Eveline Rudigier-Voigt

Nanophotonic light trapping in 3-dimensional thin-film silicon architectures

Emerging low cost and large area periodic texturing methods promote the fabrication of complex absorber structures for thin film silicon solar cells. We present a comprehensive numerical analysis of a 2 μm square periodic polycrystalline silicon absorber architecture designed in our laboratories. Simulations are performed on the basis of a precise finite element reconstruction of the experimentally realized silicon structure. In contrast to many other publications, superstrate light trapping effects are included in our model. Excellent agreement to measured absorptance spectra is obtained. For the inclusion of the absorber into a standard single junction cell layout, we show that light trapping close to the Yablonovitch limit can be realized, but is usually strongly damped by parasitic absorption.

Large-area 2D periodic crystalline silicon nanodome arrays on nanoimprinted glass exhibiting photonic band structure effects

Two-dimensional silicon nanodome arrays are prepared on large areas up to 50 cm² exhibiting photonic band structure effects in the near-infrared and visible wavelength region by downscaling a recently developed fabrication method based on nanoimprint-patterned glass, high-rate electron-beam evaporation of silicon, self-organized solid phase crystallization and wet-chemical etching. The silicon nanodomes, arranged in square lattice geometry with 300 nm lattice constant, are optically characterized by angular resolved reflection measurements, allowing the partial determination of the photonic band structure. This experimentally determined band structure agrees well with the outcome of three-dimensional optical finite-element simulations. A 16% photonic bandgap is predicted for an optimized geometry of the silicon nanodome arrays. By variation of the duration of the selective etching step, the geometry as well as the optical properties of the periodic silicon nanodome arrays can be controlled systematically.

Quasicrystalline-structured light harvesting nanophotonic silicon films on nanoimprinted glass for ultra-thin photovoltaics

We present nanophotonic light harvesting crystalline silicon (c-Si) thin films on glass exhibiting ten-fold transversely quasicrystalline lattice geometry on 6 x 8 mm2 area. The c-Si architectures with a nearest neighbor distance of 650 nm are fabricated by nanoimprinting the desired quasicrystalline geometry into sol-gel coated glass substrates followed by Si deposition of 240 nm to 270 nm thickness, self-organized solid phase crystallization and selective chemical etching. Broadband absorption measurements on these quasicrystalline-structured c-Si architectures yield a very significant improvement in light trapping in the near infrared regime and an enhanced light coupling due to a graded index effect in comparison to the unstructured sample. The average value of maximum achievable short circuit current density jsc, max of solar cells with such quasicrystalline-structured c-Si absorber geometry (19.3 mA/cm2) is more than double in comparison to the jsc, max of unstructured planar films of the same thickness (9.3 mA/cm2) and remains stable for light incident angles up to 60°. In comparison to a 320 nm thick c-Si film on textured ZnO:Al substrate as widely used for light trapping in amorphous-microcrystalline Si thin-film photovoltaics, still a 65% increased jsc, max is observable for the presented quasicrystalline c-Si structures. The nanophotonic light trapping efficiency of these transversely quasicrystalline c-Si nanoarchitectures is among the highest values for experimentally realized structures, revealing their promising influence for broadband and isotropic light trapping for economically viable and efficient ultra-thin solar cells.

A novel light trapping concept for liquid phase crystallized poly-Si thin-film solar cells on periodically nanoimprinted glass substrates

