WO2011047875A2

WO2011047875A2 - Method for the light-induced, galvanic pulsed deposition for forming a seed layer for a metal contact of a solar cell and for the subsequent reinforcement of said seed layer or said metal contact and arrangement for carrying out the method

Info

Publication number: WO2011047875A2
Application number: PCT/EP2010/006468
Authority: WO
Inventors: Valentin Radtke; Norbert Bay; Monica Aleman
Original assignee: Fraunhofer-Gesellschaft Zur Förderung Der Aangewandten Forschung E.V.; Albert-Ludwigs-Universität Freiburg
Priority date: 2009-10-23
Filing date: 2010-10-22
Publication date: 2011-04-28
Also published as: DE102009051688A1; WO2011047875A8; CN102686784A; WO2011047875A3; KR20120110101A; EP2491166A2

Abstract

The present invention relates to a light-induced, galvanic deposition method, wherein the potential difference between a first metal contact and an auxiliary electrode is varied as a function of time according to a predefined voltage/time characteristic and a light radiation on a solar cell is varied as a function of time according to a light radiation/time characteristic.

Description

Method for light-induced galvanic pulse deposition for forming a seed layer for metal contact of a solar cell and for subsequently reinforcing this seed layer or metal contact and arrangement for carrying out the method

The present invention relates to a photoinduced galvanic pulse deposition method, with which a further metal contact can be formed for a solar cell having only a single metal contact. The metal contact to be formed is, in particular, the front-side contact of the solar cell (the existing individual contact is then the back contact or rear-side contact of the solar cell). The invention also relates to an arrangement corresponding to the method.

CONFIRMATION COPY The efficiency of a solar cell depends inter alia on the amount of electrical contact resistance of the solar cell. This is the contact between the semiconductor material of the solar cell (usually: doped silicon) and a metal which is intended to dissipate the current generated in the cell, ie a contact between the front side contact of the solar cell and the semiconductor material thereof and between the rear contact of the solar cell and the semiconductor material thereof. A reduction of this contact resistance leads to an increase in the efficiency of the solar cell.

The inventive method described below relates in particular to the contact formation on the n-side of any solar cell: In a standard p-type solar cell, this is the front side, ie the side facing the light. For this reason, the metal contacts to be formed on the front side must ensure a sufficiently high incidence of light into the solar cell, ie not overshadow the cell too much (as a rule, a plurality of individual metal contacts or metal contact sections are formed on the front side; However, in the following description of a front contact in the singular spoken: As a rule, this contact thus comprises a plurality of connected individual contacts or contact portions).

The conditions of a contact resistance which is as small as possible and at the same time an optimum ratio of free solar cell area to metal area are generally met by a microstructure of certain metals. Nevertheless, there are problems that occur individually and sometimes simultaneously: metals that do not form silicides, such as silver, have a relatively high contact resistance, and the liability of such metals on the silicon is usually unsatisfactory, that is, there is a risk that the metal contact peels off. Although metals which form silicides, for example nickel, show a lower contact resistance and also a better adhesion, the electrical conductivity of the metals themselves is much lower than, for example, that of silver. Furthermore, such metals also diffuse relatively deeply into the silicon and can adversely affect the electrical properties of the solar cell. Various methods for the production of metal contacts are known from the prior art: The standard method, which is almost exclusively used in industrial mass production, is the screen printing method. Here, the contact formation and the generation of the transverse conductivity of the

Metal finger in one operation, so the metal used must have a good intrinsic conductivity. A paste which holds Silberkoloiddispers ent ^¬ is printed by a microstructured mask on the solar cell. In the paste, in addition to the silver or glass particles and lead oxide contained ^¬ th, which are essential for the contact formation, which starts only at least 800 ° C. This Me ^¬ Thode has some drawbacks: Although the passivation must approximately layer not be opened and the adhesion to the silicon is better than the rei ^¬ nen silver due to the glass particles on the front of the solar cell, but which then formed contact is very inhomogeneous and overall has a relatively large contact resistance. Moreover, the method is only suitable if the emitter has sufficient surface area. has phosphorus concentration, that is highly doped (up to about 10 ²⁰ atoms / cm ³ ). Although this simplifies the passivation of the surface, but leads to a - compared with a low-doped emitter - lower open circuit voltage. Furthermore, the aspect ratio, ie the height to width ratio of the metal fingers of the formed contact, is unfavorable. This means a relatively large shading of the solar cell surface, so a loss of efficiency.

Other prior art methods are based on first forming the metal-semiconductor contact, hereinafter also referred to as seed layer in the context of the present invention, and then subsequently reinforcing this seed layer, that is, applying high conductivity metal to the seed layer. Thus, for example, a layer sequence of different metals, for example TiPd-Ag, is first vapor-deposited and then galvanically reinforced. The disadvantage here is that the passivation layer must first be opened in a microstructured manner, and that the process (since it requires a high vacuum and expensive materials such as Pd) is very cost-intensive.

