Resonant silicon nanoparticles for enhancement of light absorption and photoluminescence from hybrid perovskite films and metasurfaces

Menezes L, Tittl A, Ren H, Maier SA. Optical Metasurfaces for Energy Conversion

Nanostructured surfaces with designed optical functionalities, such as metasurfaces, allow efficient harvesting of light at the nanoscale, enhancing light-matter interactions for a wide variety of material combinations. Exploiting light-driven matter excitations in these artificial materials opens up a new dimension in the conversion and management of energy at the nanoscale. In this review, we outline the impact, opportunities, applications, and challenges of optical metasurfaces in converting the energy of incoming photons into frequency-shifted photons, phonons, and energetic charge carriers. A myriad of opportunities await for the utilization of the converted energy. Here we cover the most pertinent aspects from a fundamental nanoscopic viewpoint all the way to applications.

Improving Photoelectric Conversion with Broadband Perovskite Metasurface

The miniaturization and integration of optoelectronic devices require progressive size reduction of active layers, resulting in less optical absorption and lower quantum efficiency. In this work, we demonstrate that introducing a metasurface made of hybrid organic-inorganic perovskite (HOIP) can significantly enhance broadband absorption and improve photon-to-electron conversion, which roots from exciting Mie resonances together with suppressing optical transmission. On the basis of the HOIP metasurface, a broadband photodetector has been fabricated where photocurrent boosts more than 10 times in the frequency ranging from ultraviolet to visible. The device response time is less than 5.1 μs at wavelengths 380, 532, and 710 nm, and the relevant 3 dB bandwidth is over 0.26 MHz. Moreover, this photodetector has been applied as a signal receiver for transmitting 2D color images in broadband optical communication. These results accentuate the practical applications of HOIP metasurfaces in novel optoelectronic devices for broadband optical communication.

Fast, Economical, and Reproducible Sensing from a 2D Si Wire Array: Accurate Characterization by Single Wire Spectroscopy

Silicon (Si) is promising as a field enhancement material because of its high abundance, low toxicity, and high refractive index. The field enhancement effect intensifies light-matter interactions, which improves photocatalysis, solar cell performance, and sensor sensitivity. To manufacture field enhancement materials on a production scale, the fabrication technique must be simple, cost-effective, fast, and highly reproducible and must produce a high enhancement factor (EF). Herein, we report on an economical and efficient fabrication method for a field enhancement substrate consisting of a two-dimensional Si wire array (2D-SiWA). This substrate was demonstrated as a fluorescence sensor with high sensitivity (EF > 200) and composed of a large area (6.0 mm²). In addition, single wire spectroscopy was used to identify very high reproducibility of the sensor sensitivity in regular regions (97%) and a mixture of regular and irregular regions (87%) of the 2D-SiWA. The large-area Si fluorescence sensor fabrication was cost-effective and rapid and was 50× less expensive, 20×faster, and 60,000×larger than the typical electron beam lithography method.

Correlated organic-inorganic motion enhances stability and charge carrier lifetime in mixed halide perovskites

Organic cations are believed to have little influence on the charge carrier lifetime in hybrid organic-inorganic perovskites. Experiments defy this expectation. We consider formamidinium lead iodide (FAPbI₃) doping with and without Br as two prototypical systems, and perform ab initio time-domain nonadiabatic (NA) molecular dynamics simulations to investigate nonradiative electron-hole recombination. The simulations demonstrate that correlated organic-inorganic motion stabilizes the lattice and inhibits nonradiative charge recombination in FAPbI₃ upon Br doping. Br doping suppresses the rotation of FA and the vibrations of both organic and inorganic components, and leads to hole localization and the extent of localization is enhanced upon thermal impact, notably reducing the NA coupling by decreasing the overlap between the electron and hole wave functions. Doping also slightly increases the bandgap for further decreasing NA coupling and enhances the open-circuit voltage of perovskite solar cells. The small NA coupling and large bandgap beat the slow coherence loss, delaying electron-hole recombination and extending the charge carrier lifetime to 1.5 ns in Br-doped FAPbI₃, which is on the order of 1.1 ns in pristine FAPbI₃. The obtained time scales are in good agreement with experiments. Multiple phonon modes, including those of both the inorganic and organic components, couple to the electronic subsystem and accommodate the excess electronic energy lost during nonradiative charge recombination. The study reveals the unexpected atomistic mechanisms for the reduction of electron-hole recombination upon Br doping, rationalizes the experiments, and advances our understanding of the excited-state dynamics of perovskite solar cells.

