**4. Conclusion**

250 Solar Cells – New Aspects and Solutions

Figure 18 shows the photocurrent density versus potential (*j-U*) curve for the titanium dioxide photoelectrode in a perchloric acid aqueous solution under simulated solar illumination. The dissolved oxygen in the solution was eliminated by using argon gas flow into the solution before the measurement. The anodic photocurrent starts to generate at -0.14 V vs. Ag/AgCl. This onset potential is more positive than -0.24 V vs. Ag/AgCl for hydrogen evolution, and thus this electrode cannot split water into hydrogen and oxygen without external bias. Figure 19 shows the photocurrent density versus potential (*j-U*) curve for the photovoltaic photoelectrochemical solar cell equipped with a Pt-nanoparticle-modified microcrystalline Si electrode in the redox solution under simulated solar light illumination through the titanium dioxide photoelectrochemical cell. The shirt-circuit photocurrent density was decreased from 5.3 mA cm-2 for the cell under direct solar light illumination to 2.6 mA cm-2 by light attenuation with the titanium dioxide cell. The multi-photon system (Fig. 17) using the same titanium dioxide and Pt-nanoparticle-modified microcrystalline Si electrodes as those in Figs. 18 and 19 indicated the photocurrent density versus potential (*j-U*) curve of Fig. 20. This system generated anodic photocurrent at a potential that was more negative than -0.24 V vs. Ag/AgCl for hydrogen evolution. Figure 21 shows that steady photocurrent was obtained for the multiphoton system in the short-circuit condition (Fig. 17). Tiny gas bubble formed on the Pt cathode during measurement under the short-circuit condition. These results show that this multi-photon system can split water into hydrogen and oxygen with no external bias with solar light. Since two photoelectrodes of titanium dioxide and Pt-nanoparticle-modified microcrystalline Si were connected in series, photovoltage was the sum of the two electrodes' values and photocurrent was the lower of the two electrodes' values. Therefore, the photocurrent density for water splitting was determined by that of the titanium dioxide electrode and very low. The photocurrent density, and thus hydrogen production by solar water splitting, is expected to increase by using a semiconductor with a narrower band gap, such as tungsten trioxide, instead of titanium dioxide. The theoretical simulation obtained 8 mA cm-2 of shirt-circuit photocurrent density, that is, 10% of solar-to-chemical conversion efficiency for solar water splitting for the tungsten trioxide and Si multi-photon system.

0


Fig. 20. Photocurrent density versus potential (*j-U*) curve for the multi-photon system (Fig. 17) using the same titanium dioxide thin film and Pt-nanoparticle modified microcrystalline Si photoelectrodes and electrolyte solutions as those in Figs. 18 and 19 under simulated solar

*U* / V vs. Ag/AgCl

0.02

0.04

*j* / mAcm-2

light illumination.

0.06

0.08

0.1

Multicrystalline silicon wafers and microcrystalline silicon thin films, which are common and prospective low-cost semiconductor materials for solar cells, respectively, were successfully applied to produce solar hydrogen via photodecomposition of hydrogen iodide and solar water splitting. These photoelectrochemical solar cells have the following advantages: 1) simple fabrication of a cell by immersing the electrode in an electrolyte solution; 2) there is no need for a p-type semiconductor or a transparent conducting layer; and 3) direct solar-to-chemical conversion (fuel production). Modification of silicon surface with platinum nanopartilces by electroless displacement deposition and porous layer formation by metal-particle-assisted hydrofluoric acid etching improve solar cell characteristics. The solar-to-chemical conversion efficiency reached 5% for the photodecomposition of hydrogen iodide, and hydrogen gas evolution was obtained by the solar water splitting with no input of external electricity.
