Diode laser crystallization processes of Si thin-film solar cells on glass

Abstract

The crystallization of Si thin-film on glass using continuous-wave diode laser is performed. The effect of various processing parameters including laser power density and scanning speed is investigated in respect to microstructure and crystallographic orientation. Optimal laser power as per scanning speed is required in order to completely melt the entire Si film. When scan speed of 15–100 cm/min is used, large linear grains are formed along the laser scan direction. Laser scan speed over 100 cm/min forms relatively smaller grains that are titled away from the scan direction. Two diode model fitting of Suns-Voc results have shown that solar cells crystallized with scan speed over 100 cm/min are limited by grain boundary recombination (n = 2). EBSD micrograph shows that the most dominant misorientation angle is 60°. Also, there were regions containing high density of twin boundaries up to ~1.2 × 10-8/cm2. SiOx capping layer is found to be effective for reducing the required laser power density, as well as changing preferred orientation of the film from ⟨ 110 ⟩ to ⟨ 100 ⟩ in surface normal direction. Cracks are always formed during the crystallization process and found to be reducing solar cell performance significantly.