Conference Agenda

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Session Overview
Session
S-8: Exciton and polariton spectroscopy 3
Time:
Tuesday, 09/Jul/2019:
16:10 - 17:50

Session Chair: Stephane Kena-Cohen
Location: Main Hall
"Artis Centrum Hotel" (Address: Totoriu str. 23, Vilnius, LT-01120, Lithuania)

Presentations
16:10 - 16:35
Invited
ID: 137
Oral presentation

MAGNETIC FIELD INDUCED CIRCULAR POLARIZATION OF PHOSPHORESCENCE AND PHOTOLUMINESCENCE IN PT-RICH POLYMERS

Eitan Ehrenfreund1, Chuang Zhang2, Dali Sun2, Ryan McLaughlin2, Dmitry Semenov3, Stephen McGill3, Zhi Gang Yu4, Z. Valy Vardeny2

1Technion – Israel Institute of Technology, Haifa, Israel; 2University of Utah, Salt Lake City, Utah, United States of America; 3National High Magnetic Field Lab., Tallahassee, Florida, United States of America; 4Washington State University, Spokane, Washington, United States of America

Incorporating heavy atoms into polymer chains represents an effective way to generate emissive triplet excitons. Here we used magneto-optical emission spectroscopy up to 17.5 Tesla for studying the fine structure of the triplet exciton in a series of Pt-rich π-conjugated polymers with various intrachain Pt concentrations. We found that their phosphorescence emission band shows substantial field-induced circular polarization (FICPO) up to 50% with an unusual, non-monotonic field dependence at cryogenic temperature. From the field-induced energy splitting between left and right circularly polarized phosphorescence we obtained the effective g factor varying in the range of -0.13 to 0.85, which depends on the Pt concentration in the polymer chains. The FICPO of triplet emission originates from the population difference in spin sublevels, which is determined by thermal equilibrium subjected to spin-orbit coupling (SOC), exchange and Zeeman interactions. Surprisingly we also observed FICPO in the fluorescence emission that results from the singlet-triplet interaction caused by the strong SOC. From these results we could extract the various interaction parameters that describe the exciton fine structure in these Pt-contained compounds.

The figure shows the circular polarization (CP) spectra, CP(l) for (a) the primary (P) band and (b) the vibronic (V) band of the phosphorescence emission in Pt-1 film, measured at 4K in the field range of ±17.5 T. The extracted ΔE(B)=ERCP-ELCP energy split (upper panels) and the FICPO response, CP(B) (lower panels) of the P band (c) and V band (d). The red lines through the data points are fits using a model based on singlet-triplet intermixing caused by strong SOC.

  1. C. Zhang, D. Sun, R. McLaughlin, D. Semenov, S. McGill, Z-G. Yu, E. Ehrenfreund, Z.V. Vardeny, Phys. Rev. B98, 155205 (2018).

16:35 - 16:50
ID: 191
Oral presentation

CONTACT RESISTANCE IN AMBIPOLAR ORGANIC FIELD-EFFECT TRANSISTORS MEASURED BY CONFOCAL PHOTOLUMINESCENCE ELECTRO-MODULATION MICROSCOPY

Wouter Koopman1, Marco Natali2, Manuela Melucci3, Muccini Michele2, Stefano Toffanin2

1Universitat Potsdam, Potsdam, Germany; 2CNR-ISMN, Bologna, Italy; 3CNR-ISOF, Bologna, Italy

Single-layer ambipolar organic field-effect transistors (aOFETs), either implementing polymeric or a small-molecule compounds, are an ideal platform to investigate the different mechanisms in hole and electron lateral transport in a single device since the device architecture provides a controllable planar pn-junction within the transistor channel. However, a direct comparison of the injection barriers and of the channel conductivities between electrons and holes cannot be measured by standard electrical characterizationswithin the same device. In this regard, the direct observation of the recombination zone (RZ) position in an aOFET channel as a function of the drain-source and gate applied bias voltages offers an alternative method for characterizing the device electrical behavior.

