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.
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.
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.
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