WO2011110231A1

WO2011110231A1 - Method and in-line production system for the production of solar cells

Info

Publication number: WO2011110231A1
Application number: PCT/EP2010/053236
Authority: WO
Inventors: Martin Schaper; Thomas Florian; Steffen Gottschalk; Holm Kahlert; Angelika Hofmann
Original assignee: Q-Cells Se
Priority date: 2010-03-12
Filing date: 2010-03-12
Publication date: 2011-09-15

Abstract

The invention relates to a method for the production of solar cells comprising the following steps providing (1) a semiconductor solar cell substrate, printing (2) a printing mixture containing a solvent and metal particles to form a printed mixture on the semiconductor solar cell substrate, drying (3) of the printed mixture at a drying temperature and over a drying duration, wherein both drying temperature and drying duration are chosen such that the solvent contained in the printed mixture is at least predominantly evaporated after the drying duration to obtain a metal structure and firing (4) the metal structure at a temperature of more than 570°C. According to the invention a resting time (T) of more than 20 seconds, preferably of more than 60 seconds, and more preferably of more than 120 seconds for the semiconductor solar cell substrate is provided for before the solvent of the printed mixture has evaporated by 10%, preferably by 5%, and more preferably by 2% of its total weight.

Description

Title:

Method and in-line production system for the production of solar cells Description:

The invention relates to a method and to an in-line production system for the production of solar cells. Solar cells usually have a p-n-junction that formed in a semiconductor solar cell substrate. The semiconductor solar cell substrates can be formed as semiconductor wafers being monocrystalline or multicrystalline or as semiconductor thin film stacks being provided on carrier sheets for instance made of glass, steel or plastics. In the area of the p-n-junction, according to the photovoltaic effect, holes and electrons are separated by the absorption of photons leading to the flux of an electric current. In order to use this electric current, the semiconductor solar cell substrate must provide electrodes for establishing the necessary electrical contact. Usually these electrodes are formed as metal structures. In order to create such metal electrodes for solar cells, methods for printing a printing mixture containing a solvent and metallic particles to form a printed mixture on semiconductor solar cell substrates are widely used. The printed mixture is then dried at a drying temperature and over a drying duration, wherein both drying temperature and drying duration are chosen such that the solvent contained in the printed mixture is at least predominantly evaporated in order to obtain a metal structure. After drying of the printed mixture, the metal structure is fired at temperatures of more than 570 "C.

The process of printing such printing mixtures in form of screen print pastes or printing inks may incur discontinuities in the form of small gas bubbles enclosed within the printed mixture and/or in the form of undesirable recesses within the desired shape and surface topology of the printed mixture.

Especially these gas bubbles may lead to cavities within the metal structure deteriorating the performance of the electrode and/or the Back-Surface-Field structure of the solar cell. According to EP 0 654 831 A2, it is suggested to feed ultrasonic vibrations to the semiconductor solar cell substrate after the printing step. This mechanical interaction leads to the destruction of gas bubbles enclosed in the printed mixture and to the levelling of undesired recesses within shape and surface topology, providing an improved and homogeneous contact between the metal electrode structure and the underlying semiconductor surface. However, the described use of ultrasonic vibrations implies the necessity of further equipment, leading to increased production costs and the step of establishing a mechanical contact to the semiconductor solar cell substrate, implying the risk of mechanically damaging the substrate. Therefore it is an object of this invention to provide a method and an in-line production system for the production of solar cells showing lower costs and a reduced risk of damaging semiconductor solar cell substrates.

This object is achieved by a production method having the features of claim 1 and by an in-line production system according to claim 14.

According to the invention, it is provided for a resting time of more than 20 seconds, preferably of more than 60 seconds, and more preferably of more than 120 seconds for the semiconductor solar cell substrate before the solvent of the printed mixture has evaporated by 10%, preferably by 5%, and more preferably by 2% of its total weight.

