**2.3.2 Effect of porous layer**

Table 2 and Figure 11 indicate the average characteristics and typical photocurrent density versus potential (*j*-*U*) curves of photovoltaic photoelectrochemical solar cells equipped with a Pt-nanoparticle modified porous multicrystalline n-Si electrode prepared under the conditions listed in Table 1. The characteristics of photoelectrodes prepared under the conditions a and b as those for the wafers indicated in Fig. 7 show that the combination of the controlling particle density and size of Pt particles, and the formation of porous layer using metal-particle-assisted etching obtained a large increase in the conversion efficiency (S) from 3.8% for curve a in Fig. 10 and 2.9% in average of 12 samples to 5.1% in the average (Table 2). The formation of

Solar to Chemical Conversion

experimental result of ca. 15%.

Using Metal Nanoparticle Modified Low-Cost Silicon Photoelectrode 243

The increase in photocurrent density of photoelectrochemical solar cells equipped with Ptnanoparticle modified multicrystalline n-Si electrode by the Pt-particle-assisted hydrofluoric acid etching is ca. 15% lower than the 30-40% estimated with reduction of the reflectance from 33% to 5-14% at the light wavelength of 700 nm. This difference can be explained by the difference in the refractive index between air (1.000), water (1.332 at 633 nm) and Si (3.796 at 1.8 eV (689 nm)) (Lide, 2004). The reflectance of Si is calculated at 34% in the air and 23% in the water. Using 23% as the initial value of reflectance estimates the increase in photocurrent density by the etching at 12-23%. This value is consistent with the

The photovoltage of photoelectrochemical solar cells equipped with Pt-nanoparticle modified multicrystalline n-Si electrode was improved by formation of the porous layer by Pt-particle-assisted hydrofluoric acid etching (Table 2). The photovoltage increase by the etching in dark conditions for 24 h was 0.01 V (*V*OC: 0.43 V) in the average of eight samples, much lower than the 0.05 V (*V*OC: 0.47 V) by the etching in a laboratory without light control (condition a in Table 1 and 2). These results show that the microporous layer effectively increases the photovoltage of such photoelectrochemical solar cells. This increase is explained by the following two possible mechanisms. 1) Screening Pt-nanoparticles' modulation of Si surface band energies by the microporous layer: The photovoltage of an n-Si electrode modified with metal particles depends on the distribution density of metal particles and the size of the direct metal-Si contacts. While metal particles are necessary as electrical conducting channels and catalysts of electrochemical reactions, the particles modulate the Si surface band energies. Thus, larger direct metal-Si contacts than a suitable size and/or a higher distribution density of metal particles than a suitable value reduce the effective energy barrier height, and then reduce the photovoltage of solar cells. The presence of a moderately thick microporous layer between the metal particles and bulk n-Si screens the modulation and thus raises the energy barrier height of the n-Si electrode, as discussed in the previous paper (Kawakami et al., 1997). 2) Increase in density of termination of Si surface bonds with iodine atoms: As we discussed in the previous section, the low opencircuit photovoltage (0.42 V) of the flat (nonporous) multicrystalline n-Si electrodes can be caused by the insufficient density of the termination of Si surface bonds with iodine atoms. Using the hydrobromic acid and bromine electrolyte solution increased the average opencircuit photovoltage of porous n-Si electrodes prepared under the condition a in Table 1 by 0.03 V for multicrystalline and 0.02 V for single-crystalline n-Si from those of using hydroiodic acid and iodide electrolyte solution. This result indicates that the density of the termination of the multicrystalline n-Si surface bonds with iodine atoms is increased to

sufficient value for generating high *V*OC by forming the microporous layer.

In the preceding section, we prepared the efficient photovoltaic photoelectrochemical solar cells using the Pt-nanoparticle modified porous multicrystalline n-Si electrode. In this section, these electrodes were used for solar to chemical conversion via the photoelectrochemical

hydrogen. A two-compartment cell was used (Fig. 1b). The multicrystalline n-Si electrode was used as a photoanode in the mixed solution of hydroiodic acid and iodine of the anode compartment. A platinum plate was used as a counterelectrode in the perchloric acid (HClO4) solution of the cathode compartment. Both compartments were separated with a porous glass plate. Figure 12 shows the typical photocurrent density versus potential (*j*-*U*) curve for the


) and hydrogen gas (H2), that is, solar

**2.4 Solar to chemical conversion (solar hydrogen production)** 

decomposition of hydrogen iodide (HI) to iodine (I2 or I3

continuous microporous layer (Figs. 8a and 11a, and condition c in Table 1) increased photovoltage (*V*OC), and decreased fill factor (*F.F.*) of the solar cells. The formation of macroand microporous combined structure (Figs. 8b and c, and conditions d and e in Table 1, respectively) increased photocurrent density (*j*SC) and fill factor (*F.F.*), and thus increased the conversion efficiency (S) of solar cells (Fig. 11b, and conditions d and e in Table 2). The decrease of particle density and size of Pt particles (Figs. 8d and e, and conditions f and g in Table 1, respectively) increased photocurrent density (*j*SC) and conversion efficiency (S) (Fig. 11c, and conditions f and g in Table 2). The conversion efficiency of solar cells reached 7.3% of curve c in Fig. 11 and 6.1% in the average of 4 samples (Table 2), and the etching time was shortened to 6.5 h from 24 h by controlling the photoillumination intensity and the dissolved oxygen concentration during etching (condition g in Table 1 and 2).


