Effects of characteristic length scales on the exciton dynamics in rubrene single crystals

We present temperature dependent time-resolved photoluminescence (PL) investigations on well-defined morphologies of the prototypical organic semiconductor rubrene. By their respective degree of spatial constraint these morphologies directly influence the temperature dependent excitonic processes and their dynamics. While in bulk single crystals singlet exciton decay is governed by thermally activated fission at a time constant of 20 ps, this mechanism appears to be absent in rubrene microcrystals. Here the dynamics are characterized by a pronounced increase of the average exciton lifetime as confirmed by the dominating PL decay channel with an effective time constant of 100 ps. The enhanced surface-to-volume ratio indicates that the participating states might originate from microcrystal boundaries which could be reached by the substantial amount of migrating excitons prior to the onset of other decay processes. The suppression of singlet fission in these crystalline microstructures is promoted by the significantly lower activation energy of 25 meV for the 100 ps channel compared to the singlet fission barrier of 44 meV and imposes severe consequences for its utilization in, e.g., thin film photovoltaics. For the crystalline samples, an additional relaxation channel with a time constant of around 500 ps becomes relevant at very low temperatures. As this process is the only one observed for amorphous rubrene thin films it points at the local nature of the underlying decay mechanism.

Figure 1

(a) Pictures of different samples and related morphologies of the excitation volumes schematically illustrating the effect of spatial confinement and different length scales on the created excitons. (b) Time-integrated PL spectra of rubrene bulk and microcrystals as well as of a thin amorphous film together with bulk crystal and amorphous film absorption (all spectra are normalized). The inset shows the molecular structure of rubrene together with the crystallographic directions of its unit cell.

Figure 2

(a) Integrated PL intensity spectra exhibiting a dramatic increase of PL signal with decreasing temperature. The numbers in brackets depict the scaling factor for the respective PL spectrum. For the two crystalline morphologies, the first spectra (298 and 200 K for bulk; 298, 200, and 150 K for microcrystal) were multiplied by a factor of 6 and 2, 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) Bulk transients taken at 2.16 and 2.02 eV for two different excitation powers exhibit comparable decay dynamics indicating the presence of just one emitting excitonic species. (c) The initial PL peak height of the bulk morphology scales linearly with excitation power excluding bimolecular recombination processes as well as delayed luminescence. (d) Fits of the transient PL data taken at 2.16 eV for all three morphologies at room temperature, 150, and 50 K. The temperature dependent decay evidences the presence of thermally activated nonradiative decay channels.