WO2011152734A1

WO2011152734A1 - Mechanical strengthening of solar cells

Info

Publication number: WO2011152734A1
Application number: PCT/NO2011/000161
Authority: WO
Inventors: Håvard LILLEBO; Timothy Charles Lommasson; Eckerhard HOFMÜLLER
Original assignee: INNOTECH SOLAR ASA
Current assignee: INNOTECH SOLAR ASA
Priority date: 2010-05-31
Filing date: 2011-05-30
Publication date: 2011-12-08
Anticipated expiration: 2012-11-30
Also published as: NO20100785A1

Abstract

Method to protect a solar cell (1) against mechanical stress and damage, wherein a polymer layer (2) is provided on at least one side of the solar cell before the cell is assembled into a solar module. The polymer layer (2) may optionally be applied in such a manner that the electrical contacts (3) or other parts of the solar cell is kept uncovered to facilitate further processing of the cell.

Description

Mechanical strengthening of solar cells

The present invention provides a solution to strengthen the mechanical properties of crystalline silicon solar cells.

Background of the invention

Silicon solar cells are made of very thin mono or multicrystalline wafers. The typical wafer thickness is in the range of 140 μιτι to 300 μιτι, while the trend is to provide even thinner wafers to minimize the silicon usage per solar cell. The traditional method to cut silicon blocks into wafers is wire sawing, while in recent times new methods, such as cleaving, have been developed.

The wafers are processed to solar cells through a sequence of different process steps, typical such as etching, coating, annealing and structuring. The mechanical strength especially of very thin solar cells is already limited by the brittle nature of crystalline silicon. In addition, the mechanical load the wafers face during the cutting process and the subsequent processing into solar cells, including all the handling steps, weaken the mechanical strength of the cells by introducing microcracks. Microcracks are common defects in solar cells, and a certain breakage rate is usually seen in the solar cell production and the following assembly into solar modules. This is of course undesired, as it leads to

economical losses and process problems.

A solar module typically comprises a plurality of solar cells. The main processes in the module assembly are typically soldering and lamination of the solar cells.

During these processes, the cells are exposed to significant thermal and mechanical loads resulting in additional stresses. Even a single fractured cell can cause a solar module to be rejected. When included in a module, a cell with a microcrack can also lead to decreased module lifetime due to propagation of the crack during operation.

A main objective of the present invention is to improve the yield of solar module production. Another objective of the invention is to enable use of solar cells which already have cracks for the module assembly.

Yet another objective of the invention is to protect a solar cell from mechanical stress and damage during any handling or processing that occurs before the cell is laminated in to a solar module.

Summary of the invention

This is accomplished by providing a method to protect a solar cell from mechanical stress and damage, comprising the step of providing a polymer layer on at least one side of the solar cell before it is assembled into a solar module.

With a polymeric layer, the cell will be more resistive to

- Bending - a polymer layer on the cell will make the cell stiffer than an original cell.

- Shock forces - a polymer layer may have better flexibility than the brittle silicon material which a normal solar cell consists of.

- Handling - the cell will be less sensitive to manual handling. Scratching and damage are higher when exposed to manual handling. The polymer layer will preserve the solar cell better

- Impurities -Any finger marks and other impurities which may come from the process steps are easier to remove on a polymer layer. In addition the surface below the polymer layer is completely covered.

The selection of the polymer material and the location of the coating is preferably done in such a way that those cells may be processed further into solar modules with the same processes as used for non coated cells.

Thus, the present invention provides a solar cell with improved mechanical strength, which reduces the risk of cell breakage during the assembly processes significantly and hence improves the total production yield.

In addition, this process enables the use of cells which already have cracks for the module assembly by preventing the cracks from propagating. Detailed description

The solar cell according to the invention has two sides, the front side facing the sun and the rear side, with metallization on at least one of these sides for electrical contacting. A polymeric material may be coated onto the rear side, the front side or both sides. The coating may be fully covering, locally or in any type of pattern. Preferably the contact metallization should be kept open partially or fully for later contacting. In some embodiments a mask is used to provide a partial or patterned coating. In other embodiments, the coating may be applied without using a mask.

The process to produce mechanically strengthened solar cells comprises the application of polymeric material in liquid, paste, granular, film or any other suitable form. Types of polymers which are known to be suitable are acrylate, epoxy, ethylene vinyl acetate (EVA), thermoplastic urethane (TPU), silicone and others. To coat the solar cells surface any manufacturing procedure applicable for the polymeric material may be chosen, such as for example printing, spraying, casting and laminating. The type of polymeric material determines if an additional curing process may follow to the coating process to solidify the polymeric layer. The curing may be done for example by heat or UV radiation.

Example 1

Figure 1 shows one embodiment of a solar cell 1 according to the present invention. A strengthening layer 2 is covering the rear side of the solar cell 1 in such way that the electrical contacts 3 are kept free. As polymer material for the strengthening layer 2 a liquid acrylate may be used. The selected acrylate may resist the increased thermal load during the solder process and the lamination process the solar cell will face during solar module manufacturing. The polymer material may have very good adhesion properties to the rear side of the solar cell as well as to the encapsulation material of the solar module, typically EVA. Mineral filler may be added to the polymer material to improve the mechanical and thermal properties. The polymer material may be applied by a spraying process. The electrical contacts 3 of the solar cell 1 may be covered by a shadow mask to keep them uncoated. The polymer material may be UV light curable. The thickness of the cured strengthening layer may be in the range of 20 μιτι to 100 μηι. Example 2

Figure 2 shows another embodiment of a solar cell 1 according to the present invention. A strengthening layer 2 is covering the rear side of the solar cell 1 in such way that the electrical contacts 3 are kept free. The strengthening layer 2 is deposited in a grid like pattern. The polymer material may be in a paste form and may be applied in a screen printing process. A suitable polymer material may be a UV curing epoxy. The properties of the polymer material may be similar to example 1.

Example 3

A third embodiment of the present invention may utilize sheets of EVA as polymer material since this material is widely used as encapsulation material for solar modules. Contact elements may be soldered onto the electrical contacts of the solar cell. The ready soldered solar cell may be placed between two sheets of EVA, each with a thickness of 100 μηι to 500 μηη. This stack may be heated over the melting point of the EVA until the EVA melts. By cooling the stack the EVA will solidify and adhere to the solar cell with the contact elements. Cross-linking of the EVA and the removal of possible air bobbles will happen during the lamination process when the cells are assembled to solar modules.

As an alternative approach the electrical contacts of the solar cell may be kept open by using EVA sheets with precut windows. In this case the contact elements may be locally soldered in a later stage onto the electrical contacts.

Claims

1. A method to protect a solar cell from mechanical stress and damage, characterized by providing a polymer layer on at least one side of the solar cell before it is assembled into a solar module.

2. The method according to claim 1 , characterized by adding a mineral filler to the polymer.

3. The method according to claim 1 , characterized by masking a part of the cell before providing the polymer layer and removing the mask before the solar cell is assembled in the solar module.

4. The method according to claim 3, characterized in that masking a part of the cell involves masking in a pattern.

5. The method according to claim 1 , characterized in that providing the polymer layer involves applying the polymer in a pattern.

6. The method according to claim 1 , characterized by applying the polymer using a method selected from a group comprising: printing, spraying, casting and laminating.

7. The method according to claim 1 , characterized in that the polymer is at least one of a liquid, gel, paste, and granulate.

8. The method according to claim 1 , characterized by curing the polymer after providing the polymer layer.