## 3 Results and Discussion
### Properties of Solar Cells with the SiNW Array Structure on an SOI Substrate  
The SiNW over 7.5 \(\upmu\)m length can achieve strong light confinement. To reduce the influence of the silicon layer as much as possible, 9 \(\upmu\)m-long SiNW array structure was adopted in the solar cell. Figure 6A shows the dark and illuminated I-V characteristics of the solar cell with an optimum SiNW absorber layer fabricated on an SOI substrate. A \(V_{\infty}\) of 110 mV was obtained, indicating that the fabricated solar cell provided a photovoltaic effect. However, the conversion efficiency of the solar cell  
Figure 5: (**A**) Dependence of the SiNW array length on etching time. (**B**) Reflectance of the SiNW arrays with different array lengths. was extremely low (0.00017%). Therefore, the solar cell parameters were analysed to determine the cause of low efficiency. A low \(J_{\text{sc}}\) of 6.2 \(\upmu\)A/cm\({}^{2}\) was obtained because of the high series resistance of the device layer, whose sheet resistance was 5 k\(\upmu\)/sq. Figure 6B presents a cross-sectional SEM image of the SiNW array structure after deposition of the n-type a-Si:H layer. Herein, a very thick n-type a-Si:H layer was deposited onto the SiNW array structure surface to ensure that a connection formed between each p-type SiNW and n-type layer. In a high-efficiency heterojunction Si solar cell, the thickness of a-Si is on the order of tens of nanometres. Therefore, the very thick a-Si leads to a solar cell, with high resistivity. Moreover, reaming the Si layer under the SiNW arrays contributes substantially to the high resistivity. To improve the \(I_{\text{sc}}\), we must consider a new structure. Figure 6C shows the quantum efficiency (QE) of the solar cell under a negative applied bias voltage with respect to the optical wavelength. The QE of the device is extremely low in the short-wavelength region (300-700 nm). Figure 6D shows the absorption coefficient of n-type a-Si:H. The band gap of n-Si:H is 1.7 eV. The optical absorption coefficient of n-type a-Si is high, and the thickness of the n-a-Si layer is approximately 300 nm. Therefore, light with wavelengths shorter than 720 nm was absorbed by the n-a-Si. Because n-type a-Si has a high carrier concentration and high defect density, it did not contribute to power generation. To improve the QE in the short-wavelength  
Figure 6: **(A)** I–V characteristics of the solar cell with an optimum SiNW absorber layer fabricated on an SOI substrate. **(B)** Cross-sectional SEM image of the SiNW array surface after the application of the n-type a-Si:H layer. **(C)** Quantum efficiency (QE) of the solar cell under a changing negative voltage from 0–1.5 V. **(D)** Absorption coefficient of n-type a-Si measured via spectroscopic ellipsometry. region, the thickness of the n-a-Si:H layer should be reduced. Although p-SiNW can absorb light longer than 720 nm, because of sheet resistance, the QE is very low in the absence of a negative bias voltage. Additionally, the QE at each wavelength increased with increasing negative bias voltage. The n-type a-Si:H layer was deposited only on the top of the SiNW array structure because of the high density of SiNWs (Figure 3C); this configuration strongly limited the concentrations of the n-type layer precursors at the bottom of the SiNWs. Moreover, a 467 nm-wide depletion region, as indicated by the difference between the donor and acceptor concentrations (Na (p-type SiNW) = 5\(\times\)10\({}^{15}\) cm\({}^{-3}\), Nd (n-type-a-Si) = 1\(\times\)10\({}^{19}\) cm\({}^{-3}\)), formed around the surfaces of SiNWs between the n-type a-Si:H and the SiNW array structure. The negative bias enhanced the width of the depletion region and the electric fields, thereby increased the carrier collection, even though the uncoated SiNWs have a high surface recombination velocity. These results reveal that reducing the surface recombination is vital for improving the efficiency of solar cells, fabricated using SiNW array structures as the absorber layer.