Also, the formation of a seed layer of nickel is known: Here, nickel is electroless, that is deposited with chemical reducing agents from an aqueous nickel-containing nickel solution on the semiconductor and then annealed. The disadvantage here is that this process is extremely difficult to control, so it can be poorly reproduced and usually leads to inhomogeneous layers. Again, the passivation layer must first be opened microstructured. Moreover, it is known from the prior art to galvanically strengthen already applied seed layers of a metal contact, since this is a very cost-effective method. One of these methods describes the so-called light-induced electroplating, in which a solar cell having a front contact on the front side and a rear side contact is connected to an external voltage source by means of the rear side contact. Under illumination, the solar cell acts as a current or voltage source for the galvanic process, the front-side contact being the cathode (low potential) and the back-side contact the anode of the galvanic cell.

Since the rear side contact is usually made of aluminum or an aluminum alloy (here too is spoken of a back contact in the singular, although it may also be several individual electrical contacts, which are angeord ^¬ net on the back), the anodic Process of this galvanic cell, the electrochemical ^¬ solution of aluminum, so the resolution of the back ^¬ side contact. However, this must be prevented, for which purpose an additional auxiliary electrode is necessary: this, a kon ^¬ stante potential difference is applied in such a way between the back of the solar cell or the back ^¬ side contact and of these consisting of the deposited Me ^¬ tall auxiliary electrode, that the back side opposite to the auxiliary electrode is negatively po ^¬ larized (ie the backside contact must be at a lower or more negative potential than the auxiliary electrode or the anode). As a result, instead of the anodic dissolution of the back contact, the anodic dissolution of the auxiliary electrode (which is also referred to below as an anode as an alternative) takes place. is drawn) instead. Since the back contact of the solar cell is usually metallized over the entire surface, the connection of the back side contact with a voltage source is technically less demanding. The auxiliary electrode can thus be understood as a sacrificial anode.

In this light-induced galvanic arrangement, a potential difference arises between the light-facing side of the solar cell (front-side contact) and the auxiliary electrode, the magnitude of which is determined by the potential difference applied between the back and the auxiliary electrode on the one hand and between the back and the front the solar cell occurring potential difference on the other hand depends. The latter is determined by the incident on the solar cell light intensity.

However, the already existing and by the light-induced electroplating reinforced metal contacts on solar cells have some shortcomings: Thus, the electrical ^¬ cal conductivity of these metal contacts is less than the specific conductivity of the corresponding metal, the adhesion of the galvanically reinforced metal layer on the already existing metal layer is not optimal and the aspect ratio (ratio of height to width) of the metal contact is reduced by the galvanic process (however, the largest possible aspect ratio is desirable to keep the shading of the solar cell as low as possible). Finally, internal stresses in the metal contact to occur, which are undesirable, however, since they have a detrimental effect on the electrical Leitfä ^¬ ability and / or the adhesion of the metal contacts and adversely affect mechanical properties of the solar cell to win (in particular the latter in view of sinking wafer thicknesses, keyword: thin-film solar cells, in importance).

The object of the present invention is, starting from the prior art, the known light-induced galvanic amplification processes in which an existing metal contact of a solar cell by the light-induced electroplating reinforced or

is thickened so educate that the above-mentioned disadvantages are avoided. The object is therefore to achieve optimized electrical conductivity and optimized adhesion of a metal contact in a simple and reliable manner, to reduce internal stresses in the metal contact and to produce metal contact with an improved aspect ratio.

This object is achieved by the method according to claim 1 and by the arrangement according to patent claim 16. Advantageous embodiments of the method and the arrangement can be found in the dependent claims. Such a method and / or such an arrangement so for light-induced training and preferably additionally also for amplifying an electrical contact, in particular a frontseiti ^¬ gen contact, a solar cell, in particular a thin film solar cell can be According to the invention, use. Hereinafter, the present invention will now be described generally, then in the form of Ausführungsbei ^¬ play. The combinations of features, as can be seen from the individual exemplary embodiments, need not be realized in exactly these configurations in the context of the present invention, but can (based on the Expert knowledge of the person skilled in the art) can also be realized in other combinations within the scope of protection afforded by the claims.

