About 30% of the total market share of industrial manufacture of silicon solar cells is taken by single crystalline Czochralski (CZ) grown wafers. The efficiency of solar cells fabricated on boron-doped Czochralski silicon degrades due to the formation of metastable defects when excess electrons are created by illumination or minority carrier injection during forward bias. The recombination path can be removed by annealing the cell at about 200° C but recombination returns on exposure to light. Several mono-crystalline and multi-crystalline solar cells have been characterized by methods such as laser beam induced current (LBIC), Four-Probe electrical resistivity etc. to better understand the light induced degradation (LID) effect in silicon solar cells. All the measurements are performed as a function of light soaking time. Annealed states are produced by exposing the cells/wafer to temperature above 200° C for 30 minutes and light soaked state was produced by exposure to 1000 W/m2 light using AM1.5 solar simulator for 72 hours. Dark I-V data are analyzed by a software developed at NREL. This study shows that LID, typically, has two components- a bulk component that arises from boron-oxygen defects and a surface component that appears to be due to the SiNx:H-Si interface. With the analysis of dark saturation current (J02), it is seen that the surface LID increases with an increase in the q/2kT component. Results show that cell performance due to bulk effect is fully recovered upon annealing where as surface LID does not recover fully. This statement is also verified by the study of mc- silicon solar cells. Multi-crystalline silicon solar cell has very low oxygen content and, therefore, recombination sites will not be able to form. This shows that there is no bulk degradation in mc- Si solar cells but they exhibit surface degradation. The results suggest that a typical Cz-silicon solar cell with an initial efficiency of - 18% could suffer a reduction in efficiency to - 17.5% after the formation of a metastable defect, out of which - 0.4% comes from a bulk effect and - 0.1 % is linked to a surface effect.

Among other intrinsic open-volume defects, copper vacancy (VCu) has been theoretically identified as the major acceptor in p-type Cu-based semiconducting transparent oxides, which has potential as low-cost photovoltaic absorbers in semi-transparent solar cells. A series of positron annihilation experiments with pure Cu, Cu2O, and CuO presented strong presence of VCu and its complexes in the copper oxides. The lifetime data also showed that the density of VCu was becoming higher as the oxidation state of Cu increased which was consistent with the decrease in the formation energy of VCu. Doppler broadening measurements further indicated that electrons with low momentum made more contribution to the contributed as pure Cu oxidizes to copper oxides. The metastable defects are known to be generated in Cu2O upon illumination and it has been known to affect the performance of Cu2O-based hetero-junctions used in solar cells. The metastable effect was studied using positron annihilation lifetime spectroscopy and its data showed the change in the defect population upon light exposure and the minimal effect of lightinduced electron density increase in the bulk of materials to the average lifetime of the positrons. The change in the defect population is concluded to be related to the dissociation and association of VCu - VCu complexes. For example, the shorter lifetime under light was ascribed to the annihilation with smaller size vacancies, which explains the dissociation of the complexes with light illumination. Doppler broadening of the annihilation was independent of light illumination, which suggested that the chemical nature of the defects remained without change upon their dissociation and association - only the size distribution of copper vacancies varied. The delafossite metal oxides, CuMIIIO2 are emerging wide-bandgap p-type semiconductors. In this research, the formation energies of structural vacancies are calculated using Van Vechten cavity model as an attempt to study the effect of the size of the MIII cation in the delafossites starting from Cu2O. Comparison of the formation energies between Cu2O and delafossite oxides clearly showed that the equilibrium concentration of the vacancies depended strongly on the structural parameters varied by the presence of different MIII cations. In particular, the size of the MIII cation greatly influenced the defect formation energies of VCu. It was observed from our calculations, as the size increases the formation energy decreases.

Heavily doped silicon is required for devices such as PowerMOSFETs. For the devices to be as sufficient as possible it is necessary to lower the electrical resistivity of the silicon substrate as low as possible. Yet, during the growth of heavily n-type doped silicon by the Czochralski method dislocation formation occurs frequently, reducing yield. Thus this work covers the topics intrinsic point defects, electrical activity of dopant atoms, spreading of dislocations and facet growth. Each topic is discussed in regard of their possible impact on the formation of the dislocations. In doing so, the control of facet growth is found to be most crucial to prevent the formation of the dislocations.

This thesis aims at understanding the mechanisms limiting the efficiency of compensated silicon solar cells (containing boron and phosphorus in the bulk). Such dopant compensation is common in solar grade materials, especially in silicon from the metallurgical route, and can potentially lead to a degradation of the materials electronic properties. We experimentally show that a thermal oxidation can create an n-type layer at the surface of compensated p-type silicon. This n-type layer is further shown to interfere with device performance and material characterization. We investigate the impact of compensation on the minority carrier lifetime, in particular for recombination through defects. Metastable defects such as chromium-boron pairs and the boron-oxygen defect are shown to degrade the lifetime of compensated n-type silicon. The boron-oxygen defect in compensated n-type silicon is then experimentally investigated. It is shown that if not mitigated, the boron-oxygen defect leads to a strong reduction in implied VOC. The defect is also shown to be fundamentally different in compensated n-type silicon compared to p-type silicon. Its concentration does not depend on the net doping and its recombination activity is dominated by a shallow defect rather than a deep defect. Through a theoretical investigation, we show that the carrier mobility is also affected by compensation. Both theory and experiments confirm that the mobility is reduced by the combined presence of acceptors and donors. Compensation not only increases the amount of ionized impurities and decreases the amount of free carriers, it also affects the scattering cross section of ionized impurities and free carriers. Theoretical calculations show a relatively weak influence of the compensating impurities on the mobility. However experimental results suggest a stronger influence of compensating impurities. This results in mobilities slightly lower than predicted by advanced model such as Klaassen's model. A new method to measure the sum of the majority and minority carrier mobility in silicon is introduced. Measurement of the influence of dopant density, injected carriers and temperature on the mobility sum are made and compared to data available in the literature. -- provided by Candidate.