TRANSIENT ELECTROLUMINESCENCE IN TRIPLE CATION PEROVSKITE SOLAR CELLS
Center for Physical Sciences and Technology, Vilnius, Lithuania
Transient electroluminescence is a widely used technique to study fundamental processes related with charge transport properties in organic or inorganics semiconductors. In this work a detailed study of electroluminescence (EL) dynamics in CsFAPb(IxBr1-x) perovskite solar cells is presented along with comprehensive discussion about its origin within the broad temporal perspective.
Electroluminescence signal in perovskite solar cells exhibit strong time-dependent behaviour which allows to reveal several associated processes. After applying periodic square electrical pulses the initial EL emission is very weak, then it evolves with time, and eventually strong light flash appears immediately after the forward-biased electrical voltage is switched off. This intense EL overshoot flash in perovskite solar cells has not been observed so far. Intensity of this hundreds of nanosecond duration flash was up to two orders of magnitude higher than EL intensity during the electrical pulse action. The flash, as well as regular EL, were very weak initially after switching on periodic electrical pulses. Their stabilization was reached during several minutes. Similar EL growth was also observed when the cell was biased by CV voltage. This information provided new insights about working mechanism of cesium containing triple cation perovskite layers in solar cell architecture. Redistribution of methyl ammonium (MA+) excess ions in active material is taking place which leads to change of electron and hole distributions - so increasing their overlap and enabling radiative recombination. Based on this model, the EL overshoot originates from rapid mixing of electron and hole clouds after electric field turn-off.
TRIPHENYLAMINE POLYMERS AS HOLE TRANSPORTING MATERIALS FOR APPLICATION IN PEROVSKITE SOLAR CELLS
1Department of Organic Chemistry, Kaunas University of Technology, Kaunas, Lithuania; 2Panasonic Corporation, Materials Research Laboratory, Advanced Research Division, Osaka, Japan; 3Laboratory of Photonics and Interfaces, Institute of Chemical Sciences and Engineering, École Polytechnique Fédérale de Lausanne, Lausanne, Switzerland; 4Group for Molecular Engineering of Functional Materials, Institute of Chemical Sciences and Engineering, École Polytechnique Fédérale de Lausanne, Lausanne , Switzerland; 5Laboratory of Photomolecular Science, Station 6, Lausanne, Switzerland
Rapid development of technologies and their influence on people's lives is increasingly associated with energy demand. In order to meet this growing energy demand and minimize related costs new and effective energy generation methods are necessary. Among several alternative sources, photovoltaics are among the most promising, as solar energy is free and inexhaustible energy source. Perovskite solar cells often consist of several layers, each having a specific function and made of materials that meet a certain set of requirements. Some of those requirements include the stability issue, which derives from additives that are used in the process of increasing the conductivity of hole transporting materials.
Having that in mind triarylamine derivative polymers with different functional groups were synthetized as hole transport materials (HTMs) for perovskite solar cells (PSCs). The novel materials enabled efficient PSCs without the use of chemical doping to enhance the charge transport. Devices employing poly(triarylamine) with methylphenylethenyl functional groups showed better power conversion efficiency then widely used additive-free compound - poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA). Notably, devices with the foremost polymer enabled stable PSCs under 1 sun at maximum power point tracking for ~40 hours and under elevated temperature (85 °C) for more than 140 hours. The results present remarkable progress towards stable PSC under real working conditions, which is crucial for industrial application.
STABILITY RESEARCH OF OXIDIZED SPIRO-MEOTAD
Kaunas University of Technology, Kaunas, Lithuania
Population growth and ever-increasing energy consumption prompts new search for cleaner and safer alternatives to fossil fuels and nuclear energy. One of the most attractive alternatives is photovoltaic systems. Because of simple manufacturing and good performance prospects perovskite solar cells received growing interest from research community. As a result of rapid development in this field, perovskite photovoltaic systems have reached 23.7%  efficiency.
Despite relatively high performance of perovskite solar cell (PSC), the current conditions still do not meet the requirement for commercialization. There are three main stability issues with PSCs: environmental (moisture and oxygen), photo and thermal stability. In addition, selective contacts and additives in HTM can also have influence on stability .
In this work the hole-transporting materials, like spiro-OMeTAD, have been investigated under various conditions, such as thermal stress, in order to estimate overall lifetime and influence of different additives. Overall morphological stability of the amorphous state of spiro-OMeTAD has deteriorated rapidly under elevated temperature after 1000 hours at 100 °C, and a material’s change from amorphous to crystalline aggregate state. This is one of the main factors leading to a rapid decline of device performance.
 H.-S. Kim, A. Hagfeldt and N-G. Park, Morphological and compositional progress in halide perovskite solar cells, Chem. Commun., 2019,55, 1192-1200
 X. Zhao and N.-G. Park. Stability Issues on Perovskite Solar Cells. Photonics 2015, 2, 1139-1151 p.
 T. Malinauskas., D. Tomkutė-Lukšienė, R. Sens, M. Daskeviciene, R. Send, H. Wonneberger, V. Jankauskas, I. Bruder, and V. Getautis. Enhancing Thermal Stability and Lifetime of Solid-State Dye-Sensitized Solar Cells via Molecular Engineering of the Hole-Transporting Material Spiro-OMeTAD. ACS Appl. Mater. Interfaces . 2015, vol. 7( 21), 11107-11116 p.
HOLE-TRANSPORTING ENAMINES WITH TRÖGER’S BASE SCAFFOLD FOR APPLICATION IN PEROVSKITE SOLAR CELLS
1Department of Organic Chemistry, Kaunas University of Technology, Kaunas, Lithuania; 2Group for Molecular Engineering of Functional Materials, Institute of Chemical Sciences and Engineering, Ecole polytechnique fédérale de Lausanne, Lausanne, Switzerland; 3Institute of Chemical Physics, Vilnius University, Vilnius, Lithuania
Perovskite solar cells (PSCs) are a promising affordable alternative to inorganic solar cells for efficient harvesting of abundant solar energy with high power conversion efficiency. Until now, most of the high efficiency PSCs are based on either small organic molecule 2,2`,7,7`-tetrakis(N,N-di-p-methoxy-phenylamine)-9,9`-spirobifluorene (Spiro-OMeTAD) or conjugated macromolecule poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA) hole-transporting materials (HTMs), both of which are expensive due to expensive multi-step synthesis and costly purification procedures. As HTMs are essential part in solar cell manufacturing, currently they are a bottleneck for the realization of cost-effective and stable devices. Tröger’s base (TB) is an easily obtainable angle-shaped molecule and can serve as a structural core, providing non-planar orientation for its substituents and structural branches towards each other. Expanding TB structure via enamine condensation can increase both structural bulk and expand conjugated π-system multiple times, thus providing easily obtainable, cheap and efficient HTMs for PSC applications.
Here we report the synthesis of three novel enamine HTMs based on Tröger’s base scaffold. These compounds are obtained in easy three-step synthesis from commercially available materials, none of the synthetic steps requiring expensive palladium catalysts or inert conditions. Two best performing HTMs are purified via crystallization, hence eliminating the need of column chromatography. Best performing material HTM3 demonstrated 18.62 % PCE in PSC, rivaling Spiro-MeOTAD in efficiency, and also showed superior stability of non-encapsulated perovskite cell; while tested in dopant-free PSCs it outperformed Spiro-MeOTAD by 1.6 times. High glass transition temperature (Tg =176 °C) of HTM3 also hints its bright perspectives in device applications.
FLUORESCENCE QUENCHING OF DOXORUBICIN THROUGH ITS INTERACTION WITH BORON NITRIDE NANOFLAKES
1Institute of Physics of the National Academy of Sciences of Ukraine, Kiev, Ukraine; 2Center for Physical Sciences and Technology, Vilnius, Lithuania
Most of chemotherapy drugs are highly toxic. Doxorubicin (DOX) is a typical chemotherapy drug used to treat leukemia, myeloma, lymphoma, prostate, breast cancer and several others from 1970’s.
