Enhanced light absorption in silicon by phase engineering

(Nanowerk Spotlight) Silicon (Si), which crystallizes in the diamond cubic (dc) lattice at room temperature and pressure, is known to exhibit several polymorphs at high-pressures. The dc-Si is a celebrated material for current electronics and solar cell applications. Researchers are always looking for new ways to improve the optoelectronic properties of Si to design low-cost and light-weight solar cells.
In 2008, researchers from the University of California at Berkeley performed first-principles calculations and showed that the rhombohedral phase (R8) of silicon, which is stable at ambient temperature, has interesting optoelectronic properties suitable for a solar absorber application (Physical Review B, "Ab initio study of the optical properties of Si-XII").
Since then, no experimental studies have been published in utilizing the R8-Si for any application. However, researchers have thoroughly investigated the properties of indentation and diamond anvil cell- assisted phase engineered R8-Si using in-situ and ex-situ advanced characterization methods. Further, there is no success yet on the direct growth or synthesis of either thin-film or single crystal forms of R8-Si and its usage for solar cell applications.
New work led by Dr. Abhay Sagade and Dr. Kiran Mangalampalli from the SRM Institute of Science and Technology, Kattankulathur, Chennai, India, shows the formation of large-area R8-Si within the top n-layer of the p-n junction Si solar cell using spherical nanoindentation, and experimentally measured optical properties of R8-Si to demonstrate ten times enhancement in its photocurrent density.
The researchers published their results in the journal Scripta Materallia ("On the indentation-assisted phase engineered Si for solar applications").
Enhanced light absorption in silicon
Left: Variation of the refractive index of dc-Si (pristine cell) and R8-Si (indented cell). Right: Schematic of a cross-sectional view of single indentation induced R8-Si (green) in dc-Si (blue). On illumination, the light absorption ability of R8-Si is more (thick red arrows) compared to dc-Si (thin red arrows). Here, ε stands for dielectric function. (click on image to enlarge)
The key aspects of the study are: 1) the absorbing layer of R8-Si is made out of the same material (top n-layer of dc-Si); 2) as the interface between the R8-Si and the active layer (n-Si) is contamination-free, the series resistance of the cell can be minimized; and 3) since the R8-Si absorbs a greater fraction of the incident light, the thickness (~1-10 µm) can be well-controlled by optimizing the indentation conditions and therefore, the weight of the cell can be further reduced.
The researchers have shown that how the contribution of light-scattering effects are minimal and R8-Si plays an important role in absorbing the light.
"We provide the first experimental evidence of the theoretical results predicted in 2008 and our results highlight the potential of such phase engineered R8-Si integration into the dc-Si p-n junction solar cells which pave a path in realizing low-cost, light-weight and high-efficiency solar cells," conclude Sagade and Kiran.
Provided by SRM Institute of Science and Technology, Kattankulathur, Chennai as a Nanowerk exclusive

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