Soft elastomeric nanopillar stamps for enhancing absorption in organic thin-film solar cells

Toward Scalable Flexible Nanomanufacturing for Photonic Structures and Devices

Continuous and scalable nanopatterning over flexible substrates is highly desirable for both commercial and scientific interests, but is difficult to realize with traditional photolithographic processes. The recent advancements in nanofabrication methodologies enable light management with photonic structures on flexible materials, providing an increasingly popular strategy to control the light harvesting in the optoelectronic devices of photovoltaics, and in organic and inorganic light-emitting diodes. Here, the current status of nanopatterning technologies for the fabrication of optoelectronic devices is summarized. Scalable nanopatterning technologies for nanomanufacturing on flexible materials are emphasized. Critical challenges in various patterning techniques when considering the resolution, scalability, processing throughput, and the use of masks and resists are addressed. The integration of flexible nanopatterned substrates with light manipulation in organic optoelectronic devices is also discussed; this enables the control of light flux and spectra. Finally, potential development directions are highlighted.

Light Manipulation in Organic Photovoltaics

Organic photovoltaics (OPVs) hold great promise for next-generation photovoltaics in renewable energy because of the potential to realize low-cost mass production via large-area roll-to-roll printing technologies on flexible substrates. To achieve high-efficiency OPVs, one key issue is to overcome the insufficient photon absorption in organic photoactive layers, since their low carrier mobility limits the film thickness for minimized charge recombination loss. To solve the inherent trade-off between photon absorption and charge transport in OPVs, the optical manipulation of light with novel micro/nano-structures has become an increasingly popular strategy to boost the light harvesting efficiency. In this Review, we make an attempt to capture the recent advances in this area. A survey of light trapping schemes implemented to various functional components and interfaces in OPVs is given and discussed from the viewpoint of plasmonic and photonic resonances, addressing the external antireflection coatings, substrate geometry-induced trapping, the role of electrode design in optical enhancement, as well as optically modifying charge extraction and photoactive layers.

Flexible Near-Field Nanopatterning with Ultrathin, Conformal Phase Masks on Nonplanar Substrates for Biomimetic Hierarchical Photonic Structures

Multilevel hierarchical platforms that combine nano- and microstructures have been intensively explored to mimic superior properties found in nature. However, unless directly replicated from biological samples, desirable multiscale structures have been challenging to efficiently produce to date. Departing from conventional wafer-based technology, new and efficient techniques suitable for fabricating bioinspired structures are highly desired to produce three-dimensional architectures even on nonplanar substrates. Here, we report a facile approach to realize functional nanostructures on uneven microstructured platforms via scalable optical fabrication techniques. The ultrathin form (∼3 μm) of a phase grating composed of poly(vinyl alcohol) makes the material physically flexible and enables full-conformal contact with rough surfaces. The near-field optical effect can be identically generated on highly curved surfaces as a result of superior conformality. Densely packed nanodots with submicron periodicity are uniformly formed on microlens arrays with a radius of curvature that is as low as ∼28 μm. Increasing the size of the gratings causes the production area to be successfully expanded by up to 16 in(2). The "nano-on-micro" structures mimicking real compound eyes are transferred to flexible and stretchable substrates by sequential imprinting, facilitating multifunctional optical films applicable to antireflective diffusers for large-area sheet-illumination displays.

Multifunctional polymers with biomimetic compound architectures via nanoporous AAO films for efficient solar energy harvesting in dye-sensitized solar cells

Polydimethylsiloxane with a biomimetic compound architecture consisting of hierarchical nanobumps/microcone arrays as a protective cover-layer for DSSCs improves solar power conversion efficiency.

Enhanced light absorption in thin-film solar cells with light propagation direction conversion

Enhancement of optical absorption in thin-film solar cells (TF-SCs) has been the long-lasting issue to achieve high efficiencies. Grating couplers have been studied for the conversion of incident light into guided modes propagating along TF-SCs to extend optical path for higher optical absorption. However the wavelength band for the efficient conversion remained relatively narrow and the overall improvement of TF-SC efficiencies has been limited. This paper demonstrates that the grating height design as well as the phase matching condition is important for the enhancement of optical absorption in TF-SCs with the calculation of short-circuit currents as a figure of merit for optimization. The influence of the light absorption coefficients and grating coupling strengths on the light absorption bandwidth is also discussed.