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Optical properties of Si microwires combined with nanoneedles for flexible thin film photovoltaics

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Abstract

A combined wire structure, made up of longer periodic Si microwires and short nanoneedles, was prepared to enhance light absorption using one-step plasma etching via lithographical patterning. The combined wire array exhibited light absorption of up to ~97.6% from 300 to 1100 nm without an anti-reflection coating. These combined wire arrays on a Si substrate were embedded into a transparent polymer. A large-scale wire-embedded soft film was then obtained by peeling the polymer-embedded wire portion from the substrate. Optically attractive features were present in these soft films, making them suitable for use in flexible silicon solar cell applications.

©2010 Optical Society of America

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Figures (8)

Fig. 1
Fig. 1 Schematics showing the Si microwires (MWs) (a) and combined wires (b). Increasing the source gas ratio of the SF6/C4F8 leads to combined wires consisting of periodic MWs with short nanoneedles (NNs).
Fig. 2
Fig. 2 Cross-sectional SEM images of two different Si microwire samples. Sample (a) is ~20 µm in length and ~10 µm in pitch, while sample (b) is ~50 µm in length and ~7 µm in pitch. Wire diameters are all ~2 µm. Insets are plan-view images showing square-patterned, periodic arrays with packing fractions of ~3.1% (a), and ~6.4% (b).
Fig. 3
Fig. 3 Total reflection spectra of the Si microwires compared to a planar Si wafer as functions of wire length (L) and pitch (P). The reflection of the microwires has been reduced significantly compared to that of the planar Si wafer.
Fig. 4
Fig. 4 Typical SEM images showing the three different combined wires in which the microwire (MW) lengths are 35 (a), 50 (b), and 70 µm (c). In all samples, 5-µm-long, small nanoneedles (NNs) were integrated with the MWs. The pitches between MWs are all 7 µm. (d) A digital photograph showing the 8-inch waferscale uniformity. The combined wires (left) have much darker features without the shiny nature shown by the Si MWs (right). Wire lengths are ~50 µm.
Fig. 5
Fig. 5 Optical characterization of Si wire arrays. (a) Total reflection and absorption spectra of the combined wires with microwire lengths of 35 (black square), 50 (blue triangle), and 70 μm (red circle). All combined wires absorb >95% incident light in the visible region. (b) Specular reflection of the combined wires was greatly suppressed due to the presence of Si nanoneedles compared to that of Si microwires.
Fig. 6
Fig. 6 Cross-sectional SEM images showing the Si microwire-embedded-film (SWF) just separated from the silicon substrate (a) and the combined wire-embedded film coated with an Al back reflector (CWAF) (b). The inset in panel (a) shows a bird’s eye view of the wire bottoms. The inset in panel (b) clarifies the presence of nanoneedles (NNs) between the microwires (MWs) even after detachment from the substrate. (c) Periodic, uniform wire arrays embedded within the PDMS maintain their original morphologies irrespective of intentional distortions of the detached film. (d) A photograph showing the flexibility of a detached film.
Fig. 7
Fig. 7 Effective methods for enhancing light absorption are shown schematically with the following sequence SWF→CWF→SWAF→CWAF; (a) Si microwire-embedded film (SWF), (b) combined wire-embedded film (CWF), (c) Si microwire-embedded film coated with an Al back reflector (SWAF), and (d) combined wire-embedded film coated with an Al back reflector (CWAF). (a) A large portion of incident light is preferentially transmitted through the gap between wires. (b) Additional nanoneedles (NNs) suppress reflection via the GRI effect while reducing the amount of transmission due to the decreased gap. (c) The transmitted light in the SWF is reflected back into the wire array via the back reflection effect. (d) The nanostructured Al back reflector increases the light trapping in the wires by reflecting the incident light. Also, SPPs are excited along the Al-dielectric interface which is periodically nanostructured due to the concave menisci of the PDMS. A magnified view is shown in the inset.
Fig. 8
Fig. 8 Optical characterization of the wire-embedded films. (a) Total transmissions, (b) reflections, and (c) absorption spectra of the pure PDMS (black square), SWF (magenta triangle), CWF (green diamond), SWAF (blue triangle), and CWAF (red circle). The CWAF sample presents excellent absorption characteristics in which ≥90% of the incident light is absorbed in the wavelength range of 300−1100 nm. (d) Absorption enhancements of the CWF, SWAF, and CWAF samples are noted over the SWF used as reference values. The inset shows the remarkable increase in absorption enhancement due to the excitation of SPPs closer to the NIR region.
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