The basic starting point of the present invention is to use the light-induced electrodeposition process not only to reinforce an already existing metal contact, but to apply to a solar cell, which has only one metal contact of its two applied metal contacts, in which therefore the second metal contact still completely is missing. Such le ^¬ diglich exactly one metal contact (typically: the rear-side contact) having solar cell can then at the opposite to the existing metal contact side (typically: the front or front side) to be open: That is, the passivation layer of the front side of the solar cell can, be ^¬ preferably microstructured, be open. The thus opened solar cell is then immersed in an electrolytic bath with an electrolyte containing those metal ions that lead to the the existing Me ^¬ tallkontakt opposite side for forming the second metal contact. The procedure is by the present invention, described in more detail below, light-induced galvanic deposition thus formed of the electrolyte contained in the requested metal ions ^¬ a seed layer for the second metal contact directly on the semiconductor material of the solar cell. This seed layer can then by the light-induced galvanic deposition decision procedure according to the present invention can also be further extended or thickened, so that the whole second metal contact can be made with the invention ^shown SEN method. Thus, in the prior art, the light-induced electroplating has hitherto been exclusively used to reinforce existing metal contacts (eg screen printing fingers or randomly generated seed layers), according to the present invention the contact formation with the light-induced electroplating already takes place. According to the invention, contact formation is understood to mean the formation of a seed layer of a metal immediately adjacent to and / or in conjunction with a semiconductor material of the solar cell. According to the invention, both the light incident on the (initially having only a single metal contact) solar cell and at the same time between the already existing metal contact (as a rule: rear contact) of the solar cell and the

Auxiliary electrode potential (potential difference A URH) a predefined, time-varying characteristics. In particular, a time-dependent variation of the voltage-time characteristic (or the potential difference AURH) according to the invention and / or the time-dependent variation of the light irradiation time characteristic (or the light irradiation) can be a pulse-shaped variation. Thus, according to the invention, both the potential applied between the existing metal contact and the auxiliary electrode and the light irradiation have a pulse-shaped course (bigepulse method). According to the invention, therefore, both the potential difference AU _RH and the incident light intensity over time are not constant ("constant" is understood here to mean that the corresponding magnitude value is constant over time except for the naturally unavoidable statistical fluctuations), but both the potential difference AU _RH between already existing metal contact and the auxiliary electrode, as well as the light irradiation is varied in a time-dependent manner according to a predefined time characteristic. In this case, the voltage-time characteristic, with which the potential difference is varied over time, can also be understood as the corresponding current-time characteristic. In the context of the present invention, therefore, alternatively or cumulatively, a corresponding variation of current density values over time can also be realized. In the following, simplified is always spoken of a voltage-time characteristic, although this may thus alternatively or cumulatively also be a variation of current density values.

It is particularly advantageous if the two pulse sequences of the voltage-time characteristic on the one hand and the light irradiation time characteristic on the other hand are synchronized with each other: pulse sequences of the potential difference on the one hand and pulse sequences of the light irradiation on the other can be realized in the form of synchronized pulse sequences. Especially with such a synchronization of the two pulse sequences of the two influencing variables potential difference and light irradiation, a particularly advantageous contacting can be realized according to the invention.

This potential difference AU _RH must be <0 in the time average, since otherwise the galvanic dissolution of the already existing metal contact (that is to say in particular of the rear-side contact) would take place and not the galvanic dissolution of the auxiliary electrode. However, this must apply only on a temporal average, so that quite short periods are also possible (see also the following exemplary embodiments) in which these Condition is not met.

A synchronization of the voltage-time characteristic (hereinafter alternatively also referred to as the first characteristic) or a pulse sequence corresponding thereto and the light irradiation time characteristic (hereinafter also referred to as second characteristic) or a pulse sequence corresponding thereto is understood below as that both characteristics at least during a defined time interval (preferably: during their entire application) each have a periodic course (eg in the form of a pulse sequence), wherein the periodic course of the first characteristic and that of the second characteristic in the said time interval are matched.

The tuning can take place in different ways: Thus, the period of one charac ^¬ teristik can be an integer multiple of the period of the other characteristic.

Likewise, the frequency of one characteristic may be an integer multiple of the frequency of the other characteristic. However, it is also possible that both periods or frequencies are identical, but then the two characteristics are shifted from one another by a fixed, predefined time interval (for example, maxima of the first characteristic may be shifted by half a period with respect to maxima of the second characteristic) ,

The concept of synchronization However, the invention is not limited to periodic pulse trains: Thus according to the invention is a temporal Synchronisie ^¬ tion of the first characteristic and the second Cha ^¬ rakteristik also present when the two influencing large (ie, the potential difference AU _RH on the one hand and the light irradiation or the incident radiation amount on the other hand) during both successive periods or time intervals always both have the same state (such a state, for example, a defined voltage or a defined amount of light irradiated or no applied voltage or no applied quantity of light). Advantageously, in each case immediately consecutive time intervals or time segments each have one and the same time interval.

