Conference Agenda

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Session Overview
Session
POST S-1: POSTER SESSION I
Time:
Monday, 08/Jul/2019:
18:05 - 20:00

Location: Carmen Hall

Presentations
Poster
ID: 276
Poster Presentation

MESOSCOPIC ORDERING BEYOND OF INTERMOLECULAR INTERACTIONS OF RH6G SELF-ORGANIZED WITHIN THE HYBRID SOL-GEL FILMS

German Telbiz1, V. Yaschuk2, S. Bugaichuk3, A. Glyschenko4, E. Tikhonov5, L. Valkunas6, V. Gulbinas6

1Department of Physical-Inorganic Chemistry, Institute of Physical Chemistry, National Academy of Sciences of Ukraine (NASU), Kyiv, Ukraine; 2Department of Liq. Crystal Physics, Institute of Physics, National Academy of Sciences of Ukraine (NASU), Kyiv, Ukraine; 3Department of Optics, Kyiv T. Shevchenko National University, Kyiv, Ukraine; 4Department of Physics, University of Colorado at Colorado Springs, Colorado Springs, Colorado, United States of America; 5Department of Physics, Institute of Physics, National Academy of Sciences of Ukraine (NASU), Kyiv, Ukraine; 6Center for Physical Sciences and Technology, Vilnius, Lithuania

The key problem for the application of hybrid films composed of organic dye molecules embedded in inorganic matrix as photonic materials is to prevent aggregation of dye molecules and, thus, to stabilize their optical properties. We have fabricated mesoscopically ordered hybrid SiO2 and TiO2 films with embedded R6G molecules possessing high fluorescence yields of the R6G dimers caused by the strong deviation of the sandwich-type dimer from the planar geometric arrangement opening the forbidden low energy excitonic transition. The composite films were used for realization of fast dynamical gratings under degenerated two-wave mixing technique upon excitation by nanosecond laser pulses. The nonlinear refraction coefficient and the nonlinear optical susceptibility values defined from the experiments, show that the main optical nonlinearities in fast time scale originate from the electronic nonlinearity of dye molecules confined in the matrix. Our research results shows that the mesoscopic SiO2 and TiO2 films are perspective new materials for applications as all-optical light valves.

Lasing of the hybrid film as a planar asymmetrical waveguide structure was observed and studied for the first time. The stimulated emission reveals a complex structure, which depends on the nature of the waveguide. Superluminescence generated in the waveguide formed by hybrid films on a glass propagates along the exited track of the film and its spectrum sharply narrows approaching that of the conventional dye laser. Laser radiation propagating in transverse direction was surprisngly collimated, even narower than one would expect accounting for the diffraction broadening. These results show high-quality lasing on the basic mode of a waveguide that forms the central beam.


Poster
ID: 139
Poster Presentation

ULTRAFAST CHARGE TRANSFER DYNAMICS IN HIGHLY EFFICIENT NONFULLERENE ORGANIC SOLAR CELLS

Rokas Jasiūnas1, Huotian Zhang2, Feng Gao2, Vidmantas Gulbinas1

1Center for Physical Sciences and Technology, Vilnius, Lithuania; 2Biomolecular and Organic Electronics, Department of Physics, Chemistry and Biology (IFM), Linköping University, Linköping, Sweden

The ever-growing demand for carbon-neutral energy source has led photovoltaic (PV) technologies to become fastest-growing form of renewable energy. Among many different PV technologies, organic photovoltaic (OPV), is based on earth-abundant materials and showing short energy payback times, has been exclusively attractive. For almost two decades highest power conversion efficiencies (PCE) were obtained with fullerene type acceptors in donor:acceptor bulk heterojunction structured OPV devices. Other acceptor molecules, generally named nonfullerene (NF) molecules, usually shows low PCE’s, however, their resurgence was witnessed in the last three years, with power conversion efficiency skyrocketing to record-breaking >13.5% values, exceeding those reached by fullerene-based systems.
Today there are >100 different acceptor materials displaying high efficiencies. Most of them are so-called A-D-A (or D-A-D) type molecules, distinct by its two symmetrical electron accepting (donating) complexes connected via conjugated electron donating (accepting) backbone. The most striking feature of such molecules is efficient exciton splitting at low, or even negligible, driving forces made by HOMO/LUMO level mismatch at D/A interface. The lack of understanding of underlying physics behind this phenomena prevents these materials to be improved further.
To this end, we have performed conventional transient absorption measurements to investigate ultrafast processes in ten different OPV systems based on three different (fullerene and two NFs) accepting materials. To distinguish between different charge transfer mechanisms, different excitation wavelengths were chosen to excite donor or acceptor separately. It was shown that efficient ultrafast (<10ps) hole transfer takes place in all NF based systems. Nevertheless, higher exciton relaxation rate, compared to fullerene based system, suggests that transfered charge might as well be easily transfered back ordaining high geminate recombination.


Poster
ID: 234
Poster Presentation

SECOND HARMONIC GENERATION ON PENTACENE-PERFLUOROPENTACENE INTERFACES

Vipilan Sivanesan1, Frank Schreiber2, Katharina Broch2, Petra Tegeder1

1Heidelberg University, Heidelberg, Germany; 2University of Tübingen, Tübingen, Germany

Understanding the ultrafast electronically excited state dynamics in organic semiconductors after optical excitation is crucial for the optimization of organic optoelectronic devices. Moreover, the knowledge of the morphology and energetics at donor-acceptor interfaces is important for efficient charge separation in organic solar cells. Thereby, charge transfer (CT) states play a decisive role. To analyse the ultrafast processes at the interface more precisely, both well-defined structures of the samples and a very high time resolution in the experiment are required. In this work we investigated thin films of pentacene (PEN), perfluoropentacene (PFP), and various heterostructures of PEN and PFP by means of femtosecond time-resolved second harmonic generation. For the donor/acceptor configurations, depending on the molecular orientations at the interface and the excitation energies, the dynamics of CT-states were analyzed.


Poster
ID: 114
Poster Presentation

THERMAL AND OPTICAL PROPERTIES OF 6-(TERT-BUTYL)-4H-PYRAN-4-YLIDENE FRAGMENT CONTAINING LASER DYES WITH DIFFERENT ELECTRON ACCEPTORS AND BULKY MOIETIES

Elmars Zarins, Deins Alksnis, Kristine Lazdovica, Valdis Kokars

Institute of Applied Chemistry, Riga Technical University, Riga, Latvia

A series of 2-(4-dialkylamino)styryl-substituted)-6-tert-butyl-4H-pyran-4-ylidene fragment containing D-π-A type organic dyes with different attachment of amorphous state promoting bulky 1,1,1-triphenylmethyl (trityl) moieties to the amino group (DWK-type laser dyes) have been synthesized as the potential gain-medium materials for organic solid-state lasers. In order to provide more understanding on DWK-type laser dye physical property relation with compound chemical structures – alternate electron acceptors – indene-1,3-dione, 1,3-dimethylpyrimidine-2,4,6(1H,3H,5H)-trione and 1,3-diethyl-2-thioxodihydropyrimidine-4,6(1H,5H)-dione were used instead of the widely known malononitrile.

