Electrodeposition of Crystalline and Photoactive Silicon Directly from Silicon Dioxide Nanoparticles in Molten CaCl2

Silicon is a widely used semiconductor for electronic and photovoltaic devices because of its earth-abundance, chemical stability, and the tunable electrical properties by doping. Therefore, the production of pure silicon films by simple and inexpensive methods has been the subject of many investigations. The desire for lower-cost silicon-based solar photovoltaic devices has encouraged the quest for solar-grade silicon production through processes alternative to the currently used Czochralski process or other processes. Electrodeposition is one of the least expensive methods for fabricating films of metals and semiconductors. Electrodeposition of silicon has been studied for over 30 years, in various solution media such as molten salts (LiF-KF-K2SiF6 at 745 8C and BaO-SiO2-BaF2 at 1465 8C ), organic solvents (acetonitrile, tetrahydrofuran), and room-temperature ionic liquids. Recently, the direct electrochemical reduction of bulk solid silicon dioxide in a CaCl2 melt was reported. [7] A key factor for silicon electrodeposition is the purity of silicon deposit because Si for the use in photovoltaic devices is solargrade silicon (> 99.9999% or 6N) and its grade is even higher in electronic devices (electronic-grade silicon or 11N). In most cases, the electrodeposited silicon does not meet these requirements without further purification and, to our knowledge, none have been shown to exhibit a photoresponse. In fact, silicon electrodeposition is not as straightforward as metal deposition, since the deposited semiconductor layer is resistive at room temperature, which complicates electron transfer through the deposit. In many cases, for example in room-temperature aprotic solvents, the deposited silicon acts as an insulating layer and prevents a continuous deposition reaction. In some cases, the silicon deposit contains a high level of impurities (> 2%). Moreover, the nucleation and growth of silicon requires a large amount of energy. The deposition is made even more challenging if the Si precursor is SiO2, which is a very resistive material. We reported previously the electrochemical formation of silicon on molybdenum from a CaCl2 molten salt (850 8C) containing a SiO2 nanoparticle (NP with a diameter of 5– 15 nm) suspension by applying a constant reduction current. However this Si film did not show photoactivity. Here we show the electrodeposition of photoactive crystalline silicon directly from SiO2 NPs from CaCl2 molten salt on a silver electrode that shows a clear photoresponse. To the best of our knowledge, this is a first report of the direct electrodeposition of photoactive silicon. The electrochemical reduction and the cyclic voltammetry (CV) of SiO2 were investigated as described previously. [8] In this study, we found that the replacement of the Mo substrate by silver leads to a dramatic change in the properties of the silicon deposit. The silver substrate exhibited essentially the same electrochemical and CV behavior as other metal substrates, that is, a high reduction current for SiO2 at negative potentials of 1.0 V with the development of a new redox couple near 0.65 V vs. a graphite quasireference electrode (QRE) (Figure 1a). Figure 1b shows a change in the reduction current as a function of the reduction potential, and the optical images of silver electrodes before and after the electrolysis, which displays a dark gray-colored deposit after the reduction. Figure 2 shows SEM images of silicon deposits grown potentiostatically ( 1.25 V vs. graphite QRE) on silver. The amount of silicon deposit increased with the deposition time, and the deposit finally covered the whole silver surface (Figure 2). High-magnification images show that the silicon deposit is not a film but rather platelets or clusters of silicon crystals of domain sizes in the range of tens of micrometers. The average height of the platelets was around 25 mm after a 10000 s deposition (Figure 2b), and 45 mm after a 20000s deposition (Figure 2c), respectively. The edges of the silicon crystals were clearly observed. Contrary to other substrates, silver enhanced the crystallization of silicon produced from silicon dioxide reduction and it is known that silver induces the crystallization of amorphous silicon. Energy-dispersive spectrometry (EDS) elemental mapping (images shown in the bottom row of Figure 2) revealed that small silver islands exist on the top of the silicon deposits, which we think is closely related to the growth mechanism of silicon on silver. The EDS spectrum of the silicon deposit (Figure 3a) suggested that the deposited silicon was quite pure and the amounts of other elements such as C, Ca, and Cl were below the detection limit (about 0.1 atom%). Since the oxygen signal was probably from the native oxide formed on exposure of the deposit to air and silicon does not form an alloy with silver, the purity of silicon was estimated to be at least 99.9 atom%. The successful reduction of Si(4+) in silicon dioxide to elemental silicon (Si) was confirmed by Xray photoelectron spectroscopy (XPS) of the silicon deposit [*] Dr. S. K. Cho, Dr. F.-R. F. Fan, Prof. A. J. Bard Center for Electrochemistry, Department of Chemistry and Biochemistry, The University of Texas at Austin Austin, TX 78712 (USA) E-mail: ajbard@mail.utexas.edu