Intrinsic Formamidinium Tin Iodide Nanocrystals by Suppressing the Sn(IV) Impurities.

The long search for nontoxic alternatives to lead halide perovskites (LHPs) has shown that some compelling properties of LHPs, such as low effective masses of carriers, can only be attained in their closest Sn(II) and Ge(II) analogues, despite their tendency toward oxidation. Judicious choice of chemistry allowed formamidinium tin iodide (FASnI3) to reach a power conversion efficiency of 14.81% in photovoltaic devices. This progress motivated us to develop a synthesis of colloidal FASnI3 NCs with a concentration of Sn(IV) reduced to an insignificant level and to probe their intrinsic structural and optical properties. Intrinsic FASnI3 NCs exhibit unusually low absorption coefficients of 4 × 103 cm-1 at the first excitonic transition, a 190 meV increase of the band gap as compared to the bulk material, and a lack of excitonic resonances. These features are attributed to a highly disordered lattice, distinct from the bulk FASnI3 as supported by structural characterizations and first-principles calculations.

Shortwave infrared-absorbing squaraine dyes for all-organic optical upconversion devices.

Shortwave infrared (SWIR) optical sensing and imaging are essential to an increasing number of next-generation applications in communications, process control or medical imaging. An all-organic SWIR upconversion device (OUC) consists of an organic SWIR sensitive photodetector (PD) and an organic light-emitting diode (OLED), connected in series. OUCs directly convert SWIR to visible photons, which potentially provides a low-cost alternative to the current inorganic compound-based SWIR imaging technology. For OUC applications, only few organic materials have been reported with peak absorption past 1000 nm and simultaneous small absorption in the visible. Here, we synthesized a series of thermally stable high-extinction coefficient donor-substituted benz[cd]indole-capped SWIR squaraine dyes. First, we coupled the phenyl-, carbazole-, and thienyl-substituted benz[cd]indoles with squaric acid (to obtain the SQ dye family). We then combined these donors with the dicyanomethylene-substituted squaraine acceptor unit, to obtain the dicyanomethylene-functionalized squaraine DCSQ family. In the solid state, the absorbance of all dyes extended considerably beyond 1100 nm. For the carbazole- and thienyl-substituted DCSQ dyes, even the peak absorptions in solution were in the SWIR, at 1008 nm and 1014 nm. We fabricated DCSQ PDs with an external photon-to-current efficiency over 30%. We then combined the PD with a fluorescent OLED and fabricated long-term stable OUCs with peak sensitivity at 1020 nm, extending to beyond 1200 nm. Our OUCs are characterized by a very low dark luminance (<10-2 cd m-2 at below 6 V) in the absence of SWIR light, and a low turn-on voltage of 2 V when SWIR light is present.

A Small Cationic Organo-Copper Cluster as Thermally Robust Highly Photo- and Electroluminescent Material.

Organic light-emitting diodes (OLEDs) are revolutionizing display applications. In this aspect, luminescent complexes of precious metals such as iridium, platinum, or ruthenium still playing a significant role. Emissive compounds of earth-abundant copper with equivalent performance are desired for practical, large-scale applications such as solid-state lighting and displays. Copper(I)-based emitters are well-known to suffer from weak spin-orbit coupling and a high reorganization energy upon photoexcitation. Here we report a cationic organo-copper cluster [Cu4(PCP)3]+ (PCP = 2,6-(PPh2)2C6H3) that features suppressed nonradiative decays, giving rise to a robust narrow-band green luminophore with a photoluminescent (PL) efficiency up to 93%. PL decay kinetics corroborated by DFT calculations reveal a complex emission mechanism involving contributions of both thermally activated delayed fluorescence and phosphorescence. This robust compound was solution-processed into a thin film in prototype OLEDs with external quantum efficiency up to 11% and a narrow emission bandwidth (65 nm fwhm).

High-resolution remote thermometry and thermography using luminescent low-dimensional tin-halide perovskites.

Although metal-halide perovskites have recently revolutionized research in optoelectronics through a unique combination of performance and synthetic simplicity, their low-dimensional counterparts can further expand the field with hitherto unknown and practically useful optical functionalities. In this context, we present the strong temperature dependence of the photoluminescence lifetime of low-dimensional, perovskite-like tin-halides and apply this property to thermal imaging. The photoluminescence lifetimes are governed by the heat-assisted de-trapping of self-trapped excitons, and their values can be varied over several orders of magnitude by adjusting the temperature (up to 20 ns °C-1). Typically, this sensitive range spans up to 100 °C, and it is both compound-specific and shown to be compositionally and structurally tunable from -100 to 110 °C going from [C(NH2)3]2SnBr4 to Cs4SnBr6 and (C4N2H14I)4SnI6. Finally, through the implementation of cost-effective hardware for fluorescence lifetime imaging, based on time-of-flight technology, these thermoluminophores have been used to record thermographic videos with high spatial and thermal resolution.

