Modeling of contact geometry and dopant profile during laser - Silicon interaction

Laser fired contacts (LFCs) and laser doped selective emitters can be used to improve manufacturing throughput of photovoltaic devices without sacrificing device conversion efficiency. However, the laser parameters used to form these features can vary significantly. LFCs can be formed with short pulses (hundreds of nanoseconds) while selective emitters can be formed using either a pulsed or CW mode. Here, mathematical models for a pulsed laser and CW laser are used to evaluate how variations in processing parameters affects alloy formation, molten pool geometry and dopant concentration profiles. The models solve the conservation equations for mass, energy, and momentum to study the effects of heat and mass transfer and fluid flow on the formation of LFCs and emitters. Comparisons between experimental data and theoretical calculations for molten pool geometry and concentration profiles demonstrate good agreement. For LFCs, when assuming complete melting and mixing of the Al contact layer, the Al concentration varies significantly with power level, which drastically impacts the calculated pool shape. The dimensionless Peclet number is used to understand dominant heat and mass transfer mechanisms. Conduction is the dominant heat transfer mechanism at power levels around 20W for both LFCs and emitters. As the power level is increased to 50W, however, the dominant heat transfer mechanism changes to convection. Changes in laser parameters also impact fluid flow velocities and dopant concentration profile for emitters processed in CW mode, which suggests that convection-based models should be used to accurately predict concentration profiles within emitters.