Surprisingly, it has been found that no additional equipment and no additional mechanical interaction with the semiconductor solar cell substrate is necessary in order to eliminate discontinuities in form of gas bubbles that are enclosed within the printed mixture and/or in form of recesses within shape and surface topology of the printed mixture. It is sufficient to provide for a resting time during which the rheological properties of the printed mixture are not significantly changed by a significant loss of solvent due to evaporation from the printed mixture. Thereby, portions of the printed mixture adjacent to discontinuities are given time to flow resulting in substantially levelling the discontinuities. One simple way to achieve this effect is to provide for the resting time at essentially the same temperature conditions given during the printing step. In this case the evaporation of solvent from the printed mixture usually is so small that the rheological properties of the printed mixture do not change significantly. However, it would be possible to ramp up the drying temperature for the printed mixture from the very beginning but at such a small rate that the above mentioned conditions are met.

With respect to the duration of the resting time, an increase of overall solar cell efficiency can be found with an increase of resting time. However, a saturation level for the efficiency increase is reached for maximum resting time durations between 2 to 10 minutes, more specifically for 4 to 8 minutes. The specific resting time duration to reach the saturation of efficiency improvement depends on a number of parameters starting with the parameters of the printing process itself. If it was for example a screen printing process, the specific mixture in form of a paste or an ink being used, printing time, the parameters of the screen printing mesh etc. With regards to the printed mixture, thickness and structure of the printed mixture do have an influence on the resting time to reach the saturation level of improved solar cell efficiency. A preferred embodiment defines the step of drying the printed mixture by the drying duration during which at least 70%, preferably at least 80%, and more preferably at least 90% of the solvent's total weight has evaporated from the printed mixture, wherein the drying duration starts with the termination of the printing step. From a chemical point of view also at room temperature a certain amount of the solvent from the mixture will always evaporate from the mixture depending on the vapour pressure of the specific solvent. Therefore, with regards to the instant invention the process of drying the printed mixture starts immediately after the printing process event though at a very small rate. With regards to industrial production, it would take too much time to dry the printed mixture at room temperature only. Therefore, an advantageous embodiment is performing the step of drying during a predominant fraction time of the drying duration at drying temperatures above room temperature preferably at drying temperatures above 70° C and more preferably at drying temperatures above 100°C but in any case at drying temperatures of less than 570°C. Above 570°C the process step of firing the metal structure takes place. Even though the process steps of drying and firing are distinguished, these steps may take place in one single heating device ramping up the substrate's drying temperatures up to the range of firing temperatures.

Alternatively, another embodiment is performing the step of drying during a small time fraction of the drying duration at drying temperatures above room temperature preferably at drying temperatures above 70° C and more preferably at drying temperatures above 100°C but in any case at drying temperatures of less than 570° C. For this embodiment for example a relatively high temperature of more than 300 °C is chosen and the drying step is completed after about 20 seconds. Certainly, it would also be possible that the step of drying at elevated temperatures takes about half of the time of the drying duration.

It is very common and advantageous to perform the step of printing as a screen printing step using the printing mixture as a screen printing paste and using a screen printing mesh. Alternatively, the printing mixture in form of a suitable paste or ink may be printed by one method selected from the group of ink-jet printing, relief printing, intaglio printing and spray printing.

For the embodiment showing a screen printing step as a first variation it is advantageous to use a screen printing mesh having a mesh density of 30 to 120 threads per cm, preferably of 40 to 105 threads per cm, and more preferably of 60 to 90 threads per cm and with a thread diameter of 25 to 120μηη, preferably of 45 to 100μιη, and more preferably of about 50μηη. Such meshes are preferably used for the screen printing of solar cell back sides.

For the embodiment showing a screen printing step as a second variation it is advantageous to use a screen printing mesh having a mesh density of 80 to 160 threads per cm, preferably of 90 to 130 threads per cm, and with a thread diameter of 15 to 50μηη, preferably of about 25μηη. Such meshes are preferably used for the screen printing of solar cell front sides.

