Fluorene- and fluorenone-based molecules as electron-transporting SAMs for photovoltaic devices.

New semiconductors containing fluorene or fluorenone central fragments along with phosphonic acid anchoring groups were synthesized and investigated as electron transporting materials for possible application in photovoltaic devices. These derivatives demonstrate good thermal stability and suitable electrochemical properties for effective electron transport from perovskite, Sb2S3 and Sb2Se3 absorber layers. Self-assembled fluorene and fluorenone electron-transporting materials have shown improved substrate wettability, indicating bond formation between monolayer-forming compounds and the ITO, TiO2, Sb2S3, or Sb2Se3 surface. Additionally, investigated materials have compatible energetic band alignment and can passivate perovskite interface defects, which makes them interesting candidates for application in the n-i-p structure perovskite solar cell.

Fluorescence quenching in aggregates of fucoxanthin-chlorophyll protein complexes: Interplay of fluorescing and dark states.

Diatoms, a major group of algae, account for about a quarter of the global primary production on Earth. These photosynthetic organisms face significant challenges due to light intensity variations in their underwater habitat. To avoid photodamage, they have developed very efficient non-photochemical quenching (NPQ) mechanisms. These mechanisms originate in their light-harvesting antenna - the fucoxanthin-chlorophyll protein (FCP) complexes. Spectroscopic studies of NPQ in vivo are often hindered by strongly overlapping signals from the photosystems and their antennae. Fortunately, in vitro FCP aggregates constitute a useful model system to study fluorescence (FL) quenching in diatoms. In this work, we present streak-camera FL measurements on FCPa and FCPb complexes, isolated from a centric diatom Cyclotella meneghiniana, and their aggregates. We find that spectra of non-aggregated FCP are dominated by a single fluorescing species, but the FL spectra of FCP aggregates additionally contain contributions from a redshifted emissive state. We relate this red state to a charge transfer state between chlorophyll c and chlorophyll a molecules. The FL quenching, on the other hand, is due to an additional dark state that involves incoherent energy transfer to the fucoxanthin carotenoids. Overall, the global picture of energy transfer and quenching in FCP aggregates is very similar to that of major light-harvesting complexes in higher plants (LHCII), but microscopic details between FCPs and LHCIIs differ significantly.

Electric Field-Induced Quenching of MAPbI3 Photoluminescence in PeLED Architecture.

Photoluminescence (PL) measurements are a widely used technique for the investigation of perovskite-based materials and devices. Although electric field-induced PL quenching provides additional useful information, this phenomenon is quite complex and not yet clearly understood. Here, we address the PL quenching of methylammonium lead iodide (MAPbI3) perovskite in a light-emitting diode (PeLED) architecture. We distinguish two quenching mechanisms: (a) indirect quenching by slow irreversible or partially reversible material changes that occur gradually under the applied light and electric field and (b) direct quenching by the influence of the electric field on the charge carrier densities, their spatial distributions, and radiative recombination rates. Direct quenching, observed under the abrupt application of negative voltage, causes a decrease of the PL intensity. However, the PL intensity then partially recovers within tens of milliseconds as mobile ions screen the internal electric field. The screening time increases to hundreds of seconds at low temperatures, indicating activation energies for ion motion of about 80 meV. On the other hand, ultrafast time-resolved PL measurements revealed two main phases of direct quenching: an instantaneous reduction in the radiative carrier recombination rate, which we attribute to the electron and hole displacement within individual perovskite grains, followed by a second phase lasting hundreds of picoseconds, which is due to the charge carrier extraction and spatial separation of electron and hole "clouds" within the entire perovskite layer thickness.

Antimicrobial particles based on Cu2ZnSnS4 monograins.

