A novel ultrafast laser processing technique is used to create self-assembled micro/nano structures on a silicon surface for efficient light trapping. Under appropriate experimental conditions, light reflection (including scattering) of the Si surface has been reduced to less than 3% for the entire solar spectrum and the material appears completely black to the naked eye. A post-chemical cleaning is applied to remove laser-redeposited material and induced defects. Optical, morphological, and structural characterizations have been carried out on as-laser-treated and post-chemically cleaned surfaces. Finally, we report for the first time the total efficiency of over 14% and high external quantum efficiency (EQE) results on photovoltaic devices fabricated on the ultrafast-laser-induced micro/nano structured silicon wafer, which can be further improved upon process optimization.
We report a novel phenomenon of spontaneous formation of nearly
regular arrays of nanospikes atop conical microstructures by exposing
a germanium surface to femtosecond laser pulses in an environment of
SF6. Silicon laser texturing has been reported, but no information has been
published on laser microtexturing of Ge and in particular the observation
of nanospikes atop conical microstructures. The nanospikes are around
2 µm
high having a tip radius of 100 nm and are formed atop conical microstructures that are around
5 µm wide
and 10 µm
tall. The tip radius could be sharpened down to
∼10 nm by brief chemical etching. A higher laser fluence and fewer shots favour the formation of
nanospikes. By increasing the number of shots at higher fluence the surface morphology
changes from conical microstructures to tall straight-walled pillar-like structures
and the nanospikes disappear. The surface morphology of germanium has been
compared with silicon. The germanium samples turn completely black after laser
processing, i.e. they exhibit greatly reduced reflectivity throughout the visible spectrum.
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