Results showed that the bonding type of bulky moieties as well as the electron acceptor fragments both considerably influenced dye thermal properties while their optical properties were affected slightly. DWK-type dyes with 1,3-diethyl-2-thioxodihydropyrimidine-4,6(1H,5H)-dione electron acceptor showed the highest thermal stability (thermal decomposition temperatures (Td) in range from 254o to 313oC) followed by 1,3-dimethylpyrimidine-2,4,6(1H,3H,5H)-trione and (Td from 183oC to 308oC) and indene-1,3-dione fragment (Td from 183oC to 267oC) containing DWK-type dyes. Glass transition temperatures (Tg) were mostly dominated by the attachment type of trityl groups and are observed in the range from 42oC to 73oC. 1,3-Diethyl-2-thioxodihydropyrimidine-4,6(1H,5H)-dione fragment containing dyes showed a by about 10-30 nm batochromic shift of absorption (λmax = 476-540nm) and photoluminescence (λmax = 619-645nm) in comparison to compounds with 1,3-dimethylpyrimidine-2,4,6(1H,3H,5H)-trione and indene-1,3-dione acceptors in their solutions of dichloromethane. Physical property investigation in dye neat films is underway.

This work has been supported by the European Regional Development Fund within the Activity 1.1.1.2 “Post-doctoral Research Aid” of the Specific Aid Objective 1.1.1 “To increase the research and innovative capacity of scientific institutions of Latvia and the ability to attract external financing, investing in human resources and infrastructure” of the Operational Programme “Growth and Employment” (No. 1.1.1.2/VIAA/1/16/035).


Poster
ID: 195
Poster Presentation

NIR OLEDS: EMITTERS, PHOTOPHYSICS AND DEVICES

Inês G. Tavares1, Piotr Pander1, Melissa Walder2, Gareth Williams2, Fernando B. Dias1

1Physics Department, Durham University, Durham, United Kingdom; 2Chemistry Department, Durham University, Durham, United Kingdom

NIR organic emitters are attracting increasing interest from industry and academia due to their potential to create a variety of innovative applications, including information-security tags to combat counterfeit products, night vision tags and displays, fingerprint-based technologies, distance detectors, and wearable optoelectronic devices such as portable light sources for photodynamic therapy. Efficient NIR OLEDs will clearly be required before such applications can be fully realized in practice. Unfortunately the design of organic molecules with efficient luminescence in the Deep-Red / NIR region is challenging. The luminescence yield of most emitters fall off in the red and NIR regions, owing to faster non-radiative internal conversion promoted by the strong coupling between the excited electronic state and higher vibrational levels of the ground state, the well know energy-gap law. Therefore, alternative photophysical mechanisms are needed to promote strong luminescence in this spectral region. Here, we present our recent investigations focused on organic Platinum complexes with Deep-Red /NIR luminescence due to the formation of excimers. The photophysics of these complexes is investigated in detail exploring different strategies for tuning excimer emission. Pt(dpyb)Cl and its derivatives are in general able to engage in face-to-face intermolecular interactions at elevated concentrations, leading to the efficient formation of highly emissive excimers that emit deep into the red region (around 700 nm and longer). Similar, low-energy excimer-like emission, is observed in the solid state when molecules are packed appropriately. The incorporation of a variety of electron-withdrawing substituents in the pyridine rings, and/or a switch to more electron-deficient heterocycles, is explored with a view of optimizing the emission energy and yield in the NIR. Prototype solution-processed OLEDs are fabricated and characterized showing efficient electroluminescence in this region.


Poster
ID: 212
Poster Presentation

PHOTOPHYSICAL PROPERTIES OF 9,9’-BIS-METHOXYPHENYL SUBSTITUTED 3,3’-BICARBAZOLES

Armands Ruduss, Kaspars Traskovskis, Valdis Kokars

Riga Technical University, Riga, Latvia

Various carbazole derived compounds has been used as host materials for OLEDs. Excellent hole mobilities and high triplet transition energies (Et) of carbazole derivatives facilitate the energy transfer to the phosphorescent emitters in electroluminescent devices. In this report we present four 9,9’-bis-methoxyphenyl substituted 3,3’-bicarbazoles (see Figure 1) designed for use as host materials in solution processed OLEDs.

The bicarbazole derivatives were synthesized employing a FeCl3 promoted oxidative dimerization reaction. Thin films were produced via spin-coating technique. UV-Vis absorption spectra were recorded on a Perkin Elmer Lambda 35 spectrometer and photoluminescence spectra were recorded using a QuantaMaster 40 spectrofluorometer. The phosphorescence spectra were measured for solutions in 2-methyltetrahydrofuran at 77 K.

All of the synthesized compounds are soluble in common organic and thin films of good optical quality could be spin-coated from chloroform solutions. Compounds exhibit an intensive absorption band at 303-304 nm and two low-intensity bands at 340 and 355 nm (DCM solutions). No significant effect of the different methoxy- substitution on the characteristics of UV-Vis absorption spectra was observed. Solutions in DCM exhibit fluorescence with intensity maxima at 406-411 nm. A small bathochromic shift (2-5 nm) of fluorescence maxima was observed for thin films samples, except for compound 4, which exhibits a hypsochromic shift of 15 nm comparing to solution in DCM. To explore the suitability of the synthesized compounds as host materials for phosphorescent emitters Et was determined. Compounds 1 and 2 exhibit triplet transitions with energy 2.67 eV, while 3 and 4 2.71 and 2.73 eV, respectively. This is sufficient for red and green triplet emitters, but for blue emitters Et over 2.75 eV is desired (J.Am.Chem.Soc., 2004, 126 (19), 6035–6042).

Acknowledgments

Funding: This work is supported by the doctoral studies grants of Riga Technical University (Grant Nr. 34-14000DOK.MLĶF/18)


Poster
ID: 213
Poster Presentation

FLEXIBLY BRIDGED MASSIVE AROMATIC GROUPS AS FUNCTIONAL FRAGMENTS TOWARDS SOLUTION-PROCESSABLE CYCLOMETALATED IRIDIUM(III) COMPLEXES FOR OLED APPLICATION

Kaspars Traskovskis1, Valdis Kokars1, Natalija Lesina2, Igors Mihailovs2, Aivars Vembris2

1Riga Technical University, Riga, Latvia; 2Institute of Solid State Physics, University of Latvia, Riga, Latvia

Organic light emitting diode (OLED) technology is widely used in display manufacturing and has prospects for general lighting applications. One of the limiting aspects for OLEDs is the relatively large production cost of the devices, especially in the cases where large emissive surfaces are prepared. This is determined by the cost intensive vacuum-deposition technique predominantly used for the layering of active electroluminescent and charge transporting materials. Development of solution-processable emissive materials, where solubility and solid state aggregation of the molecules is prevented by attached functional groups, is one of the approaches that could make OLED technology more accessible for practical applications.

Recently we have introduced a novel structural approach towards solution-processable emitters based on phosphorescent iridium(III) cyclometalated complexes (Traskovskis et al. New J. Chem., 2019, 43, 37-47). Pseudo-spherical triphenylmethyl substituents are attached to the complex core through a flexible aliphatic bridge. The synthesized emitters show high sensitivity towards processing conditions and to the structure of charge-transporting host material. Generally, medium polarity solvents and molecular instead of polymeric host materials lead to the most efficient devices. The obtained materials have been incorporated in OLEDs with a solution-processed emissive layer. Yellow emitting device with external quantum efficiency of 7.9 %, current efficiency of 12.4 cd A−1 and power efficiency of 7.8 lm W−1 has been obtained. Maximal device brightness is exceeding 17000 cd m−2.

