Hyperdoped and microstructured silicon for solar energy harvesting

We have developed a unique technique to significantly change the optoelectronic properties of silicon through hyperdoping and texturing. By irradiating silicon with a train of amplified femtosecond laser pulses in the presence of a wide variety of dopant precursors, we can hyperdope silicon to > 1 at.% in a 300-nm thin layer. In addition, laser-induced semi-periodic surface textures have excellent anti-reflection and light-trapping properties. The technique is robust: it is effective on both crystalline and amorphous silicon and for both thin films and thick substrates. When the dopant is chosen from the heavy chalcogen (sulfur, selenium, tellurium), fs-laser doped silicon exhibits near-unity light absorptance from the ultraviolet (λ > 0.25 μm) to the near infrared (λ < 5 μm), which is far beyond silicon’s bandgap at 1.1 μm.

Femtosecond laser hyperdoping achieves dopant concentrations higher than the equilibrium solubility limit through a process called solute trapping. Femtosecond laser pulses with energy greater than the melting threshold transform the surface into a molten layer, enabling dopant precursors in the vicinity to diffuse in. As the deposited energy diffuses into the substrate, the molten layer resolidifies with a speed faster than the rate at which thermodynamic equilibrium can be established (> 1 m/s), thus trapping the dopants in their hyper-concentrated state. The dopant precursors can be in the gas phase (such as SF6) or the solid phase (such as a thin film of Se or Te thermally deposited onto a Si substrate). Femtosecond laser texturing, on the other hand, originates from the formation of laser induced period surface structures (LIPSS), and consists of semi-periodic nanometer- and micrometer-scaled structures. Recently, we identified laser parameters for independently tuning the hyperdoping and texturing processes. We envision the fs-laser technique being used to produce new materials for many applications, especially in the area of solar energy.

The femtosecond laser hyperdoping technique endows silicon with remarkable optoelectronic properties, such as near unity absorption and a strong sensitivity to infrared wavelengths of light that pass through a standard silicon wafer. Laser hyperdoping creates defect- and bandgap engineered semiconductors, and laser texturing further enhances the optical density through excellent light trapping. Hyperdoped silicon devices represent the fruit of a novel fabrication technique for Earth-abundant, semiconductor-based solar energy harvesters with the potential for both low cost and high photoconversion efficiency.