Electroplating of Al on Si for Crystalline-Si Solar Cells

Electroplating of aluminum (Al) on silicon (Si) wafers in a room-temperature or near-room-temperature ionic liquid has been developed as an alternative to conventional metallization used to make electrodes on Si solar cells. Compared to other electroplating techniques for Al using molten salts or organic aprotic solvents, room-temperature ionic liquids are a relatively new class of compounds characterized by high electrical conductivity, extremely low vapor pressure, low viscosity, low toxicity, non-flammability, a wide electrochemical window, and being liquid in a wide range of temperatures. These properties make ionic liquids ideal solvents for the electroplating of Al [1-3]. Although many metals have been used as the substrate for the electroplating of Al, no results regarding Al electroplating on Si substrates have been reported. In this work, we report the electroplating of Al on Si substrates using room-temperature AlCl3/[EMIM]Cl ionic liquids. The electroplating was performed by means of galvanostatic electrolysis and the surface morphology, composition, and crystal structure of Al deposits were characterized using scanning electron microscopy (SEM) coupled with energy dispersive X-ray analysis (EDAX) and X-ray diffractometry (XRD). In addition, the sheet resistance of Al deposits was studied to understand the effects of pre-bake conditions, deposition temperatures, and post-deposition annealing. Figure 1 shows the top-view SEM image of Al deposits obtained on a Si substrate at 40 mA and 70 ˚C for 0.5 hours. It can be seen that the as-deposited Al film is dense and homogeneous with Al crystallites on the order of 10–20 mm. As shown in the inset, the deposits only display a strong peak of Al around 1.5 KeV without any other peaks corresponding to different materials. Figure 2 shows the XRD pattern of Al deposits obtained on a Si substrate at 40 mA and 70 ˚C for 0.5 hours. Four peaks shown in the XRD pattern correspond to certain crystal orientations of Al, indicating that the composition of the deposits is pure Al metal. Figure 3 shows the sheet resistance of Al deposits with different deposition temperatures at 40 mA for 0.5 hours. It is clearly seen that the sheet resistance decreases as the deposition temperature increases. Post-deposition annealing at 350 ˚C under vacuum was also performed to further reduce the sheet resistance of Al deposits as shown in figure 4. The minimum sheet resistance obtained is ~8 milliohm/sq, corresponding to a resistivity of ~7×10–6ohm-cm. Figure captions: Figure 1. Top-view SEM of Al deposits obtained on Si substrate at 40 mA and 70 ˚C for 0.5 hours. The inset shows the corresponding EDAX analysis of the deposits. Figure 2. XRD pattern of Al deposits obtained on Si substrate at 40 mA and 70 ˚C for 0.5 hours. Figure 3. Sheet resistance of Al deposits as a function of deposition temperature at 40 mA for 0.5 hours. Figure 4. Average sheet resistances of Al deposits as a function of deposition temperature before (black curve) and after (red curve) vacuum annealing. Acknowledgment: This work was supported by the U.S. Department of Energy through the SunShot Program under grant no. DE-EE-0005322. References: [1] S. Caporali, A. Fossati, A. Lavacchi, I. Perissi, A.Tolstogouzov and U. Bardi, “Aluminum electroplated from ionic liquids as protective coating against steel corrosion”, Corrosion Science 50, 534 (2008) [2] G. Yue, S. Zhang, Y. Zhu, X. Lu, S. Li and Z. Li, “A promising method for electrodeposition of aluminum on stainless steel in ionic liquid”, AIChE Journal 55, 783 (2009) [3] T. Jiang, M. Brym, G. Dubé, A. Lasia and G. Brisard, “Electrodeposition of aluminum from ionic liquids: Part I – electrodeposition and surface morphology of aluminum from aluminum chloride (AlCl3)–1-ethyl-3-methylimidazolium chloride ([EMIm]Cl) ionic liquids,” Surface and Coatings Technology 201, 1 (2006)