Table 2. Characteristics of photovoltaic photoelectrochemical solar cells equipped with Pt-nanoparticle modified porous multicrystalline n-Si electrode prepared under the conditions in Table 1. Average values are indicated.

Fig. 11. Photocurrent density versus potential (*j*-*U*) curves of photovoltaic photoelectrochemical solar cells equipped with a Pt-nanoparticle modified porous multicrystalline n-Si electrode. Preparation conditions: curves a, b, and c, are for conditions c, d, and g listed in Table 1, respectively.

continuous microporous layer (Figs. 8a and 11a, and condition c in Table 1) increased photovoltage (*V*OC), and decreased fill factor (*F.F.*) of the solar cells. The formation of macroand microporous combined structure (Figs. 8b and c, and conditions d and e in Table 1, respectively) increased photocurrent density (*j*SC) and fill factor (*F.F.*), and thus increased the

decrease of particle density and size of Pt particles (Figs. 8d and e, and conditions f and g in

11c, and conditions f and g in Table 2). The conversion efficiency of solar cells reached 7.3% of curve c in Fig. 11 and 6.1% in the average of 4 samples (Table 2), and the etching time was shortened to 6.5 h from 24 h by controlling the photoillumination intensity and the dissolved

a 21 0.47 13.8 0.60 3.9 b 7 0.50 16.6 0.62 5.1 d 17 0.46 17.6 0.60 4.9 e 3 0.50 17.4 0.63 5.5 f 3 0.49 18.0 0.66 5.8 g 4 0.50 19.5 0.63 6.1

Table 1, respectively) increased photocurrent density (*j*SC) and conversion efficiency (

Open-circuit photovoltage *V*OC (V)

Table 2. Characteristics of photovoltaic photoelectrochemical solar cells equipped with Pt-nanoparticle modified porous multicrystalline n-Si electrode prepared under the

Fig. 11. Photocurrent density versus potential (*j*-*U*) curves of photovoltaic

c, d, and g listed in Table 1, respectively.

photoelectrochemical solar cells equipped with a Pt-nanoparticle modified porous

multicrystalline n-Si electrode. Preparation conditions: curves a, b, and c, are for conditions

oxygen concentration during etching (condition g in Table 1 and 2).

S) of solar cells (Fig. 11b, and conditions d and e in Table 2). The

Short-circuit photocurrent density *j*SC (mA cm-2)

S) (Fig.

Efficiency S (%)

Fill factor *F.F.* 

conversion efficiency (

Preparation conditions see Table 1

No. of tested samples

conditions in Table 1. Average values are indicated.

The increase in photocurrent density of photoelectrochemical solar cells equipped with Ptnanoparticle modified multicrystalline n-Si electrode by the Pt-particle-assisted hydrofluoric acid etching is ca. 15% lower than the 30-40% estimated with reduction of the reflectance from 33% to 5-14% at the light wavelength of 700 nm. This difference can be explained by the difference in the refractive index between air (1.000), water (1.332 at 633 nm) and Si (3.796 at 1.8 eV (689 nm)) (Lide, 2004). The reflectance of Si is calculated at 34% in the air and 23% in the water. Using 23% as the initial value of reflectance estimates the increase in photocurrent density by the etching at 12-23%. This value is consistent with the experimental result of ca. 15%.

