**2. Platinum nanoparticle modified porous multicrystalline silicon**

Multicrystalline silicon is the most common material of the present conventional solar cells. We applied multicrystalline Si to solar hydrogen production by the photodecomposition of hydrogen iodide. To improve the conversion efficiency and stability of photoelectrochemical solar cells, multicrystalline n-type Si wafers were catalyzed with platinum nanoparticles and antireflected with a porous Si layer. The Pt nanoparticles were deposited on the multicrystalline Si substrates by electroless displacement deposition immersing substrates in a metal salt solution including hydrofluoric acid (Yae et al., 2006a, 2007c, 2008). The porous Si layers were formed by metal-particle-assisted hydrofluoric acid etching immersing metalparticle modified Si wafers in a hydrofluoric acid solution (Yae et al., 2003, 2006a, 2009).

### **2.1 Platinum nanoparticle formation by electroless displacement deposition**

The metal nanoparticles play a tremendously important role in photoelectrochemical solar cells for photovoltaic and photochemical conversion of solar energy (Nakato et al., 1988, Nakato & Tsubomura 1992). The kind, particle density and size of nanoparticles influence the stability and conversion efficiency of solar cells. Thus, controlling the size and distribution density (particle density) of metal nanoparticles on semiconductor, multicrystalline Si in this case, is important for obtaining efficient photoelectrochemical solar cells. As well-controlled and low-cost methods of depositing metal nanoparticles on Si, we have been utilizing several methods such as simply dropping a solution of colloidal metal particles (Yae et. al., 1994a), preparing a Langmuir-Blodgett layer of nanoparticles (Jia et al., 1996, Yae et. al., 1994b, 1996), electrodeposition (Yae et. al., 2001, 2008), and electroless displacement deposition (Yae et al., 2007c, 2008). We choose the electroless displacement deposition to prepare Pt nanoparticles on multicrystalline Si wafers (Yae et al., 2006a). The electroless deposition of metals is a simple process involving only the immersion of substrates into metal-salt solutions. The electroless deposition is classified into autocatalytic and displacement depositions (Paunovic & Schlesinger, 2006). The electroless displacement deposition of metals onto Si uses a simple solution containing metal-salt and hydrofluoric acid. The deposition reaction is local galvanic reaction expressed by equations (4) and (5) consisting of the cathodic deposition of metal on Si accompanying positive hole injection into the valence band of Si and the anodic dissolution of Si. Various kinds of metals can be deposited on Si using this method (Chemla et al., 2003, Gorostiza et al., 1996, 2003, Nagaraha et al., 1993, Yae et al., 2007c, 2008). Deposition of metal on Si:

$$\rm{M^{n}} \rightarrow \rm{M} + \rm{nh^{\*}} \tag{4}$$

Anodic oxidation and dissolution of Si in an hydrofluoric acid solution:

$$\rm Si + 4HF\_2 \rm + 2h^\* \to \rm SiF\_6 \rm + 2HF + H\_2 \tag{5}$$

or

In this study, Pt-nanoparticle modified multicrystalline Si wafers and microcrystalline Si (c-Si:H) thin films (Yae et al., 2007a, b) are used as photoelectrodes in the photodecomposition of hydrogen iodide for low-cost and efficient production of solar hydrogen. We also attempt solar water splitting using a multi-photon system equipped with the microcrystalline Si thin film and metal-oxide photoelectrodes in series (Yae et al., 2007b).

Multicrystalline silicon is the most common material of the present conventional solar cells. We applied multicrystalline Si to solar hydrogen production by the photodecomposition of hydrogen iodide. To improve the conversion efficiency and stability of photoelectrochemical solar cells, multicrystalline n-type Si wafers were catalyzed with platinum nanoparticles and antireflected with a porous Si layer. The Pt nanoparticles were deposited on the multicrystalline Si substrates by electroless displacement deposition immersing substrates in a metal salt solution including hydrofluoric acid (Yae et al., 2006a, 2007c, 2008). The porous Si layers were formed by metal-particle-assisted hydrofluoric acid etching immersing metalparticle modified Si wafers in a hydrofluoric acid solution (Yae et al., 2003, 2006a, 2009).

**2. Platinum nanoparticle modified porous multicrystalline silicon** 

**2.1 Platinum nanoparticle formation by electroless displacement deposition** 

Gorostiza et al., 1996, 2003, Nagaraha et al., 1993, Yae et al., 2007c, 2008).