Preferably the first variation of the embodiment having a screen printing step is further characterized by using a semiconductor wafer as the semiconductor solar cell substrate and by printing the screen printing paste comprising aluminium particles such that the metal structure is forming a Back-Surface- Field structure or a number of electrical contact portions after firing. Suitable products that can be found on the market are Toyo Q04 of Toyo Engineering K.K., Chiba, Japan; Ferro CN53-200 of Ferro Corp., Cleveland, Ohio and Palf 100A of Monocrystal Inc., Stavropol, Russia.

Preferably the second variation of the embodiment having a screen printing step is further characterized by using a semiconductor wafer as the

semiconductor solar cell substrate and by printing the screen printing paste comprising silver particles such that the metal structure is forming a front electrode grid structure after firing. Suitable products that can be found on the market are Solamet® PV1 7 and Solamet® PV159 of DuPont™ Corp.

Wilmington, Delaware, USA and Ferro NS33-502 of Ferro Corp., Cleveland, Ohio, USA.

It is advantageous if any of the preceding embodiments and its variations is characterized by the production method being organized as an in-line production method. Preferably the in-line production method is having a production rate of 1 to 4 seconds, preferably of 2.5 to 3 seconds. Preferably the semiconductor solar cell substrate is being arranged with its printed surface in an upward direction during most of the resting time.

Thereby, the impact of gravity is supporting the process of levelling

discontinuities within the printed mixture on the substrate.

It is advantageous if the solar cell semiconductor substrate is resting in or is moved through an intermediate storage device during most of the resting time. This means that no further interaction is necessary after the printing step in order to achieve the desired levelling of the printed mixture.

Furthermore, the object of the invention is achieved by an in-line production system for solar cells comprising: a printing device for printing a printing mixture containing a solvent and metal particles on semiconductor solar cell substrates and a processing device for processing the printed mixture including the drying of the printed mixture at a drying temperature and over a drying duration, wherein both drying temperature and drying duration are chosen such that the solvent contained in the printed mixture is at least

predominantly evaporated after the drying duration to obtain a metal structure, wherein the processing device is having an intermediate storage unit receiving and storing the substrates from the printing device such that the semiconductor solar cell substrates leaving the printing device are provided with a resting time of more than 20 seconds, preferably of more than 60 seconds and more preferably of more than 120 seconds before the solvent of the printed mixture has evaporated by 10%, preferably by 5%, and more preferably by 2% of its total weight.

The printing mixtures may be provided in form of pastes or inks as described above. It is important to emphasize that according to this invention the process of drying the printed mixture, namely the evaporation of the solvent contained in the printed mixture, is considered to start immediately after the termination of the printing process. Therefore, the terminology of a drying device or a drying unit is avoided because the drying process is stretched over different stages of the in-line processing line. The processing device that is arranged inline with the printing device having a set of sequentially arranged units that show different functions. The first unit is the intermediate storage unit that is arranged and adapted such that the resting time for the printed mixture is provided.

In an advantageous embodiment the processing device is having a heating unit being arranged after the intermediate storage unit and providing drying temperatures above room temperature, preferably above 70 °C and more preferably above 100°C but in any case below 570° C. Thereby, the

predominant amount of solvent is evaporated from the printed mixture at a higher rate. Furthermore, it is preferred that the processing device is further comprising a firing unit for firing the semiconductor solar cell substrates after the heating unit at a temperature of more than 570° C. It is emphasized that a combination of heating unit and firing unit is as well possible. Either the combined unit is showing different heat zones representing drying and firing and the substrates are transported continuously through these zones or the combined unit is ramping up the temperature range from drying to firing during the substrate transport.

Preferably the intermediate storage unit comprises carriers for semiconductor solar cell substrates or the intermediate storage unit is a conveyer device. These are two possible options of easily realizing the desired function of the intermediate storage device. Depending on the production rate of the in-line system the distance of the conveyer device is to be adapted in order to provide for the desired resting time. The intermediate storage device may be arranged and adapted as a first-in-first-out storage.

