Tae-Shick Kim

Effective hole injection of organic light-emitting diodes by introducing buckminsterfullerene on the indium tin oxide anode

We demonstrate that dramatically improved hole injection can be achieved by inserting a very thin C60 film between the indium tin oxide (ITO) electrode and N,N′-diphenyl-N,N′-bis(1,1′-biphenyl)-4,4′-diamine (NPB) layer. This result is ascribed to the formation of an interfacial dipole layer of buckminsterfullerene (C60) on the ITO electrode. The dipole layer induces the surface potential shift that contributes to improve the charge injection efficiency. The chemical shift was downward to help lower the hole injection energy barrier from the ITO electrode to the NPB layer, consistent with the moderately strong electron accepting nature of C60. The enhanced-charge injection provides a simple way of reducing the power consumption of organic electronic devices for real applications.

Magnetic field sensing scheme using CoFeB∕MgO∕CoFeB tunneling junction with superparamagnetic CoFeB layer

The authors investigated the tunneling magnetoresistance (TMR) of CoFeB∕MgO∕CoFeB tunnel junctions by varying the thickness (tCoFeB) of the top CoFeB layer. Linear and hysteresis-free switching was observed in junctions with tCoFeB⩽10A, while normal tunneling behavior occurred for tCoFeB>10A. The field sensitivity and the sensing field range were found to be controlled by varying the thickness of the sensing layer. This finding means that the magnetic tunneling junction (MTJ) provides a scheme for magnetic field sensing, which has a simple sensor design and low power consumption. The magnetic properties of the sensing layer with tCoFeB⩽10A were found to show the characteristics of superparamagnetism. Although the detailed mechanism of TMR in MTJs with a superparamagnetic layer is not fully understood at present, this phenomenon is observed repeatedly. Therefore, this sensing scheme would be an alternative method for overcoming the problems in magnetic sensors with a crossed magnetization pattern.

Significantly Improved Power Efficiency of Organic Light-Emitting Diodes with Surface Dipole on Anode and Ohmic Cathode Contact

Dramatically improved power efficiency and stability of organic light-emitting diodes (OLEDs) were achieved by using buckminsterfullerene (C60) as an interlayer between indium tin oxide (ITO) anode and hole transporting layer of N, N′-diphenyl-N, N′-bis(1,1′-biphenyl)-4,4′-diamine (NPB) and electron transporting layer (ETL) at the same time. The results are ascribed to the interfacial-dipole formation of C60 on the surface of ITO anode and Ohmic cathode contact of C60. The surface dipole of C60 on the ITO anode helps to lower the hole injection energy barrier from ITO to NPB. C60 also has an Ohmic cathode contact with high electron mobility in the typical structure of C60/LiF/Al. These properties of C60 make it possible to simultaneously enhance the electron and hole injection from both cathode and anode. Lowered operating voltage by surface dipole and Ohmic cathode contact of C60 can eliminate Joule heating at both organic/cathode and organic/anode interfaces and as a result, provides the improved stability of OLEDs.