Surface Passivation Of Crystalline Silicon For Solar Cells

Photovoltaic technology is an effective way for the alleviating of current energy crisis. In this promising area, crystalline silicon solar cells play an important role and account for about 80%market share. As the development of high efficiency silicon solar cells and the decreasing of their thickness, passivation of crystalline silicon surface becomes more and more important.For the analysis of crystalline silicon surface of the metal-insulator-insulator-semiconductor (MIIS) structure, we propose a new method for this purpose with the aid of high frequency capacitance voltage curves of this structure. On the basis of this theoretical mode, we have researched on the passivation properties of three kinds of thin films, including silicon nitride, alumina and the stacks of silicon nitride with amorphous silicon thin film. For the passivation of PE-CVD silicon nitride thin film, it can passivate the silicon surface well with the increase of effective lifetime of the minority carriers from 35.3ns to 217.6us. After rapid thermal annealing of the samples at 600℃with different annealing time, the effective lifetime of the minority carriers increase after the 15 seconds annealing. And then it decreases slowly. For the higher annealing temperature (700℃), there is a sharp decrease of effective lifetime of the minority carriers, which means the detereorate of thermal stability of this kind of passivation thin film. Furthermore, the electron capature capability of silicon nitride thin film decreases with the increase of annealing time and temperature, corresponding to the decrease of the density of K+centers in this film.As far as the ALD-Al2O3 passivation is concerned, there is a maximum of the effective lifetime of the minority carriers after the rapid thermal annealing of 30 seconds at 550℃. Besides, there is also the increase of the interfacial SiO2 layer from-lnm to-4nm and the decrease of the density of the interface states after this rapid thermal processing. With the increase of the annealing temperature from 450℃to 650℃, the interfacial charge in ALD-AI2O3 thin films transforms from positive to negative, and then become into positive after 600℃annealing. As the increase of annealing time at a fixed temperature, the negative charge density will decrease after the reach of a maximum value.Amorphous silicon thin film can form high quality interface with crystalline silicon, which has low density of interface states. Deposition of silicon nitride thin films can improve the passivation of amorphous silicon thin film. For this kind of passivation by thin film stacks, there is an optimal value of the amorphous silicon thin film thickness.