PHOTOPHYSICAL STUDY OF BIS-TRIAZOLYL-PURINE NUCLEOSIDE PUSH-PULL MOLECULAR SYSTEMS AND THEIR APPLICABILITY AS SELF-CALIBRATING TRANSITION METAL ION SENSORS
1Institute of Photonics and Nanotechnology, Vilnius University, Vilnius, Lithuania; 2Laboratoire Ondes et Matière d’Aquitaine, Bordeaux University, Talence, France; 3Institute of Technology of Organic Chemistry, Faculty of Materials Science and Applied Chemistry, Riga Technical University, Riga, Latvia
The research of fluorescent nucleic acid based analogues have been a subject of high interest for decades. The fluorescent nucleoside derivatives can be used to study some properties of nucleic acids, such as their structure, charge transfer dynamics, and, by all means, their biological functions. The remarkable improvements of fluorescence properties of nucleobase and nucleoside derivatives and their possible biocompatibility leaded to the extended biological applications, where specially designed molecules play an important role in sensing and reporting the targeted analytes.
In this work we present photophysical evaluation of a series of bis-triazolyl-purine nucleoside derivatives decorated with electron-accepting or electron-donating groups. The comprehensive spectroscopic study performed by means of steady-state and time-resolved spectroscopies, including transient absorption measurements, reveals appealing optical properties of these compounds, as well as their potential application as transition metal ion sensing materials. The presented molecules with its unique structure, act as distinct emissive electronic systems and in case of push-pull molecular design demonstrate a charge transfer character. The alteration of substituents and environment parameters ensured the suitable conditions for creating self-calibrating molecular metal ion sensor, based on bis-triazolyl-purine nucleoside derivative. These sensing compounds exhibited exceptional high sensitivity enabling to detect less than 0.2 equivalents of metal ions in acetonitrile solution. Additionally, the particular fluorescence spectral response to concentrations of metal ions allows to achieve the desired selectivity of studied molecules.
TAILORED CONFORMATION AND PHOTOLUMINESCENCE PROPERTIES OF SELF-ASSEMBLED PHENAZINE-CORED PLATINUM (II) METALLACYCLES
1National Taiwan University, Taipei, Taiwan; 2Uiversity of Utah, Salt Lake City, Utah, United States of America
A series of platinum (II) metallacycles were prepared via the coordination-driven self-assembly of a phenazine-cored dipyridyl donor with a 90° Pt(II) acceptor and various dicarboxylate donors in a 1:1:2 ratio. While the metallacycles display similar absorption profiles, they exhibit a trend of blue-shifted luminescence emission with the decrease in the bite angles between the carboxylate building blocks. Comprehensive spectroscopic and dynamic studies, as well as a computational approach were conducted, revealing that the difference in the degree of constraint imposed on the excited-state planarization of the phenazine core within these metallacycles results in the distinct photophysical behaviors. As such, a small initial difference in the dicarboxylate building blocks is amplified into distinct photophysical properties of the metallacycles, which is reminiscent of the efficient functional tuning observed in natural systems. In addition to the pre-assembly approach, the photophysical properties of a metallacycle can also be modulated using a post-assembly modification to the dicarboxylate building block, suggesting another strategy for functional tuning. This research illustrated the potential of coordination-driven self-assembly for the preparation of materials with precisely tailored functionalities at the molecular level.
OPTICAL PROPERTIES OF PHOTOCHROMIC DIMETHYLDIHYDROPYRENE DERIVATIVES IN SOLUTIONS
1Center for Physical Sciences and Technology, Vilnius, Lithuania; 2Warsaw University of Technology, Warsaw, Poland
Molecular switches can be commonly applied to control different functions and properties of materials which can be used in organic electronics (as for example in new memory elements based on single molecule) or in biology to manipulate the biological systems. Photochromism of dimethyldihydropyrene derivatives is a reversible transformation under UV and visible light irradiation between two, opened-ring cyclophanediene and closed-ring dimethyldihydropyrene, isomers with different spectroscopic properties. Dimethyldihydropyrene compounds could be applied in wide range of areas such as organic electronics, for example single molecule memory elements, and biology – diagnostics, control of metabolic reactions. To achieve even more suitable physical properties for different applications, DHP molecules could be modified by adding substitutes. However, the most common problem of these modified compounds is stability. To solve this problem, there is a need of deeper understanding of processes appearing during photochemical reaction.
New dimethyldihydropyrene derivatives were synthesized, and their optical properties as well as excited state dynamics were investigated in the solutions. We focus on the emissive properties of dimethyldihydropyrene derivatives with the possibility to switch them between fluorescent and non-fluorescent states. During the first 100-300 ps after excitation under visible light the closed-ring cyclohexadiene isomer were opened. Reverse transformation took place through intermediate stage during several nanoseconds.
ADVANCED VIBRATIONAL SPECTROSCOPY AND MICROSCOPY FOR THE CHARACTERIZATION OF ORGANIC SEMICONDUCTORS
1Institute of Physical Chemistry, Universität Heidelberg, Heidelberg, Germany; 2Centre for Advanced Materials, Universität Heidelberg, Heidelberg, Germany; 3Kirchhoff-Institute of Physics, Universität Heidelberg, Heidelberg, Germany; 4Institute of Physics, EIT 2, Universität der Bundeswehr München, Neubiberg, Germany; 5Interdisciplinary Center for Scientific Computing, Universität Heidelberg, Heidelberg, Germany; 6Institute of Organic Chemistry, Universität Heidelberg, Heidelberg, Germany
Vibrational spectroscopies, such as Raman and Fourier Transform Infrared spectroscopy (FTIR), have recently regained attention as powerful tools for the characterization of organic semiconductor thin films and crystals in addition to more conventional techniques (e.g. X-ray diffraction, scanning atomic force microscopy). A range of properties such as molecular orientation, polymorphism, doping levels and intra- as well as intermolecular modes with impact on charge transport can be investigated with these techniques. However, in particular FTIR is restricted in its spatial resolution to several micrometers and cannot resolve different domains in polycrystalline thin films. Two new and fundamentally different scanning probe techniques have been developed and commercialized over the past few years that offer mapping of IR modes with a spatial resolution below 100 nm: Scattering-Type Scanning Near-Field Optical Microscopy (IR s-SNOM) and Atomic Force Microscopy-Infrared Spectroscopy (AFM-IR).
Here we compare these two new techniques with each other and to classic FTIR microscopy measurements with regard to their applicability to ordered organic semiconductors. For this purpose, we employ organic single crystals of rubrene, perfluorobutyl dicyanoperylene carboxydiimide (PDIF-CN2), TIPS-pentacene and TIPS-tetraazapentacene as model systems. All of them are well-known high-mobility semiconductors, however, with very different orientations of the main conjugated core of the molecules within the crystal. We observe significant differences in intensity and position of the IR modes depending on technique and polarization and compare them to the calculated vibrational modes for each molecule. We thus highlight the advantages and disadvantages of the three methods for the characterization of organic semiconductor crystals and polycrystalline thin films.
THE PROTIC SOLVENT CATALYZED EXCITED-STATE PROTON TRANSFER REACTION IN 7-AMINOQUINOLINE AND ITS NRH DERIVATIVES
1National Taiwan University, Taipei, Taiwan; 2Chung Shan Medical University, Taichung, Taiwan
7-Aminoquinoline (7AQ) and its amino derivatives have been designed and synthesized to study their excited-state proton transfer (ESPT) reaction. Due to the far separation between the proton donor NR-H (D) and acceptor –N (A) sites, ESPT in 7AQ derivatives, if available, should proceed with solvent catalysis process. As a result, assisted by alcohol molecules, TFA-7AQ, Ts-7AQ, Boc-7AQ, and Ac-7AQ undergoes ESPT in e.g. methanol. Study indicates the existence of equilibrium between cis and trans type NR-H-----methanol hydrogen bond (H-bond) formation, in which the cis H-bonded form undergoes ESPT, yielding an imine-like proton transfer tautomer. Systematic ESIPT dynamics among all NR-H derivatives have been carried out. Unlike most of the NR-H intramolecular system where the rate of excited-state intramolecular proton transfer (ESIPT) increases as increasing the NR-H acidity, the rate of solvent catalyzed ESPT was found to correlate with the basicity of quinoline. The results are rationalized by the fact that increase of the NR-H acidity by the stronger electron withdrawing R group concurrently decreases the basicity of the quinoline nitrogen via resonance induction effect.