According to the invention, a synchronous pulse sequence of the two characteristics is realized particularly advantageously at the same time that at the side of the solar cell opposite the metal electrode already present there is temporarily no voltage generated by external influences, ie neither by

Lichteinstrahlung a voltage is generated, nor a potential difference AU _{RH is} applied. This side (usually the front of the solar cell) or the (growing) cathode is then temporarily on open cell potential (OCP of English open cell

Potential).

The following is understood to be a synchronous pulse sequence of the two characteristics: the pulse lengths of both the light pulses and the voltage pulses must be matched to one another such that periodically recurring phases of open cell potential occur at the front side of the solar cell. The open cell potential is the potential that adjusts itself to an electrode immersed in an electrolyte and over which no Electricity flows.

For solar cells this means that the front of the solar cell (light-facing side) is not under open cell voltage as long as the solar cell is illuminated and as long as a voltage is applied to the backside of the auxiliary electrode. Only a non-illuminated solar cell with simultaneous galvanic separation of the back of the voltage source leads to the open cell potential at the

Front side (and also at the back).

No voltage generated by external influences for the open cell voltage means the following: It must not fall light on the cell or a voltage between the back and auxiliary electrode are applied, otherwise a voltage between the front and back is generated and at the front no open Cell potential is applied. It is important to be careful because the expression "do not apply voltage" can be misinterpreted as "voltage difference = 0 V". Technically, the backside must therefore be galvanically isolated from the power source. For many voltage sources this is not the case, it is then only applied a voltage of 0 V, but this does not result in the open cell potential.

In particular, the voltage-time characteristic and / or the light irradiation time

Characterist ik periodically or in the form of pulse sequences can be varied over time.

Different voltage-time characteristics, light irradiation time characteristics and / or pulse sequences are conceivable (particularly advantageous). will be described in detail below): Thus, anodic pulses can be applied to the initial phase of a pulse routine, pulses of different sizes can also be superimposed, etc.

An anodic pulse is the application of a potential to a workpiece, which results in an anodic reaction (oxidation) on the workpiece, ie in the present arrangement a positive (high) potential. A cathodic potential is that which results in a cathodic reaction (reduction) on the workpiece, in the present arrangement a negative (low) potential.

It is also possible to pulse trains themselves, d. H. Between individual pulse trains, each with a voltage-time characteristic varying in the form of individual pulses over time and / or light irradiation time characteristic, time intervals occur in which no time variation of the voltage-time characteristic and / or the light irradiation Time characteristic takes place. The generated potential difference can also be changed step by step. The shape of individual pulses can be nonlinear in this case, in addition to triangular or rectangular pulses can also sinusoidal pulses, exponentially shaped pulses and / or two, three or more degrees

polynomial voltage pulses are applied.

Extending between the side of the to be grown, the second metal contact and the auxiliary electrode a ^¬ alternate voltage runs of the applied potential difference between the first metal contact (usually: back contact) and the auxiliary electrode in the same direction, that is, a cathodic potential at the Back leads z. B. to a cathodic potential on the front.

Advantageously, the potential difference _Δϋ ΚΗ a connected to a function generator between the first metal contact and the auxiliary electrode switched voltage source can be produced according to the invention: By means of the function generator is then generated by the voltage source applied between the first metal contact and the auxiliary electrode potential difference AU _RH varies with time.

Alternatively, however, it is also possible to vary this potential difference in time by borrowing each time consecutively different

Voltage sources (which generate different voltages) are connected between the first metal contact and the auxiliary electrode. This is particularly advantageous in the course of inline processes in the production of solar cells.

According to the invention, the light irradiation time characteristic of the light irradiated to the solar cell can be varied by connecting a light source irradiating the solar cell (especially the front side) to a frequency generator. With the latter, the voltage applied to the light source voltage can then be varied time-dependent, so that the light intensity emitted by the light source has a corresponding temporal variation.

Alternatively, however, it is also possible to irradiate the solar cell with a light source operated with a constant voltage (which thus continuously emits a constant intensity) and between the solar cell and the light source irradiating it a mechanical device, in particular a mechanical chopper, to arrange: With this mechanical device can then periodically, in particular periodically, the solar cell are shielded from the incident on them light.

According to the invention, a generation of a seed layer is thus carried out with voltage and Lich-pulsed (ie bigepulster) light-induced electroplating: For this purpose, the previously by any method

Microstructured open Passivierungsschicht the front of only one metal contact on the back having solar cell in a suitable for the metal to be applied contact metal-containing electrolyte are immersed. These metal ions are preferably nickel ions, cobalt ions or tungsten ions. The inventive method results in a thin, homogeneous seed layer of, for example nickel, cobalt or tungsten, which can then be subsequently further strengthens ver ^¬ according to the invention on the basis of light induced plating. The method can be used independently of the emitter resistance, ie also for high-resistance emitters. The method can be carried out with all prior art electrolytes for the respective metal to be deposited of the second metal contact (the electrolytes listed above and below are thus to be understood as examples).

