Shinji Yae

Formation of porous silicon by metal particle enhanced chemical etching in HF solution and its application for efficient solar cells

Abstract Porous Si was formed on n-Si wafers, modified with fine Pt particles, by simply immersing the wafers in a HF solution without a bias or an oxidizing agent. The Pt particles were deposited onto n-Si wafers by electrodeposition or electroless displacement deposition. SEM images show that many pores, ranging between 0.1 and 0.8 μm in diameter and covered with a luminescent nanoporous layer, were formed only on the Pt-modified area of the n-Si surface by immersion in 7.3 M HF solution for 24 h. The weight loss of Pt-electrodeposited n-Si wafer was 0.46 mg cm−2, corresponding to ca. 2 μm in thickness. The weight loss and the structure of porous Si changed with the etching conditions, such as concentration of dissolved oxygen in the HF solution, distribution density of metal particles, and different kinds of metal particles. A photoelectrochemical solar cell equipped with a Pt-particle-modified porous n-Si electrode gave 13.3 mW cm−2 of maximum output power, which corresponds to a 13% conversion efficiency and is higher than that for the Pt-particle-modified flat n-Si electrode.

Fermi level pinning on HF etched silicon surfaces investigated by photoelectron spectroscopy

A widely used approach to obtain smooth oxide-free and (partially) H-terminated silicon (Si) surfaces is to immerse Si wafers into CP4A (a mixture of H2O, HNO3, CH3COOH and HF in a volume ratio of 22:5:3:3) and/or HF solutions of varying concentrations. It is usually assumed that such treatments result in a dramatic reduction of the surface density of states and that, therefore, no surface band bending can occur. In our experiments we investigated the electronic surface structure of a number of CP4A/HF treated n- and p-Si wafers with varying doping densities by x-ray photoelectron spectroscopy (XPS). XPS allows a straightforward detection of surface stoichiometry as well as one of band bending and surface photovoltages (SPV) on semiconductor materials because the positions of the core level peaks directly depend on the position of the Fermi level within the band gap at the surface. Our experiments show that on all surfaces investigated Fermi level pinning still exists after the samples were immersed in th...

Electrochemical deposition of fine Pt particles on n-Si electrodes for efficient photoelectrochemical solar cells

Abstract Fine platinum (Pt) particles were deposited electrochemically on n-type silicon (n-Si) electrodes from an aqueous hexachloroplatinic acid(IV) solution by the single potential step (SPS) and double potential step (DPS) methods. The distribution density of the Pt particles on n-Si was 108 cm−2 for the SPS method, whereas it increased from 109 to 1010 cm−2 by a shift of the pulse potential at the initial step of the DPS method from −1.0 to −4.0 V versus SCE and remained nearly constant at more negative potentials. The size of the Pt particles enlarged with the charge density passing across the electrode surface at a potential of −0.70 V versus SCE, which was applied throughout for the SPS method and at the second step for the DPS method. Photoelectrochemical (PEC) solar cells equipped with Pt-electrodeposited n-Si electrodes generated open-circuit photovoltages (VOC) of 0.51–0.61 V, much higher than those for n-Si electrodes coated with continuous Pt layers (ca. 0.2–0.3 V). Solar cell characteristics changed with the pulsed potential and charge density passing across the electrode surface which changed the size and distribution density of the Pt particles. The characteristics were explained well by our previous theory on metal-dot coated n-Si electrodes.

Catalytic activity of noble metals for metal-assisted chemical etching of silicon

Metal-assisted chemical etching of silicon is an electroless method that can produce porous silicon by immersing metal-modified silicon in a hydrofluoric acid solution without electrical bias. We have been studying the metal-assisted hydrofluoric acid etching of silicon using dissolved oxygen as an oxidizing agent. Three major factors control the etching reaction and the porous silicon structure: photoillumination during etching, oxidizing agents, and metal particles. In this study, the influence of noble metal particles, silver, gold, platinum, and rhodium, on this etching is investigated under dark conditions: the absence of photogenerated charges in the silicon. The silicon dissolution is localized under the particles, and nanopores are formed whose diameters resemble the size of the metal nanoparticles. The etching rate of the silicon and the catalytic activity of the metals for the cathodic reduction of oxygen in the hydrofluoric acid solution increase in the order of silver, gold, platinum, and rhodium.

Modification of semiconductor surface with ultrafine metal particles for efficient photoelectrochemical reduction of carbon dioxide

A p-type silicon (p-Si) electrode modified with small metal (Cu, Au and Ag) particles works as an ideal-type electrode for photoelectrochemical reduction of carbon dioxide, producing carbon monoxide, methane, ethylene, etc., with a large photovoltage of 0.5 V. It is discussed that two types of surface-structure control on atomic and nanometer-sized levels are important for getting a high efficiency.