For some of the subsequent structural variations a formation of an intramolecularly stacked conformation was observed where triphenylmethyl isolating group attaches to the surface of iridium(III) complex at its electrostatic extreme. This provides an efficient sterical shielding of the emissive core, diminishing effects of concentration quenching and allowing increase of solid-state emission quantum yield.

This work is supported by the ERDF 1.1.1.1. activity project No. 1.1.1.1/16/A/131 “Design and Investigation of Light Emitting and Solution Processable Organic Molecular Glasses”.


Poster
ID: 229
Poster Presentation

NEW TADF EMITTERS BASED ON BENZOYLPYRIDINE COMPOUNDS

Domantas Berenis, Virginijus Ruibys, Ona Adomėnienė, Povilas Adomėnas, Gediminas Kreiza, Saulius Juršėnas, Karolis Kazlauskas

Institute of Photonics and Nanotechnology, Vilnius University, Vilnius, Lithuania.

Organic light-emitting devices (OLEDs) are attractive for large area commercial displays and lighting applications because of their high luminous efficiency, contrast ratio, flexibility, energy-efficiency and potentially low manufacturing cost. Currently, emitters exhibiting thermally activated delayed fluorescence (TADF) properties are considered as the most promising for the latest generation OLEDs. TADF allows harvesting non-radiative triplet excitons through their conversion to emissive singlet states via thermally-assisted reverse intersystem crossing (RISC) processes. Smart engineering of donor-acceptor based molecular structure allows reducing singlet-triplet energy splitting thus enabling 100% RISC and photoluminescence quantum yield (PLQY) at room temperature.

In this work a series of six new donor-acceptor-donor compounds containing benzoylpyridine acceptor and two carbazole donor moieties were investigated as potential TADF emitters for OLED application. The studied compounds had different linking positions of the carbazole moieties on a phenyl ring (para and meta linkage) as well as different locations of nitrogen atom in the pyridine moiety (3 and 4 position). Additionally, the impact of solubility-enhancing non-conjugated t-butyl groups on the photophysical properties of the compounds was studied.

To reveal the most promising compounds for TADF-OLED application their optical properties such as PLQY and PL transients of prompt and delayed fluorescence were assessed in ambient and oxygen-free conditions. Good correlation between PLQY values of solution and thin film samples was found for different emitters. Compounds possessing para-linked donors exhibited higher PLQY yet smaller delayed to prompt emission ratio as compared to their meta-substituted counterparts. Furthermore, PLQY was found to be higher for compounds with a nitrogen atom in the 3rd position of the pyridine moiety as well as for compounds containing peripheral t-butyl groups. The obtained results suggest that the compounds with para-linked donors and nitrogen in the 3rd position in the pyridine moiety are the most promising options for fabrication of vacuum- and solution-processed TADF-OLEDs.


Poster
ID: 231
Poster Presentation

PHENANTRO[9,10- D]IMIDAZOLE HOST MATERIALS FOR EFFICIENT RED TADF OLEDS

Dovydas Banevičius1, Domantas Berenis1, Edvinas Radiunas1, Saulius Grigalevičius2, Daiva Tavgenienė2, Saulius Juršėnas1, Karolis Kazlauskas1

1Institute of Photonics and Nanotechnology, Vilnius University, Vilnius, Lithuania; 2Department of Polymer Chemistry and Technology, Kaunas University of Technology, Kaunas, Lithuania

Organic light emitting diodes, OLEDs, have recently undergone breakthrough due to the discovery of novel emitters employing delayed fluorescence generated by thermal activation (TADF) allowing to reach 100% internal efficiency without implementing heavy metal atoms. General TADF OLED problems like poor color purity and strong efficiency roll-off are still to be solved in the future, however red TADF OLEDs face more challenges, in particular, efficiency reduction with increasing wavelength as a result of energy band-gap law. Phenanthroimidazole-based compounds were demonstrated to be suitable as hosts for red phosphorescent emitters and thus capable to confine triplet excitons on low band-gap emitters.

Here we investigate a series of new phenanthroimidazole compounds as potential hosts for high-efficiency red TADF emitter TPA-DCPP (Wang 2015). The choice for proper hosts was made based on experimental evaluations of their triplet energies as well as Forster and Dexter energy transfer (from host to emitter) calculations. The obtained results indicated that only carbazolyl-substituted phenanthroimidazoles were suitable hosts for TPA-DCPP emitter.

These hosts were doped with red TPA-DCPP emitter and utilized for fabrication of TADF OLEDs. Careful optimization of doping concentration and charge carrier injection/transport layers enabled to fabricate devices emitting at ~630 nm with up to 10% external quantum efficiency (EQE). Although OLEDs based on phenanthroimidazole hosts performed as good as those based on conventional TPBi host at low current densities, they experienced increased roll-off at higher driving currents.

FIG. 1. EQE vs current density of red TADF OLED based on carbazolyl-substituted phenanthroimidazole host (rectangles) and conventional TPBi host (triangles). Inset: electroluminescence spectra of phenanthroimidazole-based TADF OLED.

This research was funded by a grant No. S-LLT-19-2 from the Research Council of Lithuania and by Ministry of Science and Technology (MOST) of Taiwan.


Poster
ID: 252
Poster Presentation

CONTROL OF TRIPLET STATE NON-RADIATIVE DECAY IN PENTA-CARBAZOLE/ARYL KETONES FOR HIGHLY EFFICIENT TADF

Gediminas Kreiza1, Dovydas Banevičius1, Karolina Maleckaitė1, Justina Jovaišaitė1, Dalius Gudeika2, Dmytro Volyniuk2, Juozas Vidas Gražulevičius2, Saulius Juršėnas1, Karolis Kazlauskas1

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) has become an important research topic over the last years due to its potential to be employed in low-cost and highly efficient organic light emitting diodes (OLEDs). TADF mechanism relies on triplet exciton upconversion to singlet states through thermally assisted reverse instersystem crossing (rISC) in compounds with small singlet and triplet energy splitting (ΔEST≤ 200 meV). Carefully designed TADF materials allow 100% harvesting of both triplet and singlet excitons formed in the emissive layer without necessity to utilize precious metals as in phosphorescent emitters.1

To achieve efficient TADF scientists mainly focus on the reduction of ΔEST by synthesizing materials with strong HOMO and LUMO spatial separation in donor-acceptor compounds. Benzophenone is considered among the most popular acceptors due to its intrinsically rapid intersystem crossing. Although utilization of benzophenone groups can result in fast rISC, this does not imply high fluorescence efficiency of the TADF molecule, since it is also governed by high singlet radiative decay rate. The latter is typically challenging to achieve as it contradicts with small ΔEST resulting in increased lifetime of delayed fluorescence. Consequently, non-radiative decay of long-lived triplet species associated with torsional/vibrational motions often dominates in benzophenone-based TADF compounds.2

To address this issue we have designed and synthesized two new TADF emitters based on five electron-donating tert-butylcarbazole groups fused with benzophenone or methyl benzoate acceptors. Thorough spectroscopic investigation revealed that replacing of loose phenyl ring with methoxy group in benzophenone moiety leads to significantly reduced intramolecular non-radiative decay rate of triplet states. This resulted in near unity fluorescence quantum yield of the compound which enabled sky-blue electroluminescence with external quantum efficiency of up to 24.6% in TADF OLEDs.

