STUDY AND PERSPECTIVE OF LOCALLY GALLIUM DOPING VIA LASER TREATMENT FOR SILICON BASED SOLAR CELLS

摘要

The state of the art suggests that to get high efficiency in multi crystalline solar cells the conventional cell processshould provide low surface recombination losses by good surface passivation and/or Back Surface Field (BSF) formation. The ptypedoping is a key issue to achieve high efficiency solar cell: n-type wafer needs high quality p-type emitter while effectiveBack Surface Field (BSF) require strong p~+ region in p-type silicon. We propose Gallium as an alternative to Boron orAluminum widely used as p-type doping sources. Due the Ga lower melting point (29.76 °C), it can be locally disposed in a fastand reliable way on silicon surface and then treated by laser to promote its diffusion into the silicon network. Moreover the Garemoval from the silicon surface is faster than Al or B oxide. Despite to the poor segregation coefficient in c-Si, Ga does notshow any metastability effect compared to boron doping in c-Si. Then pulsed Nd-YAG laser has been used to promote the Gadiffusion into 1Ωcm p-type doped silicon substrate, varying beam focus as well as intensities, performing different patterns onthe c-Si surface; hall profile to evaluate the sheet resistance and activated doping profile, Ga doped region with Ag and Ni/Cumetal contacts by Transfer Length Method to calculate contact specific resistivity has been used. Laser treatment shows aneffective Ga doping concentration with peak as high as 2×10~(19) cm~(-3) with Ga depth profile similar to the Al laser induced dopeddiffusion layers.