Quantum-Cutting Luminescent Solar Concentrators Using Ytterbium-Doped Perovskite Nanocrystals

Integrated device of luminescent solar concentrators and electrochromic supercapacitors for self-powered smart window and display

Luminescent solar concentrators are translucent photovoltaic modules potentially used for building window. To store the energy generated by them, a separate energy storage module and voltage regulator module are required, but it is clear that this pairing is unwieldy for application. To address this problem, we propose a "face-to-face" tandem integration of luminescent solar concentrators and electrochromic supercapacitors. In this case, the separated energy storage module and voltage regulator module are not required, since the electric energy produced by concentrators under sunlight can be directly stored by the supercapacitors with matched voltage window. The charged energy storage module can be used to supply low-power devices. Moreover, electrochromic supercapacitors exhibit adjustable average visible transmission under different energy storage state, making the integrated device interesting for self-powered electrochromic smart windows or display devices. As an example, a self-powered information instruction display is prepared, and text messages could be clearly and rapidly displayed in a controllable manner. The integrated device capable of photovoltaic conversion, energy storage, and electrochromism is a promising alternative for smart windows.

Stacked Scintillators Based Multispectral X-Ray Imaging Featuring Quantum-Cutting Perovskite Scintillators With 570 nm Absorption-Emission Shift

Traditional energy-integration X-ray imaging systems rely on total X-ray intensity for image contrast, ignoring energy-specific information. Recently developed multilayer stacked scintillators have enabled multispectral, large-area flat-panel X-ray imaging (FPXI), enhancing material discrimination capabilities. However, increased layering can lead to mutual excitation, which may affect the accurate discrimination of X-ray energy. This issue is tackled by proposing a novel design strategy utilizing rare earth ions doped quantum-cutting scintillators as the top layer. These scintillators create new luminescence centers via energy transfer, resulting in a significantly larger absorption-emission shift, as well as the potential to double the photoluminescence quantum yield (PLQY) and enhance light output. To verify this concept, a three-layer stacked scintillator detector is developed using ytterbium ions (Yb3+)-doped CsPbCl3 perovskite nanocrystals (PeNCs) as the top layer, which offers a high PLQY of over 100% and a significant absorption-emission shift of 570 nm. This configuration, CsAgCl2 and Cs3Cu2I5 as the middle and bottom layers, respectively, ensures non-overlapping optical absorption and radioluminescence (RL) emission spectra. By calculating the optimal thickness for each layer to absorb specific X-ray energies, the detector demonstrates distinct absorption differences across various energy bands, enhancing the identification of materials with similar densities.

Impact of Sensitizer Yb3+ on Structural and Optical Properties of AE2SiO4 (AE = Ba, Ca, Sr) Orthosilicates

In the present work, BaSrSiO4: Yb and CaSrSiO4: Yb (Yb = 0 mol %, 1 mol %, 2 mol % and 3 mol %) nanophosphors were synthesized using the co-precipitation method, and their comparative structural and optical properties were studied to demonstrate the effect of Yb ion on the characteristic properties of the prepared nanophosphors. The powder X-ray diffraction (PXRD) patterns of BaSrSiO4:Yb and CaSrSiO4:Yb suggest the formation of a single-phase orthorhombic structure with space groups Pbnm and Pna21, respectively. The Raman shifts in BaSrSiO4 (BSS) and CaSrSiO4 (CSS) indicate out-of-phase bending of Sr-O and symmetric stretching of Si-O. The FTIR spectra show bands in the spectra between 1033 and 1063 cm-1, which can be attributed to the vibration of the Si-O-Si bond in both samples. The result also supported the formation of orthosilicate. SEM revealed agglomerated chunks in the BSS samples and rod-like structures in the CSS samples. The PL spectra show a significant change in the intensity of the emission peak at 975 nm following the transition 2F7/2 → 2F5/2 for different alkali earth metals (Ca, Ba), which is related to the energy level and formation of Yb2+ ion in the respective hosts. In CaSrSiO4 doped Yb a red emitting band at 700 nm is observed which is not present in BaSrSiO4 host.

Stable and Highly Emissive Infrared Yb-Doped Perovskite Quantum Cutters Engineered by Machine Learning

Quantum cutting (QC) allows the conversion of high-energy photons into lower-energy photons, exhibiting great potential for infrared communications. Yb-doped perovskite nanocrystals can achieve an efficient QC process with extremely high photoluminescence quantum yield (PLQY) thanks to the favorable Yb3+ incorporation in the perovskite structure. However, conventionally used oleic acid-oleylamine-based ligand pairs cause instability issues due to highly dynamic binding to surface states that have curbed their potential applications. Herein, zwitterionic type C3-sulfobetaine 3-(N,N-Dimethylpalmitylammonio)propanesulfonate molecule is utilized to build a strong binding state on the nanocrystals' surface through a new phosphine oxide synthesis route. Leveraging machine learning and Bayesian Optimization workflow to determine optimal synthesis conditions, near-infrared PLQY above 190% is achieved. The high PLQY is well maintained after over three months of aging, under high-flux continuous UV irradiation, and long continuous annealing. This is the first report of highly efficient and stable perovskite quantum cutters, which will drive the study of fundamental physics phenomena and near-infrared quantum communications.

Highly DUV to NIR-II responsive broadband quantum dots heterojunction photodetectors by integrating quantum cutting luminescent concentrators

Low-cost, high-performance, and uncooled broadband photodetectors (PDs) have potential applications in optical communication etc., but it still remains a huge challenge to realize deep UV (DUV) to the second near-infrared (NIR-II) detection for a single broadband PD. Herein, a single PD affording broadband spectral response from 200 to 1700 nm is achieved with a vertical configuration based on quantum dots (QDs) heterojunction and quantum cutting luminescent concentrators (QC-LC). A broadband quantum dots heterojunction as absorption layer was designed by integrating CsPbI3:Ho3+ perovskite quantum dots (PQDs) and PbS QDs to realize the spectral response from 400 to 1700 nm. The QC-LC by employing CsPbCl3:Cr3+, Ce3+, Yb3+, Er3+ PQDs as luminescent conversion layer to collect and concentrate photon energy for boosting the DUV-UV (200-400 nm) photons response of PDs by waveguide effect. Such broadband PD displays good stability, and outstanding sensitivity with the detectivity of 3.19 × 1012 Jones at 260 nm, 1.05 × 1013 Jones at 460 nm and 2.23 × 1012 Jones at 1550 nm, respectively. The findings provide a new strategy to construct broadband detector, offering more opportunities in future optoelectronic devices.

