Large‐area fabrication of equidistant free‐standing Si crystals on nanoimprinted glass

Abstract

Significant progress in nanotechnology stimulated extensive research on novel Si-based photonic structures for optoelectronic devices and promising new technologies for the development of solar energy harvesting architectures in thin film photovoltaic applications [1 -8]. The substantial challenge is the development of inexpensive and easily scalable fabrication methods for the controlled implementation of such patterns into high quality Si thin film devices. Lithographic patterning and anisotropic etching processes as well as vapor -liquid -solid processes have been explored to prepare and successfully implement arrays of crystalline Si wires in device structures [5 -7]. However, the existing methods either rely on high-quality substrates, or face scalability challenges, or do not comply with the standards of a low-cost production process. This Letter presents a versatile and scalable preparation process for tailored periodic arrays of Si crystals on imprinted glass templates by employing exclusively low-cost large-area fabrication techniques, such as nanoimprint lithography, high-rate electron-beam (e-beam) evaporation, solid phase crystallization (SPC) and etching. Nanoimprint lithography is an elegant and scalable technique to economically prepare well-defined structures on large areas with abundant design possibilities [8 -10]. The preparation of solid phase crystallized Si is a promising approach for the fabrication of solar cells with high electronic quality, yielding a conversion efficiency of 10% for Si deposited by plasma enhanced chemical vapor deposition (PECVD) [11]. Recently, we successfully demonstrated that the PECVD process can be replaced by high-rate e-beam evaporation utilizing deposition rates of 600 nm/min [12]. The singular combination of nanoimprint lithography with the emerging high-rate deposition technique e-beam evaporation of amorphous Si enables the controlled design of periodic Si nanostructures.A two-dimensional periodic pattern with a 2 µm pitch was imprinted in a sol-gel coated glass template by repli-By combining nanoimprint lithography with the emerging high-rate deposition technique electron-beam evaporation of amorphous Si, we developed a low-cost fabrication process for the design of periodic arrays of Si crystals on large areas of 50 cm 2 in a solid phase crystallization and subsequent selective etch process. The method allows for precise control over the feature size of the crystals. The promising absorption properties of the features and the versatility and simplicity of the preparation process inspire the development of threedimensional solar cell architectures and tailored large-area photonic crystals.Cross-sectional SEM image of periodic arrays of Si crystals on nanoimprinted glass.

Double-side textured liquid phase crystallized silicon thin-film solar cells on imprinted glass

“…Nanoimprintlithography has been identified as a promising technology for photovoltaics as it permits manifold degree of freedom to systematically control size and shape of the light trapping features at low fabrication costs [15][16][17][18][19][20]4]. The challenge is now to implement nanophotonic light management concepts into LPC silicon thinfilm solar cell devices maintaining the bulk material quality including a smart interface design.…”

Deterministic composite nanophotonic lattices in large area for broadband applications

“…Subsequent thermal annealing at 600 °C for several hours in a tube furnace within an N 2 atmosphere results in solid phase crystallization of the silicon film aside from the columnar substrate features. The still amorphous Si parts around the columns are removed by wet-chemical etching in an etch solution consisting of concentrated HNO 3 (65%, 30 parts), concentrated H 3 PO 4 (85%, 10 parts), HF (50%, 1 part) and H 2 O (43 parts) resulting in nanocone-nanoholes28. The nanoholes are realized by mechanically detaching the sol-gel columns by abrasion.…”

Light harvesting architectures for electron beam evaporated solid phase crystallized Si thin film solar cells: Statistical and periodic approaches

“…5 illustrates wavelength dependent absorptance of periodic arrays of 4 μm high Si crystals in comparison to a 4 μm thick poly-Si layer on planar glass. The absorptance in the near IR region is drastically improved in comparison to a planar structure [20].…”

“…Fig. 4(b) shows a cross-sectional SEM image of the realized periodic arrays consisting of 2 μm tall cone-shaped freestanding crystalline features [20].…”

“…Concluding, the structure consists of separated crystalline Si cones on the tips of the imprinted structure, which are surrounded by a funnel-shaped amorphous matrix. The polycrystalline features on the bottom of the imprint structure are interconnected and form a polycrystalline bottom layer [20].…”