Tham khảo tài liệu 'solar cells new aspects and solutions part 8', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 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. 2003 2005 2006a 2009 . Etching mechanism 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. mW cm-2 illumination for 6 h dark condition for 12 h and then ca. mW cm-2 illumination for 6 h is suitable to produce the macro- and
