Toshinori Matsushima

Formation of Ohmic hole injection by inserting an ultrathin layer of molybdenum trioxide between indium tin oxide and organic hole-transporting layers

Current density–voltage (J-V) characteristics of hole-only devices using indium tin oxide (ITO) anode and N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′-biphenyl-4,4′-diamine (α-NPD) layers were measured with various thicknesses of a molybdenum trioxide (MoO3) buffer layer inserted between ITO and α-NPD. The device with a 0.75-nm-thick MoO3 layer forms Ohmic hole injection at the ITO∕MoO3∕α-NPD interfaces and J-V characteristics of this device are controlled by a space-charge-limited current. Results of X-ray photoelectron and ultraviolet/visible/near-infrared absorption studies revealed that this Ohmic hole injection is attributable to an electron transfer from ITO and α-NPD to MoO3.

Extremely low voltage organic light-emitting diodes with p-doped alpha-sexithiophene hole transport and n-doped phenyldipyrenylphosphine oxide electron transport layers

Organic light-emitting diodes with p-doped alpha-sexithiophene and n-doped phenyldipyrenylphosphine oxide carrier transport layers are fabricated. In the doped diodes, the authors demonstrate an extremely low driving voltage of 2.9V at a current density of 100mA∕cm2 and very high luminance at a low driving voltage: 1000cd∕m2 at 2.4V, 10000cd∕m2 at 2.8V, and 920000cd∕m2 at 4.5V. Such lowered driving voltages and enhanced luminance characteristics are attributed to the generation of free charge carriers by charge transfer from matrix to dopant molecules, resulting in an increase in electrical conductivities and formation of Ohmic contacts at metal/organic interfaces.

Marked improvement in electroluminescence characteristics of organic light-emitting diodes using an ultrathin hole-injection layer of molybdenum oxide

We show that the performance of organic light-emitting diodes (OLEDs) is markedly improved by optimizing the thickness of a hole-injection layer (HIL) of molybdenum oxide (MoO3) inserted between indium tin oxide and N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′-biphenyl-4,4′-diamine (α-NPD). From results of the electroluminescence (EL) characteristics of OLEDs with various thicknesses of a MoO3 HIL, we found that the OLED with a 0.75-nm-thick MoO3 HIL had the lowest driving voltage and the highest power conversion efficiency among the OLEDs. Moreover, the operational lifetime of the OLED was improved by about a factor of 6 by using the 0.75-nm-thick MoO3 HIL. These enhanced EL characteristics are attributable to the formation of an Ohmic contact at the interfaces composed of ITO/MoO3/α-NPD.

Enhanced hole injection and transport in molybdenum-dioxide-doped organic hole-transporting layers

We have found that molybdenum dioxide (MoO2) is an excellent dopant for enhancing electrical conductivities in organic hole-transporting layers. We fabricated hole-only devices with an alpha-sexithiophene (α-6T) layer doped with MoO2 at various concentrations to investigate how doping MoO2 into the α-6T layers influences the hole-injection and hole-transport characteristics of these layers. We observed a marked increase in electrical conductivity as a result of the MoO2 doping. The 30-mol % MoO2-doped α-6T layer had a high electrical conductivity of 8.9±1.3×10−6 S/cm. From the results of our visible/near-infrared absorption spectra study of these doped layers, we confirmed that this increase in electrical conductivity is caused by a charge transfer between MoO2 and α-6T, which leads to an increase in free hole concentration in the doped layers and the formation of an ohmic contact at an electrode/α-6T interface. In the latter part of this paper, we discuss current flow and electroluminescence (EL) charact...