In this research, Cu2ZnSnS4 (CZTS) particles were successfully fabricated via the molten salt approach from the copper, zinc and tin sulphides as raw precursors. SEM analysis revealed that CZTS particles are tetragonal-shaped with sharp edges, smooth flat plane morphology, and crystal size varying from 10.8 to 28.7 µm. The phase and crystalline structure of synthesized powders were investigated using XRD analysis, which confirms the presence of a tetragonal crystal structure kesterite phase. The chemical composition of CZTS particles was evaluated by EDX spectroscopy, which identified the nearly stoichiometric composition with an averaged formula of Cu1.88Zn1.04SnS3.97. The TG/DTA-MS and ICP-OES analysis showed the possible decomposition pathways and predicted their degradation rate in aqueous solutions. The CZTS particles possessed highly effective concentration and time-dependent antimicrobial properties against medically relevant bacteria and yeast strains. The CZTS particles (1 g L-1) exhibited over 95.7 ± 1.9% killing efficiency towards M. luteus. In contrast, higher dosages (3.5 and 5 g L-1) led to its complete inactivation and reduced the P. aeruginosa cell viability to 43.2 ± 3.2% and 4.1 ± 1.1%, respectively. Moreover, the CZTS particles (0.5 g L-1) are responsible for causing 54.8 ± 1.8% of C. krusei and 89.7 ± 2.1% of C. parapsilosis yeasts death within the 24 h of exposure, which expanded to almost 100% when yeasts were treated with two times higher CZTS concentration (1.0 g L-1). The mechanism of action has been proposed and evidenced by monitoring the 2',7'-dichlorofluorescein (DCF) fluorescence, which revealed that the overproduction of reactive oxygen species (ROS) is responsible for microorganism death.

Direct Tracking of Charge Carrier Drift and Extraction from Perovskite Solar Cells by Means of Transient Electroabsorption Spectroscopy.

The best perovskite solar cells currently demonstrate more than 25% efficiencies, yet many fundamental processes that determine the operation of these devices are still not fully understood. In particular, even though the device performance strongly depends on charge carrier transport across the perovskite layer to selective electrodes, information about this process is still very controversial. Here, we investigate charge carrier motion and extraction from an archetypical CH3NH3PbI3 (MAPI) perovskite solar cell. We use the ultrafast electric-field-modulated transient absorption technique, which allows us to evaluate the electric field dynamics from the time-resolved electroabsorption spectra and to visualize the motion of charge carriers with subpicosecond time resolution. We demonstrate that photogenerated holes drift across the mesoporous TiO2/perovskite layer during hundreds of picoseconds. On the other hand, their extraction into the spiro-OMeTAD hole transporting layer lasts for more than 1 nanosecond, suggesting that the hole extraction is limited by the perovskite/spiro-OMeTAD interface rather than by the hole transport through the perovskite layer. Additionally, we use the ultrafast time-resolved fluorescence technique that reveals fluorescence decay during tens of picoseconds, which we attribute to the spatial separation of electrons and holes.

Synthesis and physical characteristics of narrow bandgap chalcogenide SnZrSe 3.

Background: The development of organic/inorganic metal halide perovskites has seen unprecedent growth since their first recognition for applications in optoelectronic devices. However, their thermodynamic stability and toxicity remains a challenge considering wide-scale deployment in the future. This spurred an interest in search of perovskite-inspired materials which are expected to retain the advantageous material characteristics of halide perovskites, but with high thermodynamic stability and composed of earth-abundant and low toxicity elements. ABX 3 chalcogenides (A, B=metals, X=Se, S) have been identified as potential class of materials meeting the aforementioned criteria. Methods: In this work, we focus on studying tin zirconium selenide (SnZrSe 3) relevant physical properties with an aim to evaluate its prospects for application in optoelectronics. SnZrSe 3 powder and monocrystals were synthesized via solid state reaction in 600 - 800 °C temperature range. Crystalline structure was determined using single crystal and powder X-ray diffraction methods. The bandgap was estimated from diffused reflectance measurements on powder samples and electrical properties of crystals were analysed from temperature dependent I-V measurements. Results: We found that SnZrSe 3 crystals have a needle-like structure (space group - Pnma) with following unit cell parameters: a=9.5862(4) Å, b=3.84427(10) Å, c=14.3959(5) Å. The origin of the low symmetry crystalline structure was associated with stereochemical active electron lone pair of Sn cation. Estimated bandgap was around 1.15 eV which was higher than measured previously and predicted theoretically. Additionally, it was found that resistivity and conductivity type depended on the compound chemical composition. Conclusions: Absorption edge in the infrared region and bipolar dopability makes SnZrSe 3 an interesting material candidate for application in earth-abundant and non-toxic single/multi-junction solar cells or other infrared based optoelectronic devices.

