Conference Agenda

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Session Overview
S-3: Exciton and polariton spectroscopy 2 / OPV 1
Monday, 08/Jul/2019:
14:00 - 15:40

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

14:00 - 14:15
ID: 210
Oral presentation


Raj Pandya1, Richard Chen1, Qifei Gu1, Jooyoung Sung1, Christoph Schnedermann1, Andrew Musser2, Semion Saikin3, Joel Yuen-Zhou4, Akshay Rao1

1University of Cambridge, Cambridge, United Kingdom; 2University of Sheffield, Sheffield, United Kingdom; 3Harvard University, Cambridge, Massachusetts, United States of America; 4University of California San Diego, La Jolla, California, United States of America

Understanding and controlling the efficient transfer of energy in the form of excitons, lies at the core of both photosynthesis and organic semiconducting devices. However, real time visualization of energy migration has not been possible due the nanoscopic length scales and ultrafast timescales (sub-100 fs) involved. In the majority of organic materials, the typical exciton transport length is between 10 – 50 nm with only a handful of systems being suggested to exceed this. Recently, it has been shown that the energy migration length in organic semiconductor films can be increased through hybridisation of the exciton with light modes to form a new quasi-particle, the polariton. However so far, this has only been realized in the case of materials embedded within a microcavity hampering potential applications.

Here we use femtosecond transient absorption microscopy (fs – TAM) with ~10 nm localisation precision and sub-10 fs time resolution to directly image (Figure 1) energy transport at ultrafast timescales in three chemically diverse one dimensional organic semiconductors, PDA (polyene), PIC (J-aggregate) and PDI (H-aggregate). Surprisingly we find photoexcitations in these systems can move up to ~270 nm in ~50 fs. We show that this is due to the formation of exciton-polaritons (EPs) despite the absence of any cavity structure. In addition, we demonstrate the magnitude and direction of the energy migration is intricately linked to the sample morphology. Using surface reflectance and photoluminesnece coherence measurements we determine the polariton Rabi splitting to be 18-40 meV and show that a high oscillator strength, short exciton lifetime and large bath-refractive index mismatch is required for the formation of robust EPs.

Our results provide experimental evidence for a new model of energy transport in organic systems that transcends underlying chemistry and is based on room temperature polaritonic effects, with great promise for modern optoelectonic technologies.

14:15 - 14:30
ID: 181
Oral presentation


Matthijs Berghuis1, Alexei Halpin2, Quynh Le Van2, Mohammad Ramezani1, Shaojun Wang1, Jaime Gómez Rivas1

1Eindhoven University of Technology, Eindhoven, The Netherlands; 2Dutch Institute for Fundamental Energy Research, Eindhoven, The Netherlands

Organic semiconductors are the main components of organic optoelectronic devices, such as organic LEDs (OLEDs) and organic photovoltaics (OPVs). The performance of these devices is determined by properties such as absorption and emission cross section of the molecules and the excited state dynamics. To reach the best device performance, it is necessary to control and optimize the properties of the organic semiconductor. A promising method to selectively change the exciton energy levels and their properties is by strong coupling with resonances in optical cavities. This coupling leads to the hybridization of these states into two new states with different energies, called the lower and upper polaritons.

In this work demonstrate the strong coupling of the singlet excitons in tetracene crystals to collective plasmonic resonances in open cavities formed by arrays of silver nanoparticles. We show that this strong coupling can modify the energy levels in tetracene and, thereby, change the absorption and emission cross sections (Fig. a. and b.). This leads to a ten-fold increase of the fluorescence at the energy of the lower polariton band (Fig c.). Moreover, we observe an enhancement of the delayed fluorescence by a factor of six, at times longer than 100 ns after excitation. These measurements prove that we can modify the photophysical properties of organic semiconductor crystals by strong light-matter coupling and illustrate that this coupling can become a very powerful tool to design material properties relevant for optoelectronic devices.

Figure: a) dispersion of the array of silver nanoparticles showing a strong resonance. b) dispersion of the same array covered with tetracene. c) Dispersion of the fluorescence from the array with tetracene. The strongest emission is coming from the lower polariton band.

