Satoshi Aihara

Stacked Image Sensor With Green- and Red-Sensitive Organic Photoconductive Films Applying Zinc Oxide Thin-Film Transistors to a Signal Readout Circuit

A vertically stacked image sensor composed of green (G)- and red (R)-sensitive organic photoconductive films, each having a thin-film transistor (TFT) that uses a transparent zinc oxide (ZnO) channel to read out a signal generated in the organic film, was fabricated. The effective number of pixels of the ZnO-TFT circuits was 1410 (47 times 30), and their pitch was 600 mum. The current on/off ratio and turn-on voltage of the ZnO-TFT were over 105 and 1.5 V, respectively. The G- and R-sensitive organic photoconductive films showed excellent wavelength selectivity: the peak wavelength of the G-sensitive film was 540 nm, and that of the R-sensitive one was 700 nm. A color image with a resolution corresponding to the number of pixels was obtained by a shooting experiment with the fabricated image sensor, which clearly demonstrated color separation in the depth direction of the image sensor, using a stacked structure of wavelength-selective organic films with ZnO-TFT readout circuits.

A 128×96 Pixel Stack-Type Color Image Sensor: Stack of Individual Blue-, Green-, and Red-Sensitive Organic Photoconductive Films Integrated with a ZnO Thin Film Transistor Readout Circuit

A color image was produced by a vertically stacked image sensor with blue (B)-, green (G)-, and red (R)-sensitive organic photoconductive films, each having a thin-film transistor (TFT) array that uses a zinc oxide (ZnO) channel to read out the signal generated in each organic film. The number of the pixels of the fabricated image sensor is 128×96 for each color, and the pixel size is 100×100 µm2. The current on/off ratio of the ZnO TFT is over 106, and the B-, G-, and R-sensitive organic photoconductive films show excellent wavelength selectivity. The stacked image sensor can produce a color image at 10 frames per second with a resolution corresponding to the pixel number. This result clearly shows that color separation is achieved without using any conventional color separation optical system such as a color filter array or a prism.

Efficient energy transfer from polysilane molecules and its application to electroluminescence

Abstract Since the first observation of an efficient resonant energy transfer between polydihexylsilane and polydiphenylsilane, we have studied the energy transfer and radiative properties of polysilanes and their energy-matched counterparts. The synthesis of soluble polysilanes enabled us to prepare such pairs in a thin film simply by a spin-coating technique. The intermolecular energy transfer from conductive poly( m -hexoxyphenyl)phenylsilane was utilized for electroluminescence (EL) of perylene (blue), coumarin 6 (green), 4-(dicyanomethylene)-2-methyl-6-( p -dimethylaminostyryl)-4H-pyran and Zinc tetra-phenylporphyrin (red). A white EL was also observed by mixing four dye molecules. Photo-oxidation during irradiating an ultraviolet light was suppressed by synthesizing water-soluble polysilanes and embedding it inside sol–gel glass matrices. These results imply that polysilanes have a possibility to open new fields of optical applications.

Photoconductive properties of organic films based on porphine complex evaluated with image pickup tubes

We have fabricated two types of organic photoconductive films; a layered structure of tetra(4-methoxyphenyl) porphine cobalt complex (Co–TPP) and bathocuproine (BCP) (target A), and another of Co–TPP, tris-8-hydroxyquinoline aluminum (Alq3) and BCP (target B). The photoconductive properties of each film have been measured using image pickup tubes. The dark current was drastically reduced in target B compared with that in target A at the same applied electric field. The external quantum efficiency reached 20% in target B, which is twenty times higher than that in target A. Image pickup of a test chart from target B at standard TV operation was also demonstrated. High resolution and excellent tone sufficient for television use has been obtained.

