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channel is effectively suppressed in microcrystal structures due to the presence of an additional decay mechanism related to the boundaries and with only 25 meV activation energy. As a result for microcrystals, excitons reaching the grain boundaries are trapped by surfaces states and transferred to dark states or dissociated into polarons within 100 ps. A third relaxation channel within the nanosecond time regime becomes relevant for long range ordered rubrene stacks at very low temperatures. The local character of this decay path is indicated by its coincidence in time and energy with the only relaxation process observed for amorphous rubrene thin films. Our findings clearly elucidate the effects by the local environment on the dynamic photophysical behavior of organic semiconductors. With respect to spatial dimensions of organic thin film devices, this correlation and the effects emerging by confinement have to be considered in the ongoing miniaturization and optimization of organic opto-electronic devices, such as photovoltaic cells.
Experimental Details: All experimental procedures are described in the Supplementary Information.
Acknowledgements The authors thank Prof. Martin Kamp (Würzburg University) for his assistance on TEM measurements and Stephan Hirschmann (Stuttgart University) for material purification. Financial support by DFG within Research Unit (FOR 1809) “Light induced dynamics in molecular aggregates” and within the contract INST 93/623-1 FUGG is acknowledged.
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Figure 1. a Pictures of different samples and related morphologies of the excitation volumes schematically illustrating the eﬀect of spatial conﬁnement on the created excitons. b Timeintegrated PL spectra of rubrene bulk and microcrystals as well as of a thin amorphous ﬁlm together with bulk crystal and amorphous ﬁlm absorption. The inset shows the molecular structure of rubrene together with the crystallographic directions.
Figure 2. a Integrated PL intensity spectra exhibiting a dramatic increase of PL signal with decreasing temperature. For the two crystalline morphologies, the first spectra (298 and 250 K for bulk; 298-150K for micro) were multiplied by a factor of six and two, respectively. b Changes of the PL spectra normalized to the values at 298 K as a function of temperature. Both crystalline samples exhibit a PL increase upon reducing the temperature and saturation at low temperatures. Contrary, the PL signal of the amorphous ﬁlm, after initial increase, is reduced for temperatures below 150 K.
Figure 3. a Energy level scheme showing the possible relaxation pathways for singlet excitons in rubrene bulk crystals. b Fits of the transient PL data taken at 2.16 eV for all three samples at room temperature, 150 and 50 K. The temperature dependent decay indicates the presence of thermally activated non-radiative decay channels. For the sake of clarity for low temperatures only the fit transients are shown.
Table 1. Parameters determined from the analysis of the temperature dependence of the steady-state and transient PL spectra. The values refer to the energetic barriers and the lifetimes (inverse rates) as well as the associated decay mechanisms for the different rubrene samples studied in this work. Lifetimes in bold letters correspond to the dominant decay channel at room temperature.
RT. Finally heating the substrate for several hours at a constant temperature of 180 C the microcrystals were generated.
density, characterization of the precipitated microcrystals was performed by a scanning electron microscope REM Ultra-Plus (Zeiss) using beam energies between 2 and 20 keV. To analyze the absorption properties of the single crystals spectra were detected by means of a Lambda 950 UV/VIS spectrometer (Perkin Elmer). The amorphous ﬁlm was measured with a V630 UV-VIS spectrophotometer (Jasco). As the samples absorb in the visible spectral range, investigations were restricted to a wavelength range of 300 nm -800 nm (3.0 -1.6 eV). Transmission electron microscopy (TEM) studies on the micro-crystals were carried out with a FEI-TitanTM TEM with beam energies of 300 keV. For this, the microcrystals were deposited and then annealed on a small copper grid. For the time-resolved PL measurements, the output of a Ti:Sa oscillator (Spectra Physics, 100 fs, 800 nm) was frequency doubled and focused onto the sample, which was mounted inside a liquid helium cryostat, using a ﬂuence of 2.6 nJ/cm2. The PL was spectrally dispersed by a spectrograph and detected with a C 5680 streak camera (Hamamatsu). The temporal resolution of the setup described was 8 ps and the detection window limited to 2ns.
Bragg peaks could be found for the film confirming its amorphous character while the measurement of the single crystal clearly indicates the good crystalline quality of the analyzed sample. The spectrum of the smooth amorphous film exhibits Kiessing oscillations, which were used to determine the thickness of the layer. The size and shape of the microcrystals were deduced from the analysis of SEM pictures and the crystalline character was confirmed by carrying out transmission electron microscopy measurements (Fig. 5).
Integrated PL of Crystalline Samples Fig. 6 shows the comparison of the time-integrated PL spectrum of the microstructures with the ones of two bulk single crystals - one with a very smooth (bulk 1) and one with a rougher surface (bulk 2). The observed effective shift of the PL spectrum of the microcrystals is due to an enhanced relative contribution of the high-energy PL peak to the spectrum and not to the blueshift of the whole spectrum.
Amorphous Films With Different Amount of Interfaces Preparing amorphous rubrene ﬁlms of similar thickness and surface ﬂatness but with diﬀerent amounts of interfaces results in completely diﬀerent decay kinetics at room temperature (see Fig. 7a). While the dynamics of the ﬁlm on sapphire (squares) can be described by a monoexponential decay at a time constant of about 0.5 ns, the decay of the sample on glass (circles) is biexponetial with an additional constant of 100 ps. The time-integrated spectra of both samples are however almost identical (inset in Fig. 7a). This diﬀerent decay behaviour is assumed to be a consequence of the diﬀerent sample topographies as observed in AFM measurements (Fig. 7b). One can clearly see an increase of surface area introduced by dislocation boundaries for the amorphous ﬁlm on glass while the ﬁlm on sapphire only shows smooth plateaus.
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Figure 4. a X-ray diﬀraction measurements of a thin amorphous rubrene ﬁlm (above) exhibiting Kiessig oscillations. The inset shows the whole Bragg spectrum where no peaks are detectable. These results conﬁrm the x-ray amorphous character of the film. b The spectrum of a rubrene single crystal is displaying the high crystalline quality of the sample.
Figure 5. The left picture shows a SEM image of hexagonal microcrystals measured in this work with the corresponding TEM spectrum on the right.
Figure 6. Comparison of the integrated PL spectrum of the microcrystals with the corresponding spectra of two bulk crystals with different surface roughness. The effective shift of the spectral weight of the microstructures is due to an increased relative contribution of the high-energy PL peak, rather than to a blueshift.
Figure 7. a PL-transients of an amorphous ﬁlm of rubrene on glass and sapphire taken at 2.18 eV at room temperature and the corresponding integrated PL spectra (inset). While the spectra are almost identical, the decay dynamics are considerably diﬀerent. b AFM surface scans of the two amorphous ﬁlms indicating topographical diﬀerences between the two samples. While the ﬁlm on sapphire exhibits only smooth plateaus, the surface of the ﬁlm grown on glass is rugged by grain boundaries and dislocations.

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