NEGATIVELY CHARGED AND DARK EXCITONS IN CSPBBR3 PEROVSKITE NANOCRYSTALS REVEALED BY HIGH MAGNETIC FIELDS
1Experimentelle Physik 2, Technische Universität Dortmund, Dortmund, Germany; 2Rzhanov Institute of Semiconductor Physics, Siberian Branch of Russian Academy of Sciences, Novosibirsk, Russia; 3Loffe Institute, Russian Academy of Sciences, St. Petersburg, Russia; 4High Field Magnet Laboratory, Radboud University, Nijmegen, The Netherlands; 5Sorbonne Universités, UPMC Univ Paris 06, Institut des NanoSciences de Paris, Paris, France; 6Institut d’Électronique, de Microélectronique et de Nanotechnologie, Villeneuve d’Ascq, France
In the past few years, colloidal cesium lead halide nanocrystals (NCs) (CsPbX3, X = Cl, Br, I) have attracted much attention due to their striking and markedly different optical properties compared to standard II−VI (CdSe, CdTe, HgTe) or III−V (InP, InAs) colloidal semiconductor nanocrystals. Among them CsPbBr3 NCs demonstrate superior stability against photoexcitation and oxidation leading to outstanding optical properties. In particular, at cryogenic temperatures and in low excitation regime, the emission spectrum of a single uncapped CsPbBr3 NC has a single or multiple sharp lines (<1 meV line width), similar to those of core/shell CdSe/ZnS NCs. At cryogenic temperatures the recombination dynamics of CsPbBr3 NCs occurs on two markedly different time scales. Most of the photons are emitted within the first 300 ps and only a small fraction within a few ns. This raises the question about the exciton fine structure in CsPbBr3 NCs, namely, on whether the ground state is dark (spin forbidden) or bright (spin allowed). This is still an open question, which requires experimental efforts.
Here, we investigate recombination and spin dynamics of charged and neutral excitons in ensemble of colloidal CsPbBr3 NCs in temperature range from 4.2 to 300 K and in high magnetic fields up to 30 T. We unveil the presence of the dark exciton state via its long-lasting recombination dynamics shortened by magnetic fields. At cryogenic temperatures the radiative recombination of negatively charged excitons dominates the PL emission. Electron, hole, and exciton g-factors are evaluated from magneto-optical data.
HALOGEN SUBSTITUTION CONTROLS CRYSTAL SYMMETRY BREAKING IN LAYERED HYBRID METAL-HALIDE PEROVSKITES
1Physikalisch-Chemisches Institut, Ruprecht-Karls-Universität Heidelberg, Heidelberg, Germany; 2Cavendish Laboratory, University of Cambridge, Cambridge, United Kingdom; 3Cambridge Graphene Centre, University of Cambridge, Cambridge, United Kingdom; 4Department of Chemistry, University of Cambridge, Cambridge, United Kingdom
Layered hybrid organic-inorganic lead halide perovskites are semiconductors with strongly confined electronic states, for which excitonic nature and light-matter interaction is controlled by the crystal lattice symmetry and conformation. Here, we use halogen para-substituents in benzylammonium lead iodide perovskites (C6H5CH2NH3)2PbI4, (4‑FC6H4CH2NH3)2PbI4, (4-ClC6H4CH2NH3)2PbI4 and (4‑BrC6H4CH2NH3)2PbI4 to control the symmetry of the crystal structure. We synthesise single crystals and resolve the changes in structural and photophysical properties for different halogen substituents. Introducing a halogen atom at the para‑position leads to a structural change which lowers the symmetry: The orientation of the aromatic cations is determined by π‑π‑, H‑π‑ and halogen‑π‑interactions, and, in turn, the orientation of the organic cations introduce a tilting of the inorganic layer consisting of PbI6-octahedra. Hence, changes in the optical properties, such as a shift of the excitonic emission, appear due to octahedra distortion. In addition, the symmetry of the perovskite material is mainly determined by the organic layer. The larger and less electronegative the para‑substituent in (4‑XC6H4CH2NH3)2PbI4 with X = H, F, Cl, Br, the larger the symmetry-breaking effect, which is promising for control of electronic states through Rashba effects. Rashba splitting appears in materials which lack an inversion centre and leads to a splitting of the electronic energy bands due to spin-orbit coupling, which is important for e.g. spintronics. Most hybrid organic inorganic perovskite compounds adopt centrosymmetric space groups which limits their application in nonlinear optics and spintronic devices. Our results present halogen substitution in hybrid layered aromatic metal-halide perovskites as a facile approach to create non‑centrosymmetric perovskite materials with tuneable electronic states.
ANISOTROPY OF THERMAL DIFFUSIVITY IN LEAD HALIDE PEROVSKITE LAYERS REVEALED BY THERMAL GRATING TECHNIQUE
1Institute of Photonics and Nanotechnology, Vilnius University, Vilnius, Lithuania; 2Center for Organic Photonics and Electronics Research (OPERA), Kyushu University, Fukuoka, Japan; 3Japan Science and Technology Agency (JST), ERATO, Adachi Molecular Exciton Engineering Project, Fukuoka, Japan; 4Innovative Organic Device Laboratory, Institute of Systems, Information Technologies and Nanotechnologies (ISIT), Fukuoka Industry-Academia Symphonicity (FiaS), Nishi, Fukuoka, Japan; 5International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), Kyushu University, Fukuoka, Japan
Heat management of optoelectronic devices is of critical significance in lead halide perovskites due to intrinsically-low thermal conductivity of this material. Despite its importance, the thermal conductivity remains understudied, particularly in polycrystalline perovskite layers with different halides. Here, we employ a novel method for investigation of thermal properties in perovskite layers, which is based on light-induced transient diffraction grating technique. We demonstrate the applicability of thermal grating technique by determining in all-optical way the thermo-optic coefficient, speed of sound, and thermal conductivity in vapor-deposited polycrystalline layers of MAPbX3 (X = Cl, Br, I), MAPbBr2I, and MAPbCl2Br perovskites. We reveal the spatial anisotropy of thermal conductivity, which is noticeably lower in the direction along the layer surface (0.2 – 0.5 W/Km) if compared to that across the layer (0.3 – 1.1 W/Km). Finally, we demonstrate that for both directions the thermal conductivity scales linearly with the average speed of sound in the perovskite layers.
FAST EXCITATION DINAMICS AND LOW ASE THRESHOLD IN LEAD-FREE MASNI3 PEROVSKITES
1Institute of Photonics and Nanotechnology, Vilnius University, Vilnius, Lithuania; 2Center for Organic Photonics and Electronics Research (OPERA), Kyushu University, Fukuoka, Japan
Lead-halide perovskites are attractive materials for wet-cast photonic applications, but their toxicity raises serious environmental and health concerns. Therefore, the lead-free perovskites are extensively studied as a possible nontoxic substitution of similar or even superior performance.
Here, we investigate the ultrafast processes of carrier thermalisation, recombination, and diffusion in a set of MASnI3 layers, grown from solution by using different precursors . We employ a unique light-induced transient grating technique (LITG) together with the photoluminescence (PL) and differential transmission (pump-probe) measurements.
We demonstrate that highly excited Sn perovskite layers exhibit very promising electrical properties: the measured carrier diffusion coefficient and lifetime reach 0.5 – 1.6 cm2/s and 100 – 140 ps, respectively, resulting in the diffusion lengths of 100-150 nm. These values are comparable to those of vapour-deposited lead-halide perovskites . We observe the fast (within 2 – 4 ps) carrier thermalisation in the layers. We show that amplified stimulated emission (ASE) can be readily obtained in Sn perovskites, yet the ASE threshold strongly depends on layer quality; in our case it varied in the 5 - 120 μJ/cm2 range. It’s notable that the lowest ASE threshold value of 5.4 μJ/cm2is considerably lower than that reported in MAPbI3 samples with additives . These results suggest that Sn perovskites can be an effective active material for photonic devices like lasers and high power LEDs.
 T. Fujihara et. al., J. Mater. Chem. C 5 (2017) pp. 1121.
 P. Ščajev et. al., J. Phys. Chem. C 121 (2017) pp. 21600.
 P. Ščajev et. al., J. Phys. Chem. Lett. 9 (2018), pp. 3167.
EXPLOITING TWO-STEP PROCESSED MIXED 2D/3D PEROVSKITES FOR BRIGHT GREEN LIGHT EMITTING DIODES
1IMEC, Heverlee, Belgium; 2ESAT, K.U.Leuven, Heverlee, Belgium; 3Department of Polymer Science and Engineering, Zhejiang University, Hangzhou, China
Mixed 2D/3D perovskite films with self-assembled quantum wells have significantly improved the performance of perovskite light emitting diodes (PeLEDs). To date, one-step spin-coating method dominates the preparation of 2D/3D perovskite films for high-efficiency PeLEDs. However, this method has some known drawbacks, e.g. the use of toxic anti-solvent as well as narrow processing widows for anti-solvent dripping. In this work, we fabricated such films through a two-step interdiffusion method which remains rarely explored for perovskites used in PeLEDs. We studied in detail the effects of incorporating large-cation ligand, i.e. butylammonium bromide (BABr) into formamidinium lead bromide (FAPbBr3) based perovskites in terms of film composition, morphology, optoelectronic properties as well as device performance.
