**2.2.3 Antireflection effect**

The macroporous layer formation changed the surface color of multicrystalline n-Si wafers to dark gray. Figure 9 shows the reflectance spectra of multicrystalline n-Si wafers. The porous layer prepared by the etching without light control and gas bubbling for 24 h reduced the reflectance from over 30% to under 6.2% (curves a and b) (Yae et al., 2006a, 2009). The porous layers prepared by the etching under the conditions d and g of Table 1 gave reflectance between 8 and 17% (curves c and d). This value is higher than that of the wafer prepared under the non-controlled conditions, but much lower than the non-etched wafer.

### **2.3 Photovoltaic photoelectrochemical solar cells**

To evaluate electrical characteristics of photoelectrodes, we prepared photovoltaic photoelectrochemical solar cells (Fig. 1a) equipped with the Pt-nanoparticle modified porous multicrystalline n-Si photoelectrode. The multicrystalline n-Si electrode and Pt-plate counterelectrode were immersed in a redox electrolyte solution. Just before measuring the solar cell characteristics, the multicrystalline n-Si electrode was immersed in a 7.3 mol dm-3 hydrofluoric acid solution for two min under the elimination of dissolved oxygen by bubbling pure argon gas into the solution. This treatment is important to obtain high photovoltage caused by halogen atom termination of Si surface as mentioned below. A mixed solution of 7.6 mol dm-3 hydroiodic acid (HI) and 0.05 mol dm-3 iodine (I2) was used

Solar to Chemical Conversion

120 (a and b), 30 s (c).

**2.3.2 Effect of porous layer** 

insufficient for generating high photovoltage.

Fig. 10. Photocurrent density versus potential (*j*-*U*) curves of photovoltaic

etching obtained a large increase in the conversion efficiency (

photoelectrochemical solar cells equipped with Pt-nanoparticle modified multicrystalline n-Si photoelectrode having no porous layer pretreated under the same conditions as the specimens of Fig. 4. Pretreatment: method A (image a), B (b and c); Pt deposition time:

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

10 and 2.9% in average of 12 samples to 5.1% in the average (Table 2). The formation of

S) from 3.8% for curve a in Fig.

Using Metal Nanoparticle Modified Low-Cost Silicon Photoelectrode 241

crystalline (18.3 mA cm-2 and 0.60 on average, respectively). 2) Insufficient density of termination of Si surface bonds with iodine atoms: The termination of Si surface bonds with iodine atoms shifts the flat band potential of Si toward negative, and thus increases the photovoltage of photoelectrochemical solar cells using hydroiodic acid and iodine redox electrolyte (Fujitani et al., 1997, Ishida et al., 1999, Yae et al., 2006a, Zhou et al., 2001). An electrolyte solution of 8.6 mol dm-3 hydrobromic acid (HBr) and 0.05 mol dm-3 bromine (Br2) has sufficient negative redox potential to generate high open-circuit photovoltage without the termination. Using the hydrobromic acid and bromine electrolyte solution increases the photovoltage by 0.06 V for multicrystalline and 0.03 V for single-crystalline n-Si electrodes from those using hydroiodic acid and iodine electrolyte solution. This result indicates that the density of the termination of multicrystalline n-Si surface bonds with iodine atoms is

as a redox electrolyte solution of the photovoltaic photoelectrochemical solar cell. Photocurrent density versus potential (*j*-*U*) curves were obtained with a cyclic voltammetry tool. The potential of the n-Si wafer was measured with respect to the Pt counterelectrode. The multicrystalline n-Si was irradiated with a solar simulator (AM1.5G, 100 mW cm-2) through the quartz window and a redox electrolyte solution ca. 3 mm thick.

Fig. 9. Reflectance spectra of multicrystalline n-Si wafers: curve a after immersion in sodium hydroxide solution for saw damage layer removal; b, c, and d prepared under the conditions a, d, and g in Table 1, respectively.

### **2.3.1 Effect of particle density and size of platinum nanoparticles**

Figure 10 show typical photocurrent density versus potential (*j*-*U*) curves of Pt-nanoparticle modified multicrystalline n-Si photoelectrodes having no porous layer pretreated under the same conditions as the specimens of Fig. 4. The decrease in particle density and size of Ptnanoparticles increased the open-circuit photovoltage (*V*OC) and short-circuit photocurrent density (*j*SC) of photovoltaic photoelectrochemical solar cells from curve a to curve c of Fig. 10. Thus, the conversion efficiency (S) of the solar cells increased from 3.8% to 5.0%.

