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Tin-based halide perovskite materials are of interest for light-emitting applications due to their broad yellow emission, but they are known to exhibit inferior stability compared to lead-based perovskites due to the tendency of Sn2+ to be readily oxidized. Here, we investigated the ambient stability of a Dion–Jacobson tin bromide perovskite. We found that the optical properties of the samples were more significantly affected by moisture rather than the ratio of Sn2+/Sn4+. The exposure to humidity resulted in changes in the ratio of 2D HDASnBr4 and hydrated 1D HDA3SnBr8 phases (where HDA denotes hexane-1,6-diammonium), with an initial increase (up to ∼40 h) in luminescence attributed to the increasing fraction of the hydrated 1D phase which contributes to self-trapped exciton emission. With further exposure to humidity, the material starts to degrade resulting in reduced luminescence after ∼40 h. The use of additives for the suppression of oxidation of Sn2+, namely SnCl2 and tetraethyl orthosilicate (TEOS), results in prolonging the lifetime of the samples (>260 h for thin films with additives, compared to 180 h for thin films without additives). However, the persistence of luminescence with an increased time of ambient exposure in samples prepared with additives can be attributed to the changes in the evolution of phase compositions (ratios of 2D and hydrated 1D phases) over time rather than suppression of the oxidation of Sn2+. [ABSTRACT FROM AUTHOR]

Dion–Jacobson (DJ) perovskites have been demonstrated to be highly promising for improved lifetime and stability of solar cells, but their applications in light emitting diodes (LEDs) have been scarce. A large number of potential spacer ligands have not been explored, offering potential to achieve significant improvements in spectral regions where DJ perovskite-based LEDs lag behind other perovskite counterparts. Therefore, we investigated the effect of the choice of spacer cation on the properties and green light emitting diode applications of DJ perovskites. We found that optimized mixing of the spacer cations results in perovskite films with improved morphology and a change in phase distribution, leading to enhanced funneling and more efficient light emission. Devices containing mixed spacer cations 1,6-hexanediammonium (HDA) and 1,10-decanediammonium (DDA) exhibited significant enhancement of the external quantum efficiency (EQE), with a maximum value for DDA0.75HDA0.25 of 9.41% significantly higher than that obtained for pure HDA (1.23%) and DDA (7.28%), as well as increased lifetime with T50 of 70 min for the optimized composition, compared to 4 min for pure HDA and 60 min for pure DDA. The maximum EQE can be further increased by triphenylphosphine oxide passivation to 12.85%. [ABSTRACT FROM AUTHOR]

In this work, we attempt to figure out alkoxy-substitution effects on two kinds of well-known non-fullerene acceptors (NFAs), IDT-series and Y-series acceptors, by placing alkoxy side chains on the β-positions of the outer thiophene units. The resulting molecules, named IDTN-O and Y6-O, exhibit different properties compared to the original acceptors named IDTN and Y6. The HOMO and LUMO levels of IDTN-O are slightly upshifted at the same time, which causes a slightly higher open-circuit voltage (Voc) without sacrificing the short-circuit current density (Jsc) of the devices. J71:IDTN-O blend films can also achieve a better blend morphology due to the conformational locking effect and higher dipole moment induced by the alkoxy groups, which helps achieve a higher fill factor (FF). In addition, Y6-O exhibits significantly upshifted LUMO levels compared to Y6, leading to a high Voc of 0.95 V. PM6:Y6-O blend films can also maintain an optimal blend morphology through side-chain engineering, which leads to an excellent FF of 78.0%. As a result of alkoxy substitution, both IDTN-O and Y6-O-based devices can achieve better performances of 12.1% and 16.6% than IDTN and Y6-based devices (10.9% and 15.7%), which indicates that this is an effective method to optimize these two types of NFAs. [ABSTRACT FROM AUTHOR]

