GB2471128A

GB2471128A - Surface passivation of silicon wafers

Info

Publication number: GB2471128A
Application number: GB0910568A
Authority: GB
Inventors: Aina Suphellen; Kenta Nakayashiki; Erik Sauar
Original assignee: REC Solar AS
Current assignee: REC Solar AS
Priority date: 2009-06-18
Filing date: 2009-06-18
Publication date: 2010-12-22
Also published as: GB0910568D0; WO2010147483A1

Abstract

A method of producing wafer based solar panels, whereby the cells obtain an improved field passivation effect. A method for passivation of a surface comprises placing the silicon semiconductor in the deposition chamber of a dielectric chemical vapour deposition equipment, introducing precursor gases for forming a silicon nitride film on the exposed surface of the silicon semiconductor into the deposition chamber, initiating deposition of a silicon nitride film on the silicon semiconductor, doping the deposited silicon nitride film by introducing at least one gaseous compound containing an element which acts as an acceptor element into the precursor gases in the deposition chamber, and continuing depositing the silicon nitride film until the film obtains a thickness in the range of 2 - 500 nm. Preferably the acceptor dopant is boron. The invention also relates to solar cells comprising p-doped silicon nitride passivation layers produced by a similar method, whereby the silicon nitride film has an impurity density of between 109and 1015cm-2.

Description

I

Method for improved passivation and solar cell with improved passivation This invention relates to a production method of wafer based solar panels, and which the cells obtain an improved field passivation effect and solar cells with

improved field passivation effects.

Background

The world supplies of fossil oi1 are expected to be gradually exhausted in the following decades. This means that our main energy source for the last century will have to be replaced within a few decades, both to cover the present energy consumption and the coming increase in the global energy demand.

In addition, there are raised many concerns that the use of fossil energy is increasing the earth greenhouse effect to an extent that may turn dangerous. Thus the present consumption of fossil fuels should preferably be replaced by energy sources/carriers that are renewable and sustainable for our climate and environment.

One such energy source is solar light, which irradiates the earth with vastly more energy than the present and any foreseeable increase in human energy consumption. However, solar cell electricity has up to date been too expensive to be competitive with present grid power made by nuclear power, thermal power etc. This needs to change if the vast potential of the solar cell electricity is to be realised.

The cost of electricity from a solar panel is a function of the energy conversion efficiency and the production costs of the solar panel. Obviously, the search for cheaper solar electricity should thus be focused at high-efficient solar cells made by cost-effective manufacturing methods. Presently, the main production of solar cells is based on silicon as the photovoltaic material, and silicon based solar cells are expected to be the mainstay in the foreseeable future.

One important factor for enhancing the energy conversion efficiency of solar cells is electric insulation of the surface of the semiconductor. This is known as surface passivation of the solar cell, and is obtained by depositing one or more layers of a dielectric in order to reduce/avoid recombination of charge carriers at the interface between the deposited dielectric surface and semiconductor.

Silicon nitride films have seen more and more use as surface passivation layer in high-efficient solar cells due to the favourable combination of excellent passivation effect and anti-reflection effect. A further advantage of the silicon nitride film is that it may relatively easy and cost-effectively be deposited in industrially applicable manufacturing processes by use of chemical vapour deposition (CVD).

Prior art

Recently, the trend of employing thinner crystalline silicon solar cells has created a need for effective low-cost rear surface passivation due to the cell thickness reaching dimensions of the same order as the diffusion lengths of the charge carriers.

According to Dauwe et a!. [1], amorphous hydrogenated silicon nitride (SiNs) films deposited on low-resistivity p-doped crystalline silicon by low temperature (<400 °C) plasma-enhanced chemical vapour deposition (PECVD) have been shown to give very low recombination velocities (SRVs), and is thus a promising candidate for low-cost rear surface passivation films.

The low surface recombination velocities of Si/SiNs interfaces are probably due to two different reasons. The first reason is saturation of dangling bonds at the interface by atomic hydrogen released from the precursor gases (SilL1 and NH3), giving an effect due to decreased interface state density. The second reason is due to fixation of a high density of positive charges in the deposited SiNg-films, giving rise to a field-effect passivation. According to Weber and Jin [2], the fixation of positive charges in PECVD deposited SiNs-films is due to an over-stochiometric ratio of silicon in the precursor gases during the deposition of the film, typically giving a density of positive charge in the film in the order of 1012 cm2. The silane is decomposed into positively charged ions (cationic gas) and ammonia is decomposed into negatively charged ions (anionic gas).