References:

1 H. Uoyama et al., Nature, 2012, 492, 234–238.

2 P. Rajamalli et al., J. Mater. Chem. C, 2017, 5, 2919–2926.


Poster
ID: 260
Poster Presentation

DERIVATIVES OF TRI-, TETRA- PHENYLETHYLENE AND 1.8-NAPHTALIMIDE EXHIBITING AGGREGATION INDUCED EMISSION ENHANCEMENT AS EMITTERS FOR OLEDS

Oleksandr Bezvikonnyi, Dalius Gudeika, Dmytro Volyniuk, Juozas Vidas Gražulevičius

Department of Polymer Chemistry and Technology, Kaunas University of Technology, Kaunas, Lithuania

The technology of organic light emitting diodes (OLED) has advanced dramatically in recent years. Synthesis and application of organic semiconductors which enable to increase efficiency of devices are important goals for researchers working in a field of materials science. Aggregation induced emission enhancement (AIEE) is recently widely exploited in different fields. The main cause of the phenomenon is restriction of intramolecular motions. Such materials employed in emitting layers of OLEDs have a big potential.

In this work, we report on the synthesis and investigation of a series of 1,8-naphtalimides substituted by tri- or tetraphenylethylene (triPE and tetraPE, respectively) moieties. Naphthalimide derivatives are known for their strong electron accepting ability. Bipolar character can be enhanced by introduction of donating units, for example carbazole, thus improving hole mobility in the layers of the compounds. TetraPE inhibits the π-π stacking interaction causing AIEE due to rotor nature of the chromophore.

To study effect of aggregation of studied molecules on photophysical properties of their dispersions in water/THF mixtures were studied. Derivatives performed emission related to two different kinds of aggregation. AIEE was manifested at water fractions of ca.30% and above 50% (Fig. 1). Photoluminescence quantum yields of the dilute toluene solutions ranged from 0.42 to 0.98. The compounds showed bipolar charge transport with charge carrier mobilities exceeding 10-3 cm2/Vs at 5.5×105 V/cm. The introduction of phenyl linkage caused more balanced charge transport.

a b

Figure 1. Normalized PL spectra of 3 in water/THF mixtures with different water fractions (fw, vol%) (a) and PL intensity of compounds in the water/THF mixtures (b).


Poster
ID: 263
Poster Presentation

HYBRID SOLUTION-PROCESSED OLEDS EXHIBITING HIGH-QUALITY WHITE ELECTROLUMINESCENCE

Dmytro Volyniuk1, Karolis Leitonas1, Jurate Simokaitiene1, Pavel Arsenyan2, Juozas Vidas Grazulevicius1

1Department of Polymer Chemistry and Technology, Kaunas University of Technology, Kaunas, Lithuania; 2Latvian Institute of Organic Synthesis, Riga, Latvia

White hybrid organic light emitting diodes (OLEDs) were developed using the newly synthesized selenium-containing deep-red iridium phosphorescent emitters. The idea of obtaining white electroluminescence is based on mixing of blue fluorescent, green thermally activated delayed fluorescent (TADF), and deep red phosphorescent emitters. Light-emitting layers of such white hybrid OLEDs were based on three different emitters which were easily deposited by simple low-temperature solution processing (spin-coating). OLED emitters were mainly chosen according to their appropriate energy levels and electroluminescent properties. High efficiency of these white hybrid OLEDs was expected due to harvesting of triplets through TADF and phosphorescence generated under electrical excitation. The blue fluorescent emitter additionally acted as the host for green and red emitters in the light-emitting layer. To prevent full energy transfer from the host to green and red emitters, ultralow concentrations of these low-energy emitters were selected. In the best case, singlet excitons were expected to be directly recombined on blue emitter while triplet excitons to be transferred to green TADF and red phosphorescent emitters due to long lifetimes (> few microseconds) and long diffusion lengths (> 10 nm) of triplet excitons. As a result, high-quality white electroluminescence with colour rendering index reaching of 85 was observed from the hybrid partly solution-processed OLEDs. The devices exhibited maximum brightness exceeding 10000 cd/m2 and high, as for solution-processed white OLEDs, external quantum efficiency of 6.26 %.

Acknowledgements

This work was supported by the project of scientific co-operation program between Latvia, Lithuania and Taiwan.


Poster
ID: 178
Poster Presentation

PERSISTENT BUILT-IN PHOTOACTIVITY OF SINGLE-WALL CARBON NANOTUBE FILMS

Vidmantas Jašinskas1, Florian Oberndorfer2, Tobias Hertel2, Vidmantas Gulbinas1

1Center for Physical Sciences and Technology, Vilnius, Lithuania; 2Physical Chemistry Department, University of Würzburg, Würzburg, Germany

Single-wall carbon nanotubes (SWCNTs) are intriguing materials of particular interest for many technological applications because of their broadly adjustable electric and photoelectric properties. SWCNTs were tested for application in organic solar cells, in light and temperature detectors, and other devices. Seeking to adopt these nanomaterials for practical use it is necessary to understand their various physical properties. Charge carrier motion in carbon nanotube films is extremely dispersive. Intratube charge carrier motion with mobility reaching thousands of cm2/V·s occurs on a subnanosecond time scale [1], while intertube carrier jumps may last for tens of microseconds [2]. Moreover, slow processes were found to cause persistent conductivity lasting for tens of minutes [3].

Here we demonstrate that external voltage applied to semiconducting SWCNTs’ film in lateral geometry for several minutes may create a long-living photoactivity lasting for hours and days. Under illumination, such film acts as a photovoltaic element generating small photovoltage and photocurrent even without an external electric field. Applied electric field creates a built-in electric field of opposite direction, which causes drift of photogenerated charge carriers. We have investigated the kinetics of this effect at various temperatures and also show that when the polarized sample is being frozen it inherits the magnitude of the effect created at a higher temperature but follows the kinetics appropriate to a lower temperature.

REFERENCES

  1. X. Zhou et al., Phys. Rev. Lett. 95, 146805 (2005).
  2. A. Eckstein et al., Nanoscale 9, 12441 (2017).
  3. R. F. Khairoutdinov et al., The Journal of Phys. Chem. B 108, 19976 (2004).

Poster
ID: 220
Oral presentation

OPTICAL PROPERTIES AND AMPLIFIED SPONTANEOUS EMISSION OF PYRANYLIDEN MOLECULAR GLASSES IN THIN NEAT FILMS

Aivars Vembris1, Patricija Paulsone1, Elmars Zarins2, Kristine Lazdovica2, Valdis Kokars2

1Institute of Solid State Physics, University of Latvia, Riga, Latvia; 2Institute of Applied Chemistry, Riga Technical University, Riga, Latvia

Optically pumped plastic lasers are currently a rapidly developing new optical technology field with the applications in sensors, biochips and sensors. Easily processable functional materials, with high light amplification properties, are required for the development of such lasers. One of the most popular compounds in dye lasers is 4-(dicyanomethylene)-2-methyl-6-(p-imethylaminostyryl)-4H-pyran (DCM). Molecule crystallisation with the subsequent decreasing of photoluminescence quantum yield limits the use of this compound in high concentration samples. The best performance was obtained only at 2wt% DCM:Alq3 system (M. Berggren et al.). Recently we have shown that bulky trityloxyethyl groups can decrease intermolecular interaction thus improving the optical and ASE properties in thin films [2].