Large grained polycrystalline silicon (poly-Si) absorbers were realized by electron beam induced liquid phase crystallization on 2 μm periodically patterned glass substrates and processed into a-Si:H/poly-Si heterojunction thin-film solar cells. The substrates were structured by nanoimprint lithography using a UV curable hybrid polymer sol-gel resist, resulting in a glassy high-temperature stable micro-structured surface. Structural analysis yielded high quality poly-Si material with grain sizes up to several hundred micrometers. An increase of absorption and an enhancement of the external quantum efficiency in the NIR as a consequence of light trapping due to the micro-structured poly-Si/substrate interface were observed. Up to now, only moderate solar cell parameters, a maximum open-circuit voltage of 413 mV and a short-circuit current density of 8 mA cm-2, were measured being significantly lower to what can be achieved with liquid phase crystallized poly-Si thin-film solar cells on planar glass substrates indicating that the substrate texture has impact on the electrical material quality. By reduction of the SiC interlayer thickness at the micro-structured poly- Si/substrate interface defect-related parasitic absorption was considerably minimized. This encourages the implementation of nanoimprinted tailored substrate textures for light trapping in liquid phase crystallized poly-Si thinfilm solar cells.

Light harvesting quasicrystalline nanophotonic structures for crystalline silicon thin-film solar cells

We present our results on optical absorption enhancement in crystalline silicon (c-Si) absorber structured with transversely quasicrystalline lattice geometry for thin-film photovoltaics. c-Si nanoarchitectures are prepared on the nanoimprinted ten-fold symmetry quasicrystalline textured substrate. The structural features of the fabricated Si nanostructures are analyzed to confirm the defining characteristics of the quasicrystalline texturing of the absorber film. We present the optical absorption plots for a spectrum of incident light for varying angle of light incidence in these fabricated higher symmetry crystalline Si architectures. Neither any back reflector nor antireflection coating is considered in the present study, where use of such layers could further improve the light absorption. The realized quasicrystalline textured silicon nanoarchitectures with higher rotational symmetry lattice geometry are observed to improve the isotropic and broad band absorption properties of the thin film c-Si absorber and envisaged to have efficiency enhanced thin film photovoltaics effective in terms of cost and performance.

Correlation between structural and opto-electronic characteristics of crystalline Si microhole arrays for photonic light management

By employing electron paramagnetic resonance spectroscopy, transmission electron microscopy, and optical measurements, we systematically correlate the structural and optical properties with the deep-level defect characteristics of various tailored periodic Si microhole arrays, which are manufactured in an easily scalable and versatile process on nanoimprinted sol-gel coated glass. While tapered microhole arrays in a structured base layer are characterized by partly nanocrystalline features, poor electronic quality with a defect concentration of 1017 cm−3 and a high optical sub-band gap absorption, planar polycrystalline Si layers perforated with periodic arrays of tapered microholes are composed of a compact crystalline structure and a defect concentration in the low 1016 cm−3 regime. The low defect concentration is equivalent to the one in planar state-of-the-art solid phase crystallized Si films and correlates with a low optical sub-band gap absorption. By complementing the experimental characterization...

Nanophotonic light trapping in polycrystalline silicon thin-film solar cells using periodically nanoimprint-structured glass substrates

A smart light trapping scheme is essential to tap the full potential of polycrystalline silicon (poly-Si) thin-film solar cells. Periodic nanophotonic structures are of particular interest as they allow to substantially surpass the Lambertian limit from ray optics in selected spectral ranges. We use nanoimprint-lithography for the periodic patterning of sol-gel coated glass substrates, ensuring a cost-effective, large-area production of thin-film solar cell devices. Periodic crystalline silicon nanoarchitectures are prepared on these textured substrates by high-rate silicon film evaporation, solid phase crystallization and chemical etching. Poly-Si microhole arrays in square lattice geometry with an effective thickness of about 2μm and with comparatively large pitch (2 μm) exhibit a large absorption enhancement (A900nm = 52%) compared to a planar film (A900nm ~ 7%). For the optimization of light trapping in the desired spectral region, the geometry of the nanophotonic structures with varying pitch from 600 nm to 800 nm is tailored and investigated for the cases of poly-Si nanopillar arrays of hexagonal lattice geometry, exhibiting an increase in absorption in comparison to planar film attributed to nanophotonic wave optic effects. These structures inspire the design of prospective applications such as highly-efficient nanostructured poly-Si thin-film solar cells and large-area photonic crystals.