Chunlan Zhou

Fabrication and photocatalytical properties of flower-like TiO2 nanostructures

Abstract Three-dimensional (3D) flower-like anatase TiO 2 nanostructures and flower-like titanate nanostructures were successfully synthesized via hydrothermal synthesis followed by post-treatment from titanium powder. The flower-like anatase TiO 2 nanostructures were characterized in detail with scanning electron microscopy (SEM), X-ray diffraction (XRD), UV-vis spectrum and nitrogen adsorption-desorption measurement, respectively. It is found that the flower-like TiO 2 nanostructures have a high specific surface area and a large light-harvesting efficiency. The photocatalytical activity of the flower-like anatase TiO 2 nanostructures was determined by degradation of methylene blue in aqueous solution, and was compared with commercial P25 titania. It is revealed that the photocatalytical activity of the flower-like anatase TiO 2 nanostructures is enhanced a lot. The apparent rate constant of the flower-like anatase TiO 2 nanostructures is almost 2 times that of P25 titania.

Passivation mechanism of thermal atomic layer-deposited Al2O3 films on silicon at different annealing temperatures.

Thermal atomic layer-deposited (ALD) aluminum oxide (Al2O3) acquires high negative fixed charge density (Qf) and sufficiently low interface trap density after annealing, which enables excellent surface passivation for crystalline silicon. Qf can be controlled by varying the annealing temperatures. In this study, the effect of the annealing temperature of thermal ALD Al2O3 films on p-type Czochralski silicon wafers was investigated. Corona charging measurements revealed that the Qf obtained at 300°C did not significantly affect passivation. The interface-trapping density markedly increased at high annealing temperature (>600°C) and degraded the surface passivation even at a high Qf. Negatively charged or neutral vacancies were found in the samples annealed at 300°C, 500°C, and 750°C using positron annihilation techniques. The Al defect density in the bulk film and the vacancy density near the SiOx/Si interface region decreased with increased temperature. Measurement results of Qf proved that the Al vacancy of the bulk film may not be related to Qf. The defect density in the SiOx region affected the chemical passivation, but other factors may dominantly influence chemical passivation at 750°C.

Analytical solution for the photocurrent of solar cells with internal reflection

A 1 D analytical photocurrent expression of the base of a crystal silicon solar cell is derived for both the planar and the textured surfaces, with consideration of internal reflection. This expression separates the internal reflection effect from other effects in the photocurrent expression. The influences on the photocurrent by different light flux passes reflected in the wafer are also distinguished. With this analytical expression, an abnormal internal quantum efficiency (IQE) drop in the textured surface scheme as compared to the planar surface scheme from 0.4 μm to 0.6 μm is observed and explained. The numerical IQE result obtained by PC1 D is also used to confirm the correctness of this analytical solution. Finally, optimization of the local back surface field (LBSF) is performed using this analytical solution, without the help of any numerical software.

Positron beam studies of argon-irradiated polycrystal α-Zr

Doppler broadening spectroscopy was performed using a variable-energy positron beam to investigate the effect of defects induced by 150-keV Ar-ion-irradiated α-Zr bulk material. S parameter in the damaged layer of the as-irradiated sample induced by ion irradiation increased with the increasing implantation dose. Isochronal annealing between 350 and 800°C in vacuum studies was carried out to investigate the thermal stability of defects in the oxide surface and damaged layer for low-dose (1×1014cm2) and high-dose (1×1016cm2) irradiated samples. The results of S-W plot measured at different annealing temperatures showed that the positron-trapping center had changed. The Ar-decorated voids or vacancies, which formed in high-dose implantation samples by Ar ions combining with open-volume defects, are stable and do not recover until at high annealing temperatures. Comparing the annealing behavior of the high-dose and low-dose implantation samples show that the recovery process of open-volume defects such as va...

Effect of Subgrains on the Performance of Mono-Like Crystalline Silicon Solar Cells

The application of Czochralski (Cz) monocrystalline silicon material in solar cells is limited by its high cost and serious light-induced degradation. The use of cast multicrystalline silicon is also hindered by its high dislocation densities and high surface reflectance after texturing. Mono-like crystalline silicon is a promising material because it has the advantages of both mono- and multicrystalline silicon. However, when mono-like wafers are made into cells, the efficiencies of a batch of wafers often fluctuate within a wide range of >1% (absolute). In this work, mono-like wafers are classified by a simple process and fabricated into laser doping selective emitter cells. The effect and mechanism of subgrains on the performance of mono-like crystalline silicon solar cells are studied. The results show that the efficiency of mono-like crystalline silicon solar cells significantly depends on material defects that appear as subgrains on an alkaline textured surface. These subgrains have an almost negligible effect on the optical performance, shunt resistance, and junction recombination but significantly affect the minority carrier diffusion length and quantum efficiency within a long wavelength range. Finally, an average efficiency of 18.2% is achieved on wafers with hardly any subgrain but with a small-grain band.

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.

One‐step Fabrication of Nanoporous Black Silicon Surfaces for Solar Cells using Modified Etching Solution

Currently, a conventional two‐step method has been used to generate black silicon (BS) surfaces on silicon substrates for solar cell manufacturing. However, the performances of the solar cell made with such surface generation method are poor, because of the high surface recombination caused by deep etching in the conventional surface generation method for BS. In this work, a modified wet chemical etching solution with additives was developed. A homogeneous BS layer with random porous structure was obtained from the modified solution in only one step at room temperature. The BS layer had low reflectivity and shallow etching depth. The additive in the etch solution performs the function of pH‐modulation. After 16‐min etching, the etching depth in the samples was approximately 200 nm, and the spectrum‐weighted‐reflectivity in the range from 300 nm to 1200 nm was below 5%. BS solar cells were fabricated in the production line. The decreased etching depth can improve the electrical performance of solar cells because of the decrease in surface recombination. An efficiency of 15.63% for the modified etching BS solar cells was achieved on a large area, p‐type single crystalline silicon substrate with a 624.32‐mV open circuit voltage and a 77.88% fill factor.

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.

Comprehensive study of SF6/O2 plasma etching for mc-silicon solar cells

The mask-free SF6/O2 plasma etching technique is used to produce surface texturization of mc-silicon solar cells for efficient light trapping in this work. The SEM images and mc-silicon etching rate show the influence of plasma power, SF6/O2 flow ratios and etching time on textured surface. With the acidic-texturing samples as a reference, the reflection and IQE spectra are obtained under different experimental conditions. The IQE spectrum measurement shows an evident increase in the visible and infrared responses. By using the optimized plasma power, SF6/O2 flow ratios and etching time, the optimal efficiency of 15.7% on 50 × 50 mm2 reactive ion etching textured mc-silicon silicon solar cells is achieved, mostly due to the improvement in the short-circuit current density. The corresponding open-circuit voltage, short-circuit current density and fill factor are 611 mV, 33.6 mA/cm2, 76.5%, respectively. It is believed that such a low-cost and high-performance texturization process is promising for large-scale industrial silicon solar cell manufacturing.