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2010 was a no-cost extension year of our project that was concluded in 2009. We are not repeating here the details of what was accomplished, referring to the old reports instead. In summary, we studied the primary photoexcitation in rubrene crystals, and identified two exciton dissociation mechanisms that lead to free carriers. One of the two mechanisms was found to involve a long-lived state and to be very efficient, transforming a majority of the photoexcited excitons into free carriers within a thin layer close to the surface of the crystal, which we assigned to interactions of the photoexcited excitons with oxygen induced defects. The other mechanism involved a minority of the photoexcited excitons, but lead to a rapid dissociation within the lifetime of the singlet excitons. The results of this research have been published in Physical Review Letters, Physical Review B, and Applied Physics Letters.

Having recognized the importance of excitons and their dynamics in rubrene, we worked during this extension year towards developing new experimental approaches that would allow us to focus our studies on identifying the types of excitons that are photogenerated, and on studying exciton diffusion, which is the main mechanisms with which excitons can reach the defect states that allow them to dissociate.

Investigation of photoluminescence dynamics.

We extended the time-resolved measurement of the photoluminescence dynamics that we started in the previous years to study the evolution of the slow component of the photoluminescence. These results are still at a preliminary stage, but we can already say that the photoluminescence decay proves that it is caused by triplet excitons that collide with each other to re-create singlet excitons that then decay radiatively. These triplet excitons have a lifetime larger than 10 microseconds and are likely the main diffusing exciton species in rubrene.

Investigation of new ways to detect exciton diffusion and measure exciton diffusion length.

This was a completely new idea to study exciton diffusion, and we started to develop an experimental setup to test it. The idea is very simple, and consists in spatially resolved imaging of the photoluminescence under conditions of localized excitation. By comparing the spatial profile of the excitation light to that of the photoluminescence light it should be possible to tell if the photoluminescence originates from excitons that have diffused away from the excitation point. We now obtained very encouraging preliminary results, and we hope that we will be able to develop this imaging technique into a new and powerful tool for the investigation of exciton diffusion.