We have already reported that the photoluminescence electro-modulation microscopy (PLEM) is an unique optical tool for observing the charge distribution in the channel in field-effect transistor devices by the in-plane mapping of the exciton-quenching (Koopman, Nano Lett., 2013). This feature enables to observe the RZ position in real-working aOFETs.

Here we report the first experimental demonstration of direct optical imaging of the gate-shifted RZ location in aOFET by PLEM microscopy, without the requirement of detectable electroluminescence (Figure 1). Moreover, we demonstrate that the contact resistance for electron and hole injection into an ambipolar OFET can be independently determined by using a single device. By comparing these results with the threshold voltages obtained from standard electrical characterization, the channel resistance can be also determined (Koopman, ACS Appl. Mater. Inter., 2018).

In this way, it is possible to probe independently how the different functional interfaces (such as the electrode/OSC and the OSC/dielectric) modulate the charge carrier transport properties of holes and electrons within the same device, and to decouple the device-dependent and material-dependent issues that affect the electrical performance in organic field-effect transistors.


16:50 - 17:05
ID: 161
Oral presentation

TUNING THE PHOTO-PHYSICS OF MOLECULES FOR OPTO-ELECTRONIC APPLICATIONS USING PLASMONIC AND DIELECTRIC EFFECTS

Sikandar Abbas, Linda Peteanu

Department of Chemistry, Carnegie Mellon University, Pittsburgh, Pennsylvania, United States of America

Organic semiconductors are promising for the development of many applications including in solid state lighting, flexible displays and organic-based lasing. However, a critical factor limiting their utility is their inherently low stability relative to inorganic materials. Organic molecules are susceptible to oxygen damage in an irreversible process known as photo-bleaching. This effect necessitates the development of highly impermeable encapsulation strategies and limits device lifetimes. Plasmonic engineering offers one avenue to alter the decay rates of nearby fluorophores and modify the emission from single molecules and thin films. Here we demonstrate that photo-stability enhancement of more than 60-fold can be achieved by depositing organic molecules onto ultra-thin gold films having a plasmonic frequency that matches the molecular emission peak. As an example, we have successfully demonstrated that single molecules and confluent thin films of poly [2-methoxy-5(2'-ethylhexyloxy)-1,4-phenylene]vinylene and related short-chain oligomers deposited on thin gold substrates maintain their emission intensities for more than 30 minutes under constant laser illumination and in ambient air. In contrast, the same samples deposited on glass substrates are completely photo-bleached in less than 2 minutes under the same conditions. Importantly, this effect is achieved without emission quenching due to non-radiative energy transfer to the plasmon band. The photo-physics of the molecule-plasmon interaction is probed using a variety of microscopy-based tools including single molecule emission lifetime and fluorescence anti-bunching. Extensions to a broader array of emission wavelengths and to other opto-electronic materials are also described.


17:05 - 17:20
ID: 243
Oral presentation

HARNESSING ELECTRONIC ENERGY STORAGE: MOLECULAR DYADS, NANOPARTICLES, MOLECULAR MACHINES

Gediminas Jonusauskas, Nathan D. McClenaghan

CNRS - Bordeaux University, Talence, France

The efficient use of energy following light absorption is of extreme importance in natural photosynthetic assemblies as well as in artificial systems. Small supramolecular systems have been used successfully to absorb light energy and transfer it to a specific site, while reversible energy transfer processes in polypyridine complexes with transition metals have been reported to temporarily stock energy and prolong excited-state lifetimes.

Here we report the unique excited-state equilibration between three different excited states in a structurally simple bichromophoric Copper(I)-phenanthroline complex coupled through a short spacer with an auxiliary anthracene chromophore acting as an energy reservoir [Leydet 2007]. The unprecedented increasing of luminescence lifetimes in Ruthenium(II) complexes based on tridentate polypyridine ligands linked to anthracene chromophore [Ragazzon 2013] and emissive cyclometallated Iridium(III) centre connected to pyrene [Denisov 2014] already used as a sensor for molecular oxygen [Medina-Rodriguez 2016] will be also presented.

Not only the organic chromophores could be the emissive units in reversible energy transfer systems, but recently demonstrated spectacular modification of the CdSe quantum dot emission lifetime are also in the scope of our research [La Rosa 2018].

