for 40×40 cm² Solar Panels
Motivation and goals
In-process series connection plays an important role when it comes to reaching high panel efficiencies. Integrated series connection minimizes the electric losses in the panel; however, the connection occupies some of the cell surface that would otherwise produce energy. In a three-phase structuring process, the individual layers of a solar panel – i.e. transparent conducting film, silicon absorber and metal back contact – are selectively interrupted after each of them has been deposited, so that the final result is a series connection between individual solar cells. This reduces the level of electricity in the conducting layers and the ohmic losses in the contact layers. In order to keep the loss of surface area low, the connection must take place on as little space as possible. At the institute, the panels are therefore connected exclusively by means of laser-induced ablation processes. Due to the high focusability of laser light, the size of the ablated areas can be kept small. Further advantages of laser technology are its high reliability and the high degree of automation possible.
Microscope image of the connection area of a silicon thin-film solar panel. The individual laser pulses are clearly visible and form a line each if put together.
Structuring has a decisive impact on the quality and stability of thin-film solar panels. If the quality of the structuring is inadequate, entire solar panels may fail. It is therefore important to study the influence of the individual laser processes on our solar panels as well as the exact mechanisms that determine the structuring quality. Structuring by means of laser processes, however, is accompanied by complex physical phenomena.
The structuring system is equipped with four Q-switched solid-state lasers. The laser sources used have a wavelength of 1064 nm, but two of the lasers are operated at a wavelength of 532 nm and 355 nm, respectively. Frequency multiplication is achieved by non-linear optical effects. All four lasers are in the fundamental transverse mode, also called TEM00 mode. The excitation of the each laser medium is effected by laser diodes. Mechanical beam switches are used to guide the collimated beams of the different lasers on a joint beam path. The laser beams are focused on the substrate through a lens. The retainer can take solar panels of the sizes 10×10 cm², 30×30 cm² and 40×40 cm². Since the laser spot is guided across the substrate in the x direction and the substrate itself can be moved in the y direction, the laser beam can reach every point on the substrate. The substrate retainer and the deflection unit are moved by two separate linear motors with a speed of up to 1 m/s and an accuracy in the μm range.
Structuring of a 30x30 cm² solar panel. The focused laser beam is guided onto the solar panel from the glass side, which causes the film to ablate on that side.