FIELD OF THE INVENTION

The present invention relates to an n-type silicon photovoltaic cell structure and to a method for producing the structure.

BACKGROUND OF THE INVENTION

Photovoltaic cells are gradually becoming an important means of generating electrical energy. Espe ^¬ cially solar cells, photovoltaic cells designed to convert sunlight into electrical energy, are consid ^¬ ered as one of the most promising candidates for re ^¬ newable energy production.

New technologies and solar cell structures having improved efficiency and reduced fabrication costs are being developed. One of the improvements in crystalline silicon (c-Si) solar cells has been the introduction of rear surface passivation to reduce the charge carrier recombination on the back side of a silicon wafer. Surface recombination in semiconductors is a result of possibly many different mechanisms leading to trapping of charge carriers in specific en ^¬ ergy states at or close to the surface of a semicon- ductor. These energy states, or surface states as they are often called, may originate from different sources, such as impurities at the surface or the in ^¬ evitable disruption of periodicity of a semiconductor crystal at the surface. In a photovoltaic cell the quantum efficiency, and therefore the overall effi ^¬ ciency, decreases as charge carriers generated by the absorption of photons in the semiconductor recombine with the surface states and therefore cannot be col ^¬ lected in the cell electrodes to contribute to the cell current. To reduce surface recombination of charge carriers, several ways of passivating a surface of a semiconductor (or conducting doped semiconductor) have been developed. Especially ALD-grown (Atomic Layer Deposition) aluminum oxide has shown to present good passivation on rear surface of p-type silicon solar cells. However, it has been recognized that high effi ^¬ ciency crystalline silicon (c-Si) solar cells would require surface passivation on both sides of the sili- con wafer as well as an anti-reflection coating on the front side, i.e. on the side of the structure meeting incident radiation.

Sputtered titanium dioxide has been used as an anti-reflection coating in crystalline silicon cells. However, silicon nitride (SiN _x ) has replaced titanium dioxide as the antireflection coating because of its advantageous surface passivation qualities. It is typically deposited in a layer with a thickness of about 60 - 70 nanometers using plasma-enhanced chemi- cal vapor deposition (PECVD) .

However, even if its passivation effect of n- emitter of p-type c-Si solar cells is good, it does not exhibit passivation on p-emitter of n-type solar cells. This results in the need of an additional alu- minum oxide layer (A1 ₂ 0 ₃ ) . Thus, the simplest structure of SiNx based n-type solar cell is SiNx/Al ₂ 0 ₃ /Si/SiNx . As the silicon nitride layers are typically fabricated in a PECVD process one side at the time three process steps are needed for producing this kind of structure.

The inventor has therefore identified a need of a more effective photovoltaic cell structure with controlled properties.

PURPOSE OF THE INVENTION

The purpose of the invention is to provide a new n-type silicon photovoltaic cell structure having advantageous surface passivation and reflection prop- erties on both sides of a substrate comprising sili ^¬ con. Further, the purpose of the present invention is to provide methods for fabricating such a structure. SUMMARY

The n-type silicon photovoltaic cell struc ^¬ ture according to the present invention is characterized by what is presented in independent claim 1.

The method according to the present invention is characterized by what is presented in independent claim 11.

The present invention relates to an n-type silicon photovoltaic cell structure comprising a sub ^¬ strate having a first surface and a second surface and comprising silicon, wherein the second surface is situated on the essentially opposite side of the first surface of the substrate, a first deposit of materials on the first surface of the substrate and a second de ^¬ posit of materials on the second surface of the sub- strate, wherein the first deposit of materials com ^¬ prises passivating material, deposited on the first surface of the substrate, and reflection adjusting ma ^¬ terial, and the second deposit of materials comprises passivating material, deposited on the second surface of the substrate, and reflection adjusting material, and wherein the passivating material comprises aluminum oxide and the reflection adjusting material is arranged for adjusting reflection of electromagnetic ra ^¬ diation in the structure.

The present invention relates to an n-type silicon photovoltaic cell structure comprising a sub ^¬ strate having a first surface and a second surface and comprising silicon, wherein the second surface is situated on the essentially opposite side of the first surface of the substrate, a first deposit of materials on the first surface of the substrate and a second de- posit of materials on the second surface of the sub ^¬ strate, wherein the first deposit of materials and the second deposit of materials have a symmetrical form, the first deposit of materials comprises passivating material, deposited on the first surface of the sub ^¬ strate, and reflection adjusting material, and the se ^¬ cond deposit of materials comprises passivating mate ^¬ rial, deposited on the second surface of the sub ^¬ strate, and reflection adjusting material, and wherein the passivating material comprises aluminum oxide and the reflection adjusting material, having a refractive index of 2,0 - 2,6, is arranged for adjusting reflec ^¬ tion of electromagnetic radiation in the structure, and wherein the structure comprises a cover substrate on top of the first deposit of materials and a conduc ^¬ tive electrode formed on the second deposit of materi ^¬ als .

According to one embodiment of the present invention the passivating material has a single layer structure. According to one embodiment of the present invention the passivating material has a multilayer structure. According to one embodiment of the present invention the reflection adjusting material has a single layer structure. According to one embodiment of the present invention the reflection adjusting material has a multilayer structure.

According to one embodiment of the present invention the first deposit of materials and the se ^¬ cond deposit of materials have an essentially symmet- rical form. According to one embodiment of the present invention the first deposit of materials and the se ^¬ cond deposit of materials have a symmetrical form. Ac ^¬ cording to one embodiment of the present invention the first deposit of materials and the second deposit of materials have an exactly symmetrical form.

