Research On Low-cost High Efficiency Crystalline Silicon Materials And Solar Cells

As an important renewable energy, photovoltaics (PV) have a promising future and have caught the eyes of the world. Crystalline silicon solar cell has been dominating the PV market in the past ten years. About90%of solar modules are based on crystalline silicon. As a result, low cost and high efficiency solar cells are the long-term objective and trend while scientific issues related to impurities and defects in silicon and their effects on solar cells are significant.Towards low cost high efficiency solar grade crystalline silicon materials, aluminum-silicon melt method for solar grade poly-silicon, cast quasi-single crystalline (QSC) silicon technique and low bowing solar cell process for thin wafers were proposed in the study of this thesis. Based on the material prepared by these methods, systematic researches were carried out on the impurities and defects in silicon. Besides, detailed researches were conducted on the mechanism of solar cell process. In the following are the innovative results of this thesis.(1) Aluminum-silicon melt purification process was invented to fabricate solar grade poly-silicon. By means of low eutectic point of aluminum and silicon, metallurgical grade silicon can be effectively purified under a comparatively low temperature. The effect of aluminum in silicon on the performance of silicon and solar cells was also investigated. Aluminum is an active impurity in silicon. It can introduce deep energy level defects in silicon, leading to the reduction of the minority carrier lifetime (MCL) and the solar cell efficiency. In addition, aluminum-containing silicon solar cells have no light-induced degradation (LID) in efficiency.(2) The performance of QSC silicon and the corresponding solar cells were studied. It is found that QSC silicon contains fewer oxygen and thus fewer boron-oxygen complex defects. Besides, QSC silicon has better quality, i.e. few grain boundaries (GB) and dislocation density in the range of104～106cm-2. Compared to cast multicrystalline (me) silicon, The absolute efficiency of QSC silicon solar cells is1%higher, to which the material quality aspect and reflection reduction aspect contribute0.3%and0.7%, respectively. Compared to commercial boron-doped Czochralski (CZ)-silicon, QSC silicon has higher productivity but the average efficiency is0.5%lower absolutely but smaller LID in efficiency.(3) The behaviors of main impurity and defects in QSC-silicon were studied. It is revealed that small quantities of scattered dislocations have little influence on the MCL and solar cell performance, but the adverse effect aggravates with the increase of dislocation density. The dislocation aggregates is disastrous. It can significantly degrade the mechanical and electrical performance of material, the performance of solar cells, and the recovery capacity of solar cells by fabrication process. The low MCL zone at the bottom of QSC silicon ingot is mainly ascribed to the high concentration of iron in this region. Besides, there are two iron concentration peaks from the bottom, one at the inner face of crucible bottom, and the other occurring at a height of several centimeters above the initial solid-liquid interface. It is revealed by both experiments and simulation that the diffusion of iron into the crystal from both the quartz crucible and the iron-rich layer formed at the initial stage of the whole crystallization process is responsible for the generation of two-peak characteristics. The interactions between defects in QSC silicon were studied. As example, dislocations have direct influence on the boron-oxygen complex defect. It is found that high density dislocation has influence on the kinetics but not dynamics of boron-oxygen complex generation. In the samples with high density dislocations, the activation energy of boron-oxygen complex generation is0.57±0.02eV and the pre-exponential factor is1.3×105s-1, which is two orders of magnitude higher than that of dislocation-free silicon. It is believed that the dislocation-related electronic states charged with holes can cause an energy potential barrier for the capture of single-positive holes that is required for the transformation of B-O complexes from latent centers to immediate transient centers.(4) The low bowing solar cell processes for thin wafers were studied. By means of silver nano-particles as catalyst, surface reflection of silicon can be modulated. Both experiments and theoretical analysis have proved that a well-organized microporous structure on the pyramids can be obtained by optimizing the size of Ag nanoparticles and the texturing time, and the silicon wafer with such structures can effectively reduce the reflectivity of sunlight. However, based on the conventional cell fabrication process, the performance of silicon solar cells with such microporous structures gets degraded. It is closely associated with the strong surface recombination and the high phosphorus diffusion barrier induced by the microporous textures. A novel aluminum back surface field (BSF) process has been invented and studied. The reduction of bowing and crack of solar cells is achieved by modulating boron-containing aluminum paste and reducing the paste thickness. This approach can form boron-containing Al-BSF. Due to the higher solid solubility of boron in silicon, the dopant concentration in BSF layer can be increased by one order of magnitude. Therefore, at low firing temperature (≤800℃), the backside recombination velocity of silicon and the contact resistance between silicon and aluminum have been reduced while these is little influence on the solar cell performance.