exemplary embodiment of the present solar cell the solar cell is interdigitated.

In an exemplary embodiment of the present solar cell the solar cell is back-contacted.

In an exemplary embodiment of the present solar cell the hole transport layer 14 is in electrical contact with the at least one hole collection region.

In an exemplary embodiment of the present solar cell the hole transport layer 14 has a thickness of 1.5-9 nm, in particular 1.7-2.0 nm. The hole transport layer is preferably as thin as possible, in view of collection of holes, and in view of efficiency.

In an exemplary embodiment of the present solar cell the electron transport layer 14 has a thickness of 1.5-9 nm, in particular 1.7-2.0 nm. The electron transport layer is preferably as thin as possible, in view of collection of holes, and in view of efficiency.

In an exemplary embodiment of the present solar cell the hole transport layer 14a is provided on a plasma pre-treated surface layer 12a (or likewise 14b/15a). The plasma treated surface provides good collection and a good efficiency, in particular in combination with the hole/electron transport layer.

In an exemplary embodiment of the present solar cell the plasma pre-treated surface is a surface passivation layer, such as wherein the surface passivation layer is an a-Si:H pre-treated layer 12a.

In an exemplary embodiment of the present solar cell the surface passivation layer further comprises a silicon pre-treated layer provided on the a-Si:H pre-treated layer.

In an exemplary embodiment of the present solar cell the work-function loss of the combined hole transport layer 12/pre-treated layer 12a is < 1.0 eV, preferably < 0.6 eV, more preferably < 0.5 eV, such as < 0.35 eV.

In an exemplary embodiment of the present solar cell the dipole of the plasma pretreated surface is < 4 C/m, preferably < 2 C/m, more preferably < 1 C/m, such as < 0.7 C/m.

In an exemplary embodiment of the present solar cell the pre-treated layer 12a is obtained by PECVD treatment with a plasma mixture comprising a positive dopant comprising gas, such as a B-, Al-or Ga-comprising dopant gas, such as B2H6, preferably comprising SiEU, H2, and the gaseous p-dopant.

In an exemplary embodiment of the present solar cell the pre-treated layer 12a comprises nanocrystalline Si, a relaxed interface, p-dopants, amorphous Si, a positive electrical charge, or a combination thereof.

In an exemplary embodiment of the present solar cell pre-treatment is performed during 10-1000 sec, preferably 20-300 sec, such as 30-100 sec.

In an exemplary embodiment of the present solar cell a power density during pre-treat- ment is 50-350 mW/cm², preferably 70-200 mW/cm², more preferably 80-100 mW/cm², such as 90 mW/cm².

In an exemplary embodiment of the present solar cell pre-treatment is performed at a temperature < 523 K (< 250 °C), preferably < 473 K (< 200 °C), more preferably < 443 K (< 170 °C).

In an exemplary embodiment of the present solar cell a plasma pressure is from 50-400 Pa (0.5-4 mbar), preferably 100-300 Pa (1-3 mbar), more preferably 150-250 Pa (1.5-2.5 mbar), such as 220 Pa (2.2 mbar).

In an exemplary embodiment of the present solar cell the pre-treated layer is substantially free of SiCE, such as having less than 1% SiCE/pre-treated layer (atom/atom), more preferably <1000 ppm, even more preferably <100 ppm, such as < 10 ppm.

In an exemplary embodiment of the present solar cell with the proviso that no annealing of the a-Si:H layer is provided.

In an exemplary embodiment of the present solar cell with the proviso that no chemical etching of the a-Si:H layer is provided, preferably no chemical etching at all.

In an exemplary embodiment of the present solar cell the hole transport layer 14a has a current gain of 1-2 mA/cm².

In an exemplary embodiment of the present solar cell the hole transport layer 14a has a thickness of 1-7 nm, preferably 1.5-5 nm, such as 1.7-2 nm.

In an exemplary embodiment of the present solar cell the hole transport layer 14a has a an absorption coefficient <20 xlO⁴ cm'¹ in the range 3-4eV, preferably <10 xlO⁴ cm'¹.

In an exemplary embodiment of the present solar cell the electron transport layer 14b has a current gain of 1-2 mA/cm².

