Strain Effects on Flexible Perovskite Solar Cells

Flexible perovskite solar cells (f-PSCs) as a promising power source have grabbed surging attention from academia and industry specialists by integrating with different wearable and portable electronics. With the development of low-temperature solution preparation technology and the application of different engineering strategies, the power conversion efficiency of f-PSCs has approached 24%. Due to the inherent properties and application scenarios of f-PSCs, the study of strain in these devices is recognized as one of the key factors in obtaining ideal devices and promoting commercialization. The strains mainly from the change of bond and lattice volume can promote phase transformation, induce decomposition of perovskite film, decrease mechanical stability, etc. However, the effect of strain on the performance of f-PSCs has not been systematically summarized yet. Herein, the sources of strain, evaluation methods, impacts on f-PSCs, and the engineering strategies to modulate strain are summarized. Furthermore, the problems and future challenges in this regard are raised, and solutions and outlooks are offered. This review is dedicated to summarizing and enhancing the research into the strain of f-PSCs to provide some new insights that can further improve the optoelectronic performance and stability of flexible devices.

Optical Manipulation of Incident Light for Enhanced Photon Absorption in Ultrathin Organic Photovoltaics

We attempted to improve the photon absorption of the photoactive layer in organic photovoltaic (OPV) devices by device engineering without changing their thickness. Soft nanoimprinting lithography was used to introduce a 1D grating pattern into the photoactive layer. The increase in photocurrent caused by the propagating surface plasmon-polariton mode was quantitatively analyzed by measuring the external quantum efficiency in transverse magnetic and transverse electric modes. In addition, the introduction of an ultrathin substrate with a refractive index of 1.34 improved photon absorption by overcoming the mismatched optical impedance at the air/substrate interface. As a result, the power conversion efficiency (PCE) of an ultrathin OPV with a 400 nm grating period was 8.34%, which was 11.6% higher than that of an unpatterned ultrathin OPV, and the PCE was 3.2 times higher at a low incident light angle of 80°, indicating very low incident light angle dependence.

Organic Solar Cells Improved by Optically Resonant Silicon Nanoparticles

Silicon nanophotonics has become a versatile platform for optics and optoelectronics. For example, strong light localization at the nanoscale and lack of parasitic losses in infrared and visible spectral ranges make resonant silicon nanoparticles a prospect for improvement in such rapidly developing fields as photovoltaics. Here, we employed optically resonant silicon nanoparticles produced by laser ablation for boosting the power conversion efficiency of organic solar cells. Namely, we created colloidal solutions of spherical nanoparticles with a range of diameters (80-240 nm) in different solvents. We tested how the nanoparticles' position in the device, their concentration, silicon doping, and method of deposition affected the final device efficiency. The best conditions optimization resulted in an efficiency improvement from 6% up to 7.5%, which correlated with numerical simulations of nanoparticles' optical properties. The developed low-cost approach paves the way toward highly efficient and stable solution-processable solar cells.

Diffraction-optimized aperiodic surface structures for enhanced current density in organic solar cells

Novel diffractive surface structures are designed, evaluated, and optimized to achieve a significant increase in current density in organic solar cells. The surface relief phase gratings are patterned based on concentric rings, a Fermat's spiral, and an Archimedean spiral, respectively, and thus diffract incident light independent of its azimuthal angle, as they all exhibit circularly symmetric diffraction patterns. The grating pillars are arranged on the rings or spirals according to periodic or deterministic aperiodic Thue-Morse and Rudin-Shapiro sequences, tailoring the desired diffraction patterns. After evaluating the surface patterns against each other and further parameter optimization, a final current density enhancement of 5 % is achieved for one of the most promising patterns, the Thue-Morse sequence on an Archimedean spiral.

Multi-element lenslet array for efficient solar collection at extreme angles of incidence

Photovoltaics (PV) are a versatile and compact route to harness solar power. One critical challenge with current PV is preserving the optimal panel orientation angle with respect to the sun for efficient energy conversion. We experimentally demonstrate a bespoke multi-element lenslet array that allows for an increased power collection over a wide field of view by increasing the effective optical interaction length by up to 13 times specifically at large angles of incidence. This design can potentially be retrofitted onto already deployed amorphous silicon solar panels to yield an increased daily power generation by a factor of 1.36 for solar equivalent illumination. We 3D printed an optical proof of concept multi-element lenslet array to confirm an increase in power density for optical rays incident between 40 and 80 degrees. Our design indicates a novel optical approach that could potentially enable increased efficient solar collection in extreme operating conditions such as on the body of planes or the side of buildings.

