single-photon emitters have been considered for applications in quantum information processing, quantum cryptography and metrology. For the sake of integration and to provide an electron photon interface, it is of great interest to stimulate single-photon emission by electrical excitation as demonstrated for quantum dots. Because of low exciton binding energies, it has so far not been possible to detect sub-Poissonian photon statistics of electrically driven quantum dots at room temperature. However, organic molecules possess exciton binding energies on the order of 1 eV, thereby facilitating the development of an electrically driven single-photon source at room temperature in a solid-state matrix. Here we demonstrate electroluminescence of single, electrically driven molecules at room temperature. By careful choice of the molecular emitter, as well as fabrication of a specially designed organic lightemitting diode structure, we were able to achieve stable single-molecule emission and detect sub-Poissonian photon statistics.

Motivated by the possibility of modifying energy levels of a molecule without substantially changing its band gap, the impact of gradual fl uorination on the optical and structural properties of zinc phthalocyanine (F n ZnPc) thin fi lms and the electronic characteristics of F n ZnPc/C 60 ( n = 0, 4, 8, 16) bilayer cells is investigated. UV-vis measurements reveal similar Q-and B-band absorption of F n ZnPc thin fi lms with n = 0, 4, 8, whereas for F 16 ZnPc a different absorption pattern is detected. A correlation between structure and electronic transport is deduced. For F 4 ZnPc/C 60 cells, the enhanced long range order supports fi ll factors of 55% and an increase of the short circuit current density by 18%, compared to ZnPc/C 60 . As a parameter being sensitive to the organic/organic interface energetics, the open circuit voltage is analyzed. An enhancement of this quantity by 27% and 50% is detected for F 4 ZnPc-and F 8 ZnPc-based devices, respectively, and is attributed to an increase of the quasi-Fermi level splitting at the donor/acceptor interface. In contrast, for F 16 ZnPc/C 60 a decrease of the open circuit voltage is observed. Complementary photoelectron spectroscopy, external quantum effi ciency, and photoluminescence measurements reveal a different working principle, which is ascribed to the particular energy level alignment at the interface of the photoactive materials.

Photoluminescence quenching of single dibenzoterrylene (DBT) dye molecules in a polymeric organic light-emitting diode was utilized to analyze the current dynamics at nanometer resolution. The quenching mechanism of single DBT molecules results from an increase in the triplet-state population induced by charge carrier recombination on individual guest molecules. As a consequence of the long triplet-state relaxation time, its population results in a reduced photoluminescence of the dispersed fluorescent dyes. From the decrease in photoluminescence together with photon correlation measurements, we could quantify the local current density and its time-dependent evolution in the vicinity of the single-molecule probe. This optical technique establishes a non-invasive approach to map the time-resolved current density in organic light-emitting diodes on the nanometer scale.

Single molecule studies are limited to a defined class of organic dye molecules inserted into respective host materials. Basic requirements for suited material combinations include high photon emission rates and long term photostability. A majority of known aromatic host-guest systems employ crystalline organic matrices to prevent dye molecules from uncontrolled reactions with contaminants. However, in terms of device fabrication and technological potentials it is often desirable to use polymers as room temperature host matrices. Unfortunately, single dye molecule investigations in polymers at room temperature usually report orders of magnitude lower photostabilities compared to their crystalline molecular counterparts, leading to a reduced interest in organic thin film applications. In this report, we exemplary demonstrate the feasibility of engineering a host-guest system based on dibenzoterrylene dye molecules which were diluted into the polymer poly-( p-phenylene-vinylene) (PPV) possessing very low photobleaching probabilities at room temperature. By controlling the oxygen exposure during manufacturing the number of emitted photons prior to photobleaching was significantly increased from 10 6 up to 10 11 photons. Employing suited encapsulation techniques to prevent oxygen penetration after host-guest preparation, photostable devices over prolonged time periods on the order of months to years could be achieved. Therefore, this approach grants access to a variety of new polymer based combinations of host-guest systems for studying single molecular quantum emitters inside organic electronic devices and nanostructured polymer films with sufficient count rates and long-term stability at room temperature.

Optical studies of individual molecules offer the unique possibility of investigating the local environment of single quantum objects on nanometre length scales and of employing molecular systems as non-classical light sources at room temperature. Usually, single molecule excitation is based on optical stimulation by laser radiation. In this feature article, we present an alternative approach by utilizing charge injection in combination with molecular electron-hole recombination to electrically excite single fluorescent dyes. The successful implementation of this strategy promotes the realization of electrically driven single photon sources on demand operating at room temperature and being feasible on a vast number of molecules emitting at e.g. telecommunication wavelengths. Moreover, these probes render probing the charge carrier transport and recombination on molecular length scales. In this article, we will first discuss the interaction of single charge carriers with optically excited molecules characterized by a decrease of photoluminescence intensity with increasing current density. As it is demonstrated, this effect originates from strong electrical pumping of triplet state populations upon charge carrier recombination. Secondly, the prospect of using phosphorescent emitters as electrically driven single photon sources at room temperature will be discussed. Finally, we will highlight that photoluminescence quenching of single fluorescent dyes provides a sensitive tool to quantify local current densities and recombination dynamics in operational organic light emitting diode devices with molecular spatial resolution.