Takahito Oyamada

Switching effect in Cu:TCNQ charge transfer-complex thin films by vacuum codeposition

We demonstrate the operation of an organic switching device using a uniform poly-crystalline Cu:7, 7, 8, 8-Tetracyanoquinodimethane (TCNQ) charge transfer (CT)-complex thin film that is prepared by vacuum vapor codeposition. Characteristic CT-absorption at λ=600–1200 nm was observed in the complex film in the UV-visible spectrum and the cyano stretching peak in the IR spectrum shifted to a higher (more than 29 cm−1) wave number than that of a pristine TCNQ film, suggesting the formation of a CT-complex in the evaporated thin film. Reproducible electrical switching characteristics were observed in the indium tin oxide/Al/(Al2O3)/Cu:TCNQ/Al structure. The device exhibited a clear threshold from low impedance to high impedance at an applied voltage of 10.0±2.0 V and a reverse phenomenon at a negative bias of −9.5±2.0 V. In this study, we demonstrate that a thin Al2O3 layer between the aluminum (Al) anode and Cu:TCNQ layers creates reproducible switching.

Material design of hole transport materials capable of thick-film formation in organic light emitting diodes

In this study, the authors show an empirical guideline for designing hole transport materials (HTMs) that suppress rises in driving voltage even with a few hundred nanometer thick film in the organic light emitting diodes (OLEDs). In a device structure of indium tin oxide (110nm)/hole transport layer (HTL) (Xnm)∕4,4′-N,N′-bis[N-(1-naphthyl)-N-phenyl-amino]biphenyl (10nm)/tris-(8-hydroxyquinoline)aluminum (Alq3) (50nm)∕MgAg (100nm)∕Ag (10nm), the authors compared electroluminescence characteristics of the OLEDs having a thin-film HTL (X=50nm) and a thick-film HTL (X=300nm) using 13 kinds of HTMs. They observed a closed correlation between suppression of the driving voltage and the HTMs’ thermal characteristics. Highly thermally stable HTMs resulted in a small increase in the driving voltage.

Lateral organic light-emitting diode with field-effect transistor characteristics

We succeeded in observing bright electroluminescence (EL) from 1wt%-rubrene doped tetraphenylpyrene (TPPy) as an active layer in a lateral organic light-emitting diode structure that allowed field-effect transistor operation. This device configuration provides an organic light-emitting diode structure where the anode (source) and cathode (drain) electrodes are laterally arranged, providing us a chance to control the EL intensity by changing the gate bias. We demonstrated that TPPy provides compatible transistor and EL characteristics. Further, not only rubrene doping into the TPPy host but also adjusting the source-drain channel length significantly improved the EL characteristics. We observed a maximum EL quantum efficiency (ηext) of ∼0.5% with a Cr∕Au source (S)-drain (D) electrode and a slightly higher ηext of ∼0.8% with S-D electrodes of MgAu∕Au, Al∕Au, Cr∕YAu∕Au, and MgAl∕Au multilayers, aiming for simultaneous hole and electron injection.

Extremely low-voltage driving of organic light-emitting diodes with a Cs-doped phenyldipyrenylphosphine oxide layer as an electron-injection layer

We demonstrated efficient electron injection and transport in organic light-emitting diodes using an electron-transport layer (ETL) composed of a Cs and phenyldipyrenylphosphine oxide (POPy2) co-deposited layer. In particular, an ETL composed of a Cs:POPy2 layer with an atom:molar ratio of 1:2 demonstrated an extremely low driving voltage, resulting in a high current density of 100mA∕cm2 at an applied voltage of only 3.9 V. The results of Kelvin probe and absorption measurements indicated that the formation of a CsAl alloy layer at the Cs:POPy2/Al cathode interface and the charge-transfer complex between the Cs and POPy2 contributed to enhancing the efficiency of electron injection and transport, respectively.

Electroluminescence of 2,4-bis(4-(2′-thiophene-yl)phenyl)thiophene in organic light-emitting field-effect transistors

We succeeded in observing electroluminescence (EL) of 2,4-bis(4-(2′-thiophene-yl)phenyl)thio-phene (TPTPT) as an active layer in an organic field-effect transistor (OFET). In particular, an OFET with a short channel of dSD=0.8μm demonstrated higher EL efficiency than one with a much longer channel (dSD=9.8μm). We observed a maximum EL quantum efficiency (ηmax) of 6.4×10−3% in the short-channel-length device at an applied source-drain voltage of Vd=−100V and a gate voltage of Vg=−40V. From the OFET characteristics, although the TPTPT layer demonstrated typical p-type operation, the occurrence of EL clearly indicated simultaneous hole and electron injection from the source and drain electronics, respectively, under high Vd and Vg.

