The photovoltaic effect in organic solar cells is a multistep process involving both bulk and interfacial phenomena. In order to probe the local mechanisms of photocurrent production in organic semiconductors, we studied the influence of light on the junction between the tip of a scanning tunneling microscope (STM) and an organic thin film. We report here on the local through-space I-V and I-(Delta) Z characteristics ((Delta) Z=tip-sample distance) of pentacene (5A), a material with potential applications in photovoltaic solar cells. These systems behave as metal-insulator-semiconductor (MIS) tunnel junctions. The influence of light is investigated by coupling the organic nano-junctions to a laser beam. Monitoring the tip-sample distance (Delta) Z at the angstrom level (i.e. over the 0-20 angstrom range), allows to tune the I-V characteristics of these nanoscale organic tunnel junctions both in the dark and under illumination.
The performance of novel organic devices such as organic light-emitting diodes or organic field-effect transistors is intimately connected to the nature and dynamics of the charge carriers in the device components. Carrying out intercalation studies of solid model oligomers, it is experimentally demonstrated that the low lying electronic excitations in p-type doped systems are significantly confined on the individual molecules due to polaronic effects and thus deserve the name polaron excitations. These results allow for a quantitative experimental estimate of the charge carrier (polaron) extension which is of the order of 20 Angstrom . In addition, it is shown that electron correlation effects play an important role in the determination of the band gap of molecular organic semiconductors. The implications of these results for organic devices are discussed.
We evidence optically pumped spectral narrowing in ultra thin (3.5 microns) single crystals of (alpha) -octithiophene ((alpha) -8T), the luminescent conjugated octamer of thiophene. Experiment is achieved in a transverse pumping mode. It occurs at a pump energy threshold of approximately 0.1 (mu) J per pulse (33 ps, 10 Hz) to yield an intense and narrow emission line peaking at 700 nm. At pump energies higher than 15 (mu) J per pulse, a second narrow line of weaker intensity emerges at 640 nm. Gain narrowing of these two lines can be easily described in the usual framework of stimulated emission (SE). Light amplification originates from the combination of a net dipole alignment and efficient wave-guiding towards the edges of the crystal. The (alpha) -8T molecules provide both the emitter and the optical cavity. Spectral SE selection of the two SE lines at 640 and 700 nm can be monitored by simply scanning the spatial position of the pump beam onto the surface of the crystal. Similar SE phenomena are also observed in the (alpha) -4T and (alpha) 6T crystals, showing that conjugated oligothiophenes ((alpha) -nT) constitute a family of monolithic organic single crystals naturally adapted to laser action. Additionally, in a longitudinal pumping gain guiding situation, nanojoule-threshold infrared two-photon pumped up- converted stimulated emission is demonstrated in the oligothiophene crystals.
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