According to one embodiment of the present invention the refractive index of the reflection ad- justing material is 2,0 - 2,6, preferably 2,25 - 2,55, and more preferably 2,3 - 2,5. According to one embod ^¬ iment of the present invention the refractive index of the reflection adjusting material is preferably 2,25 - 2,55, and more preferably 2,3 - 2,5.

According to one embodiment of the present invention the reflection adjusting material is deposited on the passivating material deposited on the first surface of the substrate and on the passivating material deposited on the second surface of the sub ^¬ strate .

A photovoltaic cell structure can be used to convert incident electromagnetic radiation to electri ^¬ cal energy through a photovoltaic effect. A solar cell structure, a specific type of a photovoltaic cell structure, can be used to convert solar radiation into electrical energy.

The term "reflection adjusting material" is used in this specification to describe material being able to adjust reflection of electromagnetic radia ^¬ tion, e.g. light, in a desired manner in the photovol ^¬ taic cell structure. The reflection adjusting material has a refractive index, which is suitable for the pre ^¬ sent purpose. Electromagnetic radiation meeting the outer surface of the photovoltaic cell structure ac ^¬ cording to the present invention will be guided by the reflection adjusting material towards the substrate comprising silicon, i.e. the reflection adjusting material will act as an anti-reflection coating. Reflec- tion adjusting material on the other side of the sub ^¬ strate of the photovoltaic cell structure serves to increase reflection of electromagnetic radiation, which has passed through the substrate and reached the back side of the photovoltaic cell structure, back to- wards the substrate.

According to one embodiment of the present invention the first deposit of materials comprises re- flection adjusting material for reducing reflection of incident electromagnetic radiation from the outer sur ^¬ face of the photovoltaic cell structure. According to one embodiment of the present invention the second de- posit of materials comprises reflection adjusting ma ^¬ terial for increasing reflection of electromagnetic radiation, i.e. for increasing reflection of electromagnetic radiation that has passed through the sub ^¬ strate of the structure and reached the reflection ad- justing material.

The function of the reflection adjusting material is a combination of refractive index and thick ^¬ ness of the formed deposit. The reflection adjusting material situated on the side of incident electromag- netic radiation in relation to the substrate of the structure reduces reflection of incident radiation, e.g. sunlight, meeting the outer surface of the photo ^¬ voltaic cell structure in accordance with the present invention. The inventor of the present invention rec- ognized that using reflection adjusting material on the front side, in view of incident electromagnetic radiation, of the substrate as well as on the rear surface between the silicon substrate and an aluminum back electrode provides advantageous properties for the photovoltaic cell structure in accordance with the present invention. The reflection adjusting material on both sides of the substrate of the structure ac ^¬ cording to the present invention has the advantage of increasing the amount of e.g. sunlight coupled into the cell structure thus increasing its efficiency.

Further, using reflection adjusting material in the structure in accordance with the present inven ^¬ tion has an advantage of making it possible to de ^¬ crease the thickness of a silicon wafer, whereby mate- rial costs are decreased. Because of the low light ab ^¬ sorption coefficient of silicon some of the light, es ^¬ pecially on the IR part of the spectrum, will pass through the wafer and absorb in the aluminum. The reflection adjusting material between the substrate and the back electrode thus reduces the amount of e.g. sunlight that reaches the aluminum back electrode.

The refractive index or index of refraction of material or medium should in this specification be understood as the measure of the speed of light in that medium. It is expressed as a ratio of the speed of light in vacuum relative to that in the considered medium. The refractive index can be presented in the following manner: n = speed of light in a vacuum / speed of

light in medium.

The present invention further relates to a method for fabricating the n-type silicon photovoltaic cell structure according to the present invention, wherein the method comprises the steps of:

- depositing passivating material comprising aluminum oxide simultaneously on the first surface of the substrate comprising silicon and on the second surface of the substrate comprising silicon in a reac ^¬ tion space by an ALD-type process; and

- depositing reflection adjusting material simultaneously on the passivating material deposited on the first surface of the substrate comprising sili ^¬ con and on the passivating material deposited on the second surface of the substrate comprising silicon in the reaction space by an ALD-type process.

According to one embodiment of the present invention the n-type silicon photovoltaic cell struc ^¬ ture is an n-type silicon solar cell structure.

The silicon substrate or silicon wafer in a photovoltaic cell structure often comprises two areas with different thicknesses for different conduction. The thicker layer is considered the base and deter- mines the type of the cell. N-type cells have an re ^¬ type base and a thin p-conductive layer or emitter. INT- type silicon can be produced by doping silicon with compounds that contain one more valence electrons than the silicon does. Phosphorus and arsenic can be men ^¬ tioned as examples of such compounds. Since only four electrons are required to bond with the four adjacent silicon atoms, the fifth valence electron is available for conduction. P-type silicon can be produced by dop- ing silicon with a compound containing one less valence electrons than silicon. Boron can be mentioned as an example of such a compound. When silicon having four valence electrons is doped with atoms that have one less valence electrons, i.e. three valence elec- trons, only three electrons are available for bonding with four adjacent silicon atoms, therefore an incom ^¬ plete bond (hole) exists which can attract an electron from a nearby atom. Filling one hole creates another hole in a different Si atom. This movement of holes is available for conduction. In an n-type cell the emit ^¬ ter can be p-doped through boron diffusion or added aluminum .

The n-type silicon photovoltaic cell struc ^¬ ture in accordance with the present invention compris- es a substrate having an n-type base and a thin p- conductive layer.

