Properties And Suppression Of Boron And Oxygen Defect In Crystalline Silicon Solar Cells

As an "inexhaustible" renewable energy, solar photovoltaic (PV) power has received great attention and been intensively investigated in recent years. In all kinds of PV products, boron-doped crystalline silicon solar cell is the main one, which currently occupys more than 80% of the share. For the reduction of PV power cost, reducing fabrication cost and improving conversion efficiency are the most mainstream strategies of crystalline silicon solar cells. However, light-induced degradation (LID) phenomenon, e.g., efficiency of crystalline silicon solar cell degrades severely caused by illumination, detrimentally influences the efficiency of solar cell. In spite of continuing studies since LID found in 1973, it is still a serious issue to be resolved in both academic and industrial area and its nature remains unclear by now. Therefore, a systematic study on the properties and mechanism of LID suffered by different types of crystalline silicon has a great significance in scientific research and practical application.This thesis aims at investigating the nature and kinetics behavior of LID in crystalline silicon and solar cells codoped by boron and other dopants, as well as the strategies of suppressing the LID. The innovative results achieved in this thesis are addressed as following:(1) Kinetics characteristics of boron-oxygen (B-O) complexes responsible for LID in p-type compensated crystalline silicon have been investigated via a bi-exponential fitting. Based on the complex generation rate constants (Rgen) as a function of temperatures extracted accurately from the fitting, the generation activation energies (Egen) for the fast and slow processes are obtained to be 0.29 eV and 0.44 eV, respectively, which indicates that the phosphorus (P) compensation increases the migration barrier of oxygen dimer (O2i) in the fast-process. Moreover, the saturated complex concentration (N∞*) increases linearly with the increase of the hole concentration (po) and the pre-exponential factor is proportional to the square of pa in both p-type conventional and compensated crystalline silicon.These results support that the P compensation has no influence on the scattering strength of O2i during its diffusion to the nearest site of B and the presence of latent centers (LC) should be reasonable.(2) Kinetics characteristics of B-O complexes with po larger than boron concentration (NB) have been investigated. The ratio of p0/Nb can be changed by adjusting the gallium (Ga) concentration doped in crystalline silicon or the intensity of laser injected, and the N∞*,fast and N∞*,slow are found to be proportional to NB and po, respectively. This indicates that B is directly involved in the fast-process of B-O complexes generation. The N∞*.fast/N∞*,slow is also found to decrease with the increase of po. Meanwhile, the Rgen and Rgen are both proportional to the square ofpo, which verifies the assumption that the O2i diffuses a small distance in the LC by capturing hole twice. Moreover, it is found that the Egen,slow, is 0.42 eV in p-type crystalline silicon codoped by Ga and B.(3) The suppression mechanism of B-O complexes in crystalline silicon by carbon (C) codoping have been studied. It have been demonstrated by the first-principle calculation and experiments that the O2i is more likely to bond with the Cs in form of a more energetically favorable CsO2i complex, which in turn reduce the N∞*. The en obtaind from C and B codoped crystalline silicon is 0.56 eV, about 0.14 eV larger than that of conventional crystalline silicon. Two mechanisms were proposed:On the one hand, the doped C with a smaller atom radius than silicon will induce tensile stress in lattice and limits the diffusion of O2i towards Bs. On the other hand, the interstitial carbon (CO formed via the kick-out mechanism will diffuse to the neighboring sites of O2i in LC, and increases the scattering strength of O2i during its small distance diffusion, which in turn improve the migration barrier. While the dissociation activation energies and binding energies are 1.35 eV and 0.93 eV respectively in both conventional and C and B codoped crystalline silicon, which indicates that there is no more complex defect formed.(4) The impact of C codoping and regeneration process on the performance of crystalline silicon solar cells after LID have been studied. Compared to the conventional cells, the performance of C and B codoped cells before LID is worse, due to the promotion of oxygen precipitations by C, which leading to more recombination of minority carriers and channels electric leakage. While the performance of C and B codoped cells after LID is better, such as short circuit current, open circuit voltage and conversion efficiency, because of the effective suppression of B-O complexes by C. Moreover, the regeneration process of B-O complexes can also reduce the degree of LID in cells significantly. Meanwhile, C codoping will decrease the regeneration activation energy from 0.64 eV to 0.59 eV. Therefore, combination of C codoping and regeneration process is a effective strategy of suppressing the LID in crystalline silicon solar cells with low cost.