Direct identification of interfacial degradation in blue OLEDs using nanoscale chemical depth profiling

Inkjet Printing of Heterostructures: Investigation and Strategies for Control of Interfaces

Development of printed electronics requires understanding and control of the interfaces in heterostructure devices. However, investigation of the interfaces between dissimilar materials to achieve control of intermixing presents challenges. Here, we report investigation of interfaces in inkjet printed heterostructures by time-of-flight secondary ion mass spectrometry (ToF-SIMS), focused ion beam scanning electron microscopy (FIB-SEM), and energy dispersive X-ray (EDX) analysis to provide complementary insights into the intermixing phenomena. By examining various heterostructures of 0D (CsPbBr3 nanocrystal), 2D (inkjet printed graphene, iGr), and polymeric (PEDOT:PSS) materials deposited with different printing parameters, we established the effect of ink composition and printing parameters on the intermixing depth. We demonstrated that in the heterostructures where the intermixing is dominated by layer porosity, the intermixing depth does not affect the electrical properties of the device, while intermixing by layer redispersion results in the decrease of the effective layer thickness accompanied by an increase of electrical resistance. The strategy for control over the interfacial composition and morphology in printed heterostructures could enable improved design and performance of printed devices.

Atmospheric Pressure Chemical Ionization Q-Orbitrap Mass Spectrometry Analysis of Gas-Phase High-Energy Dissociation Routes of Triarylamine Derivatives

Triarylamine groups have been widely utilized in the development of high-performance charge-transporting or luminescent materials for fabricating organic light-emitting diodes (OLEDs). In this study, atmospheric pressure chemical ionization (APCI) Q-Orbitrap mass spectrometry was adopted to investigate the dissociation behaviors of these triarylamine derivatives. Specifically, taking [M+H]+ as the precursor ion, high-energy collision dissociation (HCD) experiments within the energy range from 0 to 80 eV were carried out. The results showed that triarylamine derivatives with specific structures exhibited distinct fragmentation patterns. For diarylamine, the formation of odd-electron ions was ascribed to the single-electron transfer (SET) reaction mediated by ion-neutral complexes (INCs). In the low-energy range (below 40 eV), proton transfer served as the predominant mechanism for generating even-electron ions. Conversely, in the high-energy range (60 eV and above), the INC-SET reaction dominated. The precursor ion's structure affects compliance with the "even-electron rule", which has exceptions. Here, even-electron ion fragmentation was energy-dependent and could deviate from the rule, yet did not conflict with its concept, reflecting dissociation complexity. This research provides insights for triarylamine-based OLED materials, facilitating analysis and identification, and is expected to aid OLED material development.

Optical properties and exciton transfer between N-heterocyclic carbene iridium(III) complexes for blue light-emitting diode applications from first principles

N-heterocyclic carbene (NHC) iridium(III) complexes are considered as promising candidates for blue emitters in organic light-emitting diodes. They can play the roles of the emitter as well as of electron and hole transporters in the same emission layer. We investigate optical transitions in such complexes with account of geometry and electronic structure changes upon excitation or charging and exciton transfer between the complexes from first principles. It is shown that excitation of NHC iridium complexes is accompanied by a large reorganization energy ∼0.7 eV and a significant loss in the oscillator strength, which should lead to low exciton diffusion. Calculations with account of spin-orbit coupling reveal a small singlet-triplet splitting ∼0.1 eV, whereas the oscillator strength for triplet excitations is found to be an order of magnitude smaller than for the singlet ones. The contributions of the Förster and Dexter mechanisms are analyzed via the explicit integration of transition densities. It is shown that for typical distances between emitter complexes in the emission layer, the contribution of the Dexter mechanism should be negligible compared to the Förster mechanism. At the same time, the ideal dipole approximation, although giving the correct order of the exciton coupling, fails to reproduce the result taking into account spatial distribution of the transition density. For charged NHC complexes, we find a number of optical transitions close to the emission peak of the blue emitter with high exciton transfer rates that can be responsible for exciton-polaron quenching. The nature of these transitions is analyzed.