Laser-Induced Crystallization of Amorphous Silicon for Poly-Si/SiOx Passivating Contacts

Abstract

Laser-induced crystallization (LIC) was investigated to develop a suitable method to transform 100 nm-thick amorphous silicon (a-Si) into polycrystalline silicon (poly-Si) for poly-Si/SiOx passivating contacts in silicon solar cells. The project explores the influence of laser wavelength, scanning techniques and key laser parameters – such as power P, defocus D, and pulse spots distance L – on passivation quality, conductivity, and crystallinity. A comparison between 532 nm (green) and 355 nm (UV) nanosecond pulsed lasers highlights the impact of the absorption coefficients of a-Si for different wavelengths on crystallization, passivation quality and sample elemental composition due to the laser treatments. Results showed that the 355 nm source was more suited to the project as the 532 nm source treatments frequently resulted in layer ablation and a consequent absence of the oxide layer. The investigation also assesses the role of hydrogenation by silicon nitride (SiNx) deposition followed by a high-temperature, short-duration thermal step (fast firing), testing peak temperatures ranging from 700 to 900◦C. The results showed that firing temperature had a significant impact on passivation, identifying 750◦C as the optimal temperature. Further samples were processed with the 355 nm laser source, followed by a hydrogenation process by firing at 750◦C. The highest achieved implied open-circuit voltage iVoc results were: (0.677 ± 0.015)V, for a non-lasered sample, followed by (0.672 ± 0.012)V for a laser-treated wafer with P = 1.5W, D = 7.0mm and L = 35µm. However, individual measurements in several samples revealed higher iVoc values, with a maximum of 0.695V observed for P = 1.0W, D = 6.0mm, and L = 35µm. Additionally, crystallinity and conductivity of the a-Si layer improved due to laser treatments, in comparison to reference non-lasered wafers.