Diamond Materials

Diamond Materials for Quantum Application

23. September 2014: The DFG research group FOR 1493 “Diamond Materials and Quantum Applications” goes into its second funding period. FOR1493 is a national research consortium funded by the Deutsche Forsch-ungsgemeinschaft.

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ERC Advanced Grant


Nanometer scale optical sensing of current densities in OLEDs

In the light of increased commercial and scientific applicability of organic electronic circuitry and in particular organic light emitting diodes (OLEDs), it is of great interest to study fundamental electrical excitation processes and charge carrier dynamics on molecular length scales. Single molecule studies thereby offer the possibility and material compatibility to perform measurements on the level of a single quantum object in organic electronic devices. By doping of single Dibenzoterrylene (DBT) guest molecules into the charge carrier recombination zone of polymeric OLEDs, we were able to demonstrate that a single fluorescent dopant can be used as nanoscale sensor for current densities with nanometer spatial resolution.

While monitoring laser induced photoluminescence of a single DBT molecule during current injection into the emissive layer of a PPV based OLED, we observed that the PL intensity decreases with increasing current density. In contrast to optical laser excitation, electron-hole recombination mechanisms on a single molecule result in a probability of 75% to excite the triplet state manifold compared to only 25% of singlet state stimulation. This phenomenon is related to spin statistics of two recombining fermionic particles (electron and hole) resulting in either a spin 0 or spin 1 exciton. Therefore, electron-hole recombination processes in organic matrices result in a predominant pumping to the molecule’s dark and long living triplet state which leads to a reduction of photoluminescence intensity with increasing recombination rate and current density. Related to the dielectric constant of the polymer matrix, charge carriers only recombine on a single molecular dopant once they are as close as 17 nm to the dopant. Therefore, by measuring voltage dependent single molecule quenching behaviors, a determination of current densities within the molecular environment, or capture cross section, of the molecular sensor is possible. By the study of photon correlations during charge injection it is further possible to measure current densities solely by optical means without the need of any ampere meter.

In contrast to conductive AFM techniques, this approach thereby enables the non-invasive determination of current densities on nanometer length scales in operational OLED devices.

Optical Sensing of Current Dynamics in Organic Light-Emitting Devices at the Nanometer Scale
CHEMPHYSPHEM 10.1002/cphc.201100442

M. Nothaft 1, S. Höhla 2, A. Nicolet 3, F. Jelezko 4, N. Frühauf 2, J. Pflaum 5, J. Wrachtrup 1

1 3rd Physics Institute and Research Center SCoPE, University of Stuttgart, 70550 Stuttgart (Germany)
2 Institute for Large Area Microelectronics and Research Center SCoPE, University of Stuttgart, 70550 Stuttgart (Germany)
3 Laboratoire National des Champs Magnétiques Intenses, 38042 Grenoble (France)
4 Institut für Quantenoptik, University of Ulm, 89069 Ulm (Germany)
5 Experimentelle Physik VI, University of Würzburg and ZAE Bayern, 97074 Würzburg (Germany)