Femtosecond-laser microstructuring of silicon for photovoltaic devices

Presentation Date: 

Tuesday, January 24, 2006

Location: 

Photonics West 2006 (San Jose, CA)

Presentation Slides: 

Photovoltaics research has recently focused on photovoltaic materials made by cheaper processes with minimal waste such as thin-films grown by chemical vapor deposition. Because silicon is the most common semiconductor material and the second most abundant element in the earth, silicon-based thin films are an excellent choice for photovoltaics. The drawback to crystalline silicon thin films is low absorption due to silicon’s indirect band gap. Thicker films increase processing costs and sacrifice efficiency due to defects inherent in the thin-films.

We report the creation of a thin, highly absorbing layer on silicon surfaces using the intense conditions at the focus of a high-intensity, femtosecond laser pulse. Irradiation of a silicon surface with 100-femtosecond laser pulses in the presence of sulfur containing gases creates a highly doped layer of nanocrystalline silicon in the top several hundred nanometers. The incorporation of this thin layer results in near unity absorption of light from the near-ultraviolet (250 nm) to the near-infrared (2500 nm), including wavelengths normally invisible to silicon (> 1100 nm). After thermal annealing, a photovoltaic responsive junction is formed between the nanocrystalline silicon layer and the underlying silicon.

The high absorptance of a wide range of wavelengths of light in an extremely thin surface layer makes our process ideal for modifying and enhancing crystalline silicon thin film solar cells. Preliminary results on irradiation of a 1.5-µm thick silicon film resulted in a change of absorptance from 60% to 90% in the visible and from 10% to 40% in the near-infrared.