Perovskite metasurfaces with large superstructural chirality

Recent attempts to synthesize hybrid perovskites with large chirality have been hampered by large size mismatch and weak interaction between their structure and the wavelength of light. Here we adopt a planar nanostructure design to overcome these limitations and realize all-dielectric perovskite metasurfaces with giant superstructural chirality. We identify a direct spectral correspondence between the near- and the far- field chirality, and tune the electric and magnetic multipole moments of the resonant chiral metamolecules to obtain large anisotropy factor of 0.49 and circular dichroism of 6350 mdeg. Simulations show that larger area metasurfaces could yield even higher optical activity, approaching the theoretical limits. Our results clearly demonstrate the advantages of nanostructrure engineering for the implementation of perovskite chiral photonic, optoelectronic, and spintronic devices.

Though chiral hybrid organic-inorganic perovskites are attractive for next-generation optoelectronics, imparting strong chirality through chemical synthesis has proved challenging. Here, the authors report all-dielectric perovskite metasurfaces with giant superstructural chirality via planar nanostructuring.

Metasurface-assisted broadband optical absorption in ultrathin perovskite films

Ultrathin hybrid organic-inorganic perovskite (HOIP) films have significant potential for use in integrated high-performance photoelectric devices. However, the relatively low optical absorption capabilities of thinner films, particularly in the long-wavelength region, pose a significant challenge to the further improvement of photoelectrical conversion in ultrathin HOIP films. To address this problem, we propose a combining of ultrathin HOIP film with plasmonic metasurface to enhance the absorption of the film effectively. The metasurface excites localized surface plasmon resonances and deflects the reflected light within the HOIP film, resulting in an obvious enhancement of film absorption. Finite-difference time-domain simulation results reveal that the far-field intensities, deflection angles, and electric field distributions can be effectively varied by using metasurfaces with different arrangements. Examination of the reflection and absorption spectra reveals that embedding a specifically designed metasurface into the HOIP film produces an obvious enhancement in broadband optical absorption compared with pure HOIP films. We further demonstrate that this broadband absorption promotion mechanism can be effective at a wide range of HOIP film thicknesses. Comparison of the absorption spectra at various incidence angles of ultrathin HOIP films with and without underlying metasurfaces indicates that the addition of a metasurface can effectively promote absorption under wide-angle incident light illumination. Moreover, by extending the metasurface structure to a two-dimensional case, absorption enhancements insensitive to the incident polarization states have also been demonstrated. This proposed metasurface-assisted absorption enhancement method could be applied in designing novel high-performance thin-film solar cells and photodetectors.

Photon management to reduce energy loss in perovskite solar cells

Despite the rapid development of perovskite solar cells (PSCs) over the past few years, the conversion of solar energy into electricity is not efficient enough or cost-competitive yet. The principal energy loss in the conversion of solar energy to electricity fundamentally originates from the non-absorption of low-energy photons ascribed to Shockley-Queisser limits and thermalization losses of high-energy photons. Enhancing the light-harvesting efficiency of the perovskite photoactive layer by developing efficient photo management strategies with functional materials and arrays remains a long-standing challenge. Here, we briefly review the historical research trials and future research trends to overcome the fundamental loss mechanisms in PSCs, including upconversion, downconversion, scattering, tandem/graded structures, texturing, anti-reflection, and luminescent solar concentrators. We will deeply emphasize the availability and analyze the importance of a fine device structure, fluorescence efficiency, material proportion, and integration position for performance improvement. The unique energy level structure arising from the 4fn inner shell configuration of the trivalent rare-earth ions gives multifarious options for efficient light-harvesting by upconversion and downconversion. Tandem or graded PSCs by combining a series of subcells with varying bandgaps seek to rectify the spectral mismatch. Plasmonic nanostructures function as a secondary light source to augment the light-trapping within the perovskite layer and carrier transporting layer, enabling enhanced carrier generation. Texturing the interior using controllable micro/nanoarrays can realize light-matter interactions. Anti-reflective coatings on the top glass cover of the PSCs bring about better transmission and glare reduction. Photon concentration through perovskite-based luminescent solar concentrators offers a path to increase efficiency at reduced cost and plays a role in building-integrated photovoltaics. Distinct from other published reviews, we here systematically and hierarchically present all of the photon management strategies in PSCs by presenting the theoretical possibilities and summarizing the experimental results, expecting to inspire future research in the field of photovoltaics, phototransistors, photoelectrochemical sensors, photocatalysis, and especially light-emitting diodes. We further assess the overall possibilities of the strategies based on ultimate efficiency prospects, material requirements, and developmental outlook.

