The use of lasers for scribing the thin layers found in solar panels to create and interconnect sub cells been well known for many years. The technology consists of laying down a thin layer of the lower electrode material, often a transparent conducting oxide such as tin oxide, zinc oxide or indium tin oxide, on a glass plate and laser scribing lines at typically 5-10 rnrn intervals to separate the layer into electrically isolated regions. The electricity generating layer, such as amorphous silicon, is then applied over the whole area and a laser is used again to scribe lines in this layer parallel to and as close as possible to the initial scribes in the first layer. A third, top layer, often a metal such as aluminium, is then applied and a laser beam is used for the third time to scribe lines in this layer as close to and parallel to the other lines to break the electrical continuity.
By this method an electrical series connection is made between all the cells in the panel so that the voltage generated by the whole panel is given by the product of the potential formed within each cell and the number of cells. Typically panels are divided up into 50-100 cells so that overall panel output voltages are in the 50 volt range. JP10209475 gives a thorough description of the standard laser processes used.
As well as ITO/Silicon/Aluminium structures many other materials can be used to make solar panels. Other equally effective devices are made based on Cadmium Telluride (CdTe), Copper-Indium-diselenide (CIS) and crystalline silicon on glass (CSG). In all cases lasers are used to scribe some or all of the layers involved.
The laser beams that are used to scribe individual layers are sometimes applied from the coated side of the glass sheet but can also be applied from the opposite side in which case the beams pass through the glass before interacting with the film. The 10 lasers used generally operate in the infra-red (1064 nm wavelength) region of the spectrum but lasers operating at the 2nd harmonic wavelength (532 mm) are also widely used. Even UV lasers are sometimes used. The lasers are generally pulsed with pulse lengths in the range of a few to several 100 nanoseconds and operate at pulse repetition rates in the range of a few kHz to few 100 kHz
In some cases solar panels are made on non-transparent substrates such as metal sheets. In this case irradiation through the substrate is not possible so all scribing processes require beams incident from the coated side. In some other cases solar panels are fabricated on flexible substrates such as thin metal or polymer sheets. In the former case irradiation from only the coated side is possible. In the latter case irradiation from the coated side or through the substrate are both possible.
The common characteristics of all these devices is that multiple scribes each up to one or more meters in length have to be created in order to divide up each layer on a panel. Hence total scribe lengths per layer up to well over 100 m often need to be made by solar panel process tools in acceptable panel process times. Depending on the panel size, the production line output requirements and the number of tools, for these process times are generally required to be in the range 1 to 3 minutes. This means that laser scribing rates up to many meters per second are required.
Laser tools have already been built to achieve this. In some cases the tools have stationary optics which means the panel has to be moved very rapidly in one direction and then stepped in the other. To avoid excessive panel speed optics units having multiple parallel beams are often used. As an example of this a panel with dimension of about 1.1 m×1.1 m requiring 160 separate scribes can be processed with 8 parallel beams in under 100 second with the panel moving at a maximum speed of less than 300 mm/sec.
In the case discussed above the beam unit with multiple beams are stationary and the panel is moved in two dimensions. Other arrangements are also possible. In another case the panel is held stationary and the beam or optics unit is moved in two dimensions by means of a moving gantry over the panel. An intermediate approach is also possible where the panel moves in one axis and the optics unit moves in the other on a gantry over the panel.
Another solar panel scribing approach uses a single beam to scribe all the lines but causes the beam to move at high speed using a galvanometer driven mirror scanner system. US Patent Application Publication No. US2003/0209527A1 describes such a case. A scanner system is used to move the laser beam across the full width of a 600 mm wide panel at speeds up to 4 meters/sec while the panel is moved in the orthogonal direction past the scanner unit.
Application GB0611738.6 describes an extension of this invention where a scanner unit is used to move the beam at high speed as discussed in US2003/0209527A1 but the length of the beam scan region generated by the scanner unit is limited to a fraction of the total line length required rather than the full line length. The consequence of this is that multiple bands are needed to scribe the full lengths of the lines. This means that as well as the beam motion by the scanner unit motion of the substrate in two axes with respect of the scanner unit is required in order to cover the full area.
Whatever method of beam motion with respect to the panel is used the laser scribes in the electricity generating layer and top electrode layer need to be placed reliably very close to existing scribes in lower layers to minimize the inactive area between the scribes. Because of panel distortions and size changes during manufacture it is necessary to measure the position of previous scribes and compensate by adjusting the panel or beam motion to maintain accurate relative positioning. A global measurement of overall panel expansion or shrinkage is readily made by measuring the position and angle of the first and last scribes on a panel after loading. This data can be used to correct for these global changes by adjusting the parameters that control the beam motion with respect to the panel. However, simple global distortion correction is not sufficient to allow close and accurate placement of scribes since scribe pitch can become irregular due to errors on the tool that make the first scribe or errors introduced ring subsequent panel processing an ‘on-line’ system that ensures that all scribes are accurately placed with respect to previous scribes. We call our alignment system “dynamic scribe alignment” (DSA). Application GB611718.6 describes the use of such an alignment system for the case where a scanner is used to move the beam and the panel is scribed in a series of parallel bands perpendicular to the scribe line direction. In this application we describe the use of a dynamic scribe alignment system where scanners are not used and instead one or more beams are used to scribe the panel in a series of lines or bands parallel to the scribe direction.