The Effect Of Boron Doped Al-BSF On The Performances Of The Silicon Solar Cells

With the development of solar cells technology, the thickness of wafer has kept decreasing, which put forward a higher requirement for the carrier concentration on the back of the solar cell for the sake of continuing to improve the conversion efficiency. While aluminum-back surface field (Al-BSF) as a high concentration doping layer on the back surface, in terms of reducing the back surface recombination velocity, has a very significant effect. However, due to the limited solubility of Al in c-Si, it is harder to achieve a high doping concentration in the Al-BSF, which limits the improvement of conversion efficiency of c-Si solar cells. Boron has a higher solubility in Si, it can be achieve a much higher doping concentration in the BSF by boron doping through the screen-printed alloyed-aluminum-boron paste. But Boron diffusion has a higher requirement about the sintering temperature, which requires find a suitable sintering temperature.In this thesis, we prepare the Al-BSF by boron doping into the screen-printed aluminum paste, investigate the impacts of the boron-doped volume and sintering conditions on the Al-BSF carrier concentration、BSF depth and the electrical properties. We deposited Al2O3film by ALD, and studied the effect of the passivation of the Al2O3film on the sheet resistances of the samples and the Boron-doped BSF.Through experiments, we got the following conclusions:(1) Boron-doped in the aluminum pastes can significantly improve the doping concentration to1019atoms/cm3of the Al-BSF, which can increase the depth of the P+layer and the open circuit voltage, but causes the reduction in the filling factor in some extent.(2)Lower peak of the sintering temperature will cause uneven boron diffusion, higher temperature will reduce the electrical properties of Al-BSF,800℃is an appropriate sintering temperature.(3)The passivation of the back-field can be improved by Al2O3. The annealing can enhance the passivation effect of the Al2O3. However, no significant effect on the passivation is observed for Boron-doped Al2O3.(4) When the sheet resistance is about70Ω～100Ω/□, Al2O3presents the best passivation, and the highest minority carrier lifetime is achieved. In this resistance range, the majority of the Al-Si alloys has been etched, and the P+-layer has still been saved relatively complete.