According to the invention, the already existing individual metal contact of the solar cell is connected to the auxiliary electrode in such a way that a predefined potential difference, which can be varied over time or which changes over time, can be applied between the two. This potential difference advantageously consists of a periodic sequence of voltage pulses of different polarity, different amount and / or different duration and from voltage-free periods. This can be realized by the above-described function generator. The voltage pulses are selected such that the side of the solar cell carrying the already existing metal contact is at a lower potential for a longer duration than the auxiliary electrode. At the same time, the front can be irradiated with a pulsed light source. As a result of this treatment, for example, in the region of the opening of the passivation layer on the front side of the solar cell, a seed layer of the corresponding metal forms, wherein the duration of the treatment is preferably set so that the seed layer is between 50 nm and 500 nm thick.

An essential point of the invention that the potential difference AU _RH varies dependent on time between the first metal contact and the auxiliary electrode according to a predefi ^¬ ned voltage-time characteristic (first characteristic), and that at the same time during that time-dependent variation according to the first characteristic, the light irradiation is therefore to the solar cell is varied according to a further predefined characteristic (second characteristic), the light irradiation time characteristic ik being time-dependent. Alternatively, current characteristic changes corresponding to the characteristics may also be generated.

Advantageously, the first characteristic (or its time-dependent variation) is synchronized with the second characteristic (or its time-dependent variation). Preferably, the two characteristics are designed so that during a plurality of spaced apart successive intervals defined intervals neither a potential difference AU _RH between the first metal contact and the auxiliary electrode is applied, nor a light irradiation to the solar cell.

According to the invention, the seed layer for the second metal contact thus produced is preferably used as the cathode in order to further grow the second metal contact based on the seed layer (by further maintaining a potential difference AU _RH and a light irradiation). Hereinafter, the present invention will be described by way of embodiments. Show:

Figure 1 shows the basic arrangement which is used for ^¬ leadership of the method according to the invention;

2 shows a first example of a temporal Va ^¬ riation the potential difference AU _RH Zvi ^¬ rule a solar cell rear side and the auxiliary electrode and for a temporal variation of the light radiation on the front side of the solar cell;

Figure 3 is a further corresponding example.

Fig. 1 shows an arrangement according to the invention, which is designed for performing a light-induced electrodeposition process according to the invention. In a container 1 is an electrolytic bath 6, in which an auxiliary electrode H (which serves as the anode of the electrodeposition) is arranged. Also in the galvanic bath 6 is a solar cell S having a front-side contact V and a rear-side contact R formed on the opposite side of the solar cell (rear-side contact R = first metal contact, front-side contact V = second metal contact).

The auxiliary electrode H is electrically connected to the back contact R via an insulated electrical lead 12 and an insulated electrical lead 11. Between the two lines 11 and 12, a voltage source 2 is connected, with which the applied between the auxiliary electrode H and the back contact R potential difference AU _RH can be varied time-dependent.

This time-dependent variation is accomplished by means of a function generator 3 (which is connected by means of the line 13 to the voltage source 2), the voltage generated by the voltage source 2 is varied time-dependent.

Finally, above the arrangement of auxiliary electrode H, bath 6 and solar cell S, a light source 4 is arranged, with which the side of the front side contact V of the solar cell S can be irradiated with electromagnetic radiation 5, which causes Stromer ^¬ generating in the solar cell.

Like the Fign. 2 and 3, for varying the potential difference AU _RH between the solar cell backside (back contact R) and the auxiliary electrode H, the voltage source 2 is controlled by the function generator 3 so as to realize the voltage-time characteristics shown in these figures. About the function generator the voltage source ^¬ 2 can thus be controlled so that almost any Voltage sequences between the solar cell rear side R and the auxiliary electrode H can be generated.

FIG. 2 shows a first example of a simultaneous formation of a voltage-time characteristic of the potential difference UU _RH and a light-irradiation time characteristic of a quantity of light irradiated onto the front side of a solar cell.

In the example shown, the solar cell, which has only one rear-side contact, is immersed in an electrolyte 6 which has the following composition:

NiSO ₄ · 6H ₂ 0 31g / l

NiCl ₂ 2g / l

H3BO3 40g / l

NaCi ₆ H ₂ 90 ₇ S 170g / l

NaC ₈ Hi ₇ 0 ₃ S 255g / l

Residual Ingredients: Water The pH of the electrolyte solution, which can be stirred moderately fast, is between 2 and 6, preferably between 3 and 4. The temperature of the electrolyte solution may be between room temperature (20 ° C) and 70 ° C, preferably between 40 ° C and 60 ° C.