However, when SiNg-films with excess of positive fixed charges are applied OIl p-doped semiconductors, the field effect of the positive charges will serve to withhold/fix negative charges at the surface boundary region of the semiconductor and thus create a negatively charged layer underneath the SiNs-film. This layer is known as an inversion layer. The inversion layer in p-doped semiconductors is known to create problems at the metal contacts on the back/rear of the cells, due to a short circuit effect in the vicinity of the electrode which reduces the collected current of the rear side of the solar cell.. This short circuiting effect is known as parasitic shunting.

Dauwe et al. [1] performed experiments on 1.5 Qcm FZ p-type silicon cells illuminated by AM1.5G 100 mW/cm2 at 25 °C. The tests were made for cells provided with a Si02 rear side passivation film and compared the cells energy conversion efficiency with similar cells provided with a PECVD deposited SiNs rear side passivation film. They found that the parasitic shunting effect reduced the cell energy conversation efficiency from 17.9 percent points with Si02-passivation film to 16.8 percent points with S1NX passivation film, a loss of 6.2 %. They showed further that by inserting a silicon oxide zone in the SiNX rear side passivation layer acting to electrically insulating the rear side electric contacts from the inversion layer, the parasitic shunting effect was almost eliminated giving a cell efficiency of 17.8 percent points.

US 2007/0186970 discloses use of a catalytic chemical vapours deposition process to form the silicon nitride passivation film. According to the document, there is in this process no problem with high-energy charged particles causing damage in the surface boundary of the semiconductor substrate, as is the case for plasma-enhanced chemical vapour deposition, or of degradation of the passivation film due to inclusion of charged particles.

Hoex et al. [3] shows from lifetime measurements, including a direct experimental comparison with thermal Si02, a-Si:H, and as-deposited a-SiNx:H, that A1203, which contains negative charges, provides an excellent level of surface passivation on highly B-doped c-Si with doping concentrations around 1019 cm3. The Al203 films, synthesized by plasma-assisted atomic layer deposition and with a high fixed negative charge density, limit the emitter saturation current density of B-diffused p+-emitters to 10 and -30 fAJcm2 on >100 and 54 cl/sq sheet resistance p+-emitters, respectively. These results demonstrate that highly doped p-type Si surfaces can be passivated effectively with negatively charged films.

WO 02/4 1408 discloses a method for removing the inversion layer in rear surface Si02 passivated cells by depositing a fluoride layer with negative charges onto the Si02-film.

Weber and Jin [2] teaches a method where a corona discharge is used to create and store negative charge in the silicon nitride films of silicon dioxide/silicon nitride stacks. Effective lifetime measurements on both textured and planar, as well as both boron diffused and non-diffused silicon samples passivated with silicon oxide/silicon nitride stacks, show that the creation of negative charge in the nitride layer results in an improvement in the surface passivation for all samples, with very low (<2 cm/s) effective surface recombination velocities demonstrated for planar, non-diffused samples. The manipulation of charge can be exploited to improve the conversion efficiency of silicon solar cells.

Objective of the invention The main objective of the invention is to provide a cost effective industrially applicable method for passivation of silicon based solar cells, and where the passivation effect provides effective surface recombination velocities of less than cm/s.

The objective of the invention may be realised by the features as set forth in the description of the invention below, and/or in the appended patent claims.

Description of the invention

The invention is based on the realisation that a cost effective industrial applicable method of forming a passivation film with similar excellent passivation properties as the passivation films with negative charge in the prior art above, may be obtained if the negative charge in the passivation layer may be introduced during presently industrially employed deposition processes based on chemical vapour deposition.

Thus in a first aspect, the present invention relates to a method for surface passivation of a silicon semiconductor, where the method comprises: -placing the silicon semiconductor in the deposition chamber of a dielectric chemical vapour deposition equipment, -introduce precursor gases for forming a silicon nitride film on the exposed surface of the silicon semiconductor into the deposition chamber, -initiate deposition of a silicon nitride film on the silicon semiconductor, -doping the deposited silicon nitride film by introducing at least one gaseous compound containing an element which acts as an acceptor element into the precursor gases in the deposition chamber, and -continue depositing the silicon nitride film until the film obtains a thickness in the range of 2-500 nm.