The Rb7 Bi3-3x Sb3x Cl16 Family: A Fully Inorganic Solid Solution with Room-Temperature Luminescent Members.

Low-dimensional ns2 -metal halide compounds have received immense attention for applications in solid-state lighting, optical thermometry and thermography, and scintillation. However, these are based primarily on the combination of organic cations with toxic Pb2+ or unstable Sn2+ , and a stable inorganic luminescent material has yet to be found. Here, the zero-dimensional Rb7 Sb3 Cl16 phase, comprised of isolated [SbCl6 ]3- octahedra and edge-sharing [Sb2 Cl10 ]4- dimers, shows room-temperature photoluminescence (RT PL) centered at 560 nm with a quantum yield of 3.8±0.2 % at 296 K (99.4 % at 77 K). The temperature-dependent PL lifetime rivals that of previous low-dimensional materials with a specific temperature sensitivity above 0.06 K-1 at RT, making it an excellent thermometric material. Utilizing both DFT and chemical substitution with Bi3+ in the Rb7 Bi3-3x Sb3x Cl16 (x≤1) family, we present the edge-shared [Sb2 Cl10 ]4- dimer as a design principle for Sb-based luminescent materials.

Disphenoidal Zero-Dimensional Lead, Tin, and Germanium Halides: Highly Emissive Singlet and Triplet Self-Trapped Excitons and X-ray Scintillation.

Low-dimensional metal halides have been researched as optoelectronic materials for the past two decades. Zero-dimensional halides of ns2 elements (Sn, Pb, Sb) have recently gained attention as highly efficient broadband light emitters. These compounds comprise discrete metal halide centers, isolated by bulky organic cations. Herein, we report isostructural halide complexes of Ge(II), Sn(II), and Pb(II) with a 1-butyl-1-methyl-piperidinium cation (Bmpip), featuring unusual disphenoidal coordination with a highly stereoactive lone pair. Spectrally broad, bright emission from highly localized excitons, with quantum efficiencies of up to 75%, is observed in blue to red spectral regions for bromides (for Pb, Sn, and Ge, respectively) and extends into the near-infrared for Bmpip2SnI4 (peak at 730 nm). In the case of Sn(II) and Ge(II), both singlet and triplet excitonic emission bands have been observed. Furthermore, Bmpip2SnBr4 and Bmpip2PbBr4 exhibit X-ray-excited luminescence (radioluminescence) with brightness being commensurate with that of a commercial inorganic X-ray scintillator (NaI:Tl).

Guanidinium-Formamidinium Lead Iodide: A Layered Perovskite-Related Compound with Red Luminescence at Room Temperature.

Two-dimensional hybrid organic-inorganic lead halides perovskite-type compounds have attracted immense scientific interest due to their remarkable optoelectronic properties and tailorable crystal structures. In this work, we present a new layered hybrid lead halide, namely [CH(NH2)2][C(NH2)3]PbI4, wherein puckered lead-iodide layers are separated by two small and stable organic cations: formamidinium, CH(NH2)2+, and guanidinium, C(NH2)3+. This perovskite is thermally stable up to 255 °C, exhibits room-temperature photoluminescence in the red region with a quantum yield of 3.5%, and is photoconductive. This study highlights a vast structural diversity that exists in the compositional space typically used in perovskite photovoltaics.

Superweak Coordinating Anion as Superstrong Enhancer of Cyanine Organic Semiconductor Properties.

The superweak tetrakis(nonafluoro-tert-butoxy)aluminate coordinating anion was employed to introduce pseudo-gas-phase conditions to the 2-[5-(1,3-dihydro-1,3,3-trimethyl-2H-indol-2-ylidene)-1,3-pentadien-1-yl]-1,3,3-trimethyl-3H-indolium chromophore. The resulting formation of a photoactive organic-inorganic hybrid salt has led to a highly stabilized excited state of the organic chromophore mainly due to the minimized lattice energy and Coulomb interactions. These highly beneficial features caused by the well dispersed negative charge of the anion have led to an enhanced neat spin-casted film fluorescence intensity, prolonged fluorescence lifetime, smooth thin film surfaces and a record power photovoltaic efficiency of 3.8 % when compared to organic salts of this particular chromophore containing anions with localised negative charge. Clear evidence is given that a superweak coordinating anion is an emerging key parameter in cyanine dye photochemistry. This approach can be seen as a general guideline to prepare highly efficient ionic dyes for organic semiconductor applications.

