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The formation of excitons in organic molecules by charge injection is an essential process in organic light-emitting diodes (OLEDs)1-7. According to a simple model based on spin statistics, the injected charges form spin-singlet (S1) excitons and spin-triplet (T1) excitons in a 1:3 ratio2-4. After the first report of a highly efficient OLED2 based on phosphorescence, which is produced by the decay of T1 excitons, more effective use of these excitons has been the primary strategy for increasing the energy efficiency of OLEDs. Another route to improving OLED energy efficiency is reduction of the operating voltage2-6. Because T1 excitons have lower energy than S1 excitons (owing to the exchange interaction), use of the energy difference could-in principle-enable exclusive production of T1 excitons at low OLED operating voltages. However, a way to achieve such selective and direct formation of these excitons has not yet been established. Here we report a single-molecule investigation of electroluminescence using a scanning tunnelling microscope8-20 and demonstrate a simple method of selective formation of T1 excitons that utilizes a charged molecule. A 3,4,9,10-perylenetetracarboxylicdianhydride (PTCDA) molecule21-25 adsorbed on a three-monolayer NaCl film atop Ag(111) shows both phosphorescence and fluorescence signals at high applied voltage. In contrast, only phosphorescence occurs at low applied voltage, indicating selective formation of T1 excitons without creating their S1 counterparts. The bias voltage dependence of the phosphorescence, combined with differential conductance measurements, reveals that spin-selective electron removal from a negatively charged PTCDA molecule is the dominant formation mechanism of T1 excitons in this system, which can be explained by considering the exchange interaction in the charged molecule. Our findings show that the electron transport process accompanying exciton formation can be controlled by manipulating an electron spin inside a molecule. We anticipate that designing a device taking into account the exchange interaction could realize an OLED with a lower operating voltage.

We present the herringbone structure search (HSS) method to predict the crystal structures of organic semiconductors from molecular structural formulas. The charge transport efficiency of small-molecule organic semiconductors is governed by molecular packing, often in a two-dimensional herringbone structure. This method predicts crystal structures within the herringbone framework using the density functional theory. As a demonstration, we successfully applied it to promising molecules, predicting crystal structures that agreed well with experimental data. This method offers an efficient approach for in silico screening of new organic molecules, aiding the development of high-performance organic semiconductors. [ABSTRACT FROM AUTHOR]

Recently, doped organic semiconductors with high work functions have been studied for opto-electronic device applications. However, a higher work function commonly results in poorer stability due to redox reactions with water in air, limiting device processing and applications. In this study, combinations of bulky anions and a semicrystalline polythiophene derivative were explored for chemical doping under ambient conditions. The use of tetrakis(pentafluorophenyl)borate (TFPB) anions resulted in a high work function over 5.5 eV with remarkably improved stability, with 90% of the conductivity retained after storage in air for 20 days. The stability at elevated temperatures of 100 °C and 125 °C was also dramatically improved compared with the use of other dopant ions. X-ray diffraction measurements suggest that intercalation of the TFPB anion occurs in the lamellar structure of the polymer, while in-plane π-stacking structures are present. Dedoping reactions including self-degradation at high temperatures seemed to be suppressed for this inert dopant anion, which contributed to the observed exceptional stability. Our findings provide promising insights into the design of combinations of polymers and dopants to achieve highly stable doped organic semiconductors. [ABSTRACT FROM AUTHOR]

The crystal structure of organic semiconductors is an important factor that dominates various electronic properties, including charge transport properties. However, compared with the crystal structures of inorganic semiconductors, those of organic semiconductors are difficult to determine by powder x-ray diffraction (PXRD) analysis. Our proposed machine-learning (neural-network) technique can determine the diffraction peaks buried in noise and make deconvolution of the overlapped peaks of organic semiconductors, resulting in crystal-structure determination by the Rietveld analysis. As a demonstration, we apply the method to a few high-mobility organic semiconductors and confirm that the method is potentially useful for analyzing the crystal structure of organic semiconductors. The present method is also expected to be applicable to the determination of complex crystal structures in addition to organic semiconductors. [ABSTRACT FROM AUTHOR]

We investigated, by using soft x rays, one of the most fundamental properties in multiferroics, that is, coupled or decoupled feature among coexisting multiple (anti)ferroic order parameters for two types of multiferroic hexaferrites. Circular dichroic signals observed at x-ray resonance for diffraction and absorption enable us to resolve several kinds of magnetic domains, which are spatial distributions of the respective order parameters. We examined magnetic field effects on the domains and unveiled the features of the order parameters. It is found that the coupled or decoupled features of the order parameters are explained by the symmetry analysis based on the Landau theory. These findings contribute to an understanding of magnetoelectric couplings in multiferroics and to exploiting a wide variety of their functionalities. In addition, we suggest a circular dichroic specular off-peak scattering at resonance as a powerful technique for studying a surface state of a magnetic order in an expansive range of quantum materials. [ABSTRACT FROM AUTHOR]