The reason for the increase in photocurrent density of the photoelectrochemical solar cells is the decrease of surface coverage of Pt-nanoparticles on Si. The surface coverage is 20% and 5% for Fig. 4a and b, respectively. This decrease is expected to increase the intensity of solar light reaching the Si surface by 19%. This is almost consistent with the increase in the shortcircuit photocurrent density by 17%. The average open-circuit photovoltage of 12 samples is 0.42 V. This is lower than that for Pt-nanoparticle-electrolessly-deposited single crystalline n-Si electrodes (0.50 V in the average of 76 samples). This is explained by the following two reasons. 1) Lower quality of multicrystalline Si than single crystalline: The characteristics of multicrystalline Si solar cells are commonly lower than those of single crystalline. Thus, not only photovoltage but also the short-circuit photocurrent density and fill factor (*F.F.*) of photoelectrochemical solar cells are 12.1 mA cm-2 and 0.57 lower than those of single

as a redox electrolyte solution of the photovoltaic photoelectrochemical solar cell. Photocurrent density versus potential (*j*-*U*) curves were obtained with a cyclic voltammetry tool. The potential of the n-Si wafer was measured with respect to the Pt counterelectrode. The multicrystalline n-Si was irradiated with a solar simulator (AM1.5G, 100 mW cm-2)

Fig. 9. Reflectance spectra of multicrystalline n-Si wafers: curve a after immersion in sodium hydroxide solution for saw damage layer removal; b, c, and d prepared under the conditions

Figure 10 show typical photocurrent density versus potential (*j*-*U*) curves of Pt-nanoparticle modified multicrystalline n-Si photoelectrodes having no porous layer pretreated under the same conditions as the specimens of Fig. 4. The decrease in particle density and size of Ptnanoparticles increased the open-circuit photovoltage (*V*OC) and short-circuit photocurrent density (*j*SC) of photovoltaic photoelectrochemical solar cells from curve a to curve c of Fig.

The reason for the increase in photocurrent density of the photoelectrochemical solar cells is the decrease of surface coverage of Pt-nanoparticles on Si. The surface coverage is 20% and 5% for Fig. 4a and b, respectively. This decrease is expected to increase the intensity of solar light reaching the Si surface by 19%. This is almost consistent with the increase in the shortcircuit photocurrent density by 17%. The average open-circuit photovoltage of 12 samples is 0.42 V. This is lower than that for Pt-nanoparticle-electrolessly-deposited single crystalline n-Si electrodes (0.50 V in the average of 76 samples). This is explained by the following two reasons. 1) Lower quality of multicrystalline Si than single crystalline: The characteristics of multicrystalline Si solar cells are commonly lower than those of single crystalline. Thus, not only photovoltage but also the short-circuit photocurrent density and fill factor (*F.F.*) of photoelectrochemical solar cells are 12.1 mA cm-2 and 0.57 lower than those of single

S) of the solar cells increased from 3.8% to 5.0%.

**2.3.1 Effect of particle density and size of platinum nanoparticles** 

a, d, and g in Table 1, respectively.

10. Thus, the conversion efficiency (

through the quartz window and a redox electrolyte solution ca. 3 mm thick.

crystalline (18.3 mA cm-2 and 0.60 on average, respectively). 2) Insufficient density of termination of Si surface bonds with iodine atoms: The termination of Si surface bonds with iodine atoms shifts the flat band potential of Si toward negative, and thus increases the photovoltage of photoelectrochemical solar cells using hydroiodic acid and iodine redox electrolyte (Fujitani et al., 1997, Ishida et al., 1999, Yae et al., 2006a, Zhou et al., 2001). An electrolyte solution of 8.6 mol dm-3 hydrobromic acid (HBr) and 0.05 mol dm-3 bromine (Br2) has sufficient negative redox potential to generate high open-circuit photovoltage without the termination. Using the hydrobromic acid and bromine electrolyte solution increases the photovoltage by 0.06 V for multicrystalline and 0.03 V for single-crystalline n-Si electrodes from those using hydroiodic acid and iodine electrolyte solution. This result indicates that the density of the termination of multicrystalline n-Si surface bonds with iodine atoms is insufficient for generating high photovoltage.

Fig. 10. Photocurrent density versus potential (*j*-*U*) curves of photovoltaic photoelectrochemical solar cells equipped with Pt-nanoparticle modified multicrystalline n-Si photoelectrode having no porous layer pretreated under the same conditions as the specimens of Fig. 4. Pretreatment: method A (image a), B (b and c); Pt deposition time: 120 (a and b), 30 s (c).
