Xiangbo Zeng

Microstructure characterization of transition films from amorphous to nanocrocrystalline silicon

Hydrogenated silicon (Si:H) films near the threshold of crystallinity were prepared by very high-frequency plasma-enhanced chemical vapor deposition (VHF-PECVD) using a wide range of hydrogen dilution R-H = [H-2]/[SiH4] values of 2-100. The effects of H dilution R-H on the structural properties of the films were investigated using micro-Raman scattering and Fourier transform infrared (FTIR) absorption spectroscopy. The obtained Raman spectra show that the H dilution leads to improvements in the short-range order and the medium-range order of the amorphous network and then to the morphological transition from amorphous to crystalline states. The onset of this transition locates between R-H = 30 and 40 in our case, and with further increasing R-H from 40 to 100, the nanocrystalline volume fraction increases from similar to23% to 43%, and correspondingly the crystallite size enlarges from similar to2.8 to 4.4 nm. The FTIR spectra exhibit that with R-H increasing, the relative intensities of both the SiH stretching mode component at 2100 cm(-1) and wagging mode component at 620 cm(-1) increase in the same manner. We assert that these variations in IR spectra should be associated with the formation of paracrystalline structures in the low H dilution films and nanocrystalline structures in the high H dilution films

Radial n-i-p structure SiNW-based microcrystalline silicon thin-film solar cells on flexible stainless steel

Radial n-i-p structure silicon nanowire (SiNW)-based microcrystalline silicon thin-film solar cells on stainless steel foil was fabricated by plasma-enhanced chemical vapor deposition. The SiNW solar cell displays very low optical reflectance (approximately 15% on average) over a broad range of wavelengths (400 to 1,100 nm). The initial SiNW-based microcrystalline (μc-Si:H) thin-film solar cell has an open-circuit voltage of 0.37 V, short-circuit current density of 13.36 mA/cm2, fill factor of 0.3, and conversion efficiency of 1.48%. After acid treatment, the performance of the modified SiNW-based μc-Si:H thin-film solar cell has been improved remarkably with an open-circuit voltage of 0.48 V, short-circuit current density of 13.42 mA/cm2, fill factor of 0.35, and conversion efficiency of 2.25%. The external quantum efficiency measurements show that the external quantum efficiency response of SiNW solar cells is improved greatly in the wavelength range of 630 to 900 nm compared to the corresponding planar film solar cells.

Improved open-circuit voltage of silicon nanowires solar cells by surface passivation

The surface recombination at the surface of silicon nanowires (SiNWs) deteriorates the performance of SiNWs solar cells and thus the reduction of the SiNWs surface recombination becomes a crucial issue. In this paper, we observe an improved SiNW surface passivation by hydrogenated amorphous silicon (a-Si:H). The results show that a thicker i-layer results in a higher open-circuit voltage Voc. That can be ascribed to the better passivation by thicker intrinsic a-Si:H. The dark current–voltage data reveal that the reverse leakage current and the diode ideality factor at high forward bias decrease monotonically with increasing the thickness of i-layer. Moreover, for the first time, we observe that the lower Voc is associated with the capacitance–voltage (C–V) curve shifting toward higher positive voltage. We propose that the shift of the curve is related to the capacitance affected by the surface states. Finally we prove that the improvement in the NW solar cell performance, especially the Voc, can be attributed to the reduction of the surface states on SiNWs.

Study of microstructure and defects in hydrogenated microcrystalline silicon films

Microcrystalline silicon films were deposited by very high frequency (VHF) plasma-enhanced chemical vapor deposition (PECVD) with different hydrogen dilution. The microstructure of these films was investigated using Raman spectroscopy and infrared absorption (IR) spectra. The crystalline, amorphous, and grain boundary volume fractions Xc, Xa and Xgb were estimated from Raman measurements. An interface structure factor (Rif) is proposed to characterize the grain boundary volume fractions in IR spectroscopy. The density of states (DOS) of the microcrystalline crystalline silicon films were studied by phase-shift analysis of modulated photocurrent (MPC) and photoconductivity spectroscopy. It was observed that DOS increases with increasing grain boundary volume fractions, while the values of electron mobility-lifetime product μeTe decease.

