Electrochemical Formation of a p–n Junction on Thin Film Silicon Deposited in Molten Salt

Abstract

Herein we report the demonstration of electrochemical deposition of silicon p-n junctions all in molten salt. The results show that a dense robust silicon thin film with embedded junction formation can be produced directly from inexpensive silicates/silicon oxide precursors by a two-step electrodeposition process. The fabricated silicon p-n junction exhibits clear diode rectification behavior and photovoltaic effects, indicating promise for application in low-cost silicon thin film solar cells.

Electrochemical Production of Si without Generation of CO2 Based on the Use of a Dimensionally Stable Anode in Molten CaCl2

“…Thesilicon deposited on agraphite cathode with the Ti 4 O 7 anode is essentially the same as that with the graphite anode under the same experimental conditions. [8,9] Thesimultaneous deposition was also confirmed by XRD and SEM results after electrolysis (Figure 7). It was also confirmed that the silicon products deposited at the cathode depend on the cathodic current density applied during the electrolysis.W hen the cathodic current density was set at % 10 mA cm À2 or al ower current density,silicon micro/nanowires with adiameter from hundreds of nanometers to % 3 mmc an be obtained (Figure 7a).…”

“…[3] To avoid the harsh temperature and carbon emissions,d irect molten-salt electrolysis of silicon dioxide and silicon halides has been proposed for the preparation of Si products,s uch as Si nanopowders,S in anowires,a nd Si films. [4][5][6][7][8][9][10][11] In particular, direct electrochemical bulk reduction of insulative SiO 2 to Si in molten CaCl 2 at 850 8 8Cw as first proposed by T. Nohira et al, [4] and the CaCl 2 -based molten-salt process has emerged as ap romising method for Si electrodeposition in recent years. [8,9] However,c ommonly,acarbon anode was used as ac onsumable anode in molten salt where the liberated oxygen ions react with the carbon electrode to generate CO and CO 2 , resulting in ad ecrease in the electrode size.A dditionally,i n the silicon-deposition process,a tl east % 0.19 kg of carbon must be used for every kg of silicon produced, implying large amounts of CO 2 generated in large-scale molten-salt silicon production.…”

“…[4][5][6][7][8][9][10][11] In particular, direct electrochemical bulk reduction of insulative SiO 2 to Si in molten CaCl 2 at 850 8 8Cw as first proposed by T. Nohira et al, [4] and the CaCl 2 -based molten-salt process has emerged as ap romising method for Si electrodeposition in recent years. [8,9] However,c ommonly,acarbon anode was used as ac onsumable anode in molten salt where the liberated oxygen ions react with the carbon electrode to generate CO and CO 2 , resulting in ad ecrease in the electrode size.A dditionally,i n the silicon-deposition process,a tl east % 0.19 kg of carbon must be used for every kg of silicon produced, implying large amounts of CO 2 generated in large-scale molten-salt silicon production. Moreover,t he evolved CO and CO 2 may also partially dissolve in the molten salt and can thus contaminate the cathodic products, [12,13] as also does the accumulation of carbon powders peeling off from the graphite anode after long-term electrolysis.T or educe environmental impacts and labor, one must find anon-consumable anode material that is stable in the molten salt with oxygen evolution as the anode reaction.…”

Formation of Si thin films by electrodeposition in ionic liquids for solar cell applications

“…Electrodeposition under diffusion-limited conditions is known to afford rough surfaces, 39) which was confirmed to be the case herein by the observation of current decay with progressing electrodeposition. Figure 4 demonstrates that constant-potential electrodeposition at −2.3 V vs Ag=Ag + and 1500 mC cm −2 was characterized by a current density decrease until 5 s followed by a gradual decline to an almost constant current density of 0.1 mA cm −2 until ∼10000 s. Moreover, electrodeposited Si is expected to be a semiconductor, which implies that the electrodeposited semiconductor film can hardly supply a sufficient amount of electrons to the electrode=electrolyte interface, 24) as confirmed by the data in Fig. 4.…”

“…Therefore, in the future, the annealing operation should be further improved in order not to be oxidized during the annealing process. Herein, the Al content is too high for the application in comparison with the study in molten salts, 24) implying that the p-type characteristics of doped films need to be determined from their photoresponses. Nevertheless, this study revealed that the addition of AlCl 3 to the electrolyte promotes the electrodeposition of p-type Si films, with the extension of this technique to n-type dopants such as PCl 5 allowing the fabrication of n-type Si films and thus, the formation of p-n junctions.…”