The Photocurrent‐Voltage Characteristics of the Heterojunction Combination n ‐ Si / SnO2 / Redox ‐ Electrolyte

Abstract

A model for Schottky barrier-like heterojunction photoelectrodes is presented. This model allows the calculation of the current-voltage curves for such electrodes under different conditions of illumination and electrochemical charge transfer. SnO2-coated n-type Si electrodes in contact with the redox systems KJK4 Fe (CN)e and C12C1-show experimental photocurrent-voltage curves with the behavior predicted by the model. The effect of the charge transfer overvoltage and the expected current limitation due to photon control and/or redox ion diffusion are demonstrated. The feasibility of solar energy conversion through photoelectrolysis, by means of cells based on heterojunction photoelectrodes, is discussed.Photocorrosion is a general problem of photoelectrochemical cells where small bandgap semiconducting electrodes are used. To avoid this problem, a range of coatings has been applied to the electrode surfaces. Thin layers of metals (1-5) or of transparent and conducting oxides (6-8) have been deposited onto the semiconductor by different techniques. The resulting electrodes can mostly be described as Schottky barrier-like heterojunctions with an electrolyte front contact.SnO~ is a particularly attractive material for this purpose because of its high transmittance in the visibleinfrared range, its resistance against chemical attack, and because of the possibility of incorporating foreign atoms which modify both solid-state and interracial properties in a wide range (9-12). We shall consider the following electrodes composed of n-type Si-SnO2 heterojunctions. A schematic representation of the Si-SnO2-electrolyte system is given in Fig. 1. The photovoltage Vj is generated at the junction SiSnO2, which is a device of the Schottky barrier type (13). The electrochemical reaction, on the other hand, proceeds at the SnO~-electrolyte interface and is thus separated from the photoactive junction by the protective oxide film. This physical separation is advantageous in the definition of the current-voltage characteristics of the system since it permits first analyzing separately and then combining the concepts characterizing the photovoltaic and the electrochemical part of this photoelectrochemical cell.In this work, we derive a mathematical model for the treatment of heterojunction photoelectrodes. Furthermore, we shall use this model to discuss the performance of illuminated Si-SnO~ electrodes in contact with the redox systems I~/K4 Fe(CN)6 and C12/C1-. We disregard in our treatment the special case of such thin layers (< 50A) that can be tunneled by photogenerated minority carriers. Their protection against corrosion is not likely to last for long enough time since it is hardly possible to produce such thin layers without pinholes or larger defects. The SnO2 layers of about 800A thickness used in our experiments here were tight, and we could not detect any photocorrosion of the silicon after more than 30 hr of operation under solar-like illumination (14). TheoryThe equivalent circuit corresponding to the consi~lered photoe...

Effects of catalyst material and atomic layer deposited TiO2 oxide thickness on the water oxidation performance of metal–insulator–silicon anodes

“…ALD is a widely-recognized method for depositing very thin and conformal layers that are free of pinholes or cracks that, in present experiments, would allow for catalyst or oxidant diffusion to the underlying silicon substrate. [12][13][14] Calibration experiments were carried out on 3 00 p-Si test wafers to ensure high quality deposition and dene the ALD window-the processing temperature range where nearly ideal deposition is observed because of surface saturating chemisorption of the precursors during each reaction cycle. 15 At least 15 points were measured on each wafer as in Fig.…”

Si photoanode protected by a metal modified ITO layer with ultrathin NiOxfor solar water oxidation

“…In this method, two interfaces to separate the processes of the carrier generation at one (photoelectrical junction) and the chemical fuel formation at the other (electrochemical junction) are utilized to replace the single photoelectrochemical interface formed directly by the photoanode with or without a catalyst and the electrolyte. Materials include metals or metallic silicides, 5 wide band gap semiconductors (such as TiO 2 [6][7][8][9] , ZnO, 10 WO 3 , 11 GaN, 12 and Fe 2 O 3 13 ), transparent conducting oxides (TCO, such as mixtures of SnO 2 and In 2 O 3 , 14,15 SnO 2 16,17 and Sb/Ru-SnO 2 18,19 ), transition metal and its oxides polymers (PEDOT:PSS, 27 polyaniline, 28 polyacetylene, 29 and polypyrrole 30 ). Performances of the so-far developed materials or systems greatly depend on the properties of the coating and the two interfaces, such as the barrier height for charge separation, light absorption, photostability/electrochemical stability, hole conductivity, and OER catalytic activity.…”

Thin-Film Materials for the Protection of Semiconducting Photoelectrodes in Solar-Fuel Generators

“…Islands of Pt particles were shown to protect n-Si photoanodes in a 4.8 M HBr / 0.03 M Br 2 electrolyte 130. In addition to metals and silicides, conductive oxide coatings such as ITO, SnO 2 , Sb-doped SnO 2 , TiO 2 , Fe 2 O 3 and WO3 have been shown to exhibit stable photoelectrochemical behavior in conjunction with n-Si photoanodes under halide oxidation conditions 76,[131][132][133][134]. Both ion-beam sputtered ITO on n-Si with a RuO 2 co-catalyst, and CVD-grown SnO 2 on n-Si with a Pt co-catalyst, have shown 20 h of stability in either a Cl 2 /Cl -electrolyte (pH = 6.6) or in a I 3 -/I -electrolyte (pH = 1.5), respectively 76.…”