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To achieve wavelength division multiplexing of optical communications for short distances, we have theoretically investigated a photonic crystal waveguide we call an orthogonal lattice waveguide (OLW), which may have a communication wavelength bandwidth of about 20 nm. In this paper, we fabricate CirD laser structures with air-cladding layers and OLWs of different cavity radii. We confirm that the light emitted from the OLW is the light generated in the cavity. OLW has a wavelength bandwidth of about 20 nm, which agrees with the results of our previous theoretical study. The wavelength shift is about 4 nm when the radius of the circular defect cavity increases by 0.01 a, where a is the lattice constant.

The fabrication and characteristics of bulk-heterojunction organic photodetectors (OPDs) toward near-infrared narrowband-light detection devices are investigated. For conventional OPDs with thick active layers based on poly(3-hexylthiophene) and [6,6]-phenyl C61-butyric acid methyl ester blends, increasing the PCBM concentration leads to the improvement of near-infrared detection. For inverted devices, the improvement of near-infrared detection can be achieved by varying bulk-heterojunction. The inverted OPD with a thicker active layer shows a spectral response with a maximum peak of approximately 760 nm and an IPCE of above 40 % at −5 V.

Circular cavity structures are fabricated on GaAs/ AlGaAs-based epi-wafer using SiO x hard mask to realize a proposed photonic crystal (PhC) structure with GaAs core layer having embedded InAs quantum dots (QDs) towards PhC laser fabrication. We investigate the evaluations of selective dry etching of the core layer for an epi-wafer using optical measurements. Photoluminescence (PL) intensity at 1285 nm, which is related to ground-state emission of QDs, monotonously decreases with increasing the etching depth of the core layer with QDs. The change of PL intensity for epi-wafer is good agreement with the behavior of the increased etching depth of the core layer which is evaluated by the measurements of scanning electron micrograph. We demonstrate a possibility of etching end-point detection and evaluation of etching rate during the dry etching of the core layer with QDs using in-situ optical emission spectroscopy.

Photonic crystal (PC) lasers with circular-defect (CirD) microcavity offer high-potential application in wavelength division multiplexing (WDM), and therefore, are a suitable candidate to realize intrachips optical communications. In this paper, a CirD laser for the WDM applications is designed for the first time by using the three-dimensional finite-difference time-domain method. For the realization of the WDM applications, it is critical to achieve the adequate high-quality ( Q )-factor (>7142) in the bandwidth range over 20 nm, which is estimated from the characteristics of the gain medium and the coupling of the CirD cavity with a PC waveguide. After optimizing the laser structure, we achieved a constant high Q -factor of 15 000 within the bandwidth range of 25 nm. A proof-of-principle experiment of the lasing wavelength tuning was conducted by exploiting the CirD lasers with air/AlGaOx cladding layers under optical pumping conditions. The fabricated laser showed a single-mode lasing operation with the linewidth narrower than 0.07 nm (the resolution-limit of the optical spectrum analyzer) and side mode suppression ratio of ∼20 dB. The wavelength tuning of a whispering-gallery mode over 20 nm was confirmed. The present results imply that there is a great potential of CirD laser for larger scale WDM integrated devices.

To develop a wavelength division multiplexing (WDM) device using a photonic crystal (PC), we investigate coupling strength between a circular cavity resonator and a waveguide with various structural parameters. In the WDM device, light signals with various wavelengths generated in different cavity resonators travel through a single waveguide. Careful tuning of the cavity-waveguide coupling is necessary for practical operations, since the light signals must have the same intensities in the waveguide. Carrying out three-dimensional finite-differential time-domain (FDTD) calculations along with photonic bands, we search for the optimum structure for the WDM.