In this specification the expression "passiv- ating", "passivation", "surface passivation" or other corresponding expressions should be understood as the passivation of a surface for reducing surface recombi ^¬ nation, i.e. for reducing the recombination of charge carriers on or in immediate proximity to the passivat- ed surface.

Aluminum oxide as passivating material on both sides of the n-type silicon cell provides effi ^¬ cient passivation compared to prior known materials. The method of the present invention provides an efficient way of producing an n-type silicon photo ^¬ voltaic cell structure having passivating material on both sides of an essentially planar substrate as well as reflection adjusting material reducing reflection of incident electromagnetic radiation on one side of the substrate and increasing reflection of electromag ^¬ netic radiation on the other side of the substrate. The ALD-type process advantageously enable the fabri- cation of such a structure as a one step process in the same reaction space without the need of additional deposition steps in e.g. a different reaction space. As both sides of e.g. an essentially planar substrate beneficially can be simultaneously deposited the over- all time needed as well as the material costs for pro ^¬ ducing the structure are reduced compared to prior art methods. The method used for producing the structure further enable the accurate control of the refractive index of the reflection adjusting material by control- ling the type and the cycle of precursor chemicals used .

According to one embodiment of the present invention passivating material comprises aluminum oxide: titanium (Al ₂ 0 ₃ :Ti) or a nanolaminate of aluminum oxide/titanium oxide (Al ₂ 0 ₃ /Ti0 ₂ ) .

According to one embodiment of the present invention reflection adjusting material comprises titanium oxide, titanium oxide : aluminum (Ti0 ₂ :Al) or a nanolaminate of titanium oxide/aluminum oxide (Ti0 ₂ /Al ₂ 0 ₃ ) . According to one embodiment of the pre ^¬ sent invention reflection adjusting material comprises zinc oxide (ZnO) , zinc oxide : aluminum (ZnO:Al), zinc sulfide (ZnS) , tantalum oxide (Ta ₂ 0 ₅ ) , hafnium oxide (Hf0 ₂ ) , zirconium oxide (Zr0 ₂ ) , any mixture thereof or any nanolaminate thereof with aluminum oxide (A1 ₂ 0 ₃ ) . According to one embodiment of the present invention the thickness of a separate layer in the nanolaminate structure is about 2 - 10 nm.

According to one embodiment of the present invention the deposit of materials comprises a nano ^¬ laminate structure. The nanolaminate structure may comprise alternating layers comprising two or more different compounds. E.g. if titanium oxide and alumi ^¬ num oxide are used, the formula of the nanolaminate structure can be presented in the following manner:

(titanium oxide) x: (aluminum oxide) 1-x, wherein x changes as the function of deposit thickness from 0 to 1 such that x is 0 close to the surface of the substrate and 1 on top of the deposit. The nanolaminate structure may comprise also other layers in addition or as alternatives to the aluminum oxide and titanium oxide layers, however with the pro- vision that aluminum oxide is deposited on the surface of the substrate. Thus, according to one embodiment of the present invention the thicknesses of the separate layers in the nanolaminate can gradually vary depend ^¬ ing on their distance from the surface of the sub- strate. For example, if the nanolaminate comprises layers of aluminum oxide and titanium oxide, the alu ^¬ minum oxide layers may be thicker closer to the surface of the substrate than the aluminum oxide layers farther from the surface of the substrate, and in a corresponding manner the titanium oxide layers may be thinner closer to the surface of the substrate than the titanium oxide layers farther from to the surface of the substrate.

The reflection adjusting material to be used in the structure in accordance with the present inven ^¬ tion is selected such that the refractive index is suitable for the reflection adjusting material to pre- sent reflection reducing properties, i.e. to reduce reflection of incident electromagnetic radiation from the surface of the photovoltaic cell structure, on one side of the substrate and reflection increasing prop- erties, i.e. increasing reflection of electromagnetic radiation traversed from the outer surface of the structure through the substrate to the other side of the substrate in the photovoltaic cell structure. It can be presented, as an example only, that e.g. tita- nium oxide, having a refractive index of about 2,4, serves the purpose of reducing reflection of incident electromagnetic radiation from the outer surface of the structure, i.e. functions as an anti-reflection coating between glass, having a refractive index of about 1,5, and the silicon substrate having a refrac ^¬ tive index of about 3,6. When the titanium oxide is placed between the silicon substrate and the back electrode of e.g. aluminum it will serve the purpose of increasing reflection of electromagnetic radiation passed through the substrate and the passivating mate ^¬ rial. I.e. the reflective properties or behavior of the reflection adjusting material depend on the prop ^¬ erties of the surrounding materials as is obvious for the person skilled in the art.

According to one embodiment of the present invention the first surface of the substrate and the second surface of the substrate are deposited simulta ^¬ neously with passivating material in a reaction space by an ALD-type process. According to one embodiment of the present invention the passivating material depos ^¬ ited on the first surface of the substrate and the passivating material deposited on the second surface of the substrate are deposited simultaneously with re ^¬ flection adjusting material in the reaction space by an ALD-type process.

As above described an ALD-type process is used for forming the first deposit of materials and the second deposit of materials on the substrate. The ALD-type process is a method for depositing uniform and conformal deposits or coatings over substrates of various shapes, even over complex three dimensional structures. In ALD-type methods the deposit is grown by alternately repeating, essentially self-limiting, surface reactions between a precursor and a surface to be coated.