FIG. 2 a shows the potential pulse routine or the voltage-time characteristic for the potential difference AURH which is applied between the rear-side contact R of the solar cell S and the auxiliary electrode H. The potential pulse routine shown is also referred to as so-called "reverse pulse plating".

At time t = to, no potential difference is applied between time ti> t ₀ between the rear side contact and the auxiliary electrode. At time ti, a cathodic voltage pulse Ui between see the back contact R of the solar cell S and the auxiliary electrode H applied, that is, the back is at a lower potential than the auxiliary electrode, which is at a higher potential. Ui is maintained until time t ₄ . To the

At time t ₄ > ti, an anodic pulse U _{2 of} the duration of the time interval [t ₄ , t ₅ ] with t ₅ > t _{4 is} applied, that of a voltage-free time in the time interval [t ₅ , t ₇ ] with t ₅ <t ₇ followed. In this voltage-free time, AU _RH = U _oc ■ Ui <U ₀ c <ü ₂ .

U ₀ c is the open cell voltage: there is no potential difference between two electrodes from the outside

AU _R H created, so not 0 V. At the two

Electrodes (front and back) set a potential according to the electrochemical conditions, i. the system tries to reach equilibrium. This leads to a restructuring of the electrolytic double layer, the phase boundary between the electrode and the electrolyte. It is this periodic restructuring of this bilayer that most beneficially affects the growth of metal contact on the solar cell.

This voltage-time characteristic lasting from ti to t is now repeated several times, so that the voltage-time characteristic in the interval

[ti, t ₇ ] constitutes a period of a periodic voltage-time characteristic according to the invention.

According to the invention is at the same time (ie during several of, preferably all of the previously described voltage-time characteristic periods) of the

Light source 4 light on the front of the solar cell S, which at the beginning of the process is still no metal The course over time of the incident light intensity is likewise shown in FIG. 2a. In the time interval [t ₀ , ti] the light intensity changes from I ₀ to Ii with I ₀ <Ii. The time interval [ti, t ₇ ] is now subdivided into six sections of equal length, with the following for the time periods ti to t ₇ defining these periods: ti <t ₂ <t ₃ <t ₄ <t ₅ <t ₆ <t ₇ . In the time Inter ^¬ vall [ti, t _2] is the incident light intensity Ιχ, after which the irradiated light intensity during time interval [t2, _t3] to I2> Ii is increased. This is followed by the two time intervals [t ₃ , t ₅ ] and [t ₅ , t ₇ ], in which the intensity of light intensity is identical to that in the interval [ti, t ₃ ]. For the times t with t> t ₇ , the light-irradiation time characteristic of the time interval [ti, t ₇ ] is then repeated in successive time intervals of the length t ₇ -ti.

Figure 2b shows the time course of the potential that is established at the front or in the region of the seed layer of the solar cell ^¬ S (Potentialdiffe ^¬ rence between the front and auxiliary electrode).

The above-described pulse sequence in the light intensity and at the same time in the potential difference AU _RH ensures a change between phases of nucleation (during the time interval [ti, t ₄ ] and the subsequent intervals of the pulse period corresponding to this interval) and phases in which a germination takes place ( during the time interval

[ts, t ₇ ] or the corresponding time intervals which follow this time interval in the pulse sequence). The potential reversal during the time interval

[t ₄ , t ₅ ] (and the subsequent corresponding time intervals) has a leveling effect on the Divorce. t ₄ - ti can be between 10 ^"5 seconds and 1 second, preferably between 10 ^{" 4} seconds and 0.1 seconds; t ₅ - t ₄ can be between 10 ^"6 seconds and 0.1 seconds, preferably between 10 ^{~ 5} seconds and 0.1 seconds, t ₇ - t ₅ can be between 0 seconds and 1 second, preferably between 10 ^{~ 2} seconds and 1 second. The cathodic voltage pulse Ui can be between -1.5 volts and -0.1 volts, preferably between -0.6 volts and -0.2 volts The anodic pulse U ₂ can be between + 0.1 and + 5 volts, preferably between + 0.2 volts and + 1.5 volts. The light intensity Io can be between 0 W / m ² and 100 W / m ² , preferably between 0 W / m ² and 1 W / m ^2. Ii can be between 100 W / m ² and 2000 W / m ² , preferably between 100 W / m ² and 1500 W / m ^2. I ₂ can be between 100 W / m ² and 2000 W / m ² , preferably between 200 W / m ² and 1000 W / m ² .

The above-described pulse routine in light intensity and potential difference generates a homogeneous seed layer for the second metal contact of the solar cell, the thickness of which can be controlled by the number of repetitions.