Coatings in Photovoltaic Solar Energy Worldwide Research

This paper describes the characteristics of contributions that were made by researchers worldwide in the field of Solar Coating in the period 1957–2019. Scopus is used as a database and the results are processed while using bibliometric and analytical techniques. All of the documents registered in Scopus, a total of 6440 documents, have been analyzed and distributed according to thematic subcategories. Publications are analyzed from the type of publication, field of use, language, subcategory, type of newspaper, and the frequency of the keyword perspectives. English (96.8%) is the language that is most used for publications, followed by Chinese (2.6%), and the rest of the languages have a less than < 1% representation. Publications are studied by authors, affiliations, countries of origin of the authors, and H-index, which it stands out that the authors of China contribute with 3345 researchers, closely followed by the United States with 2634 and Germany with 1156. The Asian continent contributes the most, with 65% of the top 20 affiliations, and Taiwan having the most authors publishing in this subject, closely followed by Switzerland. It can be stated that research in this area is still evolving with a great international scientific contribution in improving the efficiency of solar cells.

Recent progress of light manipulation strategies in organic and perovskite solar cells

Organic and perovskite solar cells are suffering from the insufficient utilization of incident light and thus low light harvesting efficiency despite their rapid progress in the past decade. In this regard, light manipulation strategies have attracted numerous attention to solve this inherent limit. Herein, the recent advances in light manipulation techniques in this area are overviewed. The light manipulation mechanisms are illustrated to classify the structures. Various light manipulation structures, fabrication techniques, and corresponding results are given and discussed, addressing the suppression of surface reflection, nano/micro-structure-induced light scattering, and the plasmonic effects with periodic metallic patterns and metallic nanoparticles. A brief perspective on future research is also proposed for pursuing broadband light harvesting.

Recent progress in near-field nanolithography using light interactions with colloidal particles: from nanospheres to three-dimensional nanostructures

The advance of nanotechnology is firmly rooted in the development of cost-effective, versatile, and easily accessible nanofabrication techniques. The ability to pattern complex two-dimensional and three-dimensional nanostructured materials are particularly desirable, since they can have novel physical properties that are not found in bulk materials. This review article will report recent progress in utilizing self-assembly of colloidal particles for nanolithography. In these techniques, the near-field interactions of light and colloids are the sole mechanisms employed to generate the intensity distributions for patterning. Based on both 'bottom-up' self-assembly and 'top-down' lithography approaches, these processes are highly versatile and can take advantage of a number of optical effects, allowing the complex 3D nanostructures to be patterned using single exposures. There are several key advantages including low equipment cost, facile structure design, and patterning scalability, which will be discussed in detail. We will outline the underlying optical effects, review the geometries that can be fabricated, discuss key limitations, and highlight potential applications in nanophotonics, optoelectronic devices, and nanoarchitectured materials.

Design and Fabrication of Moth-Eye Subwavelength Structure with a Waist on Silicon for Broadband and Wide-Angle Anti-Reflection Property

Reflection loss on the optical component surface is detrimental to performance. Several researchers have discovered that the eyes of moths are covered with micro- and nanostructured films that reduce broadband and wide-angle light reflection. This research proposes a new type of moth-eye subwavelength structure with a waist, which is equivalent to a gradient refractive index film layer with high–low–high hyperbolic-type fill factor distribution. The diffraction order characteristics of a moth-eye subwavelength structure are first analyzed using a rigorous coupled wave analysis. The moth-eye structural parameters are optimized within the spectral range of 2–5 μm using the finite-difference time-domain method. The experimental fabrication of the moth-eye structure with a waist array upon a silicon substrate is demonstrated by using three-beam laser interferometric lithography and an inductively coupled plasma process. The experimental and simulation results show good agreement. The experimental results show that the reflectivity of the moth-eye structure with a waist is less than 1.3% when the incidence angle is less than 30°, and less than 4% when the incidence angle is less than 60°. This research can guide the development of AR broadband optical components and wide-angle applications.

Using Femtosecond Laser Irradiation to Enhance the Vertical Electrical Properties and Tailor the Morphology of a Conducting Polymer Blend Film

We report femtosecond infrared laser-induced selective tailoring of carrier transport as well as surface morphology on a conducting polymer blend thin film. Maximal 2.4 times enhancement on vertical current transport in poly(3-hexylthiophene):phenyl-C61-butyric acid methyl ester, was achieved by this irradiation. The laser irradiation induced a photo expansion without deteriorating its molecular structure and the film morphology could be customized in the micron scale by adjusting the laser writing parameters. In the photoexpanded region, the face-on populations were about 2.2 times larger in comparison with the pristine region, which was a major contributor to the enhanced carrier transport.