The essential feature of the ALD-type methods is to sequentially expose the deposition surface (s) to two or more chemicals (precursors) and to growth reac ^¬ tions of precursors essentially on the deposition sur ^¬ face. In the ALD-type process, the substrate is alter ^¬ nately exposed to at least two precursors, one precur- sor at a time, to form on the substrate a deposit or a layer by alternately repeating essentially self- limiting surface reactions between the surface of the substrate (on the later stages, naturally, the surface of the already formed coating layer on the substrate) and the precursors. As a result, the deposited materi ^¬ al is "grown" on the substrate molecule layer by mole ^¬ cule layer.

The distinctive feature of the ALD-type pro ^¬ cess is that the surface to be deposited is exposed to two or more different precursors in an alternate man ^¬ ner with usually a purging period in between the precursor pulses. During a purging period the deposition surface is exposed to a flow of gas which does not re ^¬ act with the precursors used in the process. This gas, often called the carrier gas is therefore inert to ^¬ wards the precursors used in the process and removes e.g. surplus precursor and by-products resulting from the adsorption reactions of the previous precursor pulse. This purging can be arranged by different means. The basic requirement of the ALD-type process is that the deposition surface is purged between the introduction of a precursor for a metal and a precur- sor for a non-metal. The purging period ensures that the gas phase growth is limited and only surfaces ex ^¬ posed to the precursor gas participate in the growth. However, the purging step with an inert gas can, ac- cording to one embodiment of the present invention, be omitted in the ALD-type process when applying two pro ^¬ cess gases, i.e. different precursors, which do not react with each other. Without limiting the present invention to any specific ALD-cycle, it can be men- tioned as an example only that the purging period can be omitted between two different precursors e.g. if the different precursors are precursors for oxygen, such as water and ozone, which do not react with each other .

The alternate or sequential exposure of the deposition surface to different precursors can be car ^¬ ried out in different manners. In a batch type process the substrate is placed in a reaction space, into which precursor and purge gases are being introduced in a predetermined cycle.

Other names besides atomic layer deposition (ALD) have also been employed for these types of pro ^¬ cesses, where the alternate introduction of or expo ^¬ sure to two or more different precursors lead to the growth of the layer, often through essentially self- limiting surface reactions. These other names or pro ^¬ cess variants include atomic layer epitaxy (ALE) , atomic layer chemical vapour deposition (ALCVD) , and corresponding plasma enhanced variants. Unless other- wise stated, also these processes will be collectively addressed as ALD-type processes in this specification.

The thickness of the material produced by the ALD-type process can be increased by repeating several times a pulsing sequence comprising the aforementioned pulses containing the precursor material, and usually the purging periods. The number of how many times this sequence, called the "ALD cycle", is repeated depends on the targeted thickness of the layer.

The ALD-type process has the advantage of en ^¬ abling the production of essentially symmetrical de- posits of materials on both sides of e.g. an essen ^¬ tially planar substrate. I.e. the first deposit of ma ^¬ terials and the second deposit of materials can be es ^¬ sentially symmetrical in view of e.g. thickness and form. Thus, according to one embodiment of the present invention the thickness of the passivating material on each side of the substrate is essentially equal. Ac ^¬ cording to one embodiment of the present invention the thickness of the reflection adjusting material on each side of the substrate is essentially equal. According to one embodiment of the present invention the thick ^¬ ness of the passivating material on each side of the substrate is 1 - 20 nm, preferably 5 - 15 nm. Accord ^¬ ing to one embodiment of the present invention the thickness of the reflection adjusting material, having a single layer structure, on each side of the sub ^¬ strate is 50 - 65 nm, preferably 53 - 56 nm. According to another embodiment of the present invention the thickness of the reflection adjusting material, being in the form of a nanolaminate, on each side of the substrate is determined by the formula of: thickness (d) = 560 nm/4n, wherein n is the refractive index of the nan- olaminate.

According to one embodiment of the present invention the substrate comprising silicon is a substrate having an essentially planar form.

According to one embodiment of the present invention the substrate comprises crystalline silicon (c-Si) . According to one embodiment of the present in ^¬ vention the substrate comprises multi- and/or mono- crystalline silicon or any modification thereof. Mono- like crystalline silicon can be mentioned as an exam ^¬ ple of such a modification.

According to one embodiment of the present invention the substrate comprising silicon is electrically conductive. According to one embodiment of the present invention the substrate comprising silicon is in the form of an at least one electrically conducting layer in an arrangement for a photovoltaic cell.

According to one embodiment of the present invention the structure further comprises a conductive electrode on the second deposit of materials. The con ^¬ ductive electrode comprises metal. According to one embodiment of the present invention the structure fur- ther comprises a layer comprising aluminum on the second deposit of materials. According to one embodiment of the present invention the method further comprises the step of forming a conductive electrode on the se ^¬ cond deposit of materials. According to one embodiment of the present invention the method further comprises the step of forming a layer comprising aluminum on the second deposit of materials.

According to one embodiment of the present invention the structure further comprises a cover sub- strate on top of the first deposit of materials. Ac ^¬ cording to one embodiment of the present invention the cover substrate comprises glass or plastic. According to one embodiment of the present invention the method further comprises forming a cover substrate on top of the first deposit of materials.

According to one embodiment of the present invention the photovoltaic cell structure comprises e.g. any other layers or coatings for different pur ^¬ poses as needed.

According to one embodiment of the present invention depositing the passivating material comprises depositing aluminum oxide, aluminum oxide : titanium (Al ₂ 0 ₃ :Ti) or a nanolaminate of aluminum oxide/titanium oxide (Al ₂ 0 ₃ /Ti0 ₂ ) .