Giant Enhancement of Radiative Recombination in Perovskite Light-Emitting Diodes with Plasmonic Core-Shell Nanoparticles

The integration of nanoparticles (NPs) into functional materials is a powerful tool for the smart engineering of their physical properties. If properly designed and optimized, NPs possess unique optical, electrical, quantum, and other effects that will improve the efficiency of optoelectronic devices. Here, we propose a novel approach for the enhancement of perovskite light-emitting diodes (PeLEDs) based on electronic band structure deformation by core-shell NPs forming a metal-oxide-semiconductor (MOS) structure with an Au core and SiO2 shell located in the perovskite layer. The presence of the MOS interface enables favorable charge distribution in the active layer through the formation of hole transporting channels. For the PeLED design, we consider integration of the core-shell NPs in the realistic numerical model. Using our verified model, we show that, compared with the bare structure, the incorporation of NPs increases the radiative recombination rate of PeLED by several orders of magnitude. It is intended that this study will open new perspectives for further efficiency enhancement of perovskite-based optoelectronic devices with NPs.

Nanogap-Rich TiO2 Film for 2000-Fold Field Enhancement with High Reproducibility

Titanium dioxide (TiO₂) is a crucial semiconductor for photocatalysts, solar cells, hydrogen evolution reactions, and antivirus agents. The properties and performances of these applications can improve significantly if the integrated TiO₂ acts as a light harvester through a large field enhancement. This study investigates the electromagnetic field enhancement of a nanogap-rich TiO₂ film with a large area, prepared by a facile dry process at room temperature. Herein, the loading pressure is applied to the TiO₂ particles for closely packing them in the film. The field enhancement, as a function of the loading pressure, is explored from the fluorescence intensity enhancement of a dye molecule. An average enhancement factor >2000 is achieved, which is a remarkable record for semiconductors. Furthermore, the reproducibility is significant; the relative standard deviation value is small (∼4%). Calculations were performed using the finite-difference-time-domain method. A nanogap of 5 nm yields the highest EF for triangular-prism TiO₂ particles.

Micro- and Nanopatterning of Halide Perovskites Where Crystal Engineering for Emerging Photoelectronics Meets Integrated Device Array Technology

Tremendous efforts have been devoted to developing thin film halide perovskites (HPs) for use in high-performance photoelectronic devices, including solar cells, displays, and photodetectors. Furthermore, structured HPs with periodic micro- or nanopatterns have recently attracted significant interest due to their potential to not only improve the efficiency of an individual device via the controlled arrangement of HP crystals into a confined geometry, but also to technologically pixelate the device into arrays suitable for future commercialization. However, micro- or nanopatterning of HPs is not usually compatible with conventional photolithography, which is detrimental to ionic HPs and requires special techniques. Herein, a comprehensive overview of the state-of-the-art technologies used to develop micro- and nanometer-scale HP patterns, with an emphasis on their controlled microstructures based on top-down and bottom-up approaches, and their potential for future applications, is provided. Top-down approaches include modified conventional lithographic techniques and soft-lithographic methods, while bottom-up approaches include template-assisted patterning of HPs based on lithographically defined prepatterns and self-assembly. HP patterning is shown here to not only improve device performance, but also to reveal the unprecedented functionality of HPs, leading to new research areas that utilize their novel photophysical properties.