In an exemplary embodiment of the present solar cell the electron transport layer 14b has a thickness of 1-7 nm, preferably 1.5-5 nm, such as 1.7-2 nm.

In an exemplary embodiment of the present solar cell the electron transport layer 14b has a an absorption coefficient <20 xlO⁴ cm'¹ in the range 3-4eV, preferably <10 xlO⁴ cm'¹.

In an exemplary embodiment of the present solar cell one or more of the substrate, and further layers, is textured, in particular wherein the hole transport layer 14a is structured, such as comprising a zig/zag structure, comprising random pyramids, texturing, preferably with a height of 1-7 pm, such as 2-5 pm, and combinations thereof.

In an exemplary embodiment of the present solar cell the transition metal is selected from period 4 or period 5 transition metals, such as Ti, V, Cr, Co, Ni, Cu, Zn, Cs, Nb, Mo, W, and alloys thereof.

In an exemplary embodiment of the present solar cell the hole transport layer 14a is dopant free.

In an exemplary embodiment of the present solar cell the hole transport layer 14a is deposited on a pre-treated a-Si:H layer.

In an exemplary embodiment the present solar cell further comprises an anti-reflective coating (ARC) layer 13 of 40-200 nm, the ARC layer above an < 10 nm intrinsic silicon layer 11, the intrinsic silicon layer 11 on a 100-500 pm doped crystalline silicon substrate 10, and on a back side of the doped crystalline silicon substrate, a second 1-10 nm a-Si:H layer 12, above the second a-Si:H layer the 1-10 nm separated hole transport layer and electron transport layer, preferably with an activation energy <350 meV, the 20-300 nm transparent conducting layer 16, such as an ITO layer, and a metal contact layer 18a,b above the transparent layer.

In an exemplary embodiment the present solar cell further comprises at least one interlayer (17) beneath the hole transport layer (14a) and/or beneath the electron transport layer (14b), in particular wherein the interlayer has a thickness of 0.2-5 nm, more in particular 1-2 nm.

In an exemplary embodiment the present solar cell has a short circuit current of > 39 mA/cm², and/or an FF of > 70%, preferably FF>75%, preferably >77%, such as > 80%, and/or a Voc of 700-730 mV, and/or a conversion efficiency of > 21%.

In an exemplary embodiment of the present solar cell the solar cell is selected from singlejunction solar cells, heterojunction solar cells, multi -junction solar cells, thin film solar cells, wherein the silicon is crystalline silicon, n-doped or p-doped crystalline silicon.

In an exemplary embodiment of the present solar cell, the patterned contact stack of layers (14, 15, 16) and the patterned doped Si-layer (15) are obtained by at least one of applying hard mask, by applying ebeam, and applying lithographic patterning, in particular by applying photolithographic patterning. By using hard mask, deposition and patterning can be done in one step. Patterning performed following deposition can be done by photolithography. Lithography is a planographic method of printing, typically used in semiconductor processing. In integrated circuit manufacturing, photolithography uses light to produce minutely patterns, typically of thin films of suitable materials, on a substrate, such as a silicon wafer, or previous provided layers, to protect selected areas of it during subsequent etching, deposition, or implantation operations. Typically, light, such as ultraviolet light, is used to transfer a geometric design from an optical mask comprising the to be formed pattern to a light-sensitive chemical coated on e.g. the substrate. The patterned film is then created by removing the softer parts of the coating with appropriate solvents, also known in this case as developers. The obtained pattern in the present patterned solar cell has not much to do with materials that could be patterned in themselves, such as foams.

In a second aspect the present invention relates to a method of producing a solar cell according to the invention.

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

Figures 1-3 show schematic representations of the present solar cell.

Figs. 4a-4d show schematics of a method of producing the present solar cell.

DETAILED DESCRIPTION OF FIGURES

In the figures:

100 Si-solar cell

10 Si-bulk material

11 (i) a-Si:H

12 (i) a-Si:H

13 ARC

14 a,b MoOx

15 (n)nc-Si:H 16a,b TCO

17 MoOx-(i)a-Si:H interlayer 18a,b Metal

21 hole collection region

22 electron collection region

The figures are further detailed in the description of the experiments below.