According to one embodiment of the present invention depositing the reflection adjusting material comprises depositing titanium oxide, titanium oxide: aluminum (Ti0 ₂ :Al) or a nanolaminate of titanium oxide/aluminum oxide (Ti0 ₂ /Al ₂ 0 ₃ ) .

According to one embodiment of the present invention depositing the reflection adjusting material comprises depositing zinc oxide (ZnO) , zinc ox- ide:aluminum (ZnO:Al), zinc sulfide (ZnS) , tantalum oxide (Ta ₂ 0 ₅ ) , hafnium oxide (Hf0 ₂ ) or zirconium oxide (Zr0 ₂ ) , any mixture thereof or any nanolaminate there ^¬ of with aluminum oxide (A1 ₂ 0 ₃ ) .

An advantage of the ALD-type process used for depositing the reflection adjusting material is that its refractive index can be accurately controlled by adjusting the pulsing cycle (s) used for the deposi ^¬ tion. Without limiting the present invention to any specific material, it can be mentioned that by includ ^¬ ing aluminum oxide in a layer comprising titanium oxide in a controlled manner, a material with appropri ^¬ ate refractive index can be formed. The ALD-type pro ^¬ cess enables the inclusion of aluminum oxide into ti- tanium oxide comprising material by accurately con ^¬ trolling the introduction of a precursor for aluminum in an ALD-cycle of a precursor for titanium and a precursor for oxygen.

According to one embodiment of the present invention the growth or the deposition process in the ALD-type process is essentially thermally activated. It has been noticed that e.g. the passivation effect is enhanced when the ALD-type process is essentially thermally activated i.e. no plasma activation is em- ployed.

The thicknesses of the passivating material, as well as of the reflection adjusting material, can be increased in some embodiments of the present inven ^¬ tion by repeatedly exposing the surface to be deposit ^¬ ed to the different precursors in the reactions space such that a portion of them adsorbs onto the exposed surfaces, i.e. onto the deposition surfaces. In this way e.g. the passivation effect of the passivating ma ^¬ terial on the first and the second surface of the sub ^¬ strate may be enhanced in some embodiments of the pre ^¬ sent invention.

According to one embodiment of the present invention depositing the passivating material comprises depositing the passivating material until the thickness of the material is 1 - 20 nm, preferable 5 - 15 nm. According to one embodiment of the present in- vention depositing the reflection adjusting material comprises depositing the reflection adjusting material as a single layer structure until the thickness of the material is 50 - 65 nm, preferably 53 - 56 nm. Accord ^¬ ing to another embodiment of the present invention de- positing the reflection adjusting material comprises depositing the reflection adjusting material, in the form of a nanolaminate , such that the thickness of the material is: thickness (d) = 560 nm/4n, wherein n is the refractive index of the nan ^¬ olaminate .

The precursors for the deposition processes of the passivating material and the reflection adjust ^¬ ing material can be selected from a large group of chemicals. According to one embodiment of the present invention the precursor for titanium is selected from the group of titanium tetrachloride (TiCl ₄ ) , titanium isopropoxide (Ti (OCH (CH ₃ ) ₂ ) 4) , titanium ethoxide (Ti (OCH ₂ CH ₃ ) ₄ ) , titanium tetramethoxide (Ti(OCH ₃ ) ₄ ) and titanium iodide (Til ₄ ) . According to one embodiment of the present invention the precursor for aluminum is selected from the group of TMA ( trimethylaluminum) , TEA (triethylaluminum) , A1C1 ₃ , AlBr ₃ , AlMe ₂ Cl AlMe ₂ OiPr, A10nPr ₃ and AlOnPr. According to one embodiment of the present invention the precursor for oxygen is selected from the group of H ₂ 0, 0 ₂ , 0 ₃ , ROHd, A10Et ₃ , A10iOr ₃ , H ₂ 0 ₂ , N ₂ 0 and N ₂ 0 ₄ . According to one embodiment of the present invention the precursor for zinc is selected from the group of dimethyl zinc (DMZ) and diethyl zinc (DEZ) . According to one embodiment of the present in ^¬ vention the precursor for tantalum is selected from the group of tantalum etoxide (TaEt ₅ ) , tantalum chlo ^¬ ride (TaCl ₅ ) According to one embodiment of the pre ^¬ sent invention the precursor for hafnium is selected from the group of HfCl ₄ , HfCl ₂ [N (SiMe ₃ ) ₂ ] ₂ , Hfl ₄ , Hf(OtBu) ₄ , Hf (OtBu) 2 (mmp) ₂ , Hf(mmp) ₄ , Hf(ONEt ₂ ) ,

Hf (NMe ₂ ) 4 , Hf (NEt ₂ ) , Hf (NEtMe ) . According to one em ^¬ bodiment of the present invention the precursor for zirconium is selected from the group of ZrCl , ZrCl ₂ [N (SiMe ₃ ) ₂ ] ₂ , Zrl ₄ , Zr(OtBu) ₄ , Zr (OtBu) ₂ (mmp) ₂ ,

Zr(mmp) ₄ , Zr(ONEt ₂ ) ₄ , Zr(NMe ₂ ) ₄ , Zr(NEt2)4, Zr (NEtMe) ₄ . Selecting the other process parameters for the deposi ^¬ tion processes will be obvious to the skilled person in light of this specification.