Thus the first aspect of the invention obtains introduction of negative charges in the passivation film (silicon nitride film) by introducing atoms which functions as electron acceptors in the silicon nitride film. Negative charges in the silicon nitride film will withhold positive charges on the wafer surface; in just the same manner as positive charges in the passivation film withholds negative charges and creates the inversion layer. The positive charges on the surface of the p-type doped wafer will have a beneficial effect on the solar cell structure by providing an increased field passivation effect. The layer of positive charges in the surface region of the semiconductor wafer may be considered as an inversed inversion layer, or an accumulation layer, and will be denoted "the positive charged layer" in this application.

In a second aspect, the invention relates to solar cells, where the solar cells comprises: -a silicon semiconductor wafer or film where at least on the front or rear surface, alternatively both surfaces, have at least one layer of silicon nitride deposited by chemical vapour deposition, -the silicon nitride film contains an element which acts as an acceptor element in the film and thus form negatively charged film, and where -the concentration of dopants in the silicon nitride film results in a negative charge of the film in the range from i09 and 1015 cm2.

The invention will practically eliminate the parasitic shunting effect and obtain the beneficial combined effect of the excellent chemical passivation effect of the silicon nitride film and the field passivation effect of the fixed positive layer.

Another advantage is that this effect may be obtained by employing presently implemented infrastructure for deposition of passivation films in the photovoltaic industry, and thus constituting a very cost effective solution to the problem of parasitic shunting which may relatively easily be implemented in existing production lines.

The term "acceptor" as used herein means any dopant atom added to the silicon nitride film which can contribute to form a negatively charged film. The formation of negatively charged regions in the film is believed to be due to substitution of a silicon atom in the silicon nitride film with a dopant atom which has at least one less valence electron than the silicon atom. This substitution will result in formation of one or more molecular orbitals in one neighbouring silicon atom in the film with an electron vacancy, and this unsatisfied bond will extract an electron from the surroundings and thus form a silicon atom in the film with a net negative charge and thus form the negatively charged regions of the film.

Boron and aluminium are examples of suitable dopants.

The entire silicon nitride film may be doped by simply mixing the silicon nitride forming precursor gases with the dopant forming gas (for instance a boron or aluminium containing gas) in the deposition chamber during the entire deposition of the silicon nitride film. It also envisioned forming only a doped layer into the silicon nitride film. This may be obtained by introducing the dopant gases into the deposition chamber for only a period of the deposition process. The thickness of the doped layer and the dopant density in the layer may be controlled by regulating the time period when the dopant gases are introduced into the deposition chamber and the concentration of the dopant gases in the deposition chamber of the dielectric deposition equipment, respectively.

The invention may apply any known or conceivable chemical vapour deposition technique known to be able to form a silicon nitride film. Suitable deposition techniques includes, but are not limited by, Atmospheric pressure CVD (APCVD), Low-pressure CVD (LPCVD), Ultrahigh vacuum C\'D (UHVCVD), Aerosol assisted CVD (AACVD), Microwave plasma-assisted CVD (MPCVD), Plasma-Enhanced CVD (PECVD), Remote plasma-enhanced C\TD (RPECVD), Atomic layer CVD (ALCVD), Hot wire CVD (HWCVD), Catalytic CVD (Cat-CVD), hot filament CVD (HFCVD).

The invention may apply any known or conceivable precursor gas able to form silicon nitride by chemical vapour deposition. Examples of suitable precursor gases includes, but are not limited by, S1H4, SiCI2H2, N2, and NH3. The chemical reactions involved may be given as: 3SiH4 + 4NH3 -b Si3N + 12H2 3SiCl2H2 + 4NH3 -p Si3N4 + 6HCI + 6H2 2SiH4+N2 -2SiNH+ 3H2 SiH4 +NH3 -SINH + 3H2 The dopant forming gases may be any chemical compound which is in the gas phase at the conditions experienced in the deposition chamber of chemical vapour deposition equipment, and which contains an element from group III of the periodic table. The compound should preferably be a nitride or a hydride of the group III element in order to avoid introducing "foreign" or in any way polluting elements into the deposition chamber. Diborane, B2H6, is an example of such a compound, and which is suitable for forming the p-type doped silicon nitride film according to the first aspect of the invention.

The silicon nitride layer may be applied as a second or third dielectric layer. That is, the silicon semiconductor may be provided with one or two dielectric layer(s) before depositing the silicon nitride layer (including the doped silicon nitride layer).

The inventive idea of employing a dopant forming gas in the deposition chamber which provides a negatively charged silicon nitride film may also be applied to other dielectric films than silicon nitride. In this case the precursor gases forming the dielectric film are admixed with a dopant gas in the same manner as the precursor gases forming the silicon nitride film. The use of other negatively doped dielectric films may also be combined with the silicon nitride film according to the first or second aspect of the invention.

