In this contribution we present different results of our investigations regarding the use of aluminum foil as rear side metallization for solar cells with dielectric passivation and laser fired contacts (LFC). We investigate the impact of different laser processes on the resistance of the contacts, the adhesion properties of the foil and the efficiency potential. By fabricating highly efficient, 20×20 mm 2 sized solar cells with a conversion efficiency of 21.0 %, we demonstrate the high potential of this approach, which is equal to that of LFC-cells with common screen-printed or PVD metallization on the rear side. We investigated the optical properties of such metallized rear sides which benefit from an embedded air gap between foil and passivation layer. Finally, we present first solar cell results on industrially sized wafers (A=238 cm 2 ) demonstrating again the equal efficiency potential compared to PVD metallization.
We present a laser-based method for the metallization of silicon heterojunction solar cells by Cu-plating. It consists of first applying a dielectric layer on the transparent conductive oxide (TCO) as a plating mask. Then, a NiV seed is transferred by laser induced forward transfer (LIFT) from a plastic carrier foil onto the wafer. In the second laser step, the NiV layer is fired through the dielectric layer to form a contact to the TCO. After the laser process, Cu-fingers are produced by plating. The dielectric plating mask remains on the cell. Parasitic plating is prevented by using an advanced reverse pulse plating process. With the first solar cells we reach a maximum efficiency of 22.2% and an efficiency gain of 0.5% abs compared to lowtemperature screen printing reference cells, due to a higher short circuit current and fill factor. The 30 mm wide fingers reach specific contact resistances down to 0.6 mV cm 2 and also, pass a tape adhesion test. We further demonstrate that the laser process does not cause any measurable open circuit voltage loss and that a precise alignment of the two laser steps is not necessary.Silicon heterojunction (SHJ) solar cells are believed to play a major role in the future PV market [1] . Compared to homojunction solar cells, they offer a superior module performance due to their higher open circuit voltage and their lower temperature coefficient. Moreover, the production is relatively simple and does not require high temperature steps. However, a drawback for this technology still is the metallization. Screen printed metallization can only be applied with special expensive low temperature silver pastes [2] , because the passivation by the amorphous silicon (a-Si) layers of SHJ cells cannot withstand temperatures above 200-250 C [3] . Lowtemperature silver pastes reach resistivities between 5 and 10 mVcm [4] , which is far higher than for standard silver pastes sintered at high temperature (>800 C). The typical width of industrially produced screen printed fingers on SHJ cells lies between 60 and 80 mm.Cu-plating as a low temperature metallization is a more suitable technology. Kaneka has recently presented a plated SHJ solar cell with 25.1% efficiency, which represents the world record for both-sidecontacted SHJ solar cells [5] . Depending on the masking approach, plated fingers can be much thinner than screen printed fingers and the resistivity of plated metal is very close to that of bulk Cu (1.7 mVcm). In fact, plating has shown to outperform low temperature screen printing due to a higher fill factor (FF) [6] and a higher short circuit current (J SC ) [7] . High peel forces (3-5 N/mm) for plated copper fingers on TCO have been measured with an intermediate seed layer [8,9] . In addition, plating imposes less mechanical stress onto the wafer than screen printing, which is an advantage in view of thinner wafers expected in the future [1] .In order to plate a grid structure on the TCO surface of SHJ cells, in most cases, a structured organic plating mask is applied, which covers...
Laser processing is an important application for fabrication of silicon solar cells, e.g. buried contacts, laser fired contacts or edge isolation. At Fraunhofer ISE a liquid-jet guided laser is used for Laser Chemical Processing (LCP). Both the fundamentals of laser material ablation with this system and the application of various processes for solar cell fabrication are investigated. The applications are divided into two main areas: Microstructuring and deep laser cutting (wafering) of silicon substrates. Microstructuring contains the investigation and characterization of laser induced damage and selective emitter formation for n- and p-type emitters depending on laser parameters and liquid properties. One of the most important and industrially relevant topics at the moment is the formation of a selective highly doped emitter under the metal fingers of solar cells. Wafering deals with the evaluation of suitable laser parameters, adequate chemicals or chemical additives and the understanding of ablation processes by simulation and experimental work. In this presentation newest results concerning n-type doping for varying laser and liquid parameters will be presented with regard to cell efficiency and contact resistance. Furthermore a short overview of promising LCP applications will be given, e.g. p-type doping and wafering
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