An advantage of the present invention is that when using the ALD-type process, and especially a batch type process, for depositing the passivating material as well as the reflection adjusting material e.g. an essentially planar substrate can be deposited on both sides in a single process step in the same re ^¬ action space by changing the precursors to which the deposition surface is to be exposed to in the reac ^¬ tions space. As the need for using separate reaction tools or for stopping the process between the deposi- tion processes of each side of the essentially planar substrate reduces the time needed for producing the product and also simplifies the overall process. An advantage of the present invention and es ^¬ pecially the use of the ALD-type process is that the thickness of the deposits on each side of the sub ^¬ strate can be accurately controlled. The simultaneous deposition of both sides of the essentially planar substrate results in the formation of essentially equally thick layers or deposits on both sides of the substrate .

An advantage of the present invention is that the refractive index of the reflection adjusting mate ^¬ rial can be accurately adjusted by controlled pulsing cycles and thus the overall material to be deposited.

An advantage of the present invention is that e.g. titanium oxide and aluminum oxide formed with the ALD-type process contain hydrogen, which is beneficial for crystalline silicon cells. The hydrogen is usually released during a firing step of the front electrode manufacturing process resulting in passivation of silicon .

The embodiments of the invention described hereinbefore may be used in any combination with each other. Several of the embodiments may be combined to ^¬ gether to form a further embodiment of the invention. A structure and a method, to which the invention is related, may comprise at least one of the embodiments of the invention described hereinbefore.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a further understanding of the invention and constitute a part of this specification, illus ^¬ trate embodiments of the invention and together with the description help to explain the principles of the invention. In the drawings:

Fig. 1 is a schematic illustration of a structure according to one embodiment of the present invention; and Fig. 2 is a flow-chart illustration of a method according to one embodiment of the present in ^¬ vention . DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made in detail to the embodiments of the present invention, examples of which are illustrated in the accompanying drawings.

For reason of simplicity, item numbers will be maintained in the following exemplary embodiments in the case of repeating components.

The description below discloses some embodi ^¬ ments of the invention in such a detail that a person skilled in the art is able to utilize the invention based on the disclosure. Not all steps of the embodi ^¬ ments are discussed in detail, as many of the steps will be obvious for the person skilled in the art based on this specification.

As presented above the ALD-type process is a method for depositing uniform and conformal films or layers over substrates of various shapes. Further, as presented above in ALD-type processes the deposit is grown by alternately repeating, essentially self- limiting, surface reactions between a precursor and a surface to be coated. The prior art discloses many different apparatuses suitable for carrying out an ALD-type process. The construction of a processing tool suitable for carrying out the methods in the fol ^¬ lowing embodiments will be obvious to the skilled per- son in light of this disclosure. The tool can be e.g. a conventional ALD tool suitable for handling the pro ^¬ cess chemicals. Many of the steps related to handling such tools, such as delivering a substrate into the reaction space, pumping the reaction space down to a low pressure, or adjusting gas flows in the tool if the process is done at atmospheric pressure, heating the substrates and the reaction space etc., will be obvious to the skilled person. Also, many other known operations or features are not described here in de ^¬ tail nor mentioned, in order to emphasize relevant as ^¬ pects of the various embodiments of the invention.

The structure of Fig. 1 and the method of

Fig. 2 illustrate, respectively, a structure and the corresponding method for producing the structure according to one embodiment of the invention.

In Fig. 1 the arrows schematically indicate the direction of incident light. In the n-type silicon photovoltaic cell structure 1 of Fig. 1 the first de ^¬ posit of materials 3a and the second deposit of mate ^¬ rials 3b having an essentially symmetrical form are situated on each side of an essentially planar sub- strate 2. Both the first deposit of materials 3a and the second deposit of materials 3b comprise passivat- ing material comprising aluminum oxide deposited on the first surface 2a of the substrate and on the se ^¬ cond surface 2b of the substrate, respectively. The passivating material enable efficient passivation of both sides of the substrate comprising silicon thereby minimizing surface recombination on the interface between the substrate 2 and the passivating material. In the structure of Fig. 1 the first deposit of materials 3a and the second deposit of materials 3b further com ^¬ prise reflection adjusting material on the passivating material. In the n-type silicon photovoltaic cell structure of Fig. 1 the reflection adjusting material of the first deposit of materials 3a efficiently di- rect the incident sunlight towards the substrate com ^¬ prising silicon and the reflection adjusting material of the second deposit of materials 3b efficiently re ^¬ flects sunlight that has reached the intermediate sur ^¬ face between the passivating material and the reflec- tion adjusting material in the second deposit of mate ^¬ rials 3b back towards the substrate comprising sili ^¬ con. A conductive layer 4 comprising aluminum is formed on the second deposit of materials 3b, whereby the reflection adjusting material of the second depos ^¬ it of materials 3b lies between the passivating mate ^¬ rial of the second deposit of materials 3b and the conductive layer 4. The substrate has an n-type base and a thin p-type conductive layer, which, however, is not specifically presented in Fig. 1. The structure of Fig. 1 further comprises a cover substrate 5 on top of the first deposit of materials 3a.

Fig. 2 presents one embodiment of how to car ^¬ ry out the method for producing the structure of Fig. 1, i.e. how to produce the symmetrical structure of the first deposit of materials and the second deposit of materials on both sides of the essentially planar substrate comprising silicon in one process step.

The embodiment of Fig. 2 begins with bringing the substrate comprising silicon 2 into a reaction space (step 1)) for carrying out an ALD-type process.