With this work, we have demonstrated that high-quality smooth 2D/3D mixed perovskite films from two-step method are promising for bright PeLEDs. The champion device made with the optimal BABr:PbBr2 ratio shows a maximum external quantum efficiency (EQE) of 7.36% at a high current density of 147.7 mA cm-2 and a high brightness of 37720 Cd m-2 at 5 V. We note that a high EQE at high current density are not common in literature, indicating the advantage of this two-step processing method. Furthermore, we have shown that this method could be extended to a wide range of large-cation ligands. By simply replacing BABr with 4-fluoro-benzylammonium bromide (F-BZABr), a higher maximum EQE of 8.55% is achieved, demonstrating the versatility of this method as well as its potential of achieving better device performance with further material and device engineering.
FURAN/PHENYLENE CO-OLIGOMERS: A BRIGHT ACCESS TO HIGHLY EMISSIVE SEMICONDUCTING MATERIALS
1Novosibirsk State University, Novosibirsk, Russian Federation; 2N.N. Vorozhtsov Novosibirsk Institute of Organic Chemistry SB RAS, Russian Federation
Organic materials which combines high charge mobility and efficient solid state photoluminescence are highly demand according to realize the potential applications in fully colored active matrix displays, integrated photonic circuitry, optical sensing, and electrically driven organic lasers. However, in most cases, organic semiconductors with high charge carrier mobility due to strong pi-pi overlap suffer from photoluminescence quenching in solid state caused by aggregation effects. Nevertheless, it was reported that conjugated rod-like small molecules e.g. thiophene/phenylene co-oligomers (TPCOs), distyrylbenzenes, anthracene derivatives demonstrated high efficiency photoluminescence in solid state and charge mobility >1 cm2/Vs, which makes these materials promising candidates for application as active layers in organic optoelectronics.
Rod-like furan/phenylene co-oligomers (FPs) are highly promising materials which demonstrated remarkable luminescence and charge transport properties which can be carefully tuned upon variation of their terminal substitution and molecular length. The studied FPs demonstrated ambient stability and high photoluminescence quantum yield both in solution and vapor- or solution-grown single crystals (>45%), high solubility in organic solvents (>1 g/L). The top-contact top gate organic field effect transistors demonstrated hole mobility up to 0.35 cm2/Vs and the low threshold voltage for a long chain FPs co-oligomers. We also demonstrated that the higher the torsional rigidity, the lower the reorganization energy, especially for exciton transport for which confirms that FPs may be a promising candidates for optoelectronic applications. Moreover, light doping of short FPs by the longer FPs molecules could drastically influence on its optical and electronic properties.
TUNABLE LIQUID CRYSTAL POLARIZATION CONVERTER BASED ON OPTICAL SPIN HALL EFFECT
1Institute of Experimental Physics, Faculty of Physics, University of Warsaw, Warsaw, Poland; 2Institute of Applied Physics, Military University of Technology, Warsaw, Poland; 3Institute of Chemistry, Military University of Technology, Warshaw, Poland; 4School of Physics and Astronomy, University of Southampton, Southampton, United Kingdom; 5Skolkovo Institute of Science and Technology, Skolkovo, Russian Federation
Recently, research on new spintronic and optoelectronic systems has gained great popularity. The devices that would explore the spin state of photons become of special interest as they provide a new degree of possible manipulation mechanisms by spin-to-orbital momentum conversion of photons.
In this communication we present a new kind of an active polarization converter  consisting of a microcavity filled with a nematic liquid crystal (LC). The properties of LC are controlled by external voltage. This unique device operates in room temperature and explores giant values of TE-TM splitting that are smoothly tunable in range from -15.9 meV to 27.8 meV, an order of magnitude higher to values reported previously. One of the most interesting phenomena directly dependent on the TE-TM splitting in a cavity is the optical spin Hall effect. We show that thanks to the novel design and the unique possibility to tune the splitting even for zero incidence angle, we are able to observe not only typical quadrupole spin textures, but also never reported before patterns resembling: dipole, spin doughnuts and spin whirls.
Our novel device allows to control the spin state of photon making a new building block for the next-generation of photonic spin Hall devices. Moreover, it can be easily integrated with light emitters (like various dopants: quantum dots, dyes, thin layers of transition metal dichalcogenides) for room-temperature strong light-matter coupling and lasing.
This work was supported by the Ministry of Higher Education, Poland under projects "Diamentowy Grant": 0005/DIA/2016/45 and 0109/DIA/2015/44, and the National Science Centre grant 2016/23/B/ST3/03926.
 K. Lekenta et al., Tunable optical spin Hall effect in a liquid crystal microcavity. Light Sci. Appl. 7, 74 (2018).
STRONGLY ANISOTROPIC DIFFUSION OF EXCITONS IN LAYERED RES2 OBSERVED WITH TRANSIENT ABSORPTION MICROSCOPY
University of Cambridge, Cambridge, United Kingdom
The development of optoelectronic devices based on transition metal dichalcogenides (TMDCs) will require a deep understanding of the spatial and temporal dynamics of their charge carriers and excitons. Whilst the properties of most TMDCs are isotropic, in a few cases a reduced crystal symmetry leads to unusual anisotropic properties, a degree of freedom that has remained hitherto largely unexplored. TMDCs are also typically technologically relevant only in the mono to few layers limit. In this respect, ReS2 and ReSe2 are unique members of the TMDC family. Their disordered 1T phase make them anisotropic semiconductors allowing them to support robust linear-exciton with different dipole orientations at room temperature. Besides, these excitons persist from the bulk down to the monolayer. Here, we study the anisotropic diffusion of excitons and free carriers in ReS2 with transient absorption microscopy (TAM) for thicknesses from the monolayer to the bulk
In TAM, a 10 fs pump pulse focused to the diffraction limit generates excitons and charge carriers, a 10 fs probe pulse in the wide field then monitors their temporal and spatial dynamics. A comparison of the spatial profile of the transient absorption signal following photoexcitation allows studying the movement of excited species with a sub-diffraction-limit resolution of 10 nm (Figure 1 b).
Varying the pump and probe linear polarisations we analyse the strongly anisotropic diffusion of the two lowest exciton populations (Figure 1). We observe an increase in the exciton lifetime and diffusion coefficients with the number of layers, which we attribute to a decrease in surface traps as the sample thickness increases.
EXPLOITING LOCALIZED CHARGE ACCUMULATION IN ALLOYED HYBRID PEROVSKITES FOR HIGHLY EFFICIENT LUMINESCENCE
1Cavendish Laboratory, University of Cambridge, Cambridge, United Kingdom; 2Department of Physics, Zhejiang Normal University, Jinhua, China; 3Department of Materials Science and Metallurgy, University of Cambridge, Cambridge, United Kingdom; 4Department of Earth Sciences, University of Cambridge, Cambridge, United Kingdom; 5Adolphe Merkle Institute, University of Fribourg, Fribourg, Switzerland; 6Chimie des Matériaux Nouveaux, Université de Mons, Mons, Belgium
Metal-halide perovskites have emerged as exceptional semiconductors for optoelectronic applications. Substitution of the monovalent cations has advanced luminescence yields and device efficiencies. Here, we control the cation alloying to push optoelectronic performance through alteration of the charge carrier dynamics in mixed-halide perovskites. In contrast to single-halide perovskites, we find high luminescence yields for photo-excited carrier densities far below solar illumination conditions. Using time-resolved spectroscopy we show that the charge-carrier recombination regime changes from second to first order within the first tens of nanoseconds after excitation. Supported by microscale-mapping of the optical bandgap, electrically-gated transport measurements and first-principles calculations, we demonstrate that spatially-varying energetic disorder in the electronic states causes local charge accumulation, creating p- and n-type photo-doped regions, which unearths a strategy for efficient light emission at low charge-injection in solar cells and LEDs.
