Transparent Image Sensors Using an Organic Multilayer Photodiode

Abstract

The most fascinating advantages of organic photodiodes in the context of information-conversion applications are their ability for large-area surface light detection, the potentially wide selection of materials that allows color tuning, and the possibility for low-cost fabrication of flexible devices. The photosensitivity of organic photodiodes, either multilayer-heterojunction type or bulk-heterojunction type, is quite high, and internal photon-to-electron conversion quantum efficiency in some cases approaches unity. Thus, we have a good opportunity to utilize organic photodiodes for high-performance photosensors, even though power-conversion efficiency of organic photovoltaic solar cells under solar light irradiation remains less than 5 %. Fabrications of organic photodiodes with good photoresponse, such as fast-response photodetectors, full-color image sensors, and position-sensitive photodetectors, have been reported. The use of anisotropy of optical absorption results from molecular orientation is one of the most fascinating advantages of organic photodiodes. The combination of transparent electrodes and polarized light detection that results from optical anisotropy of molecules provides unique image-sensing devices that capture images under light illumination behind them. Figure 1 demonstrates the basic concept for reading a black and white picture using our transparent photodiodes with polarization-detection capability. Let us assume that white light or monochromatic light is transmitted through a linear polarizer to the polarization-sensitive photodetector. At present, for simplicity, we assume that the photodetector is composed of oriented molecules with large optical anisotropy. When polarized light perpendicular to the transition moments of the oriented molecules irradiates the photodetector, light passes through the detector with little optical absorption. Thus, a very small photocurrent is detected by the photodiode when the incident light is perpendicularly polarized. The transmitted polarized light is reflected back according to the reflectivity of the object. When white paper is used, a large fraction of the light is reflected back and depolarized by scattering. Thus, the light component with parallel polarization reaches the photodetector and a large photocurrent signal is detected. On the other hand, when the transmitted polarized light reaches a black object, all the light is absorbed and no photocurrent is detected. Thus, the transparent and polarization-sensitive photodetectors are useful for the detection of reflectivity of objects. We note that this principle is applicable to grayscale sensing as well as simple black and white detection. When addressable crossed-array photodiodes are used or 2D scanning of incident light is available, 2D picture images are obtainable. We have successfully developed polarization-sensitive photodiodes using a pn-heterojunction-type structure composed of 3,4,9,10-perylenetetracarboxylic-bis-benzimidazole (PTCBI) for the n-type layer and titanyl phthalocyanine (TiOPc) for the p-type layer. In this report, we demonstrate the feasibility of our transparent photodetector composed of a well-aligned PTCBI and a non-aligned TiOPc layer for image sensing. As shown in Figure 2a, the device configuration was indium tin oxide (ITO)/indium/aligned PTCBI/TiOPc/poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS)/ Au. A well-aligned PTCBI layer with the optical axis parallel to the rubbing direction was grown. A large dichroic ratio of optical absorption was obtained at the peak wavelength of 540 nm. Figure 2b shows absorption spectra of both aligned PTCBI and non-aligned TiOPc films. An aligned PTCBI showed a dichroic ratio of 4.7:1 at the peak wavelength of 540 nm, while non-aligned TiOPc showed isotropic absorpC O M M U N IC A TI O N S