Scanning electron micrographs show that with a femtosecond laser beam, a corrugated structure can be formed on the silicon surface. This surface structure absorbs more light than an untreated surface, thereby increasing the efficiency of solar cells.
A*STAR scientists scanned the femtosecond electron laser beam on the silicon surface to form a uniform nanoscale ripple structure over a wide area of ​​the silicon surface. Given that the corrugated surface is much less reflective than a smooth surface, this innovation can enable solar cells to obtain more light, thereby increasing solar cell efficiency.
At present, the use of lasers to create intermittent surface structures is the focus of research. Laser processing has its important advantage: it only heats the surface of the material without affecting the underlying structure. However, many laser processing methods have their own limitations: they process very small areas and the processed corrugations are very light.
Today, Xincai Wang and his A*STAR Singapore Institute of Manufacturing Technology and Nanyang Technological University colleagues successfully solved these limitations and demonstrated the potential of their technology: on silicon substrates, they use the technology in 30 mm × The 30 mm wide area creates a uniform corrugated structure with an average corrugation depth of 300 nanometers—three times the corrugation depth of other technologies.
“This increase in depth can effectively reduce the degree of light reflection in the corrugated structure and improve the light capture capability of the corrugated structure,†Wang said. “So, when this structure is used in photovoltaic equipment, more light can be captured, which will increase the Equipment efficiency."
This technique is simple and inexpensive, using only a cylindrical lens to expand the femtosecond laser beam width to 50 microns and then scanning it onto the surface.
When the photon energy of the laser is greater than the band gap of silicon, photons will excite electrons to transition from the valence band to the conduction band. These electrons then release energy, transferring energy to the atomic lattice and heating it. However, because the pulse duration is very short, they will generate electron waves on the surface instead. This in turn generates a light wave that interferes with the incident laser beam. Where incoming light waves and outgoing light waves interfere with each other, a portion of the silicon is removed, creating a concave area in the corrugated structure.
The researchers found that after forming the corrugated structure, the average reflectance of the silicon surface dropped from 39.7% to 12.5% , which means that due to the strong scattering of the corrugated structure, the light absorption rate of the silicon surface increased by 41% . This effect can be used to manage the "behavior" of photons in solar cells and light emitting diodes.
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