Tuning structural isomers of phenylenediammonium to afford efficient and stable perovskite solar cells and modules.

Organic halide salt passivation is considered to be an essential strategy to reduce defects in state-of-the-art perovskite solar cells (PSCs). This strategy, however, suffers from the inevitable formation of in-plane favored two-dimensional (2D) perovskite layers with impaired charge transport, especially under thermal conditions, impeding photovoltaic performance and device scale-up. To overcome this limitation, we studied the energy barrier of 2D perovskite formation from ortho-, meta- and para-isomers of (phenylene)di(ethylammonium) iodide (PDEAI2) that were designed for tailored defect passivation. Treatment with the most sterically hindered ortho-isomer not only prevents the formation of surficial 2D perovskite film, even at elevated temperatures, but also maximizes the passivation effect on both shallow- and deep-level defects. The ensuing PSCs achieve an efficiency of 23.9% with long-term operational stability (over 1000 h). Importantly, a record efficiency of 21.4% for the perovskite module with an active area of 26 cm2 was achieved.

Ultrafast Spectroscopic Analysis of Pressure-Induced Variations of Excited-State Energy and Intramolecular Proton Transfer in Semi-Aliphatic Polyimide Films.

The relationship between the photoexcitation dynamics and the structures of semi-aliphatic polyimides (3H-PIs) was investigated using ultrafast fluorescent emission spectroscopy at atmospheric and increased pressures of up to 4 GPa. The 3H-PI films exhibited prominent fluorescence with extremely large Stokes shifts (Δν > 10 000 cm-1) through an excited-state intramolecular proton transfer (ESIPT) induced by keto-enol tautomerism at the isolated dianhydride moiety. The incorporation of bulky -CH3 and -CF3 side groups at the diamine moiety of the PIs increased the quantum yields of the ESIPT fluorescence owing to an enhanced interchain free volume. In addition, 3H-PI films emitted another fluorescence at shorter wavelengths originating from closely packed polyimide (PI) chains (in aggregated forms), which was mediated through a Förster resonance energy transfer (FRET) from an isolated enol form into aggregated forms. The FRET process became more dominant than the ESIPT process at higher pressures owing to an enhancement of the FRET efficiency caused by the increased dipole-dipole interactions associated with a densification of the PI chain packing. The efficiency of the FRET rapidly increased by applying pressure up to 1 GPa owing to an effective compression of the interchain free volume and additionally gradually increased at higher pressures owing to structural and/or conformational changes in the main chains.

Oxidized Spiro-OMeTAD: Investigation of Stability in Contact with Various Perovskite Compositions.

The power conversion efficiency of perovskite solar cells (PSCs) has risen steadily in recent years; however, one important aspect of the puzzle remains to be solved-the long-term stability of the devices. We believe that understanding the underlying reasons for the observed instability and finding means to circumvent it is crucial for the future of this technology. Not only the perovskite itself but also other device components are susceptible to thermal degradation, including the materials comprising the hole-transporting layer. In particular, the performance-enhancing oxidized hole-transporting materials have attracted our attention as a potential weak component in the system. Therefore, we performed a series of experiments with oxidized spiro-OMeTAD to determine the stability of the material interfaced with five most popular perovskite compositions under thermal stress. It was found that oxidized spiro-OMeTAD is readily reduced to the neutral molecule upon interaction with all five perovskite compositions. Diffusion of iodide ions from the perovskite layer is the main cause for the reduction reaction which is greatly enhanced at elevated temperatures. The observed sensitivity of the oxidized spiro-OMeTAD to ion diffusion, especially at elevated temperatures, causes a decrease in the conductivity observed in the doped films of spiro-OMeTAD, and it also contributes significantly to a drop in the performance of PSCs operated under prolonged thermal stress.

Suppression of Electric Field-Induced Segregation in Sky-Blue Perovskite Light-Emitting Electrochemical Cells.