The photovoltage of photoelectrochemical solar cells equipped with Pt-nanoparticle modified multicrystalline n-Si electrode was improved by formation of the porous layer by Pt-particle-assisted hydrofluoric acid etching (Table 2). The photovoltage increase by the etching in dark conditions for 24 h was 0.01 V (*V*OC: 0.43 V) in the average of eight samples, much lower than the 0.05 V (*V*OC: 0.47 V) by the etching in a laboratory without light control (condition a in Table 1 and 2). These results show that the microporous layer effectively increases the photovoltage of such photoelectrochemical solar cells. This increase is explained by the following two possible mechanisms. 1) Screening Pt-nanoparticles' modulation of Si surface band energies by the microporous layer: The photovoltage of an n-Si electrode modified with metal particles depends on the distribution density of metal particles and the size of the direct metal-Si contacts. While metal particles are necessary as electrical conducting channels and catalysts of electrochemical reactions, the particles modulate the Si surface band energies. Thus, larger direct metal-Si contacts than a suitable size and/or a higher distribution density of metal particles than a suitable value reduce the effective energy barrier height, and then reduce the photovoltage of solar cells. The presence of a moderately thick microporous layer between the metal particles and bulk n-Si screens the modulation and thus raises the energy barrier height of the n-Si electrode, as discussed in the previous paper (Kawakami et al., 1997). 2) Increase in density of termination of Si surface bonds with iodine atoms: As we discussed in the previous section, the low opencircuit photovoltage (0.42 V) of the flat (nonporous) multicrystalline n-Si electrodes can be caused by the insufficient density of the termination of Si surface bonds with iodine atoms. Using the hydrobromic acid and bromine electrolyte solution increased the average opencircuit photovoltage of porous n-Si electrodes prepared under the condition a in Table 1 by 0.03 V for multicrystalline and 0.02 V for single-crystalline n-Si from those of using hydroiodic acid and iodide electrolyte solution. This result indicates that the density of the termination of the multicrystalline n-Si surface bonds with iodine atoms is increased to sufficient value for generating high *V*OC by forming the microporous layer.

### **2.4 Solar to chemical conversion (solar hydrogen production)**

In the preceding section, we prepared the efficient photovoltaic photoelectrochemical solar cells using the Pt-nanoparticle modified porous multicrystalline n-Si electrode. In this section, these electrodes were used for solar to chemical conversion via the photoelectrochemical decomposition of hydrogen iodide (HI) to iodine (I2 or I3 - ) and hydrogen gas (H2), that is, solar hydrogen. A two-compartment cell was used (Fig. 1b). The multicrystalline n-Si electrode was used as a photoanode in the mixed solution of hydroiodic acid and iodine of the anode compartment. A platinum plate was used as a counterelectrode in the perchloric acid (HClO4) solution of the cathode compartment. Both compartments were separated with a porous glass plate. Figure 12 shows the typical photocurrent density versus potential (*j*-*U*) curve for the

Solar to Chemical Conversion

cells.

photoelectrode.

with insulating epoxy resin.

conditions improve the conversion efficiency.

Using Metal Nanoparticle Modified Low-Cost Silicon Photoelectrode 245

In Section 2, it was described that platinum-nanoparticle modified porous multicrystalline silicon electrodes prepared by electroless displacement deposition and metal-particleassisted hydrofluoric acid etching can generate hydrogen (solar hydrogen) and iodine through the photoelectrochemical decomposition of hydrogen iodide in aqueous solution with no external bias at the solar-to-chemical conversion efficiency of 5.4%. The control of particle density and size of Pt particles by changing the initial surface condition of Si and deposition condition of Pt, and the control of porous layer structure by changing the etching

Hydrogenated microcrystalline silicon (c-Si:H) thin films are promising new materials for low-cost solar cells. The microcrystalline Si thin film approach has several advantages, including minimal use of semiconductor resources, large-area fabrication using low-cost chemical vapor deposition (CVD) methods, and no photodegradation of the solar cell's characteristics (Matsumura, 2001, Meier et al., 1994, Yamamoto et al., 1994). We applied microcrystalline Si thin films to solar hydrogen production by the photodecomposition of hydrogen iodide (Yae et al., 2007a, 2007b) and solar water splitting(Yae et al., 2007b). Figure 13 schematically shows a cross-section of the microcrystalline silicon thin-film photoelectrode. Photoelectrochemical solar cells require neither a p-type semiconductor layer nor a transparent conducting layer, which is necessary to fabricate solid-state solar

Fig. 13. Schematic cross-section of Pt-nanoparticle modified microcrystalline Si thin-film

**3.1 Preparation of photoelectrodes and photovoltaic photoelectrochemical solar cells**  Hydrogenated microcrystalline silicon thin films were deposited onto polished glassy carbon (Tokai Carbon) substrates by the hot-wire catalytic chemical vapor deposition (cat-CVD) method (Matsumura et al. 2003). A 40-nm-thick n-type hydrogenated microcrystalline cubic silicon carbide (n-c-3C-SiC:H) layer was deposited on the substrates using hydrogendiluted monomethylsilane and phosphine gas at temperatures of 1700°C for the rhenium filament. An intrinsic hydrogenated microcrystalline silicon (i-c-Si:H) layer, with thickness of 2-3 m, was deposited on the n-type layer using monosilane gas at 1700°C for the tantalum filament. The microcrystalline silicon thin film electrodes were prepared by connecting a copper wire to the backside of the substrate with silver paste and covering it

i-c-Si:H n-c-3C-SiC:H

Carbon

**3. Platinum nanoparticle modified microcrystalline silicon thin films** 

Pt nanoparticle

multicrystalline n-Si electrode prepared under the condition g in Table 1 and 2. The potential (*U*) of the electrode was measured versus the Pt-plate counterelectrode in the perchloric acid solution of the cathode compartment (Fig. 1b). The short-circuit photocurrent density of 21.7 mA cm-2 was obtained. The solution color at the Si surface darkened, and gas evolution occurred at the Pt cathode surface. These results clearly show that the photoelectrochemical solar cell equipped with the Pt-nanoparticle modified porous multicrystalline n-Si electrode can decompose hydrogen iodide into hydrogen and iodine with no external bias, as shown in the equations (1), (2) and (3) in the section 1.1.