Finally, our new research on the diffusion controlled excited-state equilibration of triplet energies between Ruthenium(II) tris-bipyridine complex and freely moving pyrene in molecular machines (rotaxanes) will be discussed in this report.

We are grateful for financial support from the Agence Nationale de la Recherche, University of Bordeaux, CNRS.


17:20 - 17:35
ID: 226
Oral presentation

SPECTROSCOPY OF ORGANIC SEMICONDUCTOR PROTEIN-COMPLEXES.

Jozra Garrido-Velasco, George Sutherland, Mark Geoghegan, Neil C Hunter, Jenny Clark

The University of Sheffield, Sheffield, United Kingdom

We employed picosecond transient absorption and linear absorption spectroscopy to study the photophysics of a non-biological organic semiconductor encapsulated within a synthetic protein. This pigment-protein complex is comprised of C8-BTBT [2,7-Dioctyl[1]benzothieno[3,2-b][1]benzothiophene] sequestered within a synthetic ‘maquette’ protein. C8-BTBT is a small molecule which has one of the highest reported field-effect mobilities to date and is of interest for spintronic applications. Synthetic maquette proteins have been designed to be intentionally simple and robust with widely tunable capabilities. These systems could be used in applications ranging from drug delivery to light harvesting. However, to date, only biological pigments have been placed within the protein, liming their applicability in optoelectronic or photonic devices. Here we demonstrate the feasibility of extending the family of protein-bound pigments to organic semiconductors.

The proteins enable us to form controlled ensembles of a few monomers (dimers, trimers), protected and constrained by the protein structure. In this way, we can compare monomer, polycrystalline and small ensemble C8-BTBT photophysical behavior for the first time. We find the absorption spectrum of C8-BTBT within the protein is different to both the monomer and the polycrystalline film due to the size effects. In addition, intersystem crossing, which is highly efficient in solution, is completely suppressed within the pigment-protein complex. We discuss the reasons for this suppression. These studies demonstrate the potential of using protein complexes to understand fundamental behavior in organic semiconductors and could lead to new optoelectronic, photonic or spintronic applications.


17:35 - 17:50
ID: 146
Oral presentation

ENHANCED THERMALLY ACTIVATED DELAYED FLUORESCENCE THROUGH BRIDGE MODIFICATION IN SULFONE-BASED EMITTERS EMPLOYED IN DEEP BLUE ORGANIC LIGHT-EMITTING DIODES

Rajamalli Pachai Gounder, Dongyang Chen, Wenbo Li, Ifor D.W. Samuel, Eli Zysman-Colman

University of St Andrews, St Andrews, United Kingdom

Many applications benefit from the development of luminescent materials, such as bioimaging, displays and lighting. In recent years, organic light emitting diodes (OLEDs) based on thermally activated delayed fluorescence (TADF) emitter have emerged as the third-generation of OLEDs for displays and solid-state lightings.1-4 The most exciting feature of TADF-OLEDs is the harvesting triplet excitons using a pure-organic dye molecule with an efficient reverse intersystem crossing (RISC). Here, we designed and synthesized pyridine derived sulfone cored TADF emitter, where we studied the effect of heteroatom in the device performance. An electroluminescent device of this material as the dopant shows the external quantum efficiency of 13.4% with CIE value of 0.15; 0.12. Moreover, small changes in the acceptor this device shows narrow emission with full width and half maximum (FWHM) of 74 nm, which is much narrower than most of the reported blue TADF emitters.

Reference,

1. Wong, M. Y.; Zysman-Colman, E. Adv. Mater., 2017, 1605444.

2. Rajamalli, P.; Rota Martir, D.; Zysman-Colman, E. ACS Appl. Energy Mater, 2018, 1, 649.

3. Rajamalli, P.; Rota Martir, D.; Zysman-Colman, E. J. Photon. Energy, 2018, 8, 032106.

4. Rajamalli, P.; Senthilkumar, N.; Huang, P.-Y.; Ren-Wu, C.-C., Lin, H.-W.; Cheng, C.-H. J. Am. Chem. Soc., 2017, 139, 10948.