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.

What We Talk About When We Talk About Light

UNESCO (the United Nations Educational, Scientific, and Cultural Organization) has declared 2015 the "International Year of Light and Light-Based Technologies". In celebration of this proclamation, this Outlook provides a general history of light and its applications, from the earliest moments of the Big Bang through its present impact on all forms of life on the planet. Special emphasis is placed on fundamental advances in the generation and use of artificial light, as well as the harvesting and use of light from the Sun and other natural sources. During the past century, the role of light in the fields of physics, chemistry, and biology has expanded to include emerging fields such as environmental engineering, agriculture, materials science, and biomedicine. In this regard, future research challenges and new potential applications in these areas, in the context of "the central science", are presented and discussed.

Metal Nanoparticle-Decorated Two-Dimensional Molybdenum Sulfide for Plasmonic-Enhanced Polymer Photovoltaic Devices

Atomically thin two-dimensional (2D) transition metal dichalcogenides have also attracted immense interest because they exhibit appealing electronic, optical and mechanical properties. In this work, we prepared gold nanoparticle-decorated molybdenum sulfide (AuNP@MoS₂) through a simple spontaneous redox reaction. Transmission electron microscopy, UV-Vis spectroscopy, and Raman spectroscopy were used to characterize the properties of the AuNP@MoS₂ nanomaterials. Then we employed such nanocomposites as the cathode buffer layers of organic photovoltaic devices (OPVs) to trigger surface plasmonic resonance, leading to noticeable enhancements in overall device efficiencies. We attribute the primary origin of the improvement in device performance to local field enhancement induced by the effects of localized surface plasmonic resonance. Our results suggest that the metal nanoparticle-decorated two-dimensional materials appear to have great potential for use in high-performance OPVs.

Synergistic plasmonic effects of metal nanoparticle-decorated PEGylated graphene oxides in polymer solar cells

Metal nanostructures that trigger plasmonic near-field effects are often incorporated in organic photovoltaic devices (OPVs) to improve their light-harvesting ability. These nanostructures usually can be positioned in two different locations in a device: (i) within the photon absorption layers and (ii) at the interfaces between the active layer and the metal electrodes. In this study, we developed amphiphilic gold nanoparticles (Au NPs) for use in dual plasmonic nanostructures within OPVs. We employed graphene oxide as the template to anchor the Au NPs, thereby avoiding their aggregation. Furthermore, we added poly(ethylene glycol) (PEG) bis(amine) to the synthesis medium to improve the solubility of the nanocomposites, such that they could be dispersed well in water and in several organic solvents. Accordingly, we could incorporate the PEGylated Au NP/graphene oxides readily into both the buffer layer and photoactive layer of OPVs, which, as a result, exhibited obvious enhancements in their photocurrents and overall device efficiencies. Moreover, we observed different spectral enhancement regions when we positioned the nanocomposites at different locations, reflecting the different dielectric environments surrounding the NPs; this unexpected behavior should assist in enhancing the broadband absorption of solar irradiation.

Nanoimprinting sub-100 nm features in a photovoltaic nanocomposite using durable bulk metallic glass molds

The use of bulk metallic glass (BMG) for the nanoimprint of high-aspect-ratio (>3) features into functional polymers is investigated. To accomplish this, the most critical aspect is the successful demolding of the imprinted polymer. By fluorosilane functionalization of the BMG surface and optimization of processing temperature, high aspect pore features down to 45 nm diameters are introduced into the surface of two organic photovoltaic systems: poly(3-hexylthiophene-2,5-diyl) (P3HT) and 1:1 mixtures of P3HT with Phenyl-C61-butyric acid methyl ester (PCBM). The crystallinity of P3HT demands higher forming temperatures and pressures that are difficult to obtain with conventional soft nanoimprint lithography molds. The ability to accommodate a wide range of processing conditions and the low cost of fabricating molds with nanometer-scale features point to the large potential of nanotextured BMGs as an economical and scalable imprint material for high-resolution applications.

Plasmonic nanostructures for light trapping in organic photovoltaic devices

Over the past decade, we have witnessed rapid advances in the development of organic photovoltaic devices (OPVs). At present, the highest level of efficiency has surpassed 10%, suggesting that OPVs have great potential to become competitive with other thin-film solar technologies. Because plasmonic nanostructures are likely to further improve the efficiency of OPVs, this Article reviews recent progress in the development of metal nanostructures for triggering plasmonic effects in OPVs. First, we briefly describe the physical fundamentals of surface plasmons (SPs). Then, we discuss recent approaches toward increasing the light trapping efficiency of OPVs through the incorporation of plasmonic structures. Finally, we provide a brief outlook into the future use of SPs in highly efficient OPVs.

