CMPL SEMICON
National Institute of Physics
University of the Philippines - Diliman
One way to reduce light reflection by solar cells is to alter their surface profile. Textured surface causes some of the reflected light to be trapped when it undergoes internal reflections. Better light-trapping ability contributes to better efficiency of the device. The figure on the left shows pyramids that were formed by soaking the wafers in an alkaline solution under certain conditions.
Reflectance drops with increasing etching time from 20 minutes to 60 minutes as seen in the plot below. After texturing, one can see an obvious change of shade in the wafers. Textured ones are much darker because they trap and absorb more light. The untextured sample reflected much more light across the range compared to the textured ones. The texturing method effectively decreases the amount of light reflected and increases the light absorbed by the wafers.
Nanotexturing
Porous silicon (pSi) is a sponge-like composite material composed of crystalline silicon and air. The pores of pSi is usually around the nano region. These dimensions are attained by varying the anodization parameters such as electrolyte concentration, current density, etching time and doping concentration of the silicon wafer used. Because of this , the effective refractive index of porous silicon can be varied by changing these anodization parameters leading to abundant optical applications. Shown on the figure on the right is a cross section SEM image of single and multilayer porous silicon.
Silicon nanowires (NW) has been widely studied for variousapplications like anti-reflective coatings and nanostructured surface growth. Creating such, however, has been a challenge due to the parameters that require expensive equipment and resources. The optimization and production of silicon (Si) nanowires(NW) with the use of a cost-effective, easy, and effective technique has attracted much research nowadays.
The Si NWs shown on the figure were prepared using metal-assisted electrolesschemical etching method. The samples were prepared from ptype Si (100) wafer which was used as the substrate. The substrate was submerged into silver nitrate which was used as the seeding solution then subsequently soaking in hydrofluoric acid (HF) for the anistropic downward etching of the sample. .
Si NW were grown using metal-assisted electroless chemical etching method. The figures on the left shows the SEM images of Si nanowires fabricated using one-step electroless chemical etching. It is apparent that the Si NW are non-uniformly distributed and are bundled hence no distinct structure could be observed from this point of view.
From the SEM images, it can be observed that the NWs are of controllable length and vertical orientation. This is due to the anisotropic etching of HF in the Si substrate. The Si NWs were subjected to reflectance spectroscopy. The reflectance of the Si NW in visible light was observed and it was found out that the reflectance decreases for increasing NW length.