Papers by Author: Hong Wei Diao

Abstract: Hydrogenated silicon thin film was prepared by plasma enhanced chemical vapor deposition (PECVD). The effects of the deposition pressure on the growth rate, the photoelectronic and microstructure properties of the thin films were investigated via transmission, photo/dark conductivity, Raman spectroscopy, and Fourier transform infrared spectroscopy (FTIR) measurements. The results indicate that the increase of the deposition pressure increases the bandgap and the growth rate, while makes the photosensitivity get worse, decreasing from more than ~10³ to ~10². And at the same time, the crystalline volume fraction (Xc) in the film decreases from 70% to 61%, when the deposition pressure increases from 100 Pa to 500 Pa. The order degree of the microstructure was deteriorated with pressure increasing.

Abstract: Hydrogenated amorphous silicon germanium thin films (a-SiGe:H) were prepared via plasma enhanced chemical vapor deposition (PECVD). By adjusting the flow rate of GeH4, a-SiGe:H thin films with narrow bandgap (Eg) were fabricated with high Ge incorporation. It was found that although narrow Eg was obtained, high Ge incorporation resulted in a great reduction of the thin film photosensitivity. This degradation was attributed to the increase of polysilane-(SiH2)n, which indicated a loose and disordered microstructure, in the films by systematically investigating the optical, optoelectronic and microstructure properties of the prepared a-SiGe:H thin films via transmission, photo/dark conductivity, Raman spectroscopy, and Fourier transform infrared spectroscopy (FTIR) measurements. Such investigation provided a helpful guide for further preparing narrow Eg a-SiGe:H materials with good optoelectronic properties.

Abstract: By conventional production-line process, n-type Al-doped rear junction solar cell could be easily fabricated without any other equipment and process. Since the properties of n-type silicon material are different to that of p-type silicon material and the junction is placed at the back, the process parameters should be optimized theoretically to assess the efficient potential. By modeling cells using PC1D software, the effect of some process parameters on the properties of n-type base solar cells were studied, including base resistivity, bulk lifetime, front surface field and recombination rate of front surface. The key parameters were identified and the potential industrial efficiency was calculated.

Abstract: Pyramidal texture is one traditional method to realize antireflection for c-Si solar cells, due to its low cost and simplicity. As one high efficiency silicon solar cell, amorphous/crystalline silicon heterojunction (SHJ) solar cell has attracted much attention all over the world. The heterojunction interface with very low defects and interface states is critical to the SHJ solar cell performance. In order to obtain high quality interface passivation by depositing a very thin intrinsic amorphous silicon layer on the textured Si conformally, large size pyramidal texture with no metal ion contamination is required. In this work, we utilized tetra-methyl ammonium hydroxide (TMAH) instead of NaOH in the alkaline etching to prepare pyramidal texture on N-type monocrystalline silicon to avoid the possible Na+ contamination. By optimizing the etching conditions, uniform large size pyramidal texture with pyramid size of about 10 μm was fabricated successfully. Furthermore, excellent antireflection performance was demonstrated on such textured Si surface. The average reflectance was lower than 10% in the visible and near infrared spectrum range. Such pyramidally textured Si wafers will be very suitable for SHJ solar cells.

Abstract: The energy bandgap (Eg) of silicon thin film prepared by plasma enhanced chemical vapor deposition (PECVD) is greatly dependent on the deposition conditions. Although the influence of some deposition parameters on Eg has been studied individually, it is still not clear which parameter plays the most important role. Here, a 5-factor 5-level factorial experiment was designed and carried out for the deposition parameters: the flow rate of SiH4, the flow rate of H2, the plasma power, the total gas pressure, and the substrate temperature. By making main effect analysis to the influences of such 5 factors on Eg, not only the influence of each deposition parameter was obtained, but also the most critical parameters were selected out. It was found that the gas flow rate of SiH4 and the total gas pressure played the most important roles on determining Eg of silicon thin film. That is to say, in order to obtain an expected Eg for Si thin film prepared by PECVD, much attention should be paid to optimize the two parameters. However, other parameters, including the H2 flow rate, the plasma power and the substrate temperature, can be set as default values according to the experience. Thus, the optimization workload can be reduced greatly.

Abstract: Macroporous silicon was fabricated by electrochemical etching in hydrogen fluoride (HF) electrolyte on P-type silicon wafers. By optimizing the etching condition, the obtained macroporous silicon presented pore diameter of about 2 μm and pore density of ~10⁷/cm². Such macroporous silicon gave out an excellent antireflective performance with the reflectance lower than 4% in a wide spectral range of 400-1000 nm. An a-Si:H/c-Si heterojunction solar cell was fabricated on such macroporous silicon to show its application potential.

Abstract: The energy bandgap (E_g) of a-SiGe:H thin film prepared by plasma enhanced chemical vapor deposition (PECVD) is greatly dependent on the deposition conditions. By controlling the flow rates of the supplied gas sources, the total gas pressure, the plasma power, the substrate temperature and so on, E_g can be adjusted. Although the influence of the above deposition factors has been investigated individually, which factor is the most important is still not clear. Here, a 6-factor 5-level factorial experiment was designed to investigate the influence of the deposition factors comprehensively. By making the main effect analysis to E_g, not only the influence of each deposition factor was deduced, but also the most critical factors were selected out. It was found that the flow rates of SiH₄ and GeH₄, and the total gas pressure played the important roles for the E_g adjustment of a-SiGe:H thin film. So much attention can be only paid to optimize such critical factors with other factors as some default values according to the experience. Thus, the work load can be reduced greatly.

Abstract: Hydrogen doped AZO films (HAZO) were prepared by RF magnetron sputtering. A systematic study of the effect of substrate to target distance (Dst) on the structural, electrical and optical properties of the as-grown HAZO films was carried out. Compared with the Al-doped ZnO films, the hydrogen in the atmosphere influenced the growth of the films by incorporating in the films and bombarding the surface of the film, namely, the Dst, which induced the residual stress and the roughness of the films decreased with an increase of the Dst. The films showed a smaller grain size. The surface work function of the films changed with the composition of the films, reaching a maximum at 7.5cm.

Abstract: A novel method to grow silicon dioxide layers for passivating the silicon surface is given more attention. SiO₂ was grown by applying a positive voltage across silicon wafers in a nitric acid solution at low temperature. After annealing in N₂ media at 900°C for 20min, excellent surface passivation was achieved. The maximum effective lifetime of the silicon arrived at 29.8μs and 29.75μs, which was three times the value of silicon without passivation. The effective lifetime of other types of silicon could be ten times the initial value without the silicon dioxide. A comparison study of the effect of the FGA, annealing at low temperature and annealing in N₂ or O₂ containing medium at high temperature were investigated.

Abstract: Texturization of mono-crystalline by chemical anisotropic etching is one of the most important technologies for modern silicon photovoltaic. IPA is usually added to the alkaline etchants to improve the uniformity of the random pyramid texture due to remove hydrogen bubbles sticking on the silicon wafer by improving the wettability of wafer surface. In this investigation, we carried out a systematic study on the influence of IPA concentrations on the textured surface. The etching experiments were performed on (100) silicon wafer in a mixture of 20 vol. % commercial TMAH solutions (10 wt.%) and IPA (rang from 0~12 vol. %) for etching time ranging from 10 to 70 min at 80°C. The etching mechanism in the TMAH solutions with IPA addition was explained basing on the experimental results and the theoretical considerations.