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Research on Fabrication and Hydrosilylation of Silicon Nanowires and Porous Silicon

Author: LiangFeng
Tutor: LiDongSheng;YangDeRen
School: Zhejiang University
Course: Materials Engineering
Keywords: Silicon nanowires Antireflection Porous silicon Down shiftingphotoluminescence Surface modification
CLC: TQ127.2
Type: Master's thesis
Year: 2014
Downloads: 2
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Abstract

Photovoltaic (PV) technologies represent promising routes to green and renewable energy generation. But owing to low power conversion efficiencies of solar cells, the production of solar energy remains costly. The amount of reflection from a surface is one of the main obstacles in efficient solar cell performance, and another main difficulty in improving the efficiency of PV energy conversion lies in the spectral mismatch between the energy distribution of photons in the incident solar spectrum and the bandgap of a semiconductor material. To increase the efficiency of single-junction solar cells, on the one hand, we can develop antireflection technologies to reduce surface reflection, on the other hand, the spectral converters which are capable of converting a broad spectrum of light into photons of a particular wavelength can be employed to minimize the losses in the solar-cell-based energy conversion process.Towards the two issues mentioned above, we have a systematic research on the properties of silicon nanowires and porous silicon. These properties include reduced reflection, extreme light trapping, and can act as good down-shifting photo luminescence materials. But as these materials suffer from the ageing problem that the PL intensities decay dramatically subsequent to preparation, so the feasibility of using hydrosilylation as a general tool to stabilize the photoluminescence of silicon nanowires and porous silicon. Besides, detailed researches were conducted on the mechanism of fabrication process. In the following are the innovative results of this thesis.(1) We present a systematic study to elucidate the mechanism responsible for the formation of silicon nanowires in a two step silver-assisted electroless chemical etching method. It is shown that silicon nanowire arrays with various morphologies and PL intensities can be prepared by varying multiple experimental parameters such as the resistivity of the starting silicon wafer, the concentration of oxidant (H2O2), or etching time of the process. Our study also shows a consistent trend that the length and PL intensity of silicon nanowires increases with the increasing of etching time, wafer conductivity (for P type wafer) and oxidant (H2O2) concentration.(2)The antireflection property (AR) of silicon nanowire arrays (Si NWAs) was carefully investigated. We demonstrate that the length of nanowire is crucial for light trapping, which play an important role in decreasing the reflectance of Si NWAs. The longer the nanowire, the lower the reflectance of Si NWAs, but as the length of silicon nanowire exceeds2μm, this trend changes. From the calculated reflectance and the experimental results, we further demonstrate that the top morphologies of silicon nanowires are also very important for light trapping, besides, the changes of silicon nanowires’top morphology would lead to the variety of Si NWAs’effective refractive index, which finally changes the reflectance of Si NWAs. According to statistics, the effective refractive index value of Si NWAs with lower reflectance mostly between1.6-2.0.(3) Fluorescence spectrometer was employed to characterize the photoluminescence of porous silicon fabricated by electrochemical etching process. When we applied small current, as the current density increased, the depth of the porous Si layer and the intensity of PL increased significantly. Increasing the current further, the surface of PS Was polished. PL measurements show that the emissions mainly located at680nm, the change of current doesn’t cause the position shift, and the position shifts with the Si substrates’s variation of doping type and doping level. We also note that by using constant current with periodic hold-up etching method, can yield a porous silicon layer with stronger photoluminescence intensity than the porous silicon layer prepared by usual constant current etching, and changing the holding-up time can obtain porous Si with diferrent PL position.(4) The influences on the PL properties of prous Si modified via a hydrosilylation scheme were studied. After the post-processing treating, not only the luminescence intensity of porous silicon could be greatly enhanced, but also the illumination stability could be extremely improved. It is found that the intensity and peak position of PL from hydrosilylated prous Si are greatly affected by the modifier, the porous silicon modified by dodecene shows the strongest PL intensity compare to porous silicon without being modified or modified by other modifiers. Because the surface condition plays a very important role in the illumination of porous silicon, so the Fourier transform infrared spectroscopy (FTIR) was used to characterize its chemical information. The absorption peaks in FTIR located at1750cm"1is attributed to the vibration of Si-C, which is belived to the main reason to enhance the illumination stability of prous Si after surface modification.

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CLC: > Industrial Technology > Chemical Industry > Non-metallic elements and their inorganic compounds, the chemical industry > Part Ⅳ family of non-metallic element and its inorganic compounds > Silicon and its inorganic compounds
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