**2.2.2 Porous structure control**

236 Solar Cells – New Aspects and Solutions

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.

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

2003, 2005, 2006a, 2009).

acid solution.

**2.2.1 Etching mechanism** 

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.


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

Solar to Chemical Conversion

e, f, and g in Table 1, respectively.

**2.2.3 Antireflection effect** 

Using Metal Nanoparticle Modified Low-Cost Silicon Photoelectrode 239

Fig. 8. Typical scanning electron microscopic images of Pt nanoparticle modified porous multicrystalline n-Si. Preparation conditions: images a, b, c, d, and e are for conditions c, d,

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

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

under the non-controlled conditions, but much lower than the non-etched wafer.

**2.3 Photovoltaic photoelectrochemical solar cells** 

The deposition conditions of Pt-nanoparticles and metal-particle-assisted hydrofluoric acid etching conditions are listed in Table 1. Figure 7 shows typical scanning electron microscopic images of multicrystalline n-Si wafers that were pretreated by method A (image a) or B (image b) and metal-particle-assisted hydrofluoric acid etching without light control for 24 h (conditions a and b in Table 1). Macropores, whose diameter is 0.3–1 m, were formed on whole surfaces of multicrystalline n-Si wafers. The density of pores, i.e. porosity, of n-Si wafer pretreated by method B is lower than that for method A. This is consistent with the Pt particle density on multicrystalline Si surface before etching (Fig. 4a and b). Both samples showed an orange photoluminescence under UV irradiation, thus microporous layers were formed on both samples.

Fig. 7. Typical scanning electron microscopic images of Pt nanoparticle modified porous multicrystalline n-Si. Preparation conditions: images a and b are for conditions a and b in Table 1, respectively.

Figure 8 shows typical scanning electron microscopic images of multicrystalline n-Si that were pretreated by method B and metal-particle-assisted hydrofluoric acid etching under control of the photoillumination and the dissolved oxygen concentration (conditions c to g in Table 1). A microporous layer giving photoluminescence and no macropores was formed by etching under photoillumination without any gas bubbling estimated dissolved oxygen concentration of solution is ca. 5 ppm (Fig. 8a, condition c). The etching under the dark condition with oxygen gas bubbling (the solution was saturated with oxygen) after the etching under photoillumination produced macro- and microporous combined structure on the multicrystalline n-Si wafer (Fig. 8b, condition d). The morphology of the Si surface is similar to that formed by the etching without light control and gas bubbling for 24 h (Fig. 7b, condition b). Addition of the photoillumination with oxygen bubbling to the preceding conditions enlarged the macropore size and microporous layer thickness (Fig. 8c, condition e). Shortening the immersion time of multicrystalline n-Si wafers in the Pt displacement deposition solution, i.e. reduction of particle size and particle density of Pt on the wafers, reduced the number of macropores on the etched n-Si wafers (Figs. 8d and e, conditions f and g, respectively). The structure change in the porous layer of multicrystalline n-Si by changing the photoillumination intensity and dissolved oxygen concentration is consistent with our previously reported results on single crystalline n-Si (Yae et al., 2005, 2006b, 2009).

The deposition conditions of Pt-nanoparticles and metal-particle-assisted hydrofluoric acid etching conditions are listed in Table 1. Figure 7 shows typical scanning electron microscopic images of multicrystalline n-Si wafers that were pretreated by method A (image a) or B (image b) and metal-particle-assisted hydrofluoric acid etching without light control for 24 h (conditions a and b in Table 1). Macropores, whose diameter is 0.3–1 m, were formed on whole surfaces of multicrystalline n-Si wafers. The density of pores, i.e. porosity, of n-Si wafer pretreated by method B is lower than that for method A. This is consistent with the Pt particle density on multicrystalline Si surface before etching (Fig. 4a and b). Both samples showed an orange photoluminescence under UV irradiation, thus microporous

Fig. 7. Typical scanning electron microscopic images of Pt nanoparticle modified porous multicrystalline n-Si. Preparation conditions: images a and b are for conditions a and b in

results on single crystalline n-Si (Yae et al., 2005, 2006b, 2009).

Figure 8 shows typical scanning electron microscopic images of multicrystalline n-Si that were pretreated by method B and metal-particle-assisted hydrofluoric acid etching under control of the photoillumination and the dissolved oxygen concentration (conditions c to g in Table 1). A microporous layer giving photoluminescence and no macropores was formed by etching under photoillumination without any gas bubbling estimated dissolved oxygen concentration of solution is ca. 5 ppm (Fig. 8a, condition c). The etching under the dark condition with oxygen gas bubbling (the solution was saturated with oxygen) after the etching under photoillumination produced macro- and microporous combined structure on the multicrystalline n-Si wafer (Fig. 8b, condition d). The morphology of the Si surface is similar to that formed by the etching without light control and gas bubbling for 24 h (Fig. 7b, condition b). Addition of the photoillumination with oxygen bubbling to the preceding conditions enlarged the macropore size and microporous layer thickness (Fig. 8c, condition e). Shortening the immersion time of multicrystalline n-Si wafers in the Pt displacement deposition solution, i.e. reduction of particle size and particle density of Pt on the wafers, reduced the number of macropores on the etched n-Si wafers (Figs. 8d and e, conditions f and g, respectively). The structure change in the porous layer of multicrystalline n-Si by changing the photoillumination intensity and dissolved oxygen concentration is consistent with our previously reported

layers were formed on both samples.

Table 1, respectively.

Fig. 8. Typical scanning electron microscopic images of Pt nanoparticle modified porous multicrystalline n-Si. Preparation conditions: images a, b, c, d, and e are for conditions c, d, e, f, and g in Table 1, respectively.
