Light-induced Degradation Defect And Its Regeneration State In Crystalline Silicon

The solar photovoltaic (PV) power has become more and more popular all over the world. The crystalline silicon solar cell is a mainstream product at current PV market due to its good stability and high efficiency. For commercial p-type silicon solar cells, the light-induced degradation (LID) which caused by the formation of boron-oxygen (B-O) complexes under illumination has to be resolved. Since LID was found in 1973, continuous studies have been reported. However, its properties and mechanism maintains unclear until now. Therefore, a systematic study on the LID is very important and practically significant.This thesis aims at investigating the kinetics behavior of LID in crystalline silicon, as well as the strategies of suppressing the LID, such as the regeneration state and its kinetics behavior, and the effect of germanium doping and hydrogen doping. The innovative results achieved in this thesis are addressed as following:(1) Traditional boron-doped p-type silicon, boron/phosphorus co-doped p-type silicon and boron/phosphorus co-doped n-type silicon wafers all suffer the light induced degradation. Due to the high concentration of hole, the traditional boron-doped p-type silicon and compensated p-type silicon will easily jump over the regenerated state and arrival at annealed state under illumination at elevated temperature, which is a result of competition between degrading and annealing. However, we found that it is much easy to dissociate the B-O complexes at higher temperature. And we also find that the LID effect in n-type compensated silicon can be almost eliminated by illuminating at elevated temperature range of 140-200℃. The carrier lifetime after deactivation is stable permanently when the illumination temperature is lower than 40 ℃. The activation energy of regeneration of samples is 0.64 eV. The results show no correlation between the activation energy and the total boron concentration in n-type compensated silicon. Meanwhile, we found that the lifetime of the sample degrades again at regeneration state by illuminating at elevated temperature of 55-100 ℃ in the n-type compensated silicon for the first time. This kind of transformation is a process of thermal activation. The activation energy of redegradation is 0.46 eV. There is no correlation between the activation energy and the total boron concentration.(2) The formation rate of boron-oxygen complexes can be effectively suppressed by germanium doping. With the increase of germanium concentration, this suppression effect becomes more and more obvious. The more Ge doping concentration in silicon, the less B-O complexes can form. In the case of the germanium concentration exceeding the 1021 cm-3, the sample will never degrade under illumination. The reason should be associated with the fact Ge can effectively reduce the oxygen dimer concentration. With the increase of Ge concentration, the reduction of oxygen dimer concentration become more obvious. We find that there is no oxygen dimer in the germanium highly-doped samples.(3) The formation rate of boron-oxygen complexes can be effectively suppressed by hydrogen. With the increase of hydrogen concentration, this suppression effect become more obvious. Our experiment has clarified that hydrogen may also inhibit the elimination of boron-oxygen complexes. With the increase of hydrogen concentration, the elimination rate of boron-oxygen complexes is much slower. The elimination activation energy of boron-oxygen complexes without hydrogen doping is 1.19 eV, which is smaller than the value of sample with hydrogen doping,1.32 eV.