Fabricating SU-8 Photoresist Microstructures with Controlled Convexity-Concavity and Curvature through Thermally Manipulating Capillary Action in Poly(dimethylsiloxane) Microholes

We present a simple, robust, and cheap microfabrication method, based on thermally manipulating capillary action in poly(dimethylsiloxane) (PDMS) microholes, for preparing SU-8 curved microstructures. The microstructure morphology including convexity-concavity and curvature can be controlled via tuning the formation temperature. The convex SU-8 microspherical crowns with a height of 40 μm were formed at 10 °C, whereas the concave SU-8 microspherical crowns with a height of 90 μm were formed at 100 °C. The morphology of the microstructures is dictated by the thermally controlled combination of the pressure difference across the interface, contact angle, and surface tension. The fabricated microstructures with a spherical surface can be used as a microlens array or a mold for producing a microlens array. The clear and uniform images were observed using the generated microlens arrays. The equilibrium morphology of the microstructures can be predicted by numerical simulation, which can lessen the number of experiments and thus the design cost. The proposed method has the potential to find applications in industrial fields.

Curvature-Adjustable Polymeric Nanolens Fabrication Using UV-Controlled Nanoimprint Lithography

Nanolenses are gaining importance in nanotechnology, but their challenging fabrication is thwarting their wider adoption. Of particular challenge is facile control of the lens' curvature. In this work, we demonstrate a new nanoimprinting technique capable of realizing polymeric nanolenses in which the nanolens' curvature is optically controlled by the ultraviolet (UV) dose at the pre-curing step. Our results reveal a regime in which the nanolens' height changes linearly with the UV dose. Computational modeling further uncovers that the polymer undergoes highly nonlinear dynamics during the UV-controlled nanoimprinting process. Both the technique and the process model will greatly advance nanoscale science and manufacturing technology.

Light-Trapping Electrode for the Efficiency Enhancement of Bifacial Perovskite Solar Cells

Antireflection and light-trapping coatings are important parts of photovoltaic architectures, which enable the reduction of parasitic optical losses, and therefore increase the power conversion efficiency (PCE). Here, we propose a novel approach to enhance the efficiency of perovskite solar cells using a light-trapping electrode (LTE) with non-reciprocal optical transmission, consisting of a perforated metal film covered with a densely packed array of nanospheres. Our LTE combines charge collection and light trapping, and it can replace classical transparent conducting oxides (TCOs) such as ITO or FTO, providing better optical transmission and conductivity. One of the most promising applications of our original LTE is the optimization of efficient bifacial perovskite solar cells. We demonstrate that with our LTE, the short-circuit current density and fill factor are improved for both front and back illumination of the solar cells. Thus, we observe an 11% improvement in the light absorption for the monofacial PSCs, and a 15% for the bifacial PSCs. The best theoretical results of efficiency for our PSCs are 27.9% (monofacial) and 33.4% (bifacial). Our study opens new prospects for the further efficiency enhancement for perovskite solar cells.

Bioinspired Compound Eyes for Diffused Light-Harvesting Application

Natural compound eyes endow arthropods with wide-field high-performance light-harvesting capability that enables them to capture prey and avoid natural enemies in dim light. Inspired by natural compound eyes, a curved artificial-compound-eye (cACE) photodetector for diffused light harvesting is proposed and fabricated, and its light-harvesting capability is systematically investigated. The cACE photodetector is fabricated by introducing a cACE as a light-harvesting layer on the surface of a silicon-based photodetector, with the cACE being prepared via planar artificial-compound-eye (pACE) template deformation. The distinctive geometric morphology of the as-prepared cACE effectively reduces its surface reflection and the dependence of the projected area on the incident light direction, thereby significantly improving the light-harvesting ability and output photocurrent of the silicon-based photodetector. Furthermore, the performances of cACE, pACE, and bare polydimethylsiloxane (PDMS)-attached photodetectors as diffused light detectors are investigated under different luminances. The cACE-photodetector output photocurrent is 1.395 and 1.29 times those of the bare PDMS-attached and pACE photodetectors, respectively. Moreover, this photodetector has a desirable geometric shape. Thus, the proposed cACE photodetector will facilitate development of high-performance photodetectors for luminance sensing.