One example of a high efficient and much employed dielectric film on silicon based solar cells is silicon dioxide, Si02. Silicon dioxide may be applied by chemical vapour deposition using silane and oxygen at temperature in the deposition chamber between 50 -500 °C. It is also possible to employ dichlorosilane, SiCI2H2, nitrous oxide, N20, or an organic silicon compound.

Possible chemical reactions are: SiH4 + 02 -Si02 + 2H2 SiC12H2 + 2N20 -Si02 + 2N2 + 2HC1 By mixing a gas containing an element which acts as acceptor in the silicon dioxide film into the precursor gases, it is obtained a negatively charged silicon dioxide dielectric film which will form an accumulator layer in the surface region of the silicon semiconductor. The dopant forming gas may i.e. be B2H6.

Typical thickness of the doped silicon oxide film will be from 10 to 200 nm, but may also be outside this range.

The invention may apply any conceivable dielectric film suited to be used as surface passivation on silicon semiconductors and which may be deposited by chemical vapour deposition.

It is envisioned that the silicon dioxide passivation film may be applied as the only passivation film or in combination with the silicon nitride layer of the first and second aspect of the invention. The use of doped silicon dioxide may be obtained by simply substitute the NH3 or N2 precursor gas with 02 or N20 during deposition of the dielectric layer.

Another example of suited dielectric layer is hydrogenated amorphous silicon, a-H: Si, which may be formed by chemical vapour deposition of silane gas at a temperature in the deposition chamber of 50 -500 °C: SiFL1 -a-H:Si + 2H2 The film may be doped by introduction of a boron containing gas, i.e. diborane, B2H6. It is envisioned that the hydrogenated silicon film may be used in combination with silicon dioxide, silicon nitride or both. Typical thickness of the doped hydrogenated amorphous silicon film will be from 2 to 200 nm, but may also be outside this range.

Example embodiment

An example embodiment of a solar cell made according to the invention is presented in Figure 1.

The Figure shows a silicon semiconductor wafer 1 which is doped to form a front side layer 2 of n-type doping and a rear side layer 3 of p-type doping. The front side is provided with one dielectric layer 4 of silicon nitride and a grid of front side electric contacts 5 establishing electric contact with the n-type doped layer 2.

The rear side is provided with a first dielectric layer 6 of silicon dioxide and a second dielectric layer 7 of silicon nitride. The back side is provided with a metallic layer 8 forming the back side electric contact. The electric contact 8 is made with protrusions 9 which locally extend through the dielectric layers in order to make contact with the p-type doped layer 3.

An inversion layer 10 is also indicated in the Figure. In case of prior art which employs a non-doped silicon nitride dielectric layer 7, the inversion layer will be negatively charged and will thus result in parasitic shunting due to contact with the positive contact 9. However, by employing a doped silicon nitride layer 7 according to this invention, the inversion layer 10 will be inversed to form an accumulation layer which increases the current density of the solar cell. The contact between an accumulation layer 10 and electric contact 9 is no longer a problem, it is an asset.

The silicon nitride dielectric layer 7 is deposited by use of plasma enhanced chemical vapour deposition by use of SiH4 and NH3 as precursor gases admixed with B2H6. The deposition process is run at a temperature in the deposition chamber in the range from 250 to 450°C, and the process is run until a thickness of the silicon nitride dielectric layer 7 is in the range between 40-200 nm is obtained.

References 1 Stefan Dauwe et al. (2002), "Experimental evidence of Parasitic Shunting in Silicon Nitride Rear Surface Passivated Solar Cells", Frog. Photovolt: Res.Appl., 10:271-278.

2 Weber and Jin (2009), "Improved silicon surface passivation achieved by negatively charged silicon nitride films", Applied Physics Letters, 94, 063509 3 Hoex et al. (2007), "Excellent passivation of highly doped p-type Si surfaces by the negative-charge-dielectric A1203", Applied Physics Letters, 91, 112107