The fabrication of a single-crystalline thin film of non-peripheral octahexyl-phthalocyanine (C6PcH2) was realized by performing a tetrahydrofuran (THF) vapor treatment on a the spin-coated film. The solution-mediated polymorphic transformation of C6PcH2 was promoted in the THF vapor, and a single-crystalline thin film with α-phase crystal grew in the thin film when setting an appropriate temperature. The thin film exhibited optical anisotropy, such as a high dichroic ratio, and the color of the transmitted light varied from green to transparent depending on the polarization direction. The molecular packing structure on the substrate was determined by X-ray diffraction and polarized absorption measurements.

Selectively transparent photonic crystals consisting of printed inorganic/organic hybrid dielectric films based on inorganic copper(I) thiocyanate can be produced to exhibit intense Bragg peaks over their stop band in the visible region. This work exhibits a unique approach to achieve the improvement of color purity and the strong directed emission pattern for polymer light-emitting diodes and AC-driven electroluminescent devices with printed inorganic/organic hybrid dielectric mirrors, which will be important for all-printed microlight sources.

The characteristics of organic photodetectors based on poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM) blends with various film thicknesses of active layer are investigated. The red-shift in peak wavelength of incident-photon-to-current conversion efficiency (IPCE) spectra in the devices with various film thicknesses is observed with increasing film thickness. For thick devices upon irradiation by light with a photon energy of around energy gap of an active layer, excitons are formed almost uniformly because of the small absorption coefficient of red light. A conventional device with 2 μm-thick film exhibits narrow-band light detection with red-light sensitivity and the relatively narrow spectral response of the full-width at half-maximum (FWMH) of around 50 nm. On the other hand, an inverted device with 2 μm-thick film exhibits broadband light detection. For bulk heterostructure devices, one of important factors to realize the narrowband light detection is the control of charge carrier collection efficiency at electrodes which is attributed to the device structure, including the intrachain transport in polymer.

We describe the design of photonic crystal circular defect (CirD) lasers to construct a compact optical module with a wavelength division multiplexing function for the application of inter-chip or intra-chip optical interconnects. Subsequently, we investigated the characteristics of CirD lasers including the quality factor of the cavity, the lasing threshold, and the modulation speed with a three-dimensional finite-difference time-domain method and two-dimensional rate equations. Finally, we demonstrated the single mode lasing and wavelength tuning behaviors of the CirD lasers using optical pumping technology under room-temperature continuous-wave conditions.

The internet of Things (IoT) is supported by various sensor and display devices. Microlight sources based on organic light-emitting transistors (OLETs) with an electrical switching capability, in which not only the position and extension of the emission area but also the emission color are controlled by gate voltage and drain voltage, are useful for augmented (AR) and virtual reality (VR) displays and sensor applications. OLETs with top gate architectures are advantageous as compared to bottom gate architectures due to easy of fabrication, low contact resistance and environmental stability. The fabrication of low-temperature processable, OH-group-free polymer gate dielectrics are also important for fabricating ambipolar OLETs. Poly(methyl methacrylate) (PMMA), which does not contain electron-trapping groups such as OH, is one of the promising candidate gate dielectrics in ambipolar OLETs. PMMA gate dielectric formed by using fluorine-based solvent (Asahi Glass AE-3000) is useful to prevent mixing of the dielectric layer with the emissive layer for the top-gated device fabrication. The device performance of top-gated OLETs based on a poly(p-phenylene vinylene) derivative, Super Yellow is improved owing to the fabrication without intermixing. A near-infrared emission from a phosphorescent dye, Pt-tetraphenyltetrabenzoporphyrin dopant is demonstrated.

To realize wavelength division multiplexing for high-capacity optical communications, we theoretically investigate the properties of a photonic crystal waveguide structure in which a row of air holes is displaced by a/2 (a is the lattice constant). We calculate transmission loss and coupling efficiency to circular defect cavities of this waveguide structure, which we call an orthogonal lattice waveguide (OLW). The communication bandwidth of the OLW is estimated to be about 20 nm, which is twice that of the conventional line-defect (W1) waveguide. This result indicates that improvements in characteristics of the wavelength division multiplexing are expected by using the OLW instead of W1.