Organic Nanostructured Materials for Sustainable Application in Next Generation Solar Cells

Meeting our current energy demands requires a reliable and efficient renewable energy source that will bring balance between power generation and energy consumption. Organic photovoltaic cells (OPVs), perovskite solar cells and dye-sensitized solar cells (DSSCs) are among the next-generation technologies that are progressing as potential sustainable renewable energy sources. Since the discoveries of highly conductive organic charge-transfer compounds in the 1950s, organic semiconductors have captured attention. Organic photovoltaic solar cells possess key characteristics ideal for emerging next-generation technologies such as being nontoxic, abundant, an inexpensive nanomaterial with ease of production, including production under ambient conditions. In this review article, we discuss recent methods developed towards improving the stability and average efficiency of nanostructured materials in OPVs aimed at sustainable agriculture and improve land-use efficiency. A comprehensive overview on developing cost-effective and user-friendly organic solar cells to contribute towards improved environmental stability is provided. We also summarize recent advances in the synthetic methods used to produce nanostructured active absorber layers of OPVs with improved efficiencies to supply the energy required towards ending poverty and protecting the planet.

Characterization of optical manipulation using microlens arrays depending on the materials and sizes in organic photovoltaics

Various physical structures have improved light-harvesting and power-conversion efficiency in organic photovoltaic devices, and optical simulations have supported the improvement of device characteristics. Herein, we experimentally investigated how microlens arrays manipulate light propagation in microlens films and material stacks for organic photovoltaics to understand the influence of the constituent materials and sizes of the microlens. As materials to fabricate a microlens array, poly(dimethylsiloxane) and Norland Optical Adhesive 63 were adopted. The poly(dimethylsiloxane) microlens array exhibited higher total transmittance and higher diffuse transmittance, further enhancing the effective optical path and light extinction in material stacks for organic photovoltaics. This resulted in more current generation in an organic photovoltaic device with a poly(dimethylsiloxane) microlens array than in a Norland Optical Adhesive 63 microlens array. The sizes of the microlenses were controlled from 0.5 to 10 μm. The optical characteristics of microlens array films and material stacks with microlenses generally increased with size of the microlens, leading to a 10.6% and 16.0% improvement in the light extinction and power-conversion efficiency, respectively. In addition, electron and current generation in material stacks for organic photovoltaics were calculated from light extinction. The theoretical current generation matched well with experimental values derived from organic photovoltaic devices. Thus, the optical characterization of physical structures helps to predict how much more current can be generated in organic photovoltaic cells with a certain physical structure; it can also be used for screening the physical structures of organic photovoltaic cells.

The influence of constituent materials and sizes of a microlens was experimentally and theoretically explored.

Multi-layer light trapping structures for enhanced solar collection

Light trapping is a commonly used technique for enhancing the efficiency of solar collection in many photovoltaic (PV) devices. In this paper, we present the design of multi-layer light trapping structures that can potentially be retrofitted, or directly integrated, onto crystalline or amorphous silicon solar panels for enhanced optical collection at normal and extreme angle of incidence. This approach can improve the daily optical collection performance of solar panel with and without internally integrated light trapping structure by up to 7.18% and 159.93%, respectively. These improvements predict an enhancement beyond many research level and commercially deployed light trapping technologies. We further enhance this performance by combining our multi-layer optics with high refractive index materials to achieve a daily optical collection of up to 32.20% beyond leading light trapping structures. Our additive light trapping designs could enable the upgradeability of older PV technologies and can be tailored to optimally operate at unique angular ranges for building exteriors or over a wide range of incidence angle for applications such as unmanned aerial vehicles.

Mini-Review on Bioinspired Superwetting Microlens Array and Compound Eye

Microlens arrays (MLAs) and MLA-based artificial compound eyes (ACEs) are the important miniaturized optical components in modern micro-optical systems. However, their optical performance will seriously decline once they are wetted by water droplets (such as fog, dew, and rain droplets) or are polluted by contaminations in a humid environment. In this mini-review, we summarize the research works related to the fabrication of superwetting MLAs and ACEs and show how to integrate superhydrophobic and superoleophobic microstructures with an MLA. The fabrication strategy can be split into two categories. One is the hybrid pattern composed of the MLA domain and the superwetting domain. Another is the direct formation of superwetting nanostructures on the surface of the microlenses. The superhydrophobicity or superoleophobicity endows the MLAs and ACEs with liquid repellence and self-cleaning function besides excellent optical performance. We believe that the superwetting MLAs and ACEs will have significant applications in various optical systems that are often used in the humid or liquid environment.