14:30 - 14:45
ID: 261
Oral presentation


Liwei Wang, Lewis Rothberg

University of Rochester, Rochester, United States of America

We study the single chain spectroscopy of poly(9,9-dioctylfluorenylco-bithiophene) and its biselenophene analog (F8T2 and F8Se) using confocal fluorescence microscopy. These systems have been used extensively as donors in organic photovoltaics. Using simultaneous detection by a CCD and avalanche photodiode, we record spectra, luminescence intensities and excited state lifetimes. We have done an extensive analysis to correlate these with one another and we try to interpret these in terms of the Spano formalism [1] and previous work on the bulk system [2]. In particular, we find two characteristic photoluminescence patterns in F8T2, one that appears to have intrachain J-like character (larger 0-0 vibronic band and faster decay) and the other that has more H-like characteristics. The selenium analog exhibits similar behavior but with much shorter excited state lifetimes. Of specific interest to us is understanding the extent to which energy transfer is important and how that is modulated by chain morphology. Based on the relative lack of spectral evolution, we conclude that energy transfer plays a much smaller role in these systems than in our previous work on MEH-PPV. The effects of nitrogen and oxygen ambients on the single chain behavior are also investigated.

[1] Spano, F.C.; Silva, C., Ann. Rev. Phys. Chem. 2014, 65, 477-500.

[2] Kettner, O. etal, arXiv:1808.06229 DOI 10.1016/j.synthmet.2016.06.010

14:45 - 15:10
ID: 275
Oral presentation


Derya Baran

King Abdullah University of Science and Technology (KAUST), Thuwal, Saudi Arabia

Ternary organic solar cells (TSCs) contain a single three-component photoactive layer with a wide absorption window, which is obtained without the need for multiple-stacks. Subsequently, TSCs have attracted great interest in the photovoltaics field. Through careful selection of the three (or more) active components that form the photoactive layer, all photovoltaic parameters can be simultaneously enhanced within a TSC — a strategy that has resulted in record single-junction efficiencies along with improved photo and thermal stability. In this talk, I will outline key developments in TSCs, with a focus on the central role of the third component in achieving record efficiencies as well as stable bulk-heterojunction and reliable devices. I analyse the effects of the third component on the nanomorphology of the bulk heterojunction and the photovoltaic parameters of TSCs. Finally, I provide a perspective on the advantages of ternary blends and suggest design strategies for highly efficient and stable devices for commercial photovoltaics.

15:10 - 15:25
ID: 131
Oral presentation


Tracey Clarke

University College London (UCL), London, United Kingdom

Diketopyrrolopyrrole (DPP) is one of the most common building blocks for small molecules and conjugated polymers designed for organic electronic applications. Transient absorption spectroscopy (TAS) and time-resolved electron paramagnetic resonance (TR-EPR) spectroscopy were used to examine bulk heterojunction blend films of a small diketopyrrolopyrrole-based molecule, TDPP, with the common fullerene derivatives PC60BM and PC70BM. Following pulsed laser excitation, the spectral signatures of a fullerene anion and a TDPP triplet state are observed on the picosecond timescale by TAS. The presence of these species imply the formation of a TDPP: PCBM charge transfer state that subsequently undergoes ultra-fast spin-mixing and geminate recombination to produce a TDPP triplet state. The overall photophysical mechanism is confirmed by TR-EPR spectroscopy, which unambiguously shows that the TDPP triplet is formed via spin-mixing in the TDPP: PCBM charge transfer state, rather than direct intersystem crossing from the excited singlet state.

Furthermore, ultra-low band gap polymers INDT were investigated further using transient absorption spectroscopy (TAS) and pump-push photocurrent measurements. Different fullerenes were trialled to assess the effect on charge photogeneration. The LUMO levels of the donor and acceptor are almost isoenergetic for PC60BM (implying virtually zero driving force for charge separation) and this is reflected in inefficient charge photogeneration. A ketolactam fullerene with a deeper LUMO produces a higher level of charge photogeneration. Interestingly, it was discovered that the INDT polymers may possibly generate an intramolecular CT state-like singlet exciton, which is only able to be efficiently separated in the presence of a fullerene with a deep enough LUMO.