A study of laser-induced surface defects in silicon and impact on electrical properties

Abstract

Laser processing of silicon solar cells has unique advantages that offer the potential for low-cost high-efficiency photovoltaic devices. The understanding, monitoring, and control of laser-induced defects in silicon become important challenges that limit photovoltaic efficiency. In this work, the fundamental investigation of laser-induced defects was achieved by identifying defect types and origins at different laser-fluence regimes, assessing defect concentrations, and evaluating their impact on surface electrical properties and photovoltaic device performance. Studies showed that below laser melting, little degradation of electrical properties is observed, but no defects are identified; between laser melting and ablation, point defects and oxygen incorporation mainly occur; above laser ablation, dislocations and strain are primarily generated. Laser-induced dislocation density and strain are found likely to increase exponentially with laser fluence, and laser-induced strain is identified to be a possible major source of dislocation generation. In order to understand carrier recombination and charge transport in laser-processed silicon surface, we quantified the drift mobility, conductivity, carrier lifetime, and leakage current at various laser fluences and dislocation levels. The laser-defect induced degradation of surface electrical properties is governed by a probable exponential relationship with laser fluence, suggesting that laser-processing fluences near silicon melting should be carefully chosen for minimizing induced defects and electrical property degradation. Finally, the control of laser-induced defects was demonstrated through laser or furnace post annealing of laser-processed solar cell devices. After two laser-annealing steps, the open circuit voltage, fill factor, series resistance, and shunt resistance were significantly improved, resulting in an increased photovoltaic efficiency.