Femtosecond laser hyperdoping and micro/nanotexturing of silicon for photovoltaics

We have developed a technique, optical hyperdoping, for doping semiconductors to unusually high levels and endowing them with remarkable optoelectronic properties. By irradiating silicon (Si) with a train of femtosecond laser pulses in the presence of heavy chalcogen (sulfur, selenium, and tellurium) compounds, a 100-300 nm thin layer of Si is doped to non-equilibrium levels (similar to 1 at. %). Hyperdoped silicon exhibits near-unity photon absorptance from the ultraviolet (lambda < 0.25 mu m) to the mid-infrared (lambda > 2.5 mu m), even though crystalline silicon is normally transparent to wavelengths lambda > 1.1 mu m due to its band gap at 1.1 eV. Concurrent to doping, we are also able to use fs-laser irradiation to create light-trapping surface textures on the micro- and nanometer scales. Together, optical hyperdoping and surface texturing represent a route towards high-performance thin film photovoltaic devices.