Claims

CLAIMS1. Method for surface passivation of a silicon semiconductor, where the method comprises: -placing the silicon semiconductor in the deposition chamber of a dielectric chemical vapour deposition equipment, -introduce precursor gases for forming a silicon nitride film on the exposed surface of the silicon semiconductor into the deposition chamber, -initiate deposition of a silicon nitride film on the silicon semiconductor, -doping the deposited silicon nitride film by introducing at least one gaseous compound containing an element which acts as an acceptor element into the precursor gases in the deposition chamber, and -continue depositing the silicon nitride film until the film obtains a thickness in the range of 2-500 nm.
2. Method according to claim 1, where -the dielectric layer is deposited by use of plasma-enhanced chemical vapour deposition.
3. Method according to claim 2, where -the precursor gases are SiH4 and NH3, -the temperature in the deposition chamber is kept between 50 -500 °C, and -the dopant forming gas is a gaseous compound containing boron.
4. Method according to claim 3 where -the precursor gases are SiH4 and one of N20 or an organic silicon containing precursor and 02, -the temperature in the deposition chamber is kept between 50 -500 °C, and -the dopant forming gas is a gaseous compound containing boron.
5. Method according to claim 3 or 4, where dopant forming gas is B2H6.
6. Method according to claim 5, where -the temperature in the deposition chamber is kept between 250 -400 °C, and -the thickness of the deposited film is in the range of 40 -200 nm.
7. Method according to any of the preceding claims, where the also comprises -forming a first dielectric layer onto the semiconductor of amorphous silicon or silicon dioxide, and then -depositing the silicon nitride layer as the second dielectric layer.
8. Method according to claim 7, where the deposition of the dielectric layer is performed by -employing SiH4 and one of N20 or an silicon containing organic precursor and 02 as precursor gases to form a first dielectric layer of Si02 on the semiconductor, then -employing SiH4, NH3, and B206 as precursor gases to form a p-type doped silicon nitride layer onto the first dielectric layer, and then -employing SiH4 and NH3 as precursor gases to form a non-doped silicon nitride layer onto the p-typed doped silicon nitride layer.
9. Solar cell, comprising: -a silicon semiconductor wafer or film where at least on the front or rear surface, alternatively both surfaces, have at least one layer of silicon nitride, -the silicon nitride film contains an element which acts as an acceptor element in the film and thus form a p-type doped region, and where -the concentration of p-type doped regions in the silicon nitride film results in a negative charge of the film in the range from and 1015 cm2.
10. Solar cell according to claim 9, where -the at least one deposited dielectric film is a silicon nitride film of thickness 2 - 500 nm, and -which contains elemental boron in a concentration providing a negative charge density in the range from and 1015 cm2.
11. Solar cell according to claim 9, where the semiconductor wafer contains -a first dielectric layer of amorphous silicon or silicon dioxide with thickness of 2 -500 nm, -a doped silicon nitride layer onto the first dielectric layer, which have thickness in the range of 10 -200 nm and which is doped with boron in a concentration providing a negative charge density in the range from i09 and i0 cm2, and -a final non-doped silicon nitride layer onto the doped SiliCon nitride layer, and which has thickness in the range from 2 -500 nm.
12. Solar cell according to claim 9, where the semiconductor wafer contains -a first dielectric layer of doped amorphous silicon or doped silicon dioxide with thickness of 2 -500 nm, and which is doped with boron in a concentration providing a negative charge density in the range from i� and 1015 cm2, -a doped silicon nitride layer onto the first dielectric layer, which have thickness in the range of 10 -200 nm and which is doped with boron in a concentration providing a negative charge density in the range from i09 and i0 cm2, and -a final non-doped silicon nitride layer onto the doped silicon nitride layer, and which has thickness in the range from 2 -500 nm.
13. Solar cell according to claim 9, where the semiconductor wafer contains -a first dielectric layer of doped amorphous silicon or doped silicon dioxide with thickness of 2 -500 nm, and which is doped with boron in a concentration providing a negative charge density in the range from 1O9 and 1015 cm2, and -a doped silicon nitride layer onto the first dielectric layer, which have thickness in the range of 10 -200 nm and which is doped with boron in a concentration providing a negative charge density in the range from i09 and 1015 cm2.
14. Solar cell according to claim 9, where the semiconductor wafer contains -a first dielectric layer of doped amorphous silicon or doped silicon dioxide with thickness of 2 -500 nm, and which is doped with boron in a concentration providing a negative charge density in the range from i09 and 1015 cm2, and -a non-doped silicon nitride layer onto the doped silicon nitride layer, and which has thickness in the range from 2 -500 nm.
15. Solar cell according to claim 9, where the semiconductor wafer contains -a first dielectric layer of amorphous silicon with thickness of 2 -200 nm, -a second dielectric layer of silicon dioxide with thickness of 10 -200 nm, and -a third dielectric layer of silicon nitride with thickness of 2 -500 nm, and where -at least one of the first, second or third dielectric layer is doped with boron in a concentration providing a negative charge density in the range from and 1015 cm2.