DLTS Study of Recombination Active Defects in Solar Silicon

Abstract

Crystalline silicon is by far the most prevalent material in the manufacture of solar cells, principally due to their low cost of fabrication. However, depending on the growth method used to produce silicon crystals, unwanted impurities, and defects, can remain. These defects can be electrically active, acting as traps for charge carriers, reducing carrier lifetime, and ultimately leading to reduced efficiency in finished solar cells. In this thesis, two sets of defects in crystalline silicon are studied using electrical techniques - capacitance voltage (C-V) measurements, deep level transient spectroscopy (DLTS), Laplace DLTS, and minority carrier transient spectroscopy (MCTS), with the aim of understanding how their impact on cell efficiency might be reduced. In the first part of this thesis, the electrical activity of transition metal impurities, which can be common contaminants of cheaper silicon that has not been through costly chemical purification processes, is investigated. Although certain transition metals can be removed by gettering, slow diffusing metal atoms can remain, and alternative means of reducing their electrical activity, and hence their effect on cell efficiency, are required. One potential means to achieve this is the use of hydrogen as a passivation tool, as it can bond to metallic impurities, rendering them electrically inactive. The effect of hydrogen on electrical activity has been investigated for two of the slowest diffusing metals, vanadium and molybdenum, in intentionally contaminated silicon treated with a hydrogen plasma. We find that hydrogen passivation occurs in n-type silicon, with an observed reduction in the electrical activity of vanadium atoms, but that no such bonding occurs in p-type silicon for either vanadium or molybdenum. We suggest that this difference is due to the fact that metal and hydrogen atoms are both positively charged in p-type silicon, and experience Coulombic repulsion. The possibility of forcing metal-hydrogen bonding in p-type silicon using optical excitation is then investigated and found to be successful for both vanadium and molybdenum following illuminated annealing at temperatures 800 °C, after which they are not seen to form upon subsequent annealing between 450 and 700 °C. Finally, the influence of nitrogen doping during the float zone growth process on the formation of these thermally activated defects is investigated. We show that the concentration of defects formed during annealing is significantly greater in nitrogen doped than nitrogen lean silicon, and suggest that this effect is due to the differences in the concentration of vacancies between the two materials.

Date of Award	1 Aug 2019
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Matthew Halsall (Supervisor) & Bruce Hamilton (Supervisor)