DE19919742A1 - Doped silicon substrates are coated with an anti-reflection layer, especially of silicon nitride for solar cells, by sputter deposition using silicon electrodes alternately connected as cathode and anode - Google Patents

Abstract

Anti-reflection coating for doped silicon substrates is produced by sputter deposition using silicon electrodes alternately connected as cathode and anode. Doped silicon substrates are coated with an anti-reflection layer in a vacuum chamber containing a sputtering unit with a magnet system and two or more overhead electrodes in a controlled oxygen atmosphere, the electrodes consisting of silicon and being electrically connected for alternately forming cathode and anode electrodes for the gas discharge.

Description

Solar cells are components that light in elec convert tric energy. Usually exist made of a semiconductor material (mostly silicon), that contains n- or p-type regions. The Be rich are called emitters or bases. Positive and ne generated by incident light negative charge carriers are ge at the pn junction separates and can open through metallic contacts the respective areas.

In the simplest form, solar cells consist of all-over base and emitter areas, whereby the emitter on the side facing the light lies. This allows the base through the possible  wise all-over application of metal on the Back can be contacted. The emitter comes with a grid contacted with the goal, if possible to remove many load carriers and as little as possible Light through shading on the metal contact for the Losing solar cell.

To optimize the performance of the solar cell is trying to make up the optical losses Keep it as small as possible due to the reflection. He this is sufficient by the so-called deposition Anti-reflective coatings (ARC) on the surface the solar cell. These have a refractive index whose value is between that of silicon (n = approx. 3.7) and that of air (n = 1) or that of glass (n = 1.5 in the case of encapsulation in photovoltaic modules). Furthermore, the Layer thickness of the ARC chosen so that the ener most important spectral range just de structural interference of the reflected light results. Even multi-layer systems or layers with are continuously changing refractive index possible.

In the manufacture of solar cells, Antire flex layers made of the following materials sets: silicon dioxide (refractive index n = approx. 1.5), Magnesium fluoride (n = approx.1.4), silicon nitride (n = 1.9-2.3) and titanium dioxide (n = 2.0-2.3). in the Industrial solar cell manufacturing However, the two will be the last two used. In combination with the glass The disc of the module encapsulation is its refractive index  cheaper. In addition, the refraction index for titanium dioxide and silicon nitride in the Set the limits described above. Typical Layer thicknesses are then at 65 nm-75 nm. Be agreed ARCs are also able to the waiters passivate the area of the solar cell, d. H. the Probability of charge carriers to the super area and recombine there, ver wrestles. Such layers have so far been implemented made of silicon nitride or silicon dioxide.

Titanium dioxide is produced in the solar cell industry in particular by chemical vapor deposition at atmospheric pressure (APCVD = At mospheric Pressure Vapor Deposition). The process takes place e.g. B. on the hydrolysis of Tetraisopro poxidtitanat (TPT, Ti (OC ₃ H ₇ ) ₄ ) at a temperature of 280 ° C. An APCVD system typically consists of a conveyor belt on which the wafers are transported through a heated area, in which the chemical vapor is supplied and is deposited on the silicon wafers.