Structurally assisted super black in colourful peacock spiders

Male peacock spiders (Maratus, Salticidae) compete to attract female mates using elaborate, sexually selected displays. They evolved both brilliant colour and velvety black. Here, we use scanning electron microscopy, hyperspectral imaging and finite-difference time-domain optical modelling to investigate the deep black surfaces of peacock spiders. We found that super black regions reflect less than 0.5% of light (for a 30° collection angle) in Maratus speciosus (0.44%) and Maratus karrie (0.35%) owing to microscale structures. Both species evolved unusually high, tightly packed cuticular bumps (microlens arrays), and M. karrie has an additional dense covering of black brush-like scales atop the cuticle. Our optical models show that the radius and height of spider microlenses achieve a balance between (i) decreased surface reflectance and (ii) enhanced melanin absorption (through multiple scattering, diffraction out of the acceptance cone of female eyes and increased path length of light through absorbing melanin pigments). The birds of paradise (Paradiseidae), ecological analogues of peacock spiders, also evolved super black near bright colour patches. Super black locally eliminates white specular highlights, reference points used to calibrate colour perception, making nearby colours appear brighter, even luminous, to vertebrates. We propose that this pre-existing, qualitative sensory experience—‘sensory bias’—is also found in spiders, leading to the convergent evolution of super black for mating displays in jumping spiders.

Cross-scale additive direct-writing fabrication of micro/nano lens arrays by electrohydrodynamic jet printing

High-quality micro/nanolens arrays (M/NLAs) are becoming irreplaceable components of various compact and miniaturized optical systems and functional devices. There is urgent requirement for a low-cost, high-efficiency, and high-precision technique to manufacture high-quality M/NLAs to meet their diverse and personalized applications. In this paper, we report the one-step maskless fabrication of M/NLAs via electrohydrodynamic jet (E-jet) printing. In order to get the best morphological parameters of M/NLAs, we adopted the stable cone-jet printing mode with optimized parameters instead of the micro dripping mode. The optical parameters of M/NLAs were analyzed and optimized, and they were influenced by the E-jet printing parameters, the wettability of the substrate, and the viscosity of the UV-curable adhesive. Thus, diverse and customized M/NLAs were obtained. Herein, we realized the fabrication of nanolens with a minimum diameter of 120 nm, and NLAs with different parameters were printed on a silicon substrate, a cantilever of atomic force microscopy probe, and single-layer graphene.

Microlens array enhanced upconversion luminescence at low excitation irradiance

The dearth of high upconversion luminescence (UCL) intensity at low excitation irradiance hinders the prevalent application of lanthanide-doped upconversion nanoparticles (UCNPs) in many fields ranging from optical bioimaging to photovoltaics. In this work, we propose to use microlens arrays (MLAs) as spatial light modulators to manipulate the distribution of excitation light fields in order to increase UCL, taking advantage of its nonlinear response to the excitation irradiance. We show that multicolored UCL from NaYF₄:Yb³⁺,Er³⁺@NaYF₄:Yb³⁺,Nd³⁺ and NaYF₄:Yb³⁺,Tm³⁺@NaYF₄:Yb³⁺,Nd³⁺ core/shell UCNPs can be increased by more than one order of magnitude under either 980 or 808 nm excitation, by simply placing a polymeric MLA onto the top of these samples. The observed typical green (525/540 nm) and red (654 nm) UCL bands from Er³⁺ and a blue (450/475 nm) UCL band from Tm³⁺ exhibit distinct enhancement factors due to their different multi-photon processes. Importantly, our ray tracing simulation reveals that the MLA is able to spatially confine the excitation light (980 and 808 nm) by orders of magnitude, thus amplifying UCL by more than 225-fold (the 450 nm UCL band of Tm³⁺) at low excitation irradiance. The proposed MLA method has immediate ramifications for the improved performance of all types of UCNP-based devices, such as UCNP-enhanced dye sensitized solar cells demonstrated here.

