Study And Application Of New Multicrystalline Solar Cell Etching Solution

Energy is the base for human society development. And environment is the necessary condition of continuable development. People have to search new clean energy as common energies are being used up. All kinds of new energies have their advantages and disadvantages. Photovoltaic as one kind of new energies are being cared by more and more people. The yield of solar cells increased greatly in recent years. Crystalline silicon solar cells are market mainstream all the same and the multicrystalline silicon cell occupies fifty-three percent of all the markets. Acid etching technology is more and more used in multicrystalline silicon solar cell industry. However, the auto-catalyzing character led to uncontrolled and un-repeating problems when common acid solutions are used. Then some assistant apparatuses have to be used to maintain large-scale production, which enhances the product cost. New idea that ammonia (NH3?H₂O) being added into hydrofluoric acid (HF)/nitric acid (HNO₃)/water (H₂O) system to control silicon etched rate was brought forward in this article for the first time. Orthogonal test method was used to find the influences of all components in the new etching solution (HF/HNO₃/NH3·H₂O/H₂O system) on the etching rate. It was found that ammonia was irrespective to the influences of other reagents on etching rate. Etching rate influenced by ammonia volume was discussed emphatically. It was found that etching rate increased at first and then decreased with increasing NH3·H₂O volume. Components in the new solution were calculated, and Pitzer theory was used to calculated ions active coefficients. Besides, Chemical reactive rate theory was also used. It could be seen that reductions of HNO₃ concentration and active coefficients were the main reason for the reduction of etching rate with increasing ammonia volume. Consumptions of ammonium ions, silicon and nitrate ions during etching were tested. It could be concluded that ammonium nitrate (NH₄NO₃) decomposed continually to produce nitrogen monoxide (N₂O) when silicon was etched. It was the most important conclusion in this article. Three-phase model was established. A large mount of N₂O gas covered on silicon surface to increase reactive resistivity. Moreover, N₂O gas in diffusion layer increased HF transfer resistivity. So, etching rate decreased. Air bubble disturbing model was used to discuss the disturbing effect caused by rising of N₂O. Disturbing effect reduced HF diffusion resistivity, concentration grads and diffusion layer thickness which led to the increasing of etching rate. Concentration and active coefficient of HNO₃, preventing effect of N₂O, diffusion character of HF all influenced etching rate of silicon with increasing ammonia volume. Silicon etched with etching time was also studied. Variation of etching rate was interactions between reaction heat, the heat exchanging with outside environment and fluorine in the solutions. That NH₄NO₃ decomposing accompanied with silicon etching was the most important reason leading to the difference between HF/HNO₃/H₂O system and HF/HNO₃/NH3?H₂O/H₂O system.The new etching solution was used to texturing multicrystalline silicon surface. N₂O gas decomposed by NH₄NO₃ was benefit to forming etching pits. Surface of multicrystalline was homogeneous and the crystalline direction dependence was very little. When compared with other method, the reflectivity between 300nm and 1200nm of new solution etched surface was not lowest while the solar cell was the best. The difference was analyzed from several factors such as, voltage, current, fill factor, etc.. New etching solution was stable, controllable and repeatable. It needed not temperature controlled and circular apparatuses which lowered production cost.Principle of p-n junction separated by back etching method was introduced. The new etching solution was used to the back etching technology of crystalline silicon solar cell. The low etching rate of new solution prevent front surface being etched. A new back field tested method by use of alkaline solution was designed for the first time. Al-back surface field of the solar cells were studied. When compared with plasma etching method, the back field of back etching was much more homogeneous. The internal quantum efficiency (IQE) of long wave light was higher, and so voltage was higher. The back reflector of back etching solar cell was flat which led the low reflectivity of long wave light and so the current was not so high. As a whole, the solar cell prepared by back etching was better than edge etching.At last, some other experiments to be done were brought forward. We hoped that the new etching solution could be used in crystalline solar cell production line as early as possible.