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Our project aimed at using several laser-based techniques to investigate the physical processes leading to the ionization of photoinduced excitons in a molecular single crystal, rubrene. Our investigation exploited the wavelength tunability of our picosecond laser to investigate photoexcitation under different conditions of photon energy and absorption length at the surface of a rubrene crystal.

This work was performed with the help of a postdoctoral researcher, Dr. Hikmat Najafov, who was hired for a year, starting in May 2007, but who already contributed to this research part-time earlier while working on another project, and is still contributing now because he moved to Rutgers University, to the group of Prof. Podzorov, with whom we have been collaborating for a long time on this research. It is the group of Prof. Podzorov that is responsible for the production of the high quality rubrene single crystals that we investigated. In addition, a graduate student, Byunggook Lyu, contributed to the project with most of his support coming from a teaching assistantship.

Short-pulse induced photocurrent in rubrene, when excited at low powers, shows a delayed onset, with a negligible photocurrent just after the excitation pulse illuminated the crystal, and a photocurrent maximum that is reached more than 100 microseconds later before decaying with an exponential decay constant of approximately 10 ms. By analyzing the build-up time of this current as a function of illumination intensity and wavelength, we have been able to demonstrate that this delayed current is caused by the ionization of a "reservoir state" that is formed by the interaction of a photoexcited exciton with a defect state probably connected with oxidation. This defect state is found in a surface layer that has a thickness of about 10 micrometers. We found that the build-up time of the current depends on the initial density of photoexcited excitons because of quadratic recombination of the released charge carriers. Because of the dependence of excitation density from the wavelength-dependent absorption in rubrene, this causes the build-up time to depend on both the wavelength and the light intensity.

By comparing the amount of photons in the laser pulse to the number of electrons that flowed through the electrical circuit after illumination we were able to estimate that the quantum efficiency for exciton ionization through the "reservoir" effect described above is close to unity! Such an efficient exciton ionization process is unique among molecular organic materials, where excitons have a comparatively large binding energy of the order of 0.5 eV. The fact that exciton ionization is so efficient in rubrene explains the large photocurrents that can be measured in this material and is very interesting because it may offer a clue to new ways of solving the exciton ionization bottleneck, which is one the biggest problems facing the development of efficient organic solar cells.

As mentioned above, the delayed photocurrent in rubrene is excited very efficiently in a surface layer and is the dominant effect. However, there is also a faster, smaller photocurrent component that appears in less than 10 nanoseconds after photoexcitation, with the observed rise time limited by the time-resolution of our apparatus. This fast photocurrent component corresponds to less than 10% of the delayed photocurrent caused by surface oxidation that was discussed above. By comparing the relative strengths of the fast component and the delayed component under various experimental conditions of wavelength, light-intensity, temperature, and surface treatment, we have been able to demonstrate that the fast photocurrent is caused by exciton interaction with defect states that are unrelated to the oxygen induced defect that we identified close to the surface of rubrene. It is likely that the exciton is ionized during its lifetime before radiative recombination by interaction with defects present in the bulk of the rubrene crystal. An important point is that we were able to exclude the possibility of the fast photocurrent being caused by excitons immediately ionizing (within picoseconds) into charge carriers that are subsequently trapped (in just a few nanoseconds) into shallow levels from where they are subsequently thermally re-excited to give rise to the delayed photocurrent. The only consistent way of interpreting our data is that the photoinduced exciton can be directly stabilized (without emission of any free charge carriers) at a defect state, creating the "reservoir" that only later leads to free carriers through thermal excitation. Only a few of the photoexcited excitons - approximately less than 10% under typical experimental conditions - is ionized within their lifetime as molecular excitons to produce the small photocurrent component that we observed on the nanosecond scale.

The finding that most of the excitons created in rubrene do not ionize during their lifetime is consistent with current understanding of excitons in molecular materials but is at odds with the results of other investigators that have observed the signature of free carriers within a picosecond from the excitation with femtosecond pulses. The reason for this apparent discrepancy is not yet known, and is the subject of ongoing investigations.

In conclusion, rubrene single crystals are characterized by two distinct exciton ionization mechanisms: one mechanism leads to ionization of the exciton during its lifetime of a few nanoseconds, and is probably caused by interaction with bulk defects, while the other mechanism is related to surface oxidation, with a very efficient emission of free carriers that occurs several tens of microseconds after the creation of the exciton. While the exact nature of these defects responsible for exciton ionization is not clear yet, further studies on rubrene and the nature of the defect states we identified should be very interesting for the insights they can give on important excitonic processes in molecular crystals.