Highly Emissive Self-Trapped Excitons in Fully Inorganic Zero-Dimensional Tin Halides.

The spatial localization of charge carriers to promote the formation of bound excitons and concomitantly enhance radiative recombination has long been a goal for luminescent semiconductors. Zero-dimensional materials structurally impose carrier localization and result in the formation of localized Frenkel excitons. Now the fully inorganic, perovskite-derived zero-dimensional SnII material Cs4 SnBr6 is presented that exhibits room-temperature broad-band photoluminescence centered at 540 nm with a quantum yield (QY) of 15±5 %. A series of analogous compositions following the general formula Cs4-x Ax Sn(Br1-y Iy )6 (A=Rb, K; x≤1, y≤1) can be prepared. The emission of these materials ranges from 500 nm to 620 nm with the possibility to compositionally tune the Stokes shift and the self-trapped exciton emission bands.

Luminescent and Photoconductive Layered Lead Halide Perovskite Compounds Comprising Mixtures of Cesium and Guanidinium Cations.

Interest in hybrid organic-inorganic lead halide compounds with perovskite-like two-dimensional crystal structures is growing due to the unique electronic and optoelectronic properties of these compounds. Herein, we demonstrate the synthesis, thermal and optical properties, and calculations of the electronic band structures for one- and two-layer compounds comprising both cesium and guanidinium cations: Cs[C(NH2)3]PbI4 (I), Cs[C(NH2)3]PbBr4 (II), and Cs2[C(NH2)3]Pb2Br7 (III). Compounds I and II exhibit intense photoluminescence at low temperatures, whereas compound III is emissive at room temperature. All of the obtained substances are stable in air and do not thermally decompose until 300 °C. Since Cs+ and C(NH2)3+ are increasingly utilized in precursor solutions for depositing polycrystalline lead halide perovskite thin films for photovoltaics, exploring possible compounds within this compositional space is of high practical relevance to understanding the photophysics and atomistic chemical nature of such films.

Design and Synthesis of Heteroleptic Iridium(III) Phosphors for Efficient Organic Light-Emitting Devices.

The phosphorescent emitters are essential to realize energy-efficient display and lighting panels. The solution processability is of particular interest for large-scale and low-cost production. Here, we present a series of the heteroleptic iridium (Ir) complexes, Ir(ppy)2L1, Ir(ppy)2L2, and Ir(ppy)2L3, using the new ancillary ligands, including 1-(2-chlorophenyl)-5-hydroxy-3-methyl-1H-pyrazole-4-carbaldehyde (L1), 5-hydroxy-3-methyl-1-(p-tolyl)-1H-pyrazole-4-carbaldehyde (L2), and 5-hydroxy-3-methyl-1-phenyl-1H-pyrazole-4-carbaldehyde (L3). Their photophysical and electrochemical properties were systematically characterized, followed by comparing with those predicted by density functional theory simulations using hybrid functionals. Among the three phosphors synthesized, Ir(ppy)2L1 exhibits the highest photoluminescence quantum yield (ΦPL = 89%), with an exciton lifetime of 0.34 µs. By using 4,4'-bis(carbazole-9-yl)biphenyl as the host material, we demonstrate high current efficiencies of 64 and 40 cd A-1 at 100 cd m-2 in its vacuum-evaporated and solution-processed organic light-emitting devices, respectively, revealing the promise for large-area light sources.

Harnessing Defect-Tolerance at the Nanoscale: Highly Luminescent Lead Halide Perovskite Nanocrystals in Mesoporous Silica Matrixes.

Colloidal lead halide perovskite nanocrystals (NCs) have recently emerged as a novel class of bright emitters with pure colors spanning the entire visible spectral range. Contrary to conventional quantum dots, such as CdSe and InP NCs, perovskite NCs feature unusual, defect-tolerant photophysics. Specifically, surface dangling bonds and intrinsic point defects such as vacancies do not form midgap states, known to trap carriers and thereby quench photoluminescence (PL). Accordingly, perovskite NCs need not be electronically surface-passivated (with, for instance, ligands and wider-gap materials) and do not noticeably suffer from photo-oxidation. Novel opportunities for their preparation therefore can be envisaged. Herein, we show that the infiltration of perovskite precursor solutions into the pores of mesoporous silica, followed by drying, leads to the template-assisted formation of perovskite NCs. The most striking outcome of this simple methodology is very bright PL with quantum efficiencies exceeding 50%. This facile strategy can be applied to a large variety of perovskite compounds, hybrid and fully inorganic, with the general formula APbX3, where A is cesium (Cs), methylammonium (MA), or formamidinium (FA), and X is Cl, Br, I or a mixture thereof. The luminescent properties of the resulting templated NCs can be tuned by both quantum size effects as well as composition. Also exhibiting intrinsic haze due to scattering within the composite, such materials may find applications as replacements for conventional phosphors in liquid-crystal television display technologies and in related luminescence down-conversion-based devices.