Figure 1 shows a schematic representation of the present solar cell, as explained throughout the description. Therein an ARC layer 13 is provided on a first (i)a-Si:H layer 11, which (i)a- Si:H layer is provided on an n-type c-Si substrate 10. On the back side a second (i)a-Si:H layer 12 is provided on an n-type c-Si substrate; therein a (n)nc-Si:H layer 15 is partly provided on the second (i)a-Si:H layer 12. On the partly provided (n)nc-Si:H layer 15 and on the second (i)a-Si:H layer 12 a transition metal oxide layer 14, in particular an MoOx layer, is provided. Further a non-continuous TCO layer 16 is provided on the transition metal oxide layer 14, in particular a first part of the TCO layer 16 for hole transport and a second part of the TCO layer 16 for electron transport, which first and second part are electrically, and physically, separated from one and another, as well as metal contacts 18a,b. As a result a hole transport region 21, at a left side, and an electron transport region 22, at a right side, can be distinguished.

Figure 2 shows a vertically inverted layout of figure 1, having a second (i)a-Si:H layer 12 provided on an n-type c-Si substrate; therein a (n)nc-Si:H layer 15 is partly provided on the second (i)a-Si:H layer 12. On the partly provided (n)nc-Si:H layer 15 and on the second (i)a-Si:H layer 12 a transition metal oxide layer 14, in particular an MoOx layer, is provided. Further a non-continuous TCO layer 16 is provided on the transition metal oxide layer 14, in particular a first part of the TCO layer 16 for hole transport and a second part of the TCO layer 16 for electron transport, which first and second part are electrically, and physically, separated from one and another, as well as metal contacts 18a,b. As a result a hole transport region 21, at a left front side, and an electron transport region 22, at a right back side, can be distinguished. The electron transport region 22 thus comprises the (n)nc-Si:H layer 15, while the hole transport region 21 does not comprise such (n)nc-Si:H layer 15.

Fig. 3 shows a cross-section of figure 1, showing mainly the same layers. Also at least one interlayer between the MoOx layer 14a and the (i)a-Si:H layer is formed. The interlayer typically has a thickness of 1-5 nm, such as 2-3 nm. In fig. 3 it may even be more clear than in fig. 2 that the electron transport region 22 comprises the (n)nc-Si:H layer 15, while the hole transport region 21 does not comprise such (n)nc-Si:H layer 15. It is demonstrated in fig. 3, that in the hole transport region 21, the TCO layer 16a has same size and covers same surface area in same stack as metal contact 18a. It also applies to the electron transport region 22, that the TCO layer 16b and the metal contact 18b have same size relative to each other and cover the same surface area in same stack. Additionally, the (n)nc-Si:H layer 15 in the electron transport region 22 has also the same size and surface area in same stack as TCO layer 16b and metal contact 18b. As described above and shown in fig. 3 and fig. 4d, after rear-side addition of the (n)nc-Si:H layer (15) at only the electron transport region 22, a TMO layer 14 (14a and 14b), such as MoOx, is deposited full area. This is subsequently followed by separate TCO layers 16a and 16b and metal contacts 18a and 18b.

Figs. 4a-4d show schematics of an exemplary method of producing the present solar cell. The main steps in the fabrication process of IBC solar cells are the following:

1. providing c-Si substrate 10

2. front and rear side deposition of (i)a-Si:H 11,12 on the c-Si substrate 10

3. front side antireflection coating 13 on the front (i)a-Si:H 11 [fig. 4a]

4. rear side deposition and patterning of n-type a-Si:H or nc-Si:H-based layer 15*; *deposi- tion and patterning can be done in one step (e.g. by the use of hard mask) or patterning can be done following the deposition (e.g. by using photolithography) [Fig. 4b]

5. optional rear side treatment of previously deposited layers (leading to a formation of interlayer 17)

6. rear side full area deposition of transition metal oxide (TMO) such as MoOx 14a, b [fig. 4c] 7. rear side deposition and patterning of transparent conductive oxide (TCO) and metal contacts* [fig. 4d],

The initial results of solar cells fabricated with this process show FF of 67.9%, VOC of 542 mV, JSC of 35.03 mA/cm2, and efficiency of 12.9%.