A previously implemented method for applying silicon nitride layers for the production of solar cells is thermal silicon nitride (TSiN). The silicon nitride is formed by feeding nitrogen to a silicon wafer at a very high temperature (<1000 ° C). The process typically takes place in a quartz tube in order to realize a clean environment of the wafer, since at the temperatures used a contamination of the wafer, e.g. B. by the diffusion of metals, the performance potential of the cells is greatly limited. At the high temperatures, the nitrogen dissociates and forms the silicon nitride layer with the silicon of the wafer. The separation rates are very low and the purity requirements are very high. In the field of industrial solar cell production, silicon nitride is produced using a plasma-assisted chemical vapor deposition process (PECVD-plasma enhanced chemical vapor deposition). Here, the gases silane SiH ₄ and ammonia NH ₃ or nitrogen N _{2 are} supplied in a low-pressure environment (approx. 10-100 Pascal) at a temperature of 80-400 ° C above the silicon surface to be coated. The gas molecules are dissociated by an alternating voltage field applied from the outside and a silicon nitride layer forms on the wafer. By using silane, additional hydrogen is built into the layer. Through subsequent temperature steps (with T <300 ° C), the hydrogen diffuses into the silicon, whereby an improvement in particular of relatively low-value material is achieved. This is one of the most important reasons why silicon nitride is the most popular anti-reflective material in solar cell production today. From the deposition time of the silicon nitride layer for a layer thickness of approx. 75 nm is approx. 10 min. al but before and after cycles for vacuum reconstruction and dismantling are necessary so that the total process time is about 30-40 min. A system concept is used in industrial solar cell production, in which a large number of silicon wafers (approx. 80-250) are coated at the same time in order to achieve the required throughput of approx. 100 wa / hour. Here, the solar cells are processed in a quartz tube (see Fig. 2). The plasma is generated by graphite electrodes, which are attached at a distance of approximately 13 mm either parallel or vertically to the longitudinal axis of the tube. There are small holders on these electrodes into which the wafers are inserted.

The following disadvantages arise for the production of titanium dioxide in the APCVD process:

• 1. Since the deposition of the titanium dioxide is not ge is set up, the entire reactor space is included Titanium dioxide covered. With a certain Layer thickness begins to flake off the titanium dioxide and also pollutes those on the Conveyor belt located wafer. This is necessary changes regular (daily) cleaning cycles, in which the equipment is switched off got to.
• 2. In contrast to silicon nitride, Ti passivates tandioxide does not surface and it will also no hydrogen for material passivation introduced, resulting in a lower potential for high cell efficiencies than with silicon nitride exists.

The following disadvantages arise for the production of silicon nitride using the PECVD process:

• 1. The handling of the wafers for loading the Graphite electrodes is very expensive and will mostly done manually and in particular with thin silicon wafers, as they will in the future are preferred for cost reasons there is a great risk of breakage. The manual Handling is very personnel and therefore cost intensive.
• 2. The necessary for the required layer thickness ge relatively long process time complicates the rea lization of concepts in which only one or few wafers are processed at the same time. A continuous system using the PECVD process for the required throughput therefore becomes large and complex.
• 3. The undirected deposition creates in entire reaction space and in particular on a silicon nitride for the graphite electrodes layer that after a certain period of time scrolling tends. Therefore, after about every the 70th process included a cleaning cycle be during which the system is not can continue to be operated.
• 4. Through the holder on the graphite electrodes a part of the cell area is shadowed, so that there is no separation and so on an inhomogeneous visual impression is created.

A method for producing solar cells in a controlled oxygen atmosphere using a sputtering method (US 4,358,478) is also known, a photoconductive Se-As-Te layer on a substrate made of quartz glass or ceramic and an optically transmissive ITO layer thereon. Layer is sputtered on. For this purpose, the target of the sputtering cathode contains 90% In ₂ O ₃ and 10% SnO ₂ , a mixture of argon and oxygen at a pressure of 3.5.10 ^-3 Torr being used as the process gas and the sputtered ITO layer having a thickness between 500 Å and 5000 Å.

Also known is a solar cell that operates on a Substrate is applied, which is a metal plate and an elek attached to the metal plate trically insulating layer on which a Variety of solar cells connected in series is consolidated, the electrically insulating Layer is a thin layer, one is not contains monocrystalline silicone mass, which by the plasma CVD process or with the spray gun le applied (sputter vapor deposition method) and has a carbon content of at least Has 10 atomic% (EP 0 137 512).