STRUCTURAL AND OPTICAL PROPERTIES OF ALL-INORGANIC CSPBCL3−XBRX PEROVSKITES
1Department of Nanophotonics and Metamaterials, ITMO University, St.Petersburg, Russia; 2Department of Physics, North Ossetian State University, Vladikavkaz, Russia; 3Center for Physical Sciences and Technology, Vilnius, Lithuania; 4University of Texas at Dallas, Richardson, Texas, United States of America
Recent years have witnessed the enormous interest to lead halide perovskites. Owing to their high charge mobility, tunable direct band gap, and high photoluminescence quantum yield, perovskites are successfully exploited for solar energy harvesting (Bertolotti F. 2017) and light-emitting applications (Makarov S. 2018). In this work, we present systematic structural and optical studies of all-inorganic CsPbCl3−xBrx perovskite thin films.
CsPbBr3-xClx thin films on Si and glass substrates were fabricated using a wet chemical method accompanied by chemical vapor anion exchange procedure. Firstly, we prepared CsPbBr3 films by spin-coating in a N2-filled glove box. Then, to obtain mixed-halide CsPbBr3−xClx and monohalide CsPbCl3 structures, tribromide counterparts were exposed to HCl vapor at 120 ◦C.
The halogen exchange reactions were monitored by the photoluminescence (PL) spectrum. A shift of the PL peak at room temperature (RT) varied from 521 nm (CsPbBr3) to 411 nm (CsPbCl3). Stokes shift for the samples 1-6 was calculated to be close to 22 meV value.
X-ray diffraction (XRD) spectrum of CsPbBr3 matches well with the reference pattern (Card No. 01-072-7929) (Rodova 2003). The gradual diminishing of four peaks in the 24-30◦ 2θ range until their complete disappearance demonstrating a tetragonal crystal structure of CsPbCl3 in accordance with Card No. 00-018-0366 (Swanson, M. E. 1967).
The mixed-halide samples 2-5 possess intermediate Orth-Tet phases. According to XPS quantitative analysis, Br:Cl ratio varied as follows: 3:0, 2.61:0.39, 2.1:0.9, 1.5:1.5, 0.96:2.04, 0.04:2.96.
ENGINEERING AND CHARACTERIZATION OF POLYMER COLLOIDAL NANOPARTICLES FOR RETINAL PROSTHESIS
Italian Institute of Technology, CNST@PoliMi, Milan, Italy
Hybrid interfaces composed by organic semiconductors in contact with living tissues are appealing for the controlled photo-stimulation of tissue cells in both in-vivo and in-vitro applications. Organic optoelectronic materials, such as light sensitive and conjugated polymers, provide an easy processable, biocompatible and tunable tool for the development of bio-devices and prosthesis able to restore vision. Planar organic prosthesis based on a thin film of poly(3-hexylthiophene) (P3HT) have been proven to be successful in inducing vision recovery in rat model affected by retinitis pigmentosa. Considering such a promising result, we decided to investigate P3HT nanoparticles (NPs). According to preliminary results, P3HT NPs succeeded in restoring vision acuity in blind rats. However, despite the encouraging results, the mechanisms at the basis of the retinal organic prosthesis functioning is still not well understood. In this work, we investigate the photophysical properties of light sensitive core-shell nanoparticle in which one component is P3HT. Different core materials were selected (among which: ITO, TiO2 and Au) in order to have core-shell systems with different optoelectronic properties, permitting to investigate the cores effect on the organic NPs prosthesis. Core-shell NPs provides a ground for investigating the photoexcitation mechanism and eventually controlling it by engineering ad hoc interfaces where energy level alignment can be predicted. NPs are a versatile system able to be widespread in the whole treated tissue. Moreover, NPS properties, such as tissue selectivity, light sensitivity, elasticity, etc. can be tuned, providing a versatile tool for the comprehension and improvement of the interaction between the prosthesis and the biological tissue.
SINGLET FISSION AND TRIPLET TRANSFER TO PBS QUANTUM DOTS IN TIPS-TETRACENE CARBOXYLIC ACID LIGANDS
1School of Chemical and Physical Sciences, Victoria University of Wellington, New Zealand; 2Cavendish Laboratory, University of Cambridge, United Kingdom; 3Department of Chemistry, University of Kentucky, United States of America
Singlet exciton fission allows for the generation of two triplet excitons for each photon absorbed within an organic semiconductor. Efficient harvesting of these triplets could allow for the Shockley–Queisser limit on the power conversion efficiency of single-junction photovoltaics to be broken. Here, we show that singlet fission molecules bound directly to PbS quantum dots as ligands can undergo singlet fission with near unity efficiency and can transfer triplets sequentially into the PbS with near unity efficiency. Within the PbS, the excitations recombine, giving rise of the emission of photons. This allows for the doubling of the quantum dot photoluminescence quantum efficiency when photons are absorbed by the singlet fission ligand, as compared to when directly absorbed in the quantum dot. Our approach demonstrates that it is possible to convert the exciton multiplication process of singlet fission into a photon multiplication process and provides a new path to harness singlet fission with photovoltaics.
THERMALLY-ACTIVATED DELAYED FLUORESCENCE IN POLYMER-SMALL MOLECULE EXCIPLEX BLENDS FOR SOLUTION-PROCESSED ORGANIC LIGHT-EMITTING DIODES
1Durham University, Durham, United Kingdom; 2Silesian University of Technology, Gliwice, Poland
The photophysics of an exciplex state, formed between a small molecule and a polymer, is investigated in this work. The results obtained with this blend show the strong potential of polymer-small molecule blends for triplet harvesting in organic light emitting diodes (OLEDs) via thermally-activated delayed fluorescence. The exciplex formed between poly(N-vinylcarbazole) (PVK) and 2,4,6-tris[3-(diphenylphosphinyl)phenyl]-1,3,5-triazine (PO-T2T) shows yellow-green emission and is applied in solution processed OLEDs. The excellent film forming properties in this blend allow easy spin-coating and potential use in other solution-processing techniques, such as slot die coating. In this work we critically address the reverse intersystem crossing mechanism in the presented exciplex system, including the role of the local triplet state. Moreover, we bring a clear physical meaning to the exciplex emission decay components, including the decay occurring in a power-law fashion that is often ignored in the literature.
KINETIC MODELLING OF TRANSIENT PHOTOLUMINESCENCE MEASUREMENTS OF THER-MALLY ACTIVATED DELAYED FLUORESCENT MATERIALS
1Merck KGaA, Darmstadt, Germany; 2Physics Deparment, University of Durham, Durham, United Kingdom
Thermally activated delayed fluorescence (TADF) is an established mechanism for harvesting triplet excitons in “metal-free” OLEDs[Uoyama 2012] which gives a maximum attainable internal quantum efficiency of 100%. By minimizing the energy gap between the lowest lying singlet and triplet states, a conversion of slowly relaxing triplet sates to faster relaxing singlet states via reverse intersystem crossing rate (rISC) is promoted. Although several highly efficient TADF emitters have been discovered, a reliable determination of the reverse intersystem crossing rate as a key parameter for the characterization of TADFs has not been provided yet.
In this work, we present a kinetic model for the TADF process which allows to extract not only rate constants but also time dependent state populations from transient photoluminescence experiments. Using this model, we obtain temperature dependent rISC rates from which we calculate the activation energy for the TADF process.
ACRIDINE-TRIAZINE BASED SOLUTION PROCESSABLE TADF MOLECULES FOR OLED APPLICATIONS
Sheffield University, Sheffield, United Kingdom
Currently, most highly efficient Organic-based Light Emitting Diodes (OLEDs) rely upon the use of noble metal compounds. This is because when charges combine in OLED devices, they tend to form singlet and triplet excitons in a 1:3 ratio, of which triplets are only made emissive, upon energy transfer to highly phosphorescent metallic compounds. It is well known these elements are rare, and frequently claimed that they are both expensive and toxic. As a result, there has been a push in recent years to replace these compounds, with cheaper, non-toxic, organic alternatives. The most promising approach, for highly efficient devices, is to use Thermally Activated Delayed Fluorescence (TADF). TADF allows for the formation of singlet from triplets, via a thermally activated process. A major problems with TADF, is that devices are usually formed using vacuum deposition, a wasteful, and expensive procedure. Herein the process of making a molecule solution processable is investigated in a new architecture
A series of acrydine-triazine donor-acceptor molecules, with attached carbazole groups are synthesised, to create solution processable TADF molecules with novel architectures. These compounds were subsequently analysed via steady state and time resolved fluorescent spectroscopy at both standard and reduced temperature, as well as steady state absorption and cyclic voltammetry. The compounds are found to be Blue-green emitters, with high Photo-Luminescent Quantum Yield (PLQY). A series of unusual results are observed, including an increase in PLQY with increased molecular size while in solution, and a blue shift in phosphorescence spectra, relative to the fluorescence ones. While investigation into these effects remains ongoing, the effect of rigid media appears to be significant factor in these molecules emitting effectively, indicating perhaps a geometric relaxation dependent quenching process.