Aggregation-induced emission (AIE) has attracted increasing attention in recent years in the search for luminescent materials with biomedical and optoelectronic applications. Understanding of its underlying mechanisms is fundamental for the rational design of new AIE materials. In the present work, density functional theory (DFT) calculations were carried out for a set of organic fluorophores which have been recently studied as candidates for color converters in visible light communication. Three of them were (Z)-4-benzylidene-2-methyloxazol-5(4H)-one derivatives combined with an electron-donating moiety of trimethylamine, tetraphenylethene or dimethoxy-tetraphenylethene (TPA-BMO, TPE-BMO or DM-TPE-BMO, respectively). The fourth compound was a diaminomaleonitrile-based Schiff base combined with a dimethylamine moiety (DMA-AM). Our calculations suggested that the restriction of the intramolecular vibration (RIV) mechanism is responsible for the AIE effect observed for TPE-BMO and DM-TPE-BMO in the solid state. On the other hand, the uncommon photophysical behavior reported for DMA-AM (including the AIE phenomenon) in the solid state was related to excitonic coupling effects. Fluorescence spectroscopy experiments confirmed the presence of strongly-coupled DMA-AM aggregates in the solid state. Finally, some electronic properties related to the semiconductor behavior were investigated to explore the potential of applying these fluorophores in light-emitting electronic devices. It must be highlighted that the largest electron-transfer rate constant was calculated for TPE-BMO using the semi-classical Marcus theory, while the largest hole-transfer rate constants were obtained for DMA-AM and TPA-BMO. [ABSTRACT FROM AUTHOR]

Effective ternary material systems are usually achieved using a donor/acceptor guest that is structurally similar to the donor/acceptor host to maintain the film-morphology of the host binary blend. In this study, we report an unprecedented ternary material system, PBDB-TF:PBDB-T-SF:Y6, in which PBDB-T-SF is selected as the donor polymer guest of the PBDB-TF:Y6 binary blend. The absorption spectra of both polymers are almost overlapped and their energy levels are similar. However, the side-chains between the host (PBDB-TF) and the guest (PBDB-T-SF) are oriented differently, which modulates the morphology of the polymer phases from nanoscale (10–20 nm) fine fibrils in the host binary blend to an interpenetrating mesoscale polymer-crowd (>100 nm) with reduced phase crystallinity in the ternary blend. As a result, the electrical properties of the active layer are greatly improved: the hole mobility increases more than the electron mobility, resulting in balanced hole and electron mobilities; the monomolecular and bimolecular recombinations are both reduced; the charge carrier extraction is faster and the trap-assisted recombination is reduced, resulting in a longer recombination lifetime of charge carriers (0.19 μs vs. 0.15/0.11 μs). All of these contribute to the concurrently increased device fill-factor and short-circuit current-density, and finally improved efficiency (16.4%). Solid-state 19F magic angle spinning nuclear magnetic microscopy is conducted to probe interaction between the host and guest donor polymers. This study demonstrates that mesoscale morphology such as the polymer-crowd induced by differently oriented side chains of the host–guest polymers can be a morphological feature of active layer toward high-efficiency organic photovoltaic cells. [ABSTRACT FROM AUTHOR]

PEDOT:PSS is widely used in perovskite solar cells (PSCs) as the most popular hole transporting layer (HTL). However, p-i-n type inverted planar PSCs based on the PEDOT:PSS HTL typically exhibit up to 200 mV more voltage loss compared to conventional planar PSCs (n-i-p) that generally show open circuit voltages (VOC) of around 1.1 V. In this study, we develop a simple, inexpensive and effective interfacial engineering strategy by doping PEDOT:PSS with cesium iodide (CsI) to reduce the voltage loss and achieve highly efficient inverted PSCs. SEM, AFM, and XPS measurements suggest that CsI modifies the interface between PEDOT:PSS and perovskite by reacting with PbI2 to form CsPbI3, thus facilitating interfacial contact and charge transport. Moreover, after CsI-modification (CsI-PEDOT:PSS), the hole transport properties of PEDOT:PSS and the hole extraction are enhanced, while the energy levels are more favorable and charge recombination is suppressed. Importantly, compared to pristine PEDOT:PSS that suffers from a large non-radiative recombination loss (0.375 V), CsI-PEDOT:PSS makes the device to realize an impressively low non-radiative voltage loss (only 0.287 V). As a result, inverted PSCs based on CsI-PEDOT:PSS show a small voltage loss, an excellent power conversion efficiency (PCE) of 20.22%, and no hysteresis and achieve a high VOC (1.084 V), while the control device without CsI shows an inferior efficiency of 16.57%. Both the high VOC and PCE are an improvement over literature data on modified PEDOT:PSS-based inverted PSCs. [ABSTRACT FROM AUTHOR]