Investigated materials are molecular glasses that consist from pyranyliden fragment with four different electron acceptor (malononitrile, 1H-indene-1,3(2H)-dione, 1,3-dimethylpyrimidine-2,4,6(1H,3H,5H)-trione and 1,3-diethyl-2-thioxodihydropyrimidine-4,6(1H,5H)-dioneand) and three glass forming (bis(5,5,5-triphenylpentyl)amino-, 5,5,5-triphenylpentyl-piperazin-1-yl-, bis(2-(3,3,3-triphenylpropanoate)ethyl)amino-) groups.

Thin films on a glass substrate with the thickness around 300 nm were prepared from dichloromethane solution by a spin-coating method. Absorption and photoluminescence spectra, as well as photoluminescence quantum yield, was measured for all prepared samples.

ASE excitation threshold energy was determined with the variable stripe length technique. Nd:YAG laser NL303 with pulse length 10 ns and repetition rate 10Hz was used. The excitation wavelength was the same as the wavelength of absorption maximum. The irradiated area was 4*0.4 mm2 and situated at the edge of the sample where emitted light was collected by calibrated spectrometer OceanOptics QPro.

All compounds emitted light at the red spectral region. Photoluminescence quantum yield was between 0.03 and 0.18. It mostly depended on the attached bulky group which reduced intermolecular interaction. Similar group impact was observed for the ASE excitation threshold values. The lowest excitation threshold energy value of 15 μJ/cm2 was observed for the compound with 1,3-dimethylpyrimidine-2,4,6(1H,3H,5H)-trione as electron acceptor group and bis(2-(3,3,3-triphenylpropanoate)ethyl)amino- as a bulky group.


Poster
ID: 141
Oral presentation

ELUCIDATING POLYMER TRANSIENT ABSORPTION SPECTRUM FEATURES FOR OPV

José Manuel Marín Beloqui, Tracey Clarke

University College of London (UCL), London, United Kingdom

Semiconductor polymers have attracted considerable attention in the past few decades for their feasibility in low cost, flexible, roll-to-roll compatible electronic organic devices. Despite the high efficiencies that can be obtained (up to 15% for organic photovoltaic devices), there are still several unknown questions that need to be answered. One of these questions is the presenceof two separate polaron peaks for some polymers in transient absorption spectroscopy. The presence for two different peaks in TA spectrum has been related before with polarons of different trapping status, but how these two different polarons are created, or its implications for the charge generation process, has not yet been explored.

To study the origin of these two peaks, in this work we study the transient absorption spectra of different polymer blends that present this double peak transient spectrum, such as P3HS, XINDB and ZZ115 polymers. Several modification were made to these blends to induce changes in the TA spectra features. For instance, the donor:acceptor ratio was changed, and different acceptors, like PC60BM, PC70BM, ICBA, and ICTA were used to make these modifications. This, along with the study of the kinetics differences after these modifications, helped to gain insight into the charge transfer reactions occurring in OPV solar cells. This better understanding is of utmost importance for the design of new polymers for better performing devices.


Poster
ID: 168
Oral presentation

USING THZ EMISSION SPECTROSCOPY TO PROBE CHARGE GENERATION PHOTOPHYSICS

Philipp Krauspe, Julien Rehault, Natalie Banerji

Universität Bern, Bern, Switzerland

THz Emission Spectroscopy is a novel tool to probe the charge generation mechanism through the emission of a light pulse on ultrashort times.

We bias our sample system laterally with attached electrodes and excite it with a femtosecond light pulse thus generating carriers within the material. Through the constant bias the carriers are accelerated and emit a light pulse which we detect in the THz range. Since the emission is dependent on the density of carriers and their ability to follow the applied field, the detected waveform gives insights into the charge generation and transport properties. To access this information with reasonable measurement times we design our THz detection specifically for the detection of weak THz pulses.

By varying the excitation energy and the applied bias we gain information on the field dependent generation. Exemplary, here we present emission from bulk heterojunctions to investigate fundamental properties governing the charge generation process in organic photovoltaics.


Poster
ID: 151
Poster Presentation

BROADBAND IMPULSIVE VIBRATIONAL SPECTROSCOPY

Chunyong Li, Yun Long, Andrew P. Monkman

Department of Physics, Durham University, Durham, United Kingdom

We present the setup of broadband impulsive vibrational spectroscopy (BB-IVS) that is able to record the vibrational coherence (VC) of ground and excited states in the time domain [1]. Part of the output from a femtosecond laser amplifier is used to pump a home-made non-collinear optical parametric amplifier (NOPA) that produces ~10 fs output. The output from the NOPA acts as the impulsive pump. Part of the output from the amplifier is used to pump a sapphire plate to generate broadband white light continuum (WLC). The WLC acts as the probe. A third actinic pump can be introduced into the system to pump the material to excited state so that the VC of excited state can be probed. The impulsive pump is modulated by an optical chopper that synchronized with half of the frequency of the laser to generate a situation of iterated pump ON and pump OFF, then the captured spectra of the probe will be WLC with pump and WLC without pump and so on. The relative transmission change at different wavelengths can be calculated easily. By changing the time delay between the probe and the impulsive pump, we obtain the spectra map of BB-IVS. After dispersion correction, coherent artefact truncation and cancellation of the contribution from electronic kinetics, we obtain pure VC map. Apply Fourier transformation on the pure VC to obtain information about the frequency content. Zero-padding and the multiplication with a window function before Fourier transformation are commonly used to improve the spectral quality. The BB-IVS setup and the experimental data processing are working very well for many materials, such as acetonitrile solvent, silicon carbide substrate.
[1] Matz Liebel, Philipp Kukura, J. Phys. Chem. Lett. 4, 1358-1364, 2013.


Poster
ID: 152
Poster Presentation

EXCITON-POLARITON RELAXATION IN SINGLE-WALLED CARBON NANOTUBE NETWORKS

Jan Luettgens, Jana Zaumseil

Applied Physical Chemistry, University of Heidelberg, Heidelberg, Germany

Exciton-polaritons are hybrid light-matter quasiparticles with mixed photonic and excitonic properties that form upon strong coupling of an excitonic transition and a photon mode. For organic emitters exciton-polaritons are observable at room temperature and for some even (non-equilibrium) Bose-Einstein condensation and thus polariton lasing could be achieved. Understanding and optimizing the relaxation pathways of exciton-polaritons and thus scattering toward the lowest energy state is key to reach condensation. Owing to their special optical and electronic properties (i.e. high oscillator strength, narrow emission in the near-infrared and large charge carrier mobilities) semiconducting single-walled carbon nanotubes (SWCNTs) have emerged as an excellent material to create optically and even electrically pumped exciton-polaritons at room temperature [1-3]. However, the underlying relaxation dynamics of SWCNT polaritons have not yet been investigated and it remains unclear under what conditions the fastest relaxation can be expected.

Here we investigate the impact of various parameters, such as emitter density, detuning and excitation wavelength, on the occupation distribution and relaxation of polaritons in metal-clad microcavities with purified (6,5) SWCNTs by angle-resolved reflectivity and photoluminescence measurements. We find that – similar to molecular emitters – intrinsic phonons of the carbon nanotubes, that is the Raman-active D-mode (165 meV) or G+-mode (197 meV) seem to play an important role for the relaxation dynamics. Furthermore, a high packing-density of nanotubes in a network and thus their interaction with each other appears to enhance relaxation compared to cavities with equivalent Rabi splitting but with well-separated SWCNTs in a matrix. The limits of such interactions, e.g. in highly aligned nanotube films, will be explored.