INTERPLAY BETWEEN TADF PERFORMANCE AND EMITTER CONCENTRATION
Technische Universität Dresden, Dresden, Germany
Thermally activated delayed fluorescence (TADF) has been at the forefront of research of organic light emitting materials in recent years. TADF is facilitated by reverse intersystem crossing (rISC), a process which allows an upconverting spin-flip of the triplet state into the singlet state. This is especially important for organic light emitting diodes (OLEDs), where, due to spin statistics, three quarters of all excitations formed are triplet states and in most organic materials these states are usually not contributing to light emission. The physics behind the rISC process is not completely understood yet, which prompts further research in this topic. riSC usually occurs on the microsecond timescale, which means that the triplet population remains large at steady-state operation, leading to processes detrimental to efficiency and emitter or device lifetime, namely, triplet-triplet annihilation (TTA) and triplet-polaron quenching (TPQ). Unsurprisingly, the typical lifetime of a TADF OLED is much shorter than that of a fluorescence-based OLED or phosphorescence-based OLED. Additional problems involving charge balance and charge accumulation at the emitting layer (EML) interfaces are introduced due to high-bandgap host materials and are to be addressed in the development of efficient and long-living TADF OLEDs. A deeper understanding of excited state dynamics in TADF materials and OLEDs is still required.
In this work, we investigate changes induced by increasing the concentration of blue-emitting TADF molecules from 1wt% to 80wt%, which increases the PLQY, shortens the delayed fluorescence lifetime while simultaneously red-shifting the emission spectrum. Decay transient as well as fluorescence and phosphorescence spectra analysis are presented, uncovering an increase in the rISC rate with increasing emitter concentration. The results provide new insights into the lifespan of TADF emitters as well as OLEDs. Devices having higher emitter concentration have been shown having higher EQE values as well as longer lifetimes.
ALL IN ONE? ACHIEVING DEEP BLUE EMISSION WITH AIE AND TADF PROPERTIES WITHIN THE SAME MOLECULE
1School of Chemistry, University of St Andrews, St Andrews, United Kingdom; 2School of Physics, University of St Andrews, St Andrews, United Kingdom
In recent years thermally activated delayed fluorescence phenomenon (TADF) has been extensively studied. It enables purely organic emitters to harvest the triplet excitons, thus theoretically enhances device’s internal quantum efficiency up to 100%. Aggregation induced emission (AIE) is also a hot topic since it enhances the radiative rate in the solid state. Therefore, efficient emitters that do not suffer from typical concentration quenching effects can be obtained. Until now all the efforts to combine these two phenomena into the same molecule were successful in achieving all the colours except blue. This is due to the fact that decorating TADF molecules with chromophores enhancing AIE results in extended conjugation, what leads to red-shifting of the color.
For TADF a small energy difference between the lowest singlet and triplet states (ΔEST) must exist, typically taken to be less than 200 meV. The molecular design required to achieve this is frequently based on electronically decoupled donor and acceptor moieties within the molecule, which localizes the HOMO and LUMO on these respective fragments. AIE is achieved by a substantial inhibition of the twisting of the individual groups around the single bond in the solid or aggregated state. In order to combine these phenomena into a single molecule and obtain blue emission, chromophores used must be chosen carefully. Both donor and acceptor must possess high ET, must be rigid, as well as overall π-conjugation length must be limited.
In this study we present a structure-property relationship of deep blue emitting donor-acceptor compounds, which were designed to possess high photoluminescence quantum yield in the solid-state. We present a thorough photophysical study of these molecules, supported by the results of theoretical calculations, to demonstrate that they are blue TADF AIE emitters and show potential in OLEDs.
TADF CU(I) COMPLEXES FOR OLED-DEVICES
Karlsruhe Institute of Technology (KIT), Karlsruhe, Germany
The development of emitter materials for organic light-emitting diodes (OLEDs) started with fluorescent compounds (1st generation), went over heavy metal phosphorescent triplet harvesters (2nd generation) and reached structures that possess thermally activated delayed fluorescence (TADF, 3rd generation). Especially multinuclear Cu(I) complexes show extremely high PLQYs close to 100%. The emission of light is mostly caused by TADF and related to a very small energy gap ∆E(S1-T1) of the Cu(I) complexes, which enables reverse intersystem crossing. Variation of the ancillary ligands, HOMO and LUMO allows tuning of the solubility and emission properties, respectively. However, performance degradation in the actual device remains problematic. Therefore, we aim for more rigid and stable compounds. To address these issues, we strive to substitute the oxidation-prone phosphines with more stable homologues.
MOLECULAR DESIGN STRATEGIES FOR COLOUR TUNING OF BLUE TADF EMITTERS.
Durham University, Durham, United Kingdom
The synthesis and characterisation of three new structures, based on previously reported high efficiency, blue thermally activated delayed fluorescence (TADF) material DDMA-TXO2 are described. Detail photophysical analysis elucidates the effects of different donor and acceptor strengths, meta- and para- isomerism, and steric control of the D–A dihedral angle on the optical properties and electrical performance of the emitters when compared to the reference material, 2,7-bis(9,9-dimethyl-acridin-10-yl)-9,9-dimethylthioxanthene-S,S-dioxide (DDMA-TXO2). By manipulating all of the aforementioned factors, OLED devices were fabricated with molecule III substituted with methyl groups on the acceptor. Blue devices showed a maximum EQE of 22.5%, and CIE coordinates of (0.15, 0.17), outperforming previously reported DDMA-TXO2. Significantly improved roll off is also observed for III with 19% at 1000 cd m−2.
OPTICAL SPECTROSCOPY FOR THE STUDY OF LSCS AND SF LSCS
University of Cambridge, Cambridge, United Kingdom
Luminescent Solar Concentrators (LSCs) hold the promise of reducing the price of solar energy, by concentrating sunlight on to small area photovoltaic cells, thereby reducing the number of solar cells needed per unit area. In principal LSCs could achieve concentration factors in excess of 100 for diffuse light and without the need for heliostatic tracking. However, all LSCs suffer from problems associated with reabsorption, as the chromophores used within LSCs, organic dyes and quantum dots (QDs), do not achieve substantial Stokes shifts. To overcome these problems, there is a need for new concepts that can achieve high concentration factors without wasting absorbed photon energy. At the same time, all single- junction solar cells are limited by thermalisation when they directly absorb solar photons. Here, we examine the use of Singlet Fission (SF), as a means to overcome thermalisation losses and achieve very large Stokes shifts in LSCs.
The basic concept of a SF LSC is illustrated in Figure 1. The system consists of a large fraction of SF material with a small fraction of NIR emitting QDs. Triplet excitons generated via SF are transferred to the QDs where they recombine to emit photons. Since only a small fraction (<1%) of the chromophores are QDs, the reabsorption is suppressed, and a large Stokes shift generated, between the absorption of the SF material and the emission of the QD. We present a novel method of studying spatially resolved PLQE on large-scale devices to better understand morphology dynamics of the LSC. Coupled with a new simulation approach has several advantages of those presented in the literature, as current simulations rely on escape cone approximations we examine spectroscopically and in-silico the effect of various surface morphologies and chromophore distributions.
SINGLET FISSION IN π-CONJUGATED POLYMERS
University of Oxford, Oxford, United Kingdom
Singlet fission is a process in which a singlet excitation splits into two uncoupled triplet excitations1. Carotenoids are known to exhibit this phenomenon and recent work has indicated that the process may be intramolecular2. In carotenoids, the first optically active excitation is to the 1Bu singlet state. Whether singlet fission proceeds directly from this state or involves the lower energy ‘dark’ 2Ag state remains unclear3. It is thought that coupling between the vibrational and electronic degrees of freedom play a major role in excitation relaxation dynamics (important in singlet fission) of linear conjugated polyenes, such as carotenoids. In our work we model carotenoids and other linear conjugated polyenes as a 1D chain of sites and explicitly consider only the π-electrons. The electron degrees of freedom are treated with the UV-Hubbard model. These couple to quantum Debye phonons at each site. We calculate the dynamics of a photo-excited state using time dependent density matrix renormalisation group (tDMRG)4 and various observables, e.g. spin-spin correlation and ionicity, to distinguish the character of the evolving state.
