Bulk Heterojunction Upconversion Thin Films Fabricated via One-Step Solution Deposition.

Upconversion of near-infrared light into the visible has achieved limited success in applications due to the difficulty of creating solid-state films with high external quantum efficiency (EQE). Recent developments have expanded the range of relevant materials for solid-state triplet-triplet annihilation upconversion through the use of a charge-transfer state sensitization process. Here, we report the single-step solution-processed deposition of a bulk heterojunction upconversion film using organic semiconductors. The use of a bulk heterojunction thin film enables a high contact area between sensitizer and annihilator materials in this interface-triplet-generation mechanism and allows for a facile single-step deposition process. Demonstrations of multiple deposition and patterning methods on glass and flexible substrates show the promise of this materials system for solid-state upconversion applications.

Spatially Controlled UV Light Generation at Depth using Upconversion Micelles.

UV light can trigger a plethora of useful photochemical reactions for diverse applications, including photocatalysis, photopolymerization, and drug delivery. These applications typically require penetration of high-energy photons deep into materials, yet delivering these photons beyond the surface is extremely challenging due to absorption and scattering effects. Triplet-triplet annihilation upconversion (TTA-UC) shows great promise to circumvent this issue by generating high-energy photons from incident lower-energy photons. However, molecules that facilitate TTA-UC usually have poor water solubility, limiting their deployment in aqueous environments. To address this challenge, a nanoencapsulation method is leveraged to fabricate water-compatible UC micelles, enabling on-demand UV photon generation deep into materials. Two iridium-based complexes are presented for use as TTA-UC sensitizers with increased solubilities that facilitate the formation of highly emissive UV-upconverting micelles. Furthermore, this encapsulation method is shown to be generalizable to nineteen UV-emitting UC systems, accessing a range of upconverted UV emission profiles with wavelengths as low as 350 nm. As a proof-of-principle demonstration of precision photochemistry at depth, UV-emitting UC micelles are used to photolyze a fluorophore at a focal point nearly a centimeter beyond the surface, revealing opportunities for spatially controlled manipulation deep into UV-responsive materials.

Controlling the durability and optical properties of triplet-triplet annihilation upconversion nanocapsules.

Deep penetration of high energy photons by direct irradiation is often not feasible due to absorption and scattering losses, which are generally exacerbated as photon energy increases. Precise generation of high energy photons beneath a surface can circumvent these losses and significantly transform optically controlled processes like photocatalysis or 3D printing. Using triplet-triplet annihilation upconversion (TTA-UC), a nonlinear process, we can locally convert two transmissive low energy photons into one high energy photon. We recently demonstrated the use of nanocapsules for high energy photon generation at depth, with durability within a variety of chemical environments due to the formation of a dense, protective silica shell that prevents content leakage and nanocapsule aggregation. Here, we show the importance of the feed concentrations of the tetraethylorthosilicate (TEOS) monomer and the methoxy poly(ethyleneglycol) silane (PEG-silane) ligand used to synthesize these nanocapsules using spectroscopic and microscopy characterizations. At optimal TEOS and PEG-silane concentrations, minimal nanocapsule leakage can be obtained which maximizes UC photoluminescence. We also spectroscopically study the origin of inefficient upconversion from UCNCs made using sub-optimal conditions to probe how TEOS and PEG-silane concentrations impact the equilibrium between productive shell growth and side product formation, like amorphous silica. Furthermore, this optimized fabrication protocol can be applied to encapsulate multiple TTA-UC systems and other emissive dyes to generate anti-Stokes or Stokes shifted emission, respectively. These results show that simple synthetic controls can be tuned to obtain robust, well-dispersed, bright upconverting nanoparticles for subsequent integration in optically controlled technologies.

Triplet Fusion Upconversion Nanocapsule Synthesis.

Triplet fusion upconversion (UC) allows for the generation of one high energy photon from two low energy input photons. This well-studied process has significant implications for producing high energy light beyond a material's surface. However, the deployment of UC materials has been stymied due to poor material solubility, high concentration requirements, and oxygen sensitivity, ultimately resulting in reduced light output. Toward this end, nanoencapsulation has been a popular motif to circumvent these challenges, but durability has remained elusive in organic solvents. Recently, a nanoencapsulation technique was engineered to tackle each of these challenges, whereupon an oleic acid nanodroplet containing upconversion materials was encapsulated with a silica shell. Ultimately, these nanocapsules (NCs) were durable enough to enable triplet fusion upconversion-facilitated volumetric three-dimensional (3D) printing. By encapsulating upconversion materials with silica and dispersing them in a 3D printing resin, photopatterning beyond the surface of the printing vat was made possible. Here, video protocols for the synthesis of upconversion NCs are presented for both small-scale and large-scale batches. The outlined protocols serve as a starting point for adapting this encapsulation scheme to multiple upconversion schemes for use in volumetric 3D printing applications.

