Rückkontaktzellkonzepte für großflächige, kristalline Siliziumsolarzellen

Abstract

The objective of this thesis was the development of a crystalline silicon back contact solar cell, where the cell design was to be predominantly suited for mono-crystalline, large area substrates. The production sequence was to be based on technologies applicable in an industrial environment.In the past, a large number of back contact solar cell concepts have been introduced. Reviewing their specific properties leads to the conclusion that our demands are best met by the Metallisation Wrap Through (MWT) cell concept. Therefore the focus of this thesis is the investigation and production of MWT solar cells.The design of the MWT concept resembles that of conventional solar cells that have a collecting emitter on their front side which is contacted by a finger grid. The busbars, however, are shifted to the rear side of the cell. An electrical connection between fingers on the front side and the busbars on the rear side is established by a relatively small number of holes (less than 1 hole per cm^2) in the cell area.Due to the gain in active area through the relocation of the busbars, the cell concept promises higher efficiencies on the cell level. In addition, it offers advantages when interconnecting the cells in modules. On the one hand, simplified technologies based on pick-and-place can be introduced, which gives rise to potential to cost reduction. On the other hand, loss through series resistance in the module can be greatly reduced since the connectors between the cells can be designed more generously as they cause no shadowing when placed on the rear of the cells. The latter becomes particularly important for large area cells where very high currents are generated.For the production of MWT solar cells the robust thick-film process mostly used in industry was chosen as a baseline process . This process has to be adapted in several ways to enable the manufacturing of MWT instead of conventional solar cells.An additional process step, the drilling of the interconnection vias, becomes necessary. This is usually carried out with a laser. Thereby the crystal is damaged in the regions near the holes. This laser-induced damage was analysed optically as well as by spatially resolved µPCD life-time measurements. Optical investigation showed a damage depth of more than 10 µm. µPCD life-time measurements allow the determination of the time necessary to remove the damage completely by chemical etching.Further, the metallisation process has to be adjusted compared to the conventional cell process. This includes an adapted screen-printing process that allows for a reliable metallisation of the vias and thus a good electrical connection between the front and back side of the cell. Holes metallised with this adapted process show negligible series resistance of approximately 15 mOhm. An additional challenge is the identification of a silver paste suited for the metallisation of the busbars of a MWT cell. Here pastes with a low content of glass frit proved to be particularly well suited.Even MWT cells produced with the adjusted and optimised process show I-V-characteristics that can not be described adequately with the two-diode-model. A calculation of parameters for the two-diode-model in the busbar region reveals that they differ greatly from those in the other regions of the cell, which have the design of a conventional cell. In particular, a high series resistance in the busbar region demands a description of the electrical properties of a MWT cell to be a parallel interconnection of these two different cells. After testing the model experimentally it was used to calculate the expected efficiencies of MWT solar cells with different contact designs. The purpose of this was to identify a contact arrangement which, on the one hand enables cell interconnection with conventional technology, and on the other hand promises efficiency improvement compared to conventional solar cells. Both requirements are fulfilled with a design that is close to the known PUM-design.The adjusted cell process was used to process a larger batch of 83 mono-crystalline cells (15.6 x 15.6 cm^2) using the optimised contact arrangement. This resulted in an average of efficiency of 16.2%, which is an excellent result considering the cell size. Efficiency was further improved to 16.7% (best cell) by isolating the p-n-junction at the cell edges using a cut from the front side that replaces the original cut on the rear.In principle, the production sequence is also appropriate for multi-crystalline substrates. This was demonstrated with multi-crystalline MWT solar cells with efficiencies of up to 15.4% (15.6 x 15.6 cm^2).At the end of the thesis an alternative method of contact formation is described: electroless metallisation. Here the challenge turned out to be the deposition of mechanically stable contacts with good electrical properties. The best MWT solar cell with electrolessly deposited contacts had an efficiency of 16.9% (143.5 cm^2, Cz-Si), which underlines the principal suitability of the cell concept with electroless metallisation.