Tomoki Matsubara

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.

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.

Development of FOP-HARP imaging device

The high-gain avalanche rushing amorphous photoconductor (HARP) camera tube achieves ultrahigh-sensitivity by using the avalanche multiplication. The applications of this tube extend beyond broadcasting into other fields. It is attracting a great deal of attention especially for radiation diagnosis, such as synchrotron radiation microangiography, because it can obtain high-resolution and high-contrast images with a low dose of radiation. However, in the present system, a fluorescent screen and the photoconductive film of the HARP tube are connected optically by a lens-coupling method, and low light throughput remains a big problem. To improve the light throughput by using a fiber-coupling method, we applied a fiber-optic plate (FOP) to the substrate of a HARP tube. The FOP consists of three types of glass that have differing hardnesses and elastic coefficients that make it difficult to flatten the FOP surface enough to form the HARP film. We thus introduced a new mechanical polishing method and succeeded in realizing avalanche multiplication in the FOP-HARP tube. The results of shooting experiments by applying the FOP-HARP to the microangiography showed that a spatial resolution of over 20 line pairs/mm was obtained. Moreover, rat femoral arteries of 150-200 μm in diameter could be visualized as motion pictures with a one-fourth lower concentration of contrast material than that needed for ordinary microangiography. Another potential application of the FOP-HARP is an ultrahigh-sensitivity nearinfrared (NIR) image sensor made by fiber-coupling with an image intensifier (I.I.). The image sensor provides highquality images and should be a powerful tool for NIR imaging.

Improvement photoelectric conversion efficiency of red light in HARP film

We enhanced the photoelectric conversion efficiency of red light in a 15-&mgr;m-thick HARP film without deteriorating image pick-up characteristics or reliability. To achieve a higher photoelectric conversion efficiency for red light, we designed a new film structure with an increased amount of doped Te, which has a narrower band gap than that of a-Se. The thickness of the LiF-doped layer for trapping holes was increased from that of the conventional red-extended HARP film in order to weaken the internal field that would otherwise be enhanced by trapped electrons in extra doped Te. The new red-extended HARP film achieved a photoelectric conversion efficiency for red light of about 22.5% at a wavelength of 620 nm, which is twice that of the conventional red-extended film. We confirmed an improvement in signal to shot noise ratio of 3 dB and a dramatic improvement in color reproduction when we experimented with an HDTV camera with a camera tube incorporating the new film.

Heat treatment to suppress image defect occurrence in amorphous selenium avalanche multiplication photoconductive film with improved red-light sensitivity

Amorphous selenium (a-Se) avalanche multiplication photoconductive film, HARP film, has been developed for the purpose of making a high sensitivity video camera. HARP film used for the red channel in a color camera is doped with tellurium (Te) to improve its red-light sensitivity. However, doping a-Se with Te causes impurity levels that trap electrons, and the built-in-field from the trapped electrons causes image defects when a camera using the film is shooting intense spotlights. A heat treatment to suppress the defect occurrence was found. We describe this treatment and discuss its mechanism.