The dashed curve in Fig. 12 shows the current density versus the potential (*j*-*U*) curve of Pt electrode, which was in the anode compartment, instead of the Si electrode of the above cell for hydrogen iodide decomposition (Fig. 1b). The onset potential of the anodic current was 0.25 V versus the Pt-counterelectrode in the cathode compartment. This value indicates that the Gibbs energy change for the hydrogen iodide decomposition in the present solutions is 0.25 eV. The energy gain of solar to chemical conversion using the photoelectrochemical solar cell is calculated at 5.4 mW cm-2 by the product of the Gibbs energy change per the elementary charge and the short-circuit photocurrent density of 21.7 mA cm-2 under simulated solar illumination (AM1.5G, 100 mW cm-2). Thus, we calculate the efficiency of solar to chemical conversion (solar hydrogen production) via the photoelectrochemical decomposition of hydrogen iodide at 5.4%. The average in solar-to-chemical-conversion efficiency of five samples was 4.7%.

Fig. 12. Photocurrent density versus potential (*j*-*U*) curve (solid line) for solar-to-chemical conversion type of photoelectrochemical solar cell equipped with Pt-nanoparticle modified porous multicrystalline n-Si electrode prepared under condition g in Table 1. The twocompartment cell for photodecomposition of hydrogen iodide (Fig. 1b) was used. Dashed line: Pt electrode measured in the anode compartment of the two-compartment cell instead of the Si photoelectrode.

multicrystalline n-Si electrode prepared under the condition g in Table 1 and 2. The potential (*U*) of the electrode was measured versus the Pt-plate counterelectrode in the perchloric acid solution of the cathode compartment (Fig. 1b). The short-circuit photocurrent density of 21.7 mA cm-2 was obtained. The solution color at the Si surface darkened, and gas evolution occurred at the Pt cathode surface. These results clearly show that the photoelectrochemical solar cell equipped with the Pt-nanoparticle modified porous multicrystalline n-Si electrode can decompose hydrogen iodide into hydrogen and iodine with no external bias, as shown in

The dashed curve in Fig. 12 shows the current density versus the potential (*j*-*U*) curve of Pt electrode, which was in the anode compartment, instead of the Si electrode of the above cell for hydrogen iodide decomposition (Fig. 1b). The onset potential of the anodic current was 0.25 V versus the Pt-counterelectrode in the cathode compartment. This value indicates that the Gibbs energy change for the hydrogen iodide decomposition in the present solutions is 0.25 eV. The energy gain of solar to chemical conversion using the photoelectrochemical solar cell is calculated at 5.4 mW cm-2 by the product of the Gibbs energy change per the elementary charge and the short-circuit photocurrent density of 21.7 mA cm-2 under simulated solar illumination (AM1.5G, 100 mW cm-2). Thus, we calculate the efficiency of solar to chemical conversion (solar hydrogen production) via the photoelectrochemical decomposition of hydrogen iodide at 5.4%. The average in solar-to-chemical-conversion

Fig. 12. Photocurrent density versus potential (*j*-*U*) curve (solid line) for solar-to-chemical conversion type of photoelectrochemical solar cell equipped with Pt-nanoparticle modified porous multicrystalline n-Si electrode prepared under condition g in Table 1. The twocompartment cell for photodecomposition of hydrogen iodide (Fig. 1b) was used. Dashed line: Pt electrode measured in the anode compartment of the two-compartment cell instead

the equations (1), (2) and (3) in the section 1.1.

efficiency of five samples was 4.7%.

of the Si photoelectrode.

In Section 2, it was described that platinum-nanoparticle modified porous multicrystalline silicon electrodes prepared by electroless displacement deposition and metal-particleassisted hydrofluoric acid etching can generate hydrogen (solar hydrogen) and iodine through the photoelectrochemical decomposition of hydrogen iodide in aqueous solution with no external bias at the solar-to-chemical conversion efficiency of 5.4%. The control of particle density and size of Pt particles by changing the initial surface condition of Si and deposition condition of Pt, and the control of porous layer structure by changing the etching conditions improve the conversion efficiency.