Efficient Large-Area Quantum Cutting Photoconversion Films for Silicon Solar Cells on Photovoltaic Glass Using Knife Coating

Yb3+ doped perovskite nanocrystals (PNCs) serve as efficient photoconverters, exhibiting quantum cutting emission at ∼980 nm, which aligns precisely with the optimal response region of silicon solar cells (SSCs). However, severe nonradiative recombination caused by defects in the crystal lattice and film boundaries, along with limitations in small-scale film preparation, restricts their commercial application. Here, we used Ru3+ to mitigate lattice defects in CsPbCl3 PNCs and adjusted the quantum cutting luminescence, achieving a 175% photoluminescence quantum yield (PLQY). The results show that Ru3+ ions enter the perovskite lattice, fill lead vacancies, and passivate the lattice defects. Furthermore, cysteine effectively eliminates surface defects in PNCs by forming Pb-S bonds, resulting in films with a remarkable 117% PLQY, demonstrating strong photoconversion capabilities. Uniformly knife-coated on 20 × 20 cm2 photovoltaic glass, these films increased SSC efficiency from 21.45% to 23.15%. This study showcases a cost-effective photoconverter and a scalable coating method to boost the photovoltaic efficiency of large-area SSCs.

Preferential growth and electron trap synergistically promoting photoreduction CO2 of Tm ion doping bismuth titanate nanosheets

In this study, we prepared two-dimensional Bi4Ti3O12 nanosheets doped with rare earth ions. The experimental results show that Bi4-xTmxTi3O12 exhibits the highest reduction performance among various rare earth doped Bi4Ti3O12 materials, with a CO yield of 7.25 μmol g-1h-1. Furthermore, a delayed reaction in Bi3.97Tm0.03Ti3O12 is observed upon a cessation of light irradiation. Theoretical calculations reveal that the introduction of Tm ion not only reduces the surface energy of (001) plane and make it preferential growth in Bi4Ti3O12, but also brings the intervening energy level of Tm ion (4f and 4d mixed orbital), which is closer to the conduction band of Bi4Ti3O12 and facilitates charge carrier accumulation in trap states. The electrons retained in the shallow traps promote the hysteresis reaction following a cessation of illumination. This work provides further insights into elucidating precise reduction reaction mechanisms underlying rare earth dopant on photocatalysts. This research provides enhanced insights into unraveling the precise reduction reaction mechanisms influenced by rare earth dopants in photocatalysts.

Near-infrared two-photon excited photoluminescence from Yb3+-doped CsPbClxBr3-x perovskite nanocrystals embedded into amphiphilic silica microspheres

Nonlinear absorption of metal-halide perovskite nanocrystals (NCs) makes them an ideal candidate for applications which require multiphoton-excited photoluminescence. By doping perovskite NCs with lanthanides, their emission can be extended into the near-infrared (NIR) spectral region. We demonstrate how the combination of Yb3+ doping and bandgap engineering of cesium lead halide perovskite NCs performed by anion exchange (from Cl- to Br-) leads to efficient and tunable emitters that operate under two-photon excitation in the NIR spectral region. By optimizing the anion composition, Yb3+-doped CsPbClxBr3-x NCs exhibited high values of two-photon absorption cross-section reaching 2.3 × 105 GM, and displayed dual-band emission located both in the visible (407-493 nm) and NIR (985 nm). With a view of practical applications of bio-visualisation in the NIR spectral range, these NCs were embedded into silica microspheres which were further wrapped with amphiphilic polymer shells to ensure their water-compatibility. The resulting microspheres with embedded NCs could be easily dispersed in both toluene and water, while still exhibiting a dual-band emission in visible and NIR under both one- and two-photon excitation conditions.

Speciation of Lanthanide Metal Ion Dopants in Microcrystalline All-Inorganic Halide Perovskite CsPbCl3

Lanthanides are versatile modulators of optoelectronic properties owing to their narrow optical emission spectra across the visible and near-infrared range. Their use in metal halide perovskites (MHPs) has recently gained prominence, although their fate in these materials has not yet been established at the atomic level. We use cesium-133 solid-state NMR to establish the speciation of all nonradioactive lanthanide ions (La3+, Ce3+, Pr3+, Nd3+, Sm3+, Sm2+, Eu3+, Eu2+, Gd3+, Tb3+, Dy3+, Ho3+, Er3+, Tm3+, Yb3+, Lu3+) in microcrystalline CsPbCl3. Our results show that all lanthanides incorporate into the perovskite structure of CsPbCl3 regardless of their oxidation state (+2, +3).

Trap-Mediated Sensitization Governs Near-Infrared Emission from Yb3+-Doped Mixed-Halide CsPbClxBr3-x Perovskite Nanocrystals

Understanding the photosensitization mechanisms in Yb3+-doped perovskite nanocrystals is crucial for developing their anticipated photonic applications. Here, we address this question by investigating near-infrared photoluminescence of Yb3+-doped mixed-halide CsPbClxBr3-x nanocrystals as a function of temperature and revealing its strong dependence on the stoichiometry of the host perovskite matrix. To explain the observed experimental trends, we developed a theoretical model in which energy transfer from the perovskite matrix to Yb3+ ions occurs through intermediate trap states situated beneath the conduction band of the host. The developed model provides an excellent agreement with experimental results and is further validated through the measurements of emission saturation at high excitation powers and near-infrared photoluminescence quantum yield as a function of the anion composition. Our findings establish trap-mediated energy transfer as a dominant photosensitization mechanism in Yb3+-doped CsPbClxBr3-x nanocrystals and open up new ways of engineering their optical properties for light-emitting and light-harvesting applications.

Chlorophyl-Passivated Ytterbium-Doped Perovskite Quantum-Cutting Film for High-Performance Solar Energy Conversion and Near-Infrared Light-Emitting Diode Applications

The quantum cutting ytterbium (Yb3+)-doped CsPbX3 (X = Cl, Cl, or Br) nanocrystals, exhibiting photoluminescence quantum yields (PLQYs) exceeding 100%, hold significant promise for applications in solar energy conversion technologies and near-infrared (NIR) light-emitting diodes (LEDs). This work investigates the usage of chlorophyll (CHL), a naturally existing organic pigment, as an efficient molecular passivator to improve the performance of quantum cutting films. With the assistance of CHL, the resultant perovskite film displays an increased PLQY of 176%. The commercial silicon solar cells (SSCs) with CHL-treated perovskite films demonstrate a remarkable photon-to-current conversion efficiency improvement of 1.83% for a 330.15 cm2 area SSC device. Additionally, a CHL-modified Yb3+:CsPbCl3 film was used to create 988 nm NIR LEDs with an external quantum efficiency of 3.2%. This work provides a new, eco-friendly approach for producing high-quality, large-area Yb3+-doped perovskite film for deployment in photoelectric and night vision applications.

