Ultra-low reflectance, high absorption microcrystalline silicon nanostalagmite
© Thiyagu et al; licensee Springer. 2012
Received: 28 November 2011
Accepted: 6 March 2012
Published: 6 March 2012
In this work, microcrystalline silicon nanostalagmite [μc-SiNS] arrays have been successfully fabricated on glass by catalytic etching process through a template. The template, polystyrene [PS] nanospheres, with diameter and density of 30 to approximately 50 nm and 1010/cm2, respectively, was obtained by a modified nanophase separation of PS-containing block copolymer. The length of μc-SiNS could be controlled by the duration of etching time. The μc-SiNS exhibits ultra-low reflection approximately 0.3% and absorption around 99% over 300 to 800 nm in wavelength. Reflection is also suppressed for a wide range of angles of incidence in wide range of wavelength. This indicates the extensive light-trapping effect by the μc-SiNS and could possibly harvest a large amount of solar energy at infrared regime.
Keywordsultra-low reflection microcrystalline silicon nanostalagmite polystyrene nanospheres light trapping
Silicon thin film solar cells are promising candidates for future generations of photovoltaic devices [1, 2]. They offer cost effectiveness and the possibility of deposition on flexible substrates [3–5]. However, the efficiency of thin film Si solar cell is relatively low compared to crystalline solar cell. The low absorption rate, relative poor material quality and narrow absorption spectra are the major factors. In the past few years, there is an enormously growing interest in the development of nanostructure materials to improve the light-harvesting efficiency for achieving high-efficiency Si thin film solar cell while maintaining low cost. Feasible silicon nanostructures such as silicon nanowires [SiNWs] have gained much attention due to their unique properties and possible applications in the fields of nanoelectronics [6–9], nanooptoelectronics [9, 10], nanophotovolatics [11–17] and for sensor applications . SiNWs are usually produced via vapour-liquid-solid [VLS] growth mechanism , which introduced one-dimensionality growing of nanostructure by a metal nanocatalyst droplet containing gases such as silane or grow from the gas phase by supplying Si vapour. However, the VLS growth mechanism generally requests high temperature that is not the adequate method for Si thin film nanostructure. In particular, the microcrystalline silicon [μc-Si] solar cells grown on glass or plastic substrate cannot sustain high temperature.
Nanoelectronics and nanooptoelectronics require vertically oriented, length tuneable and high density silicon nanostructures to obtain processing compatibility. To obtain such nanostructure, since some of the VLS growth does not occur, the wet chemical etching [20–23] through a predetermined template might be a possible method to achieve light-trapping structure at low cost. An efficient light management is essential to further improve the light confinement in the cells. Light trapping is the standard technique for improving thin film silicon efficiencies and exploiting the sunlight. In particular, μc-Si solar cells have gained considerable attention in the recent years. Wet chemically etched μc-Si surfaces by catalytic etching method that forms the μc-Si nanostalagmite show an ultra-low reflectance compared to Si thin film layers. This ultra-low reflectance is potentially fascinating for photovoltaic applications where enough absorption of solar light occurs in thinnest Si layer possible. The μc-SiNSs structure can be used as solar cell absorber.
In this study, we show the wet chemical etching of silicon nanostalagmite arrays with controlled length, size and density with a potentially high throughput by 30 to approximately 50 nm polystyrene [PS] spheres in a convenient method. It exhibits a black appearance, and they are almost non-reflective due to strong light scattering and absorption inside the μc-SiNSs structure. The reflectance and transmittance of the μc-SiNS are about 0.3% in average over the spectral range of 300 to 800 nm.
Results and discussion
In summary, we reported the fabrication of microcrystalline silicon nanostalagmite arrays on glass by catalytic etching process through 30 to approximately 50 nm PS nanosphere template. The length of nanostalagmite is defined by the duration of the etching process. The SiNSs arrays have low transmission, ultra-low reflection approximately 0.3% and high absorption around 99% compared to planar due to their strong light-trapping effect. Reflection is also suppressed for a wide range of angles of incidence in wide range of wavelength. This indicates the extensive light-trapping effect by the μc-SiNS and could possibly harvest a large amount of solar energy at infrared regime. The photocurrent could be largely enhanced with thin μc-Si layer in the future.
hydrogen peroxide: PECVD: plasma enhanced chemical vapour deposition
scanning electron microscope
We thank the National Science Council for financial support under grant NSC-99-ET-E-005-001-ET. The support from Bureau of Energy, Ministry of Economic Affairs, Taiwan (Republic of China) under contract no. A455DR1110 is highly acknowledged.
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