SiliconPV 2012 generation of defect-related acceptor states by laser doping

In this work we report for the first time on the creation of thermal acceptors after laser irradiation of a phosphorous doped p-type silicon substrate in ambient atmosphere. The concentration of these defects up to 1020 cm−3 is leading to a conductivity change from n- to p-type just beneath the surface. Electrochemical capacitance voltage (ECV) measurements followed by resistivity measurements confirm this conversion of conductivity. Secondary ion mass spectrometry (SIMS) measurements show significant oxygen incorporation after laser irradiation. The comparison of saturation current density and lock-in thermography measurements of processed wafers in ambient and in nitrogen atmosphere reveal that these defects are electrically active recombination centres that disappear in absence of oxygen. In simulations of the temperature profile during laser processing a correlation between the pulse energy density (EP) and the appearance of thermal acceptors has been observed. The influence of these laser induced acceptor defects on solar cell parameters has been finally investigated. An efficiency loss of 1.3% absolute pertinent to pulse energies usually applied in laser doping process under ambient atmosphere could be avoided by laser processing in nitrogen atmosphere.