A series of pyrrolopyrrole aza boron dipyrromethene derivatives have been designed and synthesised as two-photon fluorescent probes. The bisalkynyl analogues, having a donor–π–donor quadrupolar skeleton, show a red-shifted Q band at ca. 700 nm in toluene and a two-photon absorption (TPA) cross-section up to 2349 GM at 1040 nm. To enable these dyes to be used for subcellular imaging, a branched oligoethylene glycol unit has been introduced to enhance their hydrophilicity and biocompatibility, and a potential organelle-selective group, namely triphenylphosphonium or morpholine moiety has also been added with a view to targeting the mitochondria or lysosomes respectively. The resulting conjugates are essentially non-aggregated in deionised water, exhibiting an intense Q band at 668 nm and a TPA cross-section up to 384 GM at 1040 nm. These spectral features have been analysed using density functional theory calculations, which suggest that the TPA is mainly due to the S0→ S2 transition. In the subcellular imaging study, it has been found that the triphenylphosphonium-containing derivative can localise in the lysosomes of HeLa human cervical carcinoma cells, which may be a result of its positive charge and the negative log P value (−2.46, P = partition coefficient), while the morpholine-substituted analogue does not exhibit preferential subcellular localisation. [ABSTRACT FROM AUTHOR]

Cation mixing has proved to be effective in stabilizing the high-temperature phase of formamidinium (FA)-based perovskites, affording high-performance n–i–p perovskite solar cells (PSCs). However, optimum cation mixing is found to be inapplicable directly to NiO p–i–n PSCs due to the energy band misalignment. In the present study, we reveal the role of mixing cesium (Cs), methylammonium (MA) and formamidinium (FA) in the energy band alignments and the crystallization of perovskites in such a device structure. By tuning the composition of mixed cations, we have significantly improved the energy band alignments of perovskites in p–i–n NiO-based PSCs. The relative amount of Cs to MA cations also plays a decisive role in shaping the nature of perovskite precursors, thus impacting the quality of the resulting perovskite layer in NiO p–i–n PSCs. These insights and the associated engineering efforts led to a significantly improved power conversion efficiency of 18.6% based on the NiO p–i–n PSCs, in addition to their superior ambient stability to typical n–i–p PSCs. [ABSTRACT FROM AUTHOR]

Based on an analogue of green fluorescent protein chromophore benzylidene-methyloxazolone (BMO), a series of fluorophores with an additional phenyl group, BMO-PH, BMO-PF, BMO-PM and BMO-PC, have been prepared and are found to be AIE-active. Their solutions are weakly emissive and their aggregation or solid states are highly emissive. Although these compounds readily undergo efficient E/Z isomerization (EZI) upon UV irradiation in solution, the intramolecular rotation around the double bond and phenyl rotation around the single bond serve as the key non-radiative decay channels to dissipate the excited-state energies. The EZI is only the phenomenal result. In aggregates, these intramolecular motions are greatly restricted by multiple intermolecular interactions, resulting in the AIE effect. To ensure a high solid-state quantum yield, prevention of detrimental π–π stacking is of essence. An additional phenyl group to BMO is found to increase the π–π distance and weaken the π–π interaction. Thus, the quantum yields are increased. Strong electron-donating groups and extended conjugation are effective at tuning the emission color bathochromically. Based on these principles, we succeeded in increasing the solid-state quantum yield up to 50% and obtaining a red emission maximum of 635 nm. Moreover, these compounds are promising for applications in photoswitches and fluorescent patterns, and their crystals are good candidates for luminescent waveguides with low light loss efficiency. [ABSTRACT FROM AUTHOR]