Understanding the Effect of the Synthetic Method and Surface Chemistry on the Properties of CsPbBr3 Nanoparticles

Over the last decade, the attractive properties of CsPbBr3 nanoparticles (NPs) have driven ever-increasing progress in the development of synthetic procedures to obtain high-quality NPs at high concentrations. Understanding how the properties of NPs are influenced by the composition of the reaction mixture in combination with the specific synthetic methodology is crucial, both for further elucidating the fundamental characteristics of this class of materials and for their manufacturing towards technological applications. This work aims to shed light on this aspect by synthesizing CsPbBr3 NPs by means of two well-assessed synthetic procedures, namely, hot injection (HI) and ligand-assisted reprecipitation (LARP) in non-polar solvents, using PbBr2 and Cs2CO3 as precursors in the presence of already widely investigated ligands. The overall goal is to study and compare the properties of the NPs to understand how each synthetic method influences the NPs' size and/or the optical properties. Reaction composition and conditions are purposely tuned towards the production of nanocubes with narrow size distribution, high emission properties, and the highest achievable concentration. As a result, the formation of bulk crystals as precipitate in LARP limits the achievement of a highly concentrated NP solution. The size of the NPs obtained by LARP seems to be poorly affected by the ligands' nature and the excess bromide, as consequence of bromide-rich solvation agents, effectively results in NPs with excellent emission properties. In contrast, NPs synthesized by HI exhibit high reaction yield, diffusion growth-controlled size, and less striking emission properties, probably ascribed to a bromide-deficient condition.

Dendrimer-Encapsulated Halide Perovskite Nanocrystals for Self-Powered White Light-Emitting Glass

Perovskite nanocrystals (PNCs) have attracted substantial attention due to their inspiring intrinsic merits such as low cost, high performance, and solution processability, but when it comes to the usage of blends of different colored PNCs with the purpose of covering the broadband spectrum field, the high degree of instability remains a major bottleneck. Herein, we report a family of dendritic ammonium ligands that act as stiff shell-encapsulating PNCs for improving their stability and suppressing ion permeability in mixed colloidal PNC solutions. The as-synthesized ligand-encapsulated PNCs notably achieve near-unity photoluminescence quantum yields (PLQYs) and strongly resist the unwanted ion exchange reaction under aggressive anion source attack. To fabricate self-powered white-emitting glass, the stabilized mixed colored PNCs were embedded into the laminated glass, which simultaneously acted as absorbers-emitters for luminescent solar concentrators (LSCs) and emitters for white light-emitting glass.

X-ray Spectroscopy of a Rare-Earth Molecular System Measured at the Single Atom Limit at Room Temperature

We investigate the limit of X-ray detection at room temperature on rare-earth molecular films using lanthanum and a pyridine-based dicarboxamide organic linker as a model system. Synchrotron X-ray scanning tunneling microscopy is used to probe the molecules with different coverages on a HOPG substrate. X-ray-induced photocurrent intensities are measured as a function of molecular coverage on the sample, allowing a correlation of the amount of La ions with the photocurrent signal strength. X-ray absorption spectroscopy shows cogent M4,5 absorption edges of the lanthanum ion originated by the transitions from the 3d3/2 and 3d5/2 to 4f orbitals. X-ray absorption spectra measured in the tunneling regime further reveal an X-ray excited tunneling current produced at the M4,5 absorption edge of the La ion down to the ultimate atomic limit at room temperature.

Negative Thermal Quenching in Quantum-Cutting Yb3+-Doped CsPb(Cl1-xBrx)3 Perovskite Nanocrystals

Ytterbium-doped all-inorganic lead-halide perovskites (Yb3+:CsPb(Cl1-xBrx)3) show broadband absorption and exceptionally high near-infrared photoluminescence quantum yields, providing opportunities for solar spectral shaping to improve photovoltaic power conversion efficiencies. Here, we report that Yb3+:CsPb(Cl1-xBrx)3 NCs also show extremely strong negative thermal quenching of the Yb3+ luminescence, with intensities at room temperature >100 times those at 5 K for some compositions. Analysis of this temperature dependence as a function of x shows that it stems from thermally activated quantum cutting related to the temperature dependence of the spectral overlap between the PL of the perovskite (donor) and the simultaneous-pair absorption of two Yb3+ ions (acceptor). In the Yb3+:CsPbBr3 limit, this spectral overlap goes to zero at 5 K, such that only single-Yb3+ sensitization requiring massive phonon emission occurs. At room temperature, Yb3+ PL in this composition is enhanced ∼135-fold by thermally activated quantum cutting, highlighting the extreme efficiency of quantum cutting relative to single-Yb3+ sensitization. These results advance the fundamental mechanistic understanding of quantum cutting in doped perovskites, with potential ramifications for solar and photonics technologies.

Atomic-scale imaging of ytterbium ions in lead halide perovskites

Lanthanide-doped lead halide perovskites have demonstrated great potential for photoelectric applications. However, there is a long-standing controversy about the existence of lanthanide ions, e.g., whether the doping of Ln3+ is successful or not; the substituting sites of Ln3+ in lead halide perovskites are unclear. We directly identify the doped Yb3+ in CsPbCl3 perovskites by using the state-of-the-art transmission electron microscopy and three-dimensional atom probe tomography at atomic scale. Different from the previous assumptions and/or results, we evidence that Yb3+ simultaneously replace Pb2+ and occupy the lattice interstitial sites. Furthermore, we directly observe the cluster phenomenon of CsPbCl3 single crystal at near atomic scale. Density functional theory modeling further confirms and explains the mechanisms of our findings. Our findings thus provide an atomic-level understanding of the doping mechanism in perovskites and will stimulate a further thinking of the doping effect on the performance of perovskites.

