Image Sensor Arrays with Organic Photoconductors
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IMAGE SENSOR ARRAYS WITH ORGANIC PHOTOCONDUCTORS R. A. Streeta, J. Grahamb, Z. Popovicb, A. Horb, M. Mulatoa, R. Laua, and J. Hoa a Xerox PARC, 3333 Coyote Hill Road, Palo Alto, CA 94304 b Xerox Research Center of Canada, Mississauga, Ontario, Canada ABSTRACT Two-dimensional image sensor arrays incorporating an organic light sensor are reported, with potential application to digital cameras and x-ray imaging. The 512x512 element array has pixel size of 100 micron, and operates with active matrix addressing using amorphous silicon thin film transistors. The array design allows the use of a continuous layer of the organic sensor material without the need for pixel-level patterning. The sensor is a bilayer comprising a thin generator layer and a thicker hole transport layer. Generator materials benzimidazole perylene (BZP) and hydroxy gallium phthalocyanine (HOGaPc) have been studied, with a tetraphenyldiamine (TPD) transport layer. Both sensors give excellent imaging properties with low leakage current, good charge collection and high spatial resolution.
INTRODUCTION Amorphous silicon image sensors are used for x-ray imaging and are also of interest for digital cameras. It is of interest to explore alternative sensor materials that can provide different device characteristics and might lower the fabrication cost. For example, organic materials might provide a spectral response that is better matched to a specific application, and the ability to fabricate devices over a wide range thickness is useful to control the capacitance and electronic noise of the system. A linear array with a polymer sensor has already been reported.[1] We report the use of organic sensors in combination with a-Si:H active matrix addressing in a 2-dimensional image sensor. The device is a 512x512 array with 100 micron pixel size. Matrix addressing uses a-Si thin film transistors (TFT), and its fabrication is discussed elsewhere.[2] The pixel circuit contains a TFT, a storage capacitor of ~0.4 pF, the address lines, and a contact pad to the sensor occupying 67% of the pixel area. Each pixel is isolated from its neighbors and from the sensor from the underlying TFT and address lines by a silicon oxynitride passivation layer. The gap between the contact pads of adjacent pixels is 15µm. Since organic materials are hard to process, we use the high fill factor approach, in which the sensor is a continuous layer rather than patterned into individual pixel sensors.[3] Figure 1 shows the crosssection view of the design with the arrangement of the sensor, TFT and capacitor. The continuous sensor design offers the greatest possible sensor area but has the possibility of undesired interactions between pixels, which we investigate. organic sensor passivation TFT
capacitor
ITO Transport layer
light scattering conduction
Generation layer contact pad gap
Figure 1. (Left) The structure of the array pixel, with the TFT and the continuous sensor layer. (Right) Structure of sensor layer, also showing some possible interactions between pixels. C7.7.1
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