A multi-chamber is also known Multiple type glow discharge device Coating chambers and a device for continuous transportation of an elongated Substrate material web on a transport route, the passes through the coating chambers in order (EP 0 576 559), each coating chamber comprises: a process gas line for introducing egg process gas; a cathode, an electromagnetic table energy source, which is elec is connected trically; and facilities for on  close the process gas in a cathode area the chamber, taking electromagnetic energy which the cathode is supplied, the process gas dissociated and a process gas plasma in the catho generated the area, with a plasma profile unit in one of the coating chambers the transport path of the substrate material web is is arranged, has: a plasma profile; an ab shielding that is arranged so that it Plasma profile essentially includes and one immediately following cleaning / Delimitation area delimited; a pipeline to initiate a cleaning / passivie tion gas, the pipeline with the shield connection and from the process gas company device is separated, with the shield in immit telbaren proximity of the substrate is arranged to the Cleaning / passivation gas in the cleaning / Passivation area included and the on of process gas enters the cleaning / passi prevent crossing area; and a set up device for supplying electromagnetic energy for Plasma profile to the cleaning / passivation gas to dissociate and from it a plasma in the Rei to generate cleaning / passivation area.

The three coating chambers are so special arranges that the substrate material web the chambers passes through in succession, a first chamber has a plasma area that the deposition of an n-doped body made of silicon alloy effected on the substrate and a second chamber has a plasma area that the deposition  an essentially intrinsically conductive body Causes silicon alloy on the substrate, and a third chamber having a plasma area has the deposition of a p-doped body pers made of silicon alloy; being the pre direction further a first plasma profile unit, between the plasma area of the first Be stratification chamber and the plasma area of the two th coating chamber is arranged; and a second plasma profile unit, which between the plasma area of the second coating chamber and the plasma area of the third coating chamber is arranged.

A device for reactive coating of a substrate (EP 0 502 242) with an electrically insulating material, for example with silicon dioxide (SiO ₂ ), consisting of an alternating current source, which is connected to magnetron cathodes arranged in an evacuable coating chamber, and its targets, has also already been proposed are atomized and their atomized particles deposit on the substrate, where two floating outputs of the AC source are each connected to a magnetron cathode, both of which are provided side by side in a coating chamber and each have approximately the same spatial spacing from the opposite substrate, the Magnetron cathodes each have their own distribution line for the process gas and the distribution of the reactive gas flow to both distribution lines is controlled by a controller via a conductance control valve such that the measured voltage difference of the effective values of both cathodes corresponds to a nominal voltage - for which purpose the rms values of the rms voltage measurements connected to the cathodes are measured and fed to the controller via lines as direct voltages.

Finally, a method of making egg ner semiconductor device by means of microwaves derschlags known (EP 0 616 729), which an over between a layer of intrinsic semiconductors material and a layer of doped semiconductor has material, the process following the steps include: providing a coating processing chamber, placing a substrate in the chamber mer, arranging a powered supply voltage wire in the chamber near the Substrate, introduction of a semiconductor Precursor gas into the chamber; Bringing in Microwave energy in the chamber, the Ener gie acts so that it has a plasma from its own forms semiconductor gas and disassembled this to a Apply layer of intrinsic semiconductor material, Supply of the bias wire with voltage and applying a layer of doped half conductor material on the layer of Eigenleilei Termatieral, the bias wire only during a portion of the time the egg is applied with a semiconductor layer positive voltage is supplied above 50 V.

The object of the present invention is to achieve grunde, a process for coating substrate doped silicon with an anti-reflection layer to indicate the disadvantages of the known Avoids procedures and a particularly stable Coating process with significantly improved loading stratification rate with maximum Antire flex properties and at the lowest cost and least effort for cleaning the coating device allowed.

According to the invention, this object is achieved by that the anti-reflective layer by means of a Vacuum chamber arranged atomization device with a magnet system and at least two above it arranged electrodes is deposited, wherein the electrodes are made of silicon and elec trisch are switched so that they alternately Ka are the method and anode of a gas discharge.

Further details and features can be found in Pa claims described and characterized in more detail net.

By sputtering with a known one Coating device with two with Mittelfre frequency operated magnetron cathodes Disadvantages of the known coating method, that with a single magnetron cathode, which in controlled oxygen atmosphere ben, so that obviously changes in the space charge zone occur, the effect  negatively affect the degree of the solar cell the.

