Direct monitoring of bias-dependent variations in the exciton formation ratio of working organic light emitting diodes

Visualizing electroluminescence process in light-emitting electrochemical cells

Electroluminescence occurs via recombination reactions between electrons and holes, but these processes have not been directly evaluated. Here, we explore the operation dynamics of ionic liquid-based light-emitting electrochemical cells (LECs) with stable electroluminescence by multi-timescale spectroscopic measurements synchronized with the device operation. Bias-modulation spectroscopy, measuring spectral responses to modulated biases, reveals the bias-dependent behavior of p-doped layers varying from growth to saturation and to recession. The operation dynamics of the LEC is directly visualized by time-resolved bias-modulation spectra, revealing the following findings. Electron injection occurs more slowly than hole injection, causing delay of electroluminescence with respect to the p-doping. N-doping proceeds as the well-grown p-doped layer recedes, which occur while the electroluminescence intensity remains constant. With the growth of n-doped layer, hole injection is reduced due to charge balance, leading to hole-accumulation on the anode, after which LEC operation reaches equilibrium. These spectroscopic techniques are widely applicable to explore the dynamics of electroluminescence-devices.

Organic light-emitting diodes: theoretical understanding of highly efficient materials and development of computational methodology

Theoretical understanding of organic light-emitting diodes started from the quest to the nature of the primary excitation in organic molecular and polymeric materials. We found the electron correlation strength, bond-length alternation as well as the conjugation extent have strong influences on the orderings of the lowest lying excited states through the first application of density matrix renormalization group theory to quantum chemistry. The electro-injected free carriers (with spin 1/2) can form both singlet and triplet bound states. We found that the singlet exciton formation ratio can exceed the conventional 25% spin statistics limit. We proposed a vibration correlation function formalism to evaluate the excited-state decay rates, which is shown to not only give reasonable estimations for the quantum efficiency but also a quantitative account for the aggregation-induced emission (AIE). It is suggested to unravel the AIE mechanism through resonance Raman spectroscopy.