Image Sensors with Frustrated Charge Transport

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Journal of Applied Physics publishes a paper "Organic photodetectors with frustrated charge transport for small-pitch image sensors" by Z. Ma and C. K. Renshaw from University of Central Florida, Orlando, FL.

"We demonstrate a frustrated organic photodetector (F-OPD) that utilizes frustrated charge-transport to quench forward-bias current and provide a low-current, light-independent OFF state. Photocurrent is collected efficiently with −3 V reverse-bias recovering the sensitive OPD response with higher than 10-bit dynamic range. This intrinsic switching mechanism eliminates the need for thin-film transistors (TFTs) to provide readout control in high-resolution image sensors. Eliminating TFTs simplifies fabrication processing, improves fill-factor, and enables higher resolution image sensors on nonplanar, stretchable, or large-area substrates for a variety of imaging applications. We simulate image sensors and show that the performance is limited by the OFF state uniformity experimentally observed across 45 devices. We simulate performance in a 900-pixel array and show that the demonstrated F-OPDs can scale into megapixel arrays with a noise-equivalent power of less than 0.6 mW/cm2 and a dynamic range of more than 6-bits; better uniformity can substantially improve this performance for large arrays."

"The F-OPD utilizes a blocking layer integrated with the OPD to frustrate charge collection and provide a low-current OFF state under forward bias. A few volts of reverse bias switches the pixel into a conducting ON state where the OPD photocurrent is efficiently collected."

"We have demonstrated F-OPDs utilizing frustrated charge-transport to enable transistor-free pixels for organic image sensors. A blocking layer at the anode reduces forward-bias current a thousand-fold and provides a low-current, light-independent OFF state; meanwhile, a few volts of reverse-bias recovers the high sensitivity and dynamic range typical of OPDs. The F-OPD operates like conventional passive pixels but elimination of the readout transistor avoids (1) allocation of pixel area to the transistor, (2) definition of subpixel features for drain/source/gate/insulator/channel, and (3) additional gate interconnects spanning the circuit. Pixel functionality is defined by a single, monolithic stack to allow nearly 100% FF and small-pixel-pitch using minimal fabrication steps. Combined with high-resolution transfer patterning for organic circuits,12 F-OPDs could enable scaling OISs to a less than 10 μm pixel-pitch limited only by edge effects and lateral photoconductive leakage. This streamlined processing can also reduce cost for curved, flexible, lightweight, large-area, and/or attritable OISs."

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