A preferred embodiment of the in-line production system comprises several in-line sets each showing a sequential arrangement of the printing device and the processing device, wherein these in-line sets are arranged in an in-line sequence to each other. Thereby, it is possible to realize the combination of several printing steps on the front surface and/or on the back surface of a single semiconductor solar cell substrate.

For the embodiment with combined in-line sets it is advantageous for the in-line production system to show a flipping device for flipping the

semiconductor solar cell substrates from one surface to the opposite surface, wherein the flipping device is arranged between two in-line sets. Thereby, printing steps on the front surface and on the back surface may be combined.

Further advantages and properties of the method and the in-line production system for producing solar cells will be described with regards to some specific embodiments shown in the figures.

Figure 1 is showing a schematic sequence of the method steps for

producing a solar cell;

Figure 2 is showing the schematic setup of a first embodiment of an in-line production system for solar cells applying the method according to figure 1 and

Figure 3 is showing the schematic setup of a second embodiment of an in-line production system for solar cells applying the method according to figure 1. Figure 1 is showing a schematic sequence of the method steps for producing a solar cell. Starting with step 1 of providing a semiconductor solar cell substrate followed by printing step 2 by printing a printing mixture in form of a screen printing paste containing a solvent and metal particles as a printed paste on the semiconductor solar cell substrate. Due to the chemical nature of the solvent, evaporation of solvent from the printed paste starts immediately after finishing printing step 2. If the process of evaporation of solvent is considered as the process of drying of the printed paste, the process of drying is also starting immediately after termination of the printing step 2 even though usually starting with a slow rate as this process is influenced by temperature and the evaporation rate at room temperature is limited. Although it is important to emphasize that the principle mechanism of the drying process of the printed paste starts immediately, the main part of this process takes place during the following step of drying 3 by the use of increased temperature. For instance the substrate with the printed paste is brought into an environment heated to 180°C during 15 minutes or heated to 300° C during less than 20 seconds. For commercially available aluminium screen printing pastes like Toyo Q04 of Toyo Engineering K.K., Chiba, Japan, Ferro CN53-200 of Ferro Corp., Cleveland, Ohio and Palf 100A of Monocrystal Inc., Stavropol, Russia, using a screen printing mesh with a mesh density of about 60 to 70 threads per cm and a thread diameter of about 50μηη and film thicknesses between 5μηη and 50μηη the mentioned parameters of drying step 3 are sufficient such that the solvent contained in the printed paste is at least predominantly evaporated after drying step 3. In order to improve the surface contact between the printed paste and the semiconductor solar cell substrate it is decisive that after the printing step 2 it is provided for a sufficient resting time T. It is important that the environmental conditions during this resting time T are such that the rheological properties of the printed paste are not changed significantly by a significant evaporation of solvent from the printed paste. One easy way to achieve this condition is to wait before ramping up the surrounding

temperature in order to accelerate the drying process. For the conditions cited above, an effect of solar cell efficiency improvement can already be seen after 20 seconds. After a minute it may reach an overall improved efficiency of about 0.1% slowly coming to a saturation level at about 0,4 to 0,5% after about eight minutes of resting time T.

Therefore, it is emphasized that with regards to the necessary resting time T it does not matter if the substrate with the printed paste is inside or outside a drying oven device or if the drying oven and a subsequent firing oven for the final step 4 of firing the metal structure of the dried paste at a temperature of more than 570 °C are combined in one housing. As long as the conditions are met that a sufficient resting time T is given before significant amounts of the solvent have evaporated from the printed paste the rheological properties remain substantially the same. Thereby, the levelling of discontinuities within the printed paste may take place leading to a better contact of the paste to the semiconductor solar cell substrate.

