Effectiveness of p-dopants in an organic hole transporting material

We investigated the effectiveness of p-dopants to generate holes in a hole transporting material by comparing the absorption in visible-near-infrared and infrared regions and current density-voltage characteristics. CuI, MoO3, and ReO3 having different work functions were doped in a hole transporting organic material, 4,4,4-tris(N-(2-naphthyl)-N-phenylamino)-triphenylamine (2TNATA). Formation of charge transfer (CT) complexes increases linearly with increasing doping concentration for all the dopants. Dopants with higher work function (ReO3>MoO3>CuI) are more effective in the formation of CT complexes and in the generation of the charges in the doped films.The authors are grateful to the Ministry of Knowledge Economy MOKE of Korea for financial support of this work. This work was also partially supported by the Samsung SDI and Dongwoo Finechem Co

Highly efficient tandem p-i-n organic light-emitting diodes adopting a low temperature evaporated rhenium oxide interconnecting layer

High quality interconnection units (ICUs) with a high transparency and superior charge generating capability for tandem organic light-emitting diodes (OLEDs) are developed. The ICUs of rubidium carbonate-doped 4,7-diphenyl-1,10-phenanthroline/rhenium oxide (ReO3)-doped N,N-diphenyl-N,N-bis(1,1-biphenyl)-4,4-diamine layers with or without an additional ReO3 interlayer produce high transmittance (88%–92% at 420–700 nm) and spontaneous internal charge generation properties. A very high efficiency of ~129 cd/A has been demonstrated from only two stacked green p-i-n OLEDs by employing the developed ICUs. The relationship between the device efficiency and internal charge generation within the ICUs is further described by means of the capacitance measurements.The authors thank the MKE of Korea and Samsung SDI for their financial support of this work

Rubidium-Carbonate-Doped 4,7-Diphenyl-1,10-phenanthroline Electron Transporting Layer for High-Efficiency p-i-n Organic Light Emitting Diodes

We investigated the electrical properties and charge transport mechanisms of a rubidium-carbonate (Rb2CO3)-doped 4,7-diphenyl-1,10-phenanthroline (Bphen) electron transporting layer (ETL). The electron-only devices and photoemission spectroscopy analysis revealed that the formation of doping-induced gap states dominantly contributes to the improvement of carrier transport characteristics of the doped system. High-efficiency green phosphorescent p-doping/intrinsic/n-doping (p-i-n) organic light emitting diodes were demonstrated using the Rb2CO3-doped Bphen ETL and rhenium oxide (ReO3)-doped N,N-diphenyl-N,N-bis(1,1-biphenyl)-4,4-diamine hole transporting layer, exhibiting an external quantum efficiency of 19.2%, power efficiency of 76 lm/W, and low operation voltage of 3.6 V at 1000 cd/m2.The authors thank the MKE of Korea and Samsung SDI for their financial support of this work

Low driving voltage and high stability organic light-emtting diodes with rhenium oxide-doped hole transporting layer

The authors report a promising metal oxide-doped hole transporting layer (HTL) of rhenium oxide (ReO3)-doped N,N-diphenyl-N,N-bis (1,1-biphenyl)-4,4-diamine (NPB). The tris(8-hydroxyquinoline) aluminum-based organic light-emitting diodes with ReO3-doped NPB HTL exhibit driving voltage of 5.2–5.4 V and power efficiency of 2.2–2.3 lm/W at 20 mA/cm2, which is significantly improved compared to those (7.1 V and 2.0 lm/W, respectively) obtained from the devices with undoped NPB. Furthermore, the device with ReO3-doped NPB layer reveals the prolonged lifetime than that with undoped NPB. Details of ReO3 doping effects are described based on the UV-Vis absorption spectra and characteristics of hole-only devices.The authors thank the Ministry of Commerce, Industry, and Energy, Samsung SDI, and Dongwoo Finechem. Co. for their financial support of this work

Electronic and chemical properties of cathode structures using 4,7-diphenyl-1,10-phenanthroline doped with rubidium carbonate as electron injection layers

The electronic properties and chemical interactions of cathode structures using 4,7-diphenyl-1, 10-phenanthroline (Bphen) doped with rubidium carbonate (Rb2CO3) as electron injection layers were investigated. Current-voltage characteristics reveal that the devices with Bphen/Rb2CO3/Al as cathode structures possess better electron injection efficiency than those with cathode structures of Bphen/LiF/Al. Ultraviolet and x-ray photoemission spectroscopy shows that n-type doping effects resulting from Rb2CO3 and the gap states created by aluminum deposition are both keys to the improved carrier injection efficiency. Moreover, theoretical calculation indicates that the chemical reaction between aluminum and the nitrogen atoms in Bphen is the origin of the gap states.This work was partially supported by the National Science Council, the Republic of China, under contract no. NSC 95-2745-M-002-011

Characteristics of Ni-Doped IZO Layers Grown on IZO Anode for Enhancing Hole Injection in OLEDs

The preparation and characteristics of a Ni-doped indium zinc oxide (NIZO) layer were investigated to enhance hole injection in organic light emitting diodes (OLEDs). A thin NIZO layer with a thickness of 5 nm was cosputtered onto an indium zinc oxide (IZO) anode using tilted Ni and IZO dual targets dc magnetron sputtering at room temperature in a pure Ar atmosphere. Using 3 W of Ni dc power, we can obtain a NIZO (5 nm)/IZO (135 nm) double-layer anode with a sheet resistance of 30.04 / and an optical transmittance of 83.8% at a wavelength of 550 nm. In addition, it was found that the work function of the NIZO layer was higher than that of a pure IZO anode due to the presence of a NiOx phase in the NIZO layer. An increase of Ni dc power above 7 W significantly degrades the electrical and optical properties in the NIZO layer. X-ray diffraction examination demonstrated that the NIZO layer consisted of an amorphous structure regardless of the Ni dc deposition power due to low substrate temperature. Furthermore, an OLED fabricated on the NIZO layer exhibited a higher current density, luminance, and efficiency due to improved hole injection by the high work function NIZO. These results indicate that the NIZO/IZO anode scheme is a promising anode material system for enhancing hole injection from the anode into the active layer of OLEDs.The authors acknowledge financial support from LG Displays, OLED Panel Development team

High performance top-emitting organic light-emitting diodes with copper iodide-doped hole injection layer

Efficient top-emitting organic light-emitting diodes were fabricated using copper iodide (CuI) doped 1,4-bis[N-(1-naphthyl)-N′-phenylamino]-4,4′-diamine (NPB) as a hole injection layer and Ir(ppy)3 doped CBP as the emitting layer. CuI doped NPB layer functions as an efficient p-doped hole injection layer and significantly improves hole injection from a silver bottom electrode. The top-emitting device shows high current efficiency of 69 cd/A with Lambertian emission pattern. The enhanced hole injection is originated from the formation of the charge transfer complex between CuI and NPB.This research was supported by a Grant (F0004071-2007-23) from the Information Display R&D Center, one of the 21st Century Frontier R&D Program and the center for OLED funded by the Ministry of Knowledge Economy (MOKE)