Inexpensive perovskite light-emitting devices fabricated by a simple wet chemical approach have recently demonstrated very prospective characteristics such as narrowband emission, low turn-on bias, high brightness, and high external quantum efficiency of electroluminescence, and have presented a good alternative to well-established technology of epitaxially grown III-V semiconducting alloys. Engineering of highly efficient perovskite light-emitting devices emitting green, red, and near-infrared light has been demonstrated in numerous reports and has faced no major fundamental limitations. On the contrary, the devices emitting blue light, in particular, based on 3D mixed-halide perovskites, suffer from electric field-induced phase separation (segregation). This crystal lattice defect-mediated phenomenon results in an undesirable color change of electroluminescence. Here we report a novel approach towards the suppression of the segregation in single-layer perovskite light-emitting electrochemical cells. Co-crystallization of direct band gap CsPb(Cl,Br)3 and indirect band gap Cs4Pb(Cl,Br)6 phases in the presence of poly(ethylene oxide) during a thin film deposition affords passivation of surface defect states and an increase in the density of photoexcited charge carriers in CsPb(Cl,Br)3 grains. Furthermore, the hexahalide phase prevents the dissociation of the emissive grains in the strong electric field during the device operation. Entirely resistant to 5.7 × 106 V·m-1 electric field-driven segregation light-emitting electrochemical cell exhibits stable emission at wavelength 479 nm with maximum external quantum efficiency 0.7%, maximum brightness 47 cd·m-2, and turn-on bias of 2.5 V.

Molecular Design and Operational Stability: Toward Stable 3D/2D Perovskite Interlayers.

Despite organic/inorganic lead halide perovskite solar cells becoming one of the most promising next-generation photovoltaic materials, instability under heat and light soaking remains unsolved. In this work, a highly hydrophobic cation, perfluorobenzylammonium iodide (5FBzAI), is designed and a 2D perovskite with reinforced intermolecular interactions is engineered, providing improved passivation at the interface that reduces charge recombination and enhances cell stability compared with benchmark 2D systems. Motivated by the strong halogen bond interaction, (5FBzAI)2PbI4 used as a capping layer aligns in in-plane crystal orientation, inducing a reproducible increase of ≈60 mV in the V oc, a twofold improvement compared with its analogous monofluorinated phenylethylammonium iodide (PEAI) recently reported. This endows the system with high power conversion efficiency of 21.65% and extended operational stability after 1100 h of continuous illumination, outlining directions for future work.

Doped but Stable: Spirobisacridine Hole Transporting Materials for Hysteresis-Free and Stable Perovskite Solar Cells.

Four spirobisacridine (SBA) hole-transporting materials were synthesized and employed in perovskite solar cells (PSCs). The molecules bear electronically inert alkyl chains of different length and bulkiness, attached to in-plane N atoms of nearly orthogonal spiro-connected acridines. Di-p-methoxyphenylamine (DMPA) substituents tailored to the central SBA-platform define electronic properties of the materials mimicking the structure of the benchmark 2,2',7,7'-tetrakis(N,N-di-4-methoxyphenylamino)-9,9'-spirobifluorene (spiro-MeOTAD), while the alkyl pending groups affect molecular packing in thin films and affect the long-term performance of PSCs. Devices with SBA-based hole transporting layers (HTL) attain efficiencies on par with spiro-MeOTAD. More importantly, solar cells with the new HTMs are hysteresis-free and demonstrate good operational stability, despite being doped as spiro-MeOTAD. The best performing MeSBA-DMPA retained 88% of the initial efficiency after a 1000 h aging test under constant illumination. The results clearly demonstrate that SBA-based compounds are potent candidates for a design of new HTMs for PSCs with improved longevity.

Aggregation-Related Nonphotochemical Quenching in the Photosynthetic Membrane.

The photosynthetic apparatus of plants is a robust self-adjustable molecular system, able to function efficiently under varying environmental conditions. Under strong sunlight, it switches into photoprotective mode to avoid overexcitation by safely dissipating the excess absorbed light energy via nonphotochemical quenching (NPQ). Unfortunately, heterogeneous organization and simultaneous occurrence of multiple processes within the thylakoid membrane impede the study of natural NPQ under in vivo conditions; thus, usually artificially prepared antennae have been studied instead. However, it has never been shown directly that the origin of fluorescence quenching observed in these artificial systems underlies natural NPQ. Here we report the time-resolved fluorescence measurements of the dark-adapted and preilluminated-to induce NPQ-intact chloroplasts, performed over a broad temperature range. We show that their spectral response matches that observed in the LHCII aggregates, thus demonstrating explicitly for the first time that the latter in vitro system preserves essential properties of natural photoprotection.