EXAMPLES/EXPERIMENTS

In an example the following solar cell was made:

Thickness of layers, from front to back:

ARC, 65nm

(i)a-Si:H 5nm

(n)c-Si wafer, 250pm

(i)a-Si:H 5nm

(n)nc-Si:H 6nm

MoOx 5 nm

TCO, 150nm.

Metal 10 pm

Electrical properties of layers

MoOx no doping

TCO: carrier concentration 5xl0²⁰cm³

(n)a-Si:H: activation energy <350 meV

Conditions of the method PECVD: treatment frequency: 13.56 MHz

Pressure: 2.2mbar

Power density: 90 mW/cm²

Time: 130s

Gas mixture SiH^: 0.8sccm, H2: 170sccm, B2H5 (200ppm in H2):10sccm

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 single or hetero junction Si-based solar cell (100) comprising a silicon bulk material (10), a contact stack of layers (14,15,16) provided in contact with the silicon bulk material, characterized in that the contact stack of layers (14, 15, 16) is patterned into at least one hole collection region (21) and into at least one electron collection region (22), wherein the at least one electron collection region (22) comprises a patterned doped Si-layer (15), wherein the doped Si-layer (15) is a negatively doped Si-layer, such as an (n) nc-Si:H layer, and wherein the at least one hole collection region (21) does not comprise a doped Si-layer, and wherein in the at least one hole collection region (21) the hole transport layer (14a) is provided under a transparent conducting oxide material (16a), in particular wherein the transparent conducting layer in the at least one electron collection region (22) is provided as an electron transparent conducting layer (16b) and in the at least one hole collection region (21) is provided as a hole transparent conducting layer (16a) which are electrically separated from one and another, and wherein the contact stack of layers comprises at least one vertical conducting layer comprising at least one transition metal oxide (14), wherein the at least one vertical conducting layer is configured to transport electrons in the at least one electron collection region and wherein the vertical conducting layer is configured to transport holes in the at least one hole collection region, in particular wherein the at least one electron collection region (22) and the at least one hole collection region (21) are provided on the same side of the solar cell.

2. The solar cell according to claim 1, wherein the at least one hole collection region (21) covers 10-90% of a surface area of a side of the solar cell, and wherein the at least one electron collection region (22) covers 10-90% of a surface area of the same side of the solar cell, in particular of the back side of the solar cell, wherein the work-function loss of the at least one electron collection region is < 1.0 eV, and/or wherein the work-function loss of the at least one hole collection region is < 1.0 eV, and/or wherein the work-function loss of the at least one hole collection region differs from the of the at least one hole collection region by at least 10%, in particular by at least 50%.

3. The solar cell according to claim 1 or 2, wherein the solar cell is interdigitated, and/or wherein the solar cell is back-contacted, and/or wherein the hole transport layer (14) is in electrical contact with the at least one hole collection region, and/or wherein the hole transport layer (14) has a thickness of 1.5-9 nm, and/or wherein the electron transport layer (14) has a thickness of 1.5-9 nm, and/or wherein the hole transport layer (14) is provided on a plasma pre-treated surface layer (12a), and/or wherein the plasma pre-treated surface is a surface passivation layer, such as wherein the surface passivation layer is an a-Si:H pre-treated layer (12a), wherein the surface passivation layer further comprises a silicon pre-treated layer provided on the a-Si:H pre-treated layer.

4. The solar cell according to any of claims 1-3, wherein the work-function loss of the combined hole transport layer (12)/pre-treated layer (12a) is < 1.0 eV, preferably < 0.6 eV, more preferably < 0.5 eV, such as < 0.35 eV, and/or wherein the dipole of the plasma pre-treated surface is < 4 C/m, preferably < 2 C/m, more preferably < 1 C/m, such as < 0.7 C/m.

5. The solar cell according to any of claims 1-4, wherein the pre-treated layer (12a) is obtained by PECVD treatment with a plasma mixture comprising a positive dopant comprising gas, such as a B-, Al-or Ga-comprising dopant gas, such as B2H6, preferably comprising SiEU, H2, and the gaseous p-dopant, and/or wherein the pre-treated layer (12a) comprises nanocrystalline Si, a relaxed interface, p-dopants, amorphous Si, a positive electrical charge, or a combination thereof.