Environmental Effects on the Performance of Quantum Dot Luminescent Solar Concentrators

Luminescent solar concentrators (LSCs) are all-photonic, semitransparent solar devices with great potential in the emerging fields of building-integrated photovoltaics and agrivoltaics. Over the past decade, particularly with the advent of quantum dot (QD) LSCs, tremendous progress has been made in terms of photovoltaic efficiency and device size by increasing solar spectral coverage and suppressing reabsorption losses. Despite these advances in LSC design, the effects of environmental conditions such as rain, dust, and dirt deposits, which are ubiquitous in both urban and agricultural environments, on LSC performance have been largely overlooked. Here, we address these issues by systematically investigating the environmental effects on the solar harvesting and waveguiding capability of state-of-the-art QD-LSCs, namely, the presence of airborne pollutants (dust), water droplets, and dried deposits. Our results show that dust is unexpectedly insignificant for the waveguiding of the concentrated luminescence and only reduces the LSC efficiency through a shadowing effect when deposited on the outer surface, while dust accumulation on the inner LSC side increases the output power due to backscattering of transmitted sunlight. Water droplets, on the other hand, do not dim the incident sunlight, but are detrimental to waveguiding by forming an optical interface with the LSC. Finally, dried deposits, which mimic the evaporation residues of heavy rain or humidity, have the worst effect of all, combining shading and waveguide losses. These results are relevant for the design of application-specific surface functionalization/protection strategies real LSC modules.

Broad Luminescence Generated by IR Laser Excitation from CsPbBr3:Yb3+ Perovskite Ceramics

This paper demonstrates the generation of broadband emission in the visible and infrared ranges induced by a concentrated beam of infrared radiation from CsPbBr₃ ceramics doped with Yb³⁺ ions. The sample was obtained by the conventional solid-state reaction method, and XRD measurements confirmed the phase purity of the material crystallizing in the orthorhombic system. Spectroscopic measurements required further sample preparation in the form of ceramics using a high-pressure press. The research showed that as the excitation power increases, the emission intensity does not increase linearly from the beginning of the experiment. Irradiation of the material results in the accumulation of the delivered energy. Absorption of a sufficient number of photons triggers avalanche emission. It was found that the most intense luminescence is produced in a vacuum. Changes in conductivity were also observed, where the excitation was able to lower the resistivity of the material and it was highly dependent on the excitation power. The mechanism responsible for the generation of the observed phenomenon involving intervalence charge transfer (IVCT) transitions has been postulated.

Heterostructured Nanotetrapod Luminophores for Reabsorption Elimination within Luminescent Solar Concentrators

Luminescent solar concentrators (LSCs) concentrate light via luminescence within a planar-waveguide and have potential use for building-integrated photovoltaics. However, their commercialization and potential applications are currently hindered greatly by photon reabsorption, where emitted waveguided light is parasitically reabsorbed by a luminophore. Nanotetrapod semiconductor materials have been theorized to be excellent luminophores for LSCs owing to their inherently large Stokes shifts. Here we present the first nanotetrapod-based LSCs (5 × 5 × 0.3 cm³) reported in the literature. External quantum efficiencies as high as 4.9 ± 0.5% were achieved under AM1.5G conditions. We also perform an in-depth investigation by optical characterization of the different operational metrics of our nanotetrapod-based LSCs and show reabsorption to be eliminated (mean number of average reabsorption events per photon equal to 0.00) in our most extended nanotetrapod devices.

Metal-Halide Perovskite Nanocrystal Superlattice: Self-Assembly and Optical Fingerprints

Self-assembly of nanocrystals into superlattices is a fascinating process that not only changes geometric morphology, but also creates unique properties that considerably enrich the material toolbox for new applications. Numerous studies have driven the blossoming of superlattices from various aspects. These include precise control of size and morphology, enhancement of properties, exploitation of functions, and integration of the material into miniature devices. The effective synthesis of metal-halide perovskite nanocrystals has advanced research on self-assembly of building blocks into micrometer-sized superlattices. More importantly, these materials exhibit abundant optical features, including highly coherent superfluorescence, amplified spontaneous laser emission, and adjustable spectral redshift, facilitating basic research and state-of-the-art applications. This review summarizes recent advances in the field of metal-halide perovskite superlattices. It begins with basic packing models and introduces various stacking configurations of superlattices. The potential of multiple capping ligands is also discussed and their crucial role in superlattice growth is highlighted, followed by detailed reviews of synthesis and characterization methods. How these optical features can be distinguished and present contemporary applications is then considered. This review concludes with a list of unanswered questions and an outlook on their potential use in quantum computing and quantum communications to stimulate further research in this area.

Ytterbium-Doped Lead-Halide Perovskite Nanocrystals: Synthesis, Near-Infrared Emission, and Open-Source Machine Learning Model for Prediction of Optical Properties

Lead-halide perovskite nanocrystals are an attractive class of materials since they can be easily fabricated, their optical properties can be tuned all over the visible spectral range, and they possess high emission quantum yields and narrow photoluminescence linewidths. Doping perovskites with lanthanides is one of the ways to widen the spectral range of their emission, making them attractive for further applications. Herein, we summarize the recent progress in the synthesis of ytterbium-doped perovskite nanocrystals in terms of the varying synthesis parameters such as temperature, ligand molar ratio, ytterbium precursor type, and dopant content. We further consider the dependence of morphology (size and ytterbium content) and optical parameters (photoluminescence quantum yield in visible and near-infrared spectral ranges) on the synthesis parameters. The developed open-source code approximates those dependencies as multiple-parameter linear regression and allows us to estimate the value of the photoluminescence quantum yield from the parameters of the perovskite synthesis. Further use and promotion of an open-source database will expand the possibilities of the developed code to predict the synthesis protocols for doped perovskite nanocrystals.

Achieving High-Efficiency Large-Area Luminescent Solar Concentrators

Luminescent solar concentrators (LSCs) are semitransparent windows that are able to generate electricity from sunlight absorption. LSCs have shown huge promise for realizing building-integrated photovoltaics (BIPV). Unfortunately, to date, the power conversion efficiency (PCE) of LSCs is still very low which dramatically hampers their practical applications. In this Perspective, We summarize and review the latest developments of LSCs by looking at different structures. Among others, we focus more on the next developments in the field of LSCs, i.e., the possibility of high PCE, large area, mass production, and durability needed for future industrial development. We hope to promote the application of uniform testing standards and to draw attention to industrial development, toxicity, and durability. Then, we will provide a critical assessment of the field of LSCs. Finally, the challenge and solution will be discussed.