CH3NH3PbBr3 (MAPbBr3)-perovskite solar cells (Br-PSCs) have attracted much attention due to their green-long wavelength transparency and high open-circuit voltage originating from their large bandgap (2.2 eV). However, the efficiency of carbon-based Br-PSCs without organic hole transport materials (HTM) is still low due to the inappropriate quality of MAPbBr3 deposited in a relatively thick porous scaffold. Herein, an amorphous precursor route based on a two-step sequential method is exploited to conformably and seamlessly grow MAPbBr3 in the TiO2 porous scaffold. In the first step, the amorphous Pb–Br precursor containing a large amount of DMF molecules was prepared by lowering the post-treatment temperature to 25 °C, affording full pore filling and smooth surface capping. The conversion to MAPbBr3 in the second step was accelerated by the molecular exchange between DMF and MABr in IPA solution. Moreover, by solvent engineering through the addition of non-polar cyclohexane into the MABr IPA solution, the molecular exchange process was tuned in such a way to separate the nucleation and growth of MAPbBr3 crystals, leading to the preferential [001] orientation with an even surface finishing and the subsequent light absorption enhancement and trap state reduction. Using MAPbBr3 films in carbon-based PSCs has boosted their efficiency to 8.09% (Voc = 1.35 V), a record value for HTM-free Br-PSCs, also comparable to that of the best HTM-based Br-PSCs. Significantly, non-encapsulated devices showed no efficiency decay after storage in dry air (25–30 °C and 10–20% humidity) for 90 days. What is more, the efficiency was retained up to about 90% after storage for 15 days under high heat stress (air, 80 °C and 50–85% humidity). [ABSTRACT FROM AUTHOR]

CH3NH3PbI3 is commonly used in perovskite solar cells due to its long diffusion length and good crystallinity. In this paper, in the one-step approach using CH3NH3I and PbCl2 for forming the perovskite, we present a new low temperature annealing approach of gradually increasing the temperature to fabricate perovskite films. Various temperatures and temperature ranges for the formation of perovskite films have been studied. Using the gradual annealing process, we can tune the amount of chlorine in the atomic ratio of chlorine/iodine from 1.2 to 4.0%. Meanwhile, the gradual annealing process influences the quality of the perovskite film and importantly the device performance. The results show that through the optimized process, the film quality is improved with high surface coverage and good photoluminescence and reproducibility. We find that a higher amount of chlorine in the perovskite film plays a positive role in the device performance in the approach for achieving a power conversion efficiency of 14.9% with no obvious hysteresis. [ABSTRACT FROM AUTHOR]

Self-assembling of luminescent molecules into one-dimensional nanostructures is of particular interest in fabricating nanoscale electronic and photonic devices. Herein, we report the rational design and synthesis of a chiral fluorescent tetraphenylethylene derivative containing l-leucine methyl ester moiety (TPE-Leu). In solution, TPE-Leu is non-emissive and CD silent, but becomes highly emissive and CD active upon aggregation, exhibiting aggregation-induced emission (AIE) and chirality (AIC). Upon evaporation of its solution, TPE-Leu readily self-assembles into helical fluorescent micro/nanofibers, which show circularly polarized luminescence (CPL) and have the CPL dissymmetry factors in the range of 0.02–0.07. This molecular design combines the AIE effect, chirality, and self-assembling capability together, and is highly efficient in constructing novel functional micro/nanomaterials with well-defined structures and enhanced emission. [ABSTRACT FROM AUTHOR]

