Light efficiency

In the currently ongoing debate about future sources of energy, photovoltaics, as a renewable energy source, plays a key role. Technological progress is a particularly important pre-condition in obtaining the objective of grid parity, i.e. having electricity from photovoltaic sources cost as little as electricity from conventional sources.
Crystalline solar cells currently dominate the photovoltaics market at an efficiency rate of up to 20 percent. Lasers are currently used in such modules in production to cut the wafers and for laser edge isolation.
In laser edge isolation, the continuous doping on the edge of the cell is severed to prevent power loss from short circuits between the front and back sides. More and more lasers are being used in the doping processes, where the laser creates a higher local doping profile on the solar cell, to improve charge carriers mobility, particularly for the contact fingers. Over the last few years, thin film modules have been growing in significance. Experts expect them to gain a market share of approximately 20 percent over the medium term.
Thin film solar cells use a layer only a few micrometers thick, so that large amounts of material can be saved in production. Lasers play a decisive role in the production of thin film solar cells, in that they structure and connect the cells to the solar module; they ablate the module and so ensure that the solar module has the required insulation strength.

Established patterning

In the production of solar modules from thin film silicon or cadmium telluride, conductive and photoactive films are deposited on large substrate areas such as glass. After every deposition, the laser subdivides the surface in such a way that the cells created are automatically switched in series by the process sequence. It is thus possible to allow the cell and module current to be set depending on the cell width. The accurate, selective and contact-free laser processing can be reliably integrated into production lines. So-called patterning (see graphic) is a stringing together of ablations from individual light pulses, which create spot sizes between 30 to 80 micrometers, whereby in Patterning 1 the glass is ablated using pulse lengths of a few nanoseconds (10 to 80 ns).
The Transparent Conductive Oxide (TCO), out of zinc dioxide or tin dioxide, is usually processed using lasers with infrared wavelengths and a comparatively high pulse-to-pulse overlap. At typical feed rates, repetition rates of over 100 kilohertz are the result. A high overlap assures for a thorough cleaning of the incision.
Depending on the absorption coefficient of the material, a suitable wave length is chosen for a specific process window. The threshold of silicon for green laser light is much lower than the threshold of TCO. Therefore green laser light can pass the TCO layer without harming it, when ablating the absorber layer (compare Tab. 1). The ablation mechanism used is the same for Patterning 2 and Patterning 3. Sensitivity regarding the process window is presented in Patterning 2 and 3 in comparison to Patterning 1. Limitations are created for the pulse overlap from the fracture mechanics of the ablation. An overlap that prevents delamination of the semi-conductor layer on the interface where individual light pulses are used for ablations can be selected, having a range of 25 to 35 percent. At typical feed rates, a repetition rate of 35 to 45 kilohertz will result. The moderate ablation threshold of about two joules per centimeter squared allows spot diameters of 40 micrometers at pulse energy of roughly 25 microjoules and low average power. As the average power of such a green laser is a few watts, beam splitting and parallel processing are possible.

Ideal for Patterning 1, 2 and 3 patterning are small and compact diode-pumped solid-state lasers for micro processing with wavelength of 1064 and 532 nanometers and a high pulse-to-pulse stability. The pulse duration of these lasers should be between eight and 40 nanoseconds with repetition rates of one to 100 kilohertz.

Ablation protection

In order to protect solar modules from corrosion and long-term short circuits, the layering system on the edge shall be placed at a width of roughly one centimeter and subsequently encapsulates the entire module. Sandblasting is currently used to ablate the layers. Even though sandblasting has low investment costs, the procedure creates high follow-up costs resulting from wear and the removal of the sand as well as the measures which are required to protect the modules from dust contamination. Production requires clean and affordable solutions, such as those offered by the use of lasers. The excellent process characteristics in patterning can be transferred to complete ablation with an increase in average power. An ablation rate of approximately 50 square centimeters per second and higher allows for cycle times of 30 seconds in the production of standard-sized modules.
Even the complete ablation of all layers can be done with a single pulse ablation and thus the ablation rate can increase relative to the average power. Lasers with high average power and pulse energy can process any selected forms within the line cycle time. Best suited are lasers which use a fiber-guided system with square or rectangular profiles. The fiber homogenizes the laser beam width on the part and thus allows for uniform ablation. The stringing together of square pulses allows for an increase in ablation rates of more than 50 percent compared to standard fibers, meaning the overlap can be reduced in a manner safer for the process. The large work interval can be used to reduce unproductive periods with the use of a scanner. The lasers should further offer options to minimize the unproductive times for the beam guidance such as time sharing. A laser can supply several work stations at once, meaning the loading and unloading periods do not reduce the total productivity for the laser.

Future laser processes

The production of CI(G)S modules and cells presents massive challenges to laser processes because of the materials used. If the carrier substrate is glass, the work material molybdenum is processed in the initial patterning phase. Molybdenum has a high boiling point, good heat conductivity and high heat capacity, which leads to cracks and delamination when heat is introduced in the molybdenum layer. Since these weak points cannot be avoided in processing with nanosecond laser pulses, the use of these lasers is associated with a loss in quality. The photoactive material also reacts sensitively to the introduction of high levels of heat. Selenium has a lower boiling point than the other metals contained in photoactive material such as copper, indium, and gallium; also, it escapes at low temperatures from the bonds. The heat entry through “long” laser pulses can thus lead to short circuits on the edge areas, as the semiconductor without selenium is transformed into an alloy.
Picosecond lasers offer a solution. The material is ablated with ultra short pulses without significant process edge zone heating. Here high performance picosecond lasers with wave lengths of 1030, 515 and 343 nanometers for the structuring of current thin film modules based on CI(G)S technologies are available. It is assumed ultra short laser pulses will replace mechanical processes due to quality and productivity advantages.

Laser prospects

Additional future laser applications include the selective ablation of passivated layers on crystalline solar cells. Lasers with ultra short pulses and high pulse energy are particularly well-suited because of their excellent beam quality. Only disk laser technologies fulfill such criteria right now. Due to the simple scalability of the laser output, a higher production throughput can be achieved, and the high beam quality in the ultra short pulses significantly improves solar cell efficiency.
Laser technology has made inroads in photovoltaic module production and has used its selective contact-free procedures to vanquish other processes. The cost pressures associated with the production of solar modules will drive the spread of high-performance lasers with high average power for large-scale processes. In addition, new laser technologies with ultra short pulses will allow for new production techniques. In any case the cost per watt of solar cell power will shrink significantly in the future thanks to the laser.
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