[1] A. Graf et. al. Nature Commun. 7 (2016) 13078.

[2] A. Graf et. al. Nature Mater. 16 (2017) 911.

[3] C. Möhl et al. ACS Photonics 5 (2018) 2074.


Poster
ID: 153
Poster Presentation

EXCITON DYNAMICS AND INTERACTION IN SQUARAINE CO-POLYMERS OF VARYING LENGTH, PROBED BY 5TH-ORDER TWO-DIMENSIONAL ELECTRONIC SPECTROSCOPY

Pavel Malý1, Julian Lüttig1, Arthur Turkin2, Jakub Dostál1, Christoph Lambert2, Tobias Brixner1

1Institut für Physikalische und Theoretische Chemie, Universität Würzburg, Würzburg, Germany; 2Institut für Organische Chemie, Universität Würzburg, Würzburg, Germany

Exciton transport and interaction in molecular aggregates and polymers is of great interest for study of natural photosynthetic complexes, organic photovoltaics, and other systems. In this work we study exciton transport in squaraine co-polymers of varying length, using a combination of 5th order two-dimensional electronic spectroscopy (2DES) and microscopic theoretical modelling. In the experiment, the 2DES is measured in a pump–probe geometry, and the 5th-order signal is isolated by selection of the excitation frequency. The observed two-exciton kinetics includes three phases, dominated by immediate exciton–exciton (e–e) annihilation (10-100 fs), diffusive behavior (0.1-10 ps), and excitation relaxation (0.01-1 ns), respectively. We reproduce the experimental kinetics by a Frenkel-exciton model, in which the squaraine molecules are treated as three-level systems. The two- and one-exciton dynamics is calculated by Redfield theory, while the e–e annihilation is treated within Lindblad formalism, microscopically as a transfer to/mixing with higher excited states and ensuing internal conversion. The 5th-order signal is calculated by a response function formalism. We demonstrate that the key quantity for the exciton dynamics is the exciton delocalization length relative to the length of the polymer: while excitons delocalized over a large fraction of a short polymer rapidly annihilate, localized excitons on a long polymer exhibit diffusive behavior. The signal power dependence demonstrates that increased exciton density leads to a more pronounced e–e annihilation. We determine the exciton delocalization length (1.5 squaraine dimers) and transport properties such as diffusion length (about 30 squaraine dimers, or ~100 nm) and anomalous diffusion coefficient (0.36).


Poster
ID: 166
Poster Presentation

IMAGING THE CRYSTALLINE STRUCTURES OF A HETEROGENEOUS FILM BY COHERENT TWO-DIMENSIONAL MICROSCOPY

Donghai Li1, Evgenii Titov1, Sebastian Hammer2, Verena Kolb2, Sebastian Goetz1, Roland Mitric1, Jens Pflaum2, Tobias Brixner1

1Institut für Physikalische und Theoretische Chemie, Universität Würzburg, Würzburg, Germany; 2Lehrstuhl für Experimentelle Physik VI, Universität Würzburg, Würzburg, Germany

Heterogeneous systems show great application potential in bulk heterojunction (BHJ) solar cells, where a blend of donor and acceptor molecules is used to create a composite material exhibiting nanoscale phase separation. The crystalline structures in different domains have determinative yet complex influence on the electrical performance of such devices [1]. Therefore, a method is required that enables us to characterize highly localized crystalline structures in a heterogeneous system. Here, we show that coherent two-dimensional (2D) microscopy, aided by time-dependent density functional theory (TDDFT) calculations, allows for imaging the crystalline structures of a heterogeneous film.

Coherent 2D microscopy is a powerful combination of fluorescence-detected 2D spectroscopy and high numerical aperture optical microscopy, which enables us to detect coherent 2D electronic spectra as well as linear excitation spectra with sub-micron spatial resolution [2]. A structured sample is fabricated by evaporating the fluorinated zinc phthalocyanine (F16ZnPc) on a monolayer of spincoated polystyrene nanospheres and subsequent lift-off of the spheres. We quantitatively extract the excitonic spectral variations of different localized crystalline structures by a 2D spectra simulation process by numerically solving a Lindblad master equation for a system interacting with an electric field, with predefined energy structure, transition dipole strengths, and dephasing times.

By conducting the TDDFT calculation on the β-phase F16ZnPc crystalline structures with varying length, we find an evident correspondence between the observed excitonic spectral variation and the length of the crystalline structures. From such a correspondence, a microscopic structural map of the sample is obtained. In the map, we can clearly reveal distinct structural domains with different average sizes of the β-phase crystalline structures. The demonstrated approach provides a promising access to the micro- and nano-scopic structure of BHJ materials.

[1] D. Deng et al., Nat. Commun. 7, 13740 (2016)

[2] S. Goetz et al., Opt. Express 26, 3915 (2018)


Poster
ID: 171
Poster Presentation

CORRELATED ATOMIC FORCE MICROSCOPY AND SPATIALLY-RESOLVED PHOTOLUMINESCENCE SPECTROSCOPY STUDY OF MEROCYANINE AGGREGATE THIN FILMS

Lukas Böhner, Thorsten Limböck, Dirk Hertel, Klas Lindfors, Klaus Meerholz

Universität zu Köln, Köln, Germany

Merocyanine molecules are interesting materials for organic solar cells (OSCs) due to their high absorption coefficients.[1] Large area OSCs can be produced at low cost compared to conventional inorganic solar cells using liquid-phase techniques. However, the in this way resulting disordered structure of the active material in OSCs is disadvantageous in terms of exciton- and charge mobility, limiting device performance. One strategy to increase order in organic materials is the utilization of molecular aggregates wherein excitons are delocalized. Aggregation of organic dye molecules leads to drastic changes of the optical properties compared to the monomeric form accompanied with an increase in exciton- and charge mobility.[2,3] Here, we study thin films of merocyanine molecular aggregates fabricated by vacuum deposition. The morphology of the films is varied by using different substrates and controlling the deposition parameters. Combining atomic force microscopy and photoluminescence microspectroscopy we are able to correlate the structural and optical properties of the aggregates. We find that by suitable choice of the fabrication parameters, thin films consisting of crystalline domains which scale in the range of tens of micrometers can be obtained. We will present the impact of molecular order on exciton and charge transport properties.

References:

[1] H. Bürckstümmer, E. V. Tulyakova, M. Deppisch, M. R. Lenze, N. M. Kronenberg, M. Gsänger, M. Stolte, K. Meerholz, F. Würthner, Angew. Chemie - Int. Ed. 2011, 50, 11628–11632.

[2] N. J. Hestand, F. C. Spano, Acc. Chem. Res. 2017, 50, 341–350.

[3] A. Liess, A. Arjona-Esteban, A. Kudzus, J. Albert, A. M. Krause, A. Lv, M. Stolte, K. Meerholz, F. Würthner, Adv. Funct. Mater. 2018, 1805058, 1–9.


