Our research group is currently conducting basic research on organic light-emitting diodes (OLEDs). Recently, we have successfully developed a series of blue and green TADF emitters for OLEDs realizing high external quantum efficiencies through high-throughput screening based on quantum chemical calculations. However, even using highly efficient emitting materials, the device performance depends on the device structure and aggregated state of organic molecules in the device. To understand the origin of the device performance, both theoretical and experimental approaches are important. In this regards, we have also carried out multiscale simulations and solid-state NMR (ssNMR) analysis of organic amorphous thin films. The ssNMR is the powerful technique for the detailed experimental analysis of amorphous aggregated materials, which has been difficult by typical diffraction methods because organic molecules in OLEDs are in the amorphous state. However, the low sensitivity of ssNMR compared to other analytical methods has always been a crucial problem. Recently, dynamic nuclear polarization enhanced ssNMR (DNP-ssNMR) has become popular for the sensitivity enhancement technique for ssNMR. In this presentation, we show the analysis of molecular orientation of an organic semiconducting material in an amorphous thin film state using DNP-ssNMR.
Charge transports in amorphous thin films with 100 nm thickness are investigated in silico by explicitly considering organic molecules. The amorphous layer of organic molecules was constructed using molecular dynamics simulations. The rate constants for charge hopping between two organic molecules, extracted from the amorphous layers, were calculated based on quantum chemical calculations. The hopping transport in amorphous layers was simulated using a Monte Carlo method. The hole mobility was calculated to be several times larger than the electron mobility, which was consistent with the experimental results. The Monte Carlo simulation also shows that diffusion transport is dominant at low applied electric fields and that contribution of drift transport increases at high electric fields. The simulation in this study enables us to reveal molecular origin of charge transport. In the presentation, we will show the results on recently-developed new thermally activated delayed fluorescence materials and the device performances.
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