Subwavelength-Polarized Quasi-Two-Dimensional Perovskite Single-Mode Nanolaser.

When approaching the subwavelength or deep subwavelength scale, there is a fundamental trade-off between the ultimate shrinking size and the performance for miniaturized lasers. Herein, to overcome this trade-off, we investigated the excitonic gain nature of quasi-two-dimensional (quasi-2D) perovskites and revealed that both singlet excitons and polarons would make nearly the entire contribution within ∼50 ps to a high net gain of 558 cm-1. Inspired by the gain characteristic, we successfully shrank the quasi-2D perovskites laser to the subwavelength scale using only a layer of ultraviolet glue and a glass substrate in the vertical dimension. In spite of the compact and simple cavity structure, single-mode lasing with a highly linear polarization degree of 81% and a quality factor of 1635 was achieved. The extremely short cavity, excellent lasing performance, and simple structure of the quasi-2D perovskite laser are expected to provide insights into next-generation integrated laser sources.

Broadband emission of double perovskite Cs2Na0.4Ag0.6In0.995Bi0.005Cl6:Mn2+ for single-phosphor white-light-emitting diodes.

In this Letter, we report the broadband photoluminescence of lead-free double perovskite Cs2Na0.4Ag0.6In0.95Bi0.05Cl6:Mn2+. Under ultraviolet excitation, the white phosphor shows two emission peaks at 550 nm and 610 nm from self-trapped exciton and doped Mn2+ ions, respectively, leading to a broad emission spectrum over the whole visible spectrum suitable for lighting application. The white-light-emitting diodes exhibit high light quality with CIE coordinates (0.38, 0.42) and color rendering index of 82.6. The mechanism of luminescence of this double perovskite is also discussed in this Letter.

Lead-Free Halide Perovskites and Perovskite Variants as Phosphors toward Light-Emitting Applications.

Lead halide perovskites have attracted tremendous research interests in the light-emitting field because of their high defect tolerance, solution processability, tunable spectrum, and efficient emission. In terms of luminescence types, both the narrowband emission derived from free-exciton (FE) and broadband white light emission from self-trapped exciton (STE) show great advantages in light-emitting applications. Despite the fascinating characteristics, their commercialization still suffers from the presence of toxic lead (Pb) and unsatisfactory stability. In this spotlight, we mainly focus on the lead-free candidates as phosphors for possible light-emitting applications. Thanks to the chemical diversity of metal halide perovskites and perovskite variants, many excellent lead-free light-emitting materials have recently been synthesized and characterized. We first classify these materials into three types according to material structures, including (1) double perovskites A2B(I)B(III)X6, (2) vacancy ordered perovskites A2B(IV)X6, (3) miscellaneous perovskite variants or halide semiconductors, which refer to halides without clear relation to the perovskite structure. We then highlight the importance of electronic dimensionality, defect passivation, and impurity doping in developing highly efficient perovskite-based emitters. We also discuss their applications in white light-emitting diodes (W-LED). Further challenges toward practical applications and potential applications are also included in a section on outlook and future challenges.

Inorganic antimony halide hybrids with broad yellow emissions.

Lead halide perovskites exhibit unexceptionable photoelectric properties. However, these materials are unsatisfactory in terms of stability and toxicity. Herein, we report Rb7Sb3Cl16 as a new kind of lead free perovskite variants. This material can be easily obtained through hydrothermal reactions. The composition is determined through structure refinement, elemental analysis and X-ray photoelectron spectra. Rb7Sb3Cl16 exhibits a broad yellow emission at 560 nm, with a Stokes shift of 175 nm and a photoluminescence quantum yield (PLQY) around 26%. Rb7Sb3Cl16 also shows good thermal and water stability due to its inorganic composition. White light-emitting diodes (LEDs) are constructed by combining Rb7Sb3Cl16 as yellow phosphors, our previously reported Cs2SnCl6:2.75%Bi as blue phosphors, and commercial UV LED chips as the excitation source, producing a white light with the Commission Internationale de'Eclairage (CIE) color coordinates at (0.39, 0.38).