The concept of using printed inorganic/organic hybrid distributed Bragg reflectors (DBRs) utilizing inorganic semiconductor and insulating polymers in microcavity polymer electroluminescent devices is introduced to provide an approach to achieve the spectral narrowing and the strong forward directionality. The large refractive index contrast of approximately 0.5 (0.44) between inorganic copper(I) thiocyanate, CuSCN, and insulating polymer of poly(vinylidene fluoride-trifluoroethylene), P(VDF-TrFE) (cellulose acetate, CA) results in the fabrication of solution-processed inorganic-organic hybrid dielectric DBRs with high reflectivity (>90%) from nanostructures consisting of only four (five) bilayers. For DBRs composed of CuSCN/CA alternative dielectric layers, all-solution processed microcavity polymer light-emitting diode based on highly conductive poly(ethylenedioxythiophene):poly(styrenesulfonate) anode except for Ag cathode exhibits the narrowing of EL spectrum with a full width at half maximum of approximately 25 nm and the maximum luminance of above 10,000 cd/m2. From the viewpoint of dielectric DBRs based on ferroelectric polymer P(VDF-TrFE) with both low refractivity and high permittivity, we demonstrate a microcavity AC voltage-driven polymer electroluminescent device (μcACEL) which exhibits the spectral narrowing and the strong forward directionality. This work is anticipated to be useful for the development of solution-processed μcACEL with unique device architecture.

We report polarized in-plane emission at full-channel area in ambipolar heterostructure organic light-emitting transistors (OLETs) utilizing bilayer oriented fluorene-type polymers. The present work demonstrates that carrier balance between bilayers is a key factor in achieving in-plane emission at almost full-channel area when hole transport is dominant in the upper layer, which acts as an electron blocking layer, and electrons are injected into the lower layer. Hole accumulation in the upper layer affects electron injection from the electrode to the lower layer and the probability of recombination of holes in the upper layer with electrons in the lower layer near the bilayer interface. For OLETs having an oriented bilayer with the channel direction parallel to orientation of the polymer chains, a uniform in-plane emission pattern, an increased electroluminescence intensity, and a high external quantum efficiency can be achieved by improving the hole mobility in the upper layer and optimizing the carrier balance. These results are expected to be very useful for the development of surface micro-light sources.

The fabrication of inorganic/organic hybrid distributed Bragg reflector (DBR) is investigated to realize all-solution processed polymer light-emitting diodes (PLED s) with the narrowing of electroluminescence (EL) spectra. The large refractive index contrast between inorganic copper(l) thiocyanate, CuSCN and polymer, cellulose acetate results in the fabrication of DBR with high reflectivity (> 95 %) from nanostructures consisting of only five bilayers. The narrowing of EL spectrum with a full width at half maximum of approximately 25 nm is achieved from all-solution-processed PLED based on highly conductive poly(ethylenedioxythiophene):poly(styrenesulfonate) anode and super yellow as an emissive layer except for a silver cathode.

The effects of molecular packing on the field-effect mobility and external quantum efficiency of polymer light-emitting transistors based on films composed of a donor–acceptor polymer, poly(9,9-dioctylfluorene- al t-benzothiadiazole) (F8BT), are investigated. The molecular packing structures of F8BT films are modified by annealing at different temperatures, such that large grains in a mesophase are formed. In the case of F8BT films in the mesophase, highly crystalline three-dimensional configurations with adjacent chains result in effective electron transport. The electron mobility, μ e , in a device in which the F8BT film is in the mesophase is approximately ten times greater than that for F8BT films annealed at other temperatures, and is estimated to be approximately 3.4 × 10 −3 cm 2 V −1 s −1 . This same device exhibits the highest hole and electron mobilities along with the highest maximum external quantum efficiency (approximately 1.4%) due to improved electron transport and carrier balance.

