CHARACTERIZATION OF LASER FIRED CONTACTS, LASER DOPED EMITTERS, AND FIXED CHARGE PASSIVATION FOR IMPROVED SILICON SOLAR CELLS

Open Access
Author:
Hedrick, Brittany Lynn
Graduate Program:
Materials Science and Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
May 02, 2011
Committee Members:
  • Suzanne E Mohney, Thesis Advisor
  • Joseph R Flemish, Thesis Advisor
Keywords:
  • Laser Doped Emitter
  • Silicon Solar Cells
  • Fixed Charge Passivation
  • Laser Fired Contacts
Abstract:
With the introduction of laser processing into silicon solar cell manufacturing, characterization and understanding of laser-material interactions and laser fired device performance is critical. To assess laser doping of the front-side emitters and laser firing of the rear-side contacts of silicon solar cells, several experiments were carried out to create and characterize laser fired contacts (LFCs) and laser doped emitters (LDEs). Additionally, an investigation of fixed charge layers for increased surface passivation performance was performed. A cross-sectioning and selective plating method was developed to characterize the molten region of LFCs created with varying laser firing conditions. Delineation of the geometry and penetration depth of an LFC, within the silicon, was possible with this method, and the once-molten, laser fired region was measured to depths of tens of microns in the silicon substrate. Investigation of surface topography, diameter, and electrical performance of LFCs was carried out in previous work by Brennan DeCesar, and is briefly reviewed herein. Simulations of LFCs were performed by modeling devices of different diameter and doping densities to replicate trends in experimental resistance data, and to identify the concentration of Al in the fired contact region. Resistances simulated for LFCs modeled with doping densities above 1x1018/cm3 matched best with experimental data, and the resistance of the contacts is dominated by spreading resistance from the heavily doped region (formed through laser firing) into the rest of the wafer. Laser doped emitters were also fabricated and characterized through electrical characterization and cross-sectioning and junction delineation techniques. Doping of the emitters using phosphorus-doped amorphous silicon films, incorporated in the passivation layers, was found to be promising. Depths of LDEs within the silicon were much shallower than those of LFCs, due to the shorter pulse duration used in laser processing (µs rather than ms). Passivation of silicon through the introduction of negative fixed charge was investigated using alumina (Al2O3) and hafnia (HfO2) films deposited by atomic layer deposition (ALD). Layered structures with SiOx capping films deposited by plasma enhanced chemical vapor deposition (PECVD) were also investigated. Passivation quality was characterized by measuring effective carrier lifetime using microwave photoconductive decay. The longest lifetimes achieved were over 1.5 ms, measured on a sample passivated with 10 nm Al2O3 and 100 nm SiOx after a 5 min 350°C anneal. Fixed charge density was calculated through flat band voltage shifts in C-V measurements made using a mercury probe. Both the Al2O3 and HfO2 samples exhibited negative fixed charge densities. Samples passivated with thin Al2O3 layers were found to have the largest negative fixed charge densities, up to 1x1013/cm2. Additionally, the creation of an inversion layer due to fixed charge in passivation films deposited on high resistivity silicon substrates was investigated using contactless conductivity measurements and changes in sheet resistance. The presence of an inversion layer was detected by a decrease in sheet resistance and correlated to an increase in effective carrier lifetime for those samples annealed for 5 min at 450°C in a tube furnace, 450°C in a RTA, and 350°C on a hot plate.