ESCA and Photoelectrochemical Studies of p‐n Junction Silicon Electrodes Protected by Platinum Deposition for Use in Solar Energy Conversion

Abstract

Layers of mixed platinum and silicon were formed on p~-n junction silicon single crystal wafers (hereafter abbreviated as p~n-Si) by depositing Pt on the p~-Si surface, followed by heating at 320~176 ESCA studies showed that Pt silicide (PtSi, or in some cases a mixture of PtSi and Pt2Si) was formed when a 3-5 nm-thick Pt layer on Si was heated, whereas a nonstoichiometric, St-rich Pt-Si intermixed layer was formed when an ultrathin Pt layer (1.0 nm thick) on Si was heated. It was also confirmed that, in the former case, a majority of the deposited Pt remained in the form of pure metal when Pt-deposited silicon was exposed to air for ca. one day before heating. The p+n-Si photoanode covered with Pt silicide gave photocurrent-voltage characteristics nearly the same as the previously reported Pt-coated p~n-Si photoanode in a hydrogen iodide/iodine solution, indicating that Pt silicide formation does not affect the photovoltage at the p+-n junction. The photocurrent of ca. 14 mAcro -2 at the maximum power point was maintained for 400h under continued illumination and only slightly reduced after 4500h (-6.3 months). The p~n-Si electrode covered with the Pt-Si intermixed layer gave a short-circuit photocurrent higher than that covered with Pt silicide by virtue of the higher light transmittance in the layer, but was somewhat inferior in photocurrent stability. ESCA studies of the electrodes after long-term stability tests revealed that some platinum was lost in the outermost layer, suggesting that the decay of the fill factor is mainly due to the chemical change at the top surface in both the case of Pt silicide and of the Pt-Si intermixed layer.Many studies have been made on semiconductor photoelectrochemical (PEC) cells in view of the direct conversion of solar energy into storable chemical energy. It has become clear that the main difficulty in this method lies in the fact that all known semiconductor electrodes having suitable bandgaps are unstable in electrolyte solutions. Various attempts have been made to stabilize such semiconductor electrodes. A method applicable to chemical conversion is coating the electrodes with thin layers of protective materials such as noble metals (1), metal oxides (2, 3), metal silicides (4), organic materials (5-7), and boron phosphide (8). Some coating materials also act as catalysts for photoelectrode reactions (9-12).We reported previously (13) that a p~n-Si photoanode coated with 2-3 nm-thick Pd or Pt metal photoelectrolyzes hydrogen iodide into hydrogen and iodine without external bias, with a high solar-to-chemical conversion efficiency of ca. 8%. The photocurrent was stable for 500h in the case of Pt coating. Nearly the same results were obtained with n~p-

Fabrication of Metal-Semiconductor Interface in Porous Silicon and Its Photoelectrochemical Hydrogen Production

“…On the bare PS photoelectrode, a large overpotential is observed and the onset potential (E OS ) is measured to be −0.1 V. On the other hand, when the PS photocathode is impregnated with Pt electrocatalyst by the electroless deposition, the overpotential is dramatically reduced and the E OS value is enhanced to +0.1 V. Note the electrode potential (E) is against the reversible hydrogen electrode (RHE) and a positive photovoltage indicates a positive energy conversion of photon to H 2 . The enhancement of H 2 generation characteristics in the presence of Pt electrocatalyst is similar to the case of planar Si photocathode 12,17,18 and can be contributed to more facile H + reduction on Pt electrocatalyst than on bare PS surface. As E is scanned in negative direction, a current plateau is observed.…”

Stabilizing inorganic photoelectrodes for efficient solar-to-chemical energy conversion

“…Several investigators have attempted to protect photoelectrodes using coatings of electrocatalytically active metal thin lms, covalent electroactive molecular species, dark photoactive conducting polymers such as polypyrrole, polyaniline, polyacetylene, etc., or by use of less corrosive non-aqueous electrolytes. [24][25][26][27][28][29][30][31][32][33][34] To date these efforts have not been wholly satisfactory. With coatings of electroactive metal thin lms such as platinum, iridium, etc.…”

Formation of Oxides and Their Role in the Growth of Ag Nanoplates on GaAs Substrates

“…However, a long-term challenge in assembling high-performance, stable PECs with semiconductor wafers is to prevent the semiconductors from corrosion caused by the holes generated in the photon-induced charge separation processes . In the last several decades, a lot of experimental and theoretical effort has indicated that depositing a submonolayer of well-separated nanoclusters made of noble metals on the surface of a semiconductor wafer could significantly increase the stability of the PEC fabricated with the metal/semiconductor hybrid photoelectrode. − ,− The metal nanoclusters play a role in modulating the distribution of Schottky barriers over the semiconductor/electrolyte interface and directing the holes to flow into the electrolyte through the metal nanoclusters. , The elimination of charge flow through the semiconductor/electrolyte interface inhibits corrosion of the semiconductor. The ideal morphology of each metal nanocluster is a “mushroom” shape, which has a very thin root with a diameter of less than 10 nm bonding to the semiconductor substrate and a large cap with suitable geometry and size for exposing the semiconductor surface as much as possible in order to harvest sunlight …”