Organic-Photoconductive-Film-Stacked Image Sensor with Thin-Film-Transistor-Based Active Pixel Sensor Readout Circuits

Organic photoconductive films (OPFs) are attractive optoelectronic materials because they possess high absorption coefficients surpassing those of conventional silicon photodiodes and are thus promising candidates for use in photoconductive parts of flexible, large-area imaging devices. In such applications, thin-film transistor (TFT)-based signal readout circuits with amorphous oxide semiconductors (AOSs) such as indium–gallium–zinc-oxide (IGZO) are useful owing to the high field-effect mobilities of AOSs (&gt;10 cm²/Vs), because image sensor applications require high response speeds and high-pixel-density integration. For example, IGZO-TFT-based flat-panel imagers have been proposed for the detection of X-rays [1]. On the other hand, our group has developed a three-layer-stacked image sensor for realizing compact cameras with high light utilization efficiency using two indium–tin–zinc-oxide (ITZO)-TFT-based readout circuits with blue/green-selective OPFs stacked on a complementary metal oxide semiconductor image sensor (for detecting transmitted red light) with a quarter video graphics array (QVGA) resolution (320 × 240) [2]. Most studies to date, however, have used the passive pixel sensor architecture, which contains one TFT in each pixel for readout of accumulated signals as a small current; thus, images reproduced from the TFT-based sensor were susceptible to external noise. To overcome this obstacle, introducing an active pixel sensor (APS) architecture is necessary, which has three TFTs in each pixel and can perform in-pixel signal amplification [3]. In this study, we describe an excellent performance of our blue-light-sensitive OPF including a dark current low enough to be used as a photoconversion unit of an APS [4], and we demonstrate imaging operation of the OPF-stacked TFT-based APS with a pixel number of 128 × 96 and a pixel pitch of 50 μm.<br/>The TFTs used in this study had a bottom-gate staggered structure, and their channel width and length were 10 μm and 2 μm, respectively. The TFT-based readout circuit was fabricated on a glass substrate by sequentially forming gate electrodes (molybdenum (Mo) alloy, 50 nm), a gate insulator (silicon oxide, 200 nm), pixel electrodes (indium-tin-oxide, 50 nm), an active semiconductive layer (ITZO, 30 nm), via holes in the gate insulator, source/drain electrodes (Mo alloy, 70 nm), signal lines (Mo alloy/aluminum (Al)/Mo alloy, 75 nm), and a passivation layer (organic insulator, 600 nm). After fabrication of the TFT circuits, the OPF was formed on the pixel area by thermally evaporating in the order of an electron-blocking layer (2,7-bis(carbazol-9-yl)-9,9-spirobifluorene, 30 nm), a blue-light-sensitive active layer (2,9-diphenyl-dinaphtho-[2,3-<i>b</i>:2′,3′-<i>f</i>]thieno[3,2-<i>b</i>]thiophene, 200 nm), a hole-blocking layer (4,6-bis(3,5-di(pyridin-4-yl)phenyl)-2-methylpyrimidine, 50 nm), and a counter electrode (Al, 30 nm).<br/>TFT and OPF performances were then measured. The on-off ratio and the extracted field-effect mobility of the TFTs were &gt;10⁸ and 24 cm²/Vs, respectively. The OPF exhibited the blue-light sensitivity with a peak external quantum efficiency of ~59%, and the dark current density of the OPF was 88 pA/cm². Finally, an imaging experiment was conducted with an image sensor test system. Images from test charts with a blue light-emitting diode backlight were projected onto a camera lens and focused on the sensor. By applying operation voltages of a QVGA array at 60 frames per second, we successfully obtained an image output from the fabricated sensor with 128 × 96 pixels. The results obtained here are potentially applicable to not only three-layer-stacked image sensors with enhanced image quality, but also flexible, large-area TFT-based APSs.<br/><br/>[1] R. A. Lujan et al., IEEE Electron Device Lett. 33, 688 (2012).<br/>[2] T. Sakai et al., ACS Appl. Electron. Mater. 3, 3085 (2021).<br/>[3] K. Imamura et al., Jpn. J. Appl. Phys. 60, 064002 (2021).<br/>[4] K. Imamura et al., Jpn. J. Appl. Phys., in press (2022).