Figure 2 is showing the schematic setup of a first embodiment of an in-line production system for solar cells applying the method according to figure 1. The horizontal arrows depict the in-line direction of the whole production system. A first in-line set 100 of a first printing device 5 receives

semiconductor solar cell substrates from a conveyer that is symbolized by the arrow. The printing device 5 is followed by a processing device 6 that may comprise two subunits. The first one is an intermediate storage unit 60 and the second one a heating unit 61. The intermediate storage unit 60 may provide for the desired resting time T under the considerations as already discussed with regards to figure 1. The heating unit 61 provides the possibility to accelerate the drying process of the printed paste as discussed with regards to figure 1. After the heating unit 61 the printed paste has been transformed to a metal structure that is ready for firing. The first in-line set 100 is screen printing and drying silver bus bars on the backside of the semiconductor solar cell substrate. This first in-line set 100 is followed by a second in-line set 101 showing the same sequence of units. In order to avoid repetitions it is referred to the description above. This second in-line set 101 is arranged and adapted for the screen printing and drying of an aluminium structure that shall take effect as a Back-Surface- Field structure on the backside of the semiconductor solar cell substrate after a final firing step. After having printed and dried two structures on the backside of the

semiconductor solar cell substrate a flipping device 7 is arranged to flip the semiconductor solar cell substrate. The flipping device 7 is followed by a third in-line set 102 for screen printing and drying of the silver front contact electrode grid of the semiconductor solar cell substrate. Unlike the first and second in-line sets 100, 101 the processing device 6 of the third in-line set 102 has another subunit in form of a firing unit 62 following its intermediate storage unit 60 and its heating unit 61. In the firing unit 62 all three screen printed and dried metal structures are fired simultaneously. This first schematic embodiment of an in-line production system for solar cells applying the method according to figure 1 leads to firing the solar cell with its front side directed upwards.

Figure 3 is showing the schematic setup of a second embodiment of an in-line production system for solar cells applying the method according to figure 1 , wherein three in-line sets 100,101 ,102 showing the same in-line structure of a printing device 5 followed by a processing device 6 are combined such that the firing of the solar cell takes place with its front side directed downwards. The same members of shown devices are given same reference numbers. In order to avoid repetitions it is referred to the description above. In figure 3 the first in-line set 100 is arranged and adapted for the screen printing and drying of the silver front electrode grid structure. Followed by a flipping device 7 on the backside of the semiconductor solar cell substrate silver bus bars are printed and dried by the second in-line set 101. Finally on the backside an aluminium Back-Surface-Field structure is printed and dried by the third in-line set 102 finishing by firing all three metal structures

simultaneously in the firing unit 62 of the third in-line set 102.

Reference numbers:

1 method step of providing a semiconductor solar cells substrate

2 method step of printing a paste on the semiconductor substrate

3 method step of drying of the printed paste to obtain a metal structure

4 method step of firing the metal structure

T resting time

5 printing device

6 processing device

60 intermediate storage unit

61 heating unit

62 firing unit

7 flipping device

100, 101 , 102 in-line sets

Claims

1 . Method for the production of solar cells comprising the following steps:

- providing (1 ) a semiconductor solar cell substrate,

- printing (2) a printing mixture containing a liquid solvent and metal particles to form a printed mixture on the semiconductor solar cell substrate,

- drying (3) of the printed mixture at a drying temperature and over a drying duration, wherein both drying temperature and drying duration are chosen such that the solvent contained in the printed mixture is at least predominantly evaporated after the drying duration to obtain a metal structure and

- firing (4) the metal structure at a temperature of more than 570° C, characterized by

providing for a resting time (T) of more than 20 seconds, preferably of more than 60 seconds, and more preferably of more than 120 seconds for the semiconductor solar cell substrate before the solvent of the printed mixture has evaporated by 10%, preferably by 5%, and more preferably by 2% of its total weight.

2. Method according to claim 1 , characterized by the step of drying (3) the printed mixture is defined by the drying duration during which at least 70%, preferably at least 80%, and more preferably at least 90% of the solvent's total weight has evaporated from the printed mixture, wherein the drying duration starts with the termination of the printing step (2).

3. Method according to claim 2, characterized by performing the step of

drying (3) during a predominant time fraction of the drying duration at drying temperatures above room temperature preferably at drying temperatures above 70 ° C and more preferably at drying temperatures above 100° C but in any case at drying temperatures of less than 570° C.