Monodisperse Formamidinium Lead Bromide Nanocrystals with Bright and Stable Green Photoluminescence.

Bright green emitters with adjustable photoluminescence (PL) maxima in the range of 530-535 nm and full-width at half-maxima (fwhm) of <25 nm are particularly desirable for applications in television displays and related technologies. Toward this goal, we have developed a facile synthesis of highly monodisperse, cubic-shaped formamidinium lead bromide nanocrystals (FAPbBr3 NCs) with perovskite crystal structure, tunable PL in the range of 470-540 nm by adjusting the nanocrystal size (5-12 nm), high quantum yield (QY) of up to 85% and PL fwhm of <22 nm. High QYs are also retained in films of FAPbBr3 NCs. In addition, these films exhibit low thresholds of 14 ± 2 µJ cm-2 for amplified spontaneous emission.

Probing the molecular character of periodic mesoporous organosilicates via photoluminescence of Lewis acid-base adducts.

Photoluminescence decay was used as a structure-sensitive method to compare the distribution of emitting sites in periodic mesoporous organosilicates (PMOs) to their respective molecular analogs. The observed close similarity of PL decays confirms the molecular nature of PMOs and high homogeneity of emitting sites.

Fast Anion-Exchange in Highly Luminescent Nanocrystals of Cesium Lead Halide Perovskites (CsPbX3, X = Cl, Br, I).

Postsynthetic chemical transformations of colloidal nanocrystals, such as ion-exchange reactions, provide an avenue to compositional fine-tuning or to otherwise inaccessible materials and morphologies. While cation-exchange is facile and commonplace, anion-exchange reactions have not received substantial deployment. Here we report fast, low-temperature, deliberately partial, or complete anion-exchange in highly luminescent semiconductor nanocrystals of cesium lead halide perovskites (CsPbX3, X = Cl, Br, I). By adjusting the halide ratios in the colloidal nanocrystal solution, the bright photoluminescence can be tuned over the entire visible spectral region (410-700 nm) while maintaining high quantum yields of 20-80% and narrow emission line widths of 10-40 nm (from blue to red). Furthermore, fast internanocrystal anion-exchange is demonstrated, leading to uniform CsPb(Cl/Br)3 or CsPb(Br/I)3 compositions simply by mixing CsPbCl3, CsPbBr3, and CsPbI3 nanocrystals in appropriate ratios.

Nanocrystals of Cesium Lead Halide Perovskites (CsPbX3, X = Cl, Br, and I): Novel Optoelectronic Materials Showing Bright Emission with Wide Color Gamut.

Metal halides perovskites, such as hybrid organic-inorganic CH3NH3PbI3, are newcomer optoelectronic materials that have attracted enormous attention as solution-deposited absorbing layers in solar cells with power conversion efficiencies reaching 20%. Herein we demonstrate a new avenue for halide perovskites by designing highly luminescent perovskite-based colloidal quantum dot materials. We have synthesized monodisperse colloidal nanocubes (4-15 nm edge lengths) of fully inorganic cesium lead halide perovskites (CsPbX3, X = Cl, Br, and I or mixed halide systems Cl/Br and Br/I) using inexpensive commercial precursors. Through compositional modulations and quantum size-effects, the bandgap energies and emission spectra are readily tunable over the entire visible spectral region of 410-700 nm. The photoluminescence of CsPbX3 nanocrystals is characterized by narrow emission line-widths of 12-42 nm, wide color gamut covering up to 140% of the NTSC color standard, high quantum yields of up to 90%, and radiative lifetimes in the range of 1-29 ns. The compelling combination of enhanced optical properties and chemical robustness makes CsPbX3 nanocrystals appealing for optoelectronic applications, particularly for blue and green spectral regions (410-530 nm), where typical metal chalcogenide-based quantum dots suffer from photodegradation.

Hydrogen-bonded organic semiconductor micro- and nanocrystals: from colloidal syntheses to (opto-)electronic devices.