Control of Femtoliter Liquid on a Microlens: A Way to Flexible Dual-Microlens Arrays

Microlens arrays are key elements for light management in optoelectronic devices. The recent advancement in the wearable intelligent electronics has driven the development of flexible microlenses. In this work, we show a controllable and scalable surface-droplet-based strategy to create unconventional flexible polymer microlens arrays. The technique is underpinned by the morphological transition of femtoliter liquid on the surface of a microlens surrounded by a planar area. We found that the droplet liquid wetted the rim of the microlens first and gradually moved upward to the microlens surface with an increase in the liquid volume. The morphology evolution of the droplet is in good agreement with the predication from our simulations based on the interfacial energy minimization under the condition of the pinned boundary. The shape of the droplet on the microlens is well controlled by the droplet volume, aspect ratio of the microlens, and the interfacial energy of the droplets on the microlens. As a result, the obtained structures of one microlens partially covered by a droplet can be produced in arrays over a large scale, serving as templates for fabricating transparent polymer double microlens arrays for improved light emission from the optoelectronic device.

The Applications of Polymers in Solar Cells: A Review

The emerging dye-sensitized solar cells, perovskite solar cells, and organic solar cells have been regarded as promising photovoltaic technologies. The device structures and components of these solar cells are imperative to the device’s efficiency and stability. Polymers can be used to adjust the device components and structures of these solar cells purposefully, due to their diversified properties. In dye-sensitized solar cells, polymers can be used as flexible substrates, pore- and film-forming agents of photoanode films, platinum-free counter electrodes, and the frameworks of quasi-solid-state electrolytes. In perovskite solar cells, polymers can be used as the additives to adjust the nucleation and crystallization processes in perovskite films. The polymers can also be used as hole transfer materials, electron transfer materials, and interface layer to enhance the carrier separation efficiency and reduce the recombination. In organic solar cells, polymers are often used as donor layers, buffer layers, and other polymer-based micro/nanostructures in binary or ternary devices to influence device performances. The current achievements about the applications of polymers in solar cells are reviewed and analyzed. In addition, the benefits of polymers for solar cells, the challenges for practical application, and possible solutions are also assessed.

Extraordinary Focusing Effect of Surface Nanolenses in Total Internal Reflection Mode

Microscopic lenses are paramount in solar energy harvesting, optical devices, and imaging technologies. This work reports an extraordinary focusing effect exhibited by a surface nanolens (i.e., with at least one dimension of subwavelength) that is situated in an evanescent field from the total internal reflection (TIR) of light illuminated to the supporting substrate above the critical angle. Our measurements show that the position, shape, and size of the surface area with enhanced light intensity are determined by the geometry of the nanolens and the incident angle, in good agreement with simulation results. This strong focusing effect of the surface nanolens is shown to significantly promote the plasmonic effect of deposited gold nanoparticles on the lens surface inlight conversion and to vaporize surrounding water to microbubbles by using low laser power. This work further demonstrates that the light redistribution by the surface nanolens in TIR enables a range of novel applications in selectively local visualization of specimens in fluorescence imaging, optical trapping of colloids from an external flow, and selective materials deposition from photoreactions.

Formation, growth and applications of femtoliter droplets on a microlens

Formation and growth of femtoliter droplets on surface microstructures are important in many fundamental and practical interfacial processes, such as water collection, vapour condensation in cooling devices, drop self-removal on anti-icing surfaces and fabrication of droplet-templated functional microstructures. In this work, we experimentally and theoretically investigate the growth of femtoliter oil-like liquid on the microlens surrounded by a hydrophilic planar area. The droplets were produced by solvent exchange, a process where the droplets nucleate and grow from an oversaturation created by displacing a good solvent by a poor solvent of the droplet liquid. Our results showed that the droplet fully coats the lens surface and the contact angle of the droplet relative to the flat surface is finely tuned over a large range by the droplet volume. The growth of the droplet on a microlens is largely described by the constant contact radius model. To demonstrate the new opportunities provided by the controlled formation of the droplet situated on a microlens, we will show a simple and effective approach for production of arrays of composite microlenses consisting of two types of polymers with different refractive indices. A high curvature of the composite microlens results in desirable diffraction patterns with potential application for enhanced light harvesting. Moreover, we demonstrate that extraction of traces of a hydrophobic solute from the flow is much faster as the droplet is lifted up from the channel wall by the microlens, promising a time effective in situ detection process in narrow channels.