Anodic oxidation and dissolution of Si in an hydrofluoric acid solution:

Deposition of metal on Si:

or

The metal nanoparticles play a tremendously important role in photoelectrochemical solar cells for photovoltaic and photochemical conversion of solar energy (Nakato et al., 1988, Nakato & Tsubomura 1992). The kind, particle density and size of nanoparticles influence the stability and conversion efficiency of solar cells. Thus, controlling the size and distribution density (particle density) of metal nanoparticles on semiconductor, multicrystalline Si in this case, is important for obtaining efficient photoelectrochemical solar cells. As well-controlled and low-cost methods of depositing metal nanoparticles on Si, we have been utilizing several methods such as simply dropping a solution of colloidal metal particles (Yae et. al., 1994a), preparing a Langmuir-Blodgett layer of nanoparticles (Jia et al., 1996, Yae et. al., 1994b, 1996), electrodeposition (Yae et. al., 2001, 2008), and electroless displacement deposition (Yae et al., 2007c, 2008). We choose the electroless displacement deposition to prepare Pt nanoparticles on multicrystalline Si wafers (Yae et al., 2006a). The electroless deposition of metals is a simple process involving only the immersion of substrates into metal-salt solutions. The electroless deposition is classified into autocatalytic and displacement depositions (Paunovic & Schlesinger, 2006). The electroless displacement deposition of metals onto Si uses a simple solution containing metal-salt and hydrofluoric acid. The deposition reaction is local galvanic reaction expressed by equations (4) and (5) consisting of the cathodic deposition of metal on Si accompanying positive hole injection into the valence band of Si and the anodic dissolution of Si. Various kinds of metals can be deposited on Si using this method (Chemla et al., 2003,

Mn+ → M + nh+ (4)

Si + 4HF2- + 2h+ → SiF62- + 2HF + H2 (5)

$$\rm{Si} + 2H\_2O + 4h^\* \to SiO\_2 + 4H^\* \tag{5}$$

$$\text{SiO}\_2 + 6\text{HF} \rightarrow \text{SiF}\_6\text{2} + 2\text{H}^\* + 2\text{H}\_2\text{O} \tag{5}$$

Non-polished multicrystalline n-type Si wafers (cast, ca. 2 cm, 0.3 mm thick) were washed with acetone, and etched with 1 mol dm-3 sodium hydroxide or potassium hydroxide solution at 353 K for saw damage layer removal. Before deposition of Pt particles, the wafers were treated by one of two pretreatment methods (hereafter, we call these pretreatments method A and method B). Method A: the wafers were washed with acetone, immersed in a CP-4A (a mixture of hydrofluoric acid, nitric acid, acetic acid, and water) solution for three min, and then immersed in a 7.3 mol dm-3 hydrofluoric acid solution for two min. Method B: the wafers were immersed in 14 mol dm-3 nitric acid at 353 K for 30 min after method A treatment. The multicrystalline Si wafer was immersed in a 1.0x10-3 mol dm-3 hexachloroplatinic (IV) acid solution containing 0.15 mol dm-3 hydrofluoric acid at 313 K for 30-120 s.

Fig. 4. Typical scanning electron microscopic (SEM) images of multicrystalline n-Si wafers pretreated by method A (image a) or B (b and c) and immersed in the Pt displacement deposition solution for 120 (a and b) or 30 s (c).

Figure 4 shows typical scanning electron microscopic (SEM) images of Pt deposited multicrystalline Si wafers. Spherical Pt-nanoparticles were sparsely scattered on the multicrystalline Si surfaces. Thin Si oxide layer formed by immersing the multicrystalline Si wafers in nitric acid solution (method B) decreased the particle density from 4x109 to 0.9x109 cm-2 (Figs. 4a and b). Shortening the immersion time from 120 to 30 s decreased the average particle size from 87 to 67 nm (Figs. 4b and c). The size and particle density of electrolessly deposited Pt nanoparticles on multicrystalline Si can be controlled by changing the deposition conditions. This is consistent with our previously reported results on singlecrystalline silicon aside from high particle density (Yae et al., 2007c).

### **2.2 Porous silicon formation by metal-particle-assisted hydrofluoric acid etching**

The antireflection of the semiconductor surface is an effective method for improving the energy conversion efficiency of solar cells (Sze, 1981, Nelson, 2003). The surface texturization by anisotropic etching is a common antireflection method for single crystalline Si solar cells. However, multicrystalline Si cannot be uniformly texturized by the anisotropic etching caused by its variety of orientations of crystallites. The metal-particle-assisted hydrofluoric acid etching can form both macroporous and microporous layers on

Solar to Chemical Conversion

a Pt particle on the bottom.

