Laser processing for the integrated series connection of thin-film silicon solar cells

The integrated series connection of solar cells is an essential aspect for thin-film photovoltaic technology. With a series connection a high output voltage of the module is achieved while the output current is kept low. Thus, Ohmic losses in the contact materials are kept low as well. In thin-film silicon solar technology the steps to create the interconnection are commonly done by laser ablation integrated in-between the depositions of the solar cell layer materials. In three steps laser scribing is used to selectively remove layers locally in the formof lines across the module substrate. In a first step the front-contact is removed for electrical insulation and cell stripe definition. Afterwards, the absorber is removed locally exposing the front-contact beneath. Finally, the interconnection is formed when the back-contact is removed locally as well. The area that is needed for the interconnection of two neighboring cells is no longer active for current generation. Depending on the technology 5–10% of active area is lost. The reduction of this area holds an attractive potential for an increase of the module efficiency. The topic of this thesis is the investigation of the lower geometrical limits for the dead area reduction for substrate side laser processing of thin-film silicon solar cells. It is well-known that the interconnection and the laser processes can have an impact on the performance of the solar module. Therefore, the characterization of the impact on the performance is of special importance when laser processes are used that are capable of generating a reduced interconnection width. P1: for the front-contact insulation process it was found out that the scribe quality strongly depends on the used laser wavelength. Ablation mechanisms that are driven by material phase changes (scribing with 532 nmor 1064 nm) can lead to smoother scribe edges compared to mechanisms dominated by stress-induced removal (355nm) where non-uniformrip-off at the edges occurs. However, in certain processing regimes, strong ablation debris redeposition in direct vicinity of the P1 scribe is observed when small beam spot radii (<10m) are used. Such redeposition has a severe impact on the solar cell performance in this region. With proper wet-chemical cleaning the amount of redeposited debris on the front-contact and the negative impact on the solar module can be minimized. Parasitic shunting of two neighboring cell stripes by deposition of absorber material into the P1 scribe increases when the scribe width is reduced. Measurements show that the overall magnitude of the shunt is in a value range that impact on the solar module is negligible for commonly used cell topologies