Dependence of Phosphorescent Emitter Orientation on Deposition Technique in Doped Organic Films

Dependence of Phosphorescent Emitter Orientation on Deposition Technique in Doped Organic Film

From Simple Ligands to Complex Structures: Structural Diversity of Silver(I) Complexes Bearing Tetradentate (^alkylenebimpy) NHC Ligands

The synthesis of five new silver­(I) NHC complexes, bearing ^alkylenebimpy ligands with bridge lengths ranging from ethylene to hexylene is described. Their solid-state structures exhibit an unforeseen variety, including infinite strands of Ag₃ units within a helically wound ligand environment. The very first infinite chain of almost linear Ag₃ units within a cage of helically wound ligand molecules is described. All complexes are spectroscopically characterized and emit polychromatic blue light (420 nm ≤ λ ≤ 510 nm) after laser excitation at λ = 337 nm. This latter behavior might lead to applications in the field of organic light-emitting diodes

Tuning the Microcavity of Organic Light Emitting Diodes by Solution Processable Polymer–Nanoparticle Composite Layers

In this study, we present a simple method to tune and take advantage of microcavity effects for an increased fraction of outcoupled light in solution-processed organic light emitting diodes. This is achieved by incorporating nonscattering polymer–nanoparticle composite layers. These tunable layers allow the optimization of the device architecture even for high film thicknesses on a single substrate by gradually altering the film thickness using a horizontal dipping technique. Moreover, it is shown that the optoelectronic device parameters are in good agreement with transfer matrix simulations of the corresponding layer stack, which offers the possibility to numerically design devices based on such composite layers. Lastly, it could be shown that the introduction of nanoparticles leads to an improved charge injection, which combined with an optimized microcavity resulted in a maximum luminous efficacy increase of 85% compared to a nanoparticle-free reference device

Tunable Anisotropic Photon Emission from Self-Organized CsPbBr₃ Perovskite Nanocrystals

We report controllable anisotropic light emission of photons originating from vertically aligned transition dipole moments in spun-cast films of CsPbBr₃ nanocubes. By depositing films of nanocrystals on precoated substrates we can control the packing density and resultant radiation pattern of the emitted photons. We develop a technical framework to calculate the average orientation of light emitters, i.e., the angle between the transition dipole moment vector (TDM) and the substrate. This model is applicable to any emissive material with a known refractive index. Theoretical modeling indicates that oriented emission originates from an anisotropic alignment of the valence band and conduction band edge states on the ionic crystal lattice and demonstrates a general path to model the experimentally less accessible internal electric field of a nanosystem from the photoluminescent anisotropy. The uniquely accessible surface of the perovskite nanoparticles allows for perturbation of the normally isotropic emissive transition. The reported sensitive and tunable TDM orientation and control of emitted light will allow for applications of perovskite nanocrystals in a wide range of photonic technologies inaccessible to traditional light emitters

<i>V</i>_oc from a Morphology Point of View: the Influence of Molecular Orientation on the Open Circuit Voltage of Organic Planar Heterojunction Solar Cells

The film morphology and device performance of planar heterojunction solar cells based on the molecular donor material α-sexithiophene (6T) are investigated. Planar heterojunctions of 6T with two different acceptor molecules, the C₆₀ fullerene and diindenoperylene (DIP), have been prepared. The growth temperature of the 6T bottom layer has been varied between room temperature and 100 °C for each acceptor. By means of X-ray diffraction and X-ray absorption, we show that the crystallinity and the molecular orientation of 6T is influenced by the preparation conditions and that the 6T film templates the growth of the subsequent acceptor layer. These structural changes are accompanied by changes in the characteristic parameters of the corresponding photovoltaic cells. This is most prominently observed as a shift of the open circuit voltage (<i>V</i>_oc): In the case of 6T/C₆₀ heterojunctions, <i>V</i>_oc decreases from 0.4 to 0.3 V, approximately, if the growth temperature of 6T is increased from room temperature to 100 °C. By contrast, <i>V</i>_oc increases from about 1.2 V to almost 1.4 V in the case of 6T/DIP solar cells under the same conditions. We attribute these changes upon substrate heating to increased recombination in the C₆₀ case while an orientation dependent intermolecular coupling seems to change the origin of the photovoltaic gap in the DIP case