Electroluminescence Dynamics in Perovskite Solar Cells Reveals Giant Overshoot Effect.

High performance of both photovoltaic and electroluminescent devices requires low nonradiative recombination losses. In perovskites, such loses strongly depend on the carrier traps related to the mobile ions and vacancies, causing I- V hysteresis of solar cells and influencing the performance of other optoelectronic devices, such as photodetectors and LEDs. To address the dynamics of the mobile ions, here we investigate electroluminescence time evolution in perovskite solar cells under constant and pulsed voltage conditions. We propose a model, accounting for the spatial ion accumulation and explaining the complex electroluminescence dynamics both on fast (microseconds) and slow (seconds) time scales. We demonstrate the appearance of a high-intensity short electroluminescence peak (overshoot pulse) immediately after termination of the electrical pulse. The generation of a giant overshoot pulse suggests a simple way to achieve high pulsed luminescence intensity with a low current density, which opens new prospects toward optical gain and implementation of electrically pumped lasers.

A Few-Minute Synthesis of CsPbBr3 Nanolasers with a High Quality Factor by Spraying at Ambient Conditions.

Inorganic cesium lead halide perovskite nanowires, generating laser emission in the broad spectral range at room temperature and low threshold, have become powerful tools for the cutting-edge applications in the optoelectronics and nanophotonics. However, to achieve high-quality nanowires with the outstanding optical properties, it was necessary to employ long-lasting and costly methods of their synthesis, as well as postsynthetic separation and transfer procedures that are not convenient for large-scale production. Here we report a novel approach to fabricate high-quality CsPbBr3 nanolasers obtained by rapid precipitation from dimethyl sulfoxide solution sprayed onto hydrophobic substrates at ambient conditions. The synthesis technique allows producing the well-separated nanowires with a broad size distribution of 2-50 µm in 5-7 min, being the fastest method to the best of our knowledge. The formation of nanowires occurs via ligand-assisted reprecipitation triggered by intermolecular proton transfer from (CH3)2CHOH to H2O in the presence of a minor amount of water. The XRD patterns confirm an orthorhombic crystal structure of the as-grown CsPbBr3 single nanowires. Scanning electron microscopy images reveal their regular shape and truncated pyramidal end facets, while high-resolution transmission electron microscopy ones demonstrate their single-crystal structure. The lifetime of excitonic emission of the nanowires is found to be 7 ns, when the samples are excited with energy below the lasing threshold, manifesting the low concentration of defect states. The measured nanolasers of different lengths exhibit pronounced stimulated emission above 13 µJ cm-2 excitation threshold with quality factor Q = 1017-6166. Their high performance is assumed to be related to their monocrystalline structure, low concentration of defect states, and improved end facet reflectivity.

Tunable Hybrid Fano Resonances in Halide Perovskite Nanoparticles.

Halide perovskites are known to support excitons at room temperatures with high quantum yield of luminescence that make them attractive for all-dielectric resonant nanophotonics and meta-optics. Here we report the observation of broadly tunable Fano resonances in halide perovskite nanoparticles originating from the coupling of excitons to the Mie resonances excited in the nanoparticles. Signatures of the photon-exciton (" hybrid") Fano resonances are observed in dark-field spectra of isolated nanoparticles, and also in the extinction spectra of aperiodic lattices of such nanoparticles. In the latter case, chemical tunability of the exciton resonance allows reversible tuning of the Fano resonance across the 100 nm bandwidth in the visible frequency range, providing a novel approach to control optical properties of perovskite nanostructures. The proposed method of chemical tuning paves the way to an efficient control of emission properties of on-chip-integrated light-emitting nanoantennas.

Enhanced fluorescence of phthalimide compounds induced by the incorporation of electron-donating alicyclic amino groups.