The invention allows a wide variety of designs opportunities for; one of them is below described.

Mono and multicrystalline silicon wafers (Basic doping: boron, 2 W cm or 0.5 W cm spec. Wi derstand) were in the sputterver according to the invention drive under variation of the parameters performance, Gas flow, pressure, belt speed and layer thickness (50-350 nm) coated on one side. Moreover the transmission data of the layers were recorded taken (Table 1). The multicrystalline wafers were previously generated with a phosphorus doping Emitter has been provided to the cell pro enable cessation.

Table 1

Process parameters of a SiN sputtering process with two cathodes equipped with Si targets with medium frequency (40 kHz)

The quality of the process and the layers according to the invention were verified in the following tests:

• 1. The monocrystalline wafers became life continuous measurement using Microwave-Photo-Conduc dance decay used to the surfaces to be able to determine passivation properties nen.
• 2. The multicrystalline wafers were further processed into solar cells. The wafers, which had favorable conditions for an efficient cell due to their refractive index (n = approx. 2.0) and layer thickness (approx. 75 nm), were selected. 2.2 cm ² large solar cells were made from them. The contact was established by selective opening of the nitride with subsequent vapor deposition and galvanizing of the contacts.

The visual inspection alone shows that very good thickness homogeneity of the deposited Layers. By subsequent tempering from the lifetime measurements of surface recombina tion speeds of less than 1000 cm / s, egg good to very good surface passivation accordingly.  

Quite good solar cell parameters were measured for all four wafers (Table 2). The two best solar cells showed efficiencies η of 13.2%. The fact that the open circuit voltage V _oc with up to 592 mV shows no deviation from comparable cells without sputtered antireflection layer is evidence that the sputtering process according to the invention does not cause any deterioration in the material quality. The measured short-circuit current densities J _sc up to 30 mA / cm ² are quite high for the technology used and proof for good light coupling and material quality. From this it can be deduced that the sputtering did not cause any significant damage to the cell surface, ie the emitter and the space charge zone! The antireflection effect and the absorption behavior of the layer also give no significant losses.

Table 2

Performance parameters of the solar cells manufactured as an application example

Claims (6)

1. Process for coating substrates doped silicon with an anti-reflection layer for solar cells by means of an in egg in a vacuum chamber direction with a magnet system and mind at least two electrodes arranged above it a controlled oxygen atmosphere, the electrodes being made of silicon and are electrically connected so that they alternating cathode and anode of the gas outlet are. 2. The method according to claim 1, characterized records that the substrate with Bor do is silicon blank, in the above right, adjacent to the anti-reflective layer Layer introduced in the hot phosphorus furnace is, the phosphorus-doped layer is the n-type and the layer below the Phosphorus doping the p-type layer bil det. 3. Process according to claims 1 and 2, characterized in that the anti-reflective layer which has never been beaten as Si ₃ N _{4 is} sputtered with an alternating current of approximately 40 kHz from two alternately operated cathodes. 4. The method according to the preceding claims, characterized in that the anti bottom of the reflective layer A blank from an electrical substrate conductive material is applied and in the anti-reflective coating grooves except for the n-forming layer of the substrate Cut layer, for example cut are etched, the grooves with an elec trically conductive substance, for example with a metal paste. 5. Procedure according to one or more of the above forthcoming claims, characterized net that Fremdato during the spraying process me, e.g. As hydrogen or cesium, in the divorced silicon nitride layer or ange excited or ionized atoms or molecules be stored in a in the Pro integrated plasma source be to the optical and electrical egg to modify the properties of the layers. 6. Procedure according to one or more of the above forthcoming claims, characterized net that two layers sequentially behind are separated from each other, the approx 10-40 nm thick layer a high proportion Silicon and possibly added Fremdato men contains what by a low intake of nitrogen and a large proportion of the other reaction gases and the inert gas (Argon) is reached, being for the second  40-70 nm thick layer of nitrogen drove significantly increased and no more Foreign atoms are added.