Hongwei Diao

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

In situ optical emission spectroscopy diagnostics of glow discharges in SiH4/GeH4/H2

An optical emission spectroscopic study identifies transient and steady-state behavior of the excited H*α/H*β/SiH*/GeH* emission in parallel plate SiH4/GeH4/H2 plasma. The effect of deposition parameters on the radical density in plasma is determined. To clarify the radical production mechanism further, the optoelectronic properties of the hydrogenated amorphous silicon–germanium thin films deposited at different hydrogen flow rates in 3 Torr pressure are investigated. The results show a low hydrogen flow rate improves the optoelectronic properties of the thin films whereas a high flow rate leads to a much higher defect density in the thin films. The ratio of GeH* to SiH* emission intensity is related to the germanium content in the plasma. The germanium content is changed for the different hydrogen flow rates, thereby adjusting the optical bandgap, with 1.32 ± 0.20 eV at a high 200 sccm hydrogen flow rate.

Rheological properties and related screen-printing performance of low-temperature silver pastes for a-Si:H/c-Si heterojunction solar cells

Abstract Four different low-temperature silver pastes were utilized to make metal grids by screen printing for silicon heterojunction solar cells. The rheological behaviors of the low-temperature silver pastes were characterized by viscosity test, thixotropy test, oscillatory stress sweep test and creep-recovery test. The correlationship between the screen-printing performance and the rheological properties was investigated. It was found that the shear thinning behavior and the thixotropy behavior of the silver pastes were desirable for the screen-printing process. An obvious viscoelastic behavior of the silver paste was also helpful for improving its printability. Further, good recovery and low creep and recovery compliances could minimize the printing defects and the tendencies to bleed out during the screen-printing, and thus increase the aspect ratio of the printed grids.

The effect of saw mark on the over-ghosting for acidic textured multicrystalline wafers with silicon nitride anti-reflectance films

The use of light-induced plating (LIP) for metallization of solar cells is attractive because of its potential simplicity in that the current driving the metal reduction process is derived from the solar cell under illumination. However, there is a challenge when applying the LIP techniques on standard acidic textured multicrystalline silicon wafers with a silicon nitride coated surface. The over-plating can cause the decrease of the solar cells efficiency mainly through the shunt or forming schottky contact, and shading losses. The main reason of over-plating on acidics, also on the alkali textured multicrystalline silicon wafers is saw mark. The over-plating on saw-damage multicrystalline silicon is still evident even the coated SiNx:H films is proper to as the plating mask. In this work, the effect of saw marks on over-plating are examined and evaluated. Finally, the elimination of over-plating on acidic textured multicrystalline silicon cells are demonstrated

Preparation of p+-layers using water vapour as oxidant in BBr3 diffusion for silicon solar cells

A boron diffusion process using water vapour as oxidant mixed with oxygen is introduced. Boron-doped emitter sheet resistances ranging from 30 to 400 Ω/sq and saturation current densities as low as 100 to 300 fA cm−2 for Al2O3/SiNx passivated emitters were achieved. For the predeposition process using water vapour, a low sheet resistance of 20 Ω/sq with uniformity of 5%, and a high sheet resistance of 100 Ω/sq with good in-wafer and wafer-to-wafer uniformities within 10% were achieved. With wet-oxygen predeposition method, an open-circuit voltage of 680 mV and pseudo fill factor (pFF) of 81% were obtained for the Al2O3 double-sides passivation p+/n-Si structure by the measurement of Suns-Voc. These results indicate that water vapour was suitable for forming both p+-emitters and back-surface fields for highly efficient n- and p-type solar cells.

Improved Reflectance for Textured Mc-Silicon Solar Cells by SF6/O2 Plasma Etching

On account of enhancing surface light trapping by reducing reflectance, front surface texturiing is always a key to improve the solar cells performance. As a maskless plasma texturing technique, reactive ion etching using SF6/O 2 plasma brings down reflectance distinctly by fluorine ions etching the surface of mc-silicon solar cells with the assisting of O2. The textured surface with bowl-like nanostructure exhibits fine anti-reflectance effect. Optimal reflectance of mc-silicon solar cells could be obtained by adjusting gases flow ratios, plasma power and etching time.

A New Attempt at Alkaline Texturization of Monocrystaline Silicon with Anionic Surfactant as the Additive

Owing to the volatilization of isopropanol (IPA), instability in the alkaline texturization of monocrystalline silicon has been a big problem for a long time. Many additives were adapted to replace IPA, such as high boiling point alcohols. In this experiment, as a new attempt, sodium lauryl sulfate (SDS), a type of anionic surfactant, was used as the additive in NaOH solution. The etching properties of silicon in 2 wt % NaOH/15–30 mg/L SDS solution were analyzed. To improve the wettability of silicon, two types of metal salt, NaCl and Na2CO3 with concentration from 2 to 15 wt %, were applied to the 2 wt % NaOH/15 mg/L SDS solution. The results showed that the effect of NaCl was better than that of Na2CO3. Finally, the role of the additive was discussed.

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.

Improving surface passivation capability of silicon heterojunction solar cells with amorphous silicon by optical emission spectrometry

The intrinsic a-Si:H thin film is the key to the high efficiency heterojunction silicon solar cells. The transient instability after plasma ignition decreases the minority carrier lifetime of passivated silicon wafer at the same deposition process. The capability to passivate the silicon surface of a-Si:H thin films deposited at different hydrogen flow rate is investigated. The results show the passivation capability at higher hydrogen flow rate is poorer. The decrease of Hα*, Hβ* and SiH* emission intensity in plasma is not easier to passivate the silicon surface.

Calculated and experimental research of sheet resistances of crystalline silicon solar cells by dry laser doping

In this paper, the calculated and experimental research of sheet resistances of crystalline silicon solar cells by dry laser doping is investigated. The nonlinear numerical model on laser melting of crystalline silicon and liquid-phase diffusion of phosphorus atoms by dry laser doping is analyzed by finite difference method implemented in MATLAB. The calculated sheet resistances as a function of laser energy density is obtained and the calculated results are in good agreement with the corresponding experimental measurement.