The Amorphous/Crystalline Silicon Interface Research of HIT Solar Cells by Simulation

In this paper, we have focused on the influence of interface state density and band offsets on the performance of HIT solar cells by simulation. The interface state density Dit have negative influence on the open-circuit voltage VOC, fill factor FF and the short circuit current JSC, beyond different Dit respectively. VOC decreases monotonically with increasing Dit only when Dit is greater than 1010 cm-2, and for FF and JSC Dit is 1012 cm-2, 1013 cm-2, respectively. Observed reduction in VOC (Dit is from 1×1010 cm-2 to 5×1013 cm-2) may be due to the enhanced recombination possibility, which diminishes the difference in quasi Fermi energies from 0.8 eV to 0.46 eV. Reduction of JSC (Dit is from 1×1013 cm-2 to 5×1013 cm-2) is connected with the carrier recombination rate that is heightened from 2.43×1024cm-3/s to 3.1×1026cm-3/s, which is markedly by two orders of magnitude. In addition, our research results demonstrate that by increasing the conduction band offset EC (from 0.09eV to 0.15 eV), the VOC could be increased by 15.9 mV, while the performance of HIT solar cells is not affected by the valence band offset EV. The increase of EC results in the rising of Vbi and thus improves VOC. It is worth mention that, the recombination potential VRe could be decreased from 232 meV to 208 meV by the increase of EC (from 0.09eV to 0.15eV) and thus is favorable to VOC. Consequently, when the interface state density is less than 1010 cm-2 and the conduction band offset reaches 0.15eV, we can optimize the performance of HIT solar cells to achieve 24.95% efficiency.

Transport mechanism of novel silicon-riched nitride (SRN)/ silicon-riched oxide (SRO) superlattice quantum dot structure

Novel structure of silicon-riched nitride (SRN)/silicon-riched oxide (SRO) is demonstrated using RF reactive magnetron sputtering and post-annealed, to increase the density of Si nanocrystals especially in the separated layer made of isolation medium, thus improving the transport ability of Si nanocrystal material. I-V characteristics shows the current of hybrid SRN/SRO system increases up to 2 orders of magnitude at 1V compared to that of SiNx/Si3N4 structure, proving the importance of the existence of sparser Si NCs in the SRO layer to greatly improve the transport ability. Temperature dependent I-V test reveals that direct tunneling dominates at low electrical field (V<;1V) and the temperature below 180K; while at high electrical field (V>;1V) Poole-Frenkel emission is the dominated one.

Microstructure of the Silicon Film Prepared Near the Phase Transition Regime from Amorphous To Nanocrystalline

A kind of hydrogenated diphasic silicon films has been prepared by a new regime of plasma enhanced chemical vapor deposition (PECVD) near the phase transition regime from amorphous to nanocrystalline. The microstructural properties of the films have been investigated by the micro-Raman and Fourier transformed Infrared (FT-IR) spectra and atom force microscopy (AFM). The obtained Raman spectra show not only the existence of nanoscaled crystallites, but also a notable improvement in the medium-range order of the diphasic films. For the FT-IR spectra of this kind of films, it notes that there is a blueshift in the Si-H stretching mode and a redshift in the Si-H wagging mode in respect to that of typical amorphous silicon film. We discussed the reasons responsible for these phenomena by means of the phase transition, which lead to the formation of a diatomic hydrogen complex, H-2* and their congeries.

Transformation Process for Passivating Current Shunting Paths in Silicon Microwires Arrays Solar Cells

We applied a method of passivating short circuit current paths to silicon microwires solar cells. It is exhibited that the Voc and FF improved 130% and 43% respectively.

Improvement of Silicon Nanowires Solar Cells by Removing Residual Catalytic Indium

We demonstrate the improvement of silicon nanowires (SiNWs) solar cells by acid treatments of SiNWs. This leads to open-circuit voltage (Voc) increase, which is related to reduced dark current.

Effect of Assets of Silicon Film Passivation Layer on the Performance of Silicon Nanowire Solar Cells

Silicon film as a surface passivation layer is reported to reduce surface recombination on silicon nanowires (SiNWs) and thus enable to improve SiNW solar cell (SC) performance. A question yet to be answered regards the link between the silicon film assets and the solar cell performances. We investigated the effect of the properties of silicon films on the SiNWs SC performances by adjusting hydrogen dilution. Our results showed that the open-circuit voltage (Voc) and short-circuit current density (Jsc) of SiNWs SC increase until hydrogen dilution 10 and then decrease. An open-circuit voltage of 0.397 V and short-circuit current density of 18.42 mA/cm2 are achieved at optimized hydrogen dilution. Based on the analysis of silicon film properties we proposed that the increase of defect density with hydrogen dilution was the main cause for the deterioration of SiNWs SC performance.