Preparation

**2.2.2 Porous structure control** 

conditions Pretreatment

d B 120

e B 120

f B 60

g B 60

Using Metal Nanoparticle Modified Low-Cost Silicon Photoelectrode 237

the solar cell characteristics (Yae et al. 2005, 2006b, 2009). In this section, we applied this method to the Pt-nanoparticle-modified multicrystalline n-Si to improve the solar cell characteristics, and attempted to shorten the etching time by controlling etching conditions

Fig. 6. Typical cross-sectional scanning electron micrograph of silicon macropore having

The Pt-nanoparticle modified multicrytalline n-Si wafers were immersed in a 7.3 mol dm-3 hydrofluoric acid aqueous solution at 298 K. In some cases, oxygen gas bubbling was applied to the solution, and/or the n-Si wafers were irradiated with a tungsten-halogen lamp during immersion in the solution in a dark room. The reflectance of Si wafers was measured using a spectrophotometer in the diffuse reflection mode with an integrating sphere attachment.

a A 120 without light control for 24 h 24 b B 120 without light control for 24 h 24

Prorous layer formation (matalparticle-assisted hydrofluoric acid ethcing) conditions

40 mW cm-2 with no bubbling for 2 h and then in the dark with oxygen bubbling for 4 h

adding under 40 mW cm-2 with oxygen bubbling for 0.5 h to condition d

40 mW cm-2 with no bubbling for 2 h and then in the dark with oxygen bubbling for 4 h

adding under 40 mW cm-2 with oxygen bubbling for 0.5 h to condition f

bubbling for 3 h <sup>3</sup>

Total etching time (h)

6

6.5

6

6.5

Pt deposition time (s)

c B 120 under 40 mW cm-2 with no

Table 1. Preparation conditions of Pt nanoparticle modified porous multicrystalline n-Si

such as the photoillumination intensity and the dissolved oxygen concentration.

multicrystalline Si wafers, which are modified with fine metal particles, by simply immersing the wafers in an hydrofluoric acid solution without a bias and a particular oxidizing agent (Yae et al. 2006a, 2009). In previous papers, we reported that porous layer formation by this etching for 24 h decreased the reflectance of Si and increased the solar cell characteristics, which are not only photocurrent density but also photovoltage (Yae et al. 2003, 2005, 2006a, 2009).

### **2.2.1 Etching mechanism**

The metal-particle-assisted hydrofluoric acid etching of Si proceeds by a local galvanic cell mechanism requiring photoillumination onto Si or dissolved oxygen in the solution (Yae et al. 2005, 2007d, 2009, 2010). Figure 5 shows a schematic diagram of n-Si and electrochemical reaction (equations (5), (6) and (7)) potential in a hydrofluoric acid solution. The local cell reaction consists of anodic dissolution of Si (equation (5)) and cathodic reduction of oxygen (equation (6)) and/or protons (equation (7)) on catalytic Pt particles. Under the photoillumination, photogenerated holes in the Si valence band anodically dissolve Si on the whole photoirradiated surface of Si. Under the dark condition, the etching proceeds by holes injected into the Si valence band with only cathodic reduction of oxygen on Pt particles, and thus the etching is localized around the Pt particles. The localized anodic dissolution produces macropores, which have Pt particles on the bottom, on the Si surface as shown in Fig. 6. We previously revealed two points about metal-particle-assisted hydrofluoric acid etching of Si: 1) the etching rate increased with photoillumination intensity on Si wafers and dissolved oxygen concentration in hydrofluoric acid solution; and 2) the time dependence of photoillumination intensity on the Si sample in the laboratory, which is ca. 0.2 mW cm-2 illumination for 6 h, dark condition for 12 h and then ca. 0.2 mW cm-2 illumination for 6 h, is suitable to produce the macro- and microporous combined structure effective for improving

Fig. 5. Schematic diagram of silicon and electrochemical reaction potential in a hydrofluoric acid solution.

the solar cell characteristics (Yae et al. 2005, 2006b, 2009). In this section, we applied this method to the Pt-nanoparticle-modified multicrystalline n-Si to improve the solar cell characteristics, and attempted to shorten the etching time by controlling etching conditions such as the photoillumination intensity and the dissolved oxygen concentration.

Fig. 6. Typical cross-sectional scanning electron micrograph of silicon macropore having a Pt particle on the bottom.
