TAILIEUCHUNG - Solar Cells Silicon Wafer Based Technologies Part 8

Tham khảo tài liệu 'solar cells silicon wafer based technologies part 8', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 166 Solar Cells - Silicon Wafer-Based Technologies leading to an increase of SVR. In other words SVR has the impact of enlarging band gap. At the same transverse dimension triangular SiNW has larger SVR than those of the rectangular SiNW and hexagonal SiNW. As a result its larger SVR induces the largest band gap among those of the rectangular and hexagonal SiNWs and the strongest size dependence. The bandgap values versus SVR of the SiNWs are shown in Fig. 28 b . The SVR effect on the bandgap of 110 SiNWs with any cross-sectional shape and area can be described by a universal expression Yao et al. 2008 Eg eV x S nm-1 where S is the value of the SVR in unit of nm-1. The bandgap of SiNWs are usually difficult to measure but their transverse cross-sectional shape and dimension are easy to know so it is of significance to predict the bandgap values of SiNWs by using the above expression. Optical reflection and absorption in SiNWs Si NW PV devices show improved optical characteristics compared to planar devices. Fig. 29 a shows typical optical reflectance spectra of SiNW film as compared to solid Si film of the same thickness 10pm Tsakalakos et al. 2007a . As one can see the reflectance of the nanowire film is less than 5 over the majority of the spectrum from the UV to the near IR and begins to increase at 700 nm to a values of 41 at the Si band edge 1100 nm similar to the bulk Si. It is clear that the nanowires impart a significant reduction of the reflectance compared to the solid film. More striking is the fact that the transmission of the nanowire samples is also significantly reduced for wavelength greater than 700 nm Fig. 29 b . This residual absorption is attributed to strong IR light trapping4 coupled with the presence of the surface states on the nanowires that absorb below bandgap light. However the level of optical absorption does not change with passivation which further indicates that light trapping plays a dominant role in the enhanced .

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