Keunhee Park

Characteristics of Organic Light Emitting Diodes with Tetrakis(Ethylmethylamino) Hafnium Treated Indium Tin Oxide

The surface of indium tin oxide (ITO) in organic light emitting diodes (OLEDs) was treated using an atomic layer deposition with tetrakis(ethylmethylamino) hafnium (TEMAH) and O2 precursors. The treatment for 5 cycles at room temperature resulted in a significant improvement in the electroluminescent characteristics resulting from the increased hole injection. According to the various characterizations, an ultra-thin HfOx layer without any insulating properties was formed, which modified the electronic band structure of the ITO anode. When the number of cycles or temperature was increased, an electrically insulating HfOx was formed, and the resulting OLED properties deteriorated. [DOI: 10.1143/JJAP.46.L461]

Hafnium metallocene compounds used as cathode interfacial layers for enhanced electron transfer in organic solar cells

We have used hafnium metallocene compounds as cathode interfacial layers for organic solar cells [OSCs]. A metallocene compound consists of a transition metal and two cyclopentadienyl ligands coordinated in a sandwich structure. For the fabrication of the OSCs, poly[3,4-ethylenedioxythiophene]:poly(styrene sulfonate), poly(3-hexylthiophene-2,5-diyl) + [6, 6]-phenyl C61 butyric acid methyl ester, bis-(ethylcyclopentadienyl)hafnium(IV) dichloride, and aluminum were deposited as a hole transport layer, an active layer, a cathode interfacial layer, and a cathode, respectively. The hafnium metallocene compound cathode interfacial layer improved the performance of OSCs compared to that of OSCs without the interfacial layer. The current density-voltage characteristics of OSCs with an interfacial layer thickness of 0.7 nm and of those without an interfacial layer showed power conversion efficiency [PCE] values of 2.96% and 2.34%, respectively, under an illumination condition of 100 mW/cm2 (AM 1.5). It is thought that a cathode interfacial layer of an appropriate thickness enhances the electron transfer between the active layer and the cathode, and thus increases the PCE of the OSCs.

Recent Development Status of Organic Solar Cells

Currently, the alternative energy is one of the critical issues because of exhaustion of petroleum resources and its high cost. The solar cell is considered as the one of the promising alternative energy. And the solar cell can be classified to inorganic solar cell and organic solar cell. Although the efficiency of organic solar cell is very lower than the that of inorganic solar cell, organic solar cells have many advantages including low process cost, high transmittance, color variation, and flexibility. For these reasons, organic solar cells have the potential in low cost solar cell market that is challenging for inorganic solar cells. Recent researches of organic solar cell is concentrating on enhancement of efficiency, lifetime, and stability to order to commercially use. Working principles and the development issues of organic solar cells are discussed in this paper.

Admittance Spectroscopic Analysis of Polymer Light Emitting Diodes with the LiF Cathode Buffer Layer

Admittance Spectroscopic analysis was applied to study the effect of LiF buffer layer and to model the equivalent circuit for poly(2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene) (MEH-PPV)-based polymer light emitting diodes (PLEDs) with the LiF cathode buffer layer. The single layer device with ITO/MEH-PPV/Al structure can be modeled as a simple combination of two resistors and a capacitor. Insertion of a LiF layer at the Al/MEH-PPV interface shifts the lowest unoccupied molecular orbital (LUMO) level and the vacuum level of the MEH-PPV layer as a result of which the barrier height for electron injection at the Al/MEH-PPV interface is reduced. The admittance spectroscopic analysis of the devices with the LiF cathode buffer layer shows reduction in contact resistance (RC), parallel resistance (RP) and increment in parallel capacitance (CP).