Preparation of a Langmuir‐Blodgett Layer of Ultrafine Platinum Particles and Its Application to n‐Si for Efficient Photoelectrochemical Solar Cells

A Langmuir layer of ultrafine platinum particles (2--6 nm in diam) has been developed on a water surface by dropping a Pt colloid solution, prepared by refluxing an ethanol-water (1:1) solution of hexachloroplatinic(IV) acid in the presence of poly(N-vinyl-2-pyrrolidone) as a stabilizer. The layer is transferred onto a single-crystal n-type silicon (n-Si) wafer by the horizontal lifting method. The Pt particles are rather homogeneously scattered on n-Si, and the particle density can be controlled on a nanometer scale by changing the area of the Langmuir layer at the time of transfer. The open-circuit photovoltage (V[sub oc]) for photoelectrochemical (PEC) solar cells with such n-Si electrodes is inversely related to Pt-particle density, and reaches 0.635 V, much higher than that for n-Si coated with a continuous Pt layer (ca. 0.30 V) or that for the conventional p-n junction Si solid solar cell of a similar simple cell structure (ca. 0.59 V). This result is in harmony with the previously proposed theory, the above increase in V[sub oc] being explained by the decrease in the majority carrier dark saturation current density.

Hole diffusion length and temperature dependence of photovoltages for n-Si electrodes modified with LB layers of ultrafine platinum particles

The mechanism of generation of high open-circuit photovoltages (Vocs) of 0.62–0.63 V for n-Si (~ 1 Ω cm) electrodes modified with colloidal Pt particles (4 nm in diameter) is investigated by measurements of minority-carrier (hole) diffusion length (Lp) and temperature dependence of Voc. Langmuir-Blodgett (LB) layers of colloidal Pt particles are used to control the Pt density on n-Si. The Lp value is determined to be 200 μm, irrespective of whether n-Si is modified with Pt or not. The temperature dependences of Vocs at 203–298 K have been explained well by our previously proposed model. It is shown that heat treatments of the Pt-modified n-Si electrodes increase the area and the width of the direct Pt-Si contacts and thus decrease Voc, but minority-carrier controlled (ideal) solar cells are obtained if the electrodes are prepared under appropriate conditions.

New current and potential oscillations for reduction reactions on platinum electrodes in acid solutions containing high concentration hydrogen peroxide

A new electrochemical oscillation (called oscillation B) appears, in addition to a reported one (called oscillation A), for reduction reactions on platinum electrodes in acid solutions containing hydrogen peroxide in case where the H 2 O 2 concentration is made high. Also, oscillation A becomes more pronounced and more stable as the H 2 O 2 concentration increases except the case of very high concentrations (≥1.1 M). Oscillation A appears in a narrow potential region just before hydrogen evolution, whereas oscillation B appears in a potential region of hydrogen evolution. Both oscillation A and B are strongly affected by the concentrations of H + as well as H 2 O 2 , suggesting that they arise from the competition between the H 2 O 2 and H + reduction. It is concluded that the high and low current states for oscillation A correspond to the H 2 O 2 reduction and almost no H 2 O 2 reduction (nor hydrogen evolution), respectively, whereas the high and low current states for oscillation B correspond to the H 2 O 2 reduction and hydrogen evolution, respectively. Mechanisms for sudden transitions between these states are discussed qualitatively.

Efficient Photoelectrochemical Solar Cells Equipped with an n‐Si Electrode Modified with Colloidal Platinum Particles

Various platinum (Pt) colloid solutions were prepared and used for the modification of n-type silicon (n-Si) electrodes. All the photoelectrochemical (PEC) solar cells, equipped with the Pt-colloid modified n-Si electrodes thus prepared, gave the open-circuit photovoltages (V[sub oc]) of 0.550 to 0.630 V, much higher than those (0.25--0.30 V) for n-Si electrodes coated with continuous thin Pt layers, clearly showing the beneficial effect of the discontinuous metal coating. The PEC cells equipped with the textured-surface n-Si electrodes modified with the Pt colloid particles prepared by Bredigs method, yielded an energy conversion efficiency of 14.9% in a three-electrode configuration. The relation between the V[sub oc] and the size and distribution of the Pt particles is discussed.

High Catalytic Activity of Palladium for Metal-Enhanced HF Etching of Silicon

Metal-enhanced HF etching of Si is an electroless method used to produce porous Si. Such etching generally uses not only metal-modified Si but also an oxidizing agent, such as hydrogen peroxide or metal ions. Pd exhibits high activity in enhancing the HF etching of Si without an oxidizing agent even under dissolved-oxygen-free and dark conditions. Electrolessly deposited Pd particles on n-type Si enhance the HF etching of Si but produce no porous layer. Patterned Pd films localize the etching under the boundary of the Pd deposited areas, and thus Pd can produce a microetch pattern on Si with a simple immersion in the HF solution. This etching reaction is explained by electron injection into the conduction band of Si due to the Pd-enhanced anodic oxidization of Si with water and the cathodic hydrogen evolution on Pd with the injected electrons.