4. Method according to claim 2, characterized by performing the step of drying (3) during a small time fraction of the drying duration at drying temperatures above room temperature preferably at drying temperatures above 70° C and more preferably at drying temperatures above 100° C but in any case at drying temperatures of less than 570 ° C.

5. Method according to one of claims 1 to 4, characterized by the step of printing (2) being a screen printing step using the printing mixture as a screen printing paste and using a screen printing mesh.

6. Method according to claim 5, characterized by the screen printing mesh having a mesh density of 30 to 120 threads per cm, preferably of 40 to 105 threads per cm, and more preferably of 60 to 90 threads per cm and with a thread diameter of 25 to 120μηη, preferably of 45 to Ι ΟΟμηη, and more preferably of about 50μηη.

7. Method according to claim 5, characterized by the screen printing mesh having a mesh density of 80 to 160 threads per cm, preferably of 90 to 130 threads per cm, and with a thread diameter of 15 to 50μηη, preferably of about 25μηη.

8. Method according to claim 5 or 6, characterized by using a semiconductor wafer as the semiconductor solar cell substrate and by printing the screen printing paste comprising aluminium particles such that the metal structure is forming a Back-Surface- Field structure or a number of electrical contact portions after firing.

9. Method according to claim 5 or 7, characterized by using a semiconductor wafer as the semiconductor solar cell substrate and by printing the screen printing paste comprising silver particles such that the metal structure is forming a front electrode grid structure after firing.

10. Method according to one of the preceding claims, characterized by the production method being organized as an in-line production method.

11. Method according to claim 10, wherein the in-line production method is having a production rate of 1 to 4 seconds, preferably of 2.5 to 3 seconds.

12. Method according to one of the preceding claims, characterized by the

semiconductor solar cell substrate being arranged with its printed surface in an upward direction during most of the resting time (T).

13. Method according to one of the preceding claims, wherein the solar cell semiconductor substrate is resting in or is moved through an intermediate storage device during most of the resting time (T).

14. In-line production system for solar cells comprising:

- a printing device (5) for printing a printing mixture containing a

solvent and metal particles on semiconductor solar cell substrates and

- a processing device (6) for processing the printed mixture including the drying of the printed mixture at a drying temperature and over a drying duration, wherein both drying temperature and drying duration are chosen such that the solvent contained in the printed mixture is at least predominantly evaporated after the drying duration to obtain a metal structure,

characterized by

the processing device (6) is having an intermediate storage unit (60) receiving and storing the substrates from the printing device (5) such that the semiconductor solar cell substrates leaving the printing device (5) are provided with a resting time (T) of more than 20 seconds, preferably of more than 60 seconds and more preferably of more than 120 seconds before the solvent of the printed mixture has evaporated by 10%, preferably by 5%, and more preferably by 2% of its total weight.

15. In-line production system according to claim 14, characterized by the processing device (6) having a heating unit (61 ) being arranged after the intermediate storage unit (60) and providing drying temperatures above room temperature, preferably above 70 °C and more preferably above 100°C but in any case below 570°C.

16. In-line production system according to claim 14 or 15, characterized by the processing device (6) further comprising a firing unit (62) for firing the semiconductor solar cell substrates after the heating unit at a temperature of more than 570°C.

17. In-line production system according to one of claims 14 to 16, characterized by the intermediate storage unit (60) comprising carriers for semiconductor solar cell substrates or by the intermediate storage unit (60) being a conveyer device.

18. In-line production system according to one of claims 14 to 17, comprising several in-line sets (100, 101 , 102) each showing a sequential arrangement of the printing device (5) and the processing device (6), wherein these in-line sets (100,101 ,102) are arranged in an in-line sequence to each other.

19. In-line production system according to claim 18, showing a flipping device (7) for flipping the semiconductor solar cell substrates from one surface to the opposite surface, wherein the flipping device is arranged between two in-line sets (100, 101 , 102).