Light-induced Degradation In Crystalline Silicon Solar Cells

The commercial solar cells based on the conventional boron-doped Czochralski silicon (Cz-Si) usually suffer the light-induced degradation (LID) in efficiency as much as10%relatively due to the formation of boron-oxygen (B-O) complexes. It has been found that the B-0complexes, having strong recombination activity for carriers, can form in the case of minority carriers injection, whether by the sunlight illumination or by the application of a forward bias. In spite of intensive studies for nearly20years, many properties and the formation mechanism of B-0complexes are still not clearly known. So it is still of great interest for the development of high efficiency solar cells.This dissertation aims at understanding the nature of B-0complexes limiting the performance of solar cells. We also foucus on the properties and applications of gamanium-doped (Ge-doped) silicon and gallium-doped (Ga-doped) silicon which are of potential for making the high-efficiency solar cells with low LID effect. The important results achieved in this dissertation are listed as follows:(1) We find the proportionality between the saturated concentration of B-0complexes and the concentration of staggered oxygen dimer (O2ist) in Cz silicon samples with quasi-thermoequilibrium annealing and in a special sample with variable distributions of oxygen and carbon concentration. Our results give evidence to the assumption that the O2ist is a component of B-0complexes and it plays an important role in the formation process.(2) We investigate the LID in n-type boron-doped silicon by thermal donor compensation. The carrier lifetime decay curve are shown to consist of two degradation processes, i. e., a fast-and a slow-degradation process. The saturated concentration of B-0complexes (Nt*) in n-type silicon is significant lower than that in p-type silicon with a similar carrier concentration by a factor of one order. The N*increases nonlinearly with the n0, and it shows a proportionality with the illumination intensity, i.e., the excess hole concentration△p. The generation rate constant Rgen of the fast-forming center is found to be proportional to no, but the Rgen of the slow-forming center is independent on n0. The generation activation energy is shown to be about0.4eV, identical to the value found in p-type silicon. Based on the these findings, we conclude that the LID in p-and n-type boron-doped silicon should have the same nature. (3) The atomic structrures, electronical properties and the formation mechanism of B-O complexes are studied using the first-principle calculations. We find two most stable configurations of B-O complexes in the staggered form, which are sinks for O2ist, each corresponding to a metastable configuration in the square form. The calculated electronical properties of metastable configurations agree well with the experimental observation of the forming recombination centers during LID. We propose that the stable configurations act as latent centers for LID, and metastable configurations are the recombination centers. By a recombination-enhanced reaction mechanism, the transformation from latent centers to the final recombination centers cause the LID phonomenon under the excess carrier injection. The calculated carrier dependence of generation rate, characteristic activation energies are consistent with the experimental results.(4) The reason for the reducion of LID in Ge-doped silicon is studied. The first-principle calculations show that Ge atoms in silicon matrix suppress the diffusion of intersitial oxygen atoms and oxygen dimers due to a larger covalent radius. Therefore the dimer concentration is reduced by Ge-doping, causing the suppression of LID. The thermal donor (TD) formation in Ge-doped silicon is also studied. By using low-temperature infrared spectroscopy, we find that the Ge-doping retards every species of TDs. Combining with a kinetics model, the diffusion activation of oxygen tetramer complexes is found to be about1.1eV in Cz silicon and it increases to1.3eV by the hindering effect of Ge atoms.(5) We develop a novel gas doping technique for improving the homogeneity of resistivity distribution in crystal. By using the phosphine (PH3) co-doping, the major disadvantage of Ga-doped silicon for photovoltaic application, i.e., it has a large resistivity dispersity due to small segregation coefficient, can be solved. The properties of Ga/P compensated silicon grown by gas doping technique are investigated. Compared the measured majority and minority carrier mobility with the theoretical calculations given by Klaassen’s model, we find the evidence of the existence of Ga-P complexes which will benefit the mobility in compensated silicon. It also shows that a shallower pn junction in Ga/P compensated silicon can be achieved after P diffusion, compared to the conventional Ga-doped silicon. The efficiency of solar cells made from the Ga/P compensated silicon in found to be identical to, even a bit higher than that of conventional Ga-doped silicon solar cells. It demonstrates that the Ga-doped silicon with moderately P compensation is suitable for fabricating high efficiency solar cells.