- N. Monahan and X.-Y. Zhu, Annual Review of Physical Chemistry, 66, 601-608, 2015
- Jie Yu, Li-Min Fu, Long-Jiang Yu, Ying Shi, Peng Wang, Zheng-Yu Wang-Otomo, and Jian-Ping Zhang, Journal of the American Chemical Society, 139, 15984-15993, 2017
- A. J. Musser, M. Al-Hashimi, M. Maiuri, D. Brida, M. Heeney, G. Cerullo, R. H. Friend, and J. Clark, Journal of the American Chemical Society 2013 135, 12747-12754
- A. J. Daley, C. Kollath, U. Schollwock, G .Vidal, J. Stat. Mech. 2004, P04005, 2004
TRIPLET FUSION MEDIATED NIR-TO-ORANGE UPCONVERSION IN PHTHALOCYANINE SENSITIZED RUBRENE FILMS
1Institute of Photonics and Nanotechnology, Vilnius University, Vilnius, Lithuania; 2Department of Organic Chemistry, Vilnius University, Vilnius, Lithuania
Photon upconversion (UC) relying on triplet fusion in organic blends are of particular interest due to the variety of applications, such as bioimaging, anti-counterfeiting, fingerprint detection, photoredox catalysis, and photovoltaics. The supplementary UC layer in solar cells is utilized to capture and retrieve sub-bandgap photons that are beyond the absorption range of the cell thus improving its efficiency. IR-to-VIS upconversion is mostly demonstrated in organic low viscosity media such as solutions, however, practical applications frequently require solid-state architecture. Lack of efficient solid-state IR-to-VIS rigid upconverters could be attributed to limited number of efficient triplet sensitizers in the IR range, which experience severe non-radiative losses as a result of small energy gaps.
The current work focuses on the UC performance of solution-processed polymer films containing conventional and modified rubrene emitters and (Pd,Pt)phthalocyanine sensitizers. In addition to the efficient intersystem crossing, long triplet-state lifetimes and exceptional photostability the novel phthalocyanines are shown to exhibit strong Q-band absorption (up to 2.5 105 M/cm) at about 720 nm and wide transparency window 450 ‑ 630 nm for the UC emission. Furthermore, determined triplet energies (1.12 ‑ 1.18 eV) of the phthalocyanine sensitizers allow to combine them with rubrene emitter for UC application. Rubrene was modified with tert-butyl side groups to preserve high emission quantum yield at high concentrations, which are required for efficient triplet diffusion and fusion. UC polymer films containing various proportions of emitter and sensitizer were fabricated by spin-coating on pre-cleaned glass substrates in nitrogen glovebox (with O2 and H2O level <0.1 ppm). Additional lower-energy DBP emitter introduced into the UC films (at a concentration of 0.5 wt%) served as a singlet exciton sink allowing to reduce detrimental singlet fission effects in rubrene. The achieved UC quantum yields are rather promising and encourage further development of rigid IR-to-VIS upconverting films.
SYNTHESIS AND APPLICATIONS OF 1,8-NAPHTHYRIDINE-BASED TADF EMITTERS
1Department of Organic Chemistry, Vilnius University, Vilnius, Lithuania; 2Institute of Photonics and Nanotechnology, Vilnius University, Vilnius, Lithuania
The latest generation organic emitters featuring thermally activated delayed fluorescence (TADF) and small triplet-to-singlet energy gap allow harvesting of non-emissive triplet excitons by converting them to radiative singlet excitons via reverse intersystem crossing (RISC) . Rational design of donor-acceptor based TADF compounds was demonstrated to result in the RISC and overall emission quantum yield (QY) as high as 100% at room temperature. TADF emitters are currently considered as the most promising for realization of the latest generation (not yet commercially available) OLEDs expressing long-term stability and efficiency at high brightness.
In this work a series of donor-acceptor compounds based on a new naphthyridine acceptor were investigated as potential TADF emitters for OLED application. The studied compounds contained dimeric carbazole or dimethylacridine donor moieties and in some cases tert-butyl groups for easy solution processing.
Steady state and time-resolved fluorescence spectroscopy revealed sky-blue emission of the naphthyridine-based compounds, which consisted of prompt (nanosecond time domain) and delayed fluorescence (microsecond) components. The delayed component was readily quenched by an oxygen confirming its triplet origin. The contribution of delayed fluorescence was twice as high as of the prompt component indicating efficient RISC in the studied compounds. Emission QY of up to 40% were attained in solution, whereas it was boosted up to 70% for the compounds dispersed in high-triplet-energy DPEPO host demonstrating reduced non-radiative decay due to suppressed intramolecular vibrational/torsional relaxation. The compounds were also tested as TADF emitters in vacuum- and solution-processed OLEDs. The obtained results suggest that this class of TADF emitters is highly attractive for fabrication of TADF OLEDs.
. H. Uoyama et al., Nature. 492, 234 (2012).
SENSITIZED PHOTON UPCONVERSION IN FLUORENE-SUBSTITUTED ANTHRACENE COMPOUNDS
1Vilnius University, Vilnius, Lithuania; 2University of Bayreuth, Bayreuth, Germany
Photon upconversion has attracted a great deal of attention because of its wide range of applications in OLEDs, photocatalysis, bioimaging, cancer therapy or solar cells. Since the photon upconversion in organics is accomplished via the annihilation of long-lived triplet excitons, relatively low excitation power density of noncoherent light (e.g. delivered by the sun) is sufficient for the process to take place, which is essential in terms of the practical applications. The photon upconversion has already been demonstrated in different media, e.g. in solutions, liquids, rubbers, gels or solid films, however in the latter, upconversion efficiency remains low due to strong triplet exciton quenching. In this work, photon upconversion in fluorene-modified anthracene compounds expressing low concentration quenching, and thus, high fluorescence quantum yields (up to 71%) in the neat films, was studied. Platinum octaethylporphyrin was chosen to serve as a triplet exciton sensitizer for these bicomponent matrix-free upconversion films. Extensive studies of phosphorescence and upconverted emission in a wide range of temperatures enabled to reveal strong singlet and triplet exciton quenching at room temperature. By decreasing temperature we were able to reduce exciton diffusion and quenching in the films, thereby enhancing upconversion quantum efficiency over two times (from 3% to 7%). The obtained results suggest that analogous control of exciton diffusion and quenching can be achieved via chemical engineering, which could enable design of photon upconverting systems with optimal performance shifted to room or even higher temperatures.
PHENOTHIAZINE-BASED DERIVATIVES EXHIBITING DELAYED FLUORESCENCE AND ROOM TEMPERATURE PHOSPHORESCENCE
Kaunas University of Technology, Kaunas, Lithuania
Organic luminescent materials have attracted increasing attention for their potential applications in the fields of OLEDs and sensors (Sasabe et al). Employment of materials exhibiting thermally activated delayed fluorescence (TADF) in the active layers of organic light emitting diodes has been stated to be very efficient. TADF materials can achieve 100 % of use of excitons through reverse intersystem crossing because of their low energy gap between the lowest singlet excited state S1 and triplet excited state T1 (Tanaka et al). The strategy to achieve such characteristics is to employ both donor and acceptor moieties in a single molecular structure.
We report on the synthesis and properties of electroactive compounds bearing benzotrifluoride moiety as an acceptor and 3,7-ditert-butylphenothiazine species as donor. Single-step synthetic route employing Buchwald-Hartwig coupling was chosen to obtain the investigated compounds. The synthesized compounds can form molecular glasses. Their glass transition temperatures are 50 and 70 °C. Solid-state ionization potential values of these derivatives were found to be 5.50 eV. Thermal, photophysical, electrochemical and photoelectrical properties are presented. It was established that the position of trifluoromethyl-substituent on the phenyl ring mainly influences the extent and efficiency of RTP. Experimental results showed that RTP was induced by rigidity that can be caused by polymer matrix or crystallinity of the organic compounds whereas the TADF was observed in amorphous state of the compounds.
