Articles you may be interested inUltrafast exciton dynamics in free standing 200 nm thin tetracene single crystals were studied at room temperature by femtosecond transient absorption spectroscopy in the visible spectral range. The complex spectrally overlapping transient absorption traces of single crystals were systematically deconvoluted. From this, the ultrafast dynamics of the ground, excited, and transition states were identified including singlet exciton fission into two triplet excitons. Fission is generated through both, direct fission of higher singlet states S n on a sub-picosecond timescale, and thermally activated fission of the singlet exciton S 1 on a 40 ps timescale. The high energy Davydov component of the S 1 exciton is proposed to undergo fission on a sub-picoseconds timescale. At high density of triplet excitons their mutual annihilation (triplet-triplet annihilation) occurs on a <10 ps timescale.
Two dimensional band structure mapping of organic single crystals using the new generation electron energy analyzer ARTOF. Spectroscopy and Related Phenomena, Journal of Electron AbstractWe report on a novel type of photoemission detector, the Angle Resolved Time Of Flight electron energy analyzer (ARTOF), which enables electronic band structure determination under measurement conditions that are ideal for radiation-sensitive samples. This is facilitated through the combination of very high electron transmission and wide accessible angular range in one geometry. These properties make the ARTOF predestined to investigate specimens that strongly suffer from radiation damage during photoemission experiments under "standard"conditions, such as organic single crystals, as extremely low fluxes can be used while not compromising spectra accumulation times and signal-to-noise ratio. Even though organic single crystals are of increasing fundamental and applied scientific interest, knowledge of their electronic properties is still largely based on theoretical calculations due to major experimental challenges in measuring photoemission. In this work we show that the band structures of rubrene and tetracene single crystals can be obtained with unprecedented quality 2 using the ARTOF detector. The dispersion of the highest occupied band in rubrene is confirmed in accordance with an earlier report [1] and we disclose the absence of notable dispersion for the highest occupied energy level in the tetracene single crystal.
The transport of optically excited states, also referred to as excitons, defines a critical step in the light-to-charge carrier conversion in organic materials. [1] To overcome limitations by the short diffusion lengths of singlet excitons, the harvesting of triplet excitons generated via singlet fission has been suggested as a possible approach to increase the conversion efficiency in organic photovoltaic cells. [2,3] Furthermore, upon further miniaturization of up-to-date organic electronic devices the correlation between exciton transport and morphological length scales becomes increasingly important as the structural coherence significantly affects the photo-physical processes and their time-evolution. [1,4,5] Here, we demonstrate the influence of spatial confinement on the excitonic states and their dynamics in the prototypical organic semiconductor rubrene. By its unique property of providing morphologies with different characteristic length scales we observe direct structural influence on the temperature dependent Photoluminescence (PL) spectra and on the relaxation processes on 10 -12 s time scales already for excitation volumes with m lateral extensions. Our results highlight the role of the local environment which affects the exciton dynamics in organic semiconductors and which has to be considered upon tailoring the microscopic morphology of organic optoelectronic devices.Besides its remarkable photophysical behavior, characterized e.g. by singlet fission in combination with extended triplet lifetimes ( s) and extended diffusion lengths ( m), [6][7][8] rubrene offers the possibility of different, well-controllable structural morphologies in combination with sufficiently high PL quantum yields. In thin films without further surfactants rubrene grows x-ray amorphous due to an activation barrier of 210 meV [9] required for planarization of its conjugated backbone. In contrast, single crystals grown by vapor sublimation under streaming carrier gas show an exceptional structural quality characterized by m-sized crystalline facets as well as lateral extensions of several millimeters.[10]Adjusting the conditions during growth allows for a broad morphological variety of microstructures in-between, ranging from spatially defined microcrystals to self-organized pyramidal surface structures, [11] all of which imposing different boundary conditions for photogenerated excitons.A recent study on the directionality of the steady state absorption and emission of rubrene suggests that variations of the PL spectra detected on microstructures originate from the anisotropic nature of emission. [12] Though the relative intensities can be explained by this approach it is insufficient to account for the temperature dependent optical phenomena and their temporal evolution on short time scales reported in this work.Following pulsed laser excitation we probed the temperature dependence of the photoluminescence decay dynamics of three types of rubrene morphologies with different structural hierarchies -bulk crystals, microcryst...
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