We here report a molecule with chiral recognition capability by a mechanism of complexation-induced circularly polarised luminescence (CPL) in the solid thin film state. A molecule (1) containing the luminogenic unit silole and chiral phenylethanamine pendants is synthesized and characterized with standard spectroscopic methods. In a solution of 1, no circular dichroism (CD) or fluorescence emission are observed. When aggregated into nanoparticles in a poor solvent, fluorescence is turned on but CD is still silent. When complexed with chiral acids to form a thin film, molecule 1 becomes CD-active and strongly fluorescent, showing simultaneous complexation-induced CD (CICD) and aggregation-induced emission (AIE) effects. The assemblies of 1 with different enantiomers of mandelic acid emit distinctly handed circularly polarised light and display CPL dissymmetry factors with absolute value around 0.01. This work provides a new platform for creating molecular functional materials with solid-state chiral sensing ability through reading CPL output signals. [ABSTRACT FROM AUTHOR]

In this work, a heteroatom-containing luminogen (TPE-Py) with multi-functionalities was synthesized in a reasonable yield by melding a pyridinium unit with tetraphenylethene through vinyl functionality. TPE-Py is weakly emissive in solution but becomes a strong emitter when aggregated as nanoparticle suspensions in poor solvents or in the solid state, displaying a phenomenon of aggregation-induced emission. Crystallization generally weakens and red-shifts the light emission. The crystalline aggregates of TPE-Py, however, emit stronger and bluer light than their amorphous counterparts. The solid-state emission of TPE-Py can be reversibly switched between green and yellow color by grinding–fuming and grinding–heating processes with a high contrast due to the transformation from the crystalline to the amorphous state and vice versa. The large Stokes shift and well-ordered molecular arrangement of the crystalline microrods of TPE-Py make it promising as an optical waveguide material with a low optical loss coefficient of ∼0.032 dB μm−1. TPE-Py works as a good fluorescent visualizer for specific staining of mitochondria in living cells with a high photostability, thanks to its hydrophobic and cationic features. [ABSTRACT FROM AUTHOR]

In this contribution, we finely tuned the singlet-triplet energy gap (ΔEST) of AIE-active materials to modulate their fluorescence, phosphorescence and delay fluorescence via rational molecular design and investigated the possible ways to harvest their triplet energy in OLEDs. Noteworthily, two molecules o-TPA-3TPE-o-PhCN and o-TPA-3TPE-p-PhCN with larger ΔEST values (0.59 eV and 0.45 eV, respectively) emitted efficient long-lived low temperature phosphorescence in their glassy solutions and exhibited efficient crystallization-induced room temperature phosphorescence (RTP). Meanwhile, it was the first time to observe a novel crystallization-induced delay fluorescence phenomenon in another AIE-active molecule p-TPA-3TPE-p-PhCN owing to its very small ΔEST value (0.21 eV). It was also found that molecules with various ΔEST values showed significantly different temperature sensitivity. Non-doped electroluminescent (EL) devices using these molecules as light-emitting layers were fabricated, exhibiting external quantum efficiencies (EQE) higher than theoretical values of purely singlet emitter type devices. Particularly, p-TPA-3TPE-p-PhCN showed outstanding device performances with high luminance and efficiencies up to 36,900 cd m(-2), 11.2 lm W(-1), 12.8 cd A(-1) and 4.37%, respectively, considering that its solid-state quantum yield was only 42%. All the above observations suggested that tuning the ΔEST values of AIE materials is a powerful methodology to generate many more interesting and meaningful optoelectronic properties.

A series of tetraphenylethene-containing BODIPYs with emissions from visible to near-IR and large Stokes shifts up to 142 nm have been designed and synthesized. They show solvatochromic fluorescence and can be utilized as fluorescent visualizers for intracellular imaging.

Melding a benzothiazolium unit with tetraphenylethene generates a new hemicyanine luminogen with aggregation-induced emission characteristics; the luminogen exhibits crystochromism and its solid-state emission can be repeatedly tuned from yellow or orange to red by grinding-fuming or grinding-heating processes due to the transformation from the crystalline to the amorphous state and vice versa.

Turning "stone" into "gold": pyrene, a faint fluorophore in the solid state, is transformed into a bright emitter by decorating it with tetraphenylethene units; the new luminogen is thermally and morphologically stable and its light-emitting diode shows excellent performance, with external quantum efficiency and current efficiency up to 4.95% and 12.3 cd A(-1), respectively.