Papers by Author: Lei Zhao

Abstract: Photovoltaic (PV) industry in China shows more growth potential because of the favorable political support, deep market penetration will be expected to follow. A detailed and comprehensive assessment for photovoltaic (PV) power plant is extremely necessary. This paper study the feasibility and profitability of the grid connect PV power plant by analyzing a case using RETScreen photovoltaic project model. It was concluded from this study that finding the optimal tilt angle is extremely necessary for the fixed installation. All kinds of financial indicators show that the project has good profitability, whether considering greenhouse gasses reduction credits or not. In addition, the results from sensitivity and risk analysis showed that avoided cost of energy、renewable energy production and initial cost have great effects on net present value (NPV). The effects of annual cost, debt rate and debt term on NPV are so small that we can ignore their impact. This study is of high value when attempting to analyze PV power plants comprehensively.

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: The effective minority carrier lifetime (τ_eff) depends upon the quality of surface passivation, which by means of the microwave photoconductance decays (μPCD) method. The effective minority carrier lifetime (τ_eff) cannot reveal the real bulk lifetime of minority carriers (τ_b) . We have applied iodine-ethanol (I-E) treatment to silicon surface at different molar concentrations and shown that the effective concentrations ranges was 0.08mol/L~0.16 mol/L, the maximum The effective minority carrier lifetime (τ_eff) of n-type monocrystalline and p-type monocrystalline was 973.71μs and 362.6μs, respectively. We also accurately evaluate the bulk lifetime of minority carriers by measured with different thickness of silicon substrate.

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: Low cost electrochemical etching method was utilized to prepare macroporous silicon on p-type silicon substrate in dilute HF solution. By optimizing the substrate resistivity, the etching current density, and the etching time, excellent macroporous silicon was obtained on 15 Ω•cm p-type silicon substrate with the pore diameter of about 2 μm, the pore depth of about 30 μm, and the surface pore density up to ~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. The low reflectance combined with the deep pore morphology provides an attractive potential to fabricate radial p-n junction solar cells on such macroporous silicon.

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.