Toward Cost‐Effective Manufacturing of Silicon Solar Cells: Electrodeposition of High‐Quality Si Films in a CaCl₂‐based Molten Salt

Abstract

Electrodeposition of Si films from a Si-containing electrolyte is a cost-effective approach for the manufacturing of solar cells. Proposals relying on fluoride-based molten salts have suffered from low product quality due to difficulties in impurity control. Here we demonstrate the successful electrodeposition of high-quality Si films from a CaCl -based molten salt. Soluble Si -O anions generated from solid SiO are electrodeposited onto a graphite substrate to form a dense film of crystalline Si. Impurities in the deposited Si film are controlled at low concentrations (both B and P are less than 1 ppm). In the photoelectrochemical measurements, the film shows p-type semiconductor character and large photocurrent. A p-n junction fabricated from the deposited Si film exhibits clear photovoltaic effects. This study represents the first step to the ultimate goal of developing a cost-effective manufacturing process for Si solar cells based on electrodeposition.

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).…”

“…[8,9] Thep hotoelectrochemical (PEC) response of the deposited p-type silicon film was 30 to 40 %ofthat of aptype silicon wafer, as shown in Figure 7f,and about the same as afilm deposited using agraphite anode. [8,9] Elementary Al dissolved from quartz crucible has been confirmed as adopant for the p-type silicon film and its concentration is about 10 ppm. [9] Note that the small dark current in the PEC response can be mainly attributed to the pin-holes in the deposited Si film.…”

“…[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

“…25,28) Specifically, electrodeposition was conducted on the Si , and the obtained films were annealed to remove impurities. 36) Table I shows the Seebeck coefficients (600-900 µV K −1 ) and resistivities (400-800 Ω cm) of doped Si thin films, with the positive values of the former coefficients suggesting that films electrodeposited in TMHA-TFSI containing 0.5 M SiCl 4 and 8.8 × 10 −4 M AlCl 3 exhibited p-type characteristics.…”