Fabrication of Micro-Optics Elements with Arbitrary Surface Profiles Based on One-Step Maskless Grayscale Lithography

A maskless lithography method to realize the rapid and cost-effective fabrication of micro-optics elements with arbitrary surface profiles is reported. A digital micro-mirror device (DMD) is applied to flexibly modulate that the exposure dose according to the surface profile of the structure to be fabricated. Due to the fact that not only the relationship between the grayscale levels of the DMD and the exposure dose on the surface of the photoresist, but also the dependence of the exposure depth on the exposure dose, deviate from a linear relationship arising from the DMD and photoresist, respectively, and cannot be systemically eliminated, complicated fabrication art and large fabrication error will results. A method of compensating the two nonlinear effects is proposed that can be used to accurately design the digital grayscale mask and ensure a precise control of the surface profile of the structure to be fabricated. To testify to the reliability of this approach, several typical array elements with a spherical surface, aspherical surface, and conic surface have been fabricated and tested. The root-mean-square (RMS) between the test and design value of the surface height is about 0.1 μm. The proposed method of compensating the nonlinear effect in maskless lithography can be directly used to control the grayscale levels of the DMD for fabricating the structure with an arbitrary surface profile.

Fabrication of a Microlens Array with Controlled Curvature by Thermally Curving Photosensitive Gel Film beneath Microholes

A rapid method is developed for fabricating low-cost and high-numerical-aperture photosensitive-gel microlens arrays (MLAs) with well-controlled curvatures. An UV-curable photosensitive-gel film beneath the microholes of a silicon mold can be flexibly deformed by thermally manipulating the surface tension of the photosensitive gel and the pressure difference across the air-photosensitive-gel interface. The concave interface is then solidified through UV curing, forming a MLA with a concave curvature. MLAs with a focal length ranging from 51.4 to 71.9 μm and a numerical aperture (NA) of 0.49 were fabricated. The photocured MLA has high mechanical and thermal strength and is suitable as a master mold for the further production of convex MLAs. The fabricated microlenses have uniform shapes and smooth surfaces. In a demonstration of imaging and focusing performance, clear and uniform images and focused light spots were observed using concave and convex MLAs.

CMOS compatible fabrication of micro, nano convex silicon lens arrays by conformal chemical vapor deposition

We present a novel CMOS-compatible fabrication technique for convex micro-nano lens arrays (MNLAs) with high packing density on the wafer scale. By means of conformal chemical vapor deposition (CVD) of hydrogenated amorphous silicon (a-Si:H) following patterning of silicon pillars via electron beam lithography (EBL) and plasma etching, large areas of a close packed silicon lens array with the diameter from a few micrometers down to a few hundred nanometers was fabricated. The resulting structure shows excellent surface roughness and high uniformity. The optical focusing properties of the lenses at infrared wavelengths were verified by experimental measurements and numerical simulation. This approach provides a feasible solution for fabricating silicon MNLAs compatible for next generation large scale, miniaturized optical imaging detectors and related optical devices.

Shape-controllable, bottom-up fabrication of microlens using oblique angle deposition

This Letter reports a novel method for the simple fabrication of microlens arrays with a controlled shape and diameter on glass substrates. Multilayer stacks of silicon dioxide deposited by oblique angle deposition with hole mask patterns enable microlens formation. Precise control of mask height and distance, as well as oblique angle steps between deposited layers, supports the controllability of microlens geometry. The fabricated microlens arrays with designed geometry exhibit uniform optical properties.