Optical cooling of lead halide perovskite nanoparticles enhanced by Mie resonances

Halide perovskites are a family of semiconductor materials demonstrating prospective properties for optical cooling owing to efficient luminescence at room temperature and strong electron-phonon interaction. Moreover, perovskite based nanophotonic designs would allow for efficient optical cooling at the nanoscale. Here, we propose a novel strategy for the enhancement of optical cooling at the nanoscale based on optical resonance engineering in halide perovskite nanoparticles. Namely, the photoluminescence up-conversion efficiency in a nanoparticle is optimized via excitation of Mie-resonances both at emission and absorption wavelengths. The optimized theoretical photo-induced temperature decrease achieved for a hybrid halide perovskite (CH3NH3PbI3) 530 nm nanoparticle on a glass substrate is more than 100 K under CW illumination at wavelength 980 nm and moderate intensities (∼7 × 106 W cm-2). The optimized regime originates from simultaneous excitation of a magnetic quadrupole and a magnetic octupole at pump and emission wavelengths, respectively. The combination of a thermally sensitive photoluminescence signal and simplicity in the fabrication of a halide perovskite nanocavity will pave the way for implementation of nanoscale optical coolers for advanced applications.

Efficient Light Management in a Monolithic Tandem Perovskite/Silicon Solar Cell by Using a Hybrid Metasurface

Solar energy is now dealing with the challenge of overcoming the Shockley–Queisser limit of single bandgap solar cells. Multilayer solar cells are a promising solution as the so-called third generation of solar cells. The combination of materials with different bandgap energies in multijunction cells enables power conversion efficiencies up to 30% at reasonable costs. However, interfaces between different layers are critical due to optical losses. In this work, we propose a hybrid metasurface in a monolithic perovskite-silicon solar cell. The design takes advantage of light management to optimize the absorption in the perovskite, as well as an efficient light guiding towards the silicon subcell. Furthermore, we have also included the effect of a textured back contact. The optimum proposal provides an enhancement of the matched short-circuit current density of a 20.5% respect to the used planar reference.

Resonance-enhanced three-photon luminesce via lead halide perovskite metasurfaces for optical encoding

Lead halide perovskites have emerged as promising materials for photovoltaic and optoelectronic devices. However, their exceptional nonlinear properties have not been fully exploited in nanophotonics yet. Herein we fabricate methyl ammonium lead tri-bromide perovskite metasurfaces and explore their internal nonlinear processes. While both of third-order harmonic generation and three-photon luminescence are generated, the latter one is less affected by the material loss and has been significantly enhanced by a factor of 60. The corresponding simulation reveals that the improvement is caused by the resonant enhancement of incident laser. Interestingly, such kind of resonance-enhanced three-photon luminescence holds true for metasurfaces with a small period number of 4, enabling promising applications of perovskite metasurface in high-resolution nonlinear color nanoprinting and optical encoding. The encoded information ‘NANO’ is visible only when the incident laser is on-resonance. The off-resonance pumping and the single-photon excitation just produce a uniform dark or photoluminescence background.

Lead halide perovskites attract high interest as semiconductor materials but their exceptional nonlinear properties have not been fully exploited. Here Fan et al. demonstrate third-order harmonic generation and 60-fold enhanced three-photon luminescence, enabling optical encoding applications.

Extraordinary optical fields in nanostructures: from sub-diffraction-limited optics to sensing and energy conversion

Along with the rapid development of micro/nanofabrication technology, the past few decades have seen the flourishing emergence of subwavelength-structured materials and interfaces for optical field engineering at the nanoscale. Three remarkable properties associated with these subwavelength-structured materials are the squeezed optical fields beyond the diffraction limit, gradient optical fields in the subwavelength scale, and enhanced optical fields that are orders of magnitude greater than the incident field. These engineered optical fields have inspired fundamental and practical advances in both engineering optics and modern chemistry. The first property is the basis of sub-diffraction-limited imaging, lithography, and dense data storage. The second property has led to the emergence of a couple of thin and planar functional optical devices with a reduced footprint. The third one causes enhanced radiation (e.g., fluorescence), scattering (e.g., Raman scattering), and absorption (e.g., infrared absorption and circular dichroism), offering a unique platform for single-molecule-level biochemical sensing, and high-efficiency chemical reaction and energy conversion. In this review, we summarize recent advances in subwavelength-structured materials that bear extraordinary squeezed, gradient, and enhanced optical fields, with a particular emphasis on their optical and chemical applications. Finally, challenges and outlooks in this promising field are discussed.