Efficient Intermolecular Energy Transfer between Polysilanes Revealed by Time-Resolved Photoluminescence

Conventional studies on polysilanes have dealt with molecular structures on an individual basis and only a few studies have been conducted so far on the intermolecular interaction between different molecules. In order to investigate the intermolecular energy transfer process in detail, time-resolved photoluminescence spectroscopy was carried out against polydihexylsilane (PDHS), polydiphenylsilane (PDPS) and their mixture in powder form. Both the intensity and the lifetime of PDHS emission decreased, while those of PDPS emission increased in the mixed samples. Their mixing-ratio dependence revealed that the resonant energy transfer process from PDHS to PDPS due to multi polar interaction played an important role in light emission characteristics, thus revealing the possibility of their molecular design as interacting pairs and their functional applications.

Electroreductive synthesis and optical characterization of silicon nanoparticles

Electroreduction of tetrachlorosilane was carried out to fabricate silicon (Si) nanoparticles for the first time. Typical size of the obtained particles is 3 nm. The transmission electron microscopy (TEM) and the electron diffraction pattern of the particles revealed that each particle consists only of an amorphous phase. Optical absorption at 5.1 eV and visible photoluminescence (PL) around 2.74 eV were observed for the particles at room temperature.

A 128×96 Pixel, 50 µm Pixel Pitch Transparent Readout Circuit Using Amorphous In–Ga–Zn–O Thin-Film Transistor Array with Indium–Tin Oxide Electrodes for an Organic Image Sensor

Towards a signal readout circuit for a highly sensitive stack-type image sensor, an entire transparent thin-film transistor (TFT) array using an amorphous In–Ga–Zn–O channel and indium–tin oxide electrodes was fabricated. The pixel pitch and number of pixels were 50 µm and 128×96, respectively. The transmittance of the TFT array for visible light reached up to 85%. The array also showed good switching characteristics. A monochromatic image sensor with a zinc phthalocyanine organic photoconductive film was fabricated using this array, and it produced clear images at 30 frames per second with a resolution corresponding to the pixel number.

Image Pickup from Zinc Phthalocyanine/Bathocuproine Double-Layer Film Using Pickup Tube

We have demonstrated image pickup from zinc phthalocyanine/bathocuproine double-layer film incorporated into a pickup tube for both standard television systems and high-definition television (HDTV) systems. A limiting resolution of more than 800 television lines was obtained, which is sufficient for HDTV. The peak external quantum efficiency was 14.7% under 620 nm irradiation. The results indicate that organic molecules have great potential for use in imaging devices.

Temperature-dependent photoluminescence and electroluminescence properties of polysilanes

Abstract We have determined an absolute internal quantum efficiency ( η int ) of various alkyl- and phenyl-based polysilanes by photoluminescence (PL) in the temperature range between 10 and 300 K. Larger η int have been obtained usually in dialkyl-based polysilanes (e.g., 90% at 10 K and 27% at 300 K for polydihexylsilane (PDHS)). The smaller η int of polymethylphenylsilane (PMPS) turned out to be due to the resonant energy-transfer between main- and side-chains. By introducing branching into Si main-chain in the PMPS, the effect of electronic delocalization on the luminescence characteristics has been investigated. A polydimethylsilane (PDMS) deposited on an indium tin oxide (ITO)-covered glass was electroluminesced (EL) at both 77 and 300 K, indicating a possibility for light emitting applications.

Improved Photoconductive Characteristics of Solution-Processed Organic Device by Doping Silole Derivative

By doping 1,1-dimethyl-2,5-bis(N,N-dimethylaminophenyl)-3,4-diphenylsilole (silole-A) in poly(9,9-dioctylfluorene-alt-benzothiadiazole) (F8BT), an improved photoconductive characteristics was observed for a single layer organic device fabricated by a spin-coating process. A maximum external quantum efficiency (EQE) was 8.9% at −20 MV/m when the ratio of silole-A:F8BT was 75 mol%. The EQE of the reference device with F8BT only was 0.06%, and the highest EQE was approximately 160 times higher than that of the reference device. In addition, the photoluminescence (PL) quantum efficiency of the silole-A:F8BT neat film was lower than those of silole-A and F8BT neat films. The lower PL quantum efficiency indicates that the photo-induced carriers efficiently dislocate in the organic layer, resulting in the high EQE of organic photoconductive device.