FIG. 3 shows an analogous example of a light and a potential difference sequence as in the case described in FIG. 2, so that only the differences will be described below.

The voltage pulses which are applied between the solar cell rear side and the auxiliary electrode, and the light pulses are in this case specifically matched to the fact that the front side of the solar cell over sufficient periods or time intervals on open Cell potential (U ₀ c in Figure 3b). For this purpose, according to FIG. 3 a, the potential difference between the rear side and the auxiliary electrode is pulsed so that initially no potential difference is applied in the time interval [t ₀ , ti]. This is followed by a cathodic voltage pulse Ui in the time interval [tx, t ₂ ], followed by an anodic voltage pulse U ₂ in the time interval [t ₂ , t ₃ ]. In the time interval [t ₃ , t ₄ ] again there is no potential difference Δϋ _ΚΗ . The time interval [ti, t ₄ ] is then repeated periodically.

The light pulses (FIG. 3a, bottom) are selected such that during the time interval [t ₀ , ti] the light intensity I _{0 is} irradiated, followed by the intensity Ii> I ₀ in the time interval [ti, t ₂ ]. In the time interval [t ₂ , t ₄ ] the intensity is again Io The light irradiation characteristic of the time interval [ti, t ₄ ] is then repeated periodically.

During the interval [t ₃ , t ₄ ], the front side is thus at open cell potential. This time interval serves to regenerate both the electrode surface and the diffusion layer lying in front of it and has proved to be particularly advantageous. t ₂ -ti can be between 10 ^"5 seconds and 1 second, preferably between 10 ^{~ 4} seconds and 0.1 seconds t ₃ - t ₂ can be between 10 ^{" 6} seconds and 0.1 seconds, preferably between 10 ^"5 seconds and 0.1 seconds t ₄ -t ₃ can be between 10 ^-5 seconds and 1 second, preferably between 10 ^{~ 2} seconds and 1 second Ui can be between -1.5 volts and -0.1 volts, preferably between -0.6 volts and -0.2 volts U _2, between +0.1 volts and +5 volts may be, preferably, between +0.2 volts and +1.5 volts. I ₀ can lie between 0 W / m ² and 100 W / m ^2, preferably between 0 W / m ² and 1 W / m ² . Ii can be between 100 W / m ² and 200 W / m ² , preferably between 500 W / m ² and 1500 W / m ² .

The method according to the invention for producing the second metal contact can be supplemented as follows: The solar cell arrangement S (comprising the rear-side contact R, the semiconductor layer and the growing front-side contact or the seed layer) can be tempered. This means that the solar cell is exposed to higher temperatures, as a result of which improved contact bonding may occur. The tempering may be carried out before a galvanic reinforcement following the formation of the seed layer, but it may also be carried out after the galvanic reinforcement. Advantageous temperature ranges of tempering are 150 ° C. to 600 ° C., advantageous tempering times are 10 s to 20 min.

However, annealing can also be dispensed with.

Claims

claims

1. Light-induced galvanic deposition method for the galvanic formation of a second etallkontakts (V) and preferably additionally for the galvanic reinforcement of a second metal contact formed in a solar cell (S) having only a first metal contact (R), wherein the solar cell (S ) with their only one metal contact (R) and serving as an anode auxiliary electrode (H) are at least partially introduced into an electrolytic bath (6), and wherein between the first metal contact (R) of the solar cell and the auxiliary electrode, a potential difference AU RH generating that the first metal contact (R) is at least temporarily at a negative potential relative to the auxiliary electrode, and wherein the solar cell is irradiated with light (5), characterized in that both the potential difference Aü _RH between the first metal contact (R) and the auxiliary electrode (H) according to a predefined, hereinafter as Spannun gs time characteristic described time-varying (cl) and / or this time-dependent variation (cl) corresponding St om density changes are generated, as well as during this time-dependent variation (cl) and / or their corresponding current density changes according to the first characteristic at the same time the light irradiation tion on the solar cell according to a predefined, hereinafter referred to as light irradiation time characteristic designated second characteristic is time-varying (c2) and / or this time-dependent variation (c2) corresponding current density changes are generated.

2. Method according to the preceding claim, characterized in that the time-dependent variation (c1) and / or the variation of this corresponding current density changes according to the first characteristic synchronized in time with the time-dependent variation (c2) and / or corresponding to this variation current density changes according to the second Cha ^¬ characteristic is / are carried out.

