Young-Mo Koo

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.

Ohmic contact probed by dark injection space-charge-limited current measurements

The authors demonstrate through dark injection space-charge-limited current (DI-SCLC) and trap-free SCLC measurements that an indium tin oxide (ITO)/buckminsterfullerene (C60) electrode can form a quasi-Ohmic contact with N,N′-bis(naphthalen-1-yl)-N,N′-bis(phenyl) benzidine (NPB). The DI-SCLC results show a clear peak current along with a shift of the peak position as the field intensity varies, implying an Ohmic (or quasi-Ohmic) contact. A theoretical simulation of the SCLC also shows that ITO/C60 forms an Ohmic contact with NPB when the electric field intensity is higher than 30 kV/cm. The Ohmic contact makes it possible to estimate the NPB hole mobility through the use of both DI-SCLC and trap-free SCLC analysis.

Spontaneous charge transfer from indium tin oxide to organic molecules for effective hole injection

Naphthalene tetracaboxylic dianhydride (NTCDA) shows strong chemical interaction with metal atoms in an indium tin oxide (ITO) substrate to form charge transfer (CT) complexes. The CT complex at the ITO/NTCDA interface can lower the energy barrier height for hole injection from ITO into the hole transporting layer of N,N′-diphenyl-N, N′-bis(1-naphthyl)(1,1′-biphenyl)-4,4′ diamine (NPB). The operational voltage of an emissive device at a current density of 100 mA/cm2 was significantly reduced from 12.2 to 9.2 V by simply inserting a thin layer of NTCDA between the ITO and NPB. The results enable the achievement of organic light-emitting diodes that consume relatively less power.

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.