Highly Sensitive Organic Photoconductor Using Boron Sub-2,3-naphthalocyanine as a Red-sensitive Film for Stack-type Imag
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Highly Sensitive Organic Photoconductor Using Boron Sub-2,3-naphthalocyanine as a Red-sensitive Film for Stack-type Image Sensors Toshikatsu Sakai, Hokuto Seo, Tomomi Takagi, and Hiroshi Ohtake NHK Science and Technology Research Laboratories, 1-10-15 Kinuta, Setagaya-ku, Tokyo, Japan. ABSTRACT Boron sub-2,3-naphthalocyanine chloride (SubNc) was investigated as a potential redsensitive organic photoconductive film (OPF). A photoconductive cell was fabricated, and its current–voltage characteristics, both with and without light irradiation, and external quantum efficiency (EQE) were determined. The structure of the photoconductive cell was as follows, with thicknesses in nm given in parentheses: glass substrate/In–Zn–O (100)/spiro-2CBP (30)/SubNc (50)/Alq3 (30)/Al (50) (spiro-2CBP = 2,7-bis(carbazol-9-yl)-9,9-spirobifluorene; Alq3 = tris(8-hydroxyquinolinato)aluminum). The spiro-2CBP and Alq3 layers were inserted between the SubNc layer and the electrodes to block dark current injection. The three organic layers were successively deposited by evaporation in a vacuum on the In–Zn–O-patterned substrate. SubNc film absorbed light in the red region well, with an absorption peak at 695 nm. The EQE of the cell reached 80% when the applied bias was 15 V. In addition, the blocking layers effectively suppressed dark current in the OPF, which corresponded to a current density of 20 nA/cm2 at 15 V. These results indicate that SubNc is a promising candidate as a red-sensitive OPF. INTRODUCTION Organic semiconductor materials have been widely studied as promising materials for applications in photoactive devices such as photovoltaic (PV) systems [1-5] and photodetectors. There have been reports of an organic phototransistor [6], a photodetector that was color-tunable by changing the voltage applied to an organic film [7], and arrays for digital image capture using organic photoconductors [8,9]. Moreover, we have proposed a new type of image sensor that is overlaid with three different organic photoconductive layers, each of which is sensitive to only one of the three primary colors [10-13]. It is advantageous to use organic materials in stack-type sensors owing to their color selectivity. This type of sensor has great potential in the manufacture of high-resolution and highly sensitive color cameras, because their light use efficiency can be improved compared with conventional sensors that use a Bayer filter as a color filter array. In previous studies, we fabricated a stack-type color image sensor with blue (B)-, green (G)-, and red (R)-sensitive organic photoconductive films (OPFs), each having a signal readout circuit with an oxide semiconductor thin-film transistor array, and obtained color images from the fabricated sensor [12,13]. However, the photon-to-charge conversion efficiency of these OPFs was insufficient for use in image sensors. In particular, improvements in the efficiency of the G- and R-sensitive films were greatly desired. On the other hand, boron sub-2,3naphthalocyanine chloride (SubNc) has recently been studi
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