Optimization of laser processes in n+Emitter formation for c-Si solar cells

Abstract

Punctual phosphorus diffused emitters were achieved by laser patterning phosphorus doped a-SiCx:H films deposited by PECVD as a doping source. Two different lasers at wavelengths of 1064 nm and 532 nm were used. Phosphorus diffusion was confirmed by Secondary Ion Mass Spectroscopy. We explored the effect of pulse energy and number of pulses per diffused point. The results show that a fine tune of the energy pulse is critical while the number of pulses has minor effects. Scanning Electron Microscopy (SEM) pictures and optical profilometry showed a laser affected area where the c-Si is melted, ejected and solidified quickly again. Typically, the diameter of the affected area for 1064 nm laser is between two and four times greater than for 532 nm laser. Optimum parameters for both lasers were determined to obtain best J-V curves nearly to ideal diode behavior. Comparing best J-V results, lower emitter saturation current density (Jo) and contact resistance are obtained with 532 nm laser. The improvement in Jo can be related mainly to the smaller affected areas observed by SEM while lower contact resistance can be attributed to that 532 nm laser has a more superficial action resulting in higher phosphorus concentration at the surface. The expected open voltage circuit for finished solar cells using these emitters is in the range of 640 mV for 532 nm laser and 620 mV for 1064 nm one.