TOPCon Silicon Solar Cells With Selectively Doped PECVD Layers Realized by Inkjet-Printing of Phosphorus Dopant Sources

Digital, additive printing processes are increasingly used to produce electronic components due to their cost-saving opportunities and flexibility in design, compared to established thin-film technologies. Inkjet compatible materials like metals (e.g. silver, gold, copper), carbons and polymers are in evaluation and application for printed electronics. However, the range of printable materials for inkjet is limited, mainly due to the needed small particle size (< 1 μm) to avoid a nozzle clogging and print failure. Silicon is a very versatile material useful in most electronic applications like solar cells, semiconductor, and lithium-ion batteries. World record solar cells (> 25 %) were demonstrated with thin-film concepts (TOPCon), where a doped silicon contact is formed on the cell backside in laborious and costly vacuum production steps. The availability of a doped and inkjet printable silicon ink would allow the direct printing of such contacts, which would simplify the production effort. Boron-doped silicon powder (< 100 μm particle size), as a recycling product from PV-industry, was intensively milled in ceramic media. A hydrofluoric acid etching procedure was developed to reduce the oxygen content of the powder. Silicon inks were formulated and evaluated with variation in type of solvents and organic binders. Important ink properties like rheological behavior and inkjet compatibility were characterized (Dimatix DMP). The ink wetting behavior on monocrystalline silicon wafers (n-type 51015 cm-2) was characterized by wetting angle and surface energy measurements (Krüss DSA 100) with an optional plasma pre-treatment. Furnace and photonic sintering methods (line laser LIMO, flash-lamp Novacentrix) were compared to demonstrate the sintering of printed Si-films. The film microstructure was characterized by SEM, XRD and Photoluminescence (PL) Imaging (charge carrier lifetime). A two-step grinding procedure significantly reduced the mean particle size to 200 nm. By HF etching the oxygen content of the powder was reduced to 0.6 wt.-%, which equals to an oxide shell of only 0.4 nm. By choosing a proper solvent, the ink wetting was optimized to achieve good print quality for dots, straight lines, and full areas. The developed ink has 10 wt.-% Si-particle loading, a viscosity of 13.5 cP and a surface tension of 29 mNm. In single pass printing the film thickness after sintering was 0.8 μm. By sintering under forming gas, a visible sintering effect of the Si-particles was observed at rather high temperatures of 1150 °C. XRD analysis reveals no crystalline oxide formation, although the samples change color from dark grey to yellow greenish. An even better sintering effect was observed for photonic laser sintering within a millisecond processing time. The functionality of such printed Sifilms for solar cell applications was evaluated by PL Imaging and life-time measurements.