| dc.description.abstract | The crystalline silicon solar cell has been dominating the PV market since it was
introduced in the 1950´s. About 90% of solar modules currently installed are based on
crystalline silicon, with cost-effective and high efficiency solar cells the long-term
objective. To achieve cost- and energy-effective silicon materials, alternative feedstock
such as metallurgically produced compensated solar grade silicon, and scrap from ingot
growth may be used in the Cz process. Based on Cz material pulled from these types of
feedstock and other, high carbon containing materials, systematic studies of the effects of
impurities and defects on the bulk and solar cell properties, were carried out. Results of
these studies may be summarized as:
(1) The properties of p-type Cz-silicon produced with recycled top cuts and compensated
solar grade (SoG) feedstock were initially investigated. Impurity levels in two
experimental Cz ingots were characterized by glow dischage mass spectroscopy (GDMS)
and Fourier Transformed Infra Red Spectroscopy (FTIR). The combined effects of
oxygen, carbon, metallic- and dopant impurities on recombination properties were
investigated by photoluminescence imaging (PL) and compared to bulk properties of a
typical electronic grade (EG) silicon ingot. Ring pattern distribution of as-grown
micro-defects (GMDs) in the ingots, related to elevated carbon and oxygen levels were
delineated based on the results of two steps of dry oxidation. Moreover, the mechanism of
defect formation in the experimental Cz-Si during solidification and oxidation was
elaborated in light of thermodynamic theories. Meanwhile, the positive effect of
phosphorus, accredited to P-Vacancy complexes in compensated feedstock, was discussed
in relation to minority carrier lifetime, based on quasi steady state photoconductance
(QSSPC) results, as well as on the formation of oxygen-related defects during
solidification.
(2) The effects of substitutional Carbon in the Si crystal lattice, originating from recycled
materials and solidification processes were studied in detail. Three n-type Cz ingots with
different carbon levels were used in the investigation. Copper decoration was used to
quantify the number of as-grown defects, while a two step oxidation process (4 h at 750
ºC and 16 h at 1050 ºC) was used to study the evolution of as-grown defects and the
formation and morphology of oxygen precipitates, stacking faults and smaller precipitates
in the silicon during heat treatment. Carrier Density Imaging (CDI) revealed the defect
distribution and distinguished their states. Results from the study show that substitutional
carbon enhances the formation of a higher number density but smaller size as-grown
oxygen defects, while the effective minority carrier lifetime in the all samples showed
stronger dependence on the oxygen concentration than on the carbon content.
(3) Temperature-dependent electrical properties were investigated with respect to
compensation, via Hall Effect measurements in the temperature range 77-350K for the aforementioned p-type Cz-grown solar grade silicon ingots. The dependence of bulk
properties including Hall mobility, carrier density, and resistivity on temperature were
determined to steam from compensation and oxygen related micro-defects respectively.
The electrical performances were demonstrated to be strongly dependent on the
compensation level RC at low temperature (~77K). Top cut materials substituting
poly-silicon to adjust the compensation level in Cz-silicon grown using compensated
solar grade silicon gives rise to a more uniform resistivity along the silicon ingot, in
addition to improving the Hall mobility. These effects were attributed to the presence of
Al impurities, which result in similar carrier transport properties as Boron. Incompletely
ionized dopants Boron and Phosphorous were observed through the measured carrier
density, showing observable impact on Hall mobility around room temperature (RT) as
phosphorus-vacancies complexes.
(4) The experimental ingots produced from compensated feedstock and top-cut materials
were also used to manufacture solar cells. In the processing of the solar cells, the
phosphorous diffusion process was optimized to improve the bulk properties and thus to
maximize the final solar cell characteristics. The solar cells produced from the
investigated ingots showed efficiency values up to 18.5% and fill factor (FF) values up to
79%, comparable to conventional silicon produced from poly silicon. Appropriate
compensation was finally demonstrated to be an efficient way to improve solar cells
efficiency of Cz silicon produced from recycled silicon, even though higher dopant
concentration incurred relatively faster light induced degradation. | nb_NO |