Solution-processed nanocomposites containing molybdenum oxide and gold nanoparticles as anode buffer layers in plasmonic-enhanced organic photovoltaic devices

Solution-processed nanocomposites containing molybdenum oxide (MoO3) and gold nanoparticles (Au NPs) have been used as anode buffer layers in organic photovoltaic devices (OPVs). The resulting devices exhibit a remarkable enhancement in power conversion efficiency after Au NPs were incorporated into the device. Such enhancements can be attributed to the localized surface plasmon resonance induced by the metallic nanostructures. We have also found that the rate of exciton generation and the probability of exciton dissociation were increased. Furthermore, the devices made of the MoO3 buffer layer containing Au NPs exhibited superior stability. This work opens up the possibility of fabricating OPVs with both high efficiency and a prolonged lifetime.

Plasmonic Enhancement of Dye Sensitized Solar Cells in the Red-to-near-Infrared Region using Triangular Core-Shell Ag@SiO2 Nanoparticles

Recently, plasmonic metal nanoparticles have been shown to be very effective in increasing the light harvesting efficiency (LHE) of dye-sensitized solar cells (DSSCs). Most commonly, spherical nanoparticles composed of silver or gold are used for this application; however, the localized surface plasmon resonances of these isotropic particles have maxima in the 400-550 nm range, limiting any plasmonic enhancements to wavelengths below 600 nm. Herein, we demonstrate that the incorporation of anisotropic, triangular silver nanoprisms in the photoanode of DSSCs can dramatically increase the LHE in the red and near-infrared regions. Core-shell Ag@SiO2 nanoprisms were synthesized and incorporated in various quantities into the titania pastes used to prepare the photoanodes. This optimization led to an overall 32 ± 17% increase in the power conversion efficiency (PCE) of cells made using 0.05% (w/w) of the Ag@SiO2 composite. Measurements of the incident photon-to-current efficiency provided further evidence that this increase is a result of improved light harvesting in the red and near-infrared regions. The effect of shell thickness on nanoparticle stability was also investigated, and it was found that thick (30 nm) silica shells provide the best protection against corrosion by the triiodide-containing electrolyte, while still enabling large improvements in PCE to be realized.

Morphology-dependent light trapping in thin-film organic solar cells

The active layer materials used in organic photovoltaic (OPV) cells often self-assemble into highly ordered morphologies, resulting in significant optical anisotropies. However, the impact of these anisotropies on light trapping in nanophotonic OPV architectures has not been considered. In this paper, we show that optical anisotropies in a canonical OPV material, P3HT, strongly affect absorption enhancements in ultra-thin textured OPV cells. In particular we show that plasmonic and gap-mode solar cell architectures redistribute electromagnetic energy into the out-of-plane field component, independent of the active layer orientation. Using analytical and numerical calculations, we demonstrate how the absorption in these solar cell designs can be significantly increased by reorienting polymer domains such that strongly absorbing axes align with the direction of maximum field enhancement.

Modifications in morphology resulting from nanoimprinting bulk heterojunction blends for light trapping organic solar cell designs

Nanoimprinting the photoactive layer of bulk heterojunction (BHJ) organic solar cells is a promising technique for enhancing device performance via improved light absorption. Here, we demonstrate that imprinting poly(3-hexylthiophene) (P3HT) and fullerene BHJ blends leads to adverse morphological changes within the photoactive nanopattern which have been previously overlooked. In particular, nanoimprinting induces a factor of 2 difference in polymer:fullerene composition between the nanopattern posts and interconnecting flash layer that inadvertently moves the composition outside the range for optimal performance. This occurs because of the strong tendency of regioregular P3HT to crystallize since imprinting blends based on amorphous regiorandom P3HT have uniform nanopattern composition. Based on these results, we outline promising design strategies, such as nanoimprinting amorphous polymers, to serve as guidelines for fabricating high-performance nanopatterned BHJ solar cells capable of maximized light absorption.

Antireflection effects at nanostructured material interfaces and the suppression of thin-film interference

Thin-film interference is a well-known effect, and it is commonly observed in the colored appearance of many natural phenomena. Caused by the interference of light reflected from the interfaces of thin material layers, such interference effects can lead to wavelength and angle-selective behavior in thin-film devices. In this work, we describe the use of interfacial nanostructures to eliminate interference effects in thin films. Using the same principle inspired by moth-eye structures, this approach creates an effective medium where the index is gradually varying between the neighboring materials. We present the fabrication process for such nanostructures at a polymer-silicon interface, and experimentally demonstrate its effectiveness in suppressing thin-film interference. The principle demonstrated in this work can lead to enhanced efficiency and reduce wavelength/angle sensitivity in multilayer optoelectronic devices.