Due to their high thermal and environmental stability, polyimides (PIs) are one of the most attractive candidates for novel highly fluorescent polymers, though photophysical studies of PIs are challenging owing to their poor solubility in common solvents. To overcome these problems, we have synthesized and examined a series of low molecular weight model imide compounds: substituted N-cyclohexylphthalimides with alicyclic amino groups at the 3 or 4-positions of the benzene rings (x-NHPIs). Their photophysical properties were systematically investigated by steady-state UV/Visible absorption, fluorescence, and time-resolved fluorescence techniques. In solution, unsubstituted N-cyclohexylphthalimide (NHPI) showed almost no emission, while x-NHPIs exhibited enhanced fluorescence emission depending on the solvent polarity. Analysis of the solvatochromism of the x-NHPIs via Lippert-Mataga plots indicated the generation of large dipole moments in the excited singlet states originating from the intramolecular charge-transfer (ICT) states. The significant difference in the fluorescence quantum yields (Φ) between the 3-substituted (3Pi and 3Pyr) and 4-substituted NHPIs (4Pi and 4Pyr) strongly suggests that the former form a twisted ICT (TICT) state, whereas the latter form a planar ICT (PICT) state when excited. 4-Substituted NHPIs also show high fluorescence yields in the crystalline state. A particularly large Φ value was obtained for the 4Pi crystal, which we explain by the large intermolecular distances and the arrangement of molecules minimizing intermolecular interactions as well as the small non-radiative deactivation rate. These facts clearly demonstrate that the introduction of an alicyclic amino group into NHPI at the 4-position enhances the fluorescence quantum yields significantly, which suggests a new pathway for the development of novel, highly fluorescent PIs.

Nonspiro, Fluorene-Based, Amorphous Hole Transporting Materials for Efficient and Stable Perovskite Solar Cells.

Novel nonspiro, fluorene-based, small-molecule hole transporting materials (HTMs) V1050 and V1061 are designed and synthesized using a facile three-step synthetic route. The synthesized compounds exhibit amorphous nature with a high glass transition temperature, a good solubility, and decent thermal stability. The planar perovskite solar cells (PSCs) employing V1050 generated an excellent power conversion efficiency of 18.3%, which is comparable to 18.9% obtained with the state-of-the-art Spiro-OMeTAD. Importantly, the devices based on V1050 and V1061 show better stability compared to devices based on Spiro-OMeTAD when aged without any encapsulation under uncontrolled humidity conditions (relative humidity around 60%) in the dark and under continuous full sun illumination.

Twisted Intramolecular Charge Transfer States in Trinary Star-Shaped Triphenylamine-Based Compounds.

Excited state dynamics of trinary star-shaped dendritic compounds with triphenylamine arms and different cores were studied by means of time-resolved fluorescence and transient absorption. Under optical excitation, nonpolar C3 symmetry molecules form polar excited states localized on one of the molecular substituents. Conformational excited state stabilization of molecules with an electron-accepting core causes a formation of twisted internal charge transfer (TICT) states in polar solvents. A low transition dipole moment from TICT state to the ground state causes very weak fluorescence of those compounds and strong dependence on the solvent polarity. The compound formed from the triphenylamine central core and identical arms also experiences excited state twisting, however, weakly sensitive to the solvent polarity.

Hindered Amine Light Stabilizers Increase the Stability of Methylammonium Lead Iodide Perovskite Against Light and Oxygen.

Methylammonium lead iodide perovskite (MAPI) is a promising material for highly efficient photovoltaic devices. However, it suffers from photooxidation, which imposes strict requirements for its protection from oxygen during processing and operation. A hindered amine light stabilizer (HALS) has been found to exert a stabilization effect on methylammonium iodide (MAI) and MAPI against photooxidation. The HALS prevents the degradation of MAI by inhibiting the oxidation of iodide to iodine. Chemical modification of HALS allows its incorporation in MAPI films, which extends the resistivity of MAPI against photodegradation in ambient air from a couple of hours to several days, while causing no significant changes in key properties, such as optical absorption and charge transport. These results represent an important advance in the stabilization of MAPI against decomposition and demonstrate for the first time that antioxidants improve the stability of MAPI.