Simulations of Passive Matrix Polymer Image Sensors
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*EE Department, Cal Poly State University, San Luis Obispo, CA 93407, [email protected] **UNIAX Corporation, Santa Barbara, CA 93117, [email protected] ABSTRACT Two-dimensional passive photodiode matrices are hardly useful for image sensing due to the crosstalk between pixels. This crosstalk makes it difficult to recover information from individual pixels. A switching unit attached to each sensing unit has been the common solution in image sensors (such as in CMOS sensors and in TFT-PiN a-Si photosensors). A novel organic photodiode with voltage-switchable photosensitivity was developed recently. Passive photodiode matrices made with such organic photodiodes can be used for image sensing applications. This circuit simulation study demonstrates an effective scheme to extract images from passive photodiode matrices, concluding that individual photodiode parameters determine the contrast and resolution of N by M image sensors. INTRODUCTION Much interest in polymer and organic light-emitting diodes and photo-diodes derives from their potential to deliver the next generation of flat and flexible image sensors and passive matrix displays [1-9]. The devices are relatively easy to make, because they consist of a photosensitive or electroluminescent layer sandwiched between an anode, usually transparent, and a cathode. A passive matrix image sensor or display results by patterning the anode into columns and the cathode into rows to form an array of pixels from the intersections between the cathode and anode electrodes. Ap plying a bias to one anode and the opposite bias to one cathode allows light detection or causes light emission from the pixel shared by the two electrodes. Organic photodetectors demonstrated recently have sufficiently high photosensitivity, low dark current and large dynamic range to allow demonstrations of large area photodiode arrays and page size image scanners. When employed in one-dimensional linear photodiode arrays or in twodimensional active matrices, the sensors can distinguish gray levels with at least 12 bit resolution, which is attractive for high quality imaging applications. This study demonstrates a drive scheme that allows high resolution image sensing using a two-dimensional passive matrix configuration. THEORY A passive matrix image sensor consists of an array of photodiodes with devices in the same row sharing a common cathode electrode and devices in the same column sharing a common anode electrode. Figure 1 illustrates the electrical connections for a 4 by 4 pixel sensor. In order to sense the photo-current of a given pixel, applying a positive voltage to a row electrode with respect to a column electrode will reverse bias the diode located at the intersection of the two electrodes and create a current dominated by the photo-current of the diode. In practice, other pixels in the same column contribute dark-current and photo-current to the column current. Consider the diodes in column A with the pixel in the top row illuminated by light and all other pixels dark. With a positive bias,
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