ASYMMETRICALLY SUBSTITUTED BIANTHRACENE DERIVATIVES FOR TTA OLED APPLICATIONS
Institute of Photonics and Nanotechnology, Vilnius University, Vilnius, Lithuania
9,9’ – bianthracene is a special type of anthracene modification with two anthracene moieties connected together via short axis, allowing the enhancement of oscillator strength and close coupling between anthracene monomers and internal QY up to 60%. Previous studies of bianthracene derivatives disclosed complex electronic excitation relaxation properties of locally excited (LE) and internal charge transfer (ICT) states, which is also detrimental for effective triplet harvesting by TTA process.
In this work we investigate excitation relaxation dynamics by extending electronic conjugation with aryl substituents at long and short axis of bianthracene moiety. Interestingly, even in slightly polar environment ICT state is created due to symmetry braking in electronic configuration of the molecule that vastly deteriorates internal quantum efficiency and properties of emission spectra. We thoroughly investigated the dynamics of ICT sate in a wide range of liquid and solid state mediums by various steady state and ultrafast spectroscopy techniques in aryl substituted 2,6,9,10 – bianthracene molecular systems. Our analysis concluded that the ICT rate is directly related to dielectric reaction field induced by the surrounding environment. Interestingly, asymmetrization of 9,9’ – bianthracene core by aromatic substituents induced effective suppression of ICT states and unusual solvatochromic effects in emission properties. To our knowledge this is the first bianthracene derivative with a dominating LE state emission even in high polarity medium. We demonstrate effective way for designing environment independent, stable and efficient singlet emission and charge transportation molecular systems for effective triplet harvesting process and new generation blue OLED materials.
THE IMPACT OF CONFORMATIONAL DISORDER ON THE STABILITY OF TADF WAVELENGTH
1Institute of Photonics and Nanotechnology, Faculty of Physics, Vilnius University, Vilnius, Lithuania; 2Institute of Chemistry, Faculty of Chemistry and Geosciences, Vilnius University, Vilnius, Lithuania
Thermally activated delayed fluorescence (TADF) is the most promising pathway for triplet harvesting since TADF compounds are cheaper and much more stable than rare-earth metals based phosphorescent emitters.
To utilize non-emissive triplet states TADF emitters employ reverse intersystem crossing (rISC), which requires low singlet-triplet energy gap (ΔEST). TADF molecules are usually constructed from donor (D) and acceptor (A) moieties twisted by large dihedral angles to reduce spatial overlap of HOMO–LUMO electron orbitals and minimize ΔEST. However, large steric hindrance between D and A units reduces the vibronic coupling between localized (3LE) and charge transfer (3CT) triplet states, which is mandatory for efficient rISC, therefore some lability of molecular core is required.In dilute solutions, minor dispersion of D–A twist angles may be observed, having negligible effect on excited state relaxation. However, in solid films molecules are frozen in large variation of geometrical conformations with different 1CT energies and ΔEST gaps, causing temporal shifts of prompt and delayed fluorescence and resulting in multiexponential emission decay with prolonged lifetime. The impact of conformational disorder to TADF properties in solid films, its relation to rISC rate and methods to reduce it, despite its importance, still are scarcely studied.
In this work we investigate the conformational disorder in solid films of phenothiazine-pyrimidine TADF compounds and its relation to emission properties. Compounds were designed to have different lability of molecular core and different reverse intersystem crossing (rISC) rate. Phenothiazine electron-donor unit was selected for its ability to form several conformations, increasing the number of possible molecular core arrangements in solid state. We have shown, that variations of molecular core rigidity had only minor impact on conformational disorder and emission properties, while the rISC rate was found to have the crucial importance to TADF properties.
PHOTOPHYSICAL PROPERTIES OF METHYL SUBSTITUTED PHENOTHIAZINE TADF COMPOUNDS
1Institute of Photonics and Nanotechnology, Vilnius University, Vilnius, Lithuania; 2Department of Polymer Chemistry and Technology, Kaunas University of Technology, Kaunas, Lithuania
Thermally activated delayed fluorescence (TADF) is one of the methods for harvesting luminescence from triplet states via reverse intersystem crossing (rISC) in organic molecules. [Uoyama, 2012] Donor-acceptor-donor (D-A-D) molecules have strong intramolecular charge-transfer (ICT) state and it is very important for the creation of strong TADF emitter. [Taneda, 2015] A fundamental principle of planar ICT (PICT) and twisted ICT (TICT) is demonstrated to obtain selectively either room temperature phosphorescence (RTP) or thermally activated delayed fluorescence (TADF), respectively. [Chen, 2018]
In this research, we have TADF-based emitters of methyl-phenothiazine derivatives, because of their quasi-axial (ax) and quasi-equatorial (eq) conformers have very interesting properties especially for TADF. We used time-resolved fluorescence method to investigate the behavior of emission properties at different temperatures. We establish that different acceptors can change emitters properties from RTP to TADF.
DIPHENYLSULFONE BASED COMPOUNDS EXHIBITING TADF EFFECT FOR ORGANIC LIGHT EMITTING DIODES
1Kaunas University of Technology, Kaunas, Lithuania; 2National Taiwan University, Taipei, Taiwan
Thermally activated delayed fluorescence (TADF)  is a promising candidate for heavy-metal-free emitter of organic light-emitting diode (OLED), which relies on the careful balance between the exchange energy and oscillation strength by designing the molecular configuration. This approach allows to employ triplet excitons by converting them to singlet excitons for luminescence and overcome spin statistical 25% limit of internal quantum efficiency of organic light emitting diodes (OLEDs). Employment of TADF emitters for the fabrication of OLEDs became widespread in recent years due to the possibility of replacing high-cost phosphorescent emitters based on rare metals . In this presentation, we proposed two different organic molecules based on diphenyl-sulfone as the central acceptor moiety connecting to two neighboring donor moieties with different positions, which showed quite different photoelectrical and optical characteristics. When the donor and acceptor are connected in meta-position on the phenyl ring, the emission red shift together with a lower external quantum efficiency and higher driving voltage, compared to the case with para-connection. In the optimized TADF-OLED with para-connection, maximum current efficiency, power efficiency, and external quantum efficiency with 61.1 cd/A, 64 lm/W, and 24.1% were achieved with the peak wavelength of 495 nm, resulting a sky-blue emission (CIE coordination of (0.18, 0.41)). The thermal, electrochemical properties of the synthesized compounds have also been investigated.
 H. Uoyama, K. Goushi, K. Shizu, H. Nomura, C. Adachi, Nature 492 (2012) 234-238.
 Q. Zhang, J. LI, K. Shizu, S. Huang, S. Hirata, H. Miyazaki, C. Adachi, J. Am. Chem. Soc. 134 (2012) 14706-14709.
EXPLORING THE PHOTOPHYSICS OF TADF MOLECULES BEYOND OLEDS.
1Physics Department, Durham University, Durham, United Kingdom; 2Chemistry Department, Durham University, Durham, United Kingdom
The investigation of novel molecular structures showing Thermally Activated Delayed Fluorescence (TADF) is crucial to pioneer their application in biological imaging and sensing. The luminescence of TADF molecules decays in two regimes; the prompt fluorescence (PF) in the nanosecond range, and the delayed fluorescence (DF) in the microsecond range, usually in less than 10 ms, but this can be tuned to target specific applications. Essentially the DF component appears from a process that involves the reverse intersystem crossing between the triplet and singlet states, which are separated by a small energy difference. TADF is thus a thermally activated mechanism, which is strongly affected by medium properties and intra-molecular phenomena that can influence the triplet state, e.g. internal conversion, singlet-triplet energy splitting, intersystem crossing, triplet quenching mechanisms etc. The DF component is, therefore, extremely sensitive to the presence of oxygen, temperature, analytes, and also the dielectric medium. This gives considerable potential for TADF emitters to be used in imaging and sensing applications, allowing prompt and delayed fluorescence intensities and lifetimes in the same molecule to be used as an internal reference, thus enabling ratiometric and concentration independent measurements, which remain to be explored. As an example, if the TADF molecule selectively interacts with another molecule or atom, most probably this will have a significant effect on the properties of the triplet excited state of the TADF emitter. These changes will be inevitably reflected in the TADF efficiency and lifetime. These are precisely the effects we aim to explore in this project, which in practice remains largely unexplored in time-resolved fluorescence microscopies and sensing applications. Here, we report our initial studies on novel TADF emitters designed to give superior photoluminescence TADF response in different media.