The reaction space is pumped down to a pres- sure suitable for forming the first deposit of materi ^¬ als and the second deposit of materials. The reaction space can be pumped down to the suitable pressure us ^¬ ing e.g. a mechanical vacuum pump or, in the case of atmospheric pressure ALD systems and/or processes, gas flows can be set to protect the deposition zone from the atmosphere. The substrate comprising silicon 2 is also heated to a temperature suitable for forming the deposits 3a, 3b by the used method. The substrate com ^¬ prising silicon 2 can be introduced to the reaction space through e.g. an airtight load-lock system or simply through a loading hatch. The substrate 2 can be heated by e.g. resistive heating elements which also heat the entire reaction space.

After the substrate 2 and the reaction space have reached the targeted temperature and other condi ^¬ tions suitable for deposition, the first surface 2a and the second surface 2b of the substrate can be con- ditioned such that the passivating material may be es ^¬ sentially directly deposited on the first surface and the second surface, respectively. This conditioning of the first surface 2a and the second surface 2b on which the passivating material is to be deposited can include chemical purification of the surfaces of the substrate from impurities and/or oxidation. Especially removal of oxide is beneficial when the silicon sur ^¬ face has been imported into the reaction space via an oxidizing environment, e.g. when transporting the exposed silicon surface from one deposition tool to another. The details of the process for removing impurities and/or oxide from the surface of the substrate comprising silicon will be obvious to the skilled per- son in view of this specification. In some embodiments of the invention the conditioning can be done ex-situ, i.e. outside the tool suitable for ALD-type processes. An example of an ex-situ conditioning process is etch ^¬ ing for 1 min in a 1 % HF solution followed by rinsing in Dl-water.

After the silicon substrate 2 has been condi ^¬ tioned, an alternate exposure of the deposition sur ^¬ faces to different precursor chemicals is started, to form passivating material comprising aluminum oxide directly on the first 2a and the second 2b surface, respectively, of the substrate 2 (Step a) in Fig. 1) . Each exposure of the deposition surfaces to a precur ^¬ sor results in the formation of additional deposit on the deposition surfaces, as a result of adsorption re- actions of the corresponding precursor with the deposition surfaces. In this specification, unless otherwise stated, the term "deposition surface" is used to address the surface of the substrate or the surface of the already formed deposit on the substrate. I.e. the term "deposition surface" should be understood as in ^¬ cluding the surface of the substrate, which has not yet been exposed to any precursor as well as the sur- face, which has been exposed to one or more precur ^¬ sors. Hence the "deposition surface" changes during the method of forming a deposit of materials on the substrate when chemicals get adsorbed onto the sur- face .

A typical reactor suitable for ALD-type depo ^¬ sition comprises a system for introducing carrier gas, such as nitrogen or argon into the reaction space such that the reaction space can be purged from surplus chemical and reaction by-products before introducing the next precursor chemical into the reaction space. This feature together with the controlled dosing of vaporized precursors enables alternately exposing the substrate surface to precursors without significant intermixing of different precursors in the reaction space or in other parts of the reactor. In practice the flow of carrier gas is commonly continuous through the reaction space throughout the deposition process and only the various precursors are alternately intro- duced to the reaction space with the carrier gas.

Thicknesses of the deposited passivating ma ^¬ terial on the first surface 2a of the substrate and of the deposited passivating material on the second sur ^¬ face 2b of the substrate can be controlled by the num- ber of exposures of the deposition surfaces to the different precursors. The thicknesses of the passivat ^¬ ing material on each side of the substrate are in ^¬ creased until a targeted thickness is reached, after which the reflection adjusting material is deposited simultaneously on the passivating material deposited on the first surface 2a of the substrate and on the passivating material deposited on the second surface 2b of the substrate (Step b) in Fig. 1) .

Deposition of the reflection adjusting mate- rial, in one embodiment of the present invention, is carried out in the ALD-type process in the same depo ^¬ sition tool directly after the deposition of the pas- sivating material has ended. In this case the simulta ^¬ neous deposition of the reflection adjusting material on both sides of the essentially planar substrate can begin simply by changing the precursor chemicals, or the ALD-cycle, from those used for the deposition of the passivating material to those suitable for the deposition of the reflection adjusting material.

The following example describes in detail how a structure comprising a substrate having a first sur- face 2a and a second surface 2b and comprising sili ^¬ con, a first deposit of materials and a second deposit of materials, wherein the first and the second depos ^¬ its of materials both comprises passivating material comprising aluminum oxide and reflection adjusting ma- terial comprising titanium oxide, can be fabricated.

EXAMPLE 1 - Processing of both sides of an essentially planar substrate by an ALD-type process

A first deposit of materials 3a and a second deposit of materials 3b were fabricated on an essen ^¬ tially planar substrate 2 comprising a first surface 2a and a second surface 2b on the essentially opposite side of the first surface of the substrate. Firstly passivating material of aluminum oxide (A1 ₂ 0 ₃ ) was de- posited on the first surface 2a of the substrate 2 comprising silicon and simultaneously on the second surface 2b of the substrate 2 comprising silicon. Thereafter reflection adjusting material of titanium oxide (Ti0 ₂ ) was deposited on the passivating material deposited on the first surface 2a and simultaneously on the passivating material deposited on the second surface 2b; steps a) and b) of the embodiment of the invention shown in Fig. 2, thus forming the first deposit of materials and the second deposit of materi- als, respectively. The passivating material as well as the reflection adjusting material were deposited using a P400 ALD batch tool (available from Beneq OY, Fin ^¬ land) .

