Investigation On The Anti-reflective Structure And Surface Passivation With High K Dielectric Thin Film For Crystalline Si Solar Cell

Among the various types of solar cells, the crystalline silicon（c-Si） solar cells boast more than 70% of the market share of the photovoltaic products owing to the optimized manufacturing technology and the stable cell performance. However, the c-Si solar cells are still too expensive to compete with public electricity charge, and the efficiency need to be further improved. Up to now, excellent antireflective structure and high-quality surface passivation is of key importance for the performance of high efficiency industrial Si solar cells. In order to improve the performance of the c-Si solar cell, the anti-reflective structure and surface passivation for c-Si solar cell are investigated in this thesis. The main contents are as follows:（1） Ag assisted chemical etching and RIE technique using self-assembly Ni nanoparticle masks have been successfully developed to texture the Si surface to reduce optical reflectivity. The micro/nano structured surfaces were formed by etching the conventional textured sc- and mc-Si surfaces with Ag assisted chemical etching method. The reflectance of the micro/nano textured surface could be reduced to below 5% by adjusting the experimental conditions, such as reaction time, temperature and the concentration of Ag NO3. Besides, RIE technique using self-assembly Ni nanoparticle masks was used to texture the front surface of the mc-Si wafers. Ni nanoparticles were assembled on the wafer surface by annealing the Ni thin film at 900 °C, which played a dominative role in controlling the morphology of textured structure. Experimental results indicated that the surface structure of the RIE textured wafer was nanopillar array, and the reflectance was significantly reduced to below 2% in the range of wavelength from 400 nm to 1000 nm. Moreover, a damage-free Si surface was recovered by incorporating improved cleaning and damage removal treatment. And the efficiency of the fabricated mc-Si solar cell can research 12.01%. This technique is not only suitable for mc-Si solar cell, but also could be applied to sc-Si solar cell.（2） The electrostatic self-assembly technique and ultrasonic spray coating technique were developed to assemble Si O₂ nanoparticles on the surface of the Si solar cell as anti-reflectance coating. Si O₂ nanoparticles with different sizes were assembled on the textured sc- and mc-Si solar cell surface by electrostatic self-assemble technique, it was found that Si O₂ nanoparticles with small size could be assembled on the side and top of the textured structure, and the Si O₂ nanoparticles with larger size is main on the bottom. Besides, 100 nm and 200 nm Si O₂ nanoparticles were assembled on the mc-Si solar cell by ultrasonic spray coating technique, the results indicated that 100 nm and 200 nm Si O₂ nanoparticles coating could reduce the surface reflectance and improve the efficiency of the mc-Si solar cell. The efficiency could be improved obviously when 100 nm Si O₂ nanoparticles were coated on the surface of the solar cell for one circle.（3） High K oxides films, Al2O3, Hf O₂,（Hf O₂）x（Al2O3）1-x,（La2O3）x（Al2O3）1-x and（La2O3）x（Hf O₂）1-x stacked structure films, were deposited on p-type Si wafers using atomic layer deposition（ALD）. The chemical compositions and structures of the deposited high K thin films were investigated in detail by X-ray photoelectron spectroscopy（XPS） using ion beam sputtering for depth profile analysis. The XPS depth profiling indicated that the synthesized stacked structure films were Hf–Al–O, La–Al–O and Hf–La–O alloys respectively. And the obtained results also suggested that the interfacial layer were most likely composed of Hf–Si–O for（Hf O₂）x（Al2O3）1-x films, La–Si–O for（La2O3）x（Al2O3）1-x films, Hf–Si–O and La–Si–O for（La2O3）x（Hf O₂）1-x films rather than pure silicon oxide. It was also found that Hf and La are easier to react with Si to form silicate at the interface near the Si substrate. The top surface was easy to absorb CO₂ to form carbonate species on the surface as a result of its contact with air at normal conditions when La existing.（4） Advanced passivation using simple materials（Al2O3, Hf O₂） and their compounds（Hf O₂）x（Al2O3）1-x deposited by ALD was investigated. It was found that the carrier recombination rate reduced, the open circuit voltage and the conversion efficiency were improved. The fixed charge of high K film can be adjusted from negative to positive by varying the ratio of Hf and Al. The ALD-Al2O3 film has been found to provide negative fixed charge（-6.4 × 1011 cm-2）, whereas Hf O₂ film provides positive fixed charge（3.2 × 1012 cm-2）. The effective lifetimes can be improved after oxygen gas annealing for 1 min. I-V characteristics of Si solar cells with high-K dielectric materials as passivation layers indicate that the performance is significantly improved, and ALD-Hf O₂ thin film would provide better passivation properties than that of the ALD-Al2O3 thin film in this research work. And the passivation mechanism is also investigated.