The concept of using an ambipolar bilayer semiconducting heterostructure in organic light-emitting transistors (OLETs) is introduced to provide a new approach to achieve surface emission. The properties of top-gate-type bilayer OLETs with ambipolar materials based on two types of fluorene-type polymers used as an emissive layer and an electron blocking layer are investigated. Line-shaped yellow–green emission occurs near a hole-injection electrode. When hole transport is dominant in the upper layer which acts as an electron blocking layer, and electrons are injected into the lower layer, an in-plane light-emitting pattern is observed. The measured in-plane emission zone confirms that both hole and electron transport are determined to occur mainly along the different organic layers between the source and drain electrodes, and an in-plane recombination zone of electrons and holes exists near the bilayer organic interface. This work is anticipated to be useful for the development of in-plane light-emitting transistors.

SUMMARY Organic photodetectors based on poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM) blends with indium tin oxide (ITO) modified by phosphonic acid–based self-assembled monolayer (SAM) treatment in a short time are investigated. The mixed SAMs serve to tune the surface free energy and the work function of ITO by varying the blend ratio. The phosphonic acid–based SAM treatment results not only in lowering of the injection barrier at the ITO/organic layer interface but also in the lowering of the contact resistance between ITO and the organic layer, as shown by impedance spectroscopy in P3HT hole-only device with SAMs. P3HT:PCBM device with ITO modified by short treatment time of 1H,1H,2H,2H-perfluorooctane-phosphonic acid exhibits an incident-photon-to-current conversion efficiency of above 50% at –2 V, a high on/off ratio, and improved durability.

For organic photodetectors (OPDs), in order to realize high incident-photon-to-current conversion efficiency (IPCE) and high on/off ratio, the characteristics of hole-only devices and polymer photodetectors with indium–tin-oxide (ITO) modified by phosphonic acid-based self-assembled monolayer treatment in a short time are investigated. 1H,1H,2H,2H-Perfluorooctanephosphonic acid (FOPA) treatment results not only in lowering of the injection barrier at the ITO/organic layer interface but also in the lowering of the contact resistance between ITO and the organic layer. An OPD based on a blend of a donor, poly(9,9-dioctylfluorene-co-bithiophene) (F8T2), and an acceptor, fullerene derivative [6-6]phenyl-C61-butyric acid methyl ester (PCBM) with FOPA treatment exhibits blue-light sensitivity and IPCE of approximately 75% at −4 V. For the F8T2:PCBM device with FOPA, the IPCE is improved, and the dark current hardly increases. F8T2:PCBM device with FOPA treatment exhibits high on/off ratio, wide detector dynamic range and cut-off frequency of more than 10 MHz under reverse voltage.

This study demonstrates the incorporation of a Cs 2 CO 3 :conjugated polyelectrolyte blended interfacial layer between the emissive layer and a silver (Ag) cathode, for realizing all-solution processed polymer light-emitting diodes. For a device with poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT) as the emissive layer, this approach improves the maximum luminance of approximately 80,000 cd/m 2 and maximum current efficiency of 10.6 cd/A. It is clarified that the interfacial layer prevents Ag nanoparticles from penetrating into the emissive layer, resulting in yellow–green emission from F8BT. We also demonstrate the possibility of all-solution processed polymer light-emitting diodes utilizing solution-processed Cs 2 CO 3 :conjugated polyelectrolyte interfacial layer and Ag nano-ink.

The properties of polymer light-emitting transistors with Ag-nanowire (AgNW) source/drain electrodes fabricated on a polymer substrate are investigated. Organic field-effect transistors (OFETs) based on poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT) with AgNW source/drain electrodes exhibit ambipolar characteristics. For an ambipolar F8BT OFET, yellow-green light emission is observed. The maximum external quantum efficiency is 0.6%. We demonstrate the possibility of producing flexible polymer light-emitting transistors using AgNW electrodes.