Organic pigments such as indigos, quinacridones, and phthalocyanines are widely produced industrially as colorants for everyday products as various as cosmetics and printing inks. Herein we introduce a general procedure to transform commercially available insoluble microcrystalline pigment powders into colloidal solutions of variously sized and shaped semiconductor micro- and nanocrystals. The synthesis is based on the transformation of the pigments into soluble dyes by introducing transient protecting groups on the secondary amine moieties, followed by controlled deprotection in solution. Three deprotection methods are demonstrated: thermal cleavage, acid-catalyzed deprotection, and amine-induced deprotection. During these processes, ligands are introduced to afford colloidal stability and to provide dedicated surface functionality and for size and shape control. The resulting micro- and nanocrystals exhibit a wide range of optical absorption and photoluminescence over spectral regions from the visible to the near-infrared. Due to excellent colloidal solubility offered by the ligands, the achieved organic nanocrystals are suitable for solution processing of (opto)electronic devices. As examples, phthalocyanine nanowire transistors as well as quinacridone nanocrystal photodetectors, with photoresponsivity values by far outperforming those of vacuum deposited reference samples, are demonstrated. The high responsivity is enabled by photoinduced charge transfer between the nanocrystals and the directly attached electron-accepting vitamin B2 ligands. The semiconducting nanocrystals described here offer a cheap, nontoxic, and environmentally friendly alternative to inorganic nanocrystals as well as a new paradigm for obtaining organic semiconductor materials from commercial colorants.

Colloidal CsPbX3 Nanocrystals with Thin Metal Oxide Gel Coatings.

Lead halide perovskite (LHP) nanocrystals (NCs) have gathered much attention as light-emitting materials, particularly owing to their excellent color purity, band gap tunability, high photoluminescence quantum yield (PLQY), low cost, and scalable synthesis. To enhance the stability of LHP NCs, bulky strongly bound organic ligands are commonly employed, which counteract the extraction of charge carriers from the NCs and hinder their use as photoconductive materials and photocatalysts. Replacing these ligands with a thin coating is a complex challenge due to the highly dynamic ionic lattice, which is vulnerable to the commonly employed coating precursors and solvents. In this work, we demonstrate thin (<1 nm) metal oxide gel coatings through non-hydrolytic sol-gel reactions. The coated NCs are readily dispersible and highly stable in short-chain alcohols while remaining monodisperse and exhibiting high PLQY (70-90%). We show the successful coating of NCs in a wide range of sizes (5-14 nm) and halide compositions. Alumina-gel-coated NCs were chosen for an in-depth analysis, and the versatility of the approach is demonstrated by employing zirconia- and titania-based coatings. Compact films of the alumina-gel-coated NCs exhibit electronic and excitonic coupling between the NCs, leading to two orders of magnitude longer photoluminescence lifetimes (400-700 ns) compared to NCs in solution or their organically capped counterparts. This makes these NCs highly suited for applications where charge carrier delocalization or extraction is essential for performance.

Integrated photodetectors for compact Fourier-transform waveguide spectrometers.

Extreme miniaturization of infrared spectrometers is critical for their integration into next-generation consumer electronics, wearables and ultrasmall satellites. In the infrared, there is a necessary compromise between high spectral bandwidth and high spectral resolution when miniaturizing dispersive elements, narrow band-pass filters and reconstructive spectrometers. Fourier-transform spectrometers are known for their large bandwidth and high spectral resolution in the infrared; however, they have not been fully miniaturized. Waveguide-based Fourier-transform spectrometers offer a low device footprint, but rely on an external imaging sensor such as bulky and expensive InGaAs cameras. Here we demonstrate a proof-of-concept miniaturized Fourier-transform waveguide spectrometer that incorporates a subwavelength and complementary-metal-oxide-semiconductor-compatible colloidal quantum dot photodetector as a light sensor. The resulting spectrometer exhibits a large spectral bandwidth and moderate spectral resolution of 50 cm-1 at a total active spectrometer volume below 100 µm × 100 µm × 100 µm. This ultracompact spectrometer design allows the integration of optical/analytical measurement instruments into consumer electronics and space devices.

Assessing the Drawbacks and Benefits of Ion Migration in Lead Halide Perovskites.

Since the inception of the unprecedented rise of halide perovskites for photovoltaic research, ion migration has shadowed this material class with undesirable hysteresis and degradation effects, limiting its practical implementations. Unfortunately, the localized doping and electrochemical reactions triggered by ion migration cause many more undesirable effects that are often unreported or misinterpreted because they deviate from classical semiconductor behavior. In this Perspective, we provide a concise overview of such effects in halide perovskites, such as operational instability in photovoltaics, polarization-induced abnormal external quantum efficiency in light-emitting diodes, and energy channel shift and anomalous sensitivities in hard radiation detection. Finally, we highlight a unique use case of exploiting ion migration as a boon to design emerging memory technologies such as memristors for information storage and computing.