Poster
ID: 174
Poster Presentation

REVEALING EXCITON–EXCITON INTERACTION IN ARTIFICIAL LIGHT-HARVESTING COMPLEXES

Julian Lüttig1, Björn Kriete2, Maxim S. Pshenichnikov2, Tobias Brixner1

1Institut für Physikalische und Theoretische Chemie, Universität Würzburg, Würzburg, Germany; 2Zernike Institute for Advanced Materials, University of Groningen, Groningen, The Netherlands

Two-dimensional electronic spectroscopy (2DES) has become a powerful tool for the investigation of single-exciton dynamics in various molecular systems, e.g. self-assembled artificial light-harvesting complexes [1]. However, with standard 2D techniques it is not possible to directly observe the interactions between multiple excitons like exciton–exciton annihilation (EEA). Here we use the recently developed technique of exciton–exciton interaction two-dimensional (EEI2D) spectroscopy to disentangle the exciton dynamics in single-walled and double-walled tubular aggregates [2]. The technique of EEI2D spectroscopy allows us to spectrally separate the biexciton from the single-exciton dynamics and, hence, unambiguously measure the contribution of double-exciton processes to the overall signal. In order to further ease the identification of otherwise congested exciton transitions, a microfluidic system is used to selectively peel off the outer layer from our model system, double-walled nanotubes, thereby granting unobscured spectroscopic access to the isolated inner tubes. The combination of these techniques poses a powerful platform to study exciton dynamics at different supramolecular hierarchies and identify the influence of the supramolecular structure on the single- and double-exciton processes. The measured 2D spectra reveal an additional loss channel of excitons through EEA and, thus, speed up the exciton relaxation in single-walled tubular aggregates. Third- and fifth-order 2D spectra of double-walled tubular aggregates (see figure) show that the energy transfer from the outer to the inner layer for single excitons as well as for biexcitons is a driving factor for EEA that occurs on the inner tube. With the technique of EEI2D spectroscopy it is possible to disentangle the higher-order contributions and to identify the different timescales of single-exciton and biexciton dynamics.

[1] B. Kriete et al., J. Phys. Chem. Lett., 8, 2895-2901 (2017).

[2] J. Dostál et al., Nat. Comm. 9, 2466 (2018).


Poster
ID: 209
Poster Presentation

NEW BODIPY DYES DESIGNED FOR POLARITONICS

James P. Pidgeon1, Andrew J. Musser1, Marco Cavazzini2, Daniel W. Polak1, Kyriacos Georgiou1, David G. Lidzey1, Jenny Clark1

1Department of Physics and Astronomy, University of Sheffield, Sheffield, United Kingdom; 2Instituto di Scienze e Tecnologie Molecolari (ISTM), Consiglio Nazionale delle Ricerche (CNR), Milan, Italy

BODIPY (boron-dipyrromethene) dyes are a class of highly fluorescent molecules, often used for sensing or labelling. More recently, BODIPY dyes have been explored as polaritonic materials, demonstrating room-temperature polariton condensation and lasing. To date, these polariton studies have relied on dispersing the BODIPY molecules in a polymer matrix at dopant concentrations below 10% (by mass) to avoid aggregate and excimer formation. This low doping concentration limits the potential of BODIPY dyes for polariton studies, as it produces films with low optical density, limiting the achievable Rabi splitting.

To solve the problem of unwanted BODIPY aggregation, we investigate four new BODIPY structures which have been designed to either (1) enhance highly emissive J-aggregate formation or (2) hinder aggregate and excimer formation. We find that the BODIPYs designed to enhance aggregate formation produce thin films which are dominated (>50% band area) by a strong, narrow (7meV FWHM) J-aggregate absorption band, with a Stokes shift of only 2meV. Residual absorption of monomer features is weak, but strong enough to allow non-resonant pumping of microcavities. On the other hand, the BODIPYs designed to avoid aggregate formation demonstrate monomer-like fluorescence behaviour in thin films with loading up to 98% (by mass).

Here we will show time-resolved and steady-state photophysical characterisation of thin films and microcavities to compare the effect of J-aggregate formation and fluorescence quantum yield on polariton behaviour.


Poster
ID: 211
Poster Presentation

DIRECT MEASUREMENTS OF EFFICIENT EXCITON TRANSPORT IN WELL-ORDERED ORGANIC SEMICONDUCTING NANOSTRUCTURES

Alexander James Sneyd1, David Palecek1, Raj Pandya1, George R. Whittell2, Ian Manners2, Richard H. Friend1, Akshay Rao1

1University of Cambridge, Cambridge, United Kingdom; 2School of Chemistry, University of Bristol, United Kingdom

The ability to transport excitons over length scales exceeding 100 nm is highly desirable for a range of light-harvesting and optoelectronic devices. This is particularly true for thin-film organic semiconductor structures, such as those formed from conjugated polymers, where large exciton diffusion lengths mean excitons can efficiently diffuse to charge-generating hetero-interfaces. So far however, few materials with large exciton diffusion lengths have been reported, especially for conjugated polymers where disorder can be high and exciton diffusion lengths are usually limited to around 10nm.

Here, following on from recent work demonstrating >200nm diffusion lengths in self-assembled highly-ordered poly(di-n-hexylfluorene) nanofibers, we study the exciton diffusion in a range of well-ordered self-assembled nanostructures constructed from conjugated polymers such as polythiophenes and polyfluorines. The exciton dynamics in these structures are studied via photoluminescence quantum efficiency measurements, femtosecond-resolved photoluminescence spectroscopy and ultrafast pump-probe spectroscopy. We also employ a recently established methodology, femtosecond transient absorption microscopy, to directly image the exciton transport in these nanostructures with a ~10nm localisation precision and sub-10 fs time resolution. This provides us with the higher level of understanding of the energy transport over different spatial regions.

We expect our work to lay a new platform for understanding and identifying the presence of efficient exciton transport in well-ordered organic-semiconducting nanostructures. These new findings may enable more efficient bilayer organic photovoltaics based on the traditional bulk heterojunction design, and/or the next generation of light-harvesting devices where polymer structures perform as antennae coupled to high-value components.


Poster
ID: 217
Oral presentation

SUB-NANOSECOND INTERSYSTEM CROSSING IN PLANAR BENZOTHIENO[3,2-B]BENZOTHIOPHENE DERIVATIVES

Oleksandra Korychenska1, Jozra Garrido Velasco2, Theo Keane1, Andrew Musser2, Alexander Auty1, Aurelio Oriana3, Giulio Cerullo3, Tersilla Virgili3,4, Mariacecilia Pasini4, Ifor Samuel5, Eirini Lariou6, Sophia Hayes6, Lupeng Yang7, Daniel Cole8, Ahmed Iraqi1, Mark Geoghegan2, Jenny Clark2

1Department of Chemistry, University of Sheffield, Sheffield, United Kingdom; 2Department of Physics and Astronomy, University of Sheffield, Sheffield, United Kingdom; 3Dipartimento di Fisica, Politecnico di Milano, Milan, Italy; 4Istituto per lo Studio delle Macromolecole (CNR), Milan, Italy; 5School of Physics & Astronomy, University of St Andrews, St Andrews, United Kingdom; 6Department of Chemistry, University of Cyprus, Nicosia, Cyprus; 7Department of Physics, University of Cambridge, Cambridge, United Kingdom; 8School of Natural and Environmental Sciences, Newcastle University, Newcastle, United Kingdom

Intersystem crossing (ISC) is a fundamental photophysical processes that is crucial for application of organic molecules in OLEDs, solar cells and spintronics. ISC requires angular momentum conservation during a spin flip which is the basis of El-Sayed’s rule. Therefore, in organic molecules ISC rate can be increased though coupling to out-of-plane vibrations (e.g. in free-base porphyrin). We demonstrate sub-nanosecond ISC in planar derivatives of benzothieno[3,2-b]benzothiophenes (BTBT). This cannot be explained by any available theoretical model when the molecule is considered to be in its equilibrium excited-state geometry.