High-efficiency liquid luminescent solar concentrator based on CsPbBr3 quantum dots

The performance degradation is still a challenge for the development of conventional polymer luminescent solar concentrator (LSC). Liquid LSC (L-LSC) may be an alternative due to polymerization-free fabrication. Here, we have prepared a CsPbBr₃ quantum dots (QDs)-based L-LSC by injecting the QDs solution into a self-assembly quartz glass mold. The as-fabricated L-LSC performance is evaluated by optical characterization and photo-electrical measurement. The external quantum efficiency of the L-LSC is up to 13.44%. After coupling the commercial solar cell, the optimal optical efficiency reaches 2.32%. These results demonstrate that L-LSC may provide a promising direction for advanced solar light harvesting technologies.

Environmentally friendly, highly efficient, and large stokes shift-emitting ZnSe:Mn2+/ZnS core/shell quantum dots for luminescent solar concentrators

In this study, heavy-metal-free orange light-emitting ZnSe:Mn²⁺/ZnS doped-core/shell (d-C/S) quantum dots (QDs) were synthesized using a nucleation doping strategy. To synthesize high quality d-C/S QDs with high photoluminescence (PL) quantum yield (QY), the Mn²⁺ concentration was optimized. The resulting ZnSe:Mn²⁺(5%)/ZnS d-C/S QDs showed a high PL QY of 83.3%. The optical properties of the synthesized QDs were characterized by absorption and PL spectroscopy. Their structural and compositional properties were studied by X-ray diffraction, transmission electron microscopy, and energy dispersive X-ray spectroscopy. After doping Mn²⁺ into a ZnSe core, the ZnSe:Mn²⁺/ZnS d-C/S QDs showed a large Stokes shift of 170 nm. The ZnSe:Mn²⁺/ZnS d-C/S QDs were embedded in a poly(lauryl methacrylate) (PLMA) polymer matrix and the ZnSe:Mn²⁺/ZnS-based polymer film was fabricated. The fabricated ZnSe:Mn²⁺/ZnS-PLMA film was highly transparent in the visible spectral region (transmittance > 83.8% for λ ≥ 450 nm) and it exhibited bright orange light under air mass (AM) 1.5G illumination using a solar simulator. The optical path-dependent PL measurement of the ZnSe:Mn²⁺/ZnS-PLMA film showed no PL band shift and minimal PL decrease under variation of excitation position. These results indicate that the highly efficient and large Stokes shift-emitting ZnSe:Mn²⁺/ZnS QDs are promising for application to luminescent solar concentrators.

Low-Loss, High-Transparency Luminescent Solar Concentrators with a Bioinspired Self-Cleaning Surface

Luminescent solar concentrators (LSCs) have emerged as a disruptive technology that can potentially enable carbon-neutral buildings. The issues with current LSCs, however, are low optical efficiencies and limited long-term outdoor stability. Here we simultaneously address them by developing an LSC with aggregation-induced-emission (AIE) molecules embedded in a polydimethylsiloxane (PDMS) matrix. The AIE-emitter displayed a near unity emission quantum yield when embedded in the PDMS and the apparent absorption-emission Stokes shift reached 0.59 eV, effectively suppressing the reabsorption loss of waveguided photons inside an LSC. Moreover, the surface texture of the PDMS matrix was engineered using a bioinspired nanolithography method with a natural lotus leaf as the template. This allowed the fabricated AIE-PDMS LSC to inherit the superhydrophobic, self-cleaning properties of the leaf and meanwhile to possess a light-trapping capability. Our 100 cm² LSC, when coupled with commercial Si PVs, delivered efficient solar power conversion, high visible transmittance, and high working stability.

Mn-Doped Multiple Quantum Well Perovskites for Efficient Large-Area Luminescent Solar Concentrators

Luminescent solar concentrators (LSCs) can be used as large-area sunlight collectors, which show great potential in the application of building-integrated photovoltaic areas. Achieving highly efficient LSCs requires the suppression of reabsorption losses while maintaining a high photoluminescence quantum yield (PLQY) and broad absorption. Perovskites as the superstar fluorophores have recently emerged as candidates for large-area LSCs. However, highly emissive perovskites with a large Stokes shift and broad absorption have not been obtained up to now. Here, we devised a facile synthetic route to obtain Mn-doped multiple quantum well (MQW) Br-based perovskites. The Br-based perovskite host ensures broad absorption. Efficient energy transfer from the exciton to the Mn dopant produces a large Stokes shift and high PLQY simultaneously. By further coating the perovskites with Al₂O₃, the stability and PLQY are greatly elevated. A large area of liquid LSC (40 cm × 40 cm × 0.5 cm) is fabricated, which possesses an internal quantum efficiency (η_int) of 47% and an optical conversion efficiency (η_opt) reaching 11 ± 1%, which shows the highest value for large-area LSCs.

Quantum Dot-based Luminescent Solar Concentrators Fabricated through the Ultrasonic Spray-Coating Method

Luminescent solar concentrators (LSCs) are a class of wave-guiding devices that can harvest solar light and concentrate it to targeted smaller areas. When coupled with photovoltaic devices (PVs), LSCs hold the potential to be integrated into various application setups, especially for building facade integration toward net-zero-energy buildings. Developing reliable LSC fabrication methods with easy scalability, high adaptability, and device controllability has been an important research topic. In this work, we report an ultrasonic nebulization-assisted spray deposition technique to fabricate quantum dot (QD)-based LSCs (QD-LSCs). This method allows for the production of high-performance QD-LSCs with different device dimensions and geometries. In addition, the quality of the QD thin-film coating layer is relatively independent of the concentration and volume of the coating QD ink solution, allowing for deliberate programming and performance optimization of the resulting QD-LSC devices. We anticipate that this ultrasonic spray coating method can be widely applied to the manufacturing of high-quality LSC devices that are integrable to various applications.

Review for Rare-Earth-Modified Perovskite Materials and Optoelectronic Applications

In recent years, rare-earth metals with triply oxidized state, lanthanide ions (Ln³⁺), have been demonstrated as dopants, which can efficiently improve the optical and electronic properties of metal halide perovskite materials. On the one hand, doping Ln³⁺ ions can convert near-infrared/ultraviolet light into visible light through the process of up-/down-conversion and then the absorption efficiency of solar spectrum by perovskite solar cells can be significantly increased, leading to high device power conversion efficiency. On the other hand, multi-color light emissions and white light emissions originated from perovskite nanocrystals can be realized via inserting Ln³⁺ ions into the perovskite crystal lattice, which functioned as quantum cutting. In addition, doping or co-doping Ln³⁺ ions in perovskite films or devices can effectively facilitate perovskite film growth, tailor the energy band alignment and passivate the defect states, resulting in improved charge carrier transport efficiency or reduced nonradiative recombination. Finally, Ln³⁺ ions have also been used in the fields of photodetectors and luminescent solar concentrators. These indicate the huge potential of rare-earth metals in improving the perovskite optoelectronic device performances.

