Mechanical deformation of polycrystalline silicon for solar cell production

• Main Author: Wu, Min
• Other Authors: Wilshaw, P. R. ; Murphy, J. D.
• Published: University of Oxford 2014
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.730606

This thesis presents a feasibility study for producing silicon sheet for photovoltaic applications via traditional hot deformation in clean conditions using high purity materials. A special deformation rig has been designed and constructed for the compression tests. The cleanliness of the deformation conditions has been characterized and determined to be at a satisfactory level. Deformation of Czochralski, dislocation-free, single crystalline silicon along the < 100 > axis has been conducted to determine a suitable and achievable deformation condition. It has been found that single crystalline silicon can be uniaxially compressed by 30 % without severe cracking at temperatures above 1050 °C using a low average strain rate of ~10^-5 s^-1, the most uniform deformed microstructure has been obtained using an average stain rate of 1.67 x ~10^-5 s^-1. A dislocation density of order 10¹³ m^-2 has been revealed by etch pit counting in an SEM. Dislocations with ½ < 110 > screw characters have been found using the TEM diffraction contrast technique. The most favourable post-annealing temperature has been determined to be 1400 °C. High purity polycrystalline, chemical vapor deposition (CVD) grown silicon has been successfully deformed at 1150 °C by 10 % at an average strain rate of 6.94 x 10^-6 s^-1. The as-received material has been found to have a strong < 110 > fiber texture as revealed by EBSD. Recrystallization occurs during pre-annealing at 1400 °C for 30 minutes. Approximately 90 % of the material recrystallizes with a complete disappearance of the fiber texture. Stacking faults have been mainly observed in the recrystallized material by TEM. It has been observed that the deformed polycrystalline material develops a second recrystallization texture when subjected to post-annealing at 1400 °C for 15 min, resulting in large grains up to several hundred microns in size and a low dislocation concentration of 5 x 10¹⁰ m^-2. The HR-EBSD cross-correlation method has been employed to quantitatively investigate the GND density distribution in polycrystalline silicon. It has been found that the as-received material has a GND concentration up to 10¹⁵ m^-2 and recrystallization during pre-annealing reduces the value to 10¹³ m^-2. The minority carrier lifetime of the polycrystalline sample deformed at 1150 °C by 10 % followed by subsequent annealing at 1400 °C for 120 min has been measured using the QSS-PC method. A value of 1.9 μs at an injection level of 1 x 10¹² cm^-3 has been obtained, which corresponds to a minority carrier diffusion length of approximately 100 μm for electrons and 50 μm for holes. H-passivation has been found to be effective in improving the measured minority carrier lifetime. The results presented in this thesis suggest that producing silicon sheets for photovoltaic applications by conventional hot-deformation may be possible and should be a topic for further investigation.