Efficient light trapping in inverted polymer solar cells by a randomly nanostructured electrode using monodispersed polymer nanoparticles

The randomly nanotextured back electrode provides a simple and efficient route for enhancing photocurrent in polymer solar cells (PSCs) by light trapping, which can increase light absorption within a finite thickness of the active layer. In this study, we incorporated mono-disperse 60 nm polystyrene nanoparticles (PS NPs) into a 50 nm thick poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) anode buffer layer (ABL) to create a randomly nanotextured back electrode with 10 nm height variations in inverted-type PSCs. The roughened interface between the PS NP-PEDOT:PSS ABL and the Ag electrode scatters light in the visible range, leading to efficient light trapping within the device and enhanced light absorption in the active layer. Inverted PSCs with randomly nanotextured electrodes (φ(NP) = 0.31) showed short-circuit current density (J(SC)) and power conversion efficiency (PCE) values that were 15% higher than those of control devices with flat electrodes. External quantum efficiency, reflectance, and optical light scattering as a function of ϕ(NP) were examined to determine the origin of the enhancement in J(SC) and PCE.

Optical design of transparent thin metal electrodes to enhance in-coupling and trapping of light in flexible polymer solar cells

ITO-free polymer solar cells with efficiencies as high as 6.6% and 5.8% are fabricated on glass and polyethylene naphthalate (PEN) by using TeO(2) to enhance the in-coupling of light in an Ag-Ag microcavity. These cells exhibit higher performance, selective microcavity resonance as a function of the thickness of TeO(2) , and better bending stability than flexible devices made with ITO.

Plasmonic nanograting design for inverted polymer solar cells

Plasmonic nanostructures for effective light trapping in a variety of photovoltaics have been actively studied. Metallic nanograting structures are one of promising architectures. In this study, we investigated numerically absorption enhancement mechanisms in inverted polymer photovoltaics with one dimensional Ag nanograting in backcontact. An optical spacer layer of TiO₂, which also may act as an electron transport layer, was introduced between nanograting pillars. Using a finite-difference-time domain method and performing a modal analysis, we explored correlations between absorption enhancements and dimensional parameters of nanograting such as period as well as height and width. The optimal design of nanograting for effective light trapping especially near optical band gap of an active layer was discussed, and 23% of absorption enhancement in a random polarization was demonstrated numerically with the optimally designed nanograting. In addition, the beneficial role of the optical spacer in plasmonic light trapping was also discussed.

Nanoimprinted polymer solar cell

Among the various organic photovoltaic devices, the conjugated polymer/fullerene approach has drawn the most research interest. The performance of these types of solar cells is greatly determined by the nanoscale morphology of the two components (donor/acceptor) and the molecular orientation/crystallinity in the photoactive layer. A vertically bicontinuous and interdigitized heterojunction between donor and acceptor has been regarded as one of the ideal structures to enable both efficient charge separation and transport. Synergistic control of polymer orientation in the nanostructured heterojunction is also critical to improve the performance of polymer solar cells. Nanoimprint lithography has emerged as a new approach to simultaneously control both the heterojunction morphology and polymer chains in organic photovoltaics. Currently, in the area of nanoimprinted polymer solar cells, much progress has been achieved in the fabrication of nanostructured morphology, control of molecular orientation/crystallinity, deposition of acceptor materials, patterned electrodes, understanding of structure-property correlations, and device performance. This review article summarizes the recent studies on nanoimprinted polymer solar cells and discusses the outstanding challenges and opportunities for future work.

Solution-Processed Nanowire Coating for Light Management in Organic Solar Cells

We report a novel light management approach based on solution-processed nanowire (NW) coating for enhancing organic solar cell efficiency. A titanium dioxide (TiO2) NW dispersion was produced by electrospinning. The coatings with various coverage fractions were fabricated by a simple solution casting of a TiO2NW dispersion. Reduced reflectivity was observed for the NW-coated glass slide. The bulk-heterojunction organic solar cells with the NW coating showed improved power conversion efficiencies (PCEs) due to their antireflection and light trapping effects in the active layer. In addition, the PCE of the cell with the NW coating was improved compared with that without the NW coating for incident angles above 70° (increased by a maximum of 51.6% at an incident angle of 85°). These results indicate that solution-processed NW coating is a promising light management approach easily scalable and applicable to a wide range of devices, including solar cells.