Hokuto Seo

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.

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.

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.

Improvements in Photoconductive Characteristics of Organic Device Using Silole Derivative

Poly[9,9-dioctylfluorenyl-2,7-diyl]-co-1,4-benzo-(2,1,3)-thiadiazole (F8BT) is one of the suitable materials for color-selective organic photoconductive devices owing to its high carrier mobility and absorption coefficient against only the blue light. We investigated a mixing method using a silole derivative, 1,1-dimethyl-2,3,4,5-tetraphenylsilole (DMTPS), in F8BT to improve the ratio between photocurrent and dark current (ON/OFF ratio), which is an important parameter for practical organic photoconductive devices. These organic materials can be coated by a solution process, which leads us to realize a low fabrication cost and a large device area in the future. By adding DMTPS into F8BT, the maximum improvement in ON/OFF ratio of 5 times was achieved compared with the reference device with F8BT only; however, the external quantum efficiency was independent of the concentration of DMTPS less than 50 wt %. In addition, the wavelength selectivity of DMTPS:F8BT in the visible wavelength region was almost the same as that of F8BT only. This result indicates that the DMTPS:F8BT layer can absorb only the blue light, indicating the possibility of a color separation without a prism for high-resolution cameras by combining the green- and red-sensitive devices.

Continuous fabrication technology for improving resolution in RGB-stacked organic image sensor

With the goal of developing a compact, high-resolution color camera, we have been studying about a novel image sensor with three stacked organic photoconductive films: each film is sensitive to only one of the primary color components, and each has a signal readout circuit. In this type of image sensor, the acceptable focal depth is roughly estimated to be shorter than about 20 μm when the pixel pitch of the sensor is several μm. To reduce the total thickness of the stack-type sensor, a continuous fabrication technology that entails stacking continuously all layers from the bottom to the top of the sensor is necessary. In the continuously stacked sensor, the three organic layers separated by interlayer insulators are formed close to each other on a single glass substrate. In this paper, we describe the elemental technologies for the continuous fabrication of a stack-type organic image sensor consisting of improving the heat resistance of organic films and decreasing the fabrication temperature of the interlayer insulators and signal readout circuits. A 150°C heat-resistant organic photoconductive film can be obtained by using organic materials possessing high glass-transition temperatures, and low-temperature fabrication of the interlayer insulator can be accomplished by metal oxides using atomic layer deposition (ALD) at 150°C. The amorphous In-Ga-Zn-O thin-film transistors (TFT) are fabricated at a maximum temperature of 150°C by using Al2O3 gate insulator via ALD and a post-treatment. The resulting TFT has good transfer characteristics. A continuously-stacked organic image sensor can be fabricated by integrating these technologies.

Stacked color image sensor using wavelength-selective organic photoconductive films with zinc-oxide thin film transistors as a signal readout circuit

Our group has been developing a new type of image sensor overlaid with three organic photoconductive films, which are individually sensitive to only one of the primary color components (blue (B), green (G), or red (R) light), with the aim of developing a compact, high resolution color camera without any color separation optical systems. In this paper, we firstly revealed the unique characteristics of organic photoconductive films. Only choosing organic materials can tune the photoconductive properties of the film, especially excellent wavelength selectivities which are good enough to divide the incident light into three primary colors. Color separation with vertically stacked organic films was also shown. In addition, the high-resolution of organic photoconductive films sufficient for high-definition television (HDTV) was confirmed in a shooting experiment using a camera tube. Secondly, as a step toward our goal, we fabricated a stacked organic image sensor with G- and R-sensitive organic photoconductive films, each of which had a zinc oxide (ZnO) thin film transistor (TFT) readout circuit, and demonstrated image pickup at a TV frame rate. A color image with a resolution corresponding to the pixel number of the ZnO TFT readout circuit was obtained from the stacked image sensor. These results show the potential for the development of high-resolution prism-less color cameras with stacked organic photoconductive films.

Improvement in Photoconductive Properties of Coumarin 30-Evaporated Film by Fullerene Doping for Blue-Sensitive Photoconductors

We fabricated two types of organic photoconductive film consisting of coumarin 30/tris(8-hydroxyquinoline)aluminum (Alq3) and fullerene (C60, 10%) : coumarin 30 (90%)/Alq3 by vacuum evaporation and measured the effect of doping C60 as an electron acceptor on the photoconductive properties of the film. The spectral photoresponse characteristics of both films showed good wavelength selectivity in the blue-light region: the peak wavelength of a coumarin 30 neat film was 410 nm and that of a C60:coumarin 30 codeposited film was 440 nm. The external quantum efficiency of the codeposited film reached 64% at an applied voltage of 10 V, while that of the neat film remained at 10% at its highest. These results clearly indicate that the dissociation of electron-hole pairs at the coumarin 30/C60 interface occurs in the codeposited film.

Color image sensor using stacked organic photoconductive films with transparent readout circuits separated by thin interlayer insulator

We have been working on developing an image sensor with three stacked organic photoconductive films (OPFs) sensitive to only one primary color component (red—R, green—G, or blue—B); each OPF has a signal readout circuit. This type of stacked sensor is advantageous for the manufacture of compact color cameras with high-quality pictures, since color separation systems, such as prisms or color filter arrays, are eliminated because of the color selectivity of OPFs. To achieve a high-resolution stacked sensor, its total thickness should be reduced to less than 10 μm. In this study, we fabricated a color image sensor with R and G-sensitive OPFs by applying amorphous In-Ga-Zn-O thin-film transistor (TFT) readout circuits. A 10 μm-thick interlayer insulator separated the R and G-sensitive layers. The entire fabrication process for the device was implemented below 150°C to avoid damaging the OPFs. Output signals were successfully read from each OPF through the TFT circuit, and multi-color images were reproduced from the fabricated sensor.

Stacked organic photoconductive films and thin-film transistor circuits separated by thin silicon nitride for a color image sensor

We develop a color image sensor with three stacked organic photoconductive films (OPFs) sensitive to only one primary color component (red (R), green (G), or blue (B)); each OPF has a signal readout circuit. This type of stacked image sensor enables a small color camera to achieve high picture quality, because color-separation systems, such as color filter arrays, are eliminated. Previously, we demonstrated green, red, and yellow color images reproduced from a sensor with two stacked OPFs sensitive to the R or G component by applying In-Ga-Zn-O thin film transistor (IGZO TFT) circuits separated by a 10-μm-thick epoxy film as an interlayer insulator. In this study, we first improve the configuration of the IGZO TFT to reduce the compressive stress that causes the films to peel off and break the stacked structure. Then, we fabricate a structure with three stacked OPFs sensitive to the R, G, or B component with a thickness of 5.8 μm by using 2-μm-thick silicon nitrides as interlayer insulators.