Estimation of electron traps in carbon-60 field-effect transistors by a thermally stimulated current technique

The authors investigated the influence of O2 and H2O molecules absorbed in carbon-60 (C60) films on their electron trap and n-type field-effect transistor (FET) characteristics. Electron traps in the C60 films were directly measured using a thermally stimulated current (TSC) technique. The TSC results demonstrate that the absorption of O2 and H2O molecules in the C60 films induced an increase in the electron trap concentration, which degrades C60 FET characteristics. By annealing the C60 films at 100°C for 8h, the electron trap concentrations were markedly lowered, enhancing the C60 FET characteristics. An electron mobility of 0.017cm2∕Vs and a current on/off ratio of 106 were observed from the degassed C60 FETs.

Degradation Mechanisms of Solution-Processed Planar Perovskite Solar Cells: Thermally Stimulated Current Measurement for Analysis of Carrier Traps.

Degradation mechanisms of CH3 NH3 PbI3 -based planar perovskite solar cells (PSCs) are investigated using a thermally stimulated current technique. Hole traps lying above the valence-band edge of the CH3 NH3 PbI3 are detected in PSCs degraded by continuous simulated solar illumination. One source of the hole traps is the photodegradation of CH3 NH3 PbI3 in the presence of water.

Multifunctional Benzoquinone Additive for Efficient and Stable Planar Perovskite Solar Cells

Device stability of planar perovskite solar cells is improved by virtue of multifunctional BQ additive. The morphology and crystal quality of the perovskite films are improved because BQ slows the rate of perovskite crystal formation. Electron transfer from perovskite to BQ reduces charge-recombination losses, and the oxidizing ability of BQ effectively suppresses the formation of metallic lead and improves device lifetime.

Solution-Processed Organic–Inorganic Perovskite Field-Effect Transistors with High Hole Mobilities

A very high hole mobility of 15 cm2 V-1 s-1 along with negligible hysteresis are demonstrated in transistors with an organic-inorganic perovskite semiconductor. This high mobility results from the well-developed perovskite crystallites, improved conversion to perovskite, reduced hole trap density, and improved hole injection by employing a top-contact/top-gate structure with surface treatment and MoOx hole-injection layers.

Carrier injection and transport characteristics of copper phthalocyanine thin films under low to extremely high current densities

We investigated current density-voltage (J-V) characteristics of copper phthalocyanine (CuPc) thin films depending on active device areas. We prepared CuPc thin-film devices with active areas smaller than S=625μm2 using a photolithography technique. The maximum breakdown current density (JMAX) and voltage (VMAX) of the devices markedly increased as the active area was decreased from S=625 to 7.9μm2. In the smallest device, with S=7.9μm2, we obtained not only an extremely high current density of JMAX=128kA∕cm2 at VMAX=9.2V, but also unique J-V characteristics, indicating that the carrier conduction process shifted from the Fowler-Nordheim tunneling injection mechanism to shallow-trap and trap-free space-charge-limited current mechanisms.

Operational stability enhancement in organic light-emitting diodes with ultrathin Liq interlayers

Organic light-emitting diodes (OLEDs) under constant current operation suffer from a decrease of luminance accompanied by an increase of driving voltage. We report a way to greatly improve the stability of OLEDs having a green emitter exhibiting thermally activated delayed fluorescence (TADF), (4s,6s)-2,4,5,6-tetra(9H-carbazol-9-yl) isophthalonitrile (4CzIPN), by introducing ultrathin (1 to 3 nm) interlayers of 8-hydroxyquinolinato lithium (Liq) between hole-blocking layer and its surrounding emissive and electron-transport layers. Under constant current operation starting at a luminescence of 1,000 cd/m2, the time to reach 90% of initial luminance (LT90) increased eight times, resulting in LT90 = 1,380 hours after insertion of the interlayers. Combining this new concept and mixed host system, LT95 was further extended to 1315 hours that is 16 times of reference device. This is the best value reported for TADF-based OLEDs and is comparable to the operational lifetimes of well-established phosphorescence-based OLEDs. Thermally stimulated current measurements showed that the number of deep charge traps was reduced with the insertion of the ultrathin Liq interlayer, indicating that reducing the number of deep traps is important for improving the operational lifetime and that exciton-polaron annihilation may be a source of the device degradation.