Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userUnraveling the degradation mechanism of FIrpic based blue OLEDs: I. A theoretical investigation
496
0
0.0
(
0
)
Ask ChatGPT about the research
No Arabic abstract
We report a detailed ab-initio study of the of the microscopic degradation mechanism of FIrpic, a popular blue emitter in OLED devices. We simulate the emph{operando} conditions of FIrpic by adding an electron-hole pair (exciton) to the system. We perform both static calculations with the TDDFT framework and we also simulate the evolution of the system at finite temperature via Car-Parrinello molecular dynamics. We found triplet excitons are very effective in reducing the Ir-N bond breaking barrier of the picolinate moiety. After the first bond breaking, the two oxygen of picolinate swap their position and FIrpic can either remain stable in an open configuration, or loose a picolinate fragment, which at a later stage can evolve a CO$_2$ molecule. Our method can be applied to other light emitting Ir-complexes in order to quickly estimate their stability in OLED devices. In Paper~II we complement our theoretical study with a parallel experimental investigation of the key degradation steps of FIrpic in an aged device.
rate research
Read More
The impact of organic light emitting diodes (OLEDs) in modern life is witnessed by their wide employment in full-color, energy-saving, flat panel displays and smart-screens; a bright future is likewise expected in the field of solid state lighting. Cyclometalated iridium complexes are the most used phosphorescent emitters in OLEDs due to their widely tunable photophysical properties and their versatile synthesis. Blue-emitting OLEDs, suffer from intrinsic instability issues hampering their long term stability. Backed by computational studies, in this work we studied the sky-blue emitter FIrpic in both ex-situ and in-situ degradation experiments combining complementary, mutually independent, experiments including chemical metathesis reactions, in liquid phase and solid state, thermal and spectroscopic studies and LC-MS investigations. We developed a straightforward protocol to evaluate the degradation pathways in iridium complexes, finding that FIrpic degrades through the loss of the picolinate ancillary ligand. The resulting iridium fragment was than efficiently trapped in-situ as BPhen derivative 1. This process is found to be well mirrored when a suitably engineered, FIrpic-based, OLED is operated and aged. In this paper we (i) describe how it is possible to effectively study OLED materials with a small set of readily accessible experiments and (ii) evidence the central role of host matrix in trapping experiments.
Instability of perovskite photovoltaics is still a topic which is currently under intense debate, especially the role of water environment. Unraveling the mechanism of this instability is urgent to enable practical application of perovskite solar cells. Here, ab initio metadynamics is employed to investigate the initial phase of a dissolution process of CH$_3$NH$_3$PbI$_3$ (MAPbI$_3$) in explicit water. It is found that the initial dissolution of MAPbI$_3$ is a complex multi-step process triggered by the departure of I$^-$ ion from the CH$_3$NH$_3$I-terminated surface. Reconstruction of the free energy landscape indicates a low energy barrier for water dissolution of MAPbI$_3$. In addition, we propose a two-step thermodynamic cycle for MAPbI$_3$ dissolution in water at a finite concentration that renders a spontaneity of the dissolution process. The low energy barrier for the initial dissolution step and the spontaneous nature of MAPbI$_3$ dissolution in water explain why the water immediately destroys pristine MAPbI$_3$. The dissolution thermodynamics of all-inorganic CsPbI$_3$ perovskite is also analyzed for comparison. Hydration enthalpies and entropies of aqueous ions play an important role for the dissolution process. Our findings provide a comprehensive understanding to the current debate on water instability of MAPbI$_3$.
The electronic structure of nanolaminate Ti2AlN and TiN thin films has been investigated by bulk-sensitive soft x-ray emission spectroscopy. The measured Ti L, N K, Al L1 and Al L2,3 emission spectra are compared with calculated spectra using ab initio density-functional theory including dipole transition matrix elements. Three different types of bond regions are identified; a relatively weak Ti 3d - Al 3p bonding between -1 and -2 eV below the Fermi level, and Ti 3d - N 2p and Ti 3d - N 2s bonding which are deeper in energy observed at -4.8 eV and -15 eV below the Fermi level, respectively. A strongly modified spectral shape of 3s states of Al L2,3 emission from Ti2AlN in comparison to pure Al metal is found, which reflects the Ti 3d - Al 3p hybridization observed in the Al L1 emission. The differences between the electronic and crystal structures of Ti2AlN and TiN are discussed in relation to the intercalated Al layers of the former compound and the change of the materials properties in comparison to the isostructural carbides.
Paramagnetic molecules can show long spin-coherence times, which make them good candidates as quantum bits. Reducing the efficiency of the spin-phonon interaction is the primary challenge towards achieving long coherence times over a wide temperature range in soft molecular lattices. The lack of a microscopic understanding about the role of vibrations in spin relaxation strongly undermines the possibility to chemically design better performing molecular qubits. Here we report a first-principles characterization of the main mechanism contributing to the spin-phonon coupling for a class of vanadium(IV) molecular qubits. Post Hartree Fock and Density Functional Theory are used to determine the effect of both reticular and intra-molecular vibrations on the modulation of the Zeeman energy for four molecules showing different coordination geometries and ligands. This comparative study provides the first insight into the role played by coordination geometry and ligand field strength in determining the spin-lattice relaxation time of molecular qubits, opening the avenue to a rational design of new compounds.
We present computer simulations of liquid and solid phases of condensed methane at pressures below 25 GPa, between 150 and 300 K, where no appreciable molecular dissociation occurs. We used molecular dynamics (MD) and metadynamics techniques, and empirical potentials in the rigid molecule approximation, whose validity was confirmed a posteriori by carrying out it ab initio MD simulations for selected pressure and temperature conditions. Our results for the melting line are in satisfactory agreement with existing measurements. We find that the fcc crystal transforms into a hcp structure with 4 molecules per unit cell (B phase) at about 10 GPa and 150 K, and that the B phase transforms into a monoclinic high pressure phase above 20 GPa. Our results for solid/solid phase transitions are consistent with those of Raman studies but the phase boundaries estimated in our calculations are at higher pressure than those inferred from spectroscopic data.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