The substrate 2 comprising silicon was a sub ^¬ strate having an n-type base and a p-type conductive layer or emitter suitable for a photovoltaic cell structure. The essentially planar substrate was posi ^¬ tioned inside the reaction space such that the first and the second surfaces of the substrate were both ex ^¬ posed to the reaction environment enabling simultane- ous deposition of both sides of the substrate in one reaction step.

The pressure and temperature during the depo ^¬ sition of the passivating material as well as during the deposition of the reflection adjusting material were about 1 mbar (1 hPa) and about 200 °C inside the reaction space.

In this example the carrier gas discussed above, and responsible for purging the reaction space, was nitrogen (N ₂ ) . The processing temperature was suf- ficient to result in a thermally activated ALD-type growth and no plasma activation was employed in this example .

As above discussed the passivating material was firstly deposited on the first surface and the se- cond surface of the substrate, respectively. Tri- methylaluminum (TMA) was introduced to the reaction space to expose the first and the second surfaces 2a, 2b of the substrate to this first precursor, the precursor for aluminum. After letting the carrier gas purge the reaction space from surplus first precursor and reaction byproducts, the resulting surfaces were similarly exposed to the second precursor, the precursor for oxygen, here ozone (0 ₃ ) . After this, the reac ^¬ tion space was purged again. This pulsing sequence was carried out once and then repeated 119 times before the process was ended. These 120 "ALD cycles" resulted in passivating material of aluminum oxide, with a thickness of approximately 10 nm being formed on the first and second surfaces of the substrate, respec ^¬ tively.

After the passivating material of aluminum oxide had been deposited the precursors were changed to start the deposition of titanium oxide as the re ^¬ flection adjusting material on the passivating material deposited on the first surface of the substrate and on the passivating material deposited on the second surface of the substrate, respectively. Titanium tet ^¬ rachloride (TiCl ₄ ) was introduced to the reaction space to expose the surface of the passivating materi ^¬ al to a precursor for titanium. After letting the carrier gas purge the reaction space from surplus first precursor and reaction byproducts, the resulting surfaces were similarly exposed to a precursor for oxy ^¬ gen, here water (H ₂ 0) . After this, the reaction space was purged again. This pulsing sequence was carried out once and then repeated 1099 times before the pro- cess was ended and the substrates were ejected from the reaction space and from the ALD tool. These 1100 "ALD cycles" resulted in reflection adjusting material of titanium oxide, with a thickness of approximately 55 nm, being formed on the passivating material.

More specifically, exposure of the deposition surfaces to a specific precursor was carried out by switching on the pulsing valve of the P400 ALD tool controlling the flow of the precursor chemicals into the reaction space. Purging of the reaction space was carried out by closing the valves controlling the flow of precursors into the reaction space, and thereby letting only the continuous flow of carrier gas flow through the reaction space. The pulsing sequence in this example was in detail as follows for the passiv- ating material: 0.5 s exposure to trimethylaluminum, 1.0 s purge, 1.0 s exposure to ozone, 2.0 s purge. The pulsing sequence in this example was in detail as fol- lows for the reflection adjusting material: 0.5 s ex ^¬ posure to titanium tetrachloride, 1.0 s purge, 0.5 s exposure to water, 2.0 s purge. An exposure time and a purge time in these sequences signify a time a specif- ic pulsing valve for a specific precursor was kept open and a time all the pulsing valves for precursors were kept closed, respectively.

In the example above the precursor for alumi- num was trimethylaluminum, the precursors for oxygen were ozone and water and the precursor for titanium was titanium tetrachloride, but other precursors can also be used depending on the desired material struc ^¬ ture and its composition. The invention is not limited to using the aforementioned precursors in particular and the advantages of the invention can be obtained by the skilled person in light of this specification also with other precursors.

The invention is not limited to the above passivating material or reflection adjusting material but they can be chosen in accordance with what is pre ^¬ sented in this specification and their deposition cycles will be obvious for the skilled person based on this specification. The passivating material as well as the reflection adjusting material can be produced as a single layer material or as a multilayer material as will be obvious for the skilled person based on this specification.

After the first deposit of materials 3a and the second deposit of materials 3b were formed on the substrate comprising silicon in a single process step as described in example 1 above, a conductive elec ^¬ trode 4, e.g. a layer of aluminum, was fabricated on the second deposit of materials leaving the reflection adjusting material of the second deposit in between the passivating material of the second deposit of ma ^¬ terials and the conductive electrode 4. The conductive electrode 4 was fabricated on the second deposit of materials 3b by a screen printing method comprising e.g. printing an aluminum paste on the second deposit of materials, drying and curing the paste at high tem- perature using method steps, which will be obvious to the person skilled in the art. Also a cover substrate 5 was formed on top of the first deposit of materials 3a .

Test results have shown that in the structure of Fig. 1, fabricated with the method of Fig. 2 de ^¬ scribed above, the passivating material of aluminum oxide together with the reflection adjusting material of titanium oxide in a symmetrical manner on both sides of the substrate surprisingly enables advanta- geous passivation properties while also the reflection properties of the structure are optimized for the spe ^¬ cific application resulting in a more efficient photo ^¬ voltaic cell structure. An additional benefit of the method according to the embodiment of Fig. 2 is that, as presented above, both sides of the essentially pla ^¬ nar substrate can be simultaneously processed in a single ALD-type process.

It is obvious to a person skilled in the art that, with the advancement of technology, the basic idea of the invention may be implemented in various ways. The invention and its embodiments are thus not limited to the examples described above; instead they may vary within the scope of the claims.