The influence of source/drain (S/D) electrodes on the external quantum efficiency (EQE) of ambipolar organic light-emitting transistors (OLETs) based on fluorene-type polymer films is investigated. The electrical properties and the maximum EQE value of the device with indium tin oxide (ITO) S/D electrodes are almost the same as those of the device with Ag S/D electrodes. A relatively high EQE of 1% is achieved regardless of the emission site for the OLET with ITO. In contrast, the EQE of the OLET with Ag is low when the emission occurs close to the S/D electrodes. The maximum EQE of the device with Ag is obtained when the emission is observed in the middle of the channel. It is found that the exciton quenching by Ag electrodes significantly influences the low EQE of the OLET with Ag electrodes. The achievement of high EQE regardless of the emission site is attributable to both better carrier injection and lower exciton quenching at the interface of S/D electrodes for the OLET with ITO.

This study focused on the characteristics of solution-processed phosphorescent organic light-emitting diodes (PHOLEDs) by using a small molecular carbazole derivative, 3,6-di(9-carbazolyl)-9-(2-ethylhexyl)carbazole (TCz1) which has side chains for solubility in organic solvent and high triplet level as a host material for the PHOLEDs. The device with fac-tris(2-phenylpyridine) iridium(III) doped in TCz1 emitted green light, and showed the maximum luminance of approximately 25,000 cd/m2. Blending hole transport material into emissive layer, carrier balance factor in the device was optimized and the maximum current and the external quantum efficiency of 23 cd/A and 6.8% were achieved.

Organic position-sensitive detector (OPSD) based on copper phthalocyanine CuPc:fullerene C60 bulk-heterojunction with an inverted structure have been fabricated using aluminum doped ZnO (ZnO:Al) as a resistive layer, which is prepared by sol–gel method. The resistance length of the one-dimensional PSD is fixed at 5 mm, and the Ag common electrode is fabricated by vacuum evaporation within the 100-μm width. The current density–voltage characteristics with different structures of photodetector, the influence of ZnO:Al resistivity on the thickness and the position characteristics of PSDs are investigated. The experimental results indicate that the architecture, which uses an inverted structure, increases sensitivity under red light illumination compared to the conventional structure. In addition, the thickness of the ZnO:Al has influence on the position characteristics. The resistivity of ZnO:Al film with Al doping concentration of 2 mol% prepared in this study is around 150 Ωcm and it increases from less than approximately 400 nm-thickness. These characteristics seem to be correlated with the properties of ZnO:Al resistive layer. For a device with a 620 nm-thick ZnO:Al layer, the measured position values obtained from the output photocurrent agree with the actual position values under red laser light illumination. CuPc:C60 OPSD with an inverted structure exhibits red light sensitivity, high incident-photon-to-current conversion efficiency of above 80% at −3 V and linearity error of 5.9% at −2V.

Organic light emitting devices, in particular, properties of polymer light-emitting transistors with printed electrodes and bilayer printed devices with in-plane emission have been investigated and discussed. The semitransparent device based on poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT) with Ag-nanowire source/drain and transparent carbon nanotube gate electrodes exhibits ambipolar and light-emitting characteristics. For the devices with oriented poly(9,9-dioctylfluorene) (F8) films, enhanced electron and hole field-effect mobilities have been achieved by aligning the polymer chains parallel to the transport direction. The bilayer device using F8BT lower layer and oriented F8 upper layer with the channel direction parallel to the polymer orientation exhibits improved EL intensity and higher external quantum efficiency than that with the channel direction perpendicular to the polymer chains orientation. The optical pulses of more than 100 Hz frequency are generated by directly modulating a bilayer device with an in-plane emission pattern.

The emission characteristics and transient electroluminescence (EL) of organic light-emitting diodes (OLEDs) using tris(8-hydroxyquinolinato)aluminum (Alq3) as an emissive layer with LiF/Al cathode and LiF doped Al composite cathode (LiF:Al alloy cathode) were investigated. A device with LiF:Al (1:10) alloy cathode showed the maximum current efficiency of 4 cd/A. The emission characteristics of this device were almost the same as that with LiF/Al cathode. Rise time of the device with LiF:Al alloy cathode was shorter than that of a device with LiF/Al cathode, because the waveform rounding of current density occurred for the device with LiF/Al cathode.