3. The method according to any one of the preceding claims, characterized in that the first characteristic and the second characteristic are formed so that during at least one, preferably during a plurality, more preferably during a plurality of periodically at a constant time intervals consecutive, defined time interval / e neither a potential difference .DELTA.ϋ _ΚΗ between the first Me ^¬ tallkontakt (R) and the auxiliary electrode (H) on ^¬ is still a light irradiation on the So ^¬ larzelle done. , Method according to one of the preceding claims, characterized in that a passivation layer of the solar cell (S) applied on a side opposite the first metal contact (R) of the solar cell is opened at least in regions before the solar cell together with its first metal contact (R) is introduced into the electrolytic bath (6).

Method according to one of the preceding claims, characterized in that the first characteristic and / or the second characteristic, preferably the first and the second characteristic, a periodic variation and / or at least one pulse sequence comprises / include or is formed as a pulse sequence / and / or that corresponding current density changes are generated, wherein preferably a first pulse train of the first characteristic is synchronized in time with a second pulse train of the second characteristic.

Method according to the preceding claim characterized by a plurality of pulse sequences, between which at least one time period without changing the first characteristic and / or the second characteristic and / or a corresponding current density change is and / or by anodic pulses at the beginning of a pulse train of the first and / or the second Characteristic and / or by a pulse sequence of the first and / or the second characteristic with individual pulses of different duration, different size and / or different polarity and / or with different time intervals between immediately adjacent pulses and / or by triangular, rectangular or sinusoidal, two, three or higher degree polynomial and / or exponential pulses and / or by superposition of a plurality of pulses of different sizes in a pulse sequence of the first and / or the second characteristic.

7. The method according to any one of the preceding claims, characterized in that by means of the potential difference AU _RH according to the first characteristic and the light irradiation according to the second characteristic, a photovoltaic effect so generated and a current in the solar cell is induced so that a second metal contact (V) forming the solar cell (S) in the form of a seed layer thick between 10 nm and 5000 nm, preferably between 50 nm and 500 nm, preferably using the seed layer as the cathode and successively further maintaining the potential difference AURH according to the first characteristic and the light irradiation is thickened according to the second characteristic. Method according to one of the preceding claims, characterized in that the light opposite the first metal contact (R) of the solar cell side of the solar cell (S) according to the second characteristic is irradiated with light and / or that the first metal contact (R) at least a portion of the rear side contact of the Solar cell and / or arranged on a p-doped side of the solar cell electrical contact of the solar cell and / or that as the second metal contact (V) at least a portion of at least one front side contact of the solar cell, at least one on the light radiation side facing the solar cell arranged electrical contact and / or at least one arranged on an n-doped side of the solar cell electrical contact of the solar cell is formed.

Method according to one of the preceding claims, characterized in that the potential difference AU _RH between the first metal contact (R) and the auxiliary electrode (H) via a connected to a function generator (3) or integrated with a functional gene generated by the function generator, the voltage difference generated by the voltage source AU _{RH is} varied over time and / or that the potential difference AU _RH between the first metal contact (R) and the auxiliary electrode (H) is varied by switching successively a plurality of different voltage sources (2) between the first metal contact and the auxiliary electrode.

Method according to one of the preceding claims, characterized in that the second characteristic of the light irradiated to the solar cell is varied time-dependent by a solar cell irradiating light source (4) is connected to a frequency generator and / or function generator, with which the voltage applied to the light source is varied over time, preferably temporally synchronized with the variation by the function generator (3) is varied according to the preceding claim, and / or that continuously light is irradiated to the solar cell and that between the solar cell and the irradiating light source (4) is a mechanical breaker (Chopper) net, with which the solar cell is periodically, in particular periodically, shielded from the incident light.

11. The method according to any one of the preceding claims, characterized in that the electrolytic bath (6) Ni ions, co-ions and / or o-ions and / or that the light irradiation is varied to the solar cell time-dependent, by during a time interval t ₀ , ti] a first light intensity Io is irradiated to the solar cell and thereafter during a time interval

[ti, t2] a second light intensity Ii unlike I _{0 is} irradiated to the solar cell, wherein the above-described intensity change between I ₀ and Ii is preferably performed multiple times and / or periodically.

12. The method according to any one of the preceding claims, characterized in that the solar cell is annealed before, during and / or after the variation of the potential difference AU _RH according to the first characteristic and the variation of the light irradiation according to the second characteristic, wherein preferably the temperature of the annealing between 150 ° C and 600 ° C and the duration of annealing between 10 s and 20 min. Arrangement comprising an electrolytic bath (6) into which a solar cell (S) having only a first metal contact (R) but not a second metal contact (V) and serving as an anode

Auxiliary electrode (H) in each case at least partially immersed, one between the first metal contact (R) and the auxiliary electrode connected voltage source (2), and a light source (4) which is arranged so that at least partially irradiated with the solar cell, characterized in that the arrangement is designed for carrying out a method according to one of the preceding claims.