Yehua Tang

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.

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.

SiOx(C)/SiNx dual-layer anti-reflectance coating for improving solar cell efficiency

The SiO_x(C)/SiN_x dual-layer shows a better anti-reflection coating compared with standard SiN_x films, after deposition and annealing. In this paper, the effect of film thickness, sintering peak temperature, belt speed, etc. on the anti- reflectance of SiO_x(C) film were discussed. Under optimized condition, the weighted reflectance of SiO_x(C)/SiN_x dual-layer obtained 1.25% reduction; the efficiency of solar cell rises 0.35%. Sixty solar cells with dual-layer encapsulated modules gained 0.5 W larger power as well as better light stability than the ones with SiN_x single layer.

Acid texturing of large area multi-crystalline silicon wafers for solar cell fabrication

Surface texturing of silicon can improve the incident light trapping and hence increase the conversion efficiency of solar cells. The texturing of multi-crystalline silicon (mc-Si) for solar cells with HF/HNO3 acidic solution has been investigated in this work. The recipe of texturing solution was studied to eliminate grain boundaries and defects which may appear in the texturing process. The effect of etch depth on large size solar cell process was also discussed. It is suggested that appropriate etch depth may enhance the surface quality of solar cells without negative effect on the incident light trapping. The result shows that elimination of deep grain boundaries and defects and enhancement of surface quality improves cell performance by increasing the open circuit voltage and the short circuit current.