Microwave plasma-assisted silicon nanoparticles: cytotoxic, molecular, and numerical responses against cancer cells

Silicon nanoparticles (SiNPs), which have a special place in material science due to their strong luminescent property and wide applicability in various physicochemical arenas synthesised via a microwave plasma-assisted process using an argon–silane mixture.

Active tuning of the Fano resonance from a Si nanosphere dimer by the substrate effect

All-dielectric materials have aroused great interest for their unique light scattering and lower losses compared with plasmonics. Generally, optical properties made by all-dielectric materials can be passively controlled by varying the geometry, size and refractive index at the design stage. Therefore, the realization of active tuning in the field of nanophotonics is important to improve the practicality and achieve light-on-chip technology in the future. Herein, we combine the high refractive index of Si and the phase transition of VO₂ to form an active tuning hybrid nanostructure with higher quality factor by depositing Si nanospheres on the VO₂ layer with an Al₂O₃ substrate. As the temperature goes up, the refractive index of the VO₂ layer switches from high to low. The scattering intensity of the magnetic dipole resonance of Si nanospheres decreases differently depending on their size, while the intensity of the electric dipole resonance remains almost unchanged. Meanwhile, Fano resonances are observed in the Si nanosphere dimers with a continuous variable Fano lineshape when adjusting the temperature. Mie theory and substrate-induced resonant magneto-electric effects are used to analyze and explain these phenomena. Tuning of the Fano resonance is attributed to the substrate effect from the interaction between Si nanospheres and phase transition of the VO₂ layer with temperature. These light scattering properties of such a hybrid nanostructure make it promising for temperature sensing or as a light source at the nanometer scale.

Absorption enhancement in methylammonium lead iodide perovskite solar cells with embedded arrays of dielectric particles

In the field of hybrid organic-inorganic perovskite based photovoltaics, there is a growing interest in the exploration of novel and smarter ways to improve the cells light harvesting efficiency at targeted wavelength ranges within the minimum volume possible, as well as in the development of colored and/or semitransparent devices that could pave the way both to their architectonic integration and to their use in the flowering field of tandem solar cells. The work herein presented targets these different goals by means of the theoretical optimization of the optical design of standard opaque and semitransparent perovskite solar cells. In order to do so, we focus on the effect of harmless, compatible and commercially available dielectric inclusions within the absorbing material, methylammonium lead iodide (MAPI). Following a gradual and systematic process of analysis, we are capable of identifying the appearance of collective and hybrid (both localized and extended) photonic resonances which allow to significantly improve light harvesting and thus the overall efficiency of the standard device by above 10% with respect to the reference value while keeping the semiconductor film thickness to a minimum. We believe our results will be particularly relevant in the promising field of perovskite solar cell based tandem photovoltaic devices, which has posed new challenges to the solar energy community in order to maximize the performance of semitransparent cells, but also for applications focusing on architectonic integration.

Photoluminescence quenching of dye molecules near a resonant silicon nanoparticle

Luminescent molecules attached to resonant colloidal particles are an important tool to study light-matter interaction. A traditional approach to enhance the photoluminescence intensity of the luminescent molecules in such conjugates is to incorporate spacer-coated plasmonic nanoantennas, where the spacer prevents intense non-radiative decay of the luminescent molecules. Here, we explore the capabilities of an alternative platform for photoluminescence enhancement, which is based on low-loss Mie-resonant colloidal silicon particles. We demonstrate that resonant silicon particles of spherical shape are more efficient for photoluminescence enhancement than their plasmonic counterparts in spacer-free configuration. Our theoretical calculations show that significant enhancement originates from larger quantum yields supported by silicon particles and their resonant features. Our results prove the potential of high-index dielectric particles for spacer-free enhancement of photoluminescence, which potentially could be a future platform for bioimaging and nanolasers.