Quantum efficiency of femtosecond-laser sulfur hyperdoped silicon solar cells after different annealing regimes
Annealing dependent absorption and quantum efficiency spectra are determined on femtosecond-laser, sulfur hyperdoped silicon solar cells in a spectral range from 300 to 2500 nm. Although the samples, which are rapidly quenched after annealing at T = 1250 °C, do not feature the highest sub-bandgap absorption, the highest sub-bandgap quantum efficiency is measured on the same level as on non-annealed samples, featuring the highest sub-bandgap absorption. Findings on annealing dependent absorption are carried over, in order to explain the measured quantum efficiency spectra. In the sub-bandgap spectral range conversion is more efficient, when deep sulfur centers are obtained from annealing at higher temperatures at preferably rapid quenching. For those annealing regimes the above-bandgap quantum efficiency is decreased, which is ascribed to less shallow sulfur donors. Lower temperatures or slow cooling rates results in shallow donors, featuring a more efficient conversion in the above-bandgap spectral range. This is on the expense of deep sulfur centers and the corresponding sub-bandgap conversion ability. Therewith it is concluded, that the main absorption occurs only in the laser-induced sulfur emitter layer. Furthermore, to some extent a prediction is proposed on sulfur hyperdoped silicon prepared by ion implantation and subsequent pulsed laser melting, featuring rapid cooling from the melt.