6. The solar cell according to any of claims 1-5, wherein pre-treatment is performed during 10-1000 sec, preferably 20-300 sec, such as 30-100 sec, and/or wherein a power density during pre-treatment is 50-350 mW/cm², preferably 70-200 mW/cm², more preferably 80-100 mW/cm², such as 90 mW/cm², and/or wherein pre-treatment is performed at a temperature < 523 K (< 250 °C), preferably < 473 K (< 200 °C), more preferably < 443 K (< 170 °C), and/or wherein a plasma pressure is from 50-400 Pa (0.5-4 mbar), preferably 100-300 Pa (1-3 mbar), more preferably 150-250 Pa (1.5-2.5 mbar), such as 220 Pa (2.2 mbar), and/or wherein the pre-treated layer is substantially free of SiCE, such as having less than 1% SiCE/pre- treated layer (atom/atom), more preferably <1000 ppm, even more preferably <100 ppm, such as < 10 ppm, and/or with the proviso that no annealing of the a-Si:H layer is provided, and/or with the proviso that no chemical etching of the a-Si:H layer is provided, preferably no chemical etching at all.

7. The solar cell according to any of claims 1-6, wherein the hole transport layer (14a) has a current gain of 1-2 mA/cm², and/or wherein the hole transport layer (14a) has a thickness of 1-7 nm, preferably 1.5-5 nm, such as 1.7-2 nm, and/or wherein the hole transport layer (14a) has a an absorption coefficient <20 xlO⁴ cm'¹ in the range 3-4eV, preferably <10 xlO⁴ cm'¹, and/or wherein the electron transport layer (14b) has a current gain of 1-2 mA/cm², and/or wherein the electron transport layer (14b) has a thickness of 1-7 nm, preferably 1.5-5 nm, such as 1.7-2 nm, and/or wherein the electron transport layer (14b) has a an absorption coefficient <20 xlO⁴ cm'¹ in the range 3-4eV, preferably <10 xlO⁴ cm'¹.

8. The solar cell according to any of claims 1-7, wherein one or more of the substrate, and further layers, is textured, in particular wherein the hole transport layer (14a) is structured, such as comprising a zig/zag structure, comprising random pyramids, texturing, preferably with a height of 1-7 pm, such as 2-5 pm, and combinations thereof.

9. The solar cell according to any of claims 1-8, wherein the transition metal is selected from period 4 or period 5 transition metals, such as Ti, V, Cr, Co, Ni, Cu, Zn, Cs, Nb, Mo, W, and alloys thereof, and/or wherein the hole transport layer (14a) is dopant free.

10. The solar cell according to any of claims 1-9, wherein the hole transport layer (14a) is deposited on a pre-treated a-Si:H layer.

11. Solar cell according to any of claims 1-10, further comprising an anti -reflective coating (ARC) layer 13 of 40-200 nm, the ARC layer above an < 10 nm intrinsic silicon layer (11), the intrinsic silicon layer (11) on a 100-500 pm doped crystalline silicon substrate (10), and on a back side of the doped crystalline silicon substrate, a second 1-10 nm a-Si:H layer (12), above the second a-Si:H layer the 1-10 nm separated hole transport layer and electron transport layer, preferably with an activation energy <350 meV, the 20-300 nm transparent conducting layer (16), such as an ITO layer, and a metal contact layer (18a, b) above the transparent layer, and optionally at least one interlayer (17) beneath the hole transport layer (14a) and/or beneath the electron transport layer (14b), in particular wherein the interlayer has a thickness of 0.2-5 nm, more in particular 1-2 nm.

12. The solar cell according to any of claims 1-11, having a short circuit current of > 39 mA/cm², and/or an FF of > 70%, preferably FF>75%, preferably >77%, such as > 80%, and/or a Voc of 700-730 mV, and/or a conversion efficiency of > 21%.

13. The solar cell according to any of claims 1-12, wherein the solar cell is selected from singlejunction solar cells, heterojunction solar cells, multi -junction solar cells, thin film solar cells, wherein the silicon is crystalline silicon, n-doped or p-doped crystalline silicon.

14. The solar cell according to any of claims 1-13, wherein the patterned contact stack of layers (14, 15, 16) and the patterned doped Si-layer (15) are obtained by at least one of applying hard mask and applying lithographic patterning, in particular by applying photolithographic patterning.