Rational Molecular Design Enables Efficient Blue TADF−OLEDs with Flexible Graphene Substrate

Observation of thermally activated delayed fluorescence (TADF) in conjugated systems redefined the molecular design approach to realize highly efficient organic light emitting diodes (OLEDs) in the early 2010s. Enabling effective reverse intersystem crossing (RISC) by minimizing the difference between singlet and triplet excited state energies (ΔEST) is proven to be a widely applicable and fruitful approach, which results in remarkable external quantum efficiencies (EQE). The efficacy of RISC in these systems is mainly dictated by the first-order mixing coefficient (λ), which is proportional to spin-orbit coupling (HSO) and inversely proportional to ΔEST. While minimizing ΔEST has been the focus of the OLED community over the last decade, the effect of HSO in these systems is largely overlooked. Here, molecular systems with increased HSO are designed and synthesized by substituting selected heteroatoms of high-performance TADF materials with heavy-atom selenium. A new series of multicolor TADF materials with remarkable EQEs are achieved. One of these materials, SeDF-B, results in pure blue emission with EQEs approaching 20%. Additionally, flexible graphene-based electrodes are developed for OLEDs and revealed to have similar performance as standard indium tin oxide (ITO) in most cases. These devices are the first report of TADF based OLEDs that utilize graphene-based anodes

Wrinkling of graphene because of the thermal expansion mismatch between graphene and copper

Well-defined bundles of wrinkles are observed on the graphene-covered copper by using atomic force microscopy after chemical vapor deposition process. Their numerical analyses are performed by employing a set of formula deduced from classical elasticity theory of bent thin films with clamped boundary conditions. Here they are imposed by the banks of trenches associated with the reconstructed copper substrate surfaces, which suppress lateral movements of graphene monolayers and induce local biaxial stress. The wrinkling wavelength (lambda) and amplitude (A) are both measured experimentally (lambda = 100-160 nm and A = 2.5-3 nm) and calculated numerically (lambda = 167 nm and A = 3.0 nm) and found to be in good agreement. Wrinkle formation is attributed to the nonhydrostatic compression stresses induced on the graphene by the linear thermal expansion coefficient difference between graphene and copper during cooling. These mismatch stresses, which are varying strongly with the temperature, create temperature-dependent wrinkling wave formation that decreases in wavelength and increases in amplitude upon cooling below the crossover temperature of 1233 K, at which both values of linear thermal expansion coefficient are equal

Steady state thermokinetic of ultra-thin Mo2C/G heterostructures grown on the prior-graphitized cu/graphene biasing

In this work, the chemical vapor deposition synthesis of the Mo2C/graphene heterostructure above the melting temperature of Cu bias (1356 K) is studied. Two sets of Mo2C growth experiments at high CH4 flow rates (5 SCCM ≥ 3 SCCM) are performed, either using prior-graphene synthesis or having in situ graphitization, for three different Cu bias thicknesses. Raman mappings taken from all six-test samples show graphene covers not only over the Mo2C pillars but also over their untransformed Cu bias substrate regions. The only difference is that the Mo2C pillar grows over the prior graphene bias; on the other hand, the in situ graphene grown Mo2C pillar nucleates and grows over the fresh Cu bias surfaces. A steady-state laminate model for flows of Mo and C species with phase transformations is developed for the radial and vertical growth kinetics of synthesized Mo2C/graphene heterostructure. The computer simulation reproduces those experimental observations performed recently in our laboratories on the prior or no-prior graphitized (G) test modules with Cu/G bias, having three different thicknesses at 1363 K. AFM-topography and SEM photos for a prior graphitized test module of 25 µm thick Cu and 4.72 Å graphene bias show a three layered Mo2C/graphene heterostructure; the first layer is almost perfect hexagonal flat, and the other two circular shaped layers constitute the whole pillar of 140 nm height. This may be compared to a 250 µm thick Cu/4.7 Å graphene bias sample, which furnishes an ultra-thin single flat layer of 10–13 nm thick Mo2C crystallites having a perfect planar hexagonal structure