We combine theoretical calculations with the use of steady-state, transient absorption, emission and Raman spectroscopy at a range of temperatures to describe and explain ISC in these molecules. We find that ISC occurs surprisingly fast (120 ps) and slows down in non-planar derivatives. Our calculations suggest that at equilibrium geometry the spin-orbit coupling matrix element is negligible. Our results can only be explained by considering the ensemble of molecular geometries in the excited state. Including these range of conformations leads to an incredible three orders of magnitude increase in ISC rate that matches our experimental observations. These results demonstrate the need to take molecular dynamics into account when considering spin-orbit coupling and ISC. We discuss the reasons and design rules for controlling ISC in organic semiconductors.


Poster
ID: 239
Poster Presentation

ENHANCED EXCITON TRANSPORT IN BIFLUORENE SINGLE CRYSTALS

Paulius Baronas1, Gediminas Kreiza1, Patrik Scajev1, Povilas Adomenas1, Karolis Kazlauskas1, Chihaya Adachi2, Saulius Jursenas1

1Institute of Photonics and Nanotechnology, Vilnius University, Vilnius, Lithuania; 2Center for Organic Photonics and Electronics Research (OPERA), Kyushu University, Fukuoka, Japan

Exciton diffusion is of prime importance for controlling host-guest energy transfer and recombination zone in organic light emitting devices. Here, we present a study of series of bifluorene small molecule crystals, which revealed transition between two exciton diffusion regimes with only minor molecular structure changes. In terms of Förster theory for energy transport is treated as incoherent nearest-neighbor hopping, however reduced vibrational degrees of freedom in some cases introduce coherent component to exciton motion, which results in enhanced diffusion. Our previous studies of crystals grown from molecules with flexible backbones showed typical incoherent energy transport, i. e. exciton diffusion length decreased at low temperatures [1]. On the other hand, crystals of molecules bearing rigid backbones revealed opposite behavior of enhanced exciton transport at low temperatures, which could be attributed to partly coherent transport due to exciton delocalization. Direct measurement of exciton diffusion by light induced transient grating (LITG) technique at room temperature allowed us to resolve highly anisotropic diffusion with coefficients of up to D = 1 cm2/s and corresponding diffusion length of LD = 300 nm in bifluorene single crystals, while the recorded diffusion coefficients were higher for crystals of rigid molecules [2]. Enhanced exciton diffusion was found to be beneficial in doped crystals, efficiently transferring all excitation energy to small amount (<0.5 %) of highly-radiative exciton traps, resulting in efficient emission (QYPL = 0.8) [3]. Potential to accommodate charge transport and emissive properties in such crystals make them attractive for light emitting field effect transistor and possibly electrically pumped organic laser applications.

References

[1] G. Kreiza, et al., Adv. Optical Mater. 5, 1600823 (2016).

[2] P. Baronas, et al., Appl. Phys. Lett. 112, 033302 (2018)

[3] P. Baronas, et al., ACS Appl. Mater. Interfaces 10, 2768 (2018).


Poster
ID: 241
Poster Presentation

OPTICAL SPECTROSCOPY OF BIFLUORENE CRYSTALS: A JOINT EXPERIMENTAL AND THEORETICAL STUDY

Mattia Anzola1, Anna Painelli1, Gediminas Kreiza2, Paulius Baronas2, Povilas Adomenas2, Karolis Kazlauskas2, Saulius Jursenas2

1SCVSA Department, Parma University, Parma, Italy; 2Institute of Photonics and Nanotechnology, Vilnius University, Vilnius, Lithuania

Organic single crystals grown from a series of different bi-fluorene dyes have shown promising photoluminescence (PL) properties, what makes them attractive as gain medium for organic lasers[1,2]. The crystals have a well-ordered crystalline structure, with 2D molecular layers and negligible interlayer interactions. Absorption and PL spectra of crystals show a significant redshift if compared with spectra collected from isolated molecules in solution. H-aggregation is supported by the fluorescence bandshapes and the small changes in radiative rate supports weak intermolecular coupling in some bi-fluorene aggregates. To get deeper insight about excitons in bi-fluorene crystals, an extended theoretical study was performed.

Specifically, an exciton model is defined to rationalize the spectroscopic observations on the crystals. The molecular transition dipole moments are estimated from TD-DFT calculations on the isolated molecules in the crystallographic geometry. Electrostatic intermolecular interactions are then calculated in the dipole approximation to obtain the magnitude of the exciton couplings. These are finally implemented in a band-structure calculation to obtain the 2D dispersion of the exciton states, getting immediate information about absorptive and emissive states.

[1] G. Kreiza, et al., Adv. Optical Mater. 5, 1600823 (2016).

[2] P. Baronas, et al., ACS Appl. Mater. Interfaces 10, 2768 (2018).


Poster
ID: 270
Poster Presentation

LIGHT NANOSCOPY FOR NATURALLY FLUORESCENT SYSTEMS

Simona Streckaitė1, Cristian Ilioaia1, Anja Krieger-Liszkay1, Francesca Zito2, Andrew Gall1, Bruno Robert1

1Institute of Integrative Biology of the Cell (I2BC), CEA/CNRS/University Paris-Sud, Gif-sur-Yvette, Cedex, France; 2Laboratoire de Biologie Physico-Chimique des Protéines Membranaires UMR 7099 – IBPC, Paris, France

During the last two decades many methods were developed to overcome the diffraction limit of classical optical microscopy which for green excitation light and a standard objective of 1-1.45 numerical aperture is around 170-250 nm. For this reason, diffraction limited microscopy does not give access to nanoscopic scale; super-resolution techniques overcome this barrier. Most of these techniques are based on the use of random activation and localization of fluorophores (PALM, STORM), the difference between spontaneous and stimulated emission (STED), or the patterned sample illumination (SIM). Nevertheless, these techniques are limited either by the requirement of special photo-switchable or high powers withstanding fluorophores, or non-trivial computational methods to achieve the final super-resolution image. Moreover, most of them are relative expensive and complex, and for the best results a combination of few methodologies must be used [1].

As a solution, a relatively simple and cheap super-resolution imaging technique was developed in our laboratory. It is a laser scanning nanoscopy based on the fact that the energy distribution of a perfectly focused laser beam on a fluorescing sample is inhomogeneous. Thus, any small displacement of the sample, smaller than the diffraction limit, gives us different illumination. After the sample is scanned with X&Y steps of ca. 10-50 nm, an image with the resolution superior to 100 nm is reconstructed from one pixel from each of the recorded images. This method enables us to study labelled compartments in various cells, and even naturally fluorescent systems in vivo. In this work our novel light nanoscopy is applied for naturally fluorescing photosynthetic organisms (plants, algae, cyanobacteria) and various labelled bacteria cells. Also, Z-plane scanning with 3D reconstruction is shown.

[1] Danial J.H.S. et al, Advanced fluorescence microscopy techniques for the life sciences, GCSP 2016:16