Controlling the Growth Modes of Femtoliter Sessile Droplets Nucleating on Chemically Patterned Surfaces

Femtoliter droplet arrays on immersed substrates are essential elements in a broad range of advanced droplet-based technologies, such as light manipulation, sensing, and high throughput diagnosis. Solvent exchange is a bottom-up approach for producing those droplets from a pulse of oil oversaturation when a good solvent of the droplet liquid is displaced by a poor solvent. The position and arrangement of the droplets are regulated by chemical micropatterns on the substrate. Here we show experimentally and theoretically that the growth modes of droplets confined in planar micropatterns on the surface can be manipulated through the laminar flow of the solvent exchange. The control parameters are the area size of the micropatterns and the flow rate, and the observables are the contact angle and the final droplet volume. For a given pattern size, the Peclet number of the flow determines whether the growing droplets switch from an initial constant contact angle mode to a subsequent constant contact radius mode. Good agreement is achieved between the experimental results and our theoretical model that describes the dependence of the final droplet size on Pe.

Diffractive nanostructures for enhanced light-harvesting in organic photovoltaic devices

We present in-coupling gratings for improving the performance of thin film organic solar cells. The impact of the grating on the absorption in the active layer is modeled and explained using a standard cell architecture. An increase in absorption of 14.8% is predicted and is shown to be independent from the active material. The structure is then applied on blade-coated devices and yields an efficiency improvement of 12%. The angular behavior of the structures is measured showing superior performance for two dimensional gratings. By simulating the current generation for different angles and illumination conditions, we predict a total yearly increase of the generated current of 12% using an optimized grating. The fabrication of these structures, moreover, is compatible with roll-to-roll production techniques, thus making them an optimal solution for printed photovoltaics.

Complex Photonic Structures for Light Harvesting

Over the last few years, micro- and nanophotonics have roused a strong interest in the scientific community for their promising impact on the development of novel kinds of solar cells. Certain thin- and ultrathin-film solar cells are made of innovative, often cheap, materials which suffer from a low energy conversion efficiency. Light-trapping mechanisms based on nanophotonics principles are particularly suited to enhance the absorption of electromagnetic waves in these thin media without changing the material composition. In this review, the latest results achieved in this field are reported, with particular attention to the realization of prototypes, spanning from deterministic to disordered photonic architectures, and from dielectric to metallic nanostructures.

Monolithic polymer microlens arrays with high numerical aperture and high packing density

This work reports a novel method for monolithic fabrication of high numerical aperture polymer microlens arrays (high-NA MLAs) with high packing density (PD) at wafer level. The close-packed high-NA MLAs were fabricated by incorporating conformal deposition of ultrathin fluorocarbon nanofilm and melting the cylindrical polymer islands. The NA and PD of hemispherical MLAs with a hexagonal arrangement increase up to 0.6 and 89%, respectively. The increase of NA enhances the lens transmission securing the beam width down to 1.1 μm. The close-packed high-NA MLAs enable high photon collection efficiency with signal-to-noise ratio greater than 50:1.

Reducing optical losses in organic solar cells using microlens arrays: theoretical and experimental investigation of microlens dimensions

A microlens-array (MLA) diffracts/focuses light in the photovoltaic device on the other side of the substrate; photocurrent increases with the microlens height/pitch ratio.

From nanodroplets by the ouzo effect to interfacial nanolenses

Polymerizing nanodroplets at solid-liquid interfaces is a facile solution-based approach to the functionalization of large surface areas with polymeric lens-shaped nanostructures. In this work, we have applied a one-pot approach to obtain polymeric nanolenses with controlled sizes and densities. We take advantage of the formation mechanism by the direct adsorption of nanodroplets from a surfactant-free microemulsion onto an immersed hydrophobic substrate. The interfacial nanodroplets were photopolymerized to produce polymeric nanolenses on the substrate surface. The surfactant-free microemulsion of the monomer nanodroplets was obtained through the spontaneous emulsification (i.e., ouzo effect) in the tertiary system of ethanol, water, and precusor monomer. The size of nanolenses on the surface was adjusted by the nanodroplet size, following a linear relationship with the ratio of the components in the microemulsion. This simple approach is applicable to produce nanolenses over the entire surface area or on any specific area at will by depositing a drop of the microemulsion. Possessing high optical transparency, the resulting substrates may have potential application as functional biomedical supporting materials or effective light-harvesting coatings.