Efficient Electron Injection Mechanism in Organic Light-Emitting Diodes Using an Ultra Thin Layer of Low-Work-Function Metals

To achieve efficient electron injection in organic light-emitting diodes, we examine ultra thin layers (0.50 nm) of the low-work-function metals, Cs (1.9eV), Rb (2.2eV), K (2.3eV), Na (2.4eV), Li (2.9eV), and Ca (2.9eV) capped with aluminum (Al) as a cathode layer. While all the alkali metals show a decrease of driving voltage compared with a single Al cathode, the Cs layer especially shows a significant decrease, and we obtain a high current density of 1.9 A/cm2 at an applied voltage of only 10V by using this layer. We demonstrate that efficient electron injection is achieved when we use a Cs layer with a thickness of less than 3 nm, although electron injection efficiency abruptly decreases when using a Cs layer thicker than 3 nm. From the Cs thickness dependence of current-voltage characteristics, we conclude that Cs atoms form an alloy layer with aluminum atoms at the organic/Al cathode interface, organic layer/Cs:Al/Al, that significantly enhances electron injection compared with that obtained from bulk Cs layers.

Top Light-Harvesting Organic Solar Cell Using Ultrathin Ag/MgAg Layer as Anode

We fabricate a top light-harvesting organic solar cell (TL-OSC) with a Si substrate/SiO2 layer/cathode/organic layer/semitransparent metal anode structure. We obtain power conversion efficiency (ηp) comparable to that of conventional bottom-type OSCs. To harvest light from a top anode, we use a 1 nm Ag layer/4 nm MgAg layer having a transmittance of 55–80% in the visible and near-infrared regions. We also insert an ultrathin molybdenum oxide (MoO3) layer at the cathode/organic layer interface to increase hole collection efficiency from the organic layer into the cathode.

Injection and Transport of High Current Density over 1000 A/cm2 in Organic Light Emitting Diodes under Pulse Excitation

We succeeded in injecting and transporting a maximum high current density of J=1163 A/cm2 in organic light-emitting diodes using short-pulse excitation combined with a highly thermally conductive silicon substrate (thermal conductivity: 148 W m-1 K-1) and a small cathode configuration (cathode radius r=50 µm). A maximum current density almost 20 times higher than that associated with direct current (DC) operation was observed by driving an OLED with a short pulse voltage. With short-pulse excitation, the decrease in external quantum efficiency (ηext) obeyed a typical singlet–singlet exciton annihilation model well, indicating that the generation of Joule heat in OLEDs can be suppressed under pulse operation.

Electroluminescence from self-organized microdomes

The preparation of a self-organized, microstructured organic electroluminescent device is reported. A dewetting process is used to form (sub)micrometer-sized dewetted patches (“domes”) of a hole transport material (tolyl-phenyl-diaminobiphenyl, TPD) on an indium-tin-oxide electrode. The domes are regular in size and spacing. Evaporation of an electron transport material (tris-8-hydroxyquinoline aluminum, Alq3) and an Mg/Ag top electrode leads to a device with electroluminescing spots of micrometer dimensions and a spacing of a few micrometers.

Estimation of carrier recombination and electroluminescence emission regions in organic light-emitting field-effect transistors using local doping method

To elucidate the electroluminescence (EL) mechanism of organic light-emitting field-effect transistors (OLEFETs), we determined the carrier recombination and EL emission regions using the local doping method. We demonstrated that the local doping method is a useful technique for estimating the width of these regions in OLEFETs. We inserted an ultrathin rubrene doped 1,3,6,8-tetraphenylpyrene (TPPy) layer (d=10nm) as a sensing layer in a TPPy layer (80nm) and measured the luminance-drain current-drain voltage characteristics and the EL spectra depending on the position of the sensing layer. We confirmed that the EL emission region expanded almost to the height (h≃40nm) of the source-drain electrodes and was independent of the gate bias voltage (Vg). Further, we observed that the EL external quantum efficiency (ηext) significantly decreased as Vg increased, suggesting that excitons generated in a TPPy host layer by carrier recombination are quenched by the application of Vg.