Construction of highly efficient carbon dots-based polymer photonic luminescent solar concentrators with sandwich structure

The enhancement of photoluminescence (PL) emission and waveguide play a key role in improving the optical efficiency of luminescent solar concentrators (LSCs). In this work, to boosting PL emission and waveguide simultaneously, one photonic crystal (PC) structure (crystalline colloid arrays (CCAs)) was introduced into carbon dots (CDs)-based polymer LSCs. A sandwich-structured CDs-based polymer photonic LSC, comprising glass/CDs-based polymer PC film/glass, was created. First, CDs-based colloidal crystal suspensions were prepared by co-assembly of monodispersed p(MMA-NIPAm) colloids and multicolor-emitting CDs in HEMA monomer induced by the evaporation-driven assembly. The obtained suspensions not only had uniform PL and structural colors, but showed enhanced PL emission. Second, the above suspensions were sandwiched between two glass sheets and finally a photonic polymer LSC with sandwiched structure (25 × 25 × 1.8 mm³) were formed via one-step photopolymerization technique. Remarkably, the optimal CDs-based polymer photonic LSCs with sandwiched structure not only had high transparence at visible range (>60%), but exhibited PL emission enhancement (at least 2 times). Furthermore, the maximum external optical efficiency (η_opt) of 5.84% could be achieved based on yellow-emitting CDs-based polymer photonic LSC. The high external optical efficiency was mainly attributed to the PL emission enhancement and good PC waveguide.

A novel approach for designing efficient broadband photodetectors expanding from deep ultraviolet to near infrared

Broadband photodetection (PD) covering the deep ultraviolet to near-infrared (200-1000 nm) range is significant and desirable for various optoelectronic designs. Herein, we employ ultraviolet (UV) luminescent concentrators (LC), iodine-based perovskite quantum dots (PQDs), and organic bulk heterojunction (BHJ) as the UV, visible, and near-infrared (NIR) photosensitive layers, respectively, to construct a broadband heterojunction PD. Firstly, experimental and theoretical results reveal that optoelectronic properties and stability of CsPbI₃ PQDs are significantly improved through Er³⁺ doping, owing to the reduced defect density, improved charge mobility, increased formation energy, tolerance factor, etc. The narrow bandgap of CsPbI₃:Er³⁺ PQDs serves as a visible photosensitive layer of PD. Secondly, considering the matchable energy bandgap, the BHJ (BTP-4Cl: PBDB-TF) is selected as to NIR absorption layer to fabricate the hybrid structure with CsPbI₃:Er³⁺ PQDs. Thirdly, UV LC converts the UV light (200-400 nm) to visible light (400-700 nm), which is further absorbed by CsPbI₃:Er³⁺ PQDs. In contrast with other perovskites PDs and commercial Si PDs, our PD presents a relatively wide response range and high detectivity especially in UV and NIR regions (two orders of magnitude increase that of commercial Si PDs). Furthermore, the PD also demonstrates significantly enhanced air- and UV- stability, and the photocurrent of the device maintains 81.5% of the original one after 5000 cycles. This work highlights a new attempt for designing broadband PDs, which has application potential in optoelectronic devices.

Doping Colloidal Quantum Dot Materials and Devices for Photovoltaics

Colloidal semiconductor nanocrystals have generated tremendous interest because of their solution processability and robust tunability. Among such nanocrystals, the colloidal quantum dot (CQD) draws the most attention for its well-known quantum size effects. In the last decade, applications of CQDs have been booming in electronics and optoelectronics, especially in photovoltaics. Electronically doped semiconductors are critical in the fabrication of solar cells, because carefully designed band structures are able to promote efficient charge extraction. Unlike conventional semiconductors, diffusion and ion implantation technologies are not suitable for doping CQDs. Therefore, researchers have creatively developed alternative doping methods for CQD materials and devices. In order to provide a state-of-the-art summary and comprehensive understanding to this research community, we focused on various doping techniques and their applications for photovoltaics and demystify them from different perspectives. By analyzing two classes of CQDs, lead chalcogenide CQDs and perovskite CQDs, we compared different working scenarios of each technique, summarized the development in this field, and raised our own future perspectives.

Highly Luminescent and Stable Organic-Inorganic Hybrid Films for Transparent Luminescent Solar Concentrators

Here, a highly luminescent, stable, and visible-transparent organic-inorganic hybrid film was in situ synthesized in a siloxane-polyether (di-ureasil) sol-gel process by dissolving a 4-hydroxy-2-methyl-1,5-naphthyridine-3-carbonitrile (2mCND) ligand and a europium(III) ion. Doping a europium(III) complex into di-ureasil achieves an boost in photoluminescence quantum efficiency (PLQY) from 23.25 to 68.9%. In particular, the excellent photostability of the hybrid film was demonstrated after a 15 h aging experiment in strong UV-LED irradiation (∼468 mW/cm²). Compared to the polymethyl methacrylate (PMMA) matrix, di-ureasil containing a europium(III) complex shows an improved UV resistance, making it a promising candidate for various photonic applications. By integrating the hybrid film onto an acrylic substrate, a transparent luminescent solar concentrator (LSC) was fabricated, which reveals an optical conversion efficiency of ∼0.51% with a G factor of 3.1 at an optical transmission level of ∼90%. Such an LSC could be of particular interest in future transparent photovoltaic windows.

Defect-Interceded Cascading Energy Transfer and Underlying Charge Transfer in Europium-Doped CsPbCl3 Nanocrystals

Rare-earth ion (RE³⁺) doping in cesium lead chloride (CsPbCl₃) has unlocked novel prospects to explore changes in optical, magnetic, and charge carrier transport properties. This leads to a huge advancement in optoelectronic applications, yet deep understanding of the photophysics governing the energy transfer processes is lacking and demands vital attention. Herein, we probe into the mechanistic transfer processes from the band edge of the host (CsPbCl₃) to the dopant europium ion (Eu³⁺) with the aid of femtosecond fluorescence upconversion and transient absorption (TA) spectroscopy. The upconversion measurement portrays a defect-mediated cascading energy transfer from CsPbCl₃ to Eu³⁺ and further cross-relaxation among Eu³⁺ states. Moreover, TA studies reveal that there is charge transfer from the band edge of CsPbCl₃ to doping-induced shallow defect states. Furthermore, two-photon absorption study establishes no compromise in the transfer mechanism even upon bandgap excitation. This work validates that Eu-CsPbCl₃ is an apt entrant for optoelectronic applications.

Rare-Earth Doping in Nanostructured Inorganic Materials

Impurity doping is a promising method to impart new properties to various materials. Due to their unique optical, magnetic, and electrical properties, rare-earth ions have been extensively explored as active dopants in inorganic crystal lattices since the 18th century. Rare-earth doping can alter the crystallographic phase, morphology, and size, leading to tunable optical responses of doped nanomaterials. Moreover, rare-earth doping can control the ultimate electronic and catalytic performance of doped nanomaterials in a tunable and scalable manner, enabling significant improvements in energy harvesting and conversion. A better understanding of the critical role of rare-earth doping is a prerequisite for the development of an extensive repertoire of functional nanomaterials for practical applications. In this review, we highlight recent advances in rare-earth doping in inorganic nanomaterials and the associated applications in many fields. This review covers the key criteria for rare-earth doping, including basic electronic structures, lattice environments, and doping strategies, as well as fundamental design principles that enhance the electrical, optical, catalytic, and magnetic properties of the material. We also discuss future research directions and challenges in controlling rare-earth doping for new applications.

Organic building blocks at inorganic nanomaterial interfaces

This tutorial review presents our perspective on designing organic molecules for the functionalization of inorganic nanomaterial surfaces, through the model of an "anchor-functionality" paradigm. This "anchor-functionality" paradigm is a streamlined design strategy developed from a comprehensive range of materials (e.g., lead halide perovskites, II-VI semiconductors, III-V semiconductors, metal oxides, diamonds, carbon dots, silicon, etc.) and applications (e.g., light-emitting diodes, photovoltaics, lasers, photonic cavities, photocatalysis, fluorescence imaging, photo dynamic therapy, drug delivery, etc.). The structure of this organic interface modifier comprises two key components: anchor groups binding to inorganic surfaces and functional groups that optimize their performance in specific applications. To help readers better understand and utilize this approach, the roles of different anchor groups and different functional groups are discussed and explained through their interactions with inorganic materials and external environments.

Surface Chemistry Engineering of Perovskite Quantum Dots: Strategies, Applications, and Perspectives

The presence of surface ligands not only plays a key role in keeping the colloidal integrity and non-defective surface of metal halide perovskite quantum dots (PQDs), but also serves as a knob to tune their optoelectronic properties for a variety of exciting applications including solar cells and light-emitting diodes. However, these indispensable surface ligands may also deteriorate the stability and key properties of PQDs due to their highly dynamic binding and insulating nature. To address these issues, a number of innovative surface chemistry engineering approaches have been developed in the past few years. Based on an in-depth fundamental understanding of the surface atomistic structure and surface defect formation mechanism in the tiny nanoparticles, a critical overview focusing on the surface chemistry engineering of PQDs including advanced colloidal synthesis, in-situ surface passivation, and solution-phase/solid-state ligand exchange is presented, after which their unprecedented achievements in photovoltaics and other optoelectronics are presented. The practical hurdles and future directions are critically discussed to inspire more rational design of PQD surface chemistry toward practical applications.

Eco-Friendly and Efficient Luminescent Solar Concentrators Based on a Copper(I)-Halide Composite

Luminescent solar concentrators (LSCs) show great promise in reducing the cost of silicon solar cells due to their potential use for high-efficiency energy harvesting. Compared to narrow absorption organic dyes, quantum dots (QDs) are a favorable approach to acquire stable LSCs. However, the use of toxic heavy metals in QDs and the small Stokes shift largely restrict their development. Here, a toxic metal-free, highly luminescent ink based on a copper(I)-halide hybrid cluster is reported, whose quantum yield (QY) exceeds 68%. Under the interaction with halohydrocarbon, CuI and phenethylamine (PEA) can be easily dissolved and the ink can be facilely acquired. The obtained film exhibits strong orange light emission with a large Stokes shift. As a proof-of-concept experiment, (PEA)₄Cu₄I₄ has been used to fabricate LSCs. The as-prepared LSC (4 cm × 4 cm × 0.3 cm) exhibits an internal quantum efficiency (η_int) as high as 44.1%. After coupling to a solar cell, an optical conversion efficiency (η_opt) of 6.85% is acquired from this LSC. In addition, the LSC possesses high stability such as air stability, water stability, and photostability. These results demonstrate that the (PEA)₄Cu₄I₄ film can be employed as a promising candidate for large-area and high-efficiency LSCs.

Ni2+ and Pr3+ Co-doped CsPbCl3 perovskite quantum dots with efficient infrared emission at 1300 nm

Lead halide perovskite quantum dots (PQDs) show great prospects in the field of optoelectronic applications. Although having high efficiency and narrow-band emission performance in the visible light region, the infrared multicolor luminescence performance of perovskite nanocrystals is still highly desired. In this work, in order to increase the luminescence intensity and extend the infrared multicolor luminescence, transition metal and rare earth ions are co-doped into PQDs. Herein, PQDs emitting at 1300 nm are realized by Pr³⁺ doping, which has not been reported in previous literature. The luminescence and kinetic process of Ni²⁺ and Pr³⁺ co-doped CsPbCl₃ PQDs are studied, which exhibit considerably enhanced emission intensity at 400 nm and 1300 nm, with an overall quantum efficiency of photoluminescence (PLQY) of 89% and the highest infrared PLQY of 23%.

Efficient and Stable Mg2+-Doped CsPbCl3 Nanocrystals for Violet LEDs

It has been a challenge to prepare perovskite violet light-emitting diodes due to the low luminous efficiency and poor stability of the relevant materials. In this work, the defect state density of CsPbCl₃ NCs was reduced by Mg²⁺ doping, so the photoluminescence quantum efficiency (PLQY) was increased by 57 times (from 1.3% to 75.8%) compared with undoped CsPbCl₃ NCs. The doped CsPbCl₃ NCs had no phase change in the atmosphere over 180 days, and the PLQY was only dropped by 19%. Based on the highly efficient luminescence and high stability of Mg²⁺-doped CsPbCl₃ NCs, violet-light-emitting diodes were fabricated. The maximum brightness of device was 135 cd/m², and the peak EQE reached 0.1%.

Ytterbium-Doped CsPbCl3 Quantum Cutters for Near-Infrared Light-Emitting Diodes

Exploring highly efficient near-infrared (NIR) emitting materials is desirable for the advancement of next-generation smart NIR light sources. Different from most reported Cr³⁺-doped emitters with far-red emissions, Yb³⁺-activated phosphors are expected to yield pure NIR (∼1000 nm) light. Herein, a new hot-injection route using all metal-oleate salts to fabricate Yb³⁺-doped CsPbCl₃ perovskite nanocrystals (PeNCs) is reported for the first time, which produce PeNC-sensitized Yb³⁺ NIR emission with photoluminescence quantum yields (PLQYs) higher than 100%. With the help of temperature-dependent PL spectra, femtosecond transient absorption spectra, and time-resolved PL spectra, it is evidenced that the in situ produced intrinsic shallow trap states in a CsPbCl₃ host play a key role in facilitating the picosecond nonradiative cooperative energy transfer from PeNCs to two Yb³⁺ dopants simultaneously. Using the optimized Yb³⁺:CsPbCl₃ quantum cutters, a phosphor-converted NIR light-emitting diode (pc-NIR-LED) is fabricated, exhibiting an external quantum efficiency of 2%@28 mA, a high NIR output irradiance of 112 mW/cm²@400 mA, and excellent long-term stability. Finally, the designed pc-NIR-LED is demonstrated to have great potential as an invisible night-vision light source.

State of the Art and Prospects for Halide Perovskite Nanocrystals

Metal-halide perovskites have rapidly emerged as one of the most promising materials of the 21st century, with many exciting properties and great potential for a broad range of applications, from photovoltaics to optoelectronics and photocatalysis. The ease with which metal-halide perovskites can be synthesized in the form of brightly luminescent colloidal nanocrystals, as well as their tunable and intriguing optical and electronic properties, has attracted researchers from different disciplines of science and technology. In the last few years, there has been a significant progress in the shape-controlled synthesis of perovskite nanocrystals and understanding of their properties and applications. In this comprehensive review, researchers having expertise in different fields (chemistry, physics, and device engineering) of metal-halide perovskite nanocrystals have joined together to provide a state of the art overview and future prospects of metal-halide perovskite nanocrystal research.

A general approach to realizing perovskite nanocrystals with insulating metal sulfate shells

The strategy of constructing the core/shell structure is of great importance in emitting semiconductor nanocrystals. However, the coating on soft metal halide perovskite nanocrystals at the single particle level remains a challenge because of the low compatibility between perovskites and common wide-band-gap semiconductors, such as ZnS and CdS. In addition, using these semiconductors as the shell layer requires high reaction temperatures, which often lead to undesirable chemical transformation. Herein we report a general route to passivate the perovskite nanocrystals by insulating metal sulfate shells. The passivating shell is created around the as-synthesized CsPbBr3 perovskite nanocrystals by initiating the reaction between an organic ammonium sulfate and a variety of metal ions in the presence of ligands. This new method allowed for creating insulating metal sulfate shells with controllable thicknesses and without unwanted chemical transformation. Importantly, these novel core/shell-structured nanocrystals show photoluminescence quantum yields near unity, highly suppressed energy transfer in film and suppressed halide exchange in solution.

Low-Dimensional-Networked Perovskites with A-Site-Cation Engineering for Optoelectronic Devices

Low-dimensional-networked (LDN) perovskites denote materials in which the molecular structure adopts 2D, 1D, or 0D arrangement. Compared to conventional 3D structured lead halide perovskite (chemical formula: ABX₃ where A: monovalent cations, B: divalent cations, X: halides) that have been studied widely as light absorber and used in current state-or-the-art solar cells, LDN perovskite have unique properties such as more flexible crystal structure, lower ion transport mobility, robust stability against environmental stress such as moisture, thermal, etc., making them attractive for applications in optoelectronic devices. Since 2014, reports on LDN perovskite materials used in perovskite solar cells, light emitting diodes (LEDs), luminescent solar concentrators (LSC), and photodetectors have been reported, aiming to overcome the obstacles of conventional 3DN perovskite materials in these optoelectronic devices. In this review, the variable ligands used to make LDN perovskite materials are summarized, their distinct properties compared to conventional 3D perovskite materials. The research progress of optoelectronic devices including solar cells, LEDs, LSCs, and photodetectors that used different LDNs perovskite, the roles and working mechanisms of the LDN perovskites in the devices are also demonstrated. Finally, key research challenges and outlook of LDN materials for various optoelectronic applications are discussed.

Rare-earth quantum cutting in metal halide perovskites - a review

Ytterbium-doped lead halide perovskite (Yb³⁺:CsPbX₃ with x = Cl or Cl/Br) nanocrystals and thin films have shown surprisingly efficient downconversion by quantum cutting with PLQYs up to 193%. After excitation of the perovskite host with high-energy photons, the excited states of two Yb ions are rapidly populated, subsequently emitting lower-energy photons. Several synthesis routes lead to highly efficient materials, and we review the progress on both the synthesis, material quality and applicability of these downconversion layers. For solar cells they could be used to increase the power converted from high-energy photons, and first applications have already shown an increase in the power conversion efficiency of silicon and CIGS solar cells. Applications such as luminescent solar concentrators an LEDs are also explored. With further research to overcome challenges regarding power saturation and stability, this material has great potential for a simple route to enhance solar cells.

Photonics for Photovoltaics: Advances and Opportunities

Photovoltaic systems have reached impressive efficiencies, with records in the range of 20-30% for single-junction cells based on many different materials, yet the fundamental Shockley-Queisser efficiency limit of 34% is still out of reach. Improved photonic design can help approach the efficiency limit by eliminating losses from incomplete absorption or nonradiative recombination. This Perspective reviews nanopatterning methods and metasurfaces for increased light incoupling and light trapping in light absorbers and describes nanophotonics opportunities to reduce carrier recombination and utilize spectral conversion. Beyond the state-of-the-art single junction cells, photonic design plays a crucial role in the next generation of photovoltaics, including tandem and self-adaptive solar cells, and to extend the applicability of solar cells in many different ways. We address the exciting research opportunities and